Smithsonian Tropical Research Institute, Apartado Postal 0843-03092, Balboa, Republic of Panama.
College of Life and Environmental Sciences, University of Exeter, Exeter, EX4 4RJ, UK.
Ecology. 2017 May;98(5):1388-1398. doi: 10.1002/ecy.1793. Epub 2017 Apr 18.
Nitrogen (N) availability influences the productivity and distribution of plants in tropical montane forests. Strategies to acquire soil N, such as direct uptake of organic compounds or associations with root symbionts to enhance N acquisition in exchange for carbon (C), may facilitate plant species coexistence and ecosystem N retention. Alternatively, rapid microbial turnover of soil N forms in tropical soils might promote flexible plant N-uptake strategies and mediate species coexistence. We tested whether sympatric plant species with different root symbiont associations, and therefore potentially different nutrient acquisition strategies, partition chemical forms of N or show plasticity in N uptake in a tropical pre-montane forest in Panama. We traced the movement of three N forms into soil pools, microbes, and seedlings of eleven species differing in root traits. Seedlings were grown in a split-plot field transplant experiment, with plots receiving equimolar mixtures of ammonium, nitrate, and glycine, with one form isotopically labeled in each block. After 48 h, more N was recovered in microbes than in plants, while all pools (extractable organic and inorganic N, microbial biomass, and leaves) contained greater amounts of N from nitrate than from ammonium or glycine. Furthermore, C from dual-labeled glycine was not recovered in the leaves of any seedling, suggesting the studied species do not directly take up organic N or transform organic N prior to translocation to leaves. Nitrogen uptake differed by root symbiont group only for nitrate, with greater N recovery in plants with arbuscular mycorrhizal (AM) associations or proteoid roots compared to orchids. Some root trait groups differed in N recovery among N forms, with greater nitrate uptake than ammonium or glycine by AM-associated and N -fixing plants. However, only five of eleven species showed differences in uptake among N forms. These results indicate flexibility in uptake of N forms in tropical plants across root trait groups, with only a few species displaying weak preferences for a specific N form.
氮（N）供应会影响热带山地森林的植物生产力和分布。获取土壤 N 的策略，例如直接吸收有机化合物或与根共生体结合以增强 N 吸收（以换取 C），可能促进植物物种共存和生态系统 N 保留。或者，热带土壤中 N 形态的快速微生物转化可能会促进植物灵活的 N 吸收策略并调节物种共存。我们测试了在巴拿马的热带前山地森林中，具有不同根共生体关联的共生植物物种（因此可能具有不同的养分获取策略）是否会划分 N 的化学形式或在 N 吸收方面表现出可塑性。我们追踪了三种 N 形态进入土壤库、微生物和 11 种根系特征不同的物种的幼苗中的移动。幼苗在分块田间移植实验中生长，其中处理接收等摩尔比的铵、硝酸盐和甘氨酸混合物，每个块体中都有一种形式进行同位素标记。48 小时后，更多的 N 被回收在微生物中而不是植物中，而所有的 N 池（可提取的有机和无机 N、微生物生物量和叶片）都含有更多的来自硝酸盐的 N，而不是来自铵或甘氨酸。此外，双标记甘氨酸的 C 未在任何幼苗的叶片中回收，这表明所研究的物种不会直接吸收有机 N 或在转运到叶片之前将有机 N 转化。仅硝酸盐的 N 吸收因根共生体组而异，与具有丛枝菌根（AM）关联或根瘤菌的植物相比，植物中回收的 N 更多。一些根系特征组在 N 形态之间的 N 回收方面存在差异，与铵或甘氨酸相比，AM 相关和 N 固定植物的硝酸盐吸收更多。然而，只有 11 种物种中的 5 种在 N 形态之间的吸收存在差异。这些结果表明，热带植物在跨根特征组的 N 形态吸收方面具有灵活性，只有少数物种对特定 N 形态表现出微弱的偏好。
