DISSERTATION RESEARCH: Interactions among nitrogen and phosphorus through plant-microbial mutualisms in tropical rain forests

热带地区拥有一些地球上最大、增长最快、碳含量最高的森林。随着热带树木的生长，它们在生物量中储存了大量的大气二氧化碳，减缓了大气二氧化碳的增加，从而减缓了气候变化的速度。然而，所有树木的生长都需要大量的土壤养分，特别是氮和磷，目前尚不清楚土壤养分的供应是否足以满足未来树木生长的预测需求。例如，热带森林土壤中的磷浓度相对较低，一些研究表明，低磷供应可能会限制树木生长和碳储量的增加。然而，之前的研究表明，一些热带树木在获取稀缺养分方面可能比其他树木更好，因为它们与土壤微生物之间的关系增强了清除养分的能力。这项研究将探索不同的热带树木如何获得土壤养分，以及这些微生物伙伴关系是否为一些树木提供了相对于竞争对手的优势。总体而言，这项研究将提高对热带森林如何应对全球环境变化的理解。能够共生固氮的树木在热带森林中大量存在，这是一种将大气中的氮转化为生物形式的植物-微生物互惠关系。前人的研究表明，固氮树比不固氮树在吸收土壤磷(P)方面具有优势，这是通过产生富含N的磷酸酶来矿化有机磷和寄生清除无机P的丛枝菌根真菌来实现的。这一优势可能导致对土壤P的竞争，在这一竞争中，N固氮树的表现优于不固氮树。该项目将通过在哥斯达黎加奥萨半岛进行的一项实验苗木研究来探索固氮和非固氮树木之间的相互作用。在这项研究中，每个植物组的多个物种将在五个磷处理下单独或竞争生长，这些处理因输送的磷化合物而有所不同。此外，以前的工作表明，这种N和P的相互作用可能不限于固定N的树，但可能在叶N含量较高的树中明显，而不考虑固定。因此，叶面氮可能是植物个体磷吸收的间接控制，如果叶面氮浓度相似的植物聚集在一起，也可以在景观尺度上控制磷的吸收。因此，这项研究还将探索叶片N、P的获取与一组以高叶和低叶N为特征的个体冠层和林地之间的生物地球化学后果之间的关系。这两个项目的结果都有可能阐明复杂的植物-微生物介导的N和P在地上和地下以及多个空间尺度上的相互作用。

The tropics contain some of the largest, fastest growing and carbon-rich forests on Earth. As they grow, tropical trees store large amounts of atmospheric carbon dioxide in their biomass, slowing the increase in atmospheric carbon dioxide and hence the pace of climate change. Yet, all trees require large amounts of soil nutrients, especially nitrogen and phosphorus, to grow, and it remains unclear whether the supply of soil nutrients will be sufficient to meet the demands of predicted tree growth in the future. For example, soil phosphorus concentrations are relatively low in tropical forest soils, and some research suggests that low phosphorus supply may limit increases in tree growth and carbon storage. However, previous work suggests that some tropical trees may be better than others at obtaining scarce nutrients because of their relationships with soil microorganisms that enhance the ability to scavenge nutrients. This research will explore how different tropical trees access soil nutrients and whether these microbial partnerships offer some trees an advantage over their competitors. Overall, this research will improve the understanding of how tropical forests may respond to global environmental change.Trees capable of symbiotic nitrogen (N) fixation, a plant-microbial mutualism that converts atmospheric N into biological forms, are abundant in tropical forests. Previous work suggests that N fixing trees have an advantage over non-N fixing trees in acquiring soil phosphorus (P) via the production of N-rich phosphatase enzymes that mineralize organic P and by hosting arbuscular mycorrhizal fungi that scavenge for inorganic P. This advantage could lead to competition for soil P in which N fixing trees outperform non-N fixing trees. This project will explore the interactions between N fixing and non-N fixing trees via an experimental seedling study on the Osa Peninsula, Costa Rica in which multiple species from each plant group will be grown alone or in competition under five P treatments that vary by the P compound being delivered. In addition, previous work suggests that this N and P interaction may not be constrained to trees that fix N, but may be evident in trees that have high foliar N regardless of fixation. Thus, foliar N may be an indirect control of P acquisition on an individual plant basis, as well as on a landscape scale if plants with similar foliar N concentrations are aggregated. Therefore, this research will also explore the relationships between foliar N, P acquisition, and the biogeochemical consequences across a set of individual emergent canopy trees and forest plots characterized by high and low foliar N. The results from both projects have the potential to elucidate complex plant-microbially mediated N and P interactions that operate above- and belowground, and on multiple spatial scales.