Relative Abundance of and Composition within Fungal Orders Differ between Cheatgrass (Bromus tectorum) and Sagebrush (Artemisia tridentata)-Associated Soils

Abstract

Nonnative Bromus tectorum (cheatgrass) is decimating sagebrush steppe, one of the largest ecosystems in the Western United States, and is causing regional-scale shifts in the predominant plant-fungal interactions. Sagebrush, a native perennial, hosts arbuscular mycorrhizal fungi (AMF), whereas cheatgrass, a winter annual, is a relatively poor host of AMF. This shift is likely intertwined with decreased carbon (C)-sequestration in cheatgrass-invaded soils and alterations in overall soil fungal community composition and structure, but the latter remain unresolved. We examined soil fungal communities using high throughput amplicon sequencing (ribosomal large subunit gene) in the 0–4 cm and 4–8 cm depth intervals of six cores from cheatgrass- and six cores from sagebrush-dominated soils. Sagebrush core surfaces (0–4 cm) contained higher nitrogen and total C than cheatgrass core surfaces; these differences mirrored the presence of glomalin related soil proteins (GRSP), which has been associated with AMF activity and increased C-sequestration. Fungal richness was not significantly affected by vegetation type, depth or an interaction of the two factors. However, the relative abundance of seven taxonomic orders was significantly affected by vegetation type or the interaction between vegetation type and depth. Teloschistales, Spizellomycetales, Pezizales and Cantharellales were more abundant in sagebrush libraries and contain mycorrhizal, lichenized and basal lineages of fungi. Only two orders (Coniochaetales and Sordariales), which contain numerous economically important pathogens and opportunistic saprotrophs, were more abundant in cheatgrass libraries. Pleosporales, Agaricales, Helotiales and Hypocreales were most abundant across all libraries, but the number of genera detected within these orders was as much as 29 times lower in cheatgrass relative to sagebrush libraries. These compositional differences between fungal communities associated with cheatgrass- and sagebrush-dominated soils warrant future research to examine soil fungal community composition across more sites and time points as well as in association with native grass species that also occupy cheatgrass- invaded ecosystems.

Reduced vertical stratification of soil bacterial community structure and composition is associated with Bromus tectorum invasion of sagebrush steppe

“…The gradients of these soil chemical parameters across soil depth profiles also differ between the two vegetation types (Weber et al, 2015). As previously noted, such chemical gradients have the ability to alter microbial community structure and composition with soil depth (e.g.…”

“…Sagebrush also provides a habitat for unique assemblages of microorganisms in the form of biological crusts on the soil surface (Belnap, 1994) and in sub-surface soils (Weber et al, 2015). However, previous studies have demonstrated that cheatgrass invasion of aridland ecosystems can alter soil microbial community composition and structure as well as rates of microbially-mediated nutrient cycling processes (D'Antonio and Vitousek, 1992;Evans et al, 2001;Rau et al, 2011;Wisdom et al, 2005;Norton et al, 2008).…”

“…Cheatgrass and sagebrush, likely in part due to their different life cycles, are associated with soils that differ significantly in %C, %N, water content and pH (Weber et al, 2015). The gradients of these soil chemical parameters across soil depth profiles also differ between the two vegetation types (Weber et al, 2015).…”

“…For instance, during the growing season, cheatgrass competes very effectively for water against other plant species (Sperry et al, 2006), limiting water availability to microbial communities in the surrounding soil. However, after senescence, soils colonized by cheatgrass can be wetter across the soil depth profiles relative to soils colonized by sagebrush (Weber et al, 2015). Higher water contents late in the growing season in combination with the large pulses of organic carbon introduced to soils by senesced cheatgrass may contribute to higher mineralization rates observed in cheatgrass than in sagebrush soils (Norton et al, 2012).…”

“…Higher water contents late in the growing season in combination with the large pulses of organic carbon introduced to soils by senesced cheatgrass may contribute to higher mineralization rates observed in cheatgrass than in sagebrush soils (Norton et al, 2012). Sagebrush's association with AMF, which produce glomalin, a proteinaceous substance that has been positively correlated with carbon sequestration, may also differentially impact carbon storage across soil depth intervals and the microbial composition in the surrounding bulk soils (Weber et al, 2015). Additionally, pH, which tends to be alkaline and increase with soil depth in sagebrush steppe (Inouye, 2006;Weber et al, 2015), contributes to stratifying microbial habitats with increasing soil depth, influencing community structure and composition.…”