Dissertation Research: Developing Biogeochemical Tracers of Apatite Weathering by Ectomycorrhizal Fungi

磷灰石(一种存在于大多数土壤中的钙磷矿物)是大多数自然生态系统最重要的磷来源。在集约化管理的森林中，通过收获去除大量磷，了解磷灰石风化提供磷的速度对于确定这种收获是否在不施肥的情况下长期可持续至关重要。有人提出，当生物对磷或钙的需求超过供应时，与树根共生的外生菌根真菌可能会通过向土壤中释放有机酸来加速磷灰石和其他土壤矿物的风化。这一过程已经在受控的实验室实验中得到了证明，但在现场很难测量。因此，在营养胁迫的生态系统中，外生菌根风化的模式和重要性还没有得到很好的理解。该项目将利用磷灰石中稀土元素(REE)浓度的升高和铅(Pb)的独特同位素比率来追踪其风化产物。将测量从土壤样品中提取的磷灰石中的稀土和铅同位素浓度，以及外生菌根真菌生产的蘑菇中的稀土和铅同位素浓度。土壤和蘑菇将在北方八个阔叶林林分采集，这些林分最近开始了氮磷肥试验，这些林分的土壤磷灰石丰度和林龄不同。这些跨林分的测量与对比自然和人工控制的养分需求和有效性将提高我们对生态系统规模的外生菌根风化控制以及负责的真菌种类的理解。这项工作最终将改进对不同收获制度和土壤矿物学下的长期养分供应的估计，从而提高我们可持续管理此类森林的能力。其他更广泛的影响将包括让本科生、国际学生和中学教师参与研究活动。

Apatite (a calcium phosphate mineral present in most soils) is the most important source of phosphorus to most natural ecosystems. In intensively managed forests where large amounts of phosphorus are removed by harvesting, knowing the rate at which phosphorus is supplied by apatite weathering is critical to determining whether such harvesting is sustainable over the long term without fertilization. It has been suggested that when biological demand for phosphorus or calcium exceeds the supply, ectomycorrhizal fungi that grow in a mutualistic relationship with tree roots may be able to accelerate the weathering of apatite and other soil minerals by releasing organic acids into the soil. The process has been demonstrated in controlled laboratory experiments, but is difficult to measure in the field. Thus, the pattern and importance of ectomycorrhizal weathering in nutrient-stressed ecosystems is poorly understood.This project will use the elevated concentrations of rare Earth elements (REEs) and the distinctive isotopic ratios of lead (Pb) in apatite to trace its weathering products. REEs and Pb isotope concentrations will be measured in apatite extracted from soil samples and also in the mushrooms produced by ectomycorrhizal fungi. Soils and mushrooms will be collected at eight northern hardwood forest stands where nitrogen and phosphorus fertilization experiments have recently begun and which differ in soil apatite abundance and stand age. These measurements across stands with contrasting natural and manipulated nutrient demand and availability will improve our understanding of the ecosystem-scale controls on ectomycorrhizal weathering, and the species of fungi responsible. The work will ultimately lead to improved estimates of long-term nutrient supply under different harvest regimes and soil mineralogies, and therefore improve our ability to manage such forests sustainably. Additional broader impacts will include involving undergraduate students, international students, and middle school teachers in research activities.