Agricultural Waste-Derived Biochar-Based Nitrogenous Fertilizer for Slow-Release Applications

The exponential increase in population demands more food to be produced by employing modern technologies. There is a worldwide increase in the use of chemical fertilizers to rapidly enhance the crop yield. Nitrogen is a crucial plant nutrient, and nitrogenous fertilizers are the most widely used fertilizers. However, the high solubility and volatility of commonly used nitrogenous fertilizers have led to low nutrient use efficiency and alarming environmental pollution. They are lost due to the volatilization of ammonia and leaching of nitrate and release of nitrous oxide, and thus, plants only absorb approximately 20-30% of the nitrogen present in fertilizers. Slow-release fertilizers have been designed to overcome these issues and supply nutrients gradually and sustainably. Biochar, a solid material rich in carbon derived from biomass, can reduce nutrient loss in soil and extend the effectiveness of fertilizers in promoting plant uptake. In the present study, a slow-release nitrogenous fertilizer is prepared using biochar obtained by pyrolysis of a banana leaf sheath (BLS) at 500 °C for 3 h. The BLS biochar and nutrient-loaded BLS (NBLS) biochar exhibited significant water absorbance capacity, water retention capacity, swelling ratio, and equilibrium water content, which would support the maintenance of water levels in soils. The lower salt index values of the prepared fertilizer showed its potential to be used as a sustainable and clean fertilizer. The prepared BLS and NBLS biochar were also characterized by various techniques such as Fourier transform infrared (FT-IR), powder X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller (BET) methods. The FT-IR spectra of both BLS and NBLS biochar demonstrate the existence of primary, secondary, and tertiary alcohols, alkanes, alkenes, esters, and phenols. The peak at 1423 cm^-1 in NBLS biochar corresponds to the vibration of NH⁴⁺ confirming nutrient loading. A minor phase change was noticed in the intensities of NBLS biochar, which may be attributed to the absorption of nutrients into the structure of biochar. TGA analysis confirmed the stability of BLS and NBLS Biochar. SEM analysis demonstrates a highly porous structure of the biochar samples due to the release of volatile matter from the biomass. The BET-specific surface area of BLS and NBLS biochar was 43.216 and 35.014 m²/g, respectively. Nutrient release studies showed an incremental increase in the nitrogen release percentage over a period of 16 h. The gradual supply of nitrogen to the plants over an extended period demonstrated by the prepared slow-release fertilizer confirms its potential to reduce the leaching loss commonly observed in conventional chemical fertilizers.