Department of Forest Ecology and Management, Swedish University of Agricultural Sciences (SLU), SE-901 83 Umeå, Sweden.
New Phytol. 2011 Jan;189(2):515-25. doi: 10.1111/j.1469-8137.2010.03485.x. Epub 2010 Sep 29.
Trees reduce their carbon (C) allocation to roots and mycorrhizal fungi in response to high nitrogen (N) additions, which should reduce the N retention capacity of forests. The time needed for recovery of mycorrhizas after termination of N loading remains unknown. Here, we report the long-term impact of N loading and the recovery of ectomycorrhiza after high N loading on a Pinus sylvestris forest. We analysed the N% and abundance of the stable isotope (15) N in tree needles and soil, soil microbial fatty acid biomarkers and fungal DNA. Needles in N-loaded plots became enriched in (15) N, reflecting decreased N retention by mycorrhizal fungi and isotopic discrimination against (15) N during loss of N. Meanwhile, needles in N-limited (control) plots became depleted in (15) N, reflecting high retention of (15) N by mycorrhizal fungi. N loading was terminated after 20yr. The δ(15) N and N% of the needles decreased 6yr after N loading had been terminated, and approached values in control plots after 15yr. This decrease, and the larger contributions compared with N-loaded plots of a fungal fatty acid biomarker and ectomycorrhizal sequences, suggest recovery of ectomycorrhiza. High N loading rapidly decreased the functional role of ectomycorrhiza in the forest N cycle, but significant recovery occurred within 6-15yr after termination of N loading.
树木会减少向根部和菌根真菌分配碳（C），以应对高氮（N）添加，这应该会降低森林的氮保留能力。氮负荷终止后菌根恢复所需的时间尚不清楚。在这里，我们报告了氮负荷和高氮负荷后外生菌根恢复对欧洲赤松森林的长期影响。我们分析了树木针叶和土壤中的氮百分比（15）N 和稳定同位素、土壤微生物脂肪酸生物标志物和真菌 DNA。负载 N 的样地中的针叶变得富含（15）N，反映出菌根真菌的氮保留减少和氮损失过程中对（15）N 的同位素歧视。同时，氮限制（对照）样地中的针叶（15）N 耗尽，反映出菌根真菌对（15）N 的高保留。20 年后终止氮加载。氮加载终止 6 年后，针叶的 δ（15）N 和 N% 下降，15 年后接近对照样地的数值。这种下降以及与负载 N 的样地相比，真菌脂肪酸生物标志物和外生菌根序列的贡献更大，表明外生菌根的恢复。高氮负荷迅速降低了外生菌根在森林氮循环中的功能作用，但在氮负荷终止后 6-15 年内出现了显著恢复。
