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

Abstract

Nitrous oxide (N2O) emissions are a significant concern when excessive nitrogen (N) fertilizers are 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 owning to experimental constraints. In this study, we performed two microcosm incubation experiments to evaluate the effect of fresh biochar compared to eight-year field-aged biochar application in a poplar plantation on soil N2O emissions triggered by biogas slurry application. The experiments incorporated three biochar levels (B0: 0, B2: 6%, and B3: 9% by mass) and four biogas slurry application rates, each with three replicates. The results demonstrated that fresh and field-aged biochar significantly reduced soil cumulative N2O emissions by 31%–61% and 75%–99%, respectively, over seven days following biogas slurry application. However, these mitigating effects diminished over time. Fresh biochar application significantly reduced the available organic carbon levels and potential denitrification rates, suggesting that it primarily suppressed soil N2O emissions by limiting the supply of electron donors. In contrast, field-aged biochar had minimal impact on soil available organic carbon, and generally enhanced the relative abundance 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 models (PLS-SEM) corroborated the two different mechanisms regulating the inhibitory influence of fresh and aged biochar on soil N2O emissions. The lower R2 of PLS-SEM for aged biochar (R2 = 0.256) compared to that of fresh biochar (R2 = 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 effects of biochar on soil N2O emissions, owing to the dynamic changes in biochar and soil properties over time.