The increasing risk of ammonia volatilization in farmland from the recovery product of magnesium-modified biochar after nitrogen and phosphorus adsorption
Science of The Total Environment(2023)
Abstract
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.
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Key words
ammonia volatilization,biochar,farmland,magnesium-modified
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