Interactions between land-use history and earthworms control gross rates of soil methane production in an overwintering pasture

The effects of earthworms on soil CH4 emissions remain ambiguous, as previous studies revealed both positive and negative effects on net rates of CH4 production. These differences may be linked to land-use history such as pasturing intensity, as the treading of livestock and the input of faeces will affect physical and chemical soil characteristics that are important to methanogenic and methanotrophic bacteria. In order to concomitantly measure the effects of earthworms on the activities of both of these bacterial groups, we performed an isotope dilution assay to measure gross CH4 production and consumption rates. Samples from three soil types, differing mainly in pasture intensity over the past 15 years, were incubated for 8 weeks in the presence or absence of the vermicomposting species Eisenia andrei. Following the incubation, earthworms had a positive effect on gross process rates in the non-pasture control soil, had no effect in the moderate pasture soil, and had a negative effect in the intensive pasture soil. Phospholipid fatty-acid profiles and chemical characteristics of the fresh soil samples suggested that the non-pasture soil would likely produce less CH4 than the control soil. Likewise, methanogenic specific mcrA gene abundance following the incubation was two orders of magnitude higher in the two pasture soils than in the control soil. The isotope dilution assay revealed, however, higher gross CH4 production rates in the control soil. It was also found that gross CH4 transformation rates were neither correlated to net production rates nor to mcrA gene abundance. We discuss the value and limitations of the isotope dilution assay for understanding the factors and underlying mechanisms governing net CH4 emissions rates from soils.