Soil profile greenhouse gas concentrations and fluxes from a semiarid grassland and a cropland site in an agro-pastoral ectone of northern China

Despite broad information on greenhouse gas (GHG) emissions from the soil surface, only few studies have examined the depth related GHG concentration and production within the soil. We coupled chamber surface GHG flux measurements with analysis of subsurface GHG concentrations at five depths (0, 10, 30, 50 and 70 cm) using silicon tubes in a grassland and cropland soils in a semi-arid agro-pastoral ecotone over a full year. Grassland and cropland soils behaved as net CO 2 and N 2 O sources, but sinks of CH 4. CO 2 and N 2 O concentrations increased but CH 4 decreased with soil depth. Subsurface GHG fluxes calculated using Fick’s law decreased with depth. The modeled GHG and measured surface fluxes agreed much better for CO 2 than N 2 O or CH 4. Spring-thaw and rainfall events triggered N 2 O pulses, CH 4 uptake peaks and CH 4 concentration drops. Annual emissions or uptake based on chamber method were 4820–7580 kg C ha −1 yr −1, 2.4–1.5 kg C ha −1 yr −1, and 0.13–0.14 kg N ha −1 yr −1 for CO 2, CH 4, and N 2 O across the grassland and cropland, respectively. Modeled annual accumulation of GHG fluxes dropped with depth, whereas the topsoil (0–20 cm) contributed about 65% for CO 2, 68% for N 2 O and 72% for CH 4 of the total profile. The vertical distribution of GHG fluxes reflected those of soil organic carbon and root biomass. The grassland had higher annual CO 2 flux but lower CH 4 uptake than the cropland. Annual N 2 O fluxes were similar between the two ecosystems.