Mechanisms for Seasonal Fluctuations In Nitrogen Availability In a Sagebrush Ecosystem.

Nutrient availability to plants is regulated by rates of nutrient release and immobilization and by nutrient transport processes in soils. Nitrogen is released from soil organic matter into plant-available forms by the process of N-mineralization, and it is removed from the plant-available pool during N immobilization by soil microbes; however we have a very poor understanding of how environmental factors regulate these processes to produce seasonal changes in N availability. We used ¹⁵N-isotope dilution to measure gross rates of N mineralization and immobilization at 2-wk intervals during two growing seasons in the surface soils of an Idaho big sagebrush—crested wheatgrass ecosystem to quantify seasonal patterns in N-cycling rates.  We also measured soil moisture, temperature, inorganic N, net N mineralization, and soil respiration rates to determine what factors regulate gross and net N cycling rates in the field.  During both a wet and a dry year, seasonal patterns in net N mineralization were similar: net N mineralization dominated in spring and fall, and net N immobilization dominated in summer. This pattern in net mineralization could be largely explained by differences in the sensitivities of gross N mineralization and immobilization to temperature and moisture: gross N mineralization rates were largely insensitive to changes in soil temperature, but were strongly regulated by soil moisture. In contrast, gross N immobilization rates were more sensitive to temperature and less sensitive to soil moisture. Seasonal changes in the quality of microbial substrates (i.e. C:N) appeared to play a trivial role in regulating seasonal patterns in net N mineralization. Biogeochemical models may need to be modified to account for the differing sensitivities of N mineralization and immobilization to temperature and moisture.