An Alternative Default Soil Organic Carbon Method for National GHG Inventory Reporting to the UNFCCC

Estimating soil organic C stocks is challenging because of the large amount of data needed to evaluate the impact of land use and management on this terrestrial C pool. Moreover, some of the required data are rarely collected by governments through surveys programs, and are not typically available in remote sensing products. Examples include data on organic amendments, cover crops, crop rotation sequences, vegetated fallows, and fertilization practices. Due to these difficulties, only about 20% of the countries report soil organic C stock changes in their national communications to the UNFCCC. Yet, C sequestration in soils represents one of the least expensive options for reducing greenhouse gas emissions, and has the largest potential for mitigation in the agricultural sector. In order to facilitate reporting, we developed an alternative approach to the current default method provided by the Intergovernmental Panel on Climate Change (IPCC) for estimating soil organic C stock changes in mineral soils. The alternative method estimates the steady-state C stocks for a three pool model given annual crop yields or net primary production as the main input, along with monthly average temperature, total precipitation and soil texture data. Yield data are commonly available in a national agricultural census, and global datasets exists with adequate data for weather and soil texture if national datasets are not available. Tillage and irrigation data are also needed to address the impact of these practices on decomposition rates. The change in steady-state stocks is assumed to occur over a few decades. A Bayesian analysis framework has been developed to derive probability distribution functions for the parameters, and the method is being applied in a global analysis of soil organic carbon stock changes.