Deep-injected straw incorporation improves subsoil fertility and crop productivity in a wheat-maize rotation system in the North China Plain

Deep-injected straw incorporation (DI-SI) is a novel soil tillage and straw management, which allowed for the formation of maize straw layer structure in the subsoil (20-38 cm) in wheat-maize rotation systems. However, little is known about how DI-SI affects soil physicochemical and enzymatic processes, as well as crop productivity. Therefore, we conducted a field experiment during 2018-2020 in the North China Plain to determine whether DI-SI would improve soil structure, nutrients, enzyme activities, and crop yield. Three treatments were included: straw removal (SR), conventional straw incorporation (SI), and DI-SI. The results showed that DI-SI improved soil structure and water retention through decreasing the bulk density and penetration resistance in 20-40 cm soil layer. DI-SI significantly increased soil organic C (67.5%), total N (61.3%), and available nutrients (i.e. NO3- , NH4+, dissolved organic C) compared with SI in the subsoil (20-40 cm), while no differences were detected among straw managements in the topsoil (0-20 cm). This was mainly attributed to the accumulation of microbial biomass C and C-, N-acquisition enzymes in the straw layers, which promoted the straw decomposition and nutrient release. Finally, with the combined effects of soil ecological processes, DI-SI significantly increased the grain yields of wheat and maize by 12.0% and 11.8% compared with SI in the second year, respectively. Our study suggested that deep placement of straw would better promote the utilization of crop residue for farming, and deep-injected straw incorporation represents a viable strategy to improve subsoil physical, chemical, and biological processes, and subsequently crop productivity in wheat-maize rotation systems.