Threshold models for how pulse dynamics affect microbial responses to climate change in arid ecosystems

Background/Question/Methods . Species interactions strongly influence community composition and dynamics. Although competition has been considered to be a driving force governing species diversity, evidence is mounting that positive interactions among plants may dominate in some aridland environments under moderate to stressful conditions. Interactions between vascular plants and fungi may also have a significant influence on plant community structure and species diversity. Indeed, the fungal-loop model for aridland soils predicts that carbon and nitrogen cycling between crusts, soils and plants may be mediated by the dynamic needs and demands of fungi. Additional evidence shows high fungal diversity in the rhizospere, roots, and leaves of dominant C4 grasses. Thus, interactions between plants and endophytes in roots and leaves may provide a key mechanism governing desert grassland ecosystems. We investigated species interactions in a Chihuahuan Desert grassland following a lightning-caused wildfire in a long-term rainfall manipulation experiment at the Sevilleta LTER site in central New Mexico. We examined the effect of variable precipitation events on symbiotic root associated fungi (RAF) communities living within a common C4 perennial grass, Sporobolus cryptandrus.

Results/Conclusions . Molecular data indicated that the RAF communities were dominated by five groups of Ascomycete fungi: Paraphaeosphaeria spp., Fusarium spp., Moniliophthora spp., Monosporascus spp., and Sordariales spp. Only the most dominant group, Paraphaeosphaeria spp., however, was obviously responsive to water amendments, though the addition of similar volumes of water appeared to increase similarity among the RAF communities, irrespective of size and distribution of precipitation events. Microscopic examinations of the roots also revealed an increased fungal load when exposed to elevated moisture, irrespective of rainfall frequency. We suggest that the increase in these cosmopolitan and abundant groups of Ascomycete fungi, in part, represents a common but complicated strategy of water translocation in a variety of soil types, similar to that proposed and shown in the Arbuscular Mycorrhizal Fungi. These results will be integrated with recent metagenomic data from the Sevilleta to better understand how fungi and other microbes interact with dominant grasses and potentially serve as facultative brokers of water in the larger biological network in the soil.