Irrigation-induced hydrothermal variation affects greenhouse gas emissions and crop production

Production and consumption of greenhouse gases (GHGs) from agricultural soil largely depend on soil temperature and moisture. Previous studies regarding the relationships between soil temperature and moisture and GHG emissions mainly considered the upper soil depth and lacked information on the deeper soil depth. Here, we for the first time explored the relationships between hydrothermal conditions at the soil surface to a depth of 100 cm and GHG emissions in a typical winter wheat−summer maize cropping system, and then evaluated yield, crop water productivity (WP), global warming potential (GWP), and GHG intensity (GHGI) under four irrigation schedules (W1, pre-sowing irrigation; W2, pre-sowing + jointing irrigation; W3, pre-sowing + anthesis irrigation; W4, pre-sowing + jointing + anthesis irrigation). Regardless of the irrigation schedule, the soil acted as a source of GHGs for the wheat−maize system. Small differences in irrigation-related soil temperature were detected, while volumetric soil water content (VSWC) in the different soil depths varied greatly. Soil temperature and VSWC in the 0–100 cm and 0–50 cm soil depths of the wheat−maize system were positively correlated with CO2 and N2O fluxes. Soil temperature in the 0–70 cm soil depth positively affected CH4 flux, whereas positive and negative correlations were observed between VSWC in the 0–40 cm and 50–90 cm soil depths and CH4 flux, respectively. An improper irrigation period under the W3 treatment lowered crop yield, and excessive irrigation under the W4 treatment caused increased water consumption and GHG emissions. The W2 treatment improved yield by 5.6–6.2% and 2.2–3.1%, increased WP by 5.0–8.1% and 12.8–13.1%, decreased GWP by 2.8–3.1% and 6.0–6.6%, and reduced GHGI by 8.6–9.4% and 9.0–9.2% compared to the W3 and W4 treatments, respectively. This study highlights the importance of the effect of the hydrothermal conditions in different soil depths on GHG emissions and an adequate irrigation schedule for improving yield, saving water, and mitigating GHGs.