Biochar mitigates N 2 O emissions by promoting complete denitrification in acidic and alkaline paddy soils

Abstract Biochar is an efficacious amendment for mitigating nitrous oxide (N 2 O) emissions in soils. Nevertheless, the underlying mechanisms responsible for reduced N 2 O emissions by biochar in paddy soils remain inadequately elucidated. Here, using two typical paddy soils with contrasting pH values (5.40 and 7.56), the N 2 and N 2 O fluxes and the associated functional genes were investigated in soil amended with varying amounts of biochar (0%, 0.5%, and 5%, weight / weight ) via soil slurry incubation integrated with the N 2 /Ar technique and q PCR analysis. The results showed that N 2 O fluxes were significantly ( p < 0.05) reduced by 0.65–3.64 times following biochar amendment, concomitant with a significant ( p < 0.05) increase in N 2 fluxes (5.47–46.14%) in both acidic and alkaline paddy soils. As a result, the N 2 O/(N 2 O + N 2 ) ratios were significantly ( p < 0.05) reduced by 1.53–4.65 fold in both soil types. In acidic paddy soils, the enhanced denitrification rates and the decreased N 2 O/(N 2 O + N 2 ) ratios exhibited a strong correlation with increased pH values. In alkaline paddy soil, these changes were ascribed to the enhanced nosZ Clade I gene abundance and nosZ /( nirS + nirK ) ratio. Our findings reveal that biochar primarily mitigates N 2 O emissions in paddy soils by promoting its reduction to N 2 .