Fulvic acid more facilitated the soil electron transfer than humic acid

Humus substances (HSs) participate in extracellular electron transfer (EET), which is unclear in heterogeneous soil. Here, a microbial electrochemical system (MES) was constructed to determine the effect of HSs, including humic acid, humin and fulvic acid, on soil electron transfer. The results showed that fulvic acid led to the optimal electron transfer efficiency in soil, as evidenced by the highest accumulated charges and removal of total petroleum hydrocarbons after 140 days, with increases of 161% and 30%, respectively, compared with those of the control. However, the performance of MES with the addition of humic acid and humin was comparable to that of the control. Fulvic acid amendment enhanced the carboxyl content and oxidative state of dissolved organic matter, endowing a better electron transfer capacity. Additionally, the presence of fulvic acid induced an increase in the abundance of electroactive bacteria and organic degraders, extracellular polymeric substances and functional enzymes such as cytochrome c and NADH synthesis, and the expression of mtrC gene, which is responsible for EET enhancement in soil. Overall, this study reveals the mechanism by which HSs stimulate soil electron transfer at the physicochemical and biological levels and provides basic support for the application of bioelectrochemical technology in soil. Interspecific electron exchange is vital for the biostimulation degradation of organic contaminants, especially in soils where mass transfer is difficult. Humus substances (HSs) possessing various redox functional groups participate in extracellular electron transfer (EET). Yet, HSs mediation for electron conduction in soil is unclear. Here, we found that fulvic acid primarily promoted electron transfer by modifying the soil DOM oxidation state, the abundances of electroactive bacteria, and EET-related enzymes and genes and thus significantly increased the petroleum removal. This study provides an insight of the HSs into electron transfer and biodegradation of petroleum hydrocarbons in soil.