Are US Croplands a Source or Sink of Atmospheric Carbon Dioxide Based on Changes in Soil Organic Carbon Stocks?

US Croplands could potentially sequester from 275 to 750 Tg CO2 equivalants per year according to analyses based on widespread adoption of alternative management practices that increases carbon input or reduces carbon losses from soils, relative to traditional practices. However, potentials are not necessarily representative of current trends. Our objectve was to determine the national trends by combining simulation modeling with datasets that provide input driving variables, including land use and management statistics, weather data, and soil characteristics. Century was used to simulate trends during the 1990s for US Croplands in a Monte Carlo Analysis, assessing uncertainty in model input data as well as the structural uncertainty associated with parameterization and model algorithms. Probability distribution functions were developed for N fertilization, tillage practices, organic amendments, while structural uncertainty was assessed based on an empirical estimator derived from 47 long-term agricultural experiments. A total of 100 iterations were simulated for each NRI during the analysis. Using this simulation modeling approach, US Croplands were estimated to sequester from 60 to 70 Tg CO2 equivalents per yr in soil organic carbon pools, which is well below the estimated potentials. While agricultural practices have created a modest sink for carbon, sequestration rates are below the potentials due to limited adoption of alternative practices.