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2003 Progress Report: Attenuation of Non-Point Source Nitrogen Pollution in a Coastal Watershed: Implications for Nutrient ManagementEPA Grant Number: R830652
Title: Attenuation of Non-Point Source Nitrogen Pollution in a Coastal Watershed: Implications for Nutrient Management
Investigators: Piehler, Michael F. , Band, L. R. , Paerl, Hans , Whalen, Stephen C.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Sergeant, Anne
Project Period: January 20, 2003 through January 19, 2006 (Extended to January 19, 2007)
Project Period Covered by this Report: January 20, 2003 through January 19, 2004
Project Amount: $737,555
RFA: Nutrient Science for Improved Watershed Management (2002) RFA Text | Recipients Lists
Research Category: Water , Water and Watersheds
The objectives of this project are to provide detailed field and experimental data on transformations of nitrogen (N) in agricultural and forested lands. These data will then be organized into a process-based spatial model to predict the quantity and form of N delivered to the watershed.Progress Summary:
A primary component of this project is a nitrogen mass balance for representative forest and agricultural ecosystems in the lower Neuse River Watershed. Both watersheds are located in the South River Basin, which is a tributary to the Neuse River Estuary (NRE). A distinct advantage in studying these two watersheds was their proximity to each other and to the NRE.
Automated water samplers (Isco, Inc.) were installed at the headwaters to SWC at Open Grounds Farm (OGF) and at two branches of Big Creek (Big A and Big B) in June 2003. All Isco, Inc., samplers were equipped with flow sensors, rain gauges, and YSI, Inc., multiparameter sondes for measuring dissolved oxygen, temperature, specific conductivity, and pH. Water samples were collected weekly at the three sites for analysis of nutrients (ammonium [NH4], nitrate [NO3], phosphorous [P], total N); elemental analysis of particulate carbon (C), hydrogen and N; and chlorophyll a. Nutrient loads were calculated using daily discharge data at each sampling site and concurrent measurements of nutrient concentration. A critical component of this project is characterization of the delivery of nutrients to receiving waters during storm events. To quantify these loads, Isco, Inc., samplers collected water samples at flow-paced intervals during periods of increased flow velocity following rain events of greater than 1.5 inches.
Overall, instream nutrient concentrations were higher in the stream draining the agricultural watershed than the forest. In SWC, NO3 and phosphate (PO4) concentrations were higher during wet weather, which was indicative of increased nutrients in stormwater runoff. Dry weather concentrations were higher at the forest (Big A and Big B), indicating that stormwater was acting to dilute instream nutrients.
Groundwater sampling sites were identified in agricultural and forested watersheds. The agricultural fields have been selected to include two water management regimes. One maintains a high groundwater level through the use of flashboard risers, and a neighboring field has a lowered water table by eliminating the flashboard risers. Monitoring wells were installed in transects across each permanent sampling plot perpendicular to cross-site drainage ditches, and they are sampled weekly for nutrient analysis. Replicate soil cores were collected in each sampling site to a depth of 50 cm. These were sectioned into 10-cm increments, and currently are being assayed for soil physicochemical and microbiological properties: bulk density, particle density, pH, moisture content, NO3, NH4, organic content, total N, total C, nitrifier enzyme activity, and denitrifier enzyme activity.
At the time of this report, fertilization and planting were started on the farm. At the time of fertilization, soil cores were collected at all sites to a depth of 20 cm for N analysis. Passive diffusers were installed in the fields, and they are being sampled at 1- to 2-day increments for ammonia volatilization. This will continue for 2 weeks post fertilization. Soil samples will be collected weekly for nutrient analysis and at longer intervals (depending on the analysis) for indices of microbial activity, including net N mineralization, net nitrification, denitrifier enzyme assay, and nitrifier enzyme assay. We intend to take additional samples during subsequent fertilizations (late April and late May), which will involve experimental addition of urease and nitrification inhibitors to selected fields.
Denitrification in Headwater Stream Sediments
Denitrification (biological transformation of NO3 to dinitrogen gas) is important in both watersheds because it permanently removes N from the system. Other retention mechanisms, including microbial respiration, microalgal photosynthesis, and uptake by aquatic macrophytes, may retard the release of N, but do not decrease total N exported from the watershed. High biological activity, N and C in the sediments, and increased retention time of surface water resulting from stream length and flat topography in the shallow headwater streams create optimal conditions for denitrification.
Rates were measured along a transect from upstream to downstream in the forested and agricultural watershed on a seasonal basis to determine both spatial and temporal variability. Rates obtained in the winter were lower than the fall, likely because of decreased biological activity at lower temperatures. Variability was high at all sites in both watersheds because of the heterogeneous nature of the sediments and the potential for high pockets of denitrifying bacterial activity. Although concentrations of NO3 varied quite considerably between the two land uses, with NO3 being much higher at the farm, denitrification rates were similar. During the collection of sediment cores for denitrification rate measurements, sediment samples also were collected for analysis of total C and N content. High denitrification rates were correlated closely with elevated C content; however, not all sediments with high C exhibited high denitrification rates, indicating other controlling factors. Future experimental work will be focused on these controlling factors, including temperature, N, C, oxygen, and macrophyte coverage.
Nutrient Uptake in Headwater Streams
Retention of river-borne inorganic N and P by stream ecosystems can be a significant fraction of a watershed's total nutrient budget. Heterotrophic microbial respiration, microalgal photosynthesis, and uptake by aquatic macrophytes are the primary mechanisms of biotic retention of inorganic N and P. These processes provide a temporary reservoir for inorganic nutrients, and will eventually export N and P downstream in a more recalcitrant, organic form.
Agricultural streams were located upstream of the SWC station. Slocum Creek, draining a portion of the Croatan National Forest, was chosen as representative of a coastal plain, forested stream. The Big A and Big B sites could not be assessed because of the unsteady flow regime of these streams. Fourteen experiments were conducted in five streams using standard methods for measuring whole-stream nutrient uptake. Briefly, a solution of NH4Cl, KH2PO4, and NaCl (a conservative tracer) was injected into the head of an experimental stream reach at a constant rate for approximately 1 hour. Specific conductivity (a proxy for NaCl concentration) was recorded continually at the terminus of the reach. Once conductivity measurements reached a plateau, a series of water samples were collected for nutrient analysis at three to four points along the reach. Nutrient samples were filtered (0.7 µm) and analyzed for NH4, NO3, and PO4. Nutrient uptake rates were derived from the regression of the normalized nutrient concentrations along the stream reach. Studies were conducted from July through October 2003.
Of 14 nutrient addition experiments, significant trends in NH4 uptake were found in 6 of the experiments, and significant trends in PO4 uptake were found in 4 of the experiments. Nutrient uptake rates could not be calculated for the remainder of the experiments (including all experiments in Slocum Creek) because variance between replicate samples was greater than the longitudinal trend in concentration change.
NH4 uptake rate ranged from 2-403 µg N m-2 min-1. PO4 uptake rate ranged from 43 µg P m-2 min-1-842 µg P m-2 min-1. Uptake length (the distance a nutrient traveled in the water column prior to uptake) of PO4 was shorter in the ditch than the canals studied. NH4 uptake length had a larger range than PO4, with values ranging from 15-250 m and 21-175 m for NH4 and PO4, respectively.
With only one exception (Whitewater Canal), the uptake rates of NH4 at all sites were higher than the median uptake rate (21 µg m-2 min-1) of other streams reported in the literature, but were within one standard deviation of the average of the literature values (120 + 499 µg m-2 min-1). PO4 uptake rates were larger than many others reported in the literature. Two of the four PO4 uptake rates found in the agricultural streams exceeded one standard deviation of the mean uptake rate (30 + 82 µg m-2 min-1) of streams reported in the literature.Future Activities:
We will continue to investigate the magnitude of N nutrient pools, as well as the rates of transformation among pools and ecosystem imports and exports in both forested and agricultural watersheds, as described in the initial proposal. Budgetary expenditures for the reporting period were in line with the level of work that has been completed.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
|Other project views:||All 23 publications||8 publications in selected types||All 8 journal articles|
|| Poe AC, Piehler MF, Thompson SP, Paerl HW. Denitrification in a constructed wetland receiving agricultural runoff. Wetlands 2003;23(4):817-826.
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