Co-application of a biochar and an electric potential accelerates soil nitrate removal while decreasing N2O emission

Denitrification is an important mechanism for mitigating groundwater nitrate (NO3-) pollution. Our previous results showed that electric potential application (-0.5 V versus standard hydrogen electrode) accelerated subsoil NO3- reduction efficiently, but nitrous oxide (N2O) emissions were also elevated with the application of the electric potential. Biochar has previously been considered to act as an electron shuttle to mitigate soil N2O emission. Therefore, this study aimed to investigate if, and how, a combined amendment of electric potential and biochar could simultaneously accelerate soil NO3- reduction and suppress N2O emission. The results showed that the electric potential application alone (E) significantly increased the soil N2O emission by 144% compared with soil only (CK), whereas the co-application of electric potential and biochar (E + BC) markedly decreased the soil N2O emission by 83%, which was more than two times higher than the efficiency of biochar amendment alone (BC) on soil N2O mitigation (35%). Moreover, the E + BC treatment further decreased the N2O/(N2O + N-2) emission ratio by 62% on average at the end of the experiment compared with the BC treatment. These results indicate that there is an interaction between biochar and electric potential on soil N2O mitigation. Contrary to the E + BC treatment, the combined amendment of electric potential and H2O2 -oxidized biochar (E + BC_H2O2) increased the soil N2O emission by 151% compared with the BC treatment. The cathode coulombic efficiency did not differ significantly between the E + BC_H2O2 and E treatments, but the cathode coulombic efficiency in the E + BC treatment was almost double that of the E treatment. Besides, the abundance of Proteobacteria, which include most electrotrophic microorganisms, and the nosZ/(nirS + nirK) gene ratio were higher in the E + BC treatment than that in other treatments. These results indicate that the biochar interacts with electric potential treatment via shuttling electrons from electrodes to soil electrotrophic denitrifying consortia and consequently accelerates soil NO3- reduction and decreases the N2O/(N2O + N-2) emission ratio.