Collaborative Research: Testing the fungal loop hypothesis for Carbon and Nitrogen cycling in dryland ecosystems

在森林和草原中，腐烂的植被在土壤表面积累，并被分解微生物群落消化。分解的最终产物作为土壤中的营养物质，与水一起，可以被植物直接通过根系吸收。相比之下，沙漠和其他干旱生态系统必须遵循一套不同的规则。缺水意味着植物的分布更加不均匀，在罕见的降水事件发生后，植物会短暂地生长。在植物斑块之间，通常会形成由表层细菌、真菌、地衣和苔藓组成的外壳。土壤结皮真菌有一种叫做菌丝的延伸物，可以在结皮区域和植物之间建立联系。该项目将研究“真菌循环假说”的含义，该假说认为地下真菌菌丝在植物和土壤表层之间提供了一个网络，为植物保存和运输水和营养物质。为了验证这一假设的各个方面，该项目的研究人员将在三个不同的地点进行实地研究：德克萨斯州埃尔帕索附近的奇瓦瓦沙漠，德克萨斯州摩押附近的科罗拉多高原，以及新墨西哥州阿尔伯克基附近的一个地点。在这些地点，他们将研究水和营养物质通过真菌菌丝的运动，并制定一个框架，以了解真菌循环在何时何地最重要。旱地约占地球表面的40%，在地球对环境变化的总体反应中起着至关重要的作用。该项目的多地点、实地密集设计也将为两个不同机构的本科生和研究生提供研究和培训机会：德克萨斯大学埃尔帕索分校（UTEP）和新墨西哥大学（UNM）。本研究的总体目标是通过研究代表性旱地土壤中碳和氮的转运和保留来验证真菌环假说。通过在三个地点的一系列田间试验，本项目将解决三个问题：(1)在旱地地点、植物种类和生物结壳类型之间，易位率（即通过真菌菌丝在植物和生物结壳之间转移C和N）是如何变化的？(2)转运是否能改善植物和生物结壳的生长、生产力和碳氮的保留？(3)转运速率是否由植物和生物结壳的化学计量需要量决定？通过使用同位素示踪剂检查每个地点的各种植物和生物结壳功能群（例如，C3与C4草）的易位率，并结合季节变化，特别是对比春季和季风生长季节，拟议的工作将生成一个预测框架，以确定何时何地植物和生物结壳之间的C和N易位最大。这项工作还将通过实验切断菌丝连接和测量对植物和生物结壳健康的影响以及生态系统中C和N的保留来研究转运的重要性。最后，为了解决转运的机制，研究人员将通过实验操纵C和N梯度并观察对C和N水平移动的影响，并使用同位素示踪剂，来验证化学计量梯度驱动C和N通过真菌菌丝移动的假设。这种研究方法将允许对真菌在多大程度上是干旱土壤中碳和氮循环的关键调节因子进行前所未有的评估，正如真菌环假说所建议的那样。

In forests and grasslands, decaying vegetation accumulates on the soil surface and is digested by communities of decomposer microorganisms. The end-products of decomposition serve as nutrients in the soil that, along with water, can be taken up by plants directly through their root systems. By contrast, deserts and other arid ecosystems have to play by a different set of rules. The lack of water means that plants are far more patchy in their distribution and grow in brief spurts following rare precipitation events. Between plant patches, a crust often forms consisting of surface-layer bacteria, fungi, lichens, and mosses. Soil crust fungi have extensions called hyphae that can make connections between crusted areas and plants. This project will examine implications of the "fungal loop hypothesis", which posits that subsurface fungal hyphae provide a network between plants and soil crusts that conserves and transports water and nutrients to plants. To test aspects of this hypothesis, researchers on this project will conduct field research at three different sites: the Chihuahuan Desert near El Paso, TX, the Colorado Plateau near Moab, UT, and a site between those, near Albuquerque, NM. At these sites, they will study the movement of water and nutrients through fungal hyphae and develop a framework for understanding when and where the fungal loop is most important. Drylands cover about 40% of Earth's surface and play essential roles in the planet's overall response to environmental change. The multi-site, field-intensive design of this project will also enable research and training opportunities for undergraduate and graduate students at two diverse institutions: the University of Texas at El Paso (UTEP) and the University of New Mexico (UNM).The overall objective of this study is to test the fungal loop hypothesis by studying C and N translocation and retention across representative dryland sites. Using a set of field experiments at three sites, this project will address three questions: (1) How do translocation rates (i.e. transfer of C and N between plants and biocrusts through fungal hyphae) vary among dryland sites, plant species, and biocrust types? (2) Does translocation improve growth, productivity and retention of C and N for plants and biocrusts? (3) Are translocation rates determined by the stoichiometric requirements of plants and biocrusts? The proposed work will generate a predictive framework for when and where translocation of C and N between plants and biocrusts is greatest by examining translocation rates using isotopic tracers in a variety of plant and biocrust functional groups at each site (e.g., C3 vs. C4 grasses) and incorporating seasonal variation, especially to contrast spring and monsoonal growing seasons. The work will also examine the importance of translocation by experimentally severing hyphal connections and measuring the effects on plant and biocrust health as well as retention of C and N in the ecosystem. Finally, to address the mechanism of translocation, the investigators will test the hypothesis that stoichiometric gradients drive C and N movement through fungal hyphae by experimentally manipulating C and N gradients and observing the effects on the horizontal movement of C and N, also with the use of isotopic tracers. This research approach will allow for an unprecedented evaluation of the extent to which fungi are the key regulators of C and N cycling in dryland soils as suggested by the fungal loop hypothesis.