Field Recovery and Seedling-Bioassay Availability of 15N-urea 10 Years after Application to a Douglas-fir Pole Stand in Coastal British Columbia.

The long-term fate of fertilizer nitrogen in forest ecosystems is poorly understood although such information is critical for improving forest fertilization practices. We studied field recovery and seedling availability of ¹⁵N-labelled urea (4.934 atom % excess, 200 kg N ha^-1) 10 years after spring (May 1982) or fall (November 1982) application to a 38-39-year-old Douglas-fir (Pseudotsuga menzeisii (Mirb.) Franco) stand in coastal British Columbia. The 10-year sampling (October 1992) showed few differences between the fall and spring fertilizer applications in total N and ¹⁵N distribution within the tree and forest ecosystem. Total fertilizer N recovery was 59.4% of which 14.5% was recovered in the trees including coarse roots. Tissue ¹⁵N remained mobile and could be transferred to new growth. Soil recovery was 39.8%, which had decreased from 57.0% at a previous 1-year sampling, although it appears that there was little continuing tree uptake. By contrast, there was substantial uptake of residual ¹⁵N fertilizer by Douglas-fir seedlings grown for 2 years in pots with LFH over 0-10 cm mineral soil, in 4 combinations with and without ¹⁵N labelling. There were no substantive differences in availability of residual ¹⁵N from the original spring and fall fertilization, residual ¹⁵N was more available from LFH than from mineral soil, and nitrate accumulated in the pots without seedlings. Our results, similar to those from two previous long-term ¹⁵N studies, highlight the high capacity of forest ecosystems for long-term retention of externally-applied inorganic N, the importance of maximizing N uptake in the first year, and also continuing need to develop new approaches to overcome the limitations to tree uptake of mineral N.