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

Abstract

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 (0–115cm) 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 0–15cm horizon; while CH4 and N2O consumption originated from the 0–40 and 0–15cm soil horizons, respectively. Specifically, we revealed that the soil surface respiration was contributed by the 0–5 and 5–15cm horizons, accounting for 70.9 and 27.3% of the surface exchange, respectively. The CH4 consumption at 0–5, 5–15 and 15–40cm depths accounted for 54.1, 32.3 and 12.1% of the surface exchange, respectively. And the N2O consumption at 0–5 and 5–15cm depths accounted for 80.4 and 6.6% of the surface exchange, respectively. The subsoil below 15cm acted largely as a CO2 buffer; the production/consumption potentials of CH4 and N2O were very weak below 40 and 15cm depths, respectively. In conclusion, our results highlight that the topsoil (0–40cm) 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.