Agro-Environmental Potential of Biosulfate as a New Soil Fertilizer: Herbicide Retention–Release and Effects on Ligninolytic Fungi and Horticultural Plants

Sustainable agriculture is increasingly reliant on reducing anthropogenic inputs and recycling organic waste while protecting ecosystems. In this context, this study investigated the agro-environmental properties of biosulfate, focusing on its interaction with herbicides and its effects on soil fungi and horticultural plants. Two biosulfate samples obtained from urban sewage sludge from the Barletta (BIO-BA) and Foggia (BIO-FO) treatment plants were characterized by Fourier transform infrared–attenuated total reflectance (FTIR-ATR) spectroscopy and scanning electron microscopy (SEM). The adsorption/desorption of the herbicides metribuzin (MET), S-metolachlor (S-ME) and cycloxydim (CYC) on biosulfates was evaluated by studying adsorption kinetics and isotherms. All herbicides reached adsorption equilibrium within a few hours, according to pseudo-second-order kinetics, indicating a predominant chemical interaction between biosulfate and the molecules. Considering the organic C content of BIO-BA (~21%) and BIO-FO (~17%), which was less than half that commonly measured for other organic fertilizers, such as compost and digestate, their adsorption capacity was high, with Freundlich adsorption constants ranging from 772 µg g−1 (S-ME on BIO-BA) to 1464 µg g−1 (CYC on BIO-FO). A low hysteresis coefficient indicated a rather slow and incomplete release of the molecules from the biosulfate. Exposure of the fungi Pleurotus ostreatus and Pleurotus eryngii to 1, 2, 3, and 4% BIO-BA and BIO-FO stimulated mycelium growth, indicating that responses depended on fungal species and biosulfate dose. Finally, germination and early growth of lettuce and basil were generally unaffected by either biosulfate, as parameters such as germination percentage, root and shoot length, and fresh and dry biomass were not statistically different from the control. Some growth stimulation was observed in basil. Overall, biosulfate appears to be a promising soil fertilizer, as it can contribute to soil organic matter, retain xenobiotics, and exert biostimulatory effects under controlled conditions.