Treeline soil warming does not affect soil methane fluxes and the spatial micro-distribution of methanotrophic bacteria

The impact of a warmer climate on CH₄ fluxes from soils is highly uncertain, because soil warming may affect methanotrophic bacteria in two opposed ways: CH₄ assimilation in soils might be increased by the decreasing soil moisture often associated with soil warming. In contrast, CH₄ oxidation might be suppressed by higher NH4+ concentrations in warmed soils resulting from an accelerated nitrogen mineralization. We investigated effects of soil warming on soil-atmosphere CH₄ fluxes in the last two years of a six-year long field experiment at a Swiss alpine treeline. Specifically, we measured CH₄ fluxes using static chambers, and characterized N cycling by quantifying soil N₂O emissions and NH4+ and NO3- concentrations. We further labeled intact soil cores with C14H4 and traced the labeled bacteria using an auto-radiographic technique to study the potential warming-related changes in the micro-distribution of methanotrophic bacteria within the soils. Our results did not show a significant effect of soil warming on net CH₄ fluxes after five and six years of soil warming. In general, soils were a net sink for CH₄ but CH₄ emissions were observed occasionally. One reason for the unaltered CH₄ fluxes might be the negligible warming effects on soil water contents in the treeline environment with frequent rainfalls. In the warmed soils, soil moisture was lower in the litter layer, but not deeper in the soils. Therefore, soil warming did not affect gas transport rates into deeper soil layers where methanotrophic bacteria were located. Another reason might be the general absence of substantial warming effects on mineral N, with NH4+ concentrations being marginally significantly higher in warmed soils only in ion exchange resin bags (P<0.1) but not in soil extracts. Auto-radiographic image analysis of soil cores revealed an overall heterogeneous ¹⁴C distribution and a warming-induced shift of methanotrophic bacteria toward the soil surface. The absence of responses of CH₄ fluxes to warming in this alpine treeline ecosystem is likely related to the rather minimal changes in the putative drivers soil moisture and NH4+ concentration.