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

Sea level rise and more frequent and larger storms will increase saltwater flooding in coastal terrestrial ecosystems, altering soil-atmosphere CO₂ and CH₄ 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 CO₂ and CH₄ 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 CO₂ and CH₄ emissions across all landscape positions. In saturated soil, CO₂ emissions increased with salinity in the lower elevation landscape positions but not in the Upland Tundra soil. Saturated soil was necessary for large CH₄ emissions. CH₄ emissions were greatest with low salinity, or after 11 weeks of incubation when SO₄²⁻ was exhausted allowing for methanogenesis as the dominant mechanism of anaerobic respiration. In partially saturated soil, greater salinity suppressed CO₂ production in all soils. CH₄ 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 CO₂ production while suppressing CH₄ production; however, where water is impounded, CH₄ production could become large, particularly in the landscapes most likely to flood.