Long-term subsoiling and straw return increase soil organic carbon fractions and crop yield

Conservation tillage has been adopted worldwide as an attractive alternative to conventional tillage. However, suitable conservation tillage for increasing soil organic carbon (SOC) and crop yield simultaneously is still limited. Two conservation tillage methods, subsoiling to the 40 cm depth (ST) and no-tillage (NT), were combined with three straw return treatments (i.e., no return [−0], return of whole wheat (Triticum aestivum L.) straw and 1 m high maize (Zea mays L.) stubble [−1], and return of whole wheat and maize straw [−a]) to study their impacts on SOC content, labile C fractions, and crop yields, with conventional tillage (CT) used as a control in a 15-year field experiment. Subsoiling with 1 m high maize stubble return (ST-1) increased the mean annual grain yields by 18% and the mean SOC content by 39% at the 0 to 100 cm depth compared with conventional tillage with no maize straw return (CT-0) in 2016 and 2017. The mean SOC at the 0 to 100 cm depth of the NT treatment was lower than those of ST and CT because of the reduced transformation from straw to SOC and labile C fractions. One meter high maize stubble return can maintain high SOC content, C fractions, and crop yield compared with whole maize straw return. Thus, subsoiling combined with 1 m high maize stubble return was an effective conservation tillage to increase the SOC content and crop yield.