The nitrogen cyclein soil

Slide 8.2

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

N-cycle Slide 8.6

plant & animal residues

organic-N

NH4+NO2

-

NO3-

N2

1122

33

33

4455

55

N2

R-NH2

NH4+

NO3- Plants

N fixation

AmmonificationNitrification

Denitrification

immobilization

Mineralization vs. Immobilization

Fate of N if added to soil???

Slide 8.8Low C:N (high N content)

Alfalfa, peas, grass

High C:N (low N) Slide 8.9

straw, bark, sawdust

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 Volatilization - gaseous loss of N

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

Nitrogen (nitrate?) Leaching

Eutrification

Denitrification

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

Process:

nodule

Rhizobium

(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