Short-term effects of NaCl and Na2SO4 on nitrogen mineralization in the soil in three marshes of the Liaohe River estuary

Abstract NaCl and Na2SO4 are the key factors affecting nitrogen (N) mineralization in the soil of estuarine wetlands. However, it remains unclear how N mineralization responses to NaCl and Na2SO4. In the present study, samples of topsoil (0–10 cm) and subsoil (10–20 cm) were collected in three marshes, including a Phragmites australis marsh (Pa-marsh), a Suaeda salsa marsh (Ss-marsh) and a mudflat marsh (Mu-marsh), in the Liaohe River estuary. The soil samples were treated by NaCl and Na2SO4, with a set of salinity levels of 0 %Es, 2 %Es, 5 %Es, 10 %Es, 20 %Es and 100 %Es (%Es, percent of equivalent seawater), respectively. After incubating the soil samples for 2 weeks, the contents of NH4+ N and NO3− N were measured and the net N mineralization rates (Rmin) were calculated. We found that NH4+ N and NO3− N accumulations in the soil samples of the three marshes in the NaCl and Na2SO4 treatments. Both NaCl and Na2SO4 significantly affected Rmin for the top soil for Pa-marsh in all salinity treatments, whereas salt-effects for Ss- and Mu-marsh soils were only found at 10 %Es and/or 20 %Es levels in the NaCl treatments. There was an ion-specific effect of NaCl and Na2SO4 on N mineralization. NaCl increased Rmin by 114–164% and 86–139% in the topsoil and subsoil of the three marshes, respectively; Na2SO4 increased Rmin by 234% in the topsoil of Pa-marsh and had no effect on Rmin in the other soils. A salt threshold of 50–60%Es was found for NaCl and Na2SO4 impacting on N mineralization of Pa-marsh soil. Salt type, salinity level, wetland type and soil layer were the main factors impacting the N mineralization. The results suggest that the effect of salt on N mineralization could vary with salt type and soil properties.