Short-Term Response of Carbon Cycling to Salinity Pulses in a Freshwater Wetland

Sea level rise increases the frequency and intensity of storm surges and extreme tidal events in coastal freshwater wetlands. Seawater affects soil biogeochemical processes by inducing osmotic stress and stimulating SO42− reduction. The objective of this study was to determine the mechanism by which salinity alters C mineralization rates by quantifying the relative importance of ionic stress, compared with the addition of the SO42− electron acceptor, on the production of CO2 and CH4. A batch incubation study measured potential anaerobic respiration and methanogenesis with time in a freshwater wetland soil exposed to varying concentrations (3.5, 14, and 35 g kg−1) of seawater or salt (NaCl) solutions. Seawater addition induced a short-term (2-wk) stimulation of CO2 production (20–32% greater than the freshwater control) and a continuous suppression of CH4 production (up to 94% less than freshwater). Ionic stress (represented by NaCl) did not reduce CO2 production at all but did decrease CH4 production for 2 wk in both the 14 and 35 g kg−1 NaCl treatments. Our results indicate that microbial populations rebound quickly from ionic stress. The intrusion of dilute seawater (3.5 g kg−1) to freshwater wetlands can accelerate organic C mineralization through the short-term increase in SO42−–induced respiration without inhibiting methanogenesis. Overall, the organic C mineralization rate was 17% higher for 3.5 g kg−1 seawater than the freshwater control. The temporary nature of the microbial response suggests that “pulses” of seawater may have a greater influence on the rate of C cycling in freshwater wetlands than a gradual sea level rise.