Influence of Crude Oil on Denitrification in Subtidal Sediments in the Northern Gulf of Mexico

In this study the impact of hydrocarbons on nitrogen cycling in intertidal sediments from a barrier island in the north central Gulf of Mexico was investigated. Initial total petroleum hydrocarbon (TPH) concentrations of the sediments averaged 49 ± 27 mg kg−1 in June and 75 ± 48 mg kg−1 in November. Following the addition of crude oil, TPH concentrations increased to 340 ± 73 and 296 ± 50 mg kg−1 in June and November, respectively. Sediment oxygen demand (SOD), inorganic nitrogen and phosphorus fluxes, and denitrification capacity, measured with the isotope pairing technique (IPT) within a week of oiling, were consistently higher in June compared to rates in November when temperatures were 10°C lower. Despite significantly higher TPH concentrations in the oiled treatments, SOD, nutrient fluxes, nitrogen fixation, denitrification, and anammox, did not differ between the treatments. Potential denitrification rates, measured in the presence of excess nitrate (100 μM), were similar between the control and oil treatments, but significantly higher than rates measured with the IPT. The elevated rates of potential denitrification suggest the presence of an active extant microbial community capable of significant attenuation of porewater nitrate concentrations. The insignificant changes in nitrogen cycling in response to petroleum hydrocarbon additions at a site with hydrocarbons present, albeit at low concentrations, suggests resiliency in nitrogen cycling in response to additional hydrocarbon inputs in ranges investigated in this study. This study investigates the impact of crude oil on nitrogen cycling in subtidal sediments in the northern Gulf of Mexico. Hydrocarbons are present in sediments, albeit low concentrations, prior to the oiling. Changes in denitrification rates measured within a week of oiling are insignificant, suggesting some degree of resiliency in nitrogen cycling at sites with prior exposure to hydrocarbons.