Soil properties and not high CO2 affect CH4 production and uptake in periodically waterlogged arable soils

Purpose Several studies related to CH4 cycling focus on the effect of elevated atmospheric CO2 levels on soil methanogenesis and methanotrophy. However, periodically waterlogged soils are characterized by much higher CO2 concentrations, while aggregates forming in arable soils due to traditional cultivation may be a natural CH4 source for low-affinity methanotrophs. We present a comprehensive laboratory investigation into both methanogenesis and methanotrophy in three different soils under ambient and high CO2 concentrations. Materials and methods The tested materials were agricultural soils widely distributed in Europe: Eutric Cambisol, Haplic Podzol and Mollic Gleysol with determined properties: C-org, N-tot, clay, silt, sand, pH, NO3-, NH4+. We conducted two independent experiments on methanogenesis (flooded conditions) and methanotrophy (non-flooded conditions). All samples for both studies were divided into three sets which differed in the initial CO2 concentration in headspace: ambient CO2 (0.03% v/v +/- 0.005), and two high CO2 levels (5% +/- 0.4 and 10% +/- 0.5 v/v CO2). Moreover, 1% (v/v) of CH4 was added to all sets prepared for the methanotrophy test since low-affinity methanotrophy in the tested soils had been reported previously. Gas concentrations (CH4, CO2, O-2) in headspace were measured using gas chromatography method. Results and discussion We observed that the examined soils differed in their ability to produce CH4 as follows: Haplic Podzol = Eutric Cambisol > Mollic Gleysol, as well as to consume CH4: Mollic Gleysol > Haplic Podzol > Eutric Cambisol. The CH4 emissions started faster in Gleysol, but the final CH4 concentration and CH4 production rate in this soil were significantly lower than in the two other soils. The CH4 uptake rate significantly differed among the tested soils. The MANOVA confirmed the significance of the soil-type factor in determining both process rates, whereas the effect of CO2 level and the interaction between both factors were not significant. The amount of consumed O-2 during CH4 oxidation was also characteristic for examined soil, but did not differ significantly between CO2 variants. Conclusions CH4 production and consumption in three different soils (Eutric Cambisol, Haplic Podzol and Mollic Gleysol) collected from periodically waterlogged arable fields were more strongly affected by soil properties than by high CO2 concentration. In contrast, the high CO2 level significantly decreased CO2 production accompanying tested the CH4-related processes.