Nitrogen Fertilizer Application to Virginia's Eastern Shore: Refining Land Use and Fertilizer Estimates for the Seaward Side

Reactive nitrogen (Nr) inputs to the seaward side of Virginia’s Eastern Shore from agricultural fertilizer were estimated with a high-resolution GIS data layer in which active agricultural fields were represented as individual polygons. Fields within the watersheds that feed the seaside lagoons were identified from the most recent high-quality air photos from the Virginia Base Mapping Program (VBMP) and manually digitized. Identification of fields under active cultivation was based on the presence of crops or tilling patterns and the absence of large areas of standing water, woody vegetation, livestock, or suburban development in the photos. A total of 2038 individual fields were identified and outlined, and each field was assigned a crop rotation based on CropScape data [USDA, 2017] from 2013 through 2016. 84% of fields were identified as growing a corn, wheat, and soybeans rotation. Nr application was assigned to each field based on the recommendations of the Virginia Cooperative Extension. Based on this data layer, the total mass of Nr applied as fertilizer to the seaward side was estimated to be 2.02 x 106 kg per year. The average fertilizer application rate was estimated to be 123 kg of Nr per year per hectare of cropland.
This data layer is being used to help estimate of agricultural Nr inputs to the seaside lagoons for the entirety of the Virginia Eastern shore, but it can also be used on a watershed or sub-watershed scale to estimate Nr inputs to individual streams or bays. This tool will be useful in estimating agricultural Nr inputs upstream of water sampling locations, which can help researchers estimate Nr removal by biological processes in various soils, aquifers, and creeks. This data layer yields less spatial error than CropScape alone, which has lower resolution and often falsely categorizes abandoned fields or meadows as active cropland in the study area.