the nitrogencycle in soil - ung.simirjana/gradivo/ozvt/tla/nitrogen cycle.pdf · slide 8.4...
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Slide 8.4
Oxidation States of Soil N
N Form Name Oxidation stateorganic-N -3
NH4+ ammonium -3
N2 dinitrogen gas 0 (oxidation) (reduction)
NO2- nitrite +3
NO3- nitrate +5
Nitrogen Redox Processes
Oxidation: loss of e-
Reduction: gain of e-
-3 +5NH4
+ → NO3-
8 e- transfer
Changes in soil NO3-, CO2 and C:N ratios after
addition of organic residues
Relativevalues ofCO2,NO3,conc.. &C:N ration
Time (weeks)residue added
CO2
NO3
C:N 40:1C:N 90:1
C:N 12:1
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) fixed by clay minerals, 2) lost by soil erosion, 3) used by plants (NH4+), 4) volatilization
» NH4+---->NH3,
Ammonia VolatilizationUrea:
CO(NH2)2 → NH3 +CO2 + H2Ourea soil enzymes& H2O
- Most volatilization when:coarse or sandy-textured soils
low clay and low organic matter (which adsorb NH4
+)
dry alkaline surface
NitrificationNH4
+ → NO2- → NO3
-
ammonium nitrite nitrate- oxidation of N
* Autotrophic bacteria• obtain energy from N oxidation• Nitrosomonas
NH4+ → NO2
- + energy
• NitrobacterNO2
- → NO3- + energy
Nitrification (cont’d)
* Rapid in well-aerated,warm, moist soils
• aerobic organisms(O2 is required)
• little NO2- accumulation
* Acid-forming processNH4
+ +3/2O2→ NO2- + 2H+ + H2O
Denitrification
Gaseous loss of N upon N reduction
+ e- + e- + e- + e-
NO3- → NO2
- → NO → N2O → N2
nitric nitrousoxide 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)
NO3-
organisms
NO3- → NH4
+ → organic-N
- N is reduced for use in protein formation
Nitrogen FixationN2 (organisms)→ NH4
+
* Symbiotic relation between bacteria and plants:
- legumes+
- rhizobium
Bacteria: Rhizobium genus (species specific)
R. meliloti - alfalfaR. trifolii - cloverR. phaseoli - beans
- bacteria require plant to function
- inoculation of seed(coat seed with proper bacteria)
Nitrogen Fixation
(b) Process:
RhizobiumRhizobium
organicorganic--CC
organicorganic--NN NN22
C from plant photosynthesis
N from fixation of N2
⇒⇒ symbiosissymbiosis
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
lowered N fixation
Symbiotic - without nodules
* Azolla/Anabaena complexblue-green algae (N-fixer)
in leavesfloating fern in rice paddies
* Rhizosphere organismsuse root exudates (C)large areas
Nonsymbiotic N-fixation:Free-living Organisms
* Bacteria and blue-green algae
aerobic and anaerobic
small amounts: 5 - 50 kg/ha/yr
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 FixationConversion 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 drinkingwater 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 hascaused 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 nitrogencan lead to eutrophication.
Increased deposition of nitrogen from atmosphericsources 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 intothe 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) improvingnitrogen 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
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