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

Abstract

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.

Citation

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.