Soil Resource Status Affects the Responses of Nitrogen Processes to Changes in Temperature and Moisture

The mechanisms of the different responses of soil gross nitrogen (N) transformation to increasing temperature or moisture in different types of soils are still unclear. Here, we conducted two 15N tracing experiments to investigate the effects of increasing temperature (15 °C and 25 °C) or moisture (30%, 45%, and 60% water-filled pore space (WFPS)) on soil gross N transformation rates for two soils (organic (O) and mineral (A) horizon soils) in a temperate forest. As the temperature increased from 15 to 25 °C or moisture increased from 30 to 60% WFPS, total mineralization rates increased by 4.5-fold and 2.5-fold respectively, total NH4+ immobilization rates increased by 173.2-fold and 7.6-fold respectively, and autotrophic nitrification rates increased by 0.7-fold and 0.6-fold respectively in the O horizon. Under the same treatment, the changes in autotrophic nitrification rates, NH4+ immobilization rates, and mineralization rates were much smaller in the A horizon than in the O horizon. We propose that the difference between the two horizons in the responses to changing temperature and moisture was due to the different resource status. The O horizon could provide more substrates than the A horizon, resulting in a stronger response of N processes to changing temperature and moisture. Resource status also affected the competition for substrates between NH4+ immobilization and autotrophic nitrification. The N transformation rates were higher in the O horizon than in the A horizon, consistent with higher microbial biomass N, microbial respiration rate, and amoA gene abundance in the O horizon. Our results suggest that the activity switch of microbes and the competition for resources are important biotic factors regulating potential responses of soil N cycling processes to changing abiotic factors.