Mechanistic insights into oxychar’ s role in mitigating ammonia volatilization from fertilised alkaline soils

Mitigating ammonia (NH₃) volatilization from fertilised alkaline soils is crucial for promoting environmental sustainability and safeguarding public health. In this study, "oxychar" was prepared from an agricultural waste via a low-temperature partial-oxidation (LTPO) process under oxygen-sufficient environment. The optimum modification temperature was 220 °C for a duration of 3 h, yielding a maximum NH₃ uptake of 76.4 mg/g by oxychar. The enhanced adsorption capacity and stability are attributed to the increased surface oxygen-containing functional groups (OCFGs) that act as adsorption sites, as well as the presence of ultra-micropores that facilitate effective trapping. When applying optimised oxychar to 21-day incubation studies, it is found that cumulative NH₃ volatilization is reduced by 76.3 %-91.6 % with oxychar application rates of 1 %-3 %, compared to urea treatments. Soil property and microbial community analyses further indicate that oxychar promotes the hydrolysis of urea and the reduction of nitrate to ammonium by dissimilation (DNRA) processes while inhibiting nitrification by modulating microbial communities, resulting in higher ammonium and lower nitrate levels in soil compared to urea treatment. However, the enriched ammonium cannot volatilize freely due to the chemisorption of ammonium and NH₃ by oxychar via ionic and covalent bonds, respectively. Introducing oxychar to soil could be a promising approach to mitigate NH₃ volatilization from fertilised soil to ensure food security and environmental sustainability.