Methanogenesis in saltmarsh soils of the North Sea coast of Germany

Temperate saltmarshes are a potential source of atmospheric methane. We have measured the concentration and emission of methane in typical saltmarsh soils (Salic Fluvisols) and humus-rich saltmarsh soils (Thionic Fluvisols) from the German North Sea coast. We also measured the methane production rates of the latter. The methane content of typical saltmarsh soils reached 12.0 mu mol l(-1), although values of 1-4 mu mol l(-1) were usual. The sulphate concentrations of the pore-water were about 10 mM, which means sulphate reduction is not limited and methanogenesis would be suppressed. Methane concentrations were generally largest in summer. Independent of the redox potential and the degree of soil development, methane concentrations were smallest in those soils poorest in humus. Methane emission rates were almost zero. In the humus-rich saltmarsh soils, methane concentrations were roughly a thousand times larger than those in typical saltmarsh soils, reaching values of 23 mmol l(-1). The sulphate concentrations of the pore-water were often less than 1 mM, indicating limited sulphate reduction. Methane production was up to 80 mu g cm(-3) day(-1) and was not inhibited when we added sulphate. Methane emission rates reached up to 190 mu g m(-2) day(-1) in summer, with values up to 20 mu g m(-2) day(-1) at other times. The two kinds of saltmarsh soil behave quite differently: the typical saltmarsh soils act as a sink for methane; the humus-rich saltmarsh soils are a source.