Crop straw incorporation increases the soil carbon stock by improving the soil aggregate structure without stimulating soil heterotrophic respiration

Crop straw incorporation is widely recommended to maintain crop yields and improve soil organic carbon (SOC) stocks as well as soil quality. However, the long-term effects of different straw incorporation practices on the SOC stock remain uncertain. In this study, a long-term experiment (2007 to 2018) with four treatments (MW₀: maize–wheat rotation with no straw incorporation, MW₅₀: maize–wheat rotation with 50% chopped straw incorporation, MW_b50: maize–wheat rotation with 50% in situ burned harvested straw, and MF₅₀: maize–fallow rotation with 50% harvested maize straw incorporation) was set up to evaluate the response of the SOC stock to different straw incorporation methods. The results showed that the SOC stock significantly increased by 32.4, 12.2 and 17.4% under the MW₅₀, MW_b50 and MF₅₀ treatments, respectively, after continuous straw incorporation over a decade, while the SOC stock under MW₀ was significantly reduced by 22.9% after the 11 year long-term experiment. Compared to MW₀, straw incorporation significantly increased organic carbon input, and improved the soil aggregate structure and the ratio of dissolved organic carbon (DOC) to particulate organic carbon (POC), but it did not significantly stimulate soil heterotrophic respiration, resulting in the increased SOC accumulation rate and SOC stocks of bulk soil. The increased ratio of DOC to microbial biomass carbon (MBC) enhanced the relative abundances of Acidobacteria and Proteobacteria but inhibited Bacteroidetes and Chloroflexi, and the bacterial relative abundances were the main reasons for the non-significant increase or even decrease in soil heterotrophic respiration with straw incorporation. The SOC stock would reach an equilibrium based on the results of Rothamsted carbon (RothC) model simulations, with a long-term equilibrium value of 18.85 Mg ha^–1 under MW₅₀. Overall, the results of the long-term field experiment (2007–2018) and RothC model simulation suggested that maize–wheat rotation with 50% chopped straw incorporation delivered the largest benefits for the SOC stock in calcareous soils of subtropical mountain landscapes over the long term.