In situ soil net nitrogen mineralization in coastal salt marshes (Suaeda salsa) with different flooding periods in a Chinese estuary

Flooding periods can be one of the most important factors influencing nitrogen (N) biogeochemical processes in wetlands ecosystem. We conducted a field study using in situ incubation method to investigate the seasonal dynamics of soil net N mineralization in three coastal salt marshes (Suaeda salsa) with different flooding periods (i.e., short-term (STF), seasonal (SF), and tidal (TF) flooding wetland) in the Yellow River Delta. Selected soil inorganic N pools (ammonium, nitrate and inorganic N) and N transformation (mineralization, nitrification and ammonification) rates in the top 0-10 cm soils were repeatedly quantified from April to October. Clear seasonal patterns in inorganic N pools and transformation rates were observed in accord with the seasonal variations of temperature and moisture. Generally, higher levels of soil inorganic nitrogen, ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3－-N) occurred in the early-growing season (April), and NH4+-N contents got a small accumulative peak in midsummer (September). The lower rates (negative) of net mineralization (Rmin), nitrification (Rnit) and ammonification (Ramm) were observed in the early-growing season (April-June) and fall (September-October), whereas higher values (positive) in midsummer (August-September). Flooding had a significant influence on inorganic N pools (except for NH4+-N) and transformation rates (p < 0.05). Rmin values in SF wetland were significantly higher in the August-September period than those in other incubation periods. Rnit values in TF wetland exhibited a small variation and the highest value occured in the June-August period. The results of principal component analysis showed that soil samples were clearly divided into two groups before and after flow-sediment regulation. After flooding events, the Rmin and Ramm values generally increased in the three wetlands, whereas a significant decrease in Rnit values was observed in SF wetland (p < 0.05), thus the differences in NO3－-N among these wetlands were eliminated. These results suggested that seasonal variations in temperature and moisture are important factors influencing inorganic N pools and transformation rates.