42Biogeochemical exports of C, N, S and H2 from watersheds are modulated by the activity of 43 microorganisms that function over micron scales. This disparity of scales presents a substantial 44 challenge for development of predictive models describing watershed function. Here, we tested 45 the hypothesis that meander-bound regions exhibit patterns of microbial metabolic potential that 46 are broadly predictive of biogeochemical processes in floodplain soils along a river corridor. We 47intensively sampled floodplain soils located in the upper, middle, and lower reaches of the East 48River in Colorado and reconstructed 248 draft quality genomes representative at a sub-species 49level. Approximately one third of the representative genomes were detected across all three 50 locations with similar levels of abundance, and despite the very high microbial diversity and 51 complexity of the soils, ~15% of species were detected in two consecutive years. A core floodplain 52 microbiome was enriched in bacterial capacities for aerobic respiration, aerobic CO oxidation, and 53 thiosulfate oxidation with the formation of elemental sulfur. We did not detect systematic patterns 54 of gene abundance based on sampling position relative to the river. However, at the watershed 55 scale meander-bound floodplains appear to serve as scaling motifs that predict aggregate capacities 56 for biogeochemical transformations in floodplain soils. Given this, we conducted a transcriptomic 57 analysis of the middle site. Overall, the most highly transcribed genes were amoCAB and nxrAB 58 (for nitrification) followed by genes involved in methanol and formate oxidation, and nitrogen and 59 CO2 fixation. Low soil organic carbon correlated with high activity of genes involved in methanol, 60formate, sulfide, hydrogen, and ammonia oxidation, nitrite oxidoreduction, and nitrate and nitrite 61 reduction. Thus, widely represented genetic capacities did not predict in situ activity at one time 62 point, but rather they define a reservoir of biogeochemical potential available as conditions change. 63 64 65Introduction 66Watersheds are geographic areas that capture precipitation that is ultimately discharged 67 into rivers and other larger water bodies. Of particular interest are watersheds in mountainous 68 regions, as these are major sources of freshwater 1, 2 . Within mountainous watersheds, complex 69 interactions among vegetation, hydrology, geochemistry, and geology occur within and across 70 watershed compartments, including across bedrock-soil-vegetation compartments of terrestrial 71 hillslopes, across terrestrial-aquatic interfaces and within the fluvial system itself. Interactions 72 within a reactive watershed typically vary as a function of disturbance as well as landscape position 73 and topography. For example, interactions in an alpine region of a mountainous watershed are 74 likely to be quite different from a lower montane floodplain region 3 . Floodplains, which extend 75 from the river banks to the base of hillslopes, comprise the rip...