Quantification of Gross Nitrogen Transformation Rates within a Conventional Potato Rotation Using Stable Isotopes.

Groundwater nitrate (NO₃^-) contamination is a major concern within Prince Edward Island (PEI), Canada; an agriculturally intensive island that is reliant on groundwater as a source of drinking water. This NO₃^- contamination has been linked primarily to potato production systems. This study examines the key soil processes (gross mineralization, immobilization, nitrification and denitrification) controlling the availability of soil NO₃^- within each phase of a three year potato-barley-red clover rotation throughout two growing seasons in PEI using the pool dilution method. Stable isotopes provide a means of quantifying the rates of individual soil processes. By using the isotope pool dilution method, soil nitrogen (N) turnover rates can be calculated by the addition of ¹⁵N-labelled solutions to undisturbed soil cores, and the dilution of the labelled N can be quantified over a 24-hour incubation period. Monitoring temporal changes in soil mineral N concentrations over the field season, together with the pool dilution method, will be used to gain insight on soil NO₃^- availability within this production system. Soil NO₃^- concentrations for 0-15 cm depth were highest in the potato phase of the rotation early in the growing season, reflecting the high fertilizer N inputs for this crop. Soil NO₃^- concentrations were similar among the different phases of the rotation in the latter part of the growing season.  Phase of the rotation had a greater effect on soil NO₃^- concentrations than the N management within each phase of the rotation. Overall, this study aims to gain a better understanding of the controls on nitrate availability within soils, which will aid in addressing groundwater NO₃^- contamination issues and improve N-management on agricultural lands within Atlantic Canada.