Drivers of atmospheric methane uptake by montane forest soils in the southern Peruvian Andes

The soils of tropical montane forests can act as sources or sinks of atmospheric methane (CH4). Understanding this activity is important in regional atmospheric CH4 budgets, given that these ecosystems account for substantial portions of the landscape in mountainous areas like the Andes. Here we investigate the drivers of CH4 fluxes from premontane, lower and upper montane forests, experiencing a seasonal climate, in southeastern Peru. Between February 2011 and June 2013, these soils all functioned as net sinks for atmospheric CH4. Mean (standard error) net CH4 fluxes for the dry and wet season were −1.6 (0.1) and −1.1 (0.1) mg CH4 – C m−2 d−1 in the upper montane forest; −1.1 (0.1) and −1.0 (0.1) mg CH4 – C m−2 d−1 in the lower montane forest; and −0.2 (0.1) and −0.1 (0.1) mg CH4 – C m−2 d−1 in the premontane forest. Variations among forest types were best explained by available nitrate and water-filled pore space, indicating that nitrate inhibition of oxidation or diffusional constraints imposed by changes in water-filled pore space on methanotrophic communities represent important controls on soil-atmosphere CH4 exchange. Seasonality in CH4 exchange varied among forests with an increase in wet season net CH4 flux only apparent in the upper montane forest. Net CH4 flux was inversely related to elevation; a pattern that differs to that observed in Ecuador, the only other extant study site of soil-atmosphere CH4 exchange in the tropical Andes. This may result from differences in rainfall patterns between the regions, suggesting that attention should be paid to the role of rainfall and soil moisture dynamics in modulating CH4 uptake by the organic-rich soils typical of high elevation tropical forests.