Anthropogenic Nitrogen Sources to Groundwater Beneath Land Cultivated in Barley and Alfalfa: Nitrogen Isotopes, Major and Selected Trace Ion Chemistry.

Abstract:

Knowledge of local agricultural Best Management Practices (BMPs) and anticipated soil biogeochemical transformation processes provides a framework for distinguishing primary sources of anthropogenic nitrogen impacting local ground water quality. Sources of nitrogen to ground water may have distinctive isotopic signatures, however processes including mineralization, volatilization, nitrification, and denitrification, as well as mixing from different inputs may result in overlapping signatures. The dual-isotope method (δ¹⁵N_nitrate, δ¹⁸O_nitrate) supported by major and trace ion chemistry and characteristic mass ratios can be used to differentiate processes and establish links between groundwater nitrate and land-use practices and their associated reactive nitrogen sources. Knowledge of initial N inputs, identifying the primary processes responsible for transformations and conditions controlling the rates of transforms can be used to build an expected signature for nitrates in groundwater.

A correlation between agricultural BMPs, soil-water conditions, and ground water quality impacts for a study area in Victor, Idaho, was established by analyzing deep soil percolation where anthropogenic nitrogen inputs and agricultural practices are known. Soil conditions (temperature and water content) and the accumulation of deep soil percolation were monitored and conditions utilized to model expected nitrogen-cycle processes and isotopic fraction. Soil water captured at the base of the effective root zone and local ground water was analyzed for δ¹⁵N_nitrate, δ¹⁸O_nitrate, δ¹⁸O_water, δ²H_water, major ions and selected nitrate source tracers. Inputs and expected processes were compared develop an isotopic and chemical signature for the known inputs.