Biochar aging diminishes soil N2O mitigation by reducing electron-donating capacity and shifting denitrification

• Biochar aging reduces soil pH and electron donation but raises oxidized groups. • The capacity of biochar to mitigate soil N 2 O progressively diminishes with aging. • Biochar aging decreases soil autotrophic and heterotrophic nitrification. • Biochar aging increases soil N 2 O primarily from denitrification. • A lower pH and electron-donating capacities hinder the reduction of N 2 O emissions. Biochar can mitigate soil nitrous oxide (N 2 O) emissions, but how its aging in the field affects N 2 O production pathways remains poorly understood. This study examines the temporal changes in biochar properties and their influences on soil N 2 O emission processes under natural aging conditions. Biochar aged in the field for 0, 2, and 5 years was analyzed for its physicochemical characteristics, particularly oxygen-containing functional groups, and electron-donating capacity (EDC). The mechanisms involved in the soil N 2 O production pathways were assessed using 15 N tracing in soils that were amended with differently aged biochar. The results revealed that biochar aging crushed its structure, increased its atomic O:C ratios, specific surface area, C O and COO groups, but reduced the pH, mean pore diameter, and EDC. The application of biochar decreased cumulative N 2 O emissions by 19–28%, and the degree of mitigation decreased with biochar aging. The decrease in N 2 O emissions derived from autotrophic and heterotrophic nitrification induced by biochar aging were associated with the decreased soil pH, ammonia-oxidizing bacteria abundance, dissolved organic carbon, and the NH 4 + substrate content. Denitrification derived N 2 O increased by 68–132% with aging, identifying denitrification as the primary pathway responsible for the increased emissions. The soil N 2 O emission rates were not significantly correlated with NH 4 + and NO 3 – substrates, but were positively linked to the abundance of biochar’s oxidative moieties (e.g., C O and COO groups), and EDC and soil ( nirK + nirS + fungal nirK )/( nosZI + nosZII ) ratio and soil pH. These results implied that the decreased capacity to mitigate soil N 2 O emissions was attributed to a reduced soil pH, accumulation of oxidative moieties, and diminished electron donation, which in conjunction disfavored complete denitrification. Our findings revealed that biochar sustained its N 2 O mitigation capability across all aged treatments, but the mitigation efficacy decreased with biochar aging by altering its functional properties and increasing the denitrification-derived N 2 O generation.