Dissertation Research: Interactions of pH on molybdenum and phosphorus limitation of nitrogen fixation in the Amazon Basin

热带森林是高产的生态系统，占陆地初级生产的三分之一，但需要氮来支持植物生长。固氮是一个由微生物进行的过程，使热带森林能够补充这种养分。有记录的最高固氮率是在热带森林中发现的，但在世界上最大的热带森林所在地亚马逊盆地进行的测量很少。磷和钼是在固氮过程中起重要作用的两种元素。 事实上，已知磷的可用性会限制固氮作用，但没有人确定钼在自然界中是否具有类似的作用。 由于钼是稀有的，并且很容易从土壤中浸出，它的低浓度可能导致亚马逊盆地的固氮率低，那里的土壤养分耗尽，并且该地区几乎没有新的钼投入。亚马逊盆地的这些所谓的“风化”土壤也是酸性的，这种条件使固氮微生物更难获得钼。这项博士论文改进补助金的研究将调查这些因素如何相互作用以限制固氮作用，这会影响热带森林如何应对环境变化。亚马逊东南部是一个研究不足但重要的森林生态系统，其中（1）固氮作用尚未量化，（2）钼和磷浓度较低，可能是由于这些元素的大气输入量较低。该奖项的资金将支持实验，以测试亚马逊东南部自由生活的土壤微生物的固氮是否直接受到低磷或钼的限制，或通过pH值对土壤中磷和钼络合的影响间接受到限制。研究人员将进行磷，钼和石灰小区添加实验，并将测量固氮率以及磷和钼的可用性的长期变化。研究结果将在审查文件中传播，以便为地球系统模型推断全球固氮率。此外，研究人员还将与巴西学生进行培训和合作，并利用成果为对科学感兴趣的中学女生开发年度“拓展你的视野”研讨会的教学模块。

Tropical forests are highly productive ecosystems and represent one-third of terrestrial primary production, but require nitrogen to support plant growth. Nitrogen fixation is a process carried out by microorganisms that allows tropical forests to replenish this nutrient. The highest documented nitrogen fixation rates are found in tropical forests, but few measurements have been made in the Amazon Basin, home to the world's largest tropical forest. Phosphorus and molybdenum are two elements that play an important role in the nitrogen fixation process. Indeed, phosphorus availability is known to limit nitrogen fixation, but no one has determined whether molybdenum has a similar effect in nature. Because molybdenum is rare, and is easily leached from soils, it's low concentration could lead to low nitrogen fixation rates in the Amazon Basin, where soils are nutrient depleted, and where regions receive little to no new inputs of molybdenum. These so-called "weathered" soils in the Amazon Basin are also acidic, and this condition can make it even more difficult for nitrogen-fixing microorganisms to access molybdenum. Research for this Doctoral Dissertation Improvement Grant will investigate how these factors interact to constrain nitrogen fixation, which affects how tropical forests will grow in response to environmental change. The southeastern Amazon is an understudied but important forest ecosystem where (1) nitrogen fixation has not yet been quantified and (2) where molybdenum and phosphorus concentrations are low, likely due to low atmospheric inputs of these elements. Funds from this award will support experiments to test whether nitrogen fixation by free-living soil microorganisms in the southeastern Amazon is directly constrained by low phosphorus or molybdenum, or indirectly limited via pH effects on phosphorus and molybdenum complexation in soils. The researchers will conduct phosphorus, molybdenum, and lime plot addition experiments, and will measure long-term changes in nitrogen fixation rates and in the availability of phosphorus and molybdenum. Results will be disseminated in review papers for extrapolation of global nitrogen fixation rates for earth system models. In addition, the researchers will train and collaborate with Brazilian students and use the results to develop a teaching module for the annual Expanding Your Horizons workshop for middle school girls interested in science.