Excessive nitrogen (N) fertilization in rice systems has caused soil degradation and reduced N use efficiency. Green manure, especially Astragalus sinicus (Chinese milk vetch), provides a sustainable alternative, but the microbial and functional gene mechanisms underlying its interaction with reduced N input remain unclear. In this study, a field experiment was conducted at Dingdian Village, Natong Town, Long’an County, Nanning City, Guangxi Province, China (107°51′21″ E, 23°00′41″ N) during the 2018–2019 rice growing seasons. Four treatments were established: conventional N fertilization (N100), 20% N reduction (N80), green manure plus full N (GMN100), and green manure plus 20% N reduction (GMN80). Soil physicochemical traits, microbial community composition, and carbon-cycling functional genes were analyzed using high-throughput sequencing and metagenomic profiling. Compared with N100, GMN80 significantly increased soil organic matter (by 21.3%), microbial biomass carbon (by 32.6%), and available phosphorus (by 17.8%). The Shannon index rose from 4.18 to 4.63, while Proteobacteria and Actinobacteria increased by 9.5% and 7.2%, respectively. Functional genes encoding glycoside hydrolases (GH5, GH9) and carbohydrate esterases (CE1, CE10) were enriched by 25–40%, with upregulation of carbon fixation (rbcL) and methane metabolism (mcrA) genes. Integrating A. sinicus with moderate N reduction improves soil fertility, stimulates microbial diversity, and enhances carbon turnover efficiency, offering a practical pathway toward sustainable low-carbon rice production.

Effective management of straw in rice (Oryza sativa L.)–rape (Brassica napus L.) rotation systems is essential for optimising resource efficiency and improving soil quality. This two-year study investigated the impact of seven straw treatment methods on soil organic carbon (SOC) dynamics. The treatments examined were as follows: (1) control (CK); (2) rice straw (SF); (3) rapeseed straw (YF); (4) rice-straw-derived biochar (SB); (5) rapeseed-straw-derived biochar (YB); (6) mixed straw (YSF); (7) mixed biochar (YSB). Soil properties, enzyme activities and carbon fractions were subsequently analysed. During the canola growing season, the application of rice straw biochar increased oxidisable carbon (ROC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) by 25.7%, 61.7% and 67.2%, respectively, compared to the control. Notably, SB was more effective than unprocessed rice straw (SF) at increasing SOC and ROC. Furthermore, SB demonstrated superior performance in enhancing ROC (56.4%), MBC (36.0%) and DOC (12.2%) compared to hybrid biochar (YSB). SB consistently exhibited a higher carbon accumulation trend than the rapeseed-derived treatments (YF, YB and YSB). The results of the study indicated that applying rice straw biochar during the oilseed rape growing season was effective in increasing variable carbon pools and soil organic carbon accumulation.

In karst agroecosystems, the stability of soil organic carbon (SOC), which is essential for sustainable land management, is strongly influenced by the carbonate parent material. However, how land use and lithology interactively affect the vertical distribution and stabilization of SOC fractions remains unknown. This study investigated the vertical distribution (0–70 cm) of SOC fractions (particulate, POC; mineral–associated, MOC; recalcitrant, ROC; microbial biomass, MBC; dissolved, DOC; permanganate–oxidizable, LOC) in paddy and upland systems overlying limestone and dolomite. Results showed that land use and lithology jointly determined depth–dependent patterns of SOC fractions. The key finding was a depth–dependent reversal of land use effects: paddy soils had higher concentrations of SOC fractions (especially MBC and LOC) in the top 0–20 cm, whereas upland soils had higher concentrations of DOC, ROC, and POC below 50 cm. This pattern underscores an overlooked deep carbon sequestration potential in aerated uplands. Furthermore, lithology exerted fundamental control on SOC stability, with limestone soils exhibiting higher MOC/SOC and MOC/POC ratios than dolomite soils, indicating stronger organo–mineral association. Random Forest and Structural Equation Modeling revealed that soil aggregate stability is a key factor influencing the stability of SOC. These findings demonstrate that effective SOC management in karst agroecosystems must consider both surface land use decisions and deep lithological constraints. Specifically, promoting aggregate stability offers a more targeted pathway for long–term carbon stabilization in these ecologically fragile agricultural systems.