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Soil processes mediate nitrogen flux from agricultural lands - evidence from stable isotope studies

In Prince Edward Island, the discharge of nitrogen (N) rich groundwater to streams is believed to be largely responsible for the increasing frequency and severity of eutrophic conditions and anoxic events in local estuaries. Stable isotope results show seasonal variations in the provenance
of N. They also suggest that mediation through the soil organic matter (SOM) pool (mineralization and nitrification) of N from a variety of sources (fertilizers, manures and crop residues) is the dominant process controlling nongrowing season leaching losses, which comprise the bulk of annual N
losses from agricultural lands in the Province. At the watershed scale, nitrate in groundwater retains only slight hints of original N sources (i.e., manure or synthetic fertilizers), with most values falling in the range typically associated with soils. At the experimental scale, nitrate in tile
drain effluents carried somewhat distinctive isotope characteristics for plots with fertilizers, manure and check plot treatments immediately following potato harvest, but converged to values characteristic of the SOM pool by the start of the subsequent growing season. Collectively, we suggest that
practices that influence the rate of nitrification during the important non-growing season can play a key part in controlling N losses from agricultural sources. Split field trials in the Souris watershed are currently
testing the hypothesis that delaying ploughing of forage crops from fall to spring will reduce over winter nitrification of crop residues and N leaching, and leave more N available for subsequent crops. Preliminary results support the importance of tillage management in the development of strategies
to reduce N losses from crop lands.

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