Balancing Vertical and Lateral Carbon Fluxes for Cropland in the U.S. Midwest

The movement of terrestrial carbon across land-water boundaries controls the carbon balance of watersheds and freshwater ecosystems. Most terrestrial carbon balance accounting efforts focus on monitoring and analyzing vertical carbon fluxes neglecting lateral carbon fluxes. Increasing evidence shows that the quantity of lateral carbon fluxes from land to freshwaters is comparable to the terrestrial carbon sink at the global scale. Therefore, there is an urgent need to include lateral carbon fluxes into carbon balance assessment of agricultural and other terrestrial ecosystems. In this study, we combine field measurements, remote sensing data and process-based modeling to quantify and analyze the relevance of lateral carbon fluxes to the assessment of carbon balance of cropland in the U.S. Midwest. To archive this goal, we (1) simulate long-term soil organic carbon (SOC) dynamics using measured data of 10 sites within the U.S. Midwest; (2); model on-site vertical carbon dioxide (CO2) with eddy covariance observations of net ecosystem exchange (NEE) and annual crop yield at six sites across the U.S. Midwest and (3) constrain the Soil and Water Assessment Tool for Carbon (SWAT-C) model for simulating lateral flows of particulate and dissolved organic carbon (POC and DOC) leaving cropland in the Upper Mississippi River Basin using measured riverine carbon fluxes and remote sensing observations of ecosystem variables. The resulting monitoring system is also applied to highlight the importance of factoring in lateral carbon fluxes for sustainable agricultural carbon management.