Recent studies on Northern Ireland rivers have shown that summer nitrite (NO 2 ؊) concentrations greatly exceed the European Union guideline of 3 g of N liter ؊1 for rivers supporting salmonid fisheries. In fast-flowing aerobic small streams, NO 2 ؊ is thought to originate from nitrification, due to the retardation of Nitrobacter strains by the presence of free ammonia. Multiple regression analyses of NO 2 ؊ concentrations against water quality variables of the six major rivers of the Lough Neagh catchment in Northern Ireland, however, suggested that the high NO 2 ؊ concentrations found in the summer under warm, slow-flow conditions may result from the reduction of NO 3 ؊. This hypothesis was supported by field observations of weekly changes in N species. Here, reduction of NO 3 ؊ was observed to occur simultaneously with elevation of NO 2 ؊ levels and subsequently NH 4 ؉ levels, indicating that dissimilatory NO 3 ؊ reduction to NH 4 ؉ (DNRA) performed by fermentative bacteria (e.g., Aeromonas and Vibrio spp.) is responsible for NO 2 ؊ accumulation in these large rivers. Mechanistic studies in which 15 N-labelled NO 3 ؊ in sediment extracts was used provided further support for this hypothesis. Maximal concentrations of NO 2 ؊ accumulation (up to 1.4 mg of N liter ؊1) were found in sediments deeper than 6 cm associated with a high concentration of metabolizable carbon and anaerobic conditions. The 15 N enrichment of the NO 2 ؊ was comparable to that of the NO 3 ؊ pool, indicating that the NO 2 ؊ was predominantly NO 3 ؊ derived. There is evidence which suggests that the high NO 2 ؊ concentrations observed arose from the inhibition of the DNRA NO 2 ؊ reductase system by NO 3 ؊ .

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The loss of inorganic N in drainage water from grazed perennial ryegrass (Lolium perenne L. cv. Talbot) swards in Northern Ireland was studied for 9 yr. Plots (each 0.2‐ha area) were hydrologically isolated and artificially drained to V‐notch weirs with flow‐proportional monitoring of drainage water. Nitrogen, as calcium ammonium nitrate, was applied at 100, 200, 300, 400, or 500 kg N ha−1 yr−1. The efficiency of flow interception by drains decreased on average by 39% during the 9 yr. Annual loss of NO−3 in drain flow for the plot receiving 300 kg N ha−1 yr−1 ranged from 16 to 52 kg N ha−1 and was highest after a dry summer. In individual years, NO−3 in drainage water was linearly related to fertilizer N input with 5 to 23% of the added N input being lost. The shape of the NO−3 dose‐response curve did not change with time. Annual losses of NH+4 and NO−2 in drainage water were not related to fertilizer rate, and ranged from 0.2 to 4 kg N ha−1 and 8 to 540 g N ha−1, respectively. Annual flow‐weighted mean NO−3, NH+4, and NO−2 concentrations usually did not exceed the European Community maximum admissible limits for drinking water below a fertilizer N application rate of 300 kg N ha−1 yr−1. However, the European Community guideline NH+4 and NO−2 concentrations in salmonid and cyprinid waters were exceeded at application rates ≥100 kg N ha−1 yr−1.

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