Effects of understory intercropping with salt-tolerant legumes on soil organic carbon pool in coastal saline-alkali land

Phytoremediation through understory intercropping with salt-tolerant legumes (forest-green manure composite patterns) efficiently and sustainably enhances saline-alkali soils, while significantly improving the stability of monoculture forest ecosystems and the efficacy of soil upgrades. However, exactly how forest-green manure patterns regulate the dynamics of the soil organic carbon (SOC) pool and related mechanisms remain unclear. For this study, a pure forest was used as the control, and three leguminous herbaceous plants (M. sativa, S. cannabina, and C. pallida) were intercropped under two forest stand types (T. hybrid 'Zhongshanshan' and C. illinoensis). The variable characteristics and control factors of SOC and its components under different patterns were elucidated by analyzing the soil physical and chemical properties, enzyme activities, and microbial communities. The results revealed that the composite pattern improved soil salinization and increased the activities of (3-1,4-glucosidase, polyphenol oxidase, peroxidase (PER), invertase (INV), and urease, as well as the carbon pool management index and the proportion of active organic carbon. At the T. hybrid 'Zhongshanshan' experimental site, planting M. sativa effectively increased the total carbon (TC) content. The ammonium nitrogen, soil moisture content, total phosphorus, alkaline phosphatase, PER, and polyphenol oxidase were the primary driving factors that affected the SOC pool. At the C. illinoensis experimental site, S. cannabina planting was observed to increase the TC content, with the TC, exchangeable Na+, (3-1,4-N-acetylglucosaminidase, and INV being the main driving factors that impacted the SOC pool. The composite pattern can indirectly influence the SOC pool by altering the soil properties to regulate the microbial community. Further, it was found that soil inorganic carbon (SIC) was the main contributor to increasing the soil carbon pool following the short-term planting of legumes; thus, there may have been a transfer process that occurred from the SOC to SIC. Our study suggests that the forest-green manure pattern has more positive effects on improving soil quality and the carbon pool in saline-alkali land.