High Temperature and Salinity Enhance Soil Nitrogen Mineralization in a Tidal Freshwater Marsh

Soil nitrogen (N) mineralization in wetlands is sensitive to various environmental factors. To compare the effects of salinity and temperature on N mineralization, wetland soils from a tidal freshwater marsh locating in the Yellow River Delta was incubated over a 48-d anaerobic incubation period under four salinity concentrations (0, 10, 20 and 35 parts per thousand) and four temperature levels (10, 20, 30 and 40 degrees C). The results suggested that accumulated ammonium nitrogen (NH4+-N) increased with increasing incubation time under all salinity concentrations. Higher temperatures and salinities significantly enhanced soil N mineralization except for a short-term (approximate to 10 days) inhibiting effect found under 35 parts per thousand salinity. The incubation time, temperature, salinity and their interactions exhibited significant effects on N mineralization (P < 0.001) except the interactive effect of salinity and temperature (P > 0.05), while temperature exhibited the greatest effect (P < 0.001). Meanwhile, N mineralization processes were simulated using both an effective accumulated temperature model and a one-pool model. Both models fit well with the simulation of soil N mineralization process in the coastal freshwater wetlands under a range of 30 to 40 degrees C (R-2 = 0.88-0.99, P < 0.01). Our results indicated that an enhanced NH4+-N release with increasing temperature and salinity deriving from the projected global warming could have profound effects on nutrient cycling in coastal wetland ecosystems.