The nitrogen cycle
in soil
Slide 8.2
Slide 8.4
Oxidation States of Soil N
N Form
Name
Oxidation state
organic-N
-3
NH4+
ammonium
N2
dinitrogen gas
NO2-
nitrite
NO3-
nitrate
È
-3
0 (oxidation) (reduction)
+3
+5
Ç
È
È
Ç
Ç
Nitrogen Redox Processes
Oxidation: loss of eReduction: gain of e-3
+5
NH4+ → NO38 e- transfer
Slide 8.6
N-cycle
plant & animal residues
5
5
NO3-
4
N2
organic-N
2
3
1
NO2-
3
NH4+
N2
Denitrification
N fixation
immobilization
NO3-
Plants
R-NH2
Nitrification
Ammonification
NH4+
Mineralization vs. Immobilization
Fate of N if added to soil???
Low C:N (high N content)
Alfalfa, peas, grass
Slide 8.8
High C:N (low N)
straw, bark, sawdust
Slide 8.9
Changes in soil NO3-, CO2 and C:N ratios after
addition of organic residues
Relative
values of
CO2,
NO3,
conc.. &
C:N
ration
C:N 12:1
NO3
CO2
C:N 40:1
C:N 90:1
residue added
Time (weeks)
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.
2. Ammonification
A. Ammonification is the
conversion of organic N (RNH2)
into inorganic ammonia (NH3)
R-NH2 ---> NH3 + H+ ----> NH4+
heterotrophic microrganism.
B. Fates of NH4+
1)
2)
3)
4)
fixed by clay minerals,
lost by soil erosion,
used by plants (NH4+),
volatilization
» NH4+---->NH3,
Ammonia Volatilization
- gaseous loss of N
Ammonia Volatilization
Urea:
CO(NH2)2 → NH3 +CO2 + H2O
urea
soil enzymes
& H2 O
- Most volatilization when:
9 coarse or sandy-textured soils
9 low clay and low organic matter
(which adsorb NH4+)
9 dry alkaline surface
Nitrification
NH4+ → NO2- → NO3ammonium
nitrite
nitrate
- oxidation of N
* Autotrophic bacteria
• obtain energy from N oxidation
• Nitrosomonas
NH4+ → NO2- + energy
• Nitrobacter
NO2- → NO3- + energy
Nitrification (cont’d)
* Rapid in well-aerated,
warm, moist soils
• aerobic organisms
(O2 is required)
• little NO2- accumulation
* Acid-forming process
NH4+ +3/2O2→ NO2- + 2H+ + H2O
Nitrogen (nitrate?) Leaching
Æ Eutrification
Denitrification
Denitrification
Gaseous loss of N upon N reduction
+ e+ e+ e+ eNO3- → NO2- → NO → N2O → N2
nitric
oxide
nitrous
oxide
Denitrification (cont’d)
* Microorganisms responsible:
• facultative anaerobes
- prefer O2 but will use N
for a terminal e- acceptor
• mostly heterotrophic
- use organic-C for energy source
(reductions require energy)
Denitrification (cont’d)
* Denitrification enhanced by:
• low O2 (flooding)
• high O.M. (energy source)
• high NO3-
Denitrification (cont’d)
* Metabolic reduction is not denitrification
(no N gas formation)
organisms
NO3-
NO3- → NH4+ → organic-N
- N is reduced for use in protein formation
Nitrogen Fixation
N2 (organisms)→ NH4+
* Symbiotic relation between
bacteria and plants:
- legumes
+
- rhizobium
Nitrogen Fixation
Bacteria: Rhizobium genus
(species specific)
R. meliloti - alfalfa
R. trifolii - clover
R. phaseoli - beans
- bacteria require plant to function
- inoculation of seed
(coat seed with proper bacteria)
Process:
Rhizobium
nodule
(b) Process:
organic-N
N2
Rhizobium
organic-C
C from plant photosynthesis Ô
N from fixation of N2
Ò
⇒ symbiosis
Quantity of N Fixed
Alfalfa and clover provide
» 100 - 250 kg N/ha/yr
(mature stand, good fertility & pH)
Beans and peas
» less fixation but high protein food
with minimum N input
added N fertilizer
Æ lowered N fixation
Symbiotic - without nodules
* Azolla/Anabaena complex
Ï
Ï
Ï
Ï
blue-green algae (N-fixer)
in leaves
floating fern in rice paddies
* Rhizosphere organisms
use root exudates (C)
z large areas
z
Nonsymbiotic N-fixation:
Free-living Organisms
* Bacteria and blue-green algae
z
aerobic and anaerobic
z
small amounts: 5 - 50 kg/ha/yr
z
inhibited by available soil N
Nitrogen Cycle
• Nitrogen in Atmosphere = 79%
• Problem is getting N into a form
that plants can use.
• Most N in soil used for
Agriculture or Sources of
• N from OM = 37%,
Manure = 19%,
• Fixed by soil organisms =
Rainfall = 8%,
• Fertilizer = 13%,
• Sewage = 4%.
1. Nitrogen Fixation
Conversion of N2 into NH3 or R-NH2
B . Biological Fixation
1. Non-Symbiotic (independent
organism) - Azotobacter - aerobic &
Clostridium - anaerobic
about 3-30 kg ha-1
2. Symbiotic - mutually beneficial for
host organism and bacteria - complex
plant - bacteria interaction
B. Symbiotic N- Fixation
Bacteria = Rhizobia
Plant = Legume - peas, clover, alfalfa,
cowpeas, peanuts, beans, soybeans
Alfalfa - 100 kg....../acre/year
Soybeans - 500 kg......./acre/year
Beans - 20 kg...../acre/year
3. Nitrification
2 - step process
1. 2NH4+ + 3O2 ---> 2NO2- + 4H+ +
2H20 + Energy
Nitrosomonas
2. 2NO2- + O2 --> 2NO3- +
Nitrobacter
Process is acid causing due to release of
4 H+
3. Fates of Nitrate
*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.
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.
Drinking Water
• In cases where the concentration of
nitrate-nitrogen 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.
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.
The application of nitrogen fertilizers to crops has
caused increased rates of denitrification and leaching of
nitrate into groundwater.The additional nitrogen entering the
groundwater system eventually flows into streams, rivers,
lakes, and estuaries. In these systems, the added nitrogen
can lead to eutrophication.
Increased deposition of nitrogen from atmospheric
sources because of fossil fuel combustion and forest burning.
Both of these processes release a variety of solid forms of
nitrogen through combustion.
Livestock release a large amounts of ammonia into
the environment from their wastes. This nitrogen enters the
soil system and then the hydrologic system through leaching,
groundwater flow, and runoff.
Sewage waste and septic tank leaching.
For Nitrate leaching to occur water must move through the soil.
Reductions in nitrate losses can be achieved by: 1) improving
nitrogen fertilizer placement, 2) applying part of the fertilizer N
later in the growing season, and 3) using slow-release fertilizers or
nitrification inhibitors and currently recommended soil test
procedures for fertilizer management.
Nitrogen
• NH4+ and NO3-
forms taken up by
plants
• Loss of N can occur:
1) leaching of NO3- ,
2) volatilization of
NH4+ to NH3 (high
pH soils),
• 3) immobilization by
plant or microbe
uptake,
• 4) Denitrification