Depth-dependent greenhouse gas production and consumption in an upland cropping system in northern China

Vertical profiles of greenhouse gas (GHG) production and consumption within soils have not been carefully quantified. The objective of this study was to quantify the depth-dependent contributions of CO₂, CH₄ and N₂O fluxes in the soil profile to soil surface gas exchange. We simultaneously measured the soil surface GHG emissions and the subsurface fluxes (0–115 cm) in situ by using a static chamber-based method (CM) and a concentration gradient-based method (GM) respectively, over two-year period in a maize-based upland cropping system in northern China. We found that unfertilized maize-based farmland acted as CO₂ sources and CH₄ and N₂O sinks. Soil surface respiration was mostly contributed by the 0–15 cm horizon; while CH₄ and N₂O consumption originated from the 0–40 and 0–15 cm soil horizons, respectively. Specifically, we revealed that the soil surface respiration was contributed by the 0–5 and 5–15 cm horizons, accounting for 70.9 and 27.3% of the surface exchange, respectively. The CH₄ consumption at 0–5, 5–15 and 15–40 cm depths accounted for 54.1, 32.3 and 12.1% of the surface exchange, respectively. And the N₂O consumption at 0–5 and 5–15 cm depths accounted for 80.4 and 6.6% of the surface exchange, respectively. The subsoil below 15 cm acted largely as a CO₂ buffer; the production/consumption potentials of CH₄ and N₂O were very weak below 40 and 15 cm depths, respectively. In conclusion, our results highlight that the topsoil (0–40 cm) plays a critical role in CO₂ production and CH₄ and N₂O consumption in an unfertilized maize-based farmland in Taihang mountain areas of northern China. However, the mechanisms responsible for changes in stored greenhouse gas within soil pore space are not clear, and further observational and experimental research is required to understand those processes.