Thermal versus chemical stabilization of sewage sludge: Effects on soil chemical properties, microbial functionality,...

The reuse of sewage sludge in agriculture represents a sustainable strategy to improve soil fertility, promote nutrient cycling, and reduce environmental liabilities. This study evaluated the effects of thermally (TS) and chemically stabilized (CS) sewage sludge, applied at rates of 0, 10, 20, 30, and 40 Mg ha^-1, on the chemical and microbial properties of a soil cultivated with maize (Zea mays L.). A comprehensive set of soil attributes was assessed, including microbial biomass C, N, and P, enzyme activities, and the abundance of structural and functional genes (16S rRNA, 18S rRNA, nifH, phoD, amo-AOB). CS application increased total organic carbon and nitrogen contents by up to 60 % and 50 %, respectively, and elevated nitrate and ammonium concentrations to around 300 and 60 mg kg^-1. It also stimulated enzymatic activity and microbial nitrogen use efficiency, resulting in a nearly 40 % higher grain yield than TS. Conversely, TS promoted a 117 % rise in fungal abundance and greater microbial biomass C, while exhibiting lower urease and alkaline phosphatase activities. Moreover, CS raised phoD gene abundance by up to 25 % and amo-AOB by 32 % compared with TS, resulting in a more energetically balanced soil system that favored nutrient cycling. Overall, CS strengthened microbial functioning and nutrient utilization, sustaining maize productivity under tropical conditions and highlighting the potential of biosolid recycling to reconcile agricultural intensification with environmental stewardship and circular economy principles.