Native ungulate grazing modifies the composition and function of soil microbial communities

Background/Question/Methods

Soil food webs are thought to be structured by bottom-up ecological forces via the production and biochemistry of plant detritus. However, soil food webs can also be structured by top-down forces exerted by the selective foraging of native ungulates. For example, these animals preferentially consume nitrogen-rich, low-lignin plant tissues, thereby altering the biochemical composition of plant detritus that is left behind to enter soil. We reasoned that selective foraging could modify “the bottom-up” forces that structure soil food webs, linking the foraging behavior of large animals to the composition and function of soil microbial communities. We asked the question, does selective foraging by ungulates change the composition and function of saprotrophic soil microbial communities? To answer this question, we sampled the saprotrophic soil microbial community inside and outside of four 50-70 year old grazing exclosures located in sagebrush steppe. We used qPCR to quantify the biomass of fungi and bacteria, and GeoChip 5.0 to quantify the abundance of fungal and bacterial genes that encode for enzymes that mediate plant litter decay. 

Results/Conclusions

We performed two-way ANOVAs to determine the effect of herbivory by ungulates on saprotrophic fungal and bacterial microbial biomass and function. Fungal and bacterial biomass were not significantly different in response to grazing exclusion. Fungal enzymes responsible for starch degradation were more abundant in the absence of grazing (P < 0.001). For bacteria, genes encoding enzymes to degrade hemicellulose, pectin, starch, and terpenes, as well as fix nitrogen, had significantly greater abundance in the absence of grazing (P < 0.001). Additionally, total microbial respiration was significantly greater in ungrazed than in grazed conditions (P < 0.05). These microbial responses appear to result from a significantly greater shrub abundance in the absence of grazing (P < 0.001). Our observations provide evidence that grazing by native ungulates modifies the composition and function of soil microbial communities by changing the amount and biochemical composition of plant detritus entering soil.