Synthesis of struvite-enriched slow-release fertilizer using magnesium-modified biochar: Desorption and leaching mechanisms.

To improve the retention and slow-release abilities of nitrogen (N) and phosphorus (P), an 82 %-purity struvite fertilizer (MAP-BC) was synthesized using magnesium-modified biochar and a solution with a 2:1 concentration ratio of NH4+ to PO43- at a pH of 8. Batch microscopic characterizations and soil column leaching experiments were conducted to study the retention and slow-release mechanisms and desorption kinetics of MAP-BC. The slow-release mechanism revealed that the dissolution rate of high-purity struvite was the dominant factor of NP slow release. The re-adsorption of NH4+ and PO43- by biochar and unconsumed MgO prolonged slow release. Mg2+ ionized by MgO could react with PO43- released from struvite to form Mg3(PO4)2. The internal biochar exhibited electrostatic attraction and pore restriction towards NH4+, while magnesium modification and nutrient loading formed a physical antioxidant barrier that ensured long-term release. The water diffusion experiment showed a higher cumulative release rate for PO43- compared to NH4+, whereas in soil column leaching, the trend was reversed, suggesting that soil's competitive adsorption facilitated the desorption of NH4+ from MAP-BC. During soil leaching, cumulative release rates of NH4+ and PO43- from chemical fertilizers were 3.55-3.62 times faster than those from MAP-BC. The dynamic test data for NH4+ and PO43- in MAP-BC fitted the Ritger-Peppas model best, predicting release periods of 163 days and 166 days, respectively. The leaching performances showed that MAP-BC reduced leaching solution volume by 5.58 % and significantly increased soil large aggregates content larger than 0.25 mm by 24.25 %. The soil nutrients retention and pH regulation by MAP-BC reduced leaching concentrations of NP. Furthermore, MAP-BC significantly enhanced plant growth, and it is more suitable as a NP source for long-term crops. Therefore, MAP-BC is expected to function as a long-term and slow-release fertilizer with the potential to minimize NP nutrient loss and replace part of quick-acting fertilizer.

Effect of acid-modified biochar coupled with alternate wetting and drying on P leaching, soil P retention and plant P uptake in paddy fields.

H2SO4-modified biochar has been recognized as a means to achieve the advantages of carbon sequestration, and nitrogen loss reduction. However, little information is available on its effect on phosphorus (P) uptake, soil available P, and P leaching under alternate wetting and drying irrigation (IAWD). A split-plot experimental layout was carried out with two irrigation regimes (conventional continuous flooding, ICF, and alternate wetting and drying, IAWD) as main plots and three biochar additions (biochar-free control, B0, non-acidified biochar, B20, and acid-modified biochar, B20A) as subplots. Results indicated that IAWD decreased water percolation by 9.26%-14.74% and P leaching by 50.14%-106.64% and increased surface soil available P by 10.88-29.08%, resulting in 14.21-35.03% apparent phosphorus balance (APB) over the three years as compared with ICF. B20 produced a 6.23% lower grain yield in the 1st year and 5.06% and 11.02% higher yields in the 2nd and 3rd years, while B20A increased or tended to increase it throughout the three years. Both B20 and B20A significantly decreased total water percolation (9.68-28.37%), P leaching (18.26-152.00%), and increased soil available P (9.90-46.24%), dissolved P in surface soil (10.00-62.50%), and P uptake (4.31-49.71%), and thereafter enhanced apparent phosphorus balance (11.06-40.78%). Compared with B20, B20A achieved a better APB due to a 113% lower P leaching and 52.9% lower dissolved P at 60 cm soil profiles. IAWDB20A-M produced the highest APB, surface soil available and dissolved P, and the lowest P leaching, which increased grain yield, APB, surface soil available P, and dissolved P by 9.54%, 129.61%, and 53.19%, and decreased P leaching by 257% over ICFB0, respectively. Therefore, the use of H2SO4-modified biochar could produce higher grain yield with lower P leaching and higher APB for IAWD paddy systems, which is beneficial to enhancing plant P uptake, mitigating P leaching, and ensuring sustainable agricultural production.