Effect of high labile biochar on N2O emission from upland soils: A decision tree analysis and an incubation experiment

Abstract

While there are extensive studies reporting a reduction in N2O emission by biochar addition, there are some cases that showed an increase in N2O emission. To identify the primary conditions that lead to an increase in N2O emissions, we conducted a decision tree analysis of 148 observations from 25 published papers. It was verified that biochar made with rice husk, rice straw, nut-shell, pits, and stones had a higher chance of increasing N2O emissions than biochar made with wood, herbaceous, and biosolid. Moreover, it was also demonstrated that the probability of increasing N2O emission by former group biochar was higher in the low pH soils. The latter group of biochar showed the lowest probability of increasing N2O emission in soil with low soil organic carbon (SOC) content. The decision tree analysis results led us to conduct an incubation experiment to understand the underlying mechanisms under which conditions biochar increases or decreases soil N2O emissions. The soils amended with urea and 4% (w/w) biochars (cocopeat: CPB, rice husk biochar: RHB, and wood pellets: WPB) were incubated for 15 days on aerobic condition. As a result, N2O emissions increased in the soils amended with CPB and RHB, which was consistent with decision tree analysis. The high labile matter content of these biochar could stimulate both heterotrophic denitrification and nitrification-mediated processes. In the soil with the high labile matter biochar which increased soil pH throughout the experimental period (CPB treatment), NO2− was accumulated and gene abundance of nitrite-oxidizing bacteria were reduced, which might be caused by NH3 toxicity. Our results can be utilized as a practical guideline to maximize biochar's climate change mitigation potential.