Salt effects on carbon mineralization in southeastern coastal wetland soils of the United States

Abstract Widespread saltwater intrusion into freshwater coastal ecosystems could dramatically alter the fate of stored carbon as well as the rate of future soil carbon (C) sequestration. We carried out a large survey of soil C at 51 freshwater wetland sites across the coastal plain of North Carolina. We then conducted a microcosm experiment with soils from a subset of eight sites that spanned the full range of observed soil solution salinity, ranging from oligohaline (ca. 0.09–2.84 ppt) to brackish (ca. 5.8–15 ppt) conditions. Each soil in the microcosm experiment was exposed to four salinity treatments, i.e., deionized water (DIW), artificial sea water with sulfate (ASW), artificial sea water without SO 4 2− (ASW − SO 4 ), and deionized water with SO 4 2− (DIW + SO 4 ) under both oxic and anoxic conditions. Here we focused on whether background salinity affected C use efficiency (or the rate of C mineralization divided by the mass of soil C). Across all 51 surveyed sites, neither soil organic C content nor microbial biomass was correlated with soil salinity. Soil organic C content was however the primary predictor of microbial biomass and rates of C mineralization, and soil chloride concentrations were positively correlated with soil organic C for the eight selected sites. We found that C use efficiency was higher in oligohaline soils, suggesting that organic C compounds in the oligohaline wetland soils may contain more labile fractions. In the microcosm experiment, the addition of marine salts, with or without SO 4 2− ions, inhibited soil C use efficiency. Sulfate added alone had little effect on C use efficiency, but substantially inhibited CH 4 production efficiency in anoxic assays for the sites with low soil Cl − concentration. Sulfate additions had no effect on CH 4 production for soils from sites with prior salinity exposure. Ionic stress exerted larger inhibitory effect on both aerobic and anaerobic C mineralization than the SO 4 2− effect in the short-term, regardless of the prior exposure to marine salts or not, while SO 4 2− exerted far greater control on the CH 4 potential only in sites that have not experienced saltwater intrusion before. Overall our findings suggest that long-term effects of salinity on shifts in C mineralization may differ from short-term experimental responses, likely due to other factors (e.g., microbial tolerance, hydrology, temperature) under field conditions.