Biogeochemical controls on nitrogen transformations in subtropical estuarine wetlands

Changes in soil moisture and salinity are expected to alter gross nitrogen (N) transformations, which can control microbial N dynamics in estuarine wetlands. However, the effects of soil moisture and salinity on microbial N limitation remain poorly understood. To this end, we used a¹⁵N pool dilution approach to characterize the changes in soil gross N transformations with the variations of soil moisture in subtropical estuarine wetlands along a low-level salinity gradient. The results showed that soil gross N mineralization (GNM) increased with the increasing soil moisture. Ammonia immobilization (AIM) and microbial N immobilization (MIM) increased with the increasing soil salinity. High gross nitrification rates were generally found in the wetlands with relatively high ammonia content and low soil moisture. The ratios of MIM to GNM (MIM/GNM), as an indicator of microbial N limitation, decreased in response to the enhancing soil moisture and increased with the increasing soil salinity. Ammonia supply capacity (GNM-AIM) decreased with increasing soil salinity but increased with the increasing soil moisture. These results together indicated that microbial N limitation became stronger in the wetlands characterized with high soil salinity and low soil moisture. Soil moisture and salinity exhibited also indirect effects on microbial N limitation through affecting organic N content, ecological stoichiometry, microbial biomass and enzyme activity. Therefore, soil moisture and salinity levels are crucial for controlling soil N transformations with important implications on microbial N removal and retention in estuarine wetlands.