Stimulation of methane consumption by endogenous CH4 production in aerobic grassland soil

The net CH₄ flux in terrestrial ecosystems is mostly the result of simultaneous gross CH₄ production (P₄CH) and gross CH₄ consumption (C₄CH) rates. However, it is not well-understood how endogenous P₄CH may influence C₄CH and hence the net CH₄ flux outcome in upland soils. Gross CH₄ consumption may be either stimulated or outweighed by P₄CH, resulting in higher or lower net CH₄ uptake rates, respectively. To investigate gross CH₄ fluxes, we incubated intact soil cores taken from a permanent grassland with and without difluoromethane (DFM; blocks CH₄ oxidation). The gross CH₄ consumption was calculated by taking the DFM-measured rate of P₄CH into account. Most of the oxic cores exhibited a more than 30 times higher C₄CH than P₄CH rate. However, four cores showed a considerably higher gross CH₄ production, accompanied by a stimulated gross CH₄ uptake which was entirely masked in the net CH₄ flux (C₄CH/P₄CH ratios of 0.9-2.3). In three of the four cores macro-fauna (earthworms and Scarab beetle larvae) were present. Removing the animals and homogenizing the intact core structure lowered P₄CH by 27-100%. Subsequent incubation of Scarabidae larvae such as Pachnoda sp., Cetonia sp. and others revealed animal CH₄ and N₂O production rates within the range of those observed in the inhabited cores. Moreover, in-situ soil air CH₄ concentrations obtained from four depth profiles (0-50 cm depth) at the grassland site also indicated that temporarily, small-scale local hot spots of CH₄ and N₂O production existed within otherwise aerobic soil profiles. These spots indicated a stimulated CH₄ uptake after the CH₄ source (of whatever nature) had ceased. The results suggest that soil macro-fauna, anaerobic soil microsites or both combined may provide suitable conditions for CH₄ production in otherwise oxic soil environments. Hence, endogenous CH₄ production in upland soil may partially be related to the density and species composition of soil invertebrates, with a potential impact on the CH₄ sink capacity of these soils.