Date of Award


Degree Name

Doctor of Philosophy


Biological Sciences


Anthony Darrouzet-Nardi


In dryland ecosystems, plant productivity and microbial decomposition are often separated in space and time due to the asynchronous availability of soil moisture and organic matter inputs. It has been proposed that fungi play a key functional role in connecting these cycles by facilitating movement of water, carbon (C), and nitrogen (N) through a network of shared hyphae between plant roots and biological soil crust (biocrust) communities at the soil surface. This connection, also known as the â??fungal loop,â?? effectively re-couples processes of nutrient release and uptake between primary producers and minimizes ecosystem N losses due to leaching, erosion, and gaseous pathways. However, direct support for the existence of these nutrient exchanges and for the importance of fungal networks in dryland biogeochemical cycles remains scarce. In this dissertation, I addressed several direct and indirect lines of evidence underlying the fungal loop hypothesis, described in the following chapters: Ch. 2 presents a greenhouse study comparing foliar recovery and uptake of inorganic and organic N forms applied to roots of three dryland plant species and summarizes our current ecological understanding of dryland plant N uptake rates and methods of quantification; Ch. 3 identifies the abundance, composition and similarity of fungal communities in both biocrust soils and roots of black grama (Bouteloua eriopoda) and compares the responses of biocrust and root-associated fungi to different global change factors; and Ch. 4 attempts to isolate the role of fungi in nutrient translocation of N between biocrust soils and plants by impeding fungal connections to plant roots and evaluating the conditions affecting N uptake from biocrust soils to plant leaves. My findings from Ch. 2 demonstrate that dryland plants with different growth requirements can take up both inorganic and organic soil N within 12-48 hours, and there is little evidence for N niche specialization among nutrient-limited plants in this habitat. Results from Ch. 3 illustrate the relative dissimilarity of biocrust and root-associated fungal communities, the potential sensitivity of root fungal diversity to N fertilization, and the reordering of biocrust fungal communities under increased precipitation variability and combined inputs of water and N inputs. Findings from Ch. 4 did not support the central importance of fungal connections to rapid N transfer through surface soils, as we found that plant 15N uptake was not inhibited by neither fungal exclusion mesh treatments nor surface soil barriers, and significant movement was only observed after 3-10 days. We conclude that (i) nutrient uptake can occur rapidly (< 24 h) in co-occurring dryland plant species following application of water and N to the roots, (ii) there are taxonomically diverse and abundant saprotrophic fungi in biocrust soils, and functionally distinct, symbiotrophic taxa in plant roots that may have differential responses to fluctuations in N and water inputs in this system, and (iii) N transfers from surface biocrusts to plant leaves are fairly rare at the ~0.5 m2 scales we tested, and relatively slower rates of nutrient movement into plants could be driven by soil diffusion and plant root uptake processes, rather than active fungal facilitation. Overall, I did not find strong direct or indirect evidence supporting the occurrence of fungal-mediated nutrient exchanges in this semiarid grassland; however, I did gain a clearer picture of the diversity, composition, abundance and potential trophic roles of dryland fungi and the potential mechanisms underlying N translocation through soils and biotic pools. Cultivating a better understanding of the relationships between short-term nutrient uptake, soil and plant microbes, and soil nutrient movement in dryland has important implications for understanding patterns and mechanisms of nutrient cycling and retention in these globally important ecosystems.




Recieved from ProQuest

File Size

223 p.

File Format


Rights Holder

Catherine E. Cort