Sewage sludge as organic matrix in the manufacture of organomineral fertilizers: Physical forms, environmental risks, and nutrients recycling

The use of organomineral fertilizers (OMFs) has grown worldwide, mainly in tropical conditions, combining economics and environmental management. Our study used a sewage sludge (SS) rich in Zn as the organic matrix in the manufacture of OMFs, with the following objectives: i) to select the best OMF physical form; ii) to evaluate environmental risks associated with their application; iii) to recycle SS nutrients for soybean (Glycine max) production; iv) to verify their benefits to soil microbial activity and other soil functions. For each soil individually, a clay and a sandy clay loam, six treatments were tested: control (CT, without fertilization); mineral fertilization (MF); MF plus Zn and B (MFZn + B); and three distinct physical forms for the OMFs [powder (Pw), granule (G), and pellet (Pt)], all calculated to provide 100 kg of P2O5 and performed in four blocks. Amending SS-OMFs did not represent an environmental risk in terms of spreading pathogens and hazardous trace elements. In comparison with MFs, SS-OMFs enhanced nutrients accumulation, pod numbers, and nodulation, as well as soil microbial activity, but did not affect shoot and root dry masses and enzymatic activities at early stages of soybean development (60 d after planting). They were also capable to supply Zn and B to plants and their distinct physical forms did not affect microbiological indicators of soil quality or nutrients accumulation in plant tissues. Overall, SS-OMFs seems to be a promising alternative for sustainable use of SSs in soybeans avoiding their disposal in landfills, but field studies are still needed to ratify the findings under more realistic conditions.