Transcript Biogeochemical Cylcles - Department of Soil, Water, and

Lecture 9b
Nitrogen Cycle- N2 gas into NO3 Nitrogen in Atmosphere = 79%
 Problem is getting N2 into a form
that plants can use.
 Most N in soil used for
Agriculture or Sources of

N used by plants in cropland=
 OM = 37%,
 Manure = 19%,
 Fixed by soil org.= 19%
Rainfall = 8%,
 Fertilizer = 13%,
 Sewage = 4%.
Nitrogen FixationConversion of N2 into NH3 or R-NH2
A . Non-Biological Fixation
-Air Pollution -The main oxides of nitrogen present in the
atmosphere are nitric oxide (NO), nitrogen dioxide (NO2) and
nitrous oxide – the result of fuel combustion from motor vehicle
exhaust and stationary fuel combustion sources like electric
utilities and industrial boilers--oxides of nitrogen may remain in
the atmosphere for several days and during this time chemical
processes may generate nitric acid, and nitrates and nitrites as
particles.
- Rainfall additions from electrical discharge
(lightning) 2-5 lbs....../acre/year
N2 -----> NO3-
N2
Denitrification
N fixation
immobilization
NO3-
R-NH2
Plants
mineralization
Nitrification
Ammonification
NH4+
1. Nitrogen Fixation
Conversion of N2 into NH3 or R-NH2
B . Biological Fixation
1. Non-Symbiotic (independent
organism) - Azotobacter - aerobic &
Clostridium - anaerobic
about 5-50 lbs....../acre/year
2. Symbiotic - mutually beneficial for
host organism and bacteria - complex
plant - bacteria interaction
http://www.agron.iastate.edu/~loynachan/mov/
B. Symbiotic NFixation
Bacteria = Rhizobia
Plant = Legume - peas, clover, alfalfa,
cowpeas, peanuts, beans, soybeans
Alfalfa - 200 lbs....../acre/year
Soybeans - 100 lbs......./acre/year
Beans - 40 lbs...../acre/year
* Green manure is live plant material added to soil
to increase N content and SOM.
Symbiotic N Fixation
 Bacteria invades host plant root
 Response of host plant root is to grow a
nodule for the bacteria to live in.
 Bacteria takes N2 from the air and converts
it into R-NH2 which resides in Bacteria in
Nodule and some is in the form of NH4+
 Fate of N Fixed by Rhizobium:
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1) used by host plant,
2) leaks out of root to become available to
surrounding plants,
3) as roots and nodules are sloughed-off
heterotrophic organisms immobilize the N
and it eventually becomes part of the SOM.
Infection and
nodule
formation
Rhizobium
Alfalfa root nodule
Dazzo & Wopereis, 2000
Root hair curling around rhizobia
Rhizobia reproduce
in infection threads
M. Barnett
Bacteroids filling a single cell
Dazzo & Wopereis, 2000
Michael Russelle - USDA-ARS
Plant Science Research Unit
Gage and Margolin, 2000
Vance et al., 1980
Michael Russelle - USDA-ARS
Plant Science Research Unit
Nitrogen fixation
is (usually) reduced by external N
Fixed N
“Soil” N
N2
Total N
in the plant
N2
Legumes buffer the N supply
and fix what they need from the air
Legume
Grass
Legume
Grass
Fixed N
Manure N
Soil N
Michael Russelle - USDA-ARS
Plant Science Research Unit
We need to fertilize non-legumes
and can easily guess wrong
Legume
Grass
Legume
Grass
Loss
Fixed N
Fert N
Manure N
Soil N
Michael Russelle - USDA-ARS
Plant Science Research Unit
2. Ammonification
A. Ammonification in the soil is
the conversion of organic N
(RNH2) into inorganic ammonia
(NH3)
heterotrophic organ.
R-NH2 ---> NH3 + H+ ----> NH4+
B. Fates of NH4+ =
1) fixed by clay minerals,
2) lost by soil erosion,
3) used by plants (NH4+),
4) volatilization
NH4+ ----> NH3
High pH Soils > 7.5
3. Nitrification
2 - step process
1. 2NH4+ + 3O2 ---> 2NO2- + 4H+ + 2H20 + E
Nitrosomonas
2. 2NO2- + O2 --> 2NO3- + E
Nitrobacter
Process is acid causing due to release of
4 H+
3. Fates of Nitrate- NO3*Immobilization ---> Plant uptake of NO3*NO3- is not held by soil particles and is
easily leached - when ppm NO3- is > 10 ppm
the water is considered to be contaminated
* Denitrification - stimulated by anaerobic
conditions.
4. Denitrification
Involves conversion of NO3- to N2 gas
C6H12O6 + 4NO3- --> 6CO2 + 6H2O +
2N2(gas) + NO + NO2
Bacteria = anaerobic
Through nitrification and denitrification 10
- 20 % of the applied N is lost.
Nitrification inhibitors can be
applied like N-Serve. This chemical
inhibits the growth of nitrosomonas
and nitrobacter or slows conversion
of NH4+ conversion to NO3-
N2
Denitrification
N fixation
immobilization
NO3-
Plants
R-NH2
Nitrification
Ammonification
NH4+
Duxbury, 1997, Wm. C. Brown Publishers
Nitrate in drinking
water supplies
 Nitrate has been detected in
surface- and ground-water
supplies in various parts of the
state.
 Low levels of nitrate can be
found in most of the surface
waters of the state.
 In a recent statewide survey of
water wells, a small percentage
contained excessive nitrate
concentrations.
 In cases where the
concentration of nitratenitrogen exceeds the
maximum contaminant
level of 10 mg/L, as set
forth by the U.S. EPA water suppliers are
required to issue a nitrate
alert to users.
 The health of infants, the
elderly and others, and
certain livestock may be
affected by the ingestion
of high levels of nitrate.
USGS, 1998
Risk of Groundwater Contamination
by Nitrate
C:N Ratios
 Bacteria require about 5 grams of
carbon for each gram of nitrogen
assimilated or used C:N in a ratio of 5:1.
 Decomposing microorganisms have first
priority for any mineralized N.
 This use of N by decomposers results in
insufficient N for plants.
 Eventually period of N starvation is over
after all the high C:N material is
decomposed.
C:N Ratio of some organic materials
 domestic sewage -5:1
 Muni. sewage - 8:1
 legume hay -13:1
 Mun. Compost 28 : 1
 green grass - 35:1
 corn stover - 50:1
 Straw - 80:1
 Sawdust - 400:1
Break even point for C:N is 20 to 30 : 1.
N-Cycle
 Plants need NO3 This can be
supplied as NO3-,
NH4+, or organic
N (R-NH2),
 The rate at which
NO3- is available
depends on : C:N,
temp, O2, water,
Ohio State University Extension Fact Sheet
 If you had to dispose of 10 tons of sawdust
every month from a local saw mill, what would
be your solution?
Soil Inoculants to increase N
Fixation
 Inoculate soil or seeds
with N-fixing bacteria
 Introduce bacteria,
nematodes, or insects
that are predators of
pest organisms
 Add nitrification
inhibitors to reduce
bacteria that convert
ammonium to nitrate
Composting
A biological process that
breaks down organic
material (such as grass
clippings and leaves) into
more stable molecules
Stages of Composing Process
 Mesophilic stage 1
 Brief
 Temperature rises to
40 degrees C
 Sugars and readily
available microbial
food sources are
rapidly metabolized
Stages of Composing Process
 Thermophilic stage
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50 to 70 degrees C
Easily decomposed
compounds are used up
and humus-like
compounds are formed
Frequent mixing
essential to maintain
oxygen levels and
assure even heating of
all materialIf too hot may kill
organisms in the pile
Stages of Composing Process
 Mesophilic (2nd)
 Curing stage
 Temperatures fall
back to ambient
 Material recolonized
by mesophilic
organisms
Benefits to Composting
 Safe storage
 Easier handling
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Volume reduced 30 to
50%
Material more
uniform
 Nitrogen competition
avoidance
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No nitrate depression

N in organic form
 Nitrogen stabilization
Benefits to Composting
 Partial sterilization
thermophilic stage kills
most weed seeds and
pathogenic organisms in
 Detoxification
 Most organic compounds
are destroyed
 Disease suppression
 Compost suppresses soil
borne diseases by
encouraging microbial
antagonisms
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