Effect of land use and land cover change in context of growth enhancements in the United States since 1700 : net source or sink?

Many flux-based data and related models indicate that the United States is currently a strong carbon sink, but this flux imbalance does not account for carbon lost from previous disturbance. Here we take a modeling approach that involves a full carbon accounting to include the effect of previous human-induced land use changes and management during a period of abiotic changes to the environment since the 1700s. The goal is to show how land use and land cover change has affected carbon loss in the context of other environmental changes like climate, elevated CO2, and nitrogen deposition that tend to enhance growth. Here we show that land use and land cover change has led to the loss of carbon since 1700 (42 PgC), while growth enhancements since the start of the 20th century have only partially countered this loss (1.8 PgC). Fertilized croplands are the largest carbon sink in the first decade of the 21st century (477 TgC/year), followed by forests (266 TgC/year) and grazing pastures (223 TgC/year), if not accounting for land use conversion fluxes, product decomposition, and livestock respiration (i.e., net ecosystem productivity). When accounting for these disturbance fluxes, only returning forests (40 TgC/year) and grasslands (16 TgC/year) remain a carbon sink. A carbon accounting of human disturbances should be considered when determining the climate and policy implications of forest regrowth on the terrestrial carbon sink.