Biochar-nitrogen composites: synthesis, properties and use as fertilizer for maize

Abstract

Nitrogen is highly reactive and prone to be easily lost into the air and soil water. Biochar plays a crucial role in mitigating nitrogen losses in the soil-plant system, and biochar-mineral nitrogen composites have proven to be efficient in nourishing maize. The formulation of nitrogen-based composites with biochar demands careful consideration, particularly in pH control, minimum loss of N during formulation, and ensuring optimal levels and available N in the final composite. This study aimed to synthesize and assess the properties of biochar-nitrogen composites formulated with the following synthetic fertilizers: ammonium sulfate (AS), urea, and diammonium phosphate (DAP), as well as evaluate the agronomic efficiency of N composites in nourishing and promoting the maize plentiful growth. Biochars were derived from two contrasting biomass: chicken manure (N = 3.5%) and leguminous cake (N = 9%) and pyrolyzed at 300°C. The biochar pH was adjusted to ~6 using acid solutions before they blended with AS, urea, and DAP, thus, it was formulated six composites. Maize was cultivated for 50 days in greenhouse conditions, with the evaluation of dry matter (DM) and nitrogen in the shoot. Mineral nitrogen content (ammonium and nitrate) and residual nitrogen in the Red-yellow Latosol medium-texture used for maize cultivation were also analyzed. Biochar and composite properties underwent scrutiny for chemical and physicochemical attributes, as well as for N chemical species, and their solubility in composites and biochars as well. The dynamics of mineral N (ammonium and nitrate) released from the composites and raw biochars were concurrently assessed throughout maize cultivation by successively sampling soil solution. With N supplied in a single dose (400 mg kg -1 N), composites proved to be as effective as urea - split in three applications - in providing nitrogen to maize plants. Additionally, composites formulated with DAP and provided at the dose of 270 mg kg -1 N yielded the same dry matter production as other composites in which N was furnished to plants at the rate of 400 mg kg -1 . Composites containing AS effectively released most mineral N into the soil solution, although, at the end of maize cultivation, no differences in residual nitrogen in the Oxisol were observed among the various N sources, including soil samples treated with N from urea. The formulated composites demonstrated comparable agronomic efficiency to urea in promoting plentiful maize growth. Maize biomass was lower for plants nourished with raw biochars without mineral N doping, indicating that the availability of N from the carbonized matrices is reduced, and N is not released within the required timeframe for maize plants.