<b>The disturbance of soil carbon and nitrogen in big sagebrush ecosystems after mastication</b>

Big sagebrush <i>(Artemisia tridentata </i>Nutt.<i>) </i>is the dominant plant species in sagebrush steppe ecosystems across western North America. Similar to other dryland shrub-dominated ecosystems, larger soil carbon and nitrogen pools are found underneath big sagebrush plants in elevated mounds of soil called hummocks than in the interspaces between shrubs. This dynamic creates an “island of fertility” effect that produces spatial heterogeneity within these ecosystems, with under-shrub microsites having more soil carbon and nitrogen than interspace microsites. Shrub removal practices, including mastication, are commonly used to increase grass forage for livestock and wildlife. We examined the effects of mastication on soil carbon and nitrogen pools and spatial distribution, using treated sites with paired controls. At each site, we measured the height of hummocks and took soil samples from under big sagebrush individuals (live and dead) and in the interspace (under perennial grass and bare ground). We found that mastication reduced hummock height, with mean height decreasing from 8±3.4 cm in control sites to 5±3.6 cm in masticated sites. Soil carbon and nitrogen pools under shrubs were significantly greater than in the interspace, but mastication reduced nutrient pools under shrubs and interspaces by approximately 14% for mean soil carbon and 18% for mean soil nitrogen. At the ecosystem-scale, mastication reduced average soil carbon from 1,671±454 g/m² in control sites to 1,350±377 g/m² and average soil nitrogen from 138±39 g/m² to 104±27 g/m². These results improve our understanding of how mastication reduces ecosystem soil carbon and nitrogen storage, as well as disrupts the spatial distribution of organic matter under shrubs, in big sagebrush ecosystems.