Biochar–Nitrogen Composites: Synthesis, Properties, and Use as Fertilizer for Maize

Nitrogen (N) is highly reactive and prone to being easily lost into the air and soil water. Biochar–N composites have proven effective in nourishing and improving maize growth. The aim of this study was to synthesize and assess the properties of composites made from biochars (pyrolyzed at 300 °C) derived from chicken manure (N = 3.5%) and leguminous cake (N = 9%) and enriched with ammonium sulfate (AS), urea, and diammonium phosphate (DAP). The biochar pH was adjusted to approximately 6 using sulfuric and phosphoric acids prior to formulating the six tested composites. Maize was cultivated for 50 days under greenhouse conditions, with evaluations of the maize dry matter (DM) and N in the plant shoot. The biochar and composite properties underwent scrutiny for chemical and physicochemical attributes, as well as for soluble N in water and in an HCl solution. Throughout maize cultivation, the release of N as ammonium and nitrate from the composites and pure biochars in the Oxisol solution was successively assessed. Composites formulated with DAP and supplied at a dose of 270 mg kg−1 N yielded the same maize dry matter as composites in which 400 mg kg−1 N was supplied to plants. Regardless of the N source, at the end of maize cultivation, the residual N in the Oxisol was reduced and inadequate for a new cultivation, even in soils treated with urea. Notably, the biochar–N composites, particularly those formulated with DAP, were as effective as urea in nourishing and promoting robust maize growth. In contrast, the maize biomass was lower for plants fertilized with pure biochars, indicating that the N from the carbonized matrices was insufficient for optimal biomass production.