Doctor of Philosophy (PhD)
Oceanography and Coastal Sciences
Deltaic wetlands can ameliorate high NO3- loads via denitrification, a dominant NO3- reduction pathway in inland and coastal habitats. Dissimilatory Nitrate Reduction to Ammonia (DNRA), in contrast to denitrification, conserves N within the system and has been found to be favored over denitrification with increase in salinity and temperature. There are no studies in the Louisiana Deltaic plain with simultaneous estimation of denitrification and DNRA. Increased P availability has been shown to increase denitrification so it is essential to understand P biogeochemistry when studying N cycle. Here, we evaluated net denitrification using N2:Ar and direct and coupled denitrification using isotope pairing technique in eroding and emerging delta in coastal Louisiana. DNRA rates were estimated using the 15N diffusion technique.This study further evaluated potential P mobilization during bacterially mediated redox reactions using iron-reducing bacteria, Shewanella putrefaciens CN32. The results revealed that denitrification rates were positively associated and linearly controlled by temperature with maximum mean denitrification rates ranging from 40.1–124.1μmol m-2 h-1 in the summer (30 oC). Despite the presence of coupled denitrification at both sites, its contribution was generally low indicating that direct denitrification was the dominant microbial nitrate reduction pathway in all sediments types under high NO3- concentrations (50 μM). High dentrification rates were consistently present in recently formed delta plains with low organic matter, low water salinity (4-3 concentrations. DNRA rates were -1 of nitrogen removal in coastal Louisiana. This results further showed that under anaerobic condition, PO43- release from sediment spiked with Sp-CN32 was higher in the WLD (marsh 30-fold, ridge 22-fold) compared to the BLC habitats (marsh 12-fold, channel 6-fold). This increase in PO43- release was significantly correlated with Fe bound PO43- in sediments from different habitats. This study will advance our understanding of biogeochemical transformations to address knowledge gaps to propose strategies to reduce eutrophication problems in coastal Louisiana.
Upreti, Kiran, "Evaluating Seasonal Nutrient Fluxes in Emerging and Eroding Wetlands of the Louisiana Delta Plain" (2019). LSU Doctoral Dissertations. 4814.
Available for download on Wednesday, February 25, 2026