Effects of Ca/Mg-enriched biochar fertilizer on slow-release performance and maize growth

Chemical fertilizer (CF) has been widely applied to promote agricultural production in recent years. However, the high solubility of nitrogen (N) and phosphorus (P) in the traditional CF can reduce the nutrient absorption rate of crops, leading to serious environmental issues. Therefore, an alternative fertilizer is required to recover the N and P organic matter from water, in order to promote the sustainable development of agricultural production. Fortunately, biochar can serve as one type of potential cost-effective adsorbent to reclaim the nutrients (e.g., phosphate and ammonium) from wastewater. A win-win strategy can be expected to develop a biochar-based fertilizer for the remediation of wastewater. In this work, the agricultural wastes (such as corn straw and eggshell) were employed to prepare the Ca/Mg-enriched biochar fertilizer (Ca/Mg-BCF) via magnesium salt impregnation, pyrolysis transformation process, and high-efficiency reclaiming of N- and P-containing nutrients from biogas effluent. Then, a series of leaching experiments and kinetics analysis were performed to explore the N and P slow-release behavior of Ca/Mg-BCF. Meanwhile, a systematic investigation was implemented to clarify the effects of environmental factors (pH and temperature) on the N and P slow-release performance. The physical and chemical properties of Ca/Mg-BCF before and after nutrient release were also characterized with X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDX). In addition, a comparison was made to determine the effects of Ca/Mg-BCF on the seed germination and early-stage seedling growth of corn. The results demonstrated that the final accumulative release ratios of N and P from the Ca/Mg-BCF at 48 h were 88.10 and 74.22 percentage points lower than those of CF, respectively. The leaching experiment indicated that the Ca/Mg-BCF exhibited excellent slow-release performances of the total P and N with sustainable release rates. Moreover, there was no significant effect on the nutrient release from the Ca/Mg-BCF at temperatures between 10-40 ℃. The N- and P-release were stable between pH 1-7 and pH 3-10, respectively. The SEM characterization showed that the remarkable morphological transformation occurred after Ca/Mg-BCF released nutrients, due to the dissolving of struvite and the formation of CaHPO₄ and Ca₃(PO₄)₂. The P-release performance of Ca/Mg-BCF was controlled by the low solubility of Mg-P precipitates that formed on the biochar surface, and then enhanced by the ‘P-trap’ effect of Ca(OH)₂ and CaCO₃ in the re-precipitation process of PO₄³⁻. Therefore, the N and P-release performance of Ca/Mg-BCF was attributed to the multiple effects, including the dissolution rate of N/P precipitates, re-precipitation of PO₄³⁻ by Ca(OH)₂ and CaCO₃, electrostatic attraction, hydrogen bonds, pore-filling, and the confinement of biochar. In addition, the Ca/Mg-BCF greatly promoted the seed germination and the growth of corn seedlings, compared with the control and CF. Specifically, the Ca/Mg-BCF-0.8 treatment exhibited a higher seed germination rate of 100%, compared with the control (80%) and CF −0.8 treatments (37.5%) in the first five days. This was probably because the fast release rate of N and P from CF caused the salt-stress effect on the seed germination. The stem height, root length, stem diameter of the corn seedlings, and dry weight of the shoot and root increased by 9.50%, 24.63%, 14.53%, 16.59%, and 38.00% respectively, after Ca/Mg-BCF-0.8 treatment, compared with the CF. It infers that sufficient nutrients were supplied for the plant growth in a longer period, indicating that the slow-release performance of Ca/Mg-BCF was better matched with the nutrient requirement of plants. In addition, the excellent properties of biochar carriers can be expected to improve the soil environment for a favorable rhizosphere of root growth. Therefore, the Ca/Mg-BCF can be promising potential for sustainable agriculture application with multiple environmental benefits.