Species interactions may couple the resource dynamics of primary producers that are disconnected in space and time and thus affect fluxes and pools of resources. Drylands have low-density plant communities and biological crusts as primary producers. Many biocrusts fix nitrogen, but this bioavailable N may be inaccessible to plants if soil moisture is insufficient to activate plant metabolism or roots do not extend into interspaces. The fungal loop hypothesis proposes that fungi transport resources between plants and biocrusts, enhancing production.
We studied 1. which fungi are active with a watering event to find candidate taxa for translocation using Stable Isotope Probing, 2. how fungi affect the performance, including the C and N content, of plants and biocrusts, and 3. the rate of nitrogen transfer between biocrusts and plants using a 15N tracer. We transplanted bunchgrasses and biocrusts into pots in the field and placed root- or fungal-excluding mesh horizontally below biocrusts. We hypothesized that 1. dark septate endophytes would be active in roots, rooting zone soil, and biocrusts, 2. fungal connections would reduce differences in C:N ratios between plants and biocrusts due to exchange for limiting resources, and 3. nitrogen transfer would be more rapid when connections were intact.
We found several dominant Pleosporales taxa active in roots, rooting zone soil, and biocrusts, suggesting these taxa were key in coupling plants and biocrusts. We found significant differences in the total fungal communities of roots, rooting zone soil, and biocrusts (P=0.01) but higher overlap in roots and rooting zone soil. When connections were impeded, a more disparate fungal community formed in the biocrusts with little difference in the roots and rooting zone soil. With fungal connections intact, plant C:N was 15% higher (P=0.01) relative to pots without biotic connections, but biocrust C:N did not differ between connection treatment. This suggests that biocrusts are less reliant on the exchange because they are capable of fixing their own nitrogen. We found no significant differences in rates of N transfer between plants and biocrusts (P > 0.05), but a trend that intact fungal connections increased the rates of transfer. These results support the fungal loop hypothesis because the presence of fungal connections enhanced the performance of both producers. As climate change alters precipitation patterns and human activity disturbs arid ecosystems, understanding the interactions between primary producers and belowground biota will allow better predictions of shifts in productivity in arid lands.