Soil moisture determines effects of climates and soil properties on nitrogen cycling: Examination of arid and humid soils.

While soil moisture has a significant effect on nitrogen (N) cycling, how it influences the dependence of this important biological process on environmental factors is unknown. Specifically, it is unclear how the relationships of net N mineralization (N) and soil moisture vary with soil properties and climates. In turn, how the relationships of N vs. soil properties and climates vary with soil moisture is also unknown. Therefore, soil samples from the 26 sites were collected within two climatic regions (i.e., arid and humid) across China. Then a four-week microcosmic incubation experiment was conducted at five soil moisture levels (20, 40, 60, 80, and 100% field water holding capacity (FWHC)) at 25 °C to measure the dynamics of N. The results showed that increasing soil moisture significantly increased N (+212%) and the N mineralization rate constant (k) (+0.26%), and that the effects of soil moisture were greater in humid soils (+250%) than arid soils (+178%). The slopes of the relationship between N vs. soil moisture increased with soil organic carbon (SOC) (+50.6%) and total N (TN) (+65.3%) concentrations, and decreased with pH (-43.0%) and clay content (-0.09%), especially in arid regions. Additionally, N was significantly correlated with soil properties and mean annual precipitation (MAP), and the slopes of most of these relationships increased with soil moisture in arid soils (+59.2-3805%), but decreased in humid soils (-1.96-140%). The results indicated that increasing soil moisture strengthened the dependence of N on soil properties and climates in arid soils, and that increasing soil pH and clay content reduced, but SOC and TN concentrations enhanced the dependence of N on soil moisture. Therefore, with changes in rainfall distribution patterns and an increase in extreme rainfall events, there is enormous potential for N in agricultural soils in arid regions, which is regulated by soil moisture and properties. On the contrary, in humid regions, the decoupling of the effects of soil moisture and soil properties on N mineralization could be due to microbial adaptation. Moreover, the coupled effects of soil environment and properties on N cycling in different climatic regions merit great consideration in experimental research as well as in biogeochemical model development and prediction.