Poster Number 574
Wednesday, 8 October 2008
George R. Brown Convention Center, Exhibit Hall E
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.