Riparian Forest Restoration Drives the Recovery of Soil Chemistry, Microbial Community Structure, and Enzymatic Activity in the Itaipu Reservoir Protection Zone

Riparian forests play a critical role in protecting soil and water resources and maintaining ecosystem stability. In this study, we evaluated the response of soil chemical and microbial attributes to different stages of riparian forest restoration in the protection zone of the Itaipu Reservoir (Brazil). Soil samples were collected during summer and winter from sites representing four restoration stages (initial, 3, 19, and 30 years), as well as from an adjacent agricultural field and a native forest used as reference systems. We assessed soil chemical properties, microbial biomass carbon, basal respiration, enzymatic activities, and the soil microbial community structure using 16S rRNA gene sequencing. Principal component analysis (PCA) revealed a clear restoration gradient, with older restored sites progressively converging toward the native forest condition. Soil chemical properties showed gradual recovery along the restoration trajectory, with increases in soil organic carbon, cation exchange capacity, and base saturation. In contrast, the availability of P, K, Ca, and Mg declined at early restoration stages and increased with restoration age. Microbial biomass carbon increased by approximately 60% from early restoration to native forest conditions, while metabolic quotients (qCO₂) decreased, indicating greater microbial efficiency and reduced metabolic stress. Enzyme activities related to C, P, and S cycling increased by 1.5- to 3-fold with restoration age. Sequencing analyses indicated a progressive convergence of microbial community composition toward that of the native forest, driven by shifts in relative abundance and the enrichment of forest-associated taxa, such as <i>Verrucomicrobia</i> and <i>Acidobacteria</i>, at advanced restoration stages. Overall, long-term riparian forest restoration promoted substantial recovery of soil chemical fertility and microbial community structure and functioning, reinforcing the role of soil microbiota as a sensitive indicator of ecosystem resilience and restoration success.