ABSTRACT
The recent oil spill in the Gulf of Mexico had significant effects on microbial communities in the Gulf, but impacts on nitrifying communities in adjacent salt marshes have not been investigated. We studied persistent effects of oil on ammonia-oxidizing archaeal (AOA) and bacterial (AOB) communities and their relationship to nitrification rates and soil properties in Louisiana marshes impacted by the Deepwater Horizon oil spill. Soils were collected at oiled and unoiled sites from Louisiana coastal marshes in July 2012, 2 years after the spill, and analyzed for community differences based on ammonia monooxygenase genes (amoA). Terminal Restriction Fragment Polymorphism and DNA sequence analyses revealed significantly different AOA and AOB communities between the three regions, but few differences were found between oiled and unoiled sites. Community composition of nitrifiers was best explained by differences in soil moisture and nitrogen content. Despite the lack of significant oil effects on overall community composition, we identified differences in correlations of individual populations with potential nitrification rates between oiled and unoiled sites that help explain previously published correlation patterns. Our results suggest that exposure to oil, even 2 years post-spill, led to subtle changes in population dynamics. How, or if, these changes may impact ecosystem function in the marshes, however, remains uncertain.
ABSTRACT
We measured soil properties, carbon and nutrient (nitrogen, phosphorus) pools, ambient and potential denitrification, and phosphorus sorption index (PSI) in natural depressional wetlands and depressional wetlands restored through the U.S. Department of Agriculture (USDA) Wetland Reserve Program. We measured the same suite of variables in natural and USDA Conservation Reserve Program-restored riparian buffers and in agricultural fields adjacent to both systems to determine the degree to which ecosystem services are being provided through restoration in different hydrogeomorphic settings. Organic carbon and nutrient pools, PSI, and denitrification were greater in natural than in 5- to 10-yr-old restored depressional wetlands. In riparian soils, carbon and nutrient pools, PSI, and denitrification were comparable between restored and natural systems, suggesting that these services develop quickly after restoration. Restored depressional wetlands had lower soil organic C, N, and P relative to agricultural soils, whereas the opposite trend was observed in restored riparian soils. Four-year-old restored riparian buffers achieved equivalence to natural riparian buffers within 4 yr, whereas restored depressional wetlands took longer to provide these ecosystem services (i.e., PSI, denitrification, C storage) at levels comparable to natural wetlands. Restored depressional wetlands and riparian buffers provide ecosystem services lost through previous conversion to agriculture throughout the Midwest; however, the development of these services depends on hydrodynamics (pulsed versus nonpulsed), parent material, soil texture (sand, clay), and disturbance regime (prescribed fire) of the site. As restoration continues throughout the region, C sequestration and nutrient removal in these systems is expected to increase water quality at the local and regional levels.
