226 Nutrient Levels in the Yazoo River Basin, Mississippi F. Douglas Shields, Jr.* USDA-ARS National Sedimentation Laboratory, doug.shields@ars.usda.gov Sam Testa, III USDA-ARS National Sedimentation Laboratory, sam.testa@ars.usda.gov Charles M. Cooper USDA-ARS National Sedimentation Laboratory (ret.), charles.cooper@ars.usda.gov Abstract High nitrogen (N) and phosphorus (P) loadings to aquatic ecosystems are linked to environmental problems including harmful algal blooms and hypoxia. Presented is an assessment of accessible data on nutrient sources, sinks and inputs to streams within the Yazoo River Basin of northern Mississippi. Accessible data included published research and data reposing either in our laboratory, or in online data sets maintained by the U.S. government agencies. Nutrient transport differs widely for the two major physiographic regions of the basin (Delta and Hills). Our search produced total N values for 72 sites with periods of record ranging from 3.3 to 28.6 years. The global mean (mean of site means) total N concentration for the Delta was 3.3 mg/L but only 1.2 mg/L for the Hills, about two to four times higher than US EPA criteria. Total P data were found for 121 sites with periods of record ranging from 3.2 to 28.6 years. Delta mean P concentrations were inversely proportional to contributing drainage area, while Hill sites were not. The Hill mean P concentration was 0.15 mg/L while the mean for Delta sites was more than four times greater, 0.66 mg/L. These values are about 4-5 times the levels set as criteria by the US EPA for these ecoregions. Delta concentrations peak strongly in spring when agricultural fertilizers are applied. Concentrations of N and P in Hill streams do not exhibit seasonal trends, but tend to be one half to one fourth as great as the Delta levels. Keywords: Nitrogen, phosphorus, nonpoint source pollution, hypoxia, Yazoo River Introduction Productivity cycles in disturbed aquatic ecosystems are often dominated by anthropogenic inputs of N and P (Carpenter et al. 1998, Caraco and Cole 1999). Human impacts on the global N cycle are significant, primarily due to the use of synthetic fertilizer, which accounts for more than half of the human alteration of the N cycle. Overall, human fixation of atmospheric N during fertilizer manufacture, combustion of fossil fuel, and production of legumes increased globally by a factor of 2 to 3 between 1960 and 1990, thus contributing significantly to nonpoint source flows of N (Howarth et al. 2002, Committee 2000). Agricultural sources contribute more than 70% of the N and P delivered to the Gulf of Mexico by the Mississippi and Atchafalaya Rivers (Alexander et al. 2008). Corn and soybean cultivation is the largest contributor of N (52%), followed by atmospheric deposition (16%), while P originates primarily from animal manure (37%) and from fertilization of corn and soybeans (25%). Because of the detrimental results of over enrichment, nutrient transport from the continental United States into coastal waters is of great concern. Between 1960 and 2000 the flux of N from the Mississippi River into NSL - 50 Years of Soil & Water Research in a Changing Agricultural Environment Back to Contents