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

Fixation by clay refers to association of nitrogen with the soil.  Since the soil has a negative charge, the ammonium ion (NH₄⁺) can be bound to the soil particle.  Depending on the type of clay, this ion can be trapped in the actual structure of the clay mineral and become unavailable to plant uptake.

Ammonification and Mineralization is a process that converts organic N in manure, crop residues and soil organic matter to ammonia and ammonium.  Annual mineralization rates vary, though in general about 1.5-3.5% of the organic nitrogen in the soil will be mineralized each year.  The exact rates depend on soil temperature, moisture and aeration status; most rapid mineralization occurs in hot climates (68-95°F), well-aerated soils, and moist soils.  If large amounts of N-rich organic materials with narrow C:N ratios (<15-20) are added, significant levels of NH₄⁺ can be produced.  This will then be converted to nitrate via nitrification, absorbed by plants, fixed or held by the soil, or converted to ammonia and lost to the air via volatilization.  In NY, about 60-80 lbs N/acre is mineralized from soil OM each year

Nitrification is the process by which microbes use enzymes to convert ammonium (NH₄⁺) to nitrate (NO₃^-) to obtain energy.  It is a two step process, with a different species of bacteria performing each step.  Nitrate is most readily available to plants and is the preferred N form.  Nitrification is most rapid when soil is warm (67-86°F), moist, and well aerated (late May, June).  However, it will not occur when soil temperature drops below 41°F or goes above 122°F.  Because the process releases H⁺ ions, nitrification lowers soil pH.