The effect of long-term biochar amendment on N2O emissions: Experiments with N15-O18 isotopes combined with specific inhibition approaches.

Numerous studies reporting a transient decrease in soil nitrous oxide (N2O) emissions after biochar amendment have mainly used short-term experiments. Thus, long-term field trials are needed to clarify the actual impact of biochar on N2O emissions and the underlying mechanisms. To address this, both a 15N18O labeling technique and gene analyses were applied to investigate how N2O production pathways and microbial mediation were affected by long term biochar amendment in field. Then, 1-octyne and 2-phenyl l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) were used in combination with potassium chlorate to evaluate the relative contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential ammonia oxidation (PAO) and the associated N2O production. Acidic and alkaline greenhouse vegetable soils that had each received two separate treatments were collected (control, no biochar amendment; biochar, biochar amended in the field after 2 or 7 years). The results showed that biochar decreased N2O emissions by 48% in acidic soils and by 22% in alkaline soils compared to those in control. These results were explained by decreases in nitrifier denitrification- (by 74%) and heterotrophic denitrification-derived N2O production (by 58%), as further evidenced by a decrease in NO2- (by 87%) and the (nirK+nirS+fungal nirK):(nosZ-I + nosZ-II) ratio (by 5%) in both greenhouse vegetable soils. However, biochar increased nitrifier nitrification-derived N2O in both soils because of increases in pH and PAO, which were attributed to an increased abundance of AOB rather than AOA. The contribution of AOB to PAO (or N2O) exceeded 69% (or 68%) of the total in acidic soil and 88% (or 85%) of the total in alkaline soil after biochar amendment. Our findings demonstrated that the mitigation of N2O by biochar is linked to specific N2O production pathways.