1. The Study of Excess Nitrogen in the Neuse River Basin

“A Landscape Level Analysis of Potential Excess Nitrogen in East-Central North Carolina, USA”

Paper by C.T. Garten Jr. and T.L. Ashwood. 2003. Water, Air, and Soil Pollution, 146. (1-4): 3-21

Presented by Harrison Miller, Environmental Studies, Geog 370, 26 February 2008

2. Assessment of potential excess nitrogen under different land cover categories

Problem: Excess nitrogen (N) contributes to aquatic eutrophication (excessive nutrients in a body of water that cause various problems) and elevated concentrations of nitrate in ground water. This study attempted to understand the distribution, the determinants, and the seasonality of potential excess N in terrestrial ecostystems at the landscape scale.

Hypothesis: If N inputs exceed N outputs, then the difference represents N at risk of loss from the landscape to surface receiving waters and groundwaters.

3. Methods

1.) Acquistition of land cover data from remote sensing 2.)Development of a mass balance model to quantify potential

excess N unders different land covers 3.) Estimation of N fluxes under different land covers based

on literature reviews 4.) Calculation of potential excess N on a seasonal basis in

the Neuse River Basin

4. Results

N uptake by crops, herbaceous vegation (grasses), and herbaceous wetlands were similar and greater than the median value of N uptakes by forests.

Differences between rates of net N mineralization in forest and grassland soils were statistically significant.

Forest ecosystems are weak N sinks whereas wetlands are strong N sinks.

The highest rates of soil N mineralization and denitrification were concentrated in areas bordering streams of the Neuse River.

The potential excess N in the Neuse River Basin is most influenced by spatial and temporal variation in fertilization and net soil mineralization.

5. Conclusions

Large areas of land in the Neuse River Basin are borderline in potential excess N. Such areas could not be classified as either N sources or N sinks because the estimated N budget was in balance. However, with future changes in land use, these sensitive areas could become N sources and N sinks.

At the landscape scale, the timing of N inputs (i.e. spreading fertilizer) and outputs are an important determinant of potential excess N.

Potential excess N on the landscape is highest in spring and summer.

The seasonal patterns in N loads in the Neuse River depends on location. Annual N fluxes at stream monitoring stations vary considerably, depending on the year.

Criticisms

The model used in this study does not distinguish between soil ammonium and nitrate. The model also assumed that all excess N is at risk of export from the landscape. These assumptions are problematic because they will overestimate N loads to surface receiving waters because nitrate is more readily leached from soil than ammonium.

The model does not distinguish between different chemical forms of N and it does not discuss hydrologic transport of N.

Measures of N export in rivers and streams may not be the best way to determine how well the model predicts potential excess N at the landscape scale. The in-stream loss of N exports from the ground can be substantial. Some studies find that the terrestrial landscape retains most of the N imported to the Neuse River Basin or that it is removed by processes occurring naturally in the river.