Changes in Soil Nitrogen Mineralization and Nitrification at Sawyer Mesa after Fire Associated with the Dome Fire, May 1996

White, Carleton

Nitrogen is often the nutrient limiting production in conifer forests (Vitousek and Howarth 1991 ). To a large extent, the supply of nitrogen for plant growth in terrestrial ecosystems is determined by the rate at which organic-bound soil nitrogen is released in inorganic form; a process termed nitrogen mineralization (Fig. 1 ). The first inorganic form of nitrogen is ammonia (NH3), which readily converts to ammonium (NH4+) in the soil solution. Ammonium (or ammonia) can be converted to nitrate (N03·) through the two-step process of nitrification. Through the nitrification process, a portion of the ammonium-nitrogen can be lost to the atmosphere as nitrous oxide. Both ammonium and nitrate are assimilated by the soil microbial community and by higher plants. If demand for inorganic nitrogen by the microbial community is high, then concentrations of inorganic nitrogen within the soil may decline, which is termed 'immobilization.' Only when the demand by the microbial population is satisfied can concentrations of inorganic nitrogen increase within the soil, representing net mineralization.