Impact of crude oil exposure on nitrogen cycling in a previously impacted Juncus roemerianus salt marsh in the northern Gulf of Mexico

This study investigated potential nitrogen fixation, net nitrification, and denitrification responses to short-term crude oil exposure that simulated oil exposure in Juncus roemerianus salt marsh sediments previously impacted following the Deepwater Horizon accident. Temperature as well as crude oil amount and type affected the nitrogen cycling rates. Total nitrogen fixation rates increased 44 and 194 % at 30 degrees C in 4,000 mg kg(-1) tar ball and 10,000 mg kg(-1) moderately weathered crude oil treatments, respectively; however, there was no difference from the controls at 10 and 20 degrees C. Net nitrification rates showed production at 20 degrees C and consumption at 10 and 30 degrees C in all oil treatments and controls. Potential denitrification rates were higher than controls in the 10 and 30 degrees C treatments but responded differently to the oil type and amount. The highest rates of potential denitrification (12.7 +/- 1.0 nmol N g(-1) wet h(-1)) were observed in the highly weathered 4,000 mg kg(-1) oil treatment at 30 degrees C, suggesting increased rates of denitrification during the warmer summer months. These results indicate that the impacts on nitrogen cycling from a recurring oil spill could depend on the time of the year as well as the amount and type of oil contaminating the marsh. The study provides evidence for impact on nitrogen cycling in coastal marshes that are vulnerable to repeated hydrocarbon exposure.