Biochar-enriched soil mitigated N₂O and NO emissions similarly as fresh biochar for wheat production

Biochar amendment has been recommended as a potential strategy to mitigate nitrous oxide (N₂O) and nitric oxide (NO) emissions for wheat production, but its mechanism and effective duration are not well understood. The 1-octyne and 2-pheny l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) in combination with potassium chlorate were used to evaluate the relative contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential ammonia oxidation (PAO) and N₂O and NO production as affected by biochar. Acidic and alkaline soils were collected during wheat-growing season, and four treatments were installed in each soil type: CK, urea alone; BE, biochar-enriched soil for 2-6 years; FB, fresh biochar added to CK; and AB, aged biochar added to CK. The results showed that octyne and PTIO efficiently assessed AOB and AOA activities in soil incubation. The AOB-driven PAO in acidic soil was largely enhanced by increased soil pH in BE and FB treatments, whereas AOA-driven PAO was not. And the contribution of AOB to PAO exceeded 80% in alkaline soil. The N₂O and NO production were positively correlated with PAO in both soils. BE treatment decreased the direct N₂O and NO production in alkaline soil, while both BE and FB treatments decreased the N₂O and NO yields in acidic soil, indicating that biochar mitigated soil N₂O and NO emissions for wheat production. The lack of differences between AB and CK treatments indicated that aged biochar lost its initial effects on PAO, while the biochar-enriched soil amended with biochar years earlier still functioned similarly as fresh biochar.