Spatial and Temporal Variability of Nitrification Potential and Ammonia-Oxidizer Abundances in Louisiana Salt Marshes

We quantified nitrification potential and ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) abundances in 13 salt marshes in three regions across southeast Louisiana (Terrebonne Bay, western and eastern regions of Barataria Bay). Seven marshes were oiled following the 2010 Deepwater Horizon oil spill, and six sites were unoiled marshes. We evaluated spatial variability by sampling transects from marsh edge to interior, as well as temporal variability across regions. Our nitrification potential rates (up to 5,499 nM NO3-N dry g(-1) day(-1)) were much higher than other published rates for salt marshes and we found strong spatial patterns in both nitrification and soil properties. In Terrebonne soils, nitrification potentials peaked in July and increased from marsh edge to interior. Conversely, rates declined with distance from the marsh edge in western Barataria; no spatial patterns were apparent in eastern Barataria marshes. Spatial patterns corresponded to changes in relative elevation, which also influenced patterns of soil properties and ammonia-oxidizer abundances. Nitrification rates were significantly greater in July than September, though no differences were found between regions within either month. Nitrification potential was positively related to ammonia-oxidizer abundance, and significant relationships were only present in the oiled marshes. In Terrebonne marsh soils, nitrification was more strongly related to AOB abundance whereas rates were more strongly related to AOA abundance in Barataria marsh soils. Although we found strong spatial patterns in nitrification, ammonia-oxidizer abundances, and soil properties that appear to, at least in part, be regulated by differences in elevation and hydrology, we found no significant oil effects 2 years post oil-exposure.