Rainfall-driven electron acceptors loss constrain natural attenuation of petroleum hydrocarbons and development of a...

While laboratory studies often report that rainfall enhances the natural attenuation of total petroleum hydrocarbons (TPH), field observations increasingly show that frequent rainfall during the wet season suppresses TPH degradation in clay-rich soils, yet the underlying mechanisms remain unclear. Based on in-situ monitoring at a petroleum-contaminated site, this study assessed how rainfall during wet and non-wet season influences TPH biodegradation and the underlying microbial mechanisms in clay-rich soils. Despite wet-season increases in microbial activity, with dehydrogenase activity rising by 44.1-64.1%, the TPH attenuation rate constant in the 40-80 cm clay-rich soils declined by 54.6%. Rainfall lowered the soil redox potential (Eh), with Eh in clay-rich soils dropping below -100 mV, shifting microbial metabolism from aerobic to anaerobic and increasing the abundance of nitrate-dependent TPH degraders by 1.16-1.61-fold. However, nitrate availability was simultaneously constrained, as rainfall suppressed surface nitrification, reduced the nitrate concentration to 7.14 mg/kg, and increased simulated leaching rates in clay-rich soils to 1.85 times those of the non-wet season. Nitrate limitation imposed an energy bottleneck, decreasing the abundance of downstream central pathway genes (e.g., bcrABCD) by 45.0% and hindering subsequent TPH biodegradation. To address this, a rainfall-responsive calcium peroxide (CaO₂) biostimulation strategy was developed to enhance surface nitrification and replenish nitrate in clay-rich soils, which increased TPH attenuation by 180% and overall biodegradation by 188%. These findings reveal that nitrate-leaching-induced energy limitation act as the principal constraint on the natural attenuation of TPH in clay-rich soils. By leveraging this mechanism, the proposed rainfall-responsive biostimulation strategy advances a novel and dynamic framework for field remediation.