Current Projects:

NTO and DNAN Transformations Quantified Using Enriched Stable Isotope Tracers — Funded through DOD-SERDP The proposed work is designed to quantify the magnitude of NTO and DNAN loss, retention, and mineralization under conditions representing a wide range of surface water environments encountered in situ. It will provide kinetic parameters suitable for direct incorporation into fate and transport models for these insensitive high explosives (IHE). The objectives are organized around the following questions: 1) At what rate is the surface water IHE load attenuated during transport? 2) What fraction of the IHE loss from surface water is attributable to complete mineralization to inert inorganic constituents? 3) What fraction of the IHE loss from surface water is attributable to retention in the sediment as parent compound and/or organic derivatives? 4) How does sediment mineral type and natural organic matter content influence the fate of IHE? 5) Do the kinetics associated with these processes fundamentally change as a function of different IHE loads?

Past Projects:

Impact of sea level rise on sedimentary nitrogen removal processes in tidal freshwater ecosystem – Funded through NSF Grant Changes in land use and increasing population density have altered delivery rates and sources of N and OM to coastal.  Coastal habitats have responded with accelerated respiration and nutrient cycling, leading to areas of severe hypoxia and possible shifts in the N saturation status of sediments.  Because of their position in the landscape the coastal ocean is, sensitive to a changing climate as it imprints regionally and as sea level rise impinges.  Global change models predict acceleration of the hydrologic cycle that will result in increased intensity, frequency, and amount of heavy precipitation over land.  This tighter coupling between the continents coastal ocean will result in increased total flux of N and/or OM rich delivered in episodic pulses to estuaries that are undergoing salinization from rising sea level. There is currently no mechanistic picture of how the sedimentary N-cycle will respond to these two changing boundary conditions.  This project examines various geochemical controls on the ratio of N-cycling reactions that either export (denitrification and anammox) or conserve (dissimilatory nitrate reduction to ammonium; DNRA) N in coastal sediments. This project aims to examine the importance of anammox in tidal freshwater ecosystems, the effects of salinity on freshwater anammox bacteria and the sedimentary nitrogen (N) cycle, and to predict alternative N removal pathways in sedimentary communities that are threatened by sea level rise. The goal is to compare community structures and activities of nitrifiers, denitrifiers, anammox and DNRA bacteria in river sediments exposed with different tidal influences; characterize instantaneous responses of sediment microbial communities to salinity intrusion; examine changes in sediment community structures and activities in mesocosms with different salinity treatments; and estimate and predict sedimentary N removal capacity and alteration of N pathways in freshwater ecosystems under sea level rise based on new biogeochemical model.

 

Anammox in a shallow coastal aquifer – combining in situ stable isotope and molecular approaches to examine controls on rates and communities – Funded through NSF Grant Groundwater holds a large reserve of DIN with extended residence times that permit slow removal rates to impact total in-aquifer loads and the magnitude of DIN subsequently discharged to surface waters.  Current analysis suggests that 20% of global DIN conversion to N₂ (via denitrification) occurs in aquifers.  Our preliminary evidence suggests that anammox may exceed denitrification in N-rich aquifers.  Global groundwater DIN loss estimates cannot be complete without considering anammox. The proposed work will yield the first reported in situ anammox rates for groundwater and define how microbial nitrogen cycling communities (anammox, denitrification, nitrification) interact in a mutualistic or competitive fashion to regulate nitrogen fate in aquifers.  Through collaboration among a hydrologist, isotope geochemist(s), and microbial ecologist(s), the work crosses traditional disciplinary boundaries. The results provide a heretofore missing component for the assessment of anammox importance across ecosystems. The work is specifically made transferable to other systems by supplementing anammox rate information with quantifying the general geochemical and/or microbial controls on anammox.  Further, by establishing the isotopic fractionation effects for this reaction we can potentially provide an anammox diagnostic tool for researchers in other disciplines (e.g. soil scientists, oceanographers).   The overall goal of this study is to address the following questions: 1) Does anammox occur at sufficient in situ rates to warrant its consideration as an important mechanism of N loss during groundwater transport? 2) Are there indicative chemical or isotopic signatures for anammox in groundwater that can aid in its detection and quantification? 3) What are the subsurface geochemical conditions that promote anammox? 4) To what extent is anammox dependent on, or competes with, other N-cycling reactions (denitrification and nitrification) for substrate(s)?

 

Microbial Regulation of Greenhouse Gas N2O Emission from Intertidal Oyster Reefs – Funded through NSF Grant This project aims to quantify N₂O fluxes and understand the regulatory factors of N₂O emissions from oyster reefs. Sedimentary N processes in oyster reefs will also be examined to develop an oyster reef N model to predict and estimate N₂O emission under varying conditions of water quality.   The goal is to estimate potential N₂O and N₂ fluxes from oysters and determine the environmental conditions affecting oyster N₂O production; define the microbial pathway responsible for oyster N₂O production; estimate N₂O emission and examine sedimentary N processes in oyster reefs subject to different water quality; and model the importance of oyster-mediated N cycling via N₂O and N₂ fluxes at the system level and assess the relative trade-offs associated with oyster restoration

 

Linking Organic Matter Composition to Shifting Baselines in the Coastal Sedimentary Nitrogen Cycle Global change models predict acceleration of the hydrologic cycle that results in increased organic matter and nitrogen loadings from continents into coastal and estuarine waters also subject to salinity intrusion from rising sea level.  How the coastal N-cycle responds to these pulsed scenarios has implications for coastal ocean productivity, resilience to eutrophication intensification, and export of N and carbon offshore.  This project considers direct reaction-scale controls on N biogeochemistry and indirect regulation of N reactions via other respiratory pathways through competition, stimulation and/or inhibition.  We examine the mechanisms controlling the balance between reactions that retain /deliver dissolved inorganic nitrogen (DIN) in sediments (mineralization, nitrification, dissimilatory nitrate reduction to ammonium-DNRA, N-fixation) and those that remove N from sediments (denitrification and anammox).  The multidisciplinary approach targets the role of OM source/chemical character on N cycling rates, N cycling gene expressions, and linkages to other respiratory pathways (e.g. sulfate reduction) through a series of laboratory and mesocosm experiments using geochemical, microbial, stable isotopic, and modeling techniques.

 

Defense Coastal/Estuarine Research Program (DCERP2) Most current assessments of carbon cycling in the coastal landscape are typically conducted piecemeal and are habitat specific. For example, atmospheric CO₂ fluxes or carbon burial rates might be measured in an intertidal marsh or estuary, but are quantified in the absence of measuring carbon exchanges between these habitats. This type of existing approach in which carbon cycling estimates are uncoupled from cross-habitat transport, or source determination, precludes full understanding of the source-sink nature of the habitat. This approach specifically confines interpretation to static mass balances and provides limited mechanistic understanding of underlying processes that are likely to drive altered patterns of carbon cycling in the future. Results from DCERP1, however, delineated the biogeochemical connections between coastal habitats within MCBCL and their sensitivity to physical drivers such as seasonal and pulsed delivery of freshwater and nutrients. This previous work also characterized the influence of tidal forcings on marsh-estuary exchanges, identified the effects of storm-driven overwash of the backbarrier marsh, and defined the linkages between land-use and watershed loadings. The system-scale knowledge acquired during DCERP1 helped shape the structure of how carbon cycling is approached in DCERP2. The DCERP2 approach equally weights intra-habitat mass balancing with inter-habitat exchanges to yield an integrated picture at the landscape scale. This “big picture” approach uniquely allows assessment of carbon (re)distribution at expanded spatial and temporal scales. In support of this effort the Tobias lab is :1) using select biomarker distribution and compound  specific isotope analyses (CSIA) to characterize carbon reservoirs in estuarine and marsh sediments in the New River Estuary, NC;  2) apply oxygen 18 isotope techniques to partition estuarine respiration into benthic vs. water column contributions; and 3) quantify the advective fluxes of dissolved carbon between marshes and adjacent open waters.

 

Tracking the Uptake, Translocation, Cycling, and Metabolism of Munitions Compounds in Coastal Marine Ecosystems Using Stable Isotopic Tracer – Funded through DOD/ SERDP Grant The explosives 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) are common munitions constituents that have left a legacy of contamination at numerous DoD facilities. Both compounds, and/or their derivatives, are EPA priority pollutants. Both compounds are persistent in the environment. Continued use of RDX and TNT in live fire ranges indicates the likelihood that environmental exposure is ongoing. Within the contiguous 48 United States, there are approximately 41 active DoD installations located within the coastal zone. Exposure of marine/estuarine ecosystems at some sites is well documented, while other installations have a high potential for exposure but limited data on RDX or TNT concentrations in marine endmembers. Coastal habitats are highly productive, nitrogen-limited, and economically valuable ecosystems. Their response to munitions compounds, and their effect on munitions cycling, persistence, bioaccumulation, and mineralization is largely unknown. We propose to quantify the pathways and rates of RDX and TNT processing in three typical coastal ecotypes: subtidal vegetated, subtidal unvegetated, and intertidal salt marsh. Four technical objectives are proposed: 1) What are the uptake rates of these compounds at the organismal to ecosystem scales, and which ecosystem components are important regulators of processing? ; 2) What ecosystem components act as zones of storage for munitions compounds vs. those that promote metabolism?; 3) Do ecosystem characteristics (e.g. mineralization, autotrophy, redox profile, trophic structure) relate to processing or accumulation of munitions compounds?; 4) What is the ecosystem-scale residence time, or clearance rate of these compounds? We propose to use stable isotope (15N) -labeled RDXand TNT in a series of whole-ecosystem tracer experiments designed to track the uptake,translocation, cycling, accumulation, and degradation of RDX and TNT in three characteristic coastalmarine ecotypes. This novel approach, used previously for tracking inorganic nitrogen on ecosystemscales, will be applied in a sequence of ecosystem mesocosms (ecocosms) experiments and thenapplied in situ at sites subject to chronic RDX and TNT exposure. Tracking the flow of 15N from theparent compounds into bulk biotic pools and as RDX or TNT compound specific pools willdistinguish between ecosystem compartments that store these compounds vs. those that promotemetabolism/mineralization. Isotope tracer modeling will be used to quantify specific uptake rates foreach organismal pool, total ecosystem clearance rate, and total capacity of the system to assimilateRDX and TNT.