DISSERTATION RESEARCH: Biotic control of resource retention in arid lands: testing the fungal loop hypothesis

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Plants and microorganisms cycle carbon and nitrogen between the atmosphere and the biota to drive the activities that they need to survive and reproduce. The role of plants and microbes in these cycles are well-understood in mesic (moist) environments but are unresolved in drylands. In arid regions, plants and microbes usually grow for only brief periods following rainfall events. During dry times, resources like carbon or nitrogen can be lost from the ecosystem and returned to the atmosphere, and therefore unavailable for organisms when the next rain comes. One mechanism for conserving resources occurs when direct exchanges between plants and microbes make water and nutrient use more efficient. For example, fungi in roots provide water and nitrogen to plants and receive carbon from plant photosynthesis. The proposed research will test they hypothesis that in arid lands, thread-like soil fungi connect plant roots to microbe-encrusted soils to more efficiently transfer and retain nutrients. This study will improve our capacity to predict the effects of climate changes on plant-microbe interactions. Results from this study will be shared with land managers so that restoration activities protect or manipulate the soil fungi to improve production and reduce degradation to arid lands. Workshops will be designed for school children and the public to increase understanding of the unique features of organisms in arid lands.In arid lands, producers (plants and biological soil crusts) are spatially separated with patchy distributions and are temporally separated due to differential responses to soil moisture, and thus resources released by one may be lost before being accessed by the other. The fungal loop hypothesis proposes that plants and biocrusts are functionally coupled by fungi that transfer water and nutrients between them, reducing resource loss. Observations support the hypothesis, but experimental manipulations of fungi have not yet been conducted. This dissertation work directly tests the mechanisms and ecological effects of fungal connections between plants and biocrusts. First, to determine whether fungal connections between plants and biocrusts exist and if they enable more efficient resource transfer than physical processes, mesh will be used to inhibit root or fungal connections. Experiments using stable isotopes (N, C) will compare transport rates via fungi to rates via physical processes alone. Second, connection (mesh) and plant/biocrust removal treatments will be imposed to test whether fungal connections affect a) the individual performance of plants and biocrusts, b) net plant-biocrust interactions, and c) resource retention. These experiments will be replicated under two precipitation regimes to determine if the fungal loop shows context-dependency with respect to water, one of the resources driving production in arid lands. Understanding the conditions that enable efficient resource cycling will refine predictions on the drivers of productivity and resource dynamics in arid lands. Proposed improvements will determine which fungal taxa occur in biocrusts, rhizospheric soil, and within roots and how these fungi are affected when fungal connections with plants or biocrusts are intact or inhibited and under different watering regimes. Stable isotope probing will be used to identify which fungal taxa are active (versus dormant), and therefore most likely to drive resource transfers between plants and biocrusts. The primary goal of stable isotope probing is to identify fungal taxa involved in the fungal loop. Connecting fungal identity and diversity to resource transfers among producers is essential for understanding the mechanisms of resource cycling in arid ecosystems.