Soil organic carbon increment sources and crop yields under long-term conservation tillage practices in wheat-maize systems

Long-term tillage and straw incorporation significantly affect soil organic carbon (SOC) sequestration and crop yield. However, the studies on the SOC sources under multicropping system are relatively few. The objective of this study was to evaluate the effects of conservation tillage on SOC and crop yields and distinguish the SOC sources from wheat (C3) and maize (C4). Therefore, the dynamics of SOC, SOC sequestration, and crop yield were evaluated during 15 years of conservation agriculture under conventional tillage (CT), subsoiling (ST), rotary tillage (RT), and zero tillage (ZT) without or with straw incorporation (CTS, STS, RTS, and ZTS, respectively). The results indicated that the highest mean SOC concentration in the 0- to 30-cm soil was found under STS (11.80 g kg(-1)), which increased by 2.29 g kg(-1) than that under CT, whereas RT had the lowest mean SOC concentration (8.10 g kg(-1)). The increases in annual yield ranged from 0.58 (ZT) to 4.93 (ST) Mg ha(-1) during 2005-2017. In comparison with the annual yield of CT, that of STS increased by 2 Mg ha(-1) and was significantly higher than other treatments (p < .05) except ZTS and CTS. In comparison with CT, the SOC stock and carbon sequestration rate of STS were the highest and increased by 15.64 Mg ha(-1) and 1.05 Mg ha(-1) yr(-1), respectively, in the 0- to 30-cm soil. Moreover, the relative contribution of wheat residues to SOC was higher than maize residues under all treatments. Thus, subsoiling combined with C3 straw incorporation was more suitable for restoring degraded land and increasing yields.