Soil organic carbon stock changes in the contiguous United States from 1920s to 2010s

Abstract:

To investigate the changes of soil organic carbon (SOC) stocks is of great importance to understand soil carbon dynamics and develop greenhouse gas mitigation and adaptation strategies. There are research gaps in understanding how natural environmental and anthropogenic factors (such as socio-cultural and political/legislative) have provided positive and negative feedbacks on SOC stocks since the 1920s at continental scale. The objectives of this study were to 1) determine the temporal trends in SOC storage across the contiguous U.S.; 2) explore the factors that can explain if soils have acted as a carbon source or sink during the period from 1920s to 2010. We used two soil datasets: 1) National Characterization Soil Survey Database (NCSS) from 1924 to 2010, which includes a total of 14,493 site observations with mutiple soil horizons within 0-100 cm; 2) The data from the Rapid Carbon Assessment (RaCA) Project, containing a total of 6,409 site observations to the maximum depth of 100 cm (2010-2012). We also extracted environmental covariates (space-time layers) covering the U.S. from various sources (remote sensing, National Elevation Dataset, climate data from PRISM project, etc.) to those sites. Results show a fluctuating trend of SOC stocks from 4 kg m^-2 in 1920-1930 to 6 kg m^-2 in 2010 in the 0-20 cm profile, and from 9 kg m^-2 in 1920-1930 to 17 kg m^-2 in 2010 in the 0-100 cm profile, respectively. However, there had been a decrease of SOC stock from 1975 to 1985 in both the 0-20 cm and 0-100 cm profiles. Our analysis reveals relationships between SOC storage and major pivotal political/legislative and socio-cultural events as well as environmental factors. The variation of SOC across the contiguous U.S. was affected in some periods by environmental legislation while in others natural effects predominated. The SOC stock change assessment can be used to infer on the magnitude and past trends; and thus, allows some insight how past natural and anthropogenic conditions have interacted with soil carbon storage. These patterns are likely to be amplified under projected anthropogenic trajectories that are magnitude of orders larger in the future. Our results also highlight the importance to take measures to achieve a neutral carbon budget fostering soil carbon sequestration to enhance soil carbon natural capital.