We tested how excess sodium (Na) from road salt and agriculture alters carbon exchange across terrestrial-aquatic boundaries, by evaluating organic-matter decomposition. The Na ecosystem-respiration hypothesis (SER) posits that more Na in riparian forests will increase leaf decomposition rates; we extend this hypothesis to streams, adding that Na will slow rates past a threshold concentration (subsidy-stress hypothesis). We predicted that 1. streams with more urban and agricultural land cover will have more Na in riparian soil and stream water and that organic matter will decompose faster at saltier sites 2. urban sites will have more Na than agricultural in autumn and winter from road salt addition and 3. more precipitation will accelerate decomposition in streams and riparia from greater microbial activity. We recruited 56 community partners from the central Appalachians to the western edge of the Great Plains and provided standardized monitoring kits, protocols, and other resources for them to use in streams selected by participants. Community study sites span 14 ecoregions within the U.S, across a gradient of land cover from either 60% forested, 10% urban and 25% cropland, pasture, or a combination of both. To measure decomposition rates, we used standardized cotton-strip assays, deployed within a 50-meter reach in both the riparian zone and stream in late November and retrieved them one month later. Results from this community science project will be shared publicly through an open-access database to support local environmental stewardship for mitigating salt impacts to ecosystems. Our data will be part of a global decomposition study using consistent methods and are framed to evaluate how salt influences riparian–stream connections through carbon transformation.