Soil-Plant Nutrient Cycling and Environmental Quality

 

Department of Plant and Soil Sciences

 

Oklahoma State University

SOIL 5813

 

 

 

W.R. Raun, G.V. Johnson,
R.W. Mullen, K.W. Freeman,
and R.L. Westerman

 

044 N. Ag Hall

Tel: (405) 744-6418

FAX: (405) 744-5269

 

wrr@mail.pss.okstate.edu

gvj@mail.pss.okstate.edu
rwm@okstate.edu
fkyle@okstate.edu

rlw@mail.pss.okstate.edu



"In recent years the 'human rights' issue has generated much interest and debate around the world.  It is a utopian issue and a noble goal to work toward.  Nevertheless, in the real world, the attainment of human rights in the fullest sense can not be achieved so long as hundreds of millions of poverty stricken people lack the basic necessities for life.  The right to dissent does not mean much to a person with an empty stomach, a shirtless back, a roofless dwelling, the frustrations and fear of unemployment and poverty, the lack of education and opportunity, and the pain, misery and loneliness of sickness without medical care.  It is my belief that all who are born into the world have the moral right to the basic ingredients for a decent, human life." 

 

Norman E. Borlaug

1970 Nobel Peace Prize

 

 

"Learning science and thinking about science or reading a paper is not about learning what a person did.  You have to do that, but to really absorb it, you have to turn it around and cast it in a form as if you invented it yourself.  You have to look and be able to see things that other people looked at and didn't see before.  How do you do that?  There's two ways.  Either you make a new instrument, and it gives you better eyes, like Galileo's telescope.  And that's a great way to do it, make such a nice instrument that you don't have to be so smart, you just look and there it is.  Or you try to internalize it in such a way that it really becomes intuitive.  Working on the right problem is only part of what it takes to succeed.  Perseverance is another essential ingredient."

 

Steven Chu

1997 Nobel Prize, Physics

 

intuition: immediate apprehension or cognition; without evident rational thought and inference; quick and ready insight

Soil-Plant Nutrient Cycling and Environmental Quality

 

Students, 1992- 2002

 

 

Spring 1992                     Spring 1994                     Spring 1996                   Spring 1998                     Spring 2000

Mohd Akbar                     Jeri L. Anderson             Justin Carpenter            Erna Lukina                     Elbek Arslanov

John V. Altom                  Jeffrey B. Ball                 Chad Dow                     Joanne LaRuffa              Michael Blazier

Edgar N. Ascencio          Andrew C. Bennett         Mark Everett                  Curt Woolfolk                   Danielle Bradford

Senayet Assefa              Jing Chen                        Mike Goedeken              Lori Gallimore                  Kyle Freeman

Randy K. Boman             Francisco Gavi-Reyes    Eric Hanke                     Doug Cossey                  Jon Karl Fuhrman

T. Ramanarayanan          David L. Gay                   Dale Keahey                  Bryan Howell                  Prajakta Ghatpande

Ananda Ramanathan      Dallas L. Geis                  Jason Kelley                  Micah DeLeon                 Cody Gray

C. W. Richardson            James P. Johnson           Butch Koemel                Rick Kochenower           Jay Ladd

Hasil Sembiring                Tracy D. Johnston           Heather Lees                Renee Albers                  Jennifer Lepper

Sonia Morales                 Fred Kanampiu                Alan O'Dell                     Matthew Barnes             Rachelle Moussavou

                                        Xin Li                               John Ringer                   Clydette Borthick             Robert Mullen

                                        Steven Phillips                 Jerry Speir                    Wade Thomason             Susan Mullins

                                        Asrat Shiferaw               Gary Strickland             Elizabeth Dayton             Shea Murdock

                                                                                Shannon Taylor             Jeremy Dennis                Eric Palmer

                                                                                Derrel White                  Elena Jigoulina                 Heather Qualls

                                                                                Mark Wood                    Aleksandr Felitsiant         Chris Stiegler

                                                                                Jason Yoder                 Michelle Franetovich       Clemn Turner

                                                                                                                      Todd Heap                       Jason Warren

                                                                                                                      Tyson Ochsner               Damon Wright

                                                                                                                      Steven McGowen           Kathie Wynn

                                                                                                                      John Roberts                  

                                                                                                                      Matthew Rowland          

                                                                                                                      Robert Zupancic            

                                                                                                                      Shawn Zupancic           

                                                                      Olga Kachurina              

 

Spring 2002

Randy Davis

Kefyalew Girma

Micah Humphreys

Jitao Si

Jason Lawles

Adi Malapati

Shambel Moges

Jagadeesh Mosali

Jamie Patton

Yan Tang

Roger Teal

 

 



TABLE of CONTENTS

 

1. Organic Matter                                                                                                                                                                   4

Nutrient Supplying Power of Soil                                                                                                                             4

Composition of Organic Matter                                                                                                                               4

C:N Ratios as Related to Organic Matter Decomposition                                                                       4

Decomposition of Organic Matter (Mineralization)                                                                                  4

Microorganisms                                                                                                                                                                  4

2. Essential Elements                                                                                                                                                            4

Arnon's Criteria of Essentiality                                                                                                                              4

3. The Nitrogen Cycle                                                                                                                                                            4

Inorganic Nitrogen Buffering                                                                                                                                    4

Ammonia Volatilization                                                                                                                                              4

Chemical Equilibria                                                                                                                                                          4

Urea                                                                                                                                                                                             4

Urea Hydrolysis                                                                                                                                                                   4

H ion buffering capacity of the soil:                                                                                                                     4

Factors Affecting Soil Acidity                                                                                                                                  4

Acidification from N Fertilizers (R.L. Westerman)                                                                                       4

4. Nitrogen Use Efficiency                                                                                                                                                  4

N Discussion                                                                                                                                                                                4

5. Use of Stable and Radioactive Isotopes                                                                                                           4

Historical                                                                                                                                                                                4

Sources of Radiation                                                                                                                                                       4

Agronomic Applications                                                                                                                                                4

6. Exchange                                                                                                                                                                                  4

Cation Exchange Capacity (CEC):                                                                                                                               4

Effective CEC                                                                                                                                                                           4

CEC Problems                                                                                                                                                                          4

Base Saturation                                                                                                                                                                 4

Anion Exchange (Kamprath)                                                                                                                                       4

7. Phosphorus Fertilizers                                                                                                                                                 4

Rock Phosphate                                                                                                                                                                    4

Calcium Orthophosphates                                                                                                                                           4

8. Theoretical Applications in Soil Fertility                                                                                                     4

Liebig's law of the minimum (Justus von Liebig 1803-1873)                                                                            4

Bray Nutrient Mobility Concept                                                                                                                               4

Sufficiency: SLAN (Sufficiency Levels of Available Nutrients)                                                             4

Plant Response to Soil Fertility as Described by the Percent Sufficiency and the Mobility Concept          4

Mitscherlich (applicability of this growth function to soil test correlation studies)  4

Bray Modified Mitscherlich                                                                                                                                        4

Fried and Dean (1951)                                                                                                                                                          4

Base Cation Saturation Ratio                                                                                                                                  4

9. Soil Testing / Critical Level Determination                                                                                                   4

Economic and Agronomic Impacts of Varied Philosophies of Soil Testing (Olson et al., 1982)            4

Cate and Nelson (1965)                                                                                                                                                      4

Use of Price Ratios                                                                                                                                                              4

Soil Testing for Different Nutrients                                                                                                                     4

Dry Combustion (Dumas 1831)                                                                                                                                       4

Rittenberg Method (N2 gas from sample)                                                                                                            4

Inorganic Nitrogen                                                                                                                                                            4

Phosphorus Soil Index Procedures                                                                                                                          4

Total P ?                                                                                                                                                                                    4

Nutrient Interactions                                                                                                                                                    4

Spectroscopy                                                                                                                                                                         4

Soil Testing versus Non-destructive Sensor Based VRT                                                                              4

Experimental Design/Soil Testing and Field Variability                                                                           4

10. Micronutrients                                                                                                                                                                 4

Chlorine                                                                                                                                                                                    4

Boron                                                                                                                                                                                          4

Molybdenum                                                                                                                                                                          4

Iron                                                                                                                                                                                              4

Manganese                                                                                                                                                                              4

Copper                                                                                                                                                                                         4

Zinc                                                                                                                                                                                               4

11. Special Topics                                                                                                                                                                    4

Method of Placement                                                                                                                                                      4

Saline/Sodic Soils                                                                                                                                                                4

Stability Analysis                                                                                                                                                             4

Stability Analysis: discussion                                                                                                                                   4

Soil Solution Equilibria                                                                                                                                                 4

Some Rules of Thumb for Predicting the Outcome of Simple Inorganic Chemical Reactions Related to Soil Fertility                                                                                                                                                                                    4

References                                                                                                                                                                               4

12. NUTRIENT CYCLES                                                                                                                                                                 4

NITROGEN                                                                                                                                                                                    4

PHOSPHORUS                                                                                                                                                                             4

POTASSIUM                                                                                                                                                                                4

IRON                                                                                                                                                                                              4

SULFUR                                                                                                                                                                                         4

CARBON                                                                                                                                                                                       4

CALCIUM                                                                                                                                                                                     4

MAGNESIUM                                                                                                                                                                              4

BORON                                                                                                                                                                                          4

MANGANESE                                                                                                                                                                              4

COBALT                                                                                                                                                                                        4

CHLORINE                                                                                                                                                                                    4

COPPER                                                                                                                                                                                         4

ZINC                                                                                                                                                                                               4

MOLYBDENUM                                                                                                                                                                          4

ALUMINUM                                                                                                                                                                                4

SODIUM                                                                                                                                                                                        4

VANADIUM                                                                                                                                                                                 4

OXYGEN                                                                                                                                                                                        4

SILICON                                                                                                                                                                                        4

13. Example Exams                                                                                                                                                                   4

14. STATISTICAL APPLICATIONS                                                                                                                                           4

Reliability                                                                                                                                                                               4

Surface Response Model                                                                                                                                                 4

Procedure for Determining Differences in Population Means                                                              4

Randomized Complete Block Randomization                                                                                                 4

Program to output Transposed Data                                                                                                                  4

Contrast Program for Unequal Spacing                                                                                                            4

Test of Differences in Slope and Intercept Components from Two Independent Regressions            4

Linear-Plateau Program                                                                                                                                               4

Linear-Linear Program                                                                                                                                                   4

 


1. Organic Matter

 

Nutrient Supplying Power of Soil    

 

In the past 150 years, CO2 levels in the atmosphere have increased from 260 to 365 ppm (Follett and McConkey, 2000) and it is expected to rise 1.5 to 2.0 ppm per year (Wittwer, 1985).  This increase is believed to have increased the average temperature of the earth by 0.5 °C and thus various reports of global warming as a result of increased evolution of CO2 into earth's atmosphere (Perry, 1983).  It is possible to decrease the release of CO2 to the atmosphere by choosing an alternative energy source.  However, total control of the release of CO2 is not easy because there are so many different sources, including the production of cement, gasoline-driven automobiles, burning of fuels for home heating, cooking, etc. (Wallace et al., 1990).  There are, however, several benefits associated with increased atmospheric CO2 including increased water use efficiency, nitrogen use efficiency and production in many crops.

            If the expected fossil fuel CO2 released for many years could be stored as soil organic matter, vastly enhanced productive soil would result.  This option requires increased biomass to produce the needed soil organic matter, but this could be achievable due to increased CO2 supplies in the atmosphere (Wallace et al., 1990).  Obstacles to increasing the level of soil organic matter are; 1) needed organic matter supplies, 2) needed nitrogen to give around a 10:1 carbon:nitrogen ratio necessary for stable soil organic matter, and 3) efficiency in microbial activity that can result in more stable soil organic matter, instead of burn out resulting in return of CO2 to the atmosphere (Wallace et al., 1990).

            It is seldom understood that organic matter contents in soils can be increased via various management practices.  Increased use of no-till management practices can increase soil organic matter.  After ten years of no-tillage with corn, soil organic carbon in the surface 30 cm was increased by 0.25% (Blevins et al. 1983).  Probably the least understood is increased N rates in continuous crop production on resultant soil organic matter levels.  Various authors have documented that N rates in excess of that required for maximum yields result in increased biomass production (decreased harvest index values e.g., unit grain produced per unit dry matter).  This results in increased amounts of carbon from corn stalks, wheat stems, etc., that are incorporated back into soil organic matter pools.  Although this effect is well documented, the deleterious effects of increased fertilizer N rates on potential NO3 leaching and/or NO3 surface runoff should be considered where appropriate.  Use of green manures and animal wastes have obvious impacts on soil organic matter when used on a frequent basis. 

            The native fertility of forest and grassland soils in North America has declined significantly as soil organic matter was mined by crop removal without subsequent addition of plant and animal manures (Doran and Smith, 1987).  For literally thousands of years, organic matter levels were allowed to increase in these native prairie soils since no cultivation was ever employed.  As soil organic matter levels declined, so too has soil productivity while surface soil erosion losses have increased.  Because of this, net mineralization of soil organic nitrogen fell below that needed for sustained grain crop production (Doran and Smith, 1987).  Work by Campbell, 1976 demonstrates that to maintain yields with continuous cultivation, supplemental N inputs from fertilizers, animal manures or legumes are required (Figure 1.1).


 

Figure 1.1.  Influence of cultivation time on relative mineralization from soil humus and wheat residue. (From Campbell et al. (1976)).

 

            When the reddish prairie soils of Oklahoma were first cultivated in the late 1800s, there was approximately 4.0% soil organic matter in the surface 1 foot of soil.  Within that 4.0% organic matter, there were over 8000 lb of N/acre.  Following more than 100 years of continuous cultivation, soil organic matter has now declined to less than 1%.  Within that 1% organic matter, only 2000 lb of N/acre remains.

 

 

 

 

 

N removal in the Check  (no fertilization) plot of the Magruder Plots

 

      20 bu/acre * 60 lb/bu * 100 years = 120000lbs

      120000 lbs * 2%N in the grain = 2400 lbs N/acre over 100 years

      8000 lbs N in the soil (1892)

      2000 lbs N in the soil (1992)

      2400 lbs N removed in the grain

      =3600 lbs N unaccounted

 

            The effects that management systems will have on soil organic matter and the resultant nutrient supplying power of the organic pools are well known.  Various management variables and their effect on soil organic matter are listed;

 

Organic Matter Management                    Effect

_________________________________________________________

1) tillage                                                      +/-                                                conventional    -                                                                     

         zero                                                     +

2) soil drainage                                           +/-

3) crop residue placement                           +/-

4) burning                                                     -

5) use of green manures                               +

6) animal wastes and composts                    +

7) nutrient management                              +/-                                                excess N          +

_________________________________________________________

 

Composition of Organic Matter

 

            The living component which includes soil microorganisms and fauna make up a relatively small portion of total soil organic matter (1-8%).  It functions however as an important catalyst for transformations of N and other nutrients (Doran and Smith, 1987).  The majority of soil organic matter is contained in the nonliving component that includes plant, animal and microbial debris and soil humus. 

            Common components of soil organic matter and their relative rates of decay are listed in Table 1.1.  Cellulose generally accounts for the largest proportion of fresh organic material.  It generally decays rather rapidly, however, the presence of N is needed in order for this to take place.  Lignin components decompose much more slowly and thus, any nutrients bound in lignin forms will not become available for plant growth.  Although lignin is insoluble in hot water and neutral organic solvents, it can be solubilized in alkali solutions.  Because of this, we seldom find calcareous soils with extremely high organic matter.  All of the polysaccharides decompose rapidly in soils and thus serve as an immediate source of C for microorganisms.  Decomposition of these respective components is illustrated in Figure 1.2.

 

Table 1.1.  Components of soil organic matter, rate of decomposition and composition of each fraction.

____________________________________________________________________________________

Form                                        Formula                                Decomposition                 Composition

____________________________________________________________________________________

Cellulose                                 (C6H10O5)n                               rapid *                              15-50%

 

Hemicellulose                                                                                                               5-35% 

        glucose                            C6H12O6                                   moderate-slow

        galactose

        mannose

        xylose                              C5H10O5                                   moderate-slow

       


Lignin(phenyl-propane)                                                        slow                                 15-35%

 

Crude Protein                          RCHNH2COOH**                     rapid                                1-10%

 

Polysaccharides

        Chitin                               (C6H9O4.NHCOCH3)n                 rapid

        Starch                              glucose chain                       rapid

        Pectins                            galacturonic acid                  rapid

        Inulin                               fructose units

____________________________________________________________________________________

* - decomposition more rapid in the presence of N

** - amino acid glycine (one of many building blocks for proteins)


Figure 1.2. Decomposition of Miscanthus sinensis leaf litter.

 

Table 1.2.  Composition of mature cornstalks (Zea mays L.) initially and after 205 days of incubation with a mixed soil microflora, in the presence and absence of added nutrients (Tenney and Waksman, 1929)

____________________________________________________________________________________

                                                                   Initial                     Composition after 205 days (%)

                                                              composition                No nutrients              Nutrients

Constituents or fraction                                 %                             added                     added

____________________________________________________________________________________

Ether and alcohol soluble                              6                                 1                            <1

Cold water soluble                                        11                                3                             4

Hot water soluble                                           4                                 4                             5

Hemicelluloses                                             18                               15                           11

Cellulose                                                       30                               13                            6

Lignins                                                          11                               23                           24

Crude protein                                                 2                                 9                            11

Ash                                                                 7                                19                           26

____________________________________________________________________________________

 

            The composition of mature cornstalks before and after 205 days of incubation with a mixed soil in the presence and absence of added nutrients is listed in Table 1.2.  As decomposition proceeds, the water soluble fraction (sugars, starch, organic acids, pectins and tannins and array of nitrogen compounds) is readily utilized by the microflora (Parr and Papendick, 1978).  Ether and alcohol-soluble fractions (fats, waxes, resins, oils), hemicelluloses and cellulose decrease with time as they are utilized as carbon and energy sources.  Lignin, tends to persist and accumulate in the decaying biomass because of its resistance to microbial decomposition.  Decomposition rates of crop residues are often proportional to their lignin content and some researchers have suggested that the lignin content may be a more reliable parameter for predicting residue decomposition rates than the C:N ratio (Alexander, 1977).  Vigil and Kissel (1991) included the lignin-to-N ratio and total soil N concentration (in g/kg) as independent variables to predict potential N mineralization in soil.  They also noted that the break point between net N mineralization and net immobilization was calculated to be at a C/N ratio of 40.


            A simple illustration of the carbon cycle is found in Figure 1.3.  The carbon cycle revolves around CO2, its fixation and regeneration.  Chlorophyll-containing plants utilize the gas as their sole carbon source and the carbonaceous matter synthesized serves to supply the animal world with preformed organic carbon.  Upon the death of the plant or animal, microbial metabolism assumes the dominant role in the cyclic sequence (Alexander, 1977).  Without the microbial pool, more carbon would be fixed than is released, CO2 concentrations in the atmosphere would decrease and photosynthesis rates would decrease.

 

Figure 1.3. Simple illustration of the carbon cycle (from Alexander, 1977). "Higher plants use light to convert water (H2O) and carbon dioxide (CO2) to glucose (C6H12O6) and oxygen (O2)."

 

 

 

 

 

 

 

 

 

 

 

 

 

C:N Ratios as Related to Organic Matter Decomposition

 

            In general, the following C:N ratios are considered to be a general rule of thumb in terms of what is expected for immobilization and mineralization.

_____________________________________________________

            C:N Ratio                                 Effect

_____________________________________________________

            30:1                                          immobilization

            <20:1                                        mineralization

            20-30:1                                    immobilization = mineralization

_____________________________________________________

 

            Unfortunately, C:N ratios say nothing about the availability of carbon or nitrogen to microorganisms.  The reason for this is because we are not aware of what makes up the carbon (C) component.  In tropical soils, significantly higher proportions of lignin will be present in the organic matter.  Even though the percent N within the organic matter may be the same, it would be present in highly stable forms that were resistant to decomposition.  Therefore, mineralization rates in organic matter that contain high proportions of lignin will be much smaller.  The C:N ratios discussed were generally developed from data obtained in temperate climates.  Therefore their applicability to tropical soils is at best minimal. 

 

Decomposition of Organic Matter (Mineralization)

 

                                                1. percent organic matter       

                                                2. organic matter composition

                                                3. cultivation (crop, tillage, burning)   

                                                4. climate (moisture, temperature)

                                                5. soil pH

                                                6. N management (fertilization)

                                                7. soil aeration

 

            During the initial stages of decomposition of fresh organic material there is a rapid increase in the number of heterotrophic organisms accompanied by the evolution of large amounts of carbon dioxide.  If the C:N ratio of the fresh material is wide, there will be a net N immobilization.  As decay proceeds, the C:N ratio narrows and the energy supply of carbon diminishes. 

            The addition of materials that contain more than 1.5 to 1.7% N would ordinarily need no supplemental fertilizer N or soil N to meet the demands of the microorganisms during decomposition (Parr and Papendick, 1978).  Note that the 'demands of the microorganisms' is what is discussed first, with no regard as to what plant N needs might be.  The addition of large amounts of oxidizable carbon from residues with less than 1.5% N creates a microbiological demand for N which can immobilize residue N and available inorganic soil N for extended periods.  The addition of supplemental inorganic fertilizer N to low N residues can accelerate their rate of decomposition (Parr and Papendick, 1978).

            In the thousands of years prior to the time cultivation was initiated, C and N had built up in native prairie soils.  However, the C:N ratio was wide, reflecting conditions for immobilization of N.  The combined influence of tillage and the application of additional organic materials (easily decomposable wheat straw and/or corn stalks) is illustrated in Figure 1.4.  Cultivation alone unleashed a radical decomposition of the 4% organic matter in Oklahoma soils.  When easily decomposable organic materials are added back to a cultivated soil, CO2 evolution increases and NO3 is initially immobilized.  However, within one yearly cycle in a temperate climate, the net increase in NO3 is reflected in Figure 1.4 via mineralization of the freshly added straw/stalks and native organic matter pools.  With time, the percent N in added organic material increases while the C:N ratio decreases (Figure 1.5).  However, it is important to note that in order for this to happen, some form of carbon must be lost from the system.  In this case CO2 is being evolved via the microbial decomposition of organic matter.


Figure 1.4.  Effect of cultivation and addition of straw materials on immobilization and mineralization of N and associated evolution of CO2


 

 

Figure 1.5.  Changes in the nitrogen content of decomposing barley straw (From Alexander, 1977).

 

 


Figure 1.6.  Changes in soil mineral N as a function of time, and addition of manure and straw.


Table 1.2. Example calculations of total N in organic matter fractions in soils and expected amounts of N mineralized on a yearly basis.

____________________________________________________________________________________

                                                            Oklahoma                                Tropical Soil

                                                     min                    max                    min               max

____________________________________________________________________________________

 

Organic Matter, %                         1                        2                         4                   12

 

1 ha (0-15cm), kg                          2241653             2241653               2241653         2241653 (Pb = 1.47)

 

Organic, matter, kg                      22416                 44833                  89666            268998

 

% N in OM                                     0.05                   0.05                     0.05               0.05

(5%)

 

kg N in OM (Total)                        1120.8                2241.6                 4483.3           13449.9

 

% N mineralized/yr                       0.03                   0.03                     0.03               0.03

(3%)

 

TOTAL (kg N/ha/yr)                       33.6                   67.2                     134.4 ?          403.5 ?

 

____________________________________________________________________________________

 

DB= Mass of dry soil/volume of solids and voids

 

2000000 pounds/afs

ft3*0.02832 = m3

0.4535 lb/kg

1 ha = 2.471ac

1 ha = 10000m2

1 ac = 4047m2

 

2000000 lb = 907184.74 kg = 907.184 Mg

43560 ft2 * 0.5 ft = 21780 ft3  = 616.80m3

 

907.184Mg/616.80m3 = DB 1.4707

 

10000m2 * 0.15m = 1500 m3

2241653 kg /1000 = 2241.6 Mg

2241.6/1500 = DB 1.49 (g/cm3 = Mg/m3)

 

What will happen if

                     a) bulk density is changed?

                     b) % N in organic matter?

                     c) % N mineralized per year?

 

Organic Matter = 0.35 + 1.80 * (organic carbon) Ranney (1969)

 

 

 

 

 

Microorganisms

 

            The most important function of the microbial flora is usually considered to be the breakdown of organic materials, a process by which the limited supply of CO2 available for photosynthesis is replenished (Alexander, 1977).

 

Five major groups of microorganisms in the soil are:

 

            1.         Bacteria

            2.         Actinomycetes

            3.         Fungi

            4.         Algae

            5.         Protozoa

 

Soil Bacteria: 108 to 1010 / g of soil

 

Heterotroph: (chemoorganotrophic) require preformed organic                                     nutrients to serve as sources of energy and carbon.

            1. Fungi

            2. Protozoa

            3. Most Bacteria

 

Autotroph: (lithotrophic) obtain their energy from sunlight or by the                     oxidation of inorganic compounds and their carbon by the              assimilation of CO2.

 

Photoautotroph: energy derived from sunlight

            1. Algae (blue-green, cyanobacteria)

            2. Higher Plants

            3. Some Bacteria

 

Chemoautotroph: energy for growth obtained by the oxidation of                                   inorganic materials.

 

            1. Few Bacterial species (agronomic importance)

                        a. nitrobacter, nitrosomonas and thiobacillus


2. Essential Elements

 

Arnon's Criteria of Essentiality

 

1.      Element required to complete life cycle.

2.      Deficiency can only be corrected by the ion in question.

3.      Element needs to be directly involved in the nutrition of the plant and not indirectly via the need of another organism.

 

Any mineral element that functions in plant metabolism, whether or not its action is specific (Tisdale et al., 1985).

 

C, H, O, N, P, and S (constituent of proteins)

Ca, Mg, K, Fe, Mn, Mo, Cu, B, Zn, Cl, Na, Co, V, Si (essential to one or more plants)

 

'CHOPKNS CaFe MgB Mn Cl CuZn Mo'

 

 

Mobile Nutrients

 

     A. Plant

           1. deficiency symptoms appear in the lower older leaves

     B. Soil

           1. can be taken up from a large volume of soil

 

Immobile Nutrients

 

     A. Plant

           1. deficiency symptoms appear in the upper younger leaves

     B. Soil

           1. taken up from a small volume of soil

 


Deficiency Symptom                Element                      Mobility       Mobility             Form taken up

                                                                                   Soil             Plant                 by Plants 

____________________________________________________________________________________

 

overall chlorosis seen                 N Nitrogen                   Yes              Yes                    NO3-,NO2-,NH4+

first on lower leaves

 

purple leaf margins                     P Phosphorus              No                Yes                    HPO4=,H2PO4-,H3PO4

 

chlorotic leaf margins                 K Potassium                No                Yes                    K+

 

uniform chlorosis, stunting

(younger leaves)                         S Sulfur                       Yes              Yes(no)              SO4=,SO2*   

                                                                                                      N*S interaction

 

stunting - no root

elongation                                 Ca Calcium                  No                No                      Ca++

 

interveinal chlorosis,

veins remain green                     Fe Iron                         No (ls)          No                      Fe+++,Fe++

 

interveinal chlorosis                    Mg Magnesium            No (ls)          Yes/No               Mg++

 

reduced terminal

growth = chlorotic tips                B  Boron (NM)              Yes              No                      H3BO3°

 

interveinal chlorosis                    Mn Manganese            No                No                      Mn++, Mn+++

 

wilting, chlorosis, reduced

root growth                                Cl Chlorine                   Yes              Yes                    Cl -

 

young leaves, yellow &

stunted                                     Cu Copper                   No (ls)          No                      Cu++

 

interveinal chlorosis in

young leaves                             Zn Zinc                        No (ls)          No                      Zn++

 

interveinal chlorosis,

stunting                                     Mo Molybdenum           Yes/No(ls)    No                      MoO4=

 

dark green color                         Na Sodium                   No(ls)           Yes                    Na+

 

                                                C  Carbon                                                                 CO2

                                                H  Hydrogen                                                              H2O

                                                O  Oxygen                                                                H2O

____________________________________________________________________________________

 

* absorbed through plant leaves

(NM) Non Metal

(ls) Low Solubility

 

Mo availability increases with soil pH, other micronutrients show the opposite of this.

Immobile nutrients in plant; symptoms of deficiency show up in the younger leaves.

Stage of growth when deficiency symptom is apparent = later stage

 

 

3. The Nitrogen Cycle

 

NITROGEN:

·        Key building block of the protein molecule upon which all life is based

·        Indispensable component of the protoplasm of plants animals and microorganisms

·        One of the few soil nutrients lost by volatilization and leaching, thus requiring continued conservation and maintenance

·        Most frequently deficient nutrient in crop production

 

 

Nitrogen Ion/Molecule Oxidation States

 

Nitrogen ions and molecules that are of interest in soil fertility and plant nutrition cover a range of N apparent oxidation states from -3 to +5.  It is most convenient to illustrate these oxidation states using common combinations of N with H and O, because H can be assumed in the +1 oxidation state (H+1) and O in the -2 oxidation state (O=).  The apparent N oxidation state, and the electron configurations involved may be depicted as follows.

 

Hydrogen:

 

The electron configuration in the ground state is 1s1 (the first electron shell has only one electron in it), as found in H2 gas.  Since the s shell can hold only two electrons, the atom would be most stable by either gaining another electron or losing the existing one.  Gaining an electron by sharing occurs in H2, where each H atom gains an electron from the other resulting in a pair of electrons being shared.  The electron configuration about the atom, where: represent a pair of electrons, may be shown as

 

H:H and the bond may be shown as H-H

 

Hydrogen most commonly exists in ionic form and in combination with other elements where it has lost its single electron.  Thus it is present as the H+ ion or brings a + charge to the molecule formed by combining with other elements.

 

Oxygen:

 

The ground state of O, having a total of eight electrons is 1s2, 2s2, 2p4.  Both s orbitals are filled, each with two electrons.  The 2p outer or valence orbital capable of holding six electrons, has only four electrons, leaving opportunity to gain two.  The common gain of two electrons from some other element results in a valence of -2 for O (O=).  The gain of two electrons also occurs in O2 gas, where two pairs of electrons are shared as

 

O::O and the double bond may be shown as O=O

 

Nitrogen:

 

The ground state of N is 1s2, 2s2, 2p3.  It is very similar to that for oxygen, except there is one less electron in the valence 2p orbital.  Hence, the 2p orbital contains three electrons but, has room to accept three electrons to fill the shell.  Under normal conditions, electron loss to form N+, N2+ or N3+ or electron gain to form N-, N2-, or N3- should not be expected.  Instead, N will normally fill its 2p orbital by sharing electrons with other elements to which it is chemically (covalent) bound.  Nitrogen can fill the 2p orbital by forming three covalent bonds with itself as in the very stable gas N2.

 

The Nitrogen cycle is not well understood, largely because of how it is communicated.  Similar to the way we communicate the differences between normal, saline, sodic and saline-sodic soils, we should do the same for response variables in the Nitrogen cycle.  In addition to temperature and pH included below, we could add reduction/oxidation, tillage (zero vs. conventional), C:N ratios, fertilizer source and a number of other variables.  These mechanistic models would ultimately lead to many 'if-then' statements/decisions that could be used within a management strategy.

 

>50F

denitrification

volatilization

leaching

leaching

<50F                                    7.0

                                          Soil pH

 

Assuming that we could speed up the nitrogen cycle what would you change?

 

                        1. Aerated environment (need for O2)

                        2. Supply of ammonium

                        3. Moisture

                        4. Temperature (30-35C or 86-95F) <10C or 50F

                        5. Soil pH

                        6. Addition of low C:N ratio materials (low lignin)

 

Is oxygen required for nitrification?

Does nitrification proceed during the growing cycle? (low C:N ratio)

 

N Oxidation States:

 

oxidized: loses electrons, takes on a positive charge

reduced: gains electrons, takes on a negative charge

 

Ion/molecule                  Name                   Oxidation State

NH3                                    ammonia                   -3

 

 

NH4+                                   ammonium                -3

 


N2                                       diatomic N                  0

N2O                                    nitrous oxide             +1

NO                                    nitric oxide                +2

-

 

+

 

-

 
NO2-                                   nitrite                         +3

-

 
NO3-                                   nitrate                        +5

 


_____________________________________

H2S                                    hydrogen sulfide       -2

SO4=                                   sulfate                       +6

_____________________________________

 

N: 5 electrons in the outer shell

·        loses 5 electrons (+5 oxidation state NO3)

·        gains 3 electrons (-3 oxidation state NH3)

 

O: 6 electrons in the outer shell

·        is always being reduced (gains 2 electrons to fill the outer shell)

 

H: 1 electron in the outer shell

 

N is losing electrons to O because O is more electronegative

N gains electrons from H because H wants to give up electrons

_______________________________________________

 

 

N recommendations

 

1. Yield goal (2lb N/bu)

      a. Applies fertilization risk on the farmer

      b. Removes our inability to predict 'environment' (rainfall)

2. Soil test

      a. For every 1 ppm NO3, N recommendation reduced by 2lbN/ac    

Nitrite accumulation?

                        1. high pH

                        2. high NH4 levels (NH4 inhibits nitrobacter)

 

 


                             NITROGEN CYCLE


 

Inorganic Nitrogen Buffering

 

            Inorganic nitrogen buffering is defined as the ability of the soil-plant system to control the amount of inorganic N accumulation in the rooting profile when N fertilization rates exceed that required for maximum yield.

            If N rates required to detect soil profile NO3 accumulation always exceeded that required for maximum yields, what biological mechanisms are present that cause excess N applied to be lost via other pathways prior to leaching?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Nitrogen Buffering Mechanisms

1. Increased Applied N results in increased plant N loss (NH3)

2. Higher rates of applied N - increased volatilization losses

3. Higher rates of applied N - increased denitrification

4. Higher rates of applied N - increased organic C, -- increased N in organic pools

5. Increased applied N - increased grain protein

6. Increased applied N - increased forage N

7. Increased applied N - increased straw N

 

Ammonia Volatilization

 

· Urease activity                                     · Air Exchange     

· Temperature                                        · N Source and Rate

· CEC (less when high)                         · Application method

· H buffering capacity of the soil           · Crop Residues

· Soil Water Content

 

NH4+ « NH3 + H+

 

If pH and temperature can be kept low, little potential exists for NH3 volatilization.  At pH 7.5, less than 7% of the ammoniacal N is actually in the form of NH3 over the range of temperatures likely for field conditions.

 

Chemical Equilibria

 

A+B    « AB

Kf        = AB/A x B

AB       « A+B

Kd       = A x B/AB

Kf        = 1/Kd (relationship between formation and dissociation constants)

 

Formation constant (Log K°) relating two species is numerically equal to the pH at which the reacting species have equal activities (dilute solutions).

 

pKa and Log K°  are sometimes synonymous

 

Henderson-Hasselbalch

pH = pKa + log [(base)/(acid)]

 

when (base) = (acid), pH = pKa

 

Urea

 

·        Urea is the most important solid fertilizer in the world today. 

·        In the early 1960's, ammonium sulfate was the primary N product in world trade (Bock and Kissel, 1988). 

·        The majority of all urea production in the U.S. takes place in Louisiana, Alaska and Oklahoma.

·        Since 1968, direct application of anhydrous ammonia has ranged from 37 to 40% of total N use (Bock and Kissel, 1988).

·        Urea: high analysis, safety, economy of production, transport and distribution make it a leader in world N trade.

·        In 1978, developed countries accounted for 44% of the world N market (Bock and Kissel, 1988). 

·        By 1987, developed countries accounted for less than 33%.

 

Share of world N consumption by product group

                                                        1970                       1986

Ammonium sulfate                        8                             5

Ammonium nitrate                        27                           15

Urea                                               9                             37

Ammonium phosphates               1                             5

Other N products (NH3)                36                           29

Other complex N products           16                           8      

 

Urea Hydrolysis

 

increase pH (less H+ ions in soil solution)

 

CO(NH2)2 + H+ + 2H2O --------> 2NH4+ + HCO3-

pH 6.5 to 8

HCO3- + H+ ---> CO2 + H2O (added H lost from soil solution)

 

CO(NH2)2 + 2H+ + 2H2O --------> 2NH4+ + H2CO3 (carbonic acid)

pH <6.3

 

 

 

 

 

 

 

 

 

 

 

 

 


Potential for gaseous loss from applied urea, both broadcast and incorporated.

During hydrolysis, soil pH can increase to >7 because the reaction requires H+ from the soil system.

 

(How many moles of H+ are consumed for each mole of urea hydrolyzed?) 2

 

In alkaline soils less H+ is initially needed to drive urea hydrolysis on a soil already having low H+.

 

In an alkaline soil, removing more H+(from a soil solution already low in H+), can increase pH even higher

 

NH4+ + OH- ---> NH4OH ---->NH3 + H2O

 

pH = pKa + log [(base)/(acid)]

 

At a pH of 9.3 (pKa 9.3) 50% NH4 and 50% NH3

 

pH       Base (NH3)      Acid (NH4)

7.3       1                       99

8.3       10                     90

9.3       50                     50

10.3    90                     10

11.3    99                     1

 


Equilibrium relationship for ammoniacal N and resultant amount of NH3 and NH4 as affected by pH for a dilute solution.

 

As the pH increases from urea hydrolysis, negative charges become available for NH4+ adsorption because of the release of H+ (Koelliker and Kissel)

 

Decrease NH3 loss with increasing CEC (Fenn and Kissel, 1976)

assuming increase pH = increase CEC, what is happening?

 

In acid soils, the exchange of NH4+ is for H+ on the exchange complex (release of H here, resists change in pH, e.g. going up)

 

In alkaline soils with high CEC, NH4 exchanges for Ca,

precipitation of CaCO3 (CO3= from HCO3- above) and one H+ released which helps resist the increase in pH

 

However, pH was already high.


 

Soil surface pH and cumulative NH3 loss as influenced by pH buffering capacity (from Ferguson et al., 1984).

 

Ernst and Massey (1960) found increased NH3 volatilization when liming a silt loam soil.  The effective CEC would have been increased by liming but the rise in soil pH decreased the soils ability to supply H+

 

Rapid urea hydrolysis:  greater potential for NH3 loss.  Why?

management: dry soil surface, incorporate, localized placement- slows urea hydrolysis

 

H ion buffering capacity of the soil:

 

Ferguson et al., 1984

(soils total acidity, comprised of exchangeable acidity + nonexchangeable titratable acidity)

 

A large component of a soils total acidity is that associated with the layer silicate sesquioxide complex (Al and Fe hydrous oxides).  These sesquioxides carry a net positive charge and can hydrolyze to form H+ which resist an increase in pH upon an addition of a base.

 

H+ ion supply comes from:

            1. OM

            2. hydrolysis of water

            3. Al and Fe hydrous oxides

            4. high clay content

 

A soil with an increased H+ buffering capacity will also show less NH3 loss when urea is applied without incorporation.

 

1. hydroxy Al-polymers added (carrying a net positive charge) to increase H+ buffering capacity.

2. strong acid cation exchange resins added (buffering capacity changed without affecting CEC, e.g. resin was saturated with H+).

 

resin: amorphous organic substances (plant secretions), soluble in organic solvents but not in water (used in plastics, inks)

 

Consider the following

1.   H+ is required for urea hydrolysis.

2.   Ability of a soil to supply H+ is related to amount of NH3 loss.

3.   H+ is produced via nitrification (after urea is applied): acidity generated is not beneficial.

4.   What could we apply with the urea to reduce NH3 loss?

 

an acid; strong electrolyte; dissociates to produce H+;increased H+ buffering; decrease pH

 

reduce NH3 loss by maintaining a low pH in the vicinity of the fertilizer granule (e.g. H3PO4)

 

Factors Affecting Soil Acidity

Acid:              substance that tends to give up protons (H+) to some other substance

Base:            accepts protons

Anion:            negatively charged ion

Cation:          positively charged ion

 

Base cation: ? (this has been taught in the past but is not correct)

 

Electrolyte: nonmetallic electric conductor in which current is carried by the movement of ions

 

            H2SO4 (strong electrolyte)

            CH3COOH (weak electrolyte)

 

            H2O

            HA --------------> H+ + A-

      potential                 active

       acidity                   acidity

 

1. Nitrogen Fertilization

 

            A. ammoniacal sources of N

 

2. Decomposition of organic matter

 

     OM ------> R-NH2 + CO2

     CO2 + H2O --------> H2CO3 (carbonic acid)

     H2CO3 ------> H+ + HCO3- (bicarbonate)

 

     humus contains reactive carboxylic, phenolic groups that behave as weak acids which dissociate and release H+

 

3. Leaching of exchangeable bases/Removal

     Ca, Mg, K and Na (out of the effective root zone)

     -problem in sandy soils with low CEC

 

     a.  Replaced first by H and subsequently by Al (Al is one of the most abundant elements in soils.  7.1% by weight of earth's crust)

     b.  Al displaced from clay minerals, hydrolyzed to hydroxy aluminum complexes

     c.  Hydrolysis of monomeric forms liberate H+

     d.  Al(H2O)6+3  + H2O   ----->   Al(OH)(H2O)++ + H2O+

 

 

monomeric: a chemical compound that can undergo polymerization

polymerization: a chemical reaction in which two or more small  molecules combine to form larger molecules that contain repeating structural units of the original molecules

 

4. Aluminosilicate clays

     Presence of exchangeable Al

     Al+3 + H2O -----> AlOH= + H+

 

5. Acid Rain

 

Acidification from N Fertilizers (R.L. Westerman)

 

1.    Assume that the absorbing complex of the soil can be represented by CaX

2.    Ca represents various exchangeable bases with which the insoluble anions X are combined in an exchangeable form and that X can only combine with one Ca

3.    H2X refers to dibasic acid (e.g., H2SO4)

 

(NH4)2SO4 -----> NH4+ to the exchange complex, SO4= combines with the base on the exchange complex replaced by NH4+

 

Thought:  Volatilization losses of N as NH3 preclude the development of H+ ions produced via nitrification and would theoretically reduce the total potential development of acidity.

 

Losses of N via denitrification leave an alkaline residue (OH-).

 


Table X. Reaction of N fertilizers when applied to soil.

______________________________________________________________________

1. Ammonium sulfate

    a. (NH4)2SO4 + CaX ---->   CaSO4 + (NH4)2X

    b. (NH4)2X + 4O2 nitrification >2HNO3 + H2X + 2H2O

    c.  2HNO3 + CaX ---->   Ca(NO3)2 + H2X

              Resultant acidity = 4H+ /mole of (NH4)2SO4

 

2. Ammonium nitrate

    a. 2NH4NO3 + CaX ---->  Ca(NO3)2 + (NH4)2X

    b. (NH4)2X + 4O2 nitrification >2HNO3 + H2X + 2H2O

    c.  2HNO3 + CaX ---->    Ca(NO3)2 + H2X

              Resultant acidity = 2H+ /mole of NH4NO3

 

3. Urea

    a. CO(NH2)2 + 2H2O ---->   (NH4)2CO3

    b. (NH4)2CO3 + CaX ---->  (NH4)2X + CaCO3

    c.  (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2O

    d. 2HNO3 +CaX ---->  Ca(NO3)2 + H2X

    e. H2X + CaCO3 neutralization >CaX + H2O + CO2

              Resultant acidity = 2H+ /mole of CO(NH2)2

 

4. Anhydrous Ammonia

    a. 2NH3 +2H2O ---->   2NH4OH

    b. 2NH4OH + CaX ---->  Ca(OH)2 + (NH4)2X

    c.  (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2O

    d. 2HNO3 + CaX ---->  Ca(NO3)2 + H2X

    e. H2X + Ca(OH)2 neutralization > CaX + 2H2O

              Resultant acidity = 1H+/mole of NH3

 

5. Aqua  Ammonia

    a. 2NH4ON + CaX  ---->  Ca(OH)2 + (NH4)2X

    b. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2O

    c.  2HNO3 +CaX ----> Ca(NO3)2 + H2X

    d. H2X + Ca(OH)2 neutralization > CaX +2H2O

              Resultant acidity = 1H+/mole of NH4OH

 

6. Ammonium Phosphate

    a. 2NH4H2PO4 + CaX ----> Ca(H2PO4)2 + (NH4)2X

    b. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2O

    c.  2HNO3 +CaX ---->   Ca(NO3)2 + H2X

              Resultant acidity = 2H+/mole of NH4H2PO4

______________________________________________________________________

 

 

 



4. Nitrogen Use Efficiency

 

In grain production systems, N use efficiency seldom exceeds 50 percent.  Variables which influence N use efficiency include

 

           a.      Variety

           b.      N source

           c.      N application method

           d.      Time of N application

           e.      Tillage

           f.       N rate (generally decreases with increasing N applied)

           g.      Production system

 

                    1. Forage

                    2. Grain

 

Olson and Swallow, 1984 (27-33% of the applied N fertilizer was removed by the grain following 5 years)

 

           h.      Plant N loss

           i.       Soil type (organic matter)

           

 

Calculating N Use Efficiency using The Difference Method

______________________________________________________________________

Applied N              Grain Yield                 N content               N uptake      Fertilizer Recovery

kg/ha                      kg/ha                           %                            kg/ha            %

______________________________________________________________________

0                             1000                           2.0                          20                 -

50                           1300                           2.1                          27.3             (27.3-20)/50=14.6

100                         2000                           2.2                          44                 (44-20)/100=24

150                         2000                           2.3                          46                 (46-20)/150=17

______________________________________________________________________

 

Estimated N use efficiency for grain production systems ranges between 20 and 50%.  The example above does not include straw, therefore, recovery levels are lower.  However, further analysis of forage production systems (Altom et al., 1996) demonstrates that N use efficiency can be as high as 60-70%.  This is largely because the plant is harvested prior to flowering thus minimizing the potential for plant N loss.  Plant N loss is known to be greater when the plant is at flowering and approaching maturity.  It is important to observe that estimated N use efficiencies in forage production systems do not decrease with increasing N applied as is normally found in grain production systems.  This is suggestive of 'buffering' whereby increased N is lost at higher rates of applied N in grain production systems, but which cannot take place in forage production systems.

Work by Moll et al. (1982) suggested the presence of two primary components of N use efficiency: (1) the efficiency of absorption or uptake (Nt/Ns), and (2) the efficiency with which the N absorbed is utilized to produce grain (Gw/Nt) where Nt is the total N in the plant at maturity (grain + stover), Ns is the nitrogen supply or rate of fertilizer N and Gw is the grain weight, all expressed in the same units.  Other parameters defined in their work and modifications (in italics) are reported in Table 4.2.


            Recent understanding of plant N loss has required consideration of additional parameters not discussed in Moll et al. (1982).  Harper et al. (1987) documented that N was lost as volatile NH3 from wheat plants after fertilizer application and during flowering.  Maximum N accumulation has been found to occur at or near flowering in wheat and corn and not at harvest.  In order to estimate plant N loss without the use of labeled N forms, the stage of growth where maximum N accumulation is known to occur needs to be identified.  The amount of N remaining in the grain + straw or stover, is subtracted from the amount at maximum N accumulation to estimate potential plant N loss (difference method).  However, even the use of difference methods for estimating plant N loss are flawed since continued uptake is known to take place beyond flowering or the point of maximum N accumulation.

 

Figure 4.1 Total N uptake in winter wheat with time and estimated loss following flowering.

 

Francis et al. (1993) recently documented that plant N losses could account for as much as 73% of the unaccounted-for N in 15N balance calculations.  They further noted that gaseous plant N losses could be greater when N supply was increased.  Similar to work by Kanampiu et al. (1997) with winter wheat, Francis et al. (1993) found that maximum N accumulation in corn occurred soon after flowering (R3 stage of growth).  In addition, Francis et al. (1993) highlighted the importance of plant N loss on the development and interpretations of strategies to improve N fertilizer use efficiencies. 

Consistent with work by Kanampiu et al. (1997), and Daigger et al. (1976), Figure 4.1 illustrates winter wheat N accumulation over time.  Estimates of plant N loss are reported in Table 4.1. Harper et al. (1987) reported that 21% of the applied N fertilizer was lost as volatile NH3 in wheat, of which 11.4% was from both the soil and plants soon after fertilization and 9.8% from the leaves of wheat between anthesis and physiological maturity.  Francis et al. (1993) summarized that failure to include direct plant N losses when calculating an N budget leads to overestimation of N loss from the soil by denitrification, leaching and ammonia volatilization. 

 


NO3- + 2e  (nitrate reductase) NO2- + 6e (nitrite reductase) NH4+

 

Reduction of NO3- to NO2- is the rate-limiting step in the transformation of N into amino forms.

 

Does the plant wake up in the morning and turn on the TV to check the weather forecast, to see if it should assimilate NO3 and attempt to form amino acids?

Could we look at the forecast and attempt to communicate with the plant, letting it know that weather conditions will be good (or bad), thus proceeding with increased NO3 uptake?

 

Major pathways for assimilation of NH3

1.    Incorporation into glutamic acid to form glutamine, a reaction catalyzed by glutamine synthetase (Olson and Kurtz, 1982)

2.    Reaction of NH3 and CO2 to form carbamyl phosphate, which in turn is converted to the amino acid arginine.

3.    Biosynthesis of amides by combination of NH3 with an amino acid.  In this way aspartic acid is converted to the amide, asparagine.

 

Table 4.1. Means over N rate and variety for protein, NUE components and estimated plant N loss