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Nitrogen (N) fertilizer enhances crop production, but field runoff impacts water quality in adjacent freshwaters. Planting winter cover crops reduces nitrate-N losses during the fallow period, but less is known about impacts on ammonium-N. From 2016–2023, we sampled biweekly from the Shatto Ditch and Kirkpatrick Ditch Watersheds in Indiana (USA) to compare the impact of cover crops on dissolved inorganic nitrogen at the field-, edge-of-field, and watershed-scales. We measured soil ammonium-N and nitrate-N, biomass, and organic matter in fall and spring. Cover crops reduced soil ammonium-N at Shatto and soil nitrate-N in both watersheds. Tile losses and watershed yields of ammonium-N occurred on scales orders of magnitude lower than nitrate-N. Tile ammonium-N losses from cover cropped fields ranged from 97 % lower to 31 % higher at Shatto, and 45 % lower to 75 % higher at Kirkpatrick compared to those without. Cover crops reduced field-scale nitrate-N losses at Shatto by 58–87 %, but losses at Kirkpatrick ranged 99 % lower to 15 % higher. Tile flow explained interannual variation in nitrate-N losses, while field-scale ammonium-N losses were driven by soil and microbial interactions and mobilization during storms. Watershed-scale ammonium-N and nitrate-N yields correlated with runoff (Kendall τ=0.45 and 0.39, respectively). While nitrate-N yields mirrored runoff, ammonium-N yields exhibited a step-functional increase, pointing to the importance of storms as a driver of loss. As Midwest crop production adapts to fluctuating environmental conditions, we demonstrate how applying cover crops over a multi-year period can mitigate ammonium-N losses 

Abstract Plastic litter is accumulating in ecosystems worldwide. Rivers are a major source of plastic litter to oceans. However, rivers also retain and transform plastic pollution. While methods for calculating particle transport dynamics in rivers are well established, they are infrequently used to quantify the transport and retention of microplastics (i.e., particles < 5 mm) in flowing waters. Measurements of microplastic movement in rivers are needed for a greater understanding of the fate of plastic litter at watershed and global scales, and to inform pollution prevention strategies. Our objectives were to (1) quantify the abundance of microplastics within different river habitats and (2) adapt organic matter “spiraling” metrics to measure microplastic transport concurrent with fine particulate organic matter (FPOM). We quantified microplastic and FPOM abundance across urban river habitats (i.e., surface water, water column, benthos), and calculated downstream particle velocity, index of retention, turnover rate, and spiraling length for both particle types. Microplastic standing stock was assessed using a habitat‐specific approach, and estimates were scaled up to encompass the study reach. Spatial distribution of particles demonstrated that microplastics and FPOM were retained together, likely by hydrodynamic forces that facilitate particle sinking or resuspension. Microplastic particles had a higher downstream particle velocity and lower index of retention relative to FPOM, suggesting that microplastics were retained to a lesser degree than FPOM in the study reaches. Microplastics also showed lower turnover rates and longer spiraling lengths relative to FPOM, attributed to the slow rates of plastic degradation. Thus, rivers are less retentive of microplastics than FPOM, although both particles are retained in similar locations. Because microplastics are resistant to degradation, individual particles can be transported longer distances prior to mineralization than FPOM, making it likely that microplastic particles will encounter larger bodies of water and interact with various aquatic biota in the process. These empirical assessments of particle transport will be valuable for understanding the fate and transformation of microplastic particles in freshwater resources and ultimately contribute to the refinement of global plastic budgets.