PO 13. Describe how the following affect the fate of N in soil: the Nitrogen Cycle.

Volatilization is the production and loss of ammonia gas from ammonium.  Ammonia volatilization increases with soil pH, as the high H⁺ concentration promotes the conversion of nitrate to ammonium.  Volatilization losses may be high for unincorporated urea fertilizer or manure (urine).  The high level of evaporation assists this loss.  Incorporation of manure and fertilizers can reduce ammonia losses by 25-75%.

Denitrification occurs when NO₃^- is converted into gaseous forms of N.  The process is common in poorly drained (anaerobic) soils, even those that are tile-drained, and in warm conditions.

Immobilization is the reverse of mineralization.  Microbes compete with crops for NH₄⁺ and NO₃^- for their own survival; when nitrogen is scarce the microbes convert inorganic N forms into their own organic forms, preventing plants from taking the N up.  This commonly occurs in aerated soils (as opposed to denitrification, which occurs in anaerobic soils), particularly with high carbon-to-nitrogen (C:N) ratio.  This happens when materials like straw, sawdust, etc. are incorporated.  Immobilization ties up available N in microbial tissue, which must be "re-mineralized" to become available to plants again.