Considerations for the mitigation of nitrate contamination: stable isotopes and insights into the importance of soil processes

Nutrient management is widely promoted to minimize the impact of intensive fertilizer use on groundwater quality, however watershed-scale stable isotope studies in eastern North America suggest nitrogen transport to groundwater is dominated by non-growing season fluxes derived principally from the mineralization and nitrification of soil organic matter. In the current field scale study, δ¹⁵N ratios of nitrate in tile drain effluents from experimental potato plots treated with 300 kg/ha ammonia nitrate and those with no fertilizer both average +4.7‰, close to the +4.0‰ ratios observed in soils of the same plots, and distinct from values near 0‰ for inorganic fertilizer. A source apportionment model using δ¹⁵N and δ¹⁸O in nitrate suggests that even with heavy fertilizer application, less than 10% of non-growing season N flux is derived from direct leaching of fertilizer, the remainder representing N from various sources, including residual fertilizer that has been assimilated into the broader soil organic matter pool and subsequently released via mineralization and nitrification. Factors controlling these losses could be as closely related to cropping practices as initial N application rates, providing potential opportunities for more efficiently utilizing N available in the soil profile and reducing initial N application rates.