|First Hour Exam, February 8, 2002|
Soil-Plant Nutrient Cycling and Environmental Quality
An example of protonation is when NH3 reacts with H+
to become stable NH4
The adverse effects of weather can be overcome by applying more
H+ does not have an electron, and that is why it has
a + charge, having only 1 proton
Subsoils can provide significant amounts of NH4 for crop
NH4 is most likely to accumulate in semi-arid
Nitrogen use efficiency for worldwide cereal production averages
Work by Francis et al (1993) reported that plant N losses accounted
for 52 to 73% of the unaccounted-for N in 15N balance
Denitrification in soils is controlled largely by the supply of
water-soluble or readily decomposable organic matter
Denitrification losses are generally greater when N is applied in
conventional till soils as compared to zero till soils.
The vast majority of NOx 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.
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
niter is an evaporite that forms in arid to semi-arid climates. We
hypothesize that in these environments during times of moisture,
and nitrification take place more rapidly than during
of low moisture, as microbes, like almost all life, are dependent
water to survive and carry out their life processes.
events in these environments are infrequent and often
there is the potential for leaching of nitrate out of the root
However, because of the extreme desiccation of the soil one would
expect the wetting front to move to great depths, resulting in the
of nitrate at the leading edge of the wetting front (cm to m
soil). Over a
period of hundreds to thousands of years nitrate
accumulate at semi-shallow depths and react with the base cations in
soil to form soluble nitrate salts such as soda niter.
Compare NH4 in arid/semi-arid environments vs. humid
formation, transformations, and fates (Randy Davis)
As organic matter is accumulated at the soil surface,
rapid due to favorable environmental conditions.
As quickly as NH4 is
it is immediately taken up by plants leaving little or none to
As water is limiting in this type of environment smaller
of plant life are present. Consequently
there is little organic
accumulation. However, as N
in residue is mineralized it also
nitification. As water is the
limiting nutrient it is also
that other plant nutrients, i.e. NO3, be transported by water. NH4
also accumulate leading to possible loss as NH3.
What are the effects of soil pH on nitrification? (Yan Tang)
organisms are sensitive to H+.
pure culture, their activity is reduced below pH 6.0 and becomes
below pH 5.0.
soils with pH 4.0 or less, however, may contain some NO3- and it
that the organisms derived from acid soils are frequently more
pH is 6.6 to 8.0 or higher.
Name 3 different pathways by which N is lost from ecosystems and
the environmental conditions in which they are apt to happen? (Jason
Ammonia volatilization happens when there is a high pH generally
7.5 and drying weather conditions in semi-arid and arid climates
Leaching occurs when NO3- is moved by water below
the root zone.
occurs in environments where there is ample rainfall, soil type
influences how rapid NO3- moves through the soil.
Denitrification. When the soil is very wet, water fills in the spaces
soil particles. This leaves very little room for oxygen. Some
microorganisms can get the oxygen they need from the oxygen portion
nitrite (NO2-) and nitrate (NO3-)
forms of nitrogen. When this
nitrogen (N2) and nitrous oxide (N2O) gas are formed
What happens to NH4+ and NO3-
in soil? (Jitao)
+ OH- <-->
NH3 + H2O?
drying and removing H2O shift the equilibrium in favor of the
reaction to the right.
2NO3- + 4H+ + 2 H2O
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)
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
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 50oF 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
A farmer applied excessive rates of nitrogen fertilizer with the intention
increase yield. He applied the fertilizer at planting when the moisture
temperture was optimal. Describe two problems arising from these
rates in the soil system (Kefyalew Girma)
Excess nitrogen as NO3-N is fairly easy to leach from
The consequence of this is nitrogen level build up in ground
that is then used by animals or humans.
Nitrification of the added fertilizer can lower the pH of the
This in turn may enhance Al and/or Mn toxicity.
In what way does nitrogen cycle pose problem to environment?
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.
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
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)
think it would not be a good idea because if you incorporate a lot of
paper it will
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.
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.
What does inorganic nitrogen buffering mean?
Define 4 of the buffering mechanisms? (Shambel Moges)
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.
of the important mechanisms include:
increased applied N results in increased plant N loss (NH3)
higher rate of applied N results in increased volatilization losses
higher rate of applied N can result in increased denitrificaiton
application of higher N rates will increase grain protein, forage
and straw N.
(from lecture material)
The application of excessive N rates in cereal production has been
‘dead zone’ or hypoxia in the Gulf of Mexico
eutrophication of lakes and streams
nitrate contamination of groundwater
global warming via increased production of nitrous oxide from
negative backlash of the green revolution
What is the relationship between greenhouse warming and
denitrification in soils? Explain?
What concentrations of soil NH4, NO2, and NO3
(mg/kg) would you expect to find in an undisturbed, native prairie
of these are expected to accumulate in a native prairie soil.
What concentrations of NH4 (in soil) would you expect to
itself would not be toxic, excluding the salt effect at high rates.
What would be toxic would be NH3
What are likely the three most important factors controlling what
actually happens in the nitrogen cycle?
soil organic matter, temperature, and pH
soil organic matter, moisture, and pH
soil organic matter, pH, and temperature
soil organic matter, temperature, and moisture
soil organic matter, pH, and, volatilization
6. Burford and Bremner found the following
denitrification capacities were highly correlated with water
denitrification capacities were highly correlated with
denitrification under anaerobic conditions in controlled largely by
the supply of readily decomposable organic matter
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
the countries of Colombia, Peru, Pakistan, Nepal, and France