Differential temperature and moisture sensitivities of N mineralization and immobilization regulate seasonal dynamics of net N mineralization in a semi-arid ecosystem

Background/Question/Methods

Nutrient availability is regulated by rates of nutrient release and immobilization and by nutrient transport processes. 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 a 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.

Results/Conclusions

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 insensitive to changes in soil temperature, but were strongly related to changes in soil moisture. In contrast, gross N immobilization rates were insensitive to soil moisture but strongly related to temperature. 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.