SOIL5813

Soil-Plant Nutrient Cycling and Environmental Quality

First Hour Exam

February 8, 2002

Name: ______________

Miscellaneous:

T          F          An example of protonation is when NH₃ reacts with H⁺ to become stable NH₄

T          F          The adverse effects of weather can be overcome by applying more N

T          F          H⁺ does not have an electron, and that is why it has a + charge, having only 1 proton

T          F          Subsoils can provide significant amounts of NH₄ for crop growth

T          F          NH₄ is most likely to accumulate in semi-arid environments

T          F          Nitrogen use efficiency for worldwide cereal production averages 67%

T          F          Work by Francis et al (1993) reported that plant N losses accounted for 52 to 73% of the unaccounted-for N in ¹⁵N balance calculations

T          F          Denitrification in soils is controlled largely by the supply of water-soluble or readily decomposable organic matter

T          F          Denitrification losses are generally greater when N is applied in conventional till soils as compared to zero till soils. 

T          F          The vast majority of NO_x is released as nitric oxide, NO, which converts to nitrogen dioxide, NO₂, within minutes by reaction with ozone and peroxy radicals.  NO₂ is recycled to NO by photolysis.

STUDENT Questions

1. Under what climatic conditions does soda niter (sodium nitrate) form? In other words, what climatic conditions would lead to the accumulation of nitrate in the subsoil?  (Jamie Patton)

Soda niter is an evaporite that forms in arid to semi-arid climates.  We

can hypothesize that in these environments during times of moisture,

mineralization and nitrification take place more rapidly than during

periods of low moisture, as microbes, like almost all life, are dependent

on water to survive and carry out their life processes.  Because the

precipitation events in these environments are infrequent and often

torrential, there is the potential for leaching of nitrate out of the root

zone.  However, because of the extreme desiccation of the soil one would

not expect the wetting front to move to great depths, resulting in the

accumulation of nitrate at the leading edge of the wetting front (cm to m

in the soil).    Over a period of hundreds to thousands of years nitrate

would accumulate at semi-shallow depths and react with the base cations in

the soil to form soluble nitrate salts such as soda niter.

2. Compare NH₄ in arid/semi-arid environments vs. humid environments.  Include formation, transformations, and fates (Randy Davis)

Humid:  As organic matter is accumulated at the soil surface, mineralization

is rapid due to favorable environmental conditions.  As quickly as NH4 is

produced it is immediately taken up by plants leaving little or none to

undergo nitrification.

Arid/Semi:  As water is limiting in this type of environment smaller

populations of plant life are present.  Consequently there is little organic

matter accumulation.  However, as N in residue is mineralized it also

undergoes nitification.  As water is the limiting nutrient it is also

important that other plant nutrients, i.e. NO3, be transported by water. NH4

may also accumulate leading to possible loss as NH3.

3. What are the effects of soil pH on nitrification? (Yan Tang)

Nitrifying organisms are sensitive to H+.

In pure culture, their activity is reduced below pH 6.0 and becomes

negligible below pH 5.0.

Some soils with pH 4.0 or less, however, may contain some NO3- and it

appears that the organisms derived from acid soils are frequently more

tolerant of H+.

Optimum pH is 6.6 to 8.0 or higher.

4. Name 3 different pathways by which N is lost from ecosystems and

describe the environmental conditions in which they are apt to happen? (Jason Lawles)

a) Ammonia volatilization happens when there is a high pH generally

above 7.5 and drying weather conditions in semi-arid and arid climates

b) Leaching occurs when NO₃^- is moved by water below the root zone.

Leaching occurs in environments where there is ample rainfall, soil type

also influences how rapid NO₃^- moves through the soil.

c) Denitrification. When the soil is very wet, water fills in the spaces

between soil particles. This leaves very little room for oxygen. Some

soil microorganisms can get the oxygen they need from the oxygen portion

of the nitrite (NO₂^-) and nitrate (NO₃^-) forms of nitrogen. When this

happens, nitrogen (N₂) and nitrous oxide (N₂O) gas are formed and escape

to the atmosphere.

5. What happens to NH₄⁺ and NO₃^- in soil? (Jitao)

1.About NH₄⁺:

• Immobilization. When residue does not contain enough N to meet the needs of microbes decaying it, the microbes will utilize NH₄⁺ in the residue and any additional mineral-n present in the soil. This process is called immobilization.
• Cation exchange. When NH₄⁺ accumulated in the soil, NH₄⁺ will successfully compete for exchange sites on clay and humus occupied by other cation. Then NH₄⁺ becomes immobile in the soil.
• Volatilization. When the pH is high, the N could be loss by volatilization of the gas NH₃.

     NH₄⁺ + OH^-  <-->  NH₃ + H₂O?

     Surface drying and removing H₂O shift the equilibrium in favor of the reaction to the right.

• Plant uptake. Plants can uptake NH₄⁺ directly. As NH₄⁺ is immobile in the soil, the density of the root is very important for the plant uptake.
• Nitrification. NH₄⁺ change to NO₃^-. This process is called nitrification.

2NH₄⁺ +4O₂  ßà  2NO₃^- + 4H⁺ + 2 H₂O

1.About NO₃^-:

• Immobilization. When residue does not contain enough N to meet the needs of microbes decaying it, the microbes will utilize NO₃^- in the residue and any additional mineral-n present in the soil. This process is called immobilization.

6. Some states do not recommend soil testing for inorganic nitrogen for all general purposes.  Give a reason why you think they do not.  Do you think they should soil test for inorganic nitrogen?  If so, why? (Roger Teal)

I have designed this question to be opinionated because I don’t think that everyone realizes that nitrogen analysis is not always a given in soil testing. 

Although there is no wrong answer to this question, the rationale behind not testing for nitrogen without a request is: inorganic nitrogen soil test would not be accurate.  In these regions, the soil temperature stays above 50^oF most of the time, the soils are acid with no more than 1% OM and low CEC (kaolinite), and the annual rainfall is above 50 inches.  In these conditions the inorganic nitrogen levels are consistently changing with high potential for leaching and immobilization 

7. A farmer applied excessive rates of nitrogen fertilizer with the intention

to increase yield. He applied the fertilizer at planting when the moisture

and temperture was optimal. Describe two problems arising from these

excessive rates in the soil system (Kefyalew Girma)

a.  Excess nitrogen as NO₃-N is fairly easy to leach from the

    soil. The consequence of this is nitrogen level build up in ground

    water that is then used by animals or humans.

b.  Nitrification of the added fertilizer can lower the pH of the

    soil. This in turn may enhance Al and/or Mn toxicity.

8.  In what way does nitrogen cycle pose problem to environment? (Adi Malapati)

If excess amounts of N in the form of nitrate and ammonium are present on the surface of soils, these get washed away as surface runoff in to the nearby water bodies.  Nitrogen entering into the water bodies can cause’ eutropication’.  Ammonium is not only toxic to fish but it also uses up the dissolved oxygen in the water during the nitrification process.

N in the form of nitrate poses a particular environmental concern because of its leaching properties.  Nitrate can reach the ground water through leaching and can create health problems if its content is more than 10 ppm

In saturated soils or the wet lands where rice is grown under water logged conditions denitrification is the major process because anerobic conditions prevail.  Nitrous and nitric oxides are the  product of this process and both these are greenhouse gases.

9.  You have a paper factory in a small country where you don't have big disposal sites and you have a lot of waste paper and sawdust from a paper factory that you have to dispose of. One of your subordinates told you that paper is organic in nature, and wanted to incorporate it in the soil in adjacent wheat and corn fields. Is this a good idea?  Yes or No?  Why? (Jagadeesh)

I think it would not be a good idea because if you incorporate a lot of paper it will

tend to immobilize the nitrogen as the C:N ratio would be very high.  Plants would likely be highly stressed due to the limited supply of N.

10.  Explain Ammonia (NH₃) volatilization in terms of pH (generally)  by means of a graph or a chemical equation with an explanation for whichever you choose.  For example
under what conditions will ammonia be volatilized? (Micah Humphreys)

11.  What does inorganic nitrogen buffering mean?  Define 4 of the buffering mechanisms? (Shambel Moges)

Inorganic nitrogen buffering is 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.

Some of the important mechanisms include:

1.         increased applied N results in increased plant N loss (NH3)

2.         higher rate of applied N results in increased volatilization losses

3.         higher rate of applied N can result in increased denitrificaiton losses

4.         application of higher N rates will increase grain protein, forage and straw N.

Miscellaneous (from lecture material)

1.  The application of excessive N rates in cereal production has been blamed for

a. ‘dead zone’ or hypoxia in the Gulf of Mexico

b. eutrophication of lakes and streams

c. nitrate contamination of groundwater

d. global warming via increased production of nitrous oxide from denitrification

e. negative backlash of the green revolution

2.  What is the relationship between greenhouse warming and denitrification in soils?  Explain?

• Increasing use of N fertilizers has sent more Nitrous oxide into the atmosphere via denitrification (nitrous oxide generated by bacteria using nitrate)
• Reactions of nitrous oxide with excited oxygen contribute to the destruction of ozone
• Ozone molecules serve to screen out dangerous ultraviolet light, and nitrous oxide (from denitrification) can destroy ozone
• The atmospheric lifetime of nitrous oxide is longer than a century, and every one of its molecules absorbs roughly 200 times more outgoing radiation than does a single carbon dioxide molecule.

3.  What concentrations of soil NH₄, NO₂, and NO₃ (mg/kg) would you expect to find in an undisturbed, native prairie environment?  Explain.

5 ppm NH₄

5 ppm NO₃

<1 ppm NO₂

none of these are expected to accumulate in a native prairie soil.

4.  What concentrations of NH₄ (in soil) would you expect to be toxic?

NH₄ itself would not be toxic, excluding the salt effect at high rates.  What would be toxic would be NH₃

5.  What are likely the three most important factors controlling what actually happens in the nitrogen cycle?

a.  soil organic matter, temperature, and pH

b.  soil organic matter, moisture, and pH

c.  soil organic matter, pH, and temperature

d.  soil organic matter, temperature, and moisture

e.  soil organic matter, pH, and, volatilization

6.  Burford and Bremner found the following

a.  denitrification capacities were highly correlated with water soluble carbon

b.  denitrification capacities were highly correlated with mineralizable carbon

c.  denitrification under anaerobic conditions in controlled largely by the supply of readily decomposable organic matter

d.  denitrifying bacteria responsible for reduction of nitrate to gaseous forms of nitrogen are facultative anaerobes that have the ability to use both oxygen and nitrate as hydrogen acceptors

BONUS:(5 points)

Outline the countries of Colombia, Peru, Pakistan, Nepal, and France