Electrodes donate electrons for nitrate reduction in a soil matrix via DNRA and denitrification

Abstract

Microbial strains and indigenous microbiota in soil slurries have been reported to use electrons from electrodes for nitrate (NO₃⁻) reduction. However, few studies have confirmed this in a soil matrix hitherto. This study investigated if, and how, an electric potential affected NO₃⁻ 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 NO₃⁻-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 NO₃⁻-reduction rate and significantly increased the rates of production of ammonium (NH₄⁺), dinitrogen (N₂), and nitrous oxide (N₂O). The amount of NO₃⁻-N reduced under the electric potential treatment was comparable to the sum of the amounts of N observed in the increased N₂O, N₂, NH₄⁺, and nitrite (NO₂⁻) pools. An open-air experiment showed that the electric potential treatment promoted soil NO₃⁻ reduction with a spatial scale of at least 38 cm. These results demonstrated that an electric potential treatment could enhance NO₃⁻ reduction via both denitrification and dissimilatory NO₃⁻ 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 NO₃⁻ reduction in the vadose zone and consequently reducing the risk of NO₃⁻ leaching.