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

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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.