Nitrogen Cycling in Freshwaters II
Presiding: P Mulholland, Oak Ridge National Laboratory; D Scott, U.S. Geological Survey
NB22E-01 10:30h
The Influence of Sandbar Vegetation on Hyporheic Nitrogen Processing
High nitrate concentrations due to agricultural runoff are common in Midwestern rivers. While floodplains can facilitate nitrate removal through denitrification, only a small proportion of riverine flow reaches floodplains. Conditions required for denitrification also occur within in-channel sandbars, which contact river flow year round. In this study, we addressed the role of vegetated and unvegetated sandbars in nitrogen processing. Sixteen sandbars (half with vegetation, half without) were sampled along the southern Wisconsin River in July, August and September of 2004. On each sandbar, wells were installed to the water table at 5 m intervals along the length of each bar. Water samples from each well were filtered and analyzed for nitrate and ammonium. Nitrate declined significantly below riverine concentrations in all vegetated sandbars but was variable in unvegetated bars. Ammonium concentrations were significantly higher than riverine concentrations in all sandbars, with no significant difference due to vegetation. The presence of sandbar vegetation facilitated nitrate loss, potentially fueling denitrification through carbon subsidies. Nitrate declines coupled with increased ammonium concentrations suggest that dissimilatory nitrate reduction to ammonia (DNRA) was also an active process in sandbars. Therefore, vegetation stimulated nitrate decline in riverine sandbars, likely due to a combination of denitrification and DNRA.
NB22E-02 10:45h
Nitrogen Dynamics in Hyporheic Zone Sediments
Understanding uptake and transformation of NO3 in hyporheic sediments in streams with varying levels of anthropogenic N inputs addresses a basic question of how human disturbance (urbanization and agriculture) affects these processes. Co-injections of bromide and 15N-NO3 were conducted over 24 hours in multiple streams (native, urban, and agricultural areas) in the southwestern USA. Groundwater wells were sampled along a longitudinal gradient within the15N injection sites. Water and gas samples were taken prior to, during, and after injections and were analyzed for O2, major cations and anions, DOC and 15N (gaseous and aqueous). Transient storage parameters were calculated using OTIS-P and As/A values ranged from 4.04E-01 to 3.92E-03 and alpha (storage zone exchange coefficients) ranged from 3.35E-04 to 6.35E-05 sec-1 in native and impacted sites, respectively. Wells contained >80% surface water at three sites with varying N-loads. These wells also had higher enrichment levels of 15N for various transformation products of NO3 than stream water. These data suggest that hyporheic zones may be a significant site of uptake and processing of NO3 in human disturbed streams.
NB22E-03 INVITED 11:00h
Nitrogen-Sulfur Coupling in Freshwater Sediments: Implications for Nitrate Removal by Wetlands, Lakes and Streams
Nitrate disappearance in sediments is usually assumed to be due to respiratory denitrification, an important process for improving water quality. While performing tracer experiments in aquatic sediments of Michigan, we found that nitrate removal coincided with sulfate production, and that sulfate removal commenced only after nitrate was depleted. Because this activity appears to be biological, we hypothesize that the nitrate removal is due to sulfur oxidizing bacteria that use nitrate to oxidize reduced sulfur forms to sulfate. Sulfur oxidizers do not necessarily denitrify nitrate to dinitrogen, but may instead produce ammonium via a form of dissimilatory nitrate reduction to ammonium (DNRA). Push-pull experiments in streams, lakes and wetlands revealed a persistent pattern of sulfate production during nitrate removal. The data also suggest that nitrate introduction is associated with ammonium production and sulfide removal when compared to controls (bromide introduction only). If this is a form of DNRA, nitrate removed via this pathway would remain in a biologically available form, in contrast to respiratory denitrification. The sulfur-oxidizing pathway of nitrate removal has important implications for water quality by coupling nitrate removal to sulfur cycling.
NB22E-04 11:15h
Denitrification Rates in a Midwestern Stream Containing High Nitrate: In Situ Assessment Using Tracers in Dome-Shaped Incubation Chambers
The extent to which in-stream processes alter or remove nutrient loads in agriculturally-impacted streams is critically important to the function of a watershed and the delivery of those loads to coastal waters. Accurate assessment and characterization of key biogeochemical processes, such as nitrification and denitrification, are needed for establishing links between land use and watershed response and for developing predictive tools for agricultural management practices. In this study, patch-scale rates of in-stream benthic processes were determined within a second order stream reach (Sugar Creek, IN) dominated by drainage from tiled, row-crop fields. Rates of denitrification, nitrification, and net oxygen production/consumption were quantified using open-bottomed incubation chambers (0.6 m diam. acrylic hemispheres inserted 5-10 cm into the stream-channel sediment) that were fitted with water sampling/mixing ports, a volume compensation bladder, and pore-water piezometers. Incubations were conducted by injecting tracers (NaBr as a conservative tracer, along with 15N-enriched nitrate, nitrite or ammonium) into the chambers and collecting samples at 1-5 hr intervals for up to 48 hrs. Also assessed were in situ responses to increased nitrate concentrations, light vs. dark incubations, rates in various sediment types, and companion incubations during a reach-scale, in-stream tracer test with 15N-enriched nitrate. Overall, chamber denitrification rates ranged from about 25 to 155 Μmol N m-2 hr-1 for nitrate concentrations from 90 to 1300 ΜM and were in general agreement with modeled rates from the reach-scale test. Increased nitrate concentrations resulted in increased denitrification rates, increased nitrite and nitrous oxide fluxes, and decreased methane fluxes. Oxygen consumption rates and nitrate loss rates based solely on concentration changes were much more variable than denitrification rates based on the isotope tracer results. Measured rates of nitrification were relatively low, even with added ammonium. The chamber incubations confirmed earlier studies that denitrification was a substantial sink for nitrate in this watershed and provided additional information about in situ responses to experimental manipulations.
NB22E-05 11:30h
Denitrification in an Urban Greenbelt: Contrasting Aquatic and Terrestrial Patches
The Indian Bend Wash flood-control project relies on a greenbelt to safely convey floods through Scottsdale, AZ. A chain of shallow artificially maintained lakes sitting in a larger, protected floodplain of irrigated turf grass characterizes the greenbelt. We conducted a series of experiments exploring how the creation of novel patch types affected denitrification in the wash. DEA analyses on sediments collected from eight lake and six stream segments as well as soil samples from eight floodplain transects demonstrated that mass-specific potential denitrification rates were significantly higher in lakes than in streams or floodplains. Nutrient limitation bioassays revealed that NO3-N limited denitrification in lake sediments while floodplain soils were limited by the availability anaerobic conditions created by water additions. Although rain events are rare in the desert, irrigation is common and we conclude that annual denitrification in the floodplain can be substantial. Because lake water NO3-N concentrations often exceed 1 mg/L, the finding that NO3-N limited denitrification in the sediments was surprising. However, further experiments using intact cores demonstrated that it was not so much the availability of NO3-N in the overlying water as the rate the nitrate diffused into the sediments that limited denitrification in the lakes.
NB22E-06 11:45h
Denitrification in Agriculturally Influenced Coastal Plain Streams
Agricultural runoff from coastal plain watersheds contributes nitrogen to downstream estuarine and coastal waters. Nitrogen fuels eutrophication, which has resulted in increased algal biomass, hypoxia, and fish kills in the Neuse River Estuary, North Carolina. Denitrification is the sole mechanism of permanent nitrogen removal along the riverine to estuarine continuum, but its contribution to nitrogen attenuation in this system is not well understood. Denitrification rates measured seasonally in stream bed sediments were variable but showed a distinct spring maximum, which was likely associated with rising temperatures and added nitrogen from fertilizer application (0-150 umol N m-2 h-1 during the summer, fall and winter and 150-300 umol N m-2 h-1 in the spring). Reach-scale uptake experiments showed the potential for 65-98% retention of the watershed DIN load in ephemeral drainage ditches. Results indicated that nitrogen retention was high despite low hyporheic exchange that is typically associated with channelized streams with low gradients, straight channels and homogenous stream bed sediments. Comparison of direct denitrification rate measurements to reach scale uptake rates and a watershed mass balance showed considerable potential for nitrogen removal via denitrification in agricultural stream sediments.