Fig. 3. (A) Land-use change in large river basins such as the Potomac River watershed can be characterized by four stages: frontier, agricultural expansion, industrial urbanization, and post-industrial economy. (B) Conceptualized scale of nutrient cycling as a function of four stages in land-use change: from the frontier era in the 1600s, when nutrient cycles were well bounded by small watersheds, to the current post-industrial era, which is characterized by increasing urbanization and large-scale (up to global) nutrient fluxes.