Identification and verification of key functional groups of biochar influencing soil N2O emission

Biochars containing abundant and diverse functional groups can influence soil nitrous oxide (N2O) emissions. It remains unclear how these functional groups alter soil N2O emissions and which specific functional groups are responsible. In this study, biochars containing differing functional group compositions were initially screened for their ability to mitigate N2O emissions. Then, the mechanisms of the targeted functional group were assessed by loading the targeted functional group onto graphene, to exclude the potential effects of other functional groups, in order to test specific functional group effects on N2O production and reduction in an agricultural soil and on a typical denitrifier strain (Thiobacillus denitrificans). The results showed that the abundance of the C=O (carboxyl and carbonyl functional groups) was significantly correlated (P < 0.01) with soil N2O emissions and the N2O/(N2O+N2) emission ratio, indicating that the carboxyl functional group plays a significant role in reducing the mitigation potential of biochar on soil N2O emissions. Compared to the carboxyl-free graphene, the carboxyl-loaded graphene significantly decreased the genetic potential for N2O reduction and increased the N2O/(N2O+N2) emission ratio of the soil and the Thiobacillus denitrificans strain. The addition of an electron transfer inhibitor (N, N′-dicyclohexylcarbodiimide) eliminated the negative effect of the carboxyl functional group in terms of reducing N2O emissions, suggesting that the inhibitory effect of the carboxyl functional group on N2O reduction was likely caused by disruption of the electron transfer from biochar to N2O-reducing microbes. Our results imply that the carboxyl functional group on biochar’s surface can negatively affect the potential of biochar to mitigate soil N2O emissions.