Department of Ecosystem and Conservation Sciences, W. A. Franke College of Forestry and Conservation, University of Montana, Missoula, Montana, 59812, USA.
Smithsonian Tropical Research Institute, Apartado, 0843-03092, Balboa, Ancón, Republic of Panama.
Ecology. 2019 Apr;100(4):e02646. doi: 10.1002/ecy.2646. Epub 2019 Mar 13.
Tropical forests play a dominant role in the global carbon (C) cycle, and models predict increases in tropical net primary productivity (NPP) and C storage in response to rising atmospheric carbon dioxide (CO ) concentrations. The extent to which increasing CO will enhance NPP depends in part on the availability of nitrogen (N) and phosphorus (P) to support growth. Some tropical trees can potentially overcome nutrient limitation by acquiring N via symbiotic dinitrogen (N ) fixation, which may provide a benefit in acquiring P via investment in N-rich phosphatase enzymes or arbuscular mycorrhizal (AM) fungi. We conducted a seedling experiment to investigate the effects of elevated CO and soil nutrient availability on the growth of two N -fixing and two non-N -fixing tropical tree species. We hypothesized that under elevated CO and at low nutrient availability (i.e., low N and P), N fixers would have higher growth rates than non-N fixers because N fixers have a greater capacity to acquire both N and P. We also hypothesized that differences in growth rates between N fixers and non-N fixers would decline as nutrient availability increases because N fixers no longer have an advantage in nutrient acquisition. We found that the N fixers had higher growth rates than the non-N fixers under elevated CO and at low nutrient availability, and that the difference in growth rates between the N and non-N fixers declined as nutrient availability increased, irrespective of CO . Overall, N fixation, root phosphatase activity, and AM colonization decreased with increasing nutrient availability, and increased under elevated CO at low nutrient availability. Further, AM colonization was positively related to the growth of the non-N fixers, whereas both N fixation and root phosphatase activity were positively related to the growth of the N fixers. Though our results indicate all four tree species have the capacity to up- or down-regulate nutrient acquisition to meet their stoichiometric demands, the greater capacity for the N fixers to acquire both N and P may enable them to overcome nutritional constraints to NPP under elevated CO , with implications for the response of tropical forests to future environmental change.
热带森林在全球碳（C）循环中起着主导作用，模型预测，随着大气二氧化碳（CO ）浓度的升高，热带净初级生产力（NPP）和 C 储量将会增加。增加的 CO 会在多大程度上促进 NPP 的增加，部分取决于氮（N）和磷（P）的供应情况，以支持植物生长。一些热带树木可以通过共生固氮作用获得 N 来潜在地克服养分限制，这可能通过投资于富含 N 的磷酸酶或丛枝菌根（AM）真菌来获取 P 提供一种益处。我们进行了一项幼苗实验，以研究升高的 CO 和土壤养分供应对两种固氮和两种非固氮热带树种生长的影响。我们假设，在升高的 CO 和低养分供应（即低 N 和 P）下，固氮植物的生长速度将高于非固氮植物，因为固氮植物具有更大的获取 N 和 P 的能力。我们还假设，随着养分供应的增加，固氮植物和非固氮植物之间的生长速度差异将会减小，因为固氮植物在养分获取方面不再具有优势。我们发现，在升高的 CO 和低养分供应下，固氮植物的生长速度高于非固氮植物，并且固氮植物和非固氮植物之间的生长速度差异随着养分供应的增加而减小，而与 CO 无关。总的来说，固氮作用、根磷酸酶活性和 AM 定殖随着养分供应的增加而减少，而在低养分供应下升高的 CO 则增加了。此外，AM 定殖与非固氮植物的生长呈正相关，而固氮作用和根磷酸酶活性与固氮植物的生长呈正相关。尽管我们的结果表明，所有四种树种都有能力上调或下调养分获取以满足其化学计量需求，但固氮植物获取 N 和 P 的能力更强，可能使它们能够在升高的 CO 下克服对 NPP 的营养限制，这对热带森林对未来环境变化的反应具有重要意义。
