Moisture-related opposing N2O and CH4 gas exchange in mineral upland tundra soils of West Greenland: Linking gas fluxes to soil horizons

Arctic tundra soils are sources or sinks of the greenhouse gases (GHGs) nitrous oxide (N 2 O), methane (CH 4 ) and carbon dioxide (CO 2 ), but the simultaneous regulation of these GHGs by soil types remains unclear. We measured GHG fluxes across mineral soil types differing in organic-layer thickness (OLT) and moisture in a West Greenlandic upland tundra. Drier soils functioned primarily as N 2 O sources and CH 4 sinks, with both functions weakening as moisture increased; consequently, wet soils emitted CH 4 and took up N 2 O. Soil classification based on topsoil horizons served as an indicator of net CH 4 exchange magnitude and direction but failed to predict N 2 O and CO 2 fluxes. To identify soil-horizon controls on net GHG fluxes, we conducted aerobic in-situ measurements using undisturbed soil samples. In dry upland tundra soils, the highest GHG production and consumption rates occurred in organic (O) horizons, whereas mineral horizons exhibited significantly lower rates. Upper O horizons produced most of the CO 2 and N 2 O, while CH 4 consumption was highest in lower O horizons. In wetland soils, all horizons produced CH 4 , and N 2 O was produced in topsoil and consumed in subsoil horizons. In all soils, CO 2 and N 2 O production correlated positively with carbon and nitrogen concentrations and with OLT, whereas in O-horizon soils, CH 4 consumption decreased with OLT but increased with nitrogen availability. Lower moisture (20–40 % volumetric water content, VWC) correlated with higher CH 4 consumption and higher N 2 O and CO 2 production. Our study underscores the importance of soil types and horizons for upscaling GHG fluxes to landscapes.