Interactions of NaCl and Na2SO4 on soil organic C mineralization after addition of maize straws

NaCl and Na2SO4 often dominate salt compositions in saline soils. While either salt alone affects soil organic matter mineralization, their interactions on soil organic matter dynamics are unknown. This study aimed to investigate interactive effects of the two salts on organic C mineralization and microbial biomass C of the saline soils after addition of maize straws. Both NaCl and Na2SO4 were applied at 0, 40 and 80 mmol Na kg−1 soil and the incubation was undertaken at soil water content of 15% and 20% (w/w) in dark at 28.5 °C for 70 days. The study found significant interactions of NaCl and Na2SO4 on CO2–C evolution during the early incubation periods—a suppressing effect at days 1–2 but a stimulating effect at days 6–8 and 17–20, and thereafter the salt interactions were influenced by water content. The interactions of water content with NaCl or Na2SO4 on CO2–C evolution were observed through the incubation periods except days 1–2, showing that the salt effects were dependent on water content. Total CO2 evolution over the 70-day-long incubation decreased with increasing NaCl but increased with increasing Na2SO4 compared to the nil-salted treatment. Salt interactions on soil microbial biomass C were observed at days 7, 21, but not at day 49. Microbial biomass C increased at day 7 in the soils treated with either NaCl or Na2SO4 but decreased where the two salts were combined. At day 21, microbial biomass C increased with NaCl but decreased with Na2SO4 regardless whether the counterpart salt was added. The results suggest that soil organic C mineralization can be affected by the interactions of NaCl and Na2SO4, possibly through the salt-induced changes in microbial biomass community structure.