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    Background and aims As low initial uptake and essentially zero later uptake limit efficacy ofN fertilization for temperate conifers, we investigated factors limiting long-term tree uptake of residual 15N-labeled fertilizer. Methods We used a pot bioassay to assess availability of 15N from soil sampled 10 years after fertilization of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand with 15N-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. Results Percentage recovery of residual 15N in seedlings was not affected by the original season of fertilization (spring vs. fall), but differed by the source of 15N excess. LFH was a better source of residual 15N; 12.4% of residual LFH 15N was taken up by seedlings and 7.6 % transferred to soil, whereas mineral soil yielded only 8.3 % of residual 15N to seedling uptake and 2.4 % to LFH. Extractable inorganic N was 2–3 orders of magnitude higher in fallow pots. Conclusions Ten-year residual fertilizer 15N 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.

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    Swanston, Christopher W.; Preston, Caroline M. 2014. Availability of residual fertilizer 15N from forest floor and mineral soil to Douglas-fir seedlings ten years after fertilization. Plant Soil. 381(1-2): 381-394.


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    Douglas-fir, Forest fertilization, Seedling bioassay, 15N, Nitrogen

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