Influences on Soil Nitrogen Mineralization: Implications for Soil Restoration and Revegetation
Nitrogen is a macronutrient essential to the growth of plants and is also one of the most deficient nutrients in most soils. Insufficient levels of available soil nitrogen limit microbial growth and decay and growth of the plants themselves. Because site disturbance adversely affects the flow of nitrogen through soil-plant-microbial systems, the re-establishment of the cycle of nitrogen flow in the soil is crucial to revegetation attempts. Mineralizable nitrogen--nitrogen in forms readily usable by plants--is dependent upon a number of factors. According to Stanford and Smith (1972), those factors include soil moisture, pH level, temperature, microbial biomass, and amounts of other nutrients. Three of the more significant and interrelated influences--microbial biomass, temperature, and moisture--are discussed here. This is certainly not an exhaustive list of factors influencing soil nitrogen mineralization, but instead, a general overview of the more significant considerations.
Microbial decay of organic material is the main release process of soil nitrogen. Nitrogen is released from microbial decay in the form of ammonium, but can also be immobilized by microbes which take up nitrate and ammonium to satisfy their own requirements. The mobilization or immobilization of nitrogen is dependent upon the amount of N the decomposing material itself provides; insufficient nitrogen released from decaying organic matter will result in the absorption of ammonium and nitrate by the microbes. An excess of N will satisfy the requirements of the microbes and provide a surplus to be released into the soil for use by the plants. Heterotrophic organisms also oxidize ammonium into nitrate; inorganic oxidation such as nitrification is important because it mobilizes soil nitrogen, making readily usable forms of nitrogen available to plants. Decay of organic matter by microorganisms is directly influenced by temperature and moisture, as discussed in the following paragraphs (Singer and Munns, 1996).
Temperature is a primary factor influencing soil nitrogen mineralization as it directly affects microbial decay as the main release process of N. Generally speaking, favorable temperature conditions for plant growth closely resemble optimal conditions for microbial decay (Myrold, 1987). Powers (1980) determined that in the standard temperature range of forest soils, N mineralization increases as mean summer soil temperature rises, due to increased microbial activity at higher temperatures. In a study of the effects of simulated fall and spring temperature conditions on soil N, Campbell et. al. (1971) found that temperatures which fluctuated daily reduce microbial organisms in the soil, especially the population of bacteria. The reduction of microbial organisms produced marked differences in the rate of...