Effects of fresh and aged biochar on N₂O emission from a poplar plantation soil

Nitrous oxide (N2O) emissions pose a serious environmental problem when nitrogen (N) fertilizer is excessively applied to plantation systems to enhance tree growth. Although biochar can improve soil fertility and mitigate soil N losses, our understanding of its interaction with N fertilizer and its long-term effects remains limited owing to experimental constraints. In this study, two microcosm incubation experiments were performed to evaluate the effect of fresh biochar, compared to 8-year field-aged biochar application in a poplar plantation, on soil N2O emissions triggered by biogas slurry application. The experiments incorporated three biochar levels and four biogas slurry application rates, each with three replicates. The results demonstrated that fresh and aged biochar significantly reduced soil cumulative N2O emissions by 31%-61% and 75%-99%, respectively, over 7 d following biogas slurry application. However, these mitigating effects diminished over incubation time. The application of fresh biochar significantly reduced soil available organic carbon and potential denitrification rates, suggesting that it primarily suppressed soil N2O emissions by limiting the supply of electron donors. In contrast, aged biochar had minimal impact on soil available organic carbon and generally enhanced the abundances of bacterial amoA, nirS, nirK, and nosZ genes. This suggests that the aged biochar potentially suppressed soil N2O emissions by promoting complete denitrification. Partial least squares structure equation model (PLS-SEM) analysis corroborated the two different mechanisms regulating the inhibitory influence of fresh and aged biochar on soil N2O emissions. The lower R-2 of PLS-SEM analysis for aged biochar (R-2 = 0.256) compared to that for fresh biochar (R-2 = 0.798) indicates that other factors, such as biochar properties, potentially affect soil N2O emissions and warrant further investigation. This study highlights the need to evaluate the long-term effect of biochar on soil N2O emissions, owing to the dynamic changes in biochar and soil properties over time.