The Impact of Biostimulation Agents on Diesel-Degrading Microbes in Cold-Regions Soils Has Seasonal Specificity

On-site bioremediation is a cost-effective option for northern petroleum-contaminated sites, and nutrient biostimulation is commonly implemented. Microcosm experiments were conducted using soils from a sub-Arctic site dosed with Arctic diesel (D), with and without biostimulants, under three different temperature regimes reflective of summer and spring/fall. The biostimulation of alkane degraders and bacteria overall was assessed by alkane hydroxylase (alkB) and 16S rRNA genomics and transcriptomics and hydrocarbon degradation trends. At 7 °C, both water (W) and nitrogen and phosphorus (NP) additions provided initial stimulation of petroleum hydrocarbon degraders and the overall community. This resulted in 42 and 57% hydrocarbon degradation for DW and DWNP after 36 days compared with 24% in systems with neither stimulant. Under constant frozen conditions (−5 °C) adding WNP was less stimulatory than adding W for diesel degradation (80 days DW: 22%, DWNP:15%), and for the bacteria in both diesel-amended systems and diesel-free controls. Under freeze–thaw, whereas W and NP had additive impacts on bacteria overall, they had indistinguishable impacts on hydrocarbon degraders and hydrocarbon degradation (∼40%), suggesting freeze–thaw-induced changes in soil structure and associated hydrocarbon bioavailability as rate-limiting. Our study shows that impacts of biostimulation by addition of W or WNP have seasonal specificity.