Effects of salinity and moisture on sediment net nitrogen mineralization in salt marshes of a Chinese estuary

The interactive effects of salinity and moisture on the net nitrogen mineralization were seldom studied in coastal wetlands. To reveal the phenomenons and mechanisms of salinity and moisture effects, we conducted a 30-days laboratory experiment with different salt addition levels (0, ppt, 5 ppt and 35 ppt) and moisture levels (10%, 50% and 100% of water holding capacity (WHC)), and the key N transformation rates and enzymes activities were measured. Our results showed that during the incubation, the rates of soil net nitrogen mineralization (R_min) and nitrification (R_nit) under all treatments, increased during 0-1 day, decreased during 1-3 days, then increased and kept around zero. As incubation time increased, urease activities increased, arylamidase activities decreased and fluorescein diacetate activities increased first and then decreased. Increasing salinity under high moisture (100% WHC) treatments and increasing moisture under high salinity (35 ppt) treatments would promote R_min and R_nit. Whereas, increasing salinity under low moisture (10% WHC) treatments, and increasing moisture under low salinity (0 ppt) treatments would suppress R_min and R_nit. The responses of enzyme activities to salinity and moisture gradients almost correspond with R_min, indicating salinity and moisture could affect N transformation by the regulation of related microbial enzyme activities.