Availability of residual fertilizer 15 N from forest floor and mineral soil to Douglas-fir seedlings ten years after fertilization

As low initial uptake and essentially zero later uptake limit efficacy of N fertilization for temperate conifers, we investigated factors limiting long-term tree uptake of residual (15) N-labeled fertilizer. We used a pot bioassay to assess availability of (15) N from soil sampled 10 years after fertilization of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand with (15) N-urea (200 kg N ha(-1)). Douglas-fir seedlings were grown for 2 years in organic (designated LFH) and mineral soil (0-10 cm) layers reconstructed from control and fertilized plots; residual fertilizer N amounted to 10 % of LHF and 5 % of MIN N. Percentage recovery of residual (15) N in seedlings was not affected by the original season of fertilization (spring vs. fall), but differed by the source of (15) N excess. LFH was a better source of residual (15) N; 12.4 % of residual LFH (15) N was taken up by seedlings and 7.6 % transferred to soil, whereas mineral soil yielded only 8.3 % of residual (15) N to seedling uptake and 2.4 % to LFH. Extractable inorganic N was 2-3 orders of magnitude higher in fallow pots. Ten-year residual fertilizer (15) N was clearly cycling between LFH and mineral soil and available to seedlings, indicating that other factors such as denitrification, leaching, and asynchrony of soil N mineralization and tree uptake limit long-term residual N fertilizer uptake in the field.