Development and evaluation of biochar-based secondary and micronutrient enriched slow release nano-fertilizer for reduced nutrient losses

Four diverse biochars were impregnated into secondary and micronutrient solutions to develop slow release nano-fertilizer via a three-step synthesis process and to verify the appropriateness of biochar as a support material for nutrient release patterns to crops on a slow release basis. The nano range (8.33–24.33 nm), such as the structure of the synthesized nano-fertilizer, was confirmed by a transmission electron microscope. It exhibited a nearly neutral pH (5.94–6.61) with adequate percentages of C, H, N, and O. The developed nano-fertilizer showed an excellent water absorbency character (32.46–45.76%) having a lower density (0.731–0.762 g cm⁻³). It represented excellent swelling (4.15–4.61%) capacity with a lower salt index (0.04–0.07) and equilibrium water content (68–74%). It also augmented the soil water retention (42.47–53.16%) capacity than commercial compound fertilizer (22.43%) and soil (19.52%) after 45 days which is very desirable for supplying water under water stress conditions. It demonstrated lower Ca⁺², Mg⁺², Zn⁺², Fe^+2/+3, and Na₂O release than commercial fertilizer, representing its low release behavior. The nutrient release pattern study depicted that after 90 days, the concentration of Ca⁺² released 87.4–90.7%, Mg⁺² 81.8–85.3%, Na⁺² 96.2–99.1%, Fe⁺² 2.8–5.2%, and Zn⁺² 0.5–1.6%. Thus, among the mineral nutrients, the leaching capacity was highest for Na₂O followed by Ca⁺² > Mg⁺² > Fe^+2/+3 > Zn⁺². Besides, among the four diverse biochars, the pine needle biochar showed the best nutrient retention/sorption capacity followed by Lantana camara, black gram, and maize stalk biochars for slow release of Ca⁺², Mg⁺², Fe^+2/+3, and Na₂O nutrients. But exceptionally, Lantana camara biochar showed significant Zn⁺² nutrient retention/sorption capacity followed by pine needle, black gram, and maize stalk biochars. Thus, feedstock types also manipulated the nutrient release pattern of the secondary and micronutrient from the developed nano-fertilizer. Hence, biochar-based nano-fertilizer has a greater possibility to decrease nutrient leaching, enhance water retention, and thus augment plant nutrient and water use efficiencies.