PO 13. Describe how the following affect the fate of N in soil: the Nitrogen Cycle.
- Fixation by clay
- Ammonification and mineralization: R-NH2 → NH3 → NH4+ (organic N à ammonia à ammonium)
- Nitrification: NH4+ (ammonium) → NO2- (nitrite) → NO3- (nitrate)
- Volatilization: CO(NH2)2 (urea) → NH4+ (ammonium) → NH3 (ammonia)
- Denitrification: NO3- (nitrate) → NO2- (nitrite) → NO (nitric oxide gas) → N2O (nitrous oxide gas) → N2 (dinitrogen gas)
- Immobilization: NH4+ (ammonium) and NO3- (nitrate) → R-NH2 (organic N)
- Leaching
- Plant uptake
- Symbiotic fixation: N2 → NH3 → R-NH2 → amino acids → proteins
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Leaching is the loss of NO3- from the soil with water movement. Since nitrate is an anion, it does not attach to soil particles and thus easily leaches from the soil. Total losses are determined by water movement and nitrate contents of the soil. Plant uptake occurs when nitrate is available and conditions are aerobic (i.e. not wet or flooded). Symbiotic fixation is the conversion of N2 from the atmosphere to plant protein. Atmospheric N is fixed in a symbiotic process carried out by microorganisms, the Rhizobium bacteria which form root nodules in legumes. This nitrogen becomes available when N fixers die. The process requires energy and the enzyme nitrogenase (Fe, Mo, P, S), so if a plant-available N source is present, the crop will use that instead of fixing N from the air. |
