Land use influences on soil fertility, carbon stocks, and microbial activity in the Amazonian floodplain, Brazil

Understanding how land use controls deep soil carbon storage and biogeochemical processes is essential for managing tropical floodplain ecosystems. This study evaluated the effects of land use on soil properties at the Amazon River estuary floodplains in Brazil. Soils were sampled to 2 m under açaí (Euterpe oleracea) agroforestry, upper and lower floodplain (várzea) forests, regenerated forest, and mangrove systems. Soil cores were collected at four depth increments (0–50, 50–100, 100–150, and 150–200 cm), with three replicate cores per land-use × depth combination, using vibrocore samplers. Samples were analyzed for chemical properties, soil organic carbon (SOC) content and stocks (SOCS), microbial biomass carbon (MBC), β-glucosidase activity, soil moisture, and bulk density. SOC and SOCS differed significantly among land uses and depths, with the highest values observed in mangrove soils (2.8% and 52 Mg C ha⁻¹). Principal component (PC) analysis revealed a distinct land-use pattern, explaining 75.9 and 10.3 of the total variance by PC1 and PC2, respectively. Significantly, soil properties were also affected by land use, as shown by the PERMANOVA (R² = 0.35, p = 0.001). Regression analyses also showed significant pH-nutrient-carbon relations, while SOC, available phosphorus, cation exchange capacity, and MBC were significantly correlated. Combined microbial, chemical, and physical indicators showed that land use and soil depth influenced carbon storage and nutrient cycling. Altogether, these results identify mangrove soils as important reservoirs of deep carbon and expose the dynamic hydrologic-vegetation-soil process interactions controlling soil fertility in Amazonian floodplains.