Grassland ecosystems cover over a third of Earth's vegetated surface. Except for those converted to agriculture, most grasslands are used for livestock grazing. For the first time, this NSF CAREER project will study interactions between grazing mammals and soil microbes, organisms that together control grassland nitrogen (N) cycling and soil fertility. Hypotheses to be tested include: 1) whether large grazers promote dispersal of soil microbes, and 2) whether this affects different steps of the soil N-cycle based on the diversity of microorganisms that control each step. To do this, project personnel, including students and citizen scientists, will work together to measure soil microbe biodiversity and N-cycling activity in bison- and cattle- grazed and ungrazed areas of tallgrass prairie. These experiments will be conducted across distances ranging from 10-cm (local soil microbial habitat) to 1000-km (Flint Hills, KS region). Results from this work will advance general knowledge of the factors regulating grassland soil fertility, including those supporting ecosystem N retention at local to regional scales. Understanding N retention is important for protecting public drinking water quality. Outcomes will also include broad and timely communication of this knowledge to students and citizens, and engagement of students and citizens in the scientific research process. <br/><br/>Despite the importance of grassy ecosystems globally and the key role of grazing animals in these ecosystems, there is no coherent perspective on the generalized effects of large ungulate grazing on grassland N cycling processes, beyond the ?grazing lawn? phenomenon, in which a higher proportion of N is in plant-available form due to consumer-driven nutrient recycling. The overarching hypothesis of this work is that the distribution of grassland soil N cycling microbes is more strongly affected by landscape-scale than local factors, with greater functional consequences for guilds with lower functional redundancy. In other words, the presence of large grazers at the landscape scale will affect microbial diversity more strongly than will edaphic factors at the soil core scale, and this will be associated with differences in soil nitrification potential (a more specialized N-cycling function) more than differences in denitrification or decomposition potential (sequentially less specialized functions). Evidence to evaluate this hypothesis will be collected in collaboration with land managers across the Flint Hills, KS, region, and will be disseminated through communication with ranchers, high school classes and college classes, as well as traditional academic outlets. In addition to this regional-scale pattern evaluation, experimental work will parse the independent effects of microbial community composition and soil pH, organic matter and N availability on N cycling processes. Larger-scale spatial, temporal and conceptual knowledge gain, and dissemination of this knowledge to a broad audience, are the main goals.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.