PARENT SESSION
Oral Session #41: Agroecology and Urban Ecology. Presiding: M. Liebman.
Wednesday, August 8, 2001. 8:00 AM to 12:15 PM. Hall of Ideas L&M.
Human and natural impacts on soil phosphorus accumulation in an urbanizing agricultural watershed.
BENNETT, ELENA1, CARPENTER, STEPHEN1, 1
ABSTRACT- Phosphorus (P), a limiting nutrient in lakes and primary cause of lake eutrophication, enters aquatic ecosystems mainly as nonpoint pollution from upland soils. P accumulating in watershed soils is increasing potential P runoff to surface waters worldwide. We know little about the spatial pattern of soil P accumulation, the causes of this pattern, or its effects on P runoff. Here, we asked which natural factors (e.g., soil type), historical factors (e.g., land use through time), and current management factors (e.g., fertilizer use) are significant predictors of soil P concentrations. We hypothesized that, at the watershed scale, the spatial pattern of predicting factors, and therefore also of P storage, may be best understood along an urban-rural gradient. We used a gradient definition based on driving time to the urban center and population density to map an urban-rural gradient across Dane County, Wisconsin and measured soil P concentrations at 500 sites along this gradient. While mean soil test P values were similar across the gradient (mean=43±22 ppm in urban areas and 74±51 ppm in rural areas), the variation increased significantly across the gradient from urban to rural sites, with a higher probability of encountering elevated P in rural areas. Current land use, historical land use, and soil type (r2= 0.57, p<0.001), rather than management techniques such as fertilizer application rates (r2=0.09, p<0.000), appear to be the dominant factors influencing this relationship.
KEY WORDS: phosphorus, nonpoint pollution, landscape, human impacts