Linking landscape management indices and water quality improvement in agricultural watersheds

Background/Question/Methods Food and fiber production systems now cover one quarter of the Earth's land surface. There is concern, however, about the environmental off-site impacts of agricultural activity on the integrity of natural ecosystems. Agricultural practices, for example, currently account for 70% of the nitrogen exported from of the Mississippi River Basin, with corn and soybean cultivation being the largest contributor.

The research presented here is part of an ongoing effort to examine the relationships between different land-use practices and nitrogen exports from agricultural watersheds in an effort to reduce the impact of agricultural activity on distant water bodies. Specifically, we quantify the effects of perennial farmlands, annual croplands, and a Cropland Diversity Index, on riverine nitrate-nitrogen (NN) concentrations from the beginning to the end of the 20^th Century in representative watersheds across the continental U.S. We also identify a spatial relationship between cropping systems and the Normalized Difference Vegetation Index (NDVI), or ‘greenness' index, for a given region.

Results/Conclusions We find that riverine NN concentrations throughout the 20^th Century are statistically related to the fraction of a watershed's area in annual crops, human population density, and cattle density (F = 12.02, p < 0.0001). This result indicates that agricultural land use had an impact on NN concentrations before commercial fertilizers were widely applied. Conversely, there is an inverse association between the fraction of a watershed's area in perennial farmland and the concentration of NN exiting the watershed (r² = 0.65, p < 0.0001).

A surprising result shows that the Cropland Diversity Index of an agricultural region is inversely related to NN concentrations exiting a watershed (r² = 0.45, p < 0.0001 ). This suggests that homogeneous croplands are responsible for a substantial proportion of the increase in NN export in the 20^th Century.

A spatial linkage is also demonstrated between the regional NDVI measurement of a landscape and its dominant agricultural cropping system. Annual croplands in the Midwest demonstrate a lower NDVI whereas similar regions dominated by perennial farmlands demonstrate a higher NDVI during the early spring and late fall.

In summary, the NN concentration of a watershed is directly related to the area of annual cropland and inversely related to the diversity of the cropland and the area of perennial farmland. These perennial farmlands generally demonstrate a higher ‘greenness' index and have implications for how we might effectively manage soil and water resources.