Cover Crops Enhance Aboveground Biomass, Soil Biological Activity and Soil Organic Carbon in Tropical Agroecosystems

In tropical agroecosystems, soil organic carbon (SOC) accrual is constrained by rapid organic matter turnover and seasonal water limitation. Cover crops are widely used to improve soil C inputs, but their effects on SOC fractions and soil biological activity can vary according to species composition and management duration. Understanding how different second‐season systems affect SOC stocks, particulate organic matter (POM), mineral‐associated organic matter (MAOM) and organic matter chemistry is important for improving C accumulation in tropical soils. Here, we assessed SOC stocks, POM, MAOM, organic functional groups and soil biological activity. The study was conducted at two sites, Rio Verde, Goiás (after 5 years of management) and Rondonópolis, Mato Grosso (after 9 years of management). In both sites, soybean was grown annually in the main season, followed by different second‐season systems. In Rio Verde, the second‐season systems were (i) maize, (ii) ruzigrass, (iii) maize–ruzigrass intercropping and (iv) cover crop mix (millet, showy rattlebox and ruzigrass). In Rondonópolis, the second‐season systems were (i) fallow, (ii) maize–ruzigrass intercropping, (iii) ruzigrass, (iv) showy rattlebox and (v) cover crop mix (showy rattlebox, millet, ruzigrass and pigeon pea). Ruzigrass/soybean and mix/soybean systems contributed more than 6 Mg C ha −1 year −1 of aboveground biomass. The mix of cover crops, which combined greater residue input with more diverse aboveground biomass quality, was associated with a more chemically diverse organic matter pool, higher biological activity (+40%–50%) and greater POM. The cover crop mix increased POM by 53%, while MAOM responses varied according to site and soil depth. Structural equation modelling indicated positive associations among aboveground biomass, soil biological activity, MAOM and SOC stocks. Overall, our findings provide field‐based evidence that aboveground biomass quantity and biological activity are linked to SOC accrual and to changes in the distribution of C between POM and MAOM, showing that diverse cover crops can enhance soil carbon accrual in tropical agroecosystems.