Is adding biochar be better than crop straw for improving soil aggregates stability and organic carbon contents in film mulched fields in semiarid regions? -Evidence of 5-year field experiment

Plastic film mulching is used widely to increase crop yields in semiarid areas, but improving the soil fertility in film mulched fields is also important for achieving sustainable high yields in northwest of China. In this study, a completely randomized two-factor field design experiment was conducted in Pengyang, Ningxia, China during 2017-2021. In order to investigate the effects of plastic film mulching with straw/biochar addition on the soil aggregate characteristics, organic carbon content, and maize yield. Six treatments were established as follows: control (C), straw (S), biochar (B), plastic film mulching (F), plastic film mulching with added straw (FS) or biochar (FB). After 5 years of continuous production, each straw and biochar addition treatments significantly improved the soil aggregate distribution and stability, and the average aggregate content >0.25 mm increased significantly by 47.32%. Compared with the treatments without plastic film mulching, the mean weight diameter and geometric mean diameter of the soil particles increased by 9.19% and 4.15%, respectively, under the plastic film mulching treatments. The organic carbon content of the 0-60 cm soil layer increased significantly under each straw and biochar addition treatment compared with the without straw. The aggregate organic carbon contents under each treatment increased as the aggregate particle size increased, where the straw and biochar addition treatments significantly increased the organic carbon content of the aggregates, whereas the contents decreased under the plastic film mulching treatments. The contributions of the soil aggregates >0.25 mm to the organic carbon contents of the 0-60 cm soil layer were significantly higher under FS (37.63%) and FB (56.45%) than F. Structural equation modeling showed that straw/biochar added, plastic film mulching, and a greater soil organic carbon content could significantly promote yield increases, where the straw and biochar addition treatments significantly increased the average maize by 14.6% on average. In conclusion, carbon input as straw, especially biochar, had a positive effect on improving the soil organic carbon content and maize yield under plastic film mulching farmland in a semiarid region.