Trends in Agricultural Sources of Nitrogen in the Altamaha River Watershed

We compiled USDA Census of Agriculture data on livestock production and agricultural land use along with USGS estimates of fertilizer use to evaluate trends in agricultural sources of nitrogen (N) to the watershed of the Altamaha River. Between 1954 and 2002, the estimated contribution of N from the three major livestock crops (cattle, chickens, and pigs) remained fairly stable, averaging approximately 1,000 kg N/km. However, the source of the waste shifted from primarily cattle in 1954 to a mix of cattle and chickens in 2002, due in large part to a ten-fold increase in the number of chickens in the watershed. We did not have fertilizer data through 2002, but between 1954 and 1991 estimated fertilizer N use doubled, from 503 to 1,055 kg N/km (calculated based on the area of the watershed). These changes in N sources were accompanied by decreases in the amount of cultivated land in the watershed. The amount of land classified as harvested cropland decreased from 6,528 in 1954 to 2,448 km in 1992, which suggests there has been an increase in the amount of fertilizer applied per unit area. The amount of pastureland showed an even larger decrease, from 11,421 in 1954 to 3,294 km in 2002, which we take as evidence of increased confinement of livestock. When these numbers are considered in the context of all of the sources of N to the watershed, we estimate that the proportion of agriculturalderived N accounted for approximately 66% of the N input to the watershed in 2000, with the remainder coming from human waste and atmospheric deposition.