Variability and controls of soil CO₂ fluxes under different tillage and crop residue managements in a wheat-maize...

The spatial and temporal variability of soil CO₂ emissions from agricultural soils is inherently high. While tillage and crop residue practices play vital roles in governing soil CO₂ emission, their effects on the variability of soil CO₂ fluxes across depths and seasons are still poorly understood. To address this, an experiment consisting of four treatments, namely conventional tillage with (CT+) and without crop residue application (CT-), as well as no tillage with (NT+) and without crop residue application (NT-), was conducted to investigate soil CO₂ fluxes at top 40 cm soils with 10-cm depth intervals in a winter wheat-summer maize rotation system in the North China Plain. Our results showed soil CO₂ fluxes increased with depth in both the wheat- and maize-growing seasons. However, the dominant factors in regulating soil CO₂ fluxes changed with soil depth and seasons. In the wheat-growing season, increase in soil CO₂ fluxes with depth was attributed to the increase of dissolved organic carbon-to-nitrogen ratio (DOC/DON) and a decline in soil DON concentration along the soil profile. These factors explained about 55-96% of the total variation in soil CO₂ fluxes at different soil depths. In the maize-growing season, the dominant factors were soil DOC/DON ratio, soil DON concentrations, and soil moisture. These factors explained approximately 79-96% of the total variation in soil CO₂ fluxes along the soil depth. Greater soil CO₂ fluxes (except at 30-40 cm depth) were observed in NT- than CT- treatments. Furthermore, crop residue application enhanced soil CO₂ fluxes across different depths, but the enhancement was more prominent in CT+ than NT+. Moreover, soil CO₂ fluxes in the maize-growing season were greater than those in the wheat-growing season. Our results demonstrate that the effects of tillage regimes and crop residue management practices on soil CO₂ emissions are not confined only to the plough layer but can extend to soils of over 30 cm depths. We also need to revisit the general conventional view that no tillage can significantly reduce soil CO₂ emissions compared with conventional tillage for better climate change mitigation.