Developing biochar-based slow-release N-P-K fertilizer for controlled nutrient release and its impact on soil health and yield

There has been an augmented attention for broad application of biochar-based slow-release fertilizer (SRF) to agricultural soils in recent years. It was synthesized four dissimilar biochar-based slow-release N-P-K fertilizer using nutrient impregnation technique and evaluated for nutrient release patterns, leaching behavior, yield, and soil health. Biochar prepared at 600 °C, impregnated into nutrient solution for 72 h, mixed with starch-PVA binder at 1:5 ratio to achieve an even coating followed by morpho-chemical characterization through scanning electron microscope which revealed that biochar pores appear to have locked with salt crystals of N-P-K nutrients. The highly porous microstructure of the four biochar allowed it to efficiently sorb NO₃⁻, NH₄⁺, PO₄³⁻, and K₂O and form a nutrient-impregnated SRF. The nutrient release pattern study depicted that after 90 days of leaching the NO₃⁻ released 55.47–50.84%, NH₄⁺ 55.47–50.84%, PO₄³⁻ 65.31–68.52%, and K₂O 74.33–77.27%. Thus, leaching capacity was highest in NO₃⁻ followed by K₂O > PO₄³⁻  > NH₄⁺. Besides, among the four diverse biochar, the pine needle biochar (PNB) showed best nutrient retention/sorption capacity and lowest with maize stalk biochar (MSB). The SRF had lower nutrient release pattern than the fertilizer alone, demonstrating its slow-release behavior. After leaching with water equivalent to 462.18 mm rainfall (160 mL), approximately 47.60–58.27% NO₃⁻, 47.84–65.40% NH₄⁺, and 58.05–59.07% K₂O was recovered in 40–50-cm column depth which indicated that SRF retained the nutrients in upper soil column and reduced its leaching potential. It also indicated that fertilizer was mobile as compared to the SRF. Biochar slowed the downward mobility of N and K in packed soil column. But, interestingly, phosphorus movement was enhanced by SRF in column and it increased its release potential in soil column. The crop yield (2.89–8.82%) and yield attribute characters were positively increased/enhanced by the biochar-based SRF than fertilization which was highest with BGB-SRF (black gram biochar-SRF) followed by MSB-SRF, LCB-SRF (Lantana camara biochar-SRF), and PNB-SRF. The nitrogen use efficiency followed as BGB-SRF (38.3%) > MSB-SRF (37.5%) > LCB-SRF (36.2%) > PNB-SRF (35.7%) than fertilizer (22.8%). The biochar-based SRF also improved the soil quality by increasing available nutrient (5.20–15.71%), oxidizable carbon (19.01–37.18%), and decreasing soil pH (11.74–3.73%). Soil quality improvement facilitated superior maize and black gram grain nutrient uptake (24.44–5.11%). Hence, the biochar-based SRF could be used as N-P-K-based slow-release fertilizer to maximize the functions of the N-P-K fertilizer when added to a sandy soil and minimize its environmental impact.