Soil erosion is a serious threat to soil fertility in agricultural watersheds and makes it difficult to quantify nutrient and carbon fluxes within these systems. Currently, there is much debate on how much eroded carbon is potentially mineralizable to atmospheric CO2. To answer this, soil was sampled at 5-cm and 10-cm depths along hillslopes in six agricultural watersheds at the Neal Smith National Wildlife Refuge in Prairie City, Iowa. Watersheds with three different treatments of perennial grass filter strips (PFS) demonstrated to reduce erosion and sediment accumulation at their bases. We hypothesized that PFS also reduce potentially mineralizable carbon at the base of the watershed hillsopes. To assess this, the amount of mineralizable carbon in 60 soil samples at room temperature and 60% of water holding capacity were measured using an infrared gas analyzer. Results led to rejecting the hypothesis: potentially mineralizable carbon was greater in watershed hillslopes with PFS vs. watershed hillslopes without PFS. This indicates erosion may be sever in watersheds without PFS where eroded carbon is either rapidly transported out of the watershed or mineralized to CO2. However, results also showed that mineralizable carbon was more concentrated near the center of hillslopes for all PFS treatments.