Spatio-temporal mapping reveals changes in soil organic carbon stocks across the contiguous United States since 1955
Chenconghai Yang, Feixue Shen, Li Xiang, Wenkai Cui, Lei Zhang, Lin Yang, Chenghu Zhou
Abstract
Understanding spatio-temporal patterns of soil organic carbon (SOC) is critical for global climate change mitigation and sustainable soil management. However, information on long term dynamics of SOC over large area is lacking. Supported by soil samples collected over years and environmental covariates, space and time digital soil mapping (ST-DSM) has become an important and effective method to reveal the spatio-temporal changes of SOC. The contiguous United States (CONUS) has abundant and well-documented soil samples with time labels, which lays the groundwork for us to estimate the long-term SOC dynamics in multiple soil layers over that region with high resolution. Specifically, we propose leveraging time-series soil data from World Soil Information Service (WoSIS) and International Soil Carbon Network (ISCN) to build ST-DSM models at different soil depths based on matching environmental covariates and machine learning techniques (random forest framework). Then, multi-depth ST-DSM models are employed to generate spatial prediction of SOC in different layers (0–15 cm, 15–30 cm, 30–60 cm and 60–100 cm) from 1955 to 2014 at 250 m resolution and 5-year intervals (1955–1959, 1960–1964,…., 2010–2014). Meanwhile, predictive uncertainties are quantified via Quantile Regression Forest (QRF). Furthermore, we analyze the dynamic trends in SOC stocks across various depths and land uses. The results indicate that over 60 years, overall SOC stocks in 0–100 cm demonstrate a multi-stage change of “rising-fluctuating”. SOC stocks in 0–100 cm rose from 68.40 Pg (1 Pg = 10^15 g) to 70.33 Pg with an increase rate of 32.25 Tg (1 Tg = 10^12 g) per year. SOC stocks in 0–15 cm layer declined slightly before 1970 and increased thereafter; the 15–100 cm layer fluctuated with an overall rising trend; notably, the 60–100 cm layer exhibited a steady upward trend with minimal fluctuations since 1980s, suggesting a greater SOC accumulation potential in soil beneath 15 cm. Across different land uses, the topsoil of cropland may be a source of carbon, while forests are important carbon sinks. SOC stocks in surface soil (0–15 cm) in cropland areas notably decreased, and subsurface and deep soil in cropland (15–30 cm, 30–60 cm and 60–100 cm) showed a fluctuating rising trend. SOC stocks in forest show an overall fluctuating rising trend. Pastureland, unmanaged natural grasslands and sparse/no vegetation areas once showed some carbon sequestration capacity in the past, but they have transitioned to slower accumulation. This research enhances our understanding of soil carbon dynamics at a national scale and provides references for the development of effective soil management and climate mitigation strategies. Soil organic carbon stocks above 1 m increased from 68.40 Pg to 70.33 Pg, exhibiting a multi-stage change of “rising-fluctuating” for the last 60 years across the Contiguous United States, according to a spatial-temporal analysis of soil organic carbon dataset spanning from 1955 to 2014.