Rhizosphere activity and atmospheric methane concentrations drive variations of methane fluxes in a temperate forest soil

Aerated soils represent an important sink for atmospheric methane (CH⁠4), due to the effect of methanotrophic bacteria, thus mitigating current atmospheric CH⁠4 increases. Whilst rates of CH⁠4 oxidation have been linked to types of vegetation cover, there has been no systematic investigation of the interaction between plants and soil in relation to the strength of the soil CH⁠4 sink. We used quasi-continuous automated chamber measurements of soil CH⁠4 and CO⁠2 flux from soil collar treatments that selectively include root and ectomycorrhizal (ECM) mycelium to investigate the role of rhizosphere activity as well as the effects of other environmental drivers on CH⁠4 uptake in a temperate coniferous forest soil. We also assessed the potential impact of measurement bias from sporadic chamber measurements in altering estimates of soil CO⁠2 efflux and CH⁠4 uptake. Results show a clear effect of the presence of live roots and ECM mycelium on soil CO⁠2 efflux and CH⁠4 uptake. The presence of ECM hyphae alone (without plant roots) showed intermediate fluxes of both CO⁠2 and CH⁠4 relative to soils that either contained roots and ECM mycelium, or soil lacking root- and ECM mycelium. Regression analysis confirmed a significant influence of soil moisture as well as temperature on flux dynamics of both CH⁠4 and CO⁠2 flux. We further found a surprising increase in soil CH⁠4 uptake during the night, and discuss diurnal fluctuations in atmospheric CH⁠4 (with higher concentrations during stable atmospheric conditions at night) as a potential driver of CH⁠4 oxidation rates. Using the high temporal resolution of our data set, we show that low-frequency sampling results in systematic bias of up-scaled flux estimates, resulting in under-estimates of up to 20% at our study site, due to fluctuations in flux dynamics on diurnal as well as longer time scales.