Print PageAltered fluxes of nitrogen (N) and phosphorus (P) have been driven directly by human-mediated fluxes and indirectly due to changes in land and water systems that alter rates of biogeochemical transformations and transport vectors for nutrients. The Northeastern United States as a region underwent many biophysical and regulatory changes over the 20th century, making it an excellent case study for understanding human-driven alterations to biogeochemical cycles over time. From 1920 to 2000, this region experienced significant losses of agricultural land and increases in forest and urban land cover. Furthermore, major national and state legislation, including the Clean Water Act, was passed during the 20th century to control pollution, and major technological advances in wastewater treatment were made.
Using data from the historic Census of Agriculture, the National Atmospheric Deposition Program, and primary literature, we created a comprehensive time series database of anthropogenic N and P inputs to the Northeast. Inputs are estimated for each county at five year time intervals. Inputs include atmospheric deposition of nitrogen, enhanced biological N fixation, fertilizer, manure, and domestic waste. We used this database and data on hydrology, climate, and stream nutrient loads estimated from USGS NWIS to develop a nutrient transport model for the Northeast. We used this model to estimate nutrient loads for all HUC 8 watersheds in our study region at 5 year time steps.
Results/Conclusions
Over a multi-decade timeframe, we found temporal trends in N and P fluxes at the regional scale as well as a major restructuring of the spatial patterns of these fluxes. Inputs of P increased during the first half of the century and decreased after the 1960s primarily due to decreased fertilizer and manure P as well as detergent phosphate bans. Inputs of N in fertilizer increased, reflecting a shift in fertilizer composition. Agricultural inputs of N and P dominate regionally, but atmospheric N deposition has become increasingly important, in absolute and relative terms, since 1980. Locally, agricultural inputs have become less important as urban centers expanded and atmospheric deposition of N increased, but they remained significant contributors of N and P. In the 1920’s, five urban centers were the only major hotspots of loading in the region. By the end of the century, hotspots of agricultural nutrient loading appear in areas of intensive crop and livestock agriculture, particularly in the Chesapeake Bay region, and in areas of suburbanization.
