Methane Emissions Are Highly Variable Across Wetland Habitats in Natural and Restored Tidal Freshwater Wetlands

It is increasingly clear that hydrology, vegetation, and soil characteristics are important in controlling methane (CH4) emissions from wetlands. However, few studies have examined CH4 emissions between wetland habitats dominated by different plants, or between naturally occurring and restorred wetlands, which are hoped to offer a means of sequestering carbon (C) and offsetting greenhouse gas emission. Our goal was to assess the variation of CH4 fluxes between different habitats in natural and restored tidal freshwater wetlands. One natural site with three habitats (low marsh, high marsh, and swamp habitat) and restored site with two habitats (low and high marsh) were selected. Both sites are tidal freshwater wetlands (measured salinity <0.3 Practical Salinity Units (PSU)) located on the Patuxent River, Maryland, USA. Gas flux was quantified using static chambers once a month during day and night from May to August 2016 during a single growing season. At 12.5 and 40 cm soil depth, soil redox potential, temperature, porewater CH4, and total iron (Fe) concentrations were measured on the same days as flux. Restored habitats had significantly higher soil redox potential (i.e., less reducing conditions) than their natural counterparts (P < 0.05). Methane flux had high spatiotemporal variations across natural and restored wetland habitats (ranged from −87,901 mg CH4 m−2 day−1 in restored high marsh during June to 6860 mg CH4 m−2 day−1 in natural swamp habitat during May). Moreover, natural wetland habitats, on average, were significantly warmer (P < 0.05) than their restored counterparts at both soil depths (12.5 and 40 cm), where porewater CH4 concentrations had positive and significant correlation with wetland soil temperatures (r = 0.36 and P < 0.001 – the warmer the soil, the higher the porewater CH4 available). For a more precise global C budget, soil temperature, spatiotemporal CH4 variations between wetland types, and tidal effects should be considered in accounting for CH4 fluxes in wetlands in general and tidal freshwater wetlands specifically.