Electrodes Donate Electrons for Nitrate Reduction in a Soil Matrix via DNRA and Denitrification.

Microbial strains and indigenous microbiota in soil slurries have been reported to use electrons from electrodes for nitrate (NO) reduction. However, few studies have confirmed this in a soil matrix hitherto. This study investigated if, and how, an electric potential affected NO reduction in a soil matrix. The results showed that, compared to a control treatment, applying an electric potential of -0.5 V versus the standard hydrogen electrode (SHE) significantly increased the relative abundance of NO-reducing microbes (e.g., Alcaligenaceae and Pseudomonadaceae) and the abundances of the nrfA, nirK, nirS, and nosZ genes in soil matrices. Meanwhile, the electric potential treatment doubled the NO reduction rate and significantly increased the rates of production of ammonium (NH), dinitrogen (N), and nitrous oxide (NO). The amount of NO-N reduced under the electric potential treatment was comparable to the sum of the amounts of N observed in the increased NO, N, NH, and nitrite (NO) pools. An open-air experiment showed that the electric potential treatment promoted soil NO reduction with a spatial scale of at least 38 cm. These results demonstrated that an electric potential treatment could enhance NO reduction via both denitrification and dissimilatory NO reduction to ammonium (DNRA) in the soil matrix. The mechanisms revealed in this study have implications for the future development of potential techniques for enhancing NO reduction in the vadose zone and consequently reducing the risk of NO leaching.