Soil biological indicators demonstrate sustained organic matter functionality under sewage sludge–based organomineral fertilization in tropical agroecosystems

Sustainable intensification of tropical agriculture discussed in the context of soil organic matter (SOM) management aimed at maintaining biological functionality and environmental integrity. Sewage sludge–based organomineral fertilizers combine organic carbon inputs with mineral nutrient supply and may influence microbial processes regulating SOM dynamics and nutrient cycling. This study evaluated soil biological indicators under organomineral fertilization in a no-till system with soybean followed by off-season maize in a clayey tropical soil. A field experiment was conducted using three physical forms of organomineral fertilizer (powder, granule, and pellet) applied at phosphorus rates equivalent to 70% and 100% of crop demand, compared with mineral fertilization and a non-fertilized control. Soil biological quality was assessed through β-glucosidase, acid phosphatase, and arylsulfatase activities, microbial biomass carbon, and easily extractable glomalin after soybean harvest and after the maize cycle. During the soybean cycle, microbial biomass carbon ranged from 470 to 540 mg C kg^-1 soil, while glomalin levels remained stable at approximately 3.22 mg g^-1 across treatments, indicating no suppressive effects of organomineral fertilization. Acid phosphatase activity varied with fertilizer physical form, with higher values under powder and granule than pellet, reflecting differences in organic phosphorus accessibility. After the maize cycle, acid phosphatase activity under organomineral fertilization increased up to 742 mg pNP kg^-1 soil h^-1, exceeding mineral fertilization, indicating enhanced biologically mediated phosphorus cycling. Overall, soil biological indicators demonstrate that sewage sludge–based organomineral fertilization sustains microbial processes essential to SOM functionality, supporting its potential as a sustainable management strategy in tropical agroecosystems.