EH 
An Ecological Assessment of the Louisiana Tensas River Basin Chapter 4 
m 
Nitrogen and Phosphorus Export to Streams 
Despite the many benefits, there is a potential negative 
impact of fertilizer application on agricultural fields. The 
problem was first identified decades ago as part of the 
study of lake eutrophication. Lake eutrophication is a 
process by which excess nutrients in lake water make it 
easier for undesirable plants to thrive, which in turn 
consume other resources and adversely affect lake water 
quality for other purposes. The potential effects of the 
export of nitrogen and phosphorus from farmlands to 
streams have been intensively studied for several de¬ 
cades. It is now possible to survey the scientific literature 
to determine how much nitrogen and phosphorus export 
can be expected for different types of land uses in 
different areas. 
A literature survey of North American nutrient export 
studies (Young and others, 1996, in the Journal of 
Environmental Management) provided coefficients for 
estimated export (kg/ha/yr) for nitrogen and phosphorus 
under different types of land uses. To estimate total 
nutrient export potential on the Tensas River Basin, the 
reported median coefficients for comparable agricultural 
uses were multiplied by the amount of land cover in the 
agriculture land cover classes. The coefficient-times-land 
use model was developed in 1980 for the United States 
Environmental Protection Agency by Rechow and others 
(US EPA 440/5-80-011, Washington, DC). The coeffi¬ 
cients reported for nitrogen varied from 2.6 to 6.2 kg/ha/ 
yr, with a median value of 3.9 kg/ha/yr. The values 
reported for phosphorus ranged from 0.3 to 1.5 kg/ha/yr, 
with a median value of 0.7 kg/ha/yr. 
The scientific literature provides a simple predictive 
model based only on nitrogen and phosphorus loadings 
to streams. Of course, this model does not reflect actual 
fertilizer application rates which determine local export 
amounts. However, over an area such as the Tensas 
River Basin, the models are valuable as a screening tool 
to rank subwatersheds based on potential impacts, 
assuming that average fertilizer rates are used through¬ 
out the region. In a nutshell, if there are no agricultural 
lands in a watershed, then fertilizer application is near 
zero. Such a watershed has less risk of impacts than a 
watershed for which 30 percent of the area is used for 
agriculture. One major drawback of this simple model is 
that it ignores fertilizer applications in urban areas, where 
areas such as lawns, gardens, and golf courses can 
receive heavy fertilizer doses several times a year. 
When this model was applied to the Tensas River Basin 
the potential nitrogen loading was 4.96 kg/ha/yr and the 
potential phosphorus loading was 1.34 kg/ha/yr. These 
are very high values when compared to values found 
elsewhere in the U.S. These values were calculated 
from the agricultural landuse data shown in the landcover 
data in Chapter 3 (Figure 3.4). These data show 
225,708 hectares of land used for intensive agriculture, 
which does not include grasslands for grazing, orchards, 
and vegetation in towns. This is 60% of the total land 
area which is consistent with the human use index 
shown in Chapter 3 (Figure 3.6). Again, this model only 
shows the amount of nitrogen and phosphorus that may 
be available for transport into the water system. 
