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

Abstract

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 0–10cm 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.