Straw-induced soil pH decreased drives aluminum speciation shift and enhances soil organic carbon stability: A 12-year paddy experiment

Crop straw application is a critical strategy for enhancing soil organic carbon (SOC) content. However, the impacts of crop straw application on soil acidity and aluminum (Al) fractions have been less explored, and the consequences of these changes for SOC dynamics remain incompletely understood. In this study, a long-term field experiment (12 years) was conducted in a paddy field with continuous crop straw application. A crop straw application treatment [NPK with crop straw (NPK+S)], a conventional mineral fertilization control (NPK), and a non-fertilized control were included. The results show that compared with NPK treatment, the NPK+S treatment can reduce soil pH value by 0.27 units. This is because crop straw promotes the ammonia oxidation process and the increase of cumulative CO 2 emissions in paddy field soil. Among them, the number of amoA -AOA gene and cumulative CO 2 emissions of NPK+S treatment increased by 82.20% and 27.25% relative to the NPK treatment, respectively. Crop straw application significantly increased the exchangeable aluminum (Ex-Al) and organically bound aluminum (Or-Al) content while markedly decreasing the hydroxide aluminum (Hy-Al) content relative to the NPK treatment. Crop straw application also enhanced the stability of SOC. Specifically, compared with NPK treatment, the humification degree of dissolved organic matter (DOM) and the content of mineral-associated organic carbon (MAOC) in NPK+S treatment increased significantly by 14.80% and 44.29%, respectively, which is related to the transformation of soil Al fractions. Therefore, when utilizing crop straw in paddy paddies, a trade-off between soil acidity and organic carbon sequestration must be carefully considered. • The long-term crop straw addition decreased pH in paddy soil by promoted ammonia oxidation and CO 2 emission. • The long-term crop straw application enhanced the soil active Al content. • The long-term crop promoted the formation of MAOC by facilitating the interaction between DOM and Al 3+ and the accumulation of Or-Al. • The enhanced stability of SOC traded off the soil acidification under crop straw addition.