Transcript N and S - Plant, Environmental and Soil Sciences

Nitrogen and Sulfur
Chapter 13
What are the 18 essential elements plants need?
Nitrogen
Importance of N to Plants
Taken up as NH4+ or NO3Component of biomolecules
Deficiency 
→
chlorosis and
poor growth
Oversupply 
→
rank, abnormal
growth
Most N in atmosphere
Some in vegetation but 20 x as much in soil
Most soil N in organic matter
Only 1 to 2 % as inorganic ions
Nitrogen Cycle
Plants can use
Immobilization and Mineralization
N mineralization refers to the conversion
of organic-N to inorganic-N (True / False)
And what is N immobilization?
True, and immobilization is the reverse, incorporation into biomass.
The upper 6 inches of an acre of soil
contains 3 % organic matter. If 2 % of
the organic matter is mineralized
annually and it contains 5 % N, how
much N is made available? Assume
2,000,000 lbs per AFS.
If you do the multiplication, I think you will arrive at 60 # N per acre per year,
which is a fair amount. Of course, soil microbes are competing for it and there
are several other processes going on in the N cycle that affect the fate of
mineralized N. Nevertheless, N mineralization is an important source of plantavailable N.
N mineralization is an important
source of N for plant growth
(True / False)
+
4
NH Fixation
+
4
The green box is biomass, the brown box organic
materials subject to mineralization and the first
baby blue box, the pool of mineralized N (NH4+).
We are at the box with star, NH4+ fixation.
NH trapped between units of 2:1 minerals
Vermiculite > illite > smectite
+
4
NH slowly released


NH4+ is about the same
size as K+ and so is
subject to interlayer
entrapment.
The effect is
greatest with
vermiculite
because adjacent
crystals are not
already collapsed onto one another as with illite with
its interlayer K+. Some NH4+ fixation about interlayer
edges in illite.
NH3 Volatilization
NH4+ + OH- ↔ H2O + NH3 ↑
High pH
Increase volatility?
This is a spot where some appreciation of
High CEC
chemical equilibrium is useful. The notion is
Incorporation that if OH concentration is high, the reaction
tends to go to the right, with production of NH ,
Dry soil
which can be lost from solution to soil air and
-
3
If you use an NH4+
type fertilizer, the
deeper it’s put, the
less likely it’s to go.
diffuse out of the soil. As for CEC, since NH4+
is a cation, it is adsorbed by – charged colloids and
the greater the density of – charge, the greater is
its adsorption. So, more adsorbed, less in solution
subject to reaction going to the right.
Nitrification
Microbial oxidation of NH4+
2NH4+ + 3O2 → 2NO2- + 4H+ + 2H2O + E
2NO2- + O2 → 2NO3- + E
So, a two-step process with nitrite
produced first, then nitrite oxidized
to nitrate. The microbes carrying out
the nitrification process derive energy (E)
from it.
Carried out by autotrophic bacteria
Step 1
Nitrosomonas
Step 2
Nitrobacter
Nitrification acidifies soil
Doesn’t the first reaction indicate so?
Rapidly in warm,
moist, well-aerated
and fertile soils
Chemical inhibitors
that reduce
Nitrosomonas
activity can be used
If you had your choice, would you rather
inhibit the first or second of the two reaction?
If you inhibited the second, you would build up nitrite, no?
NO3- Leaching
These are the negative consequences. Since
nitrate is a anion, it is not subject to much
adsorption, at least not in soils (like around here
in the sub-tropics or in the temperate region) with
mostly negative charge. So, it leaches quickly
if water is percolating.
Loss of nutrient from soil
Contaminates ground and surface waters
May lead to
This is, basically, fertilization of the water body,
leading to increased algal and aquatic plant growth.
In turn, there is more organic matter in the water
body, and if there is more organic matter, there is
more microbial activity, perhaps, tending to deplete
O2. Other negatives, too.
N is considered more limiting in salt water, P in
freshwater. Think, Gulf of Mexico hypoxia.
Eutrophication, especially marine systems
Methemoglobinemia, blue baby syndrome,
from reduction of NO3 to NO2
Happens in GI tract by bugs. Nitrite tends to bind to hemoglobin,
reducing O2 capacity.
Denitrification
Reduction of NO3- to NO, N2O or N2
NO3- → NO2- → NO → N2O → N2
volatile losses
Anaerobic respiration process. Go check
back with chapter on soil air.
So, nitrate is reduced,
leading to the production
first of nitrite, which is reduced
to a gaseous form of N, principally
either nitrous oxide or N2.
Reaction kinetics, anyone?
Factors affecting denitrification
3
NO
Oxidizable substrates
Anaerobic conditions
How affect rate of denitrification?
You gotta have all three, and the more nitrate and organic matter, the more
denitrification you get. Make sense?
Where does denitrification occur?
Riparian zones?
Look at figure. MWD = moderately well-drained,
SPD = somewhat poorly, etc. How come
more where developed?
Wetlands and rice fields?
Sure, you
wouldn’t use
nitrate with rice,
but if you used
an ammonium
like fertilizer and
left it at the
surface, it would
nitrify, move
into the lower
anaerobic soil,
and denitrify.
So, put it deeper
where it won’t
nitrify, no?
Even upland agricultural soils?
Spatially and temporally variable but up to
60 kg / ha annually
Sure, even uplands and
all the time, like deep in
aggregates where anoxic
conditions may exist.
+ / - environmental effects
Acid deposition due to HNO3 formed from
NO and N2O
N2O is a greenhouse gas
NO3- removed so less potential for nutrient
enrichment Of water bodies, that is, and less potential of babies
going blue.
This is the major way N gets into the soil
naturally.
Biological N Fixation
N2 + 6H+ + 6e → NH3
NH3 into amino acids
Then various pathways.
Certain bacteria, actinomycetes and
cyanobacteria carry out N fixation
~ 140,000,000 Mg N fixed annually
Very, very important process for the biological well-being of the Earth as
we know it. Most think it’s next most important to photosynthesis.
A
B is what
happens
when the
N-fixers
in the soil
have been
autoclaved
away. A
is living
large and
symbiotically
with N-fixers.
B
Biological N Fixation
Nitrogenase enzyme complex involved
Reaction requires energy
Therefore, biological N fixation is aided by
association with plants which supply
photosynthetic products (True / False).
Answer: True. The Ns in N2 are triple bonded. Strong bond that needs a lot
of energy to break.
Biological N Fixation
+
4
3
Inhibited by NH and NO
Zero response of N-fixing
clover to N fertilization.
So, do you fertilize N fixing plants with N?
Symbiotic Fixation with Legumes
Rhizobium and Bradyrhizobium genera of
bacteria involved
Form nodules
on roots of
legumes
OK, these guys chemically
reduce N2. You can’t
do that in an oxidizing
environment, yet these
guys are aerobes.
They maintain an anoxic
zone, by excluding O2.
Cut a nodule open and
you get this.
Biological N Fixation
Symbiosis specific between legume and
bacteria species
Can inoculate if right species absent
That is, maybe plant N-fixing plant if it’s been grown there before and the soil
has a good population of Rhizobia to infect the seedling root. If not, coat the
seed with inoculum to best ensure nodule development.
There is also
Symbiotic Fixation with Nonlegumes
Nodulated / nonnodulated associations
Actinomycetes and others as well as
bacteria
External (rhizosphere)
Wouldn’t you say that the next best
thing to being in the plant would be
being in the rhizosphere with all that
rhizo-deposition going on, you know
energy for the N-fixing bugs?
Sulfur
Component of certain amino acids and
vitamins
Deficiencies result in chlorosis and
stunted growth
Whereas if a plant is deficient in N
(low available soil N), N is translocated from older to newer tissue,
S is less mobile within the plant.
So, if you had to choose which
plant, A or B, was suffering a S
deficiency, which would it be?
A
B
Sources of S
Organic S
Soil minerals
CaSO4, FeS and adsorbed SO42Atmospheric S
Sulfur
Cycle
Focus on
Mineralized S may be
either in oxidized form,
SO42-, or reduced, S2-.
In an oxidizing (aerated)
environment, the reduced
form(s) are oxidized to
sulfate, and in a reducing environment, the oxidized form is reduced.
Oxidation / Reduction
Mineralization of organic-S releases
incompletely oxidized forms of S
Oxidation to SO42- is largely biological
H2S + 2O2 → 2H2O + 2H2SO4
Carried out by autotrophic Thiobacillus
Reduction under anaerobic conditions
SO42- + 8H+ + 8e → S2- + 4H2O
This is coupled with oxidation of
organic matter
Recall this matter from the chapter on soil air.
Acidification Problems
Acid sulfate soils
Mined soils
Acid deposition on forest soils
Environmental problems due to S pertain to acidity. Wetland soils, dredged
materials, etc. may contain an appreciable amount of chemically reduced S. When
these soils / materials become aerated, the reduced S undergoes oxidation,
producing high levels of H2SO4. Some mined waste similarly contains minerals
with reduced S that, when exposed to air or in contact with aerated soil, behave
the same way, releasing H2SO4. Then there is also the matter of acid rain.
Acid sulfate soils
Mined soils
Oxidation of FeS and
FeS2 creates very low pH
Soil with reduced S is
drained or reduced S is
excavated
4FeS + 9O2 + 4H2O →
2Fe2O3 + 4H2SO4
Acidic mine drainage.
Acid deposition on forest soils
H2SO4 + HNO3
Adds H+ in addition
to natural
acidification
processes
Accelerates natural
leaching loss of
nutrients