Salinity and Moisture Influence CO2 and CH4 Emissions From High‐Latitude Coastal Soils

Abstract Sea level rise and more frequent and larger storms will increase saltwater flooding in coastal terrestrial ecosystems, altering soil‐atmosphere CO 2 and CH 4 exchange. Understanding these impacts is particularly relevant in high‐latitude coastal soils that hold large carbon stocks but where the interaction of salinity and moisture on greenhouse gas flux remains unexplored. Here, we quantified the effects of salinity and moisture on CO 2 and CH 4 fluxes from low‐Arctic coastal soils from three landscape positions (two Wetlands and Upland Tundra) distinguished by elevation, flooding frequency, soil characteristics, and vegetation. We used a full factorial laboratory incubation experiment of three soil moisture levels (40%, 70%, or 100% saturation) and four salinity levels (freshwater, 3, 6, or 12 ppt). Salinity and soil moisture were important controls on CO 2 and CH 4 emissions across all landscape positions. In saturated soil, CO 2 emissions increased with salinity in the lower elevation landscape positions but not in the Upland Tundra soil. Saturated soil was necessary for large CH 4 emissions. CH 4 emissions were greatest with low salinity, or after 11 weeks of incubation when SO 4 2− was exhausted allowing for methanogenesis as the dominant mechanism of anaerobic respiration. In partially saturated soil, greater salinity suppressed CO 2 production in all soils. CH 4 fluxes were overall quite low, but increased between 3 and 6 ppt in the Tundra. In the future, a small increase in floodwater salinity may increase CO 2 production while suppressing CH 4 production; however, where water is impounded, CH 4 production could become large, particularly in the landscapes most likely to flood.