International Research Fellowship Program: Does the Foliar Nitrogen Isoptope Variability in Rain Forest Trees Reflect Niche Partitioning, Biophysical Processes or Climate?

国际研究奖学金计划使美国科学家和工程师能够在国外进行9到24个月的研究。该项目的奖项提供了联合研究的机会，并提供了使用国外独特或互补的设施、专业知识和实验条件的机会。这一奖项将支持乔丹·马约尔博士与本杰明·特纳博士在巴拿马史密森热带研究所合作的为期24个月的研究奖学金。植物的氮素有效性是控制世界大部分地区森林生产力和生态系统动态的主要因素。热带森林被认为是相对富含氮的，但它们的生长和适应能力对氮循环和气候的自然和人为变化做出了反应。由于氮素循环的复杂性和植物对氮素限制的各种类型的响应，迫切需要使用氮的重稳定同位素与轻稳定同位素的比率来综合生物地球化学过程和植物群落对氮素有效性波动的响应。然而，为了对叶N同位素比率的使用和解释有信心，必须更好地理解潜在的机制过程。该仪器被安置在本杰明·特纳博士的土壤生物地球化学实验室内，将通过测量巴拿马热带雨林的三个实验系统中所有三种形式的植物有效N以及树木的根和叶组织的稳定同位素比率来满足上述研究需要。选择的实验系统是针对植物N同位素值的主要假想控制因素：降雨量、土壤养分生物有效性和与根相关的菌根真菌的类型。这三个实验系统中的每一个都旨在针对具体的假设，当组合在一起时，就热带森林树木N同位素模式的变异原因提供了一个统一的理论。人类活动可以通过增加化肥或大气沉积的N投入，或通过加快气候变化或景观退化导致的N的内部循环，来增加生态系统中N的有效性。这项联合研究将为解释相对简单的叶分析提供信息，以便更好地理解N周期中复杂的、时间整合的差异，并容易发现它们的变化。在全球迅速变化的时代，这些自然指数无疑将被证明对森林监测工作有用。除了在全球范围内区分树木N同位素比率的根本原因的好处外，这些数据还将允许对特定形式的土壤N进行树木专门化测试--这是一种假设的机制，允许在生物多样性的雨林中共存，而不是竞争排除其他相似的树种。

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award will support a 24-month research fellowship by Dr. Jordan Mayor to work with Dr. Benjamin Turner at the Smithsonian Tropical Research Institute in Panama. The availability of nitrogen (N) to plants is a primary control over forest productivity and ecosystem dynamics throughout much of the world. Tropical forests are presumed to be relatively N-rich yet their growth and adaptations respond to natural and anthropogenic changes in N-cycling and climate. Due to the complexity of the N cycle and the various types of plant responses to N limitations, there is a strong research need to use the ratio of heavy to light stable isotope measurements of N to integrate both biogeochemical processes and plant community responses to fluctuations in N availability. However, to be confident in the use and interpretation of foliar N isotope ratios, underlying mechanistic processes must be better understood. The PI, while housed within the Soils Biogeochemistry laboratory of Dr. Benjamin Turner, will meet the above research need by measuring stable isotope ratios from all three forms of plant available soil N, along with tree root and leaf tissue, across three experimental systems throughout Panamanian rainforest. The experimental systems were selected to target the main hypothesized controls over plant N isotope values: precipitation, soil nutrient bioavailability, and the type of root-associated mycorrhizal fungi. Each of the three experimental systems is designed to target specific hypotheses and, when combined, provide for a unified theory on the causes of variability in tree N isotope patterns in tropical forests. Human activities can increase N availability in ecosystems either by increasing N inputs through fertilizer addition or atmospheric deposition, or by speeding up internal recycling of N as a result of altered climate or landscape degradation. This joint research will inform the interpretation of relatively simple foliar analyses so that complex, time-integrated differences in N-cycles can be better understood and changes to them easily detected. In a time of rapid global change, these natural indexes will undoubtedly prove useful to forest monitoring efforts. In addition to the benefits of differentiating the underlying causes of tree N isotope ratios globally, these data will also permit testing for tree specialization on specific forms of soil N - a hypothesized mechanism for permitting the coexistence, rather than competitive exclusion, of otherwise similar tree species in biodiverse rainforest.