The increasing risk of ammonia volatilization in farmland from the recovery product of magnesium-modified biochar after nitrogen and phosphorus adsorption

Many studies have shown that magnesium modified biochar (MgBC) can recover nutrients from wastewater and be applied as an excellent slow-release fertilizer in farmland. However, the recovery products (NP-loaden MgBC), represented by struvite or magnesium phosphate, have a high degree of self-alkalinity, which may significantly increase the ammonia (NH₃) volatilization in farmland. In this study, the optimal adsorption parameters, self-alkaline regulation process and co-adsorption mechanism of MgBC for ammonium ion (NH₄⁺) and phosphate ion (PO₄³⁻) were studied through batch experiments. A field experiment was conducted with three treatments, including local conventional fertilization (N₁B₀) and the application of 5 t·ha⁻¹ or 10 t·ha⁻¹ NP-loaden MgBC in combination with local conventional fertilization (N₁B₁ and N₁B₂, respectively), to determine the impact of NP-loaden MgBC on NH₃ volatilization, surface water c(NH₄⁺-N) and pH. The results indicated that the maximum NH₄⁺ and PO₄³⁻ synergistic recovery of MgBC under the optimal adsorption parameters (dosage of 0.6 g·L⁻¹; initial NH₄⁺ and PO₄³⁻ concentrations of 120 and 60 mg·L⁻¹ and pH of 8) were 59.96 and 98.60 mg·g⁻¹, respectively. Self-regulating alkaline MgBC maintained pH suitable for struvite, and precipitation mechanism controlled the adsorption. The presence of NP-loaden MgBC raised the pH levels in surface water during the basal fertilization stage and increased c(NH₄⁺-N) in surface water during the topdressing stages. This, in turn, led to a significant increase in NH₃ volatilization loss during the entire rice-growing period, with N₁B₁ and N₁B₂ experiencing a 23.87 % and 48.91 % increase respectively, compared to N₁B₀. Therefore, it is imperative to take into account the adverse impact of NP-laden MgBC on NH₃ loss in paddy fields when considering its application in future field studies.