Geology topics

C. Tobias

Publications and source records attributed to C. Tobias.

3 recordsLinked to original sources

USGS2015

Role of anaerobic ammonium oxidation (anammox) in nitrogen removal from a freshwater aquifer

Anaerobic ammonium oxidation (anammox) couples the oxidation of ammonium with the reduction of nitrite, producing N 2 . The presence and activity of anammox bacteria in groundwater were investigated at multiple locations in an aquifer variably affected by a large, wastewater-derived contaminant plume. Anammox bacteria were detected at all locations tested using 16S rRNA gene sequencing and quantification of hydrazine oxidoreductase ( hzo ) gene transcripts. Anammox and denitrification activities were quantified by in situ 15 NO 2 – tracer tests along anoxic flow paths in areas of varying ammonium, nitrate, and organic carbon abundances. Rates of denitrification and anammox were determined by quantifying changes in 28 N 2 , 29 N 2 , 30 N 2 , 15 NO 3 – , 15 NO 2 – , and 15 NH 4 + with groundwater travel time. Anammox was present and active in all areas tested, including where ammonium and dissolved organic carbon concentrations were low, but decreased in proportion to denitrification when acetate was added to increase available electron supply. Anammox contributed 39–90% of potential N 2 production in this aquifer, with rates on the order of 10 nmol N 2 –N L –1 day –1 . Although rates of both anammox and denitrification during the tracer tests were low, they were sufficient to reduce inorganic nitrogen concentrations substantially during the overall groundwater residence times in the aquifer. These results demonstrate that anammox activity in groundwater can rival that of denitrification and may need to be considered when assessing nitrogen mass transport and permanent loss of fixed nitrogen in aquifers.

Environmental Science & Technology↗

USGS2006

Denitrification across landscapes and waterscapes: A synthesis

Denitrification is a critical process regulating the removal of bioavailable nitrogen (N) from natural and human-altered systems. While it has been extensively studied in terrestrial, freshwater, and marine systems, there has been limited communication among denitrification scientists working in these individual systems. Here, we compare rates of denitrification and controlling factors across a range of ecosystem types. We suggest that terrestrial, freshwater, and marine systems in which denitrification occurs can be organized along a continuum ranging from (1) those in which nitrification and denitrification are tightly coupled in space and time to (2) those in which nitrate production and denitrification are relatively decoupled. In aquatic ecosystems, N inputs influence denitrification rates whereas hydrology and geomorphology influence the proportion of N inputs that are denitrified. Relationships between denitrification and water residence time and N load are remarkably similar across lakes, river reaches, estuaries, and continental shelves. Spatially distributed global models of denitrification suggest that continental shelf sediments account for the largest portion (44%) of total global denitrification, followed by terrestrial soils (22%) and oceanic oxygen minimum zones (OMZs; 14%). Freshwater systems (groundwater, lakes, rivers) account for about 20% and estuaries 1% of total global denitrification. Denitrification of land-based N sources is distributed somewhat differently. Within watersheds, the amount of land-based N denitrified is generally highest in terrestrial soils, with progressively smaller amounts denitrified in groundwater, rivers, lakes and reservoirs, and estuaries. A number of regional exceptions to this general trend of decreasing denitrification in a downstream direction exist, including significant denitrification in continental shelves of N from terrestrial sources. Though terrestrial soils and groundwater are responsible for much denitrification at the watershed scale, per-area denitrification rates in soils and groundwater (kg N·km −2 ·yr −1 ) are, on average, approximately one-tenth the per-area rates of denitrification in lakes, rivers, estuaries, continental shelves, or OMZs. A number of potential approaches to increase denitrification on the landscape, and thus decrease N export to sensitive coastal systems exist. However, these have not generally been widely tested for their effectiveness at scales required to significantly reduce N export at the whole watershed scale.

Ecological Applications↗

USGS2003

Sediment DIN fluxes and preferential recycling of benthic microalgal nitrogen in a shallow macrotidal estuary

Sediment -water fluxes of NH 4 + , NO 3 - , dissolved inorganic carbon, and O 2 were measured in cores collected from the upper Rowley River estuary , Massachusetts, and used to calculate rates of organic nitrogen (N) mineralization, nitrification, and coupled and direct denitrification (DNF). The cores contained 15 N label in benthic microalgae (BMA) and in NO 3 - in the overlying water as a result of an ongoing whole- estuary 15 NO 3 - enrichment study (NISOTREX II). The tracer allowed for estimation of gross NO 3 - regeneration in sediments and the contribution of BMA derived N to total mineralization. The mean mineralization rate between sites was 16.0 ± 2.0 mmol N m -2 d -1 . Approximately 13 to 56% of the mineralized N was nitrified at rates ranging from 1.8 to 10.1 mmol N m -2 d -1 . Total denitrification was dominated by direct DNF (3.6 mmol N m -2 d -1 ) furthest upstream, where NO 3 - concentrations were highest. Coupled DNF was most important (8.0 mmol N m -2 d -1 ) in the sediments with high nitrification and low water column NO 3 - . A gross NO 3 - flux from sediments to water of 0.9 to 2.1 mmol N m -2 d -1 was estimated from the isotope dilution of δ 15 NO 3 - in the overlying water of the cores. The isotope dilution seen in the cores was also detected as a deviation from conservative δ 15 NO 3 - mixing along estuarine transects. Incorporation of this NO 3 - regeneration into the DNF calculations effectively increased the estimate of direct DNF by up to 50% and decreased the coupled DNF estimate by up to 220%. Increasing δ 15 NH 4 + in the water of the cores indicated that the 15 N-labelled BMA were preferentially mineralized over bulk sediment organic N. Additional 15 N enrichments in the sediment bacterial biomarker diaminopimelic acid showed a link among 15 N-labeled BMA, active bacteria, and 15 NH 4 + released to the overlying water. Based on δ 15 NH 4 + enrichments in the cores, BMA accounted for approximately 50 to 100% of the N mineralized. An isotopic enrichment of δ 15 NH 4 + above background in the estuary was observed at a magnitude consistent with the core-based rates of BMA mineralization. These results provide further evidence that BMA are not unidirectional sinks for water column-dissolved organic nitrogen , but instead act to turn over N between sediments and estuarine water on the scale of days.

Marine Ecology Progress Series↗