Manure and Fertilizer Inputs to Land in the Chesapeake Bay Watershed, 1950-2012

The proportion of nutrient inputs reaching streams and rivers is directly affected by when and where those nutrients enter the landscape. This study supports the U.S. Geological Survey's efforts to describe spatial and temporal patterns in nutrient inputs to the landscape in the Chesapeake Bay watershed, thereby informing efforts to understand changes in riverine and estuarine conditions. The magnitude, spatial variability, and changes over time in nutrient inputs from manure and fertilizer were evaluated in the context of changes in land use and agricultural practices from 1950 through 2012 at three spatial scales: the entire Chesapeake Bay watershed, the 53 8-digit hydrologic units (HUC8s) contained within the watershed, and a set of 7 regions that were determined by aggregating geographically similar HUC8s. The expected effect of reported agricultural Best Management Practices (BMPs) on agricultural nutrient inputs from 1985 through 2012 was also investigated. Nitrogen (N) and phosphorus (P) inputs from manure increased gradually over time at the scale of the entire watershed. Fertilizer-N inputs showed steeper increases, with greater inter-annual fluctuations. Fertilizer-P inputs increased moderately from 1950 through the mid-1970s and declined thereafter. Nutrient inputs and farming practices varied geographically, with implications for the potential impact of these inputs on downstream water quality and ecosystem health. Temporal and spatial patterns in the intensity of agricultural nutrient inputs were consistent with the magnitude and concentration of livestock populations and the intensity of row crop agriculture. Implementation of animal and land- use change BMPs were expected to have little effect on agricultural nutrient inputs. Understanding these patterns is important for explaining changes in nutrient loads to the rivers and streams of the Chesapeake Bay watershed, and their impacts on the water quality and ecosystem health of Chesapeake Bay itself.