Tillage Management Alters Carbon Sink Capacity in Arid Phaeozems: Insights from a Carbon Balance Perspective

To comprehensively explore the net carbon balance within cropland systems subject to diverse tillage practices (Down-slope cultivation (CK), Subsoiling tillage (SF), Ridge to district field (RF), Ridge to district field + subsoiling tillage (RF-S), Transverse slope planting (TP), Transverse slope planting + ridge to district field (TP-R), Transverse slope planting + subsoiling tillage (TP-S)), a series of well-designed field experiments were meticulously carried out. The CO2 emission intensity of soil heterotrophic respiration, CH4 emission intensity, carbon loss in runoff, carbon emissions from farmland materials, dry matter mass and carbon content of different crop organs after harvest were measured for the six different tillage practices. Moreover, the annual and seasonal variations in farmland soil carbon pools under different treatments were analyzed using the net carbon flux (NCF) of the cropland system. The results indicated that, under different tillage practices, the CO2 emission intensity of soil heterotrophic respiration in each regime across different years generally exhibited a pattern of increasing initially and then decreasing, reaching its peak during the filling stage (pod-setting stage). The RF regime significantly reduced the CO2 emissions from soil heterotrophic respiration (p < 0.05). The CH4 emissions in each regime across different years also demonstrated an overall tendency of rising initially and subsequently declining, with an alternating positive–negative pattern, reaching its peak during the jointing stage (branching stage). The SF regime significantly decreased the CH4 emissions (p < 0.05). The regimes with cross-slope tillage significantly reduced the carbon loss in runoff (p < 0.05). Throughout every year, the NPP of crops under the TP-S regime attained its peak value (p < 0.05). The RF regime effectively increased the NPP of crops, reduced the soil heterotrophic respiration CO2 emissions and the carbon loss in runoff, and its NCF value reached the maximum level (p < 0.05), presenting a weak carbon “source”. Overall, ridged-field (RF) effectively curbs greenhouse gas emissions, boosts farmland carbon sequestration, and mitigates soil fertility decline.