Effect of crude oil and chemical additives on metabolic activity of mixed microbial populations in fresh marsh soils

Hydrocarbons increase abundance of hydrocarbon-degrading microorganisms, but also decrease microbial diversity. This could disrupt ecosystem dynamics by altering soil organic matter mineralization and resultant nutrient remineralization rates. Crude oil, which is known to contain toxins and reduce microbial diversity, was hypothesized to reduce gross metabolic activity of mixed microbial populations in wetland soils. Soil respiration and Eh were compared, for 6 months, among microcosms containing marsh soils that differed in soil organic matter (Panicum hemitomon Shult. or Sagittaria lancifolia L. dominated marshes), crude oil (Arabian crude, Louisiana crude, or no oil), and additives (a cleaner, a dispersant, fertilizer, or no additive). No treatment slowed activity; instead, Louisiana plus fertilizer and all Arabian treatments temporarily accelerated activity. Additional C respired from oiled microcosms exceeded C added as crude oil by 1.4 to 3.5 times. Thus, much additional C originated from soil organic matter rather than crude oil. Crude oils temporarily lowered soil Eh, which is consistent with accelerated metabolism and demand for electron accepters. The lack of inhibition observed at the community level does not necessarily indicate an absence of toxicity. Instead, tolerant species with metabolic versatility probably maintained activity. Stimulation probably resulted from removal of micronutrient limitation, rather than removal of grazing pressure or macronutrient limitation. Regardless, accelerated soil organic matter mineralization surely accelerated nutrient remineralization. This might explain some reports of crude oil stimulating plant growth. These results are not inconsistent with theoretical and experimental conclusions regarding effects of biodiversity on ecosystem stability and productivity, nor are they inconsistent with conclusions that crude oils contain components that are toxic to microbes, vegetation, and fauna. However, these data do indicate that crude oils also contain components that temporarily stimulate metabolic activity of surviving microbes.