Nitrogen Cycling in Freshwaters III
Presiding: J Tank, University of Notre Dame; S Findlay, Institute of Ecosystem Studies
NB23E-01 13:30h
Linking Denitrification to Channel Geomorphology in Agricultural Streams
Agricultural streams in east-central Illinois have elevated nitrate concentrations (10 to 20 mg N L-1) contributing to greater denitrification rates in sediments. However, reduced retention times and extensive stream channelization limit the effectiveness of denitrification as a removal mechanism for water column nitrate. In this study, we compared geomorphic controls of denitrification in six channel reaches varying in geomorphology, organic matter, and nitrate concentration. Denitrification rates were measured monthly between June 2003 and February 2005 using the acetylene inhibition procedure (chloramphenicol was added to control for denitrifier growth). We found in meandering and channelized reaches of greater nitrate concentrations and fine substrate, denitrification rates averaged 19 mg N m-2 h-1. In areas of lesser nitrate concentrations and coarse substrates, denitrification rates were comparably lower at 3.0 mg N m-2 h-1 in meandering reaches and 0.80 mg N m-2 h-1 in channelized reaches. Our study concluded that denitrification rates were strongly linked to geomorphic variability, organic matter, substrate composition, and nitrate concentration. A strong geomorphic connection appeared with substrate composition showing that greater nitrogen removal in channel conditions of fine substrate are more effective.
NB23E-02 INVITED 13:45h
In-situ Investigations of N Transformations in Coastal Massachusetts Streams
As part of the LINX II experiments, 15NO3 was added to streams in forest, agricultural, and suburban areas in northeastern Massachusetts to determine the role of land use on nitrogen export from small watersheds. Denitrification was measured in-situ by increasing the stream 15NO3 to 20,000 permil for 24 hours. Dissolved gas samples analyzed for 15N2 showed stream enrichments of up to 60 permil in suburban and agricultural streams. Agricultural streams showed enrichments of 12,000 permil of 15N2O . Although denitrification rates were much higher in suburban and agricultural streams, nitrate uptake rates were highest in the forested streams. Uptake in forested streams resulted from storage in benthic compartments rather than removal by denitrification.
NB23E-03 14:00h
The Influence of Leaf Fall and Organic Carbon Availability on Nitrogen Cycling in a Headwater Stream
The study of allochthonous carbon has a long and distinguished history in stream ecology. Despite this legacy, relatively little is known regarding the influence of leaf litter on nutrient dynamics. We conducted 15N-NO3 tracer additions to a headwater stream in upstate New York before and after autumn leaf fall to assess the influence of leaf litter on nitrogen spiraling. In addition, we amended the stream with labile dissolved organic carbon (as acetate) midway through each experiment to examine whether organic carbon availability differentially stimulated nitrogen cycling. Leaf standing stocks increased from 53 to 175 g dry mass m-2 and discharge more than tripled (6 to 20 L s-1) between the pre- and post-leaf fall period. In contrast, nitrate concentration fell from approximately 50 to less then 10 ug L-1. Despite higher discharge, uptake length was shorter following leaf fall under both ambient (250 and 72 m, respectively) and DOC amended (125 and 45 m) conditions. Uptake velocity increased dramatically following leaf fall, despite a slight decline in the areal uptake rate. Dissolved N2 gas samples were also collected to estimate denitrification rates under each experimental condition. The temporal extent of increased nitrogen retention will also be explored.
NB23E-04 INVITED 14:15h
Land-use Impacts on Nitrate Removal and Retention in six Kansas Streams
We used short-term 15N-nitrate tracer additions to determine the rates of nitrogen transformations in six low-order Kansas streams draining prairie, agricultural, and urban lands. Baseline nitrate concentrations ranged from 0.001 to 3 mg/L nitrate-N, and were highest in the two urban streams and lower in the agricultural and prairie streams. A linear relationship existed between stream nitrate concentration and uptake rate (p > 0.001), uptake length (p > 0.001) and nitrification (p > 0.001). Denitrification made up only a small portion of uptake in all of the streams. These relationships do not suggest a saturation of nitrate uptake within the range of concentrations encountered in this study. Regeneration of tracer DIN was minimal after 1, 3, and 7 days. Increases in uptake in the urban streams appear to be attributable to increased autotrophic assimilation and denitrification. Land-use within the watersheds influenced stream nitrate concentration and subsequently the rates of nutrient cycling within the streams.
NB23E-05 14:30h
Comparison of Whole-stream and Hyporheic-zone Estimates of Denitrification Determined Simultaneously During an Isotope Tracer Injection in a Nitrate-Rich Stream
15N labeled nitrate is increasingly being used as a reactive tracer in stream tracer tests to estimate whole-stream denitrification averaged at a spatial scale large enough to allow comparisons across disparate stream ecosystems. No matter how valuable, these whole-stream estimates are not very informative about controlling processes and will have limited transfer value unless processes controlling denitrification are investigated simultaneously at finer scales. Insights about the processes that influence the whole-stream rates could be especially informative if simultaneous rate measurements are made representing variable hydrologic and biogeochemical conditions near reactive surfaces in the stream and in the streambed. Our approach was to investigate factors that control denitrification by simultaneously measuring denitrification in-situ in a variety of streambed environments by sampling evolution of the (15NO3-) tracer during transport through shallow hyporheic flow paths. Here we report results from two tracer studies conducted in Sugar Creek, western Indiana, in a basin dominated by corn and soybean agriculture. The two tracer experiments were conducted in September 2001 and September 2003, when streamflows (40 and 20 L s-1) and stream NO3- concentrations (70 and 175 Μmoles L-1) in the two reaches were near their annual minimum values. The experiments involved co-injection of conservative (Br), reactive (15NO3-), and dissolved gas (SF6) tracers into streamflow allowing quantification of advection, dispersion, gas evasion, hydrologic retention in "storage" zones, and also allowing in-situ estimation of denitrification within selected hyporheic flow paths. The experiments resulted in estimates of both whole-stream and hyporheic-zone rates of denitrification and related nitrogen reactions. The streambed of Sugar Creek is covered in most areas with a relatively thin layer (ranging from <1 to 3 cm) of fine granular and organic sediment and periphyton, overlying a thicker layer (30 to 100 cm) of relatively coarse granular sediment with a median grain size of 2 mm. In-situ sampling in the streambed was conducted simultaneously with the tracer injection using the USGS MINIPOINT sampler to withdraw small-volume water samples from depths ranging between 1.5 and 15 cm in the hyporheic zone at 6 locations along the two stream reaches. A total of 22 in-situ estimates of denitrification were derived from those samples. Zero-order denitrification rates in the hyporheic zone ranged broadly across two orders of magnitude (2 to 230 Μmoles L-1 h-1) with a trend of decreasing rates with increasing depth below the streambed. The hyporheic-zone dentrification rates (54 to 430 Μmoles m-2 h-1) bracketed the reach-scale rates (120 to 305 Μmoles m-2 h-1), demonstrating that denitrification in hyporheic flow paths significantly contributed to whole-stream denitrification in this nitrate-rich Midwestern stream.
http://water.usgs.gov/nrp/jharvey/site/index.html
NB23E-06 INVITED 14:45h
Factors Controlling Denitrification and NO3 Uptake in Three Tropical Puerto Rican Streams Using a 15N Tracer Approach.
An intensive study that was part of the LINX II project was conducted to determine nitrogen transformations of 3 low-order streams with contrasting land use. Short term (24-hour) additions of K15NO3 and NaBr were performed on forested, urban, and agricultural streams to determine anthropogenic impacts on N retention in tropical streams. Stream water and organic matter samples were collected before, during, and after the experiment. We hypothesized that there would be greater uptake into biomass and higher denitrification (15NO3 to 15N2 and 15N2O) in streams with greater human impacts and stream nitrate concentrations. Background nitrate concentrations ranged from 148 ug N/L in the forested stream to 259 ug N/L in the agricultural stream to 875 ug N/L in the urban stream. Denitrification to 15N2 was undetectable in the forest stream, was small in the agricultural stream, and was highest in the urban stream. Denitrification to 15N2O was detectable in all 3 streams and followed the same increasing pattern with ambient nitrate concentrations. Incorporation of 15N into biomass was lowest at the forest stream, intermediate at the agricultural stream, and highest at the urban stream. Nitrate loading associated with change in land use resulted in greater denitrification and uptake in biomass.