Incapability of biochar to mitigate biogas slurry induced N2O emissions: Field investigations after 7 years of biochar application in a poplar plantation

Abstract

Nitrous oxide (N2O) is a potent atmospheric greenhouse gas that is largely emitted from soils due to the enhanced use of reactive nitrogen in agriculture and plantations. In this study, we evaluated the N2O mitigation ability of biochar after 7 years of application in a poplar plantation. The field experiment was based on combinations of three biochar (0, 80, and 120 t ha−1) and four biogas slurry (0, 125, 250, and 375 m3 ha−1) rates following a factorial design. N2O flux rates were measured for seven consecutive months using in situ static chambers. Soil physicochemical characteristics, potential nitrification rate (PNR), denitrification (DEA), and N2O reduction were recorded once each in September 2019 and January 2020 via lab incubations. In addition, qPCR assays were used to assess the abundance of key nitrifying and denitrifying functional genes. Biochar application after 7 years had no significant effects on N2O flux rates, PNR, and DEA rates. However, a triggering effect of biogas slurry on soil N2O emission was observed, although there was no correlation between biogas slurry rates and N2O emission rates. Factorial ANOVA showed a significant effect of biogas slurry and its interaction with biochar on the relative abundance of bacterial denitrifying and nitrifying functional genes. Additionally, significant correlations of N2O emission rates with PNR rates and NO3− concentration indicated that nitrification was the dominant pathway of N2O emission. Thus, a single biochar application did not mitigate N2O emission rates induced by biogas slurry on a long-term scale.