Streamflow nitrogen loss following forest fertilization in a southern Vancouver Island watershed

Water quality was monitored in the Lens Creek watershed on southern Vancouver Island to determine nitrogen loss following fall application of 224 kg N/ha urea fertilizer on a second-growth Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) forest. Peak nitrogen concentrations measured in two small tributary streams were 14 mg/L as urea, 1.9 mg/L as ammonia, and 9.3 mg/L as nitrate. For the first 14 months, estimated nitrogen outputs in excess of background amounts were 5.9 and 14.5% of the total applied nitrogen for the two subsidiary watersheds with 46 and 80% of their drainage areas fertilized, respectively. These losses were considerably higher than amounts of less than 1% previously reported for western North America. Increased levels of urea N and ammonia N were short-lived, while nitrate N remained above background levels for the study duration. Reasons for the high nitrogen loss include nitrification of the urea during 7 weeks of mild, dry weather following fertilization, presence of alder and swampy areas adjacent to the streams, high soil permeability, steep slopes, and abundant, above average early winter rainfall. The watersheds had been previously fertilized, but any influence of this first fertilization on nitrogen loss during the present study is unknown. Lens Creek water quality was not adversely affected by the fertilization in terms of drinking water standards or toxicity to fish.