Measuring organic carbon (OC) losses from soils presents a challenge because of the intricate interplay of human-induced and biophysical processes that govern its transfer from terrestrial to aquatic ecosystems. This study employed a modeling approach to enhance our comprehension of human activities' influence on particulate OC (POC) and dissolved OC (DOC) losses from a typical agricultural watershed to the riverine ecosystem through surface and subsurface flows in the Upper Maurice Watershed in the Mid-Atlantic Region. We calibrated and validated an eco-hydrological model (i.e., SWAT-C) using historical (2001-2020) data on streamflow, sediment, POC, DOC loads, and crop yields. Simulation outcomes from 2001 to 2020 reveal that surface runoff was the primary contributor to the total DOC load (65%), followed by lateral flow (30%), and then groundwater (5%). Meanwhile, POC load was linked to erosion processes induced by surface runoff. Our findings indicate that agricultural land-use types exhibited the highest annual average DOC and POC loads. Forests and grasslands displayed intermediate loads, while barren land had the lowest load. Concerning seasonal fluctuations, agricultural land-use types exhibited distinct DOC and POC load patterns when compared to forest and grassland types indicating the dominant role of management practices in determining SOC losses. Notable seasonal variations were observed among the three primary crop rotations, namely corn-soybean (CS), corn-soybean-soybean (CSS), and corn-soybean-winter wheat (CSW), which can be attributed to management practices and residue quality (e.g., C: N ratio). We additionally examined management practices' impact on SOC budgets, considering various combinations of tillage, irrigation, and fertilization levels in the watershed by comparing two baselines: (1) pre-treatment and (2) post-treatment. The results showed maximal SOC sequestration with full irrigation, no-till (NT), and full fertilization in the three rotations compared with both baselines. In contrast, the largest SOC depletion arose from combining conservation tillage (CT) and no fertilization, irrespective of irrigation in both scenarios. Our study demonstrated SWAT-C's effectiveness in measuring lateral DOC and POC fluxes from agricultural watersheds to riverine ecosystems. The model can also simulate land use and management impacts on SOC changes and is a valuable decision-support tool for watershed carbon management plans
