METHANE AND REDUCED SULFUR GAS-PRODUCTION BY FRESH AND DRIED WETLAND SOILS

Wetland soil microorganisms are probably influenced by fluctuating water levels and by longer-term climatic change. This study investigated production of CH4 and reduced S gases [H2S, CH3SH, (CH3)(2)S, COS, and CS2] by both freshly collected and air-dried, rewet soils from four wetlands in the Mississippi deltaic plain. Soils from three depth intervals (0-10, 10-20, and 20-30 cm) from a swamp forest (Typic Hydraquent), a freshwater marsh (Terric Medisaprist), an intermediate salinity marsh (Typic Medisaprist), and a salt marsh (Typic Medisaprist) were incubated under anaerobic conditions. At repeated time intervals, the headspace gas was analyzed and then purged with N-2. Peak production rates of both CH4 and H2S occurred earlier in the fresh soils than in the dried soils in almost all cases. Presumably, this time lag associated with drying was caused by an increase in potentially reducible Mn and Fe. Nevertheless, drying increased labile C levels, thus total CH4 production was similar during the entire 69-d incubation. Production of CH4 was positively correlated with organic matter content (fresh, R(2) = 0.52; dried, R(2) = 0.64) and labile C (fresh, R(2) = 0.83; dried, R(2) = 0.89). Due to their relatively low concentrations, inclusion of soil oxidants in a model did not significantly improve the fit of a model that already contained a term for labile C. Production of S gases was positively correlated with initial SO42- concentrations (fresh, R(2) = 0.,75; dried, R(2) = 0.42). Drying significantly decreased total H2S production only for the salt marsh soils. In contrast, the drying treatment increased CH3SH production; only these soils produced (CH3)(2)S, COS, and CS2.