Uncertainty Analysis for a US Inventory of Soil Organic Carbon Stock Changes

F. Jay Breidt, Department of Statistics, Colorado State University

Soil organic matter is an important sink for carbon, and the size of this sink is impacted by agricultural management such as organic amendments and tillage practices. Appropriate management can increase carbon sequestration in soils and mitigate greenhouse emissions of carbon dioxide to the atmosphere. Accounting for the amount of carbon sequestration is difficult due to the long temporal scales and fine spatial scales of interest and the complexity of the dynamics of carbon as it cycles between the atmosphere and biosphere, including soil organic matter pools. CENTURY, a biogeophysical process model, is used to model carbon dynamics in a national-level inventory of soil organic carbon stock changes. Inputs to CENTURY include weather, soils data, cropping history, tillage practices, fertilizer usage, and organic amendments, all of which are available to some degree from different national databases at different spatial scales. CENTURY is used to simulate soil organic matter dynamics at points used by the National Resources Inventory (NRI), a nationally-representative two-stage area sample, for which a standard design-based variance estimator provides a consistent estimate of uncertainty. At these NRI points, detailed soils data and cropping history are available, but tillage, mineral fertilizer use and organic amendments are not. Monte Carlo methods are used in conjunction with the design-based methods to provide accounting for both the sampling uncertainty of NRI and the uncertainty of inputs not available from NRI. Finally, external validation data of actual soil organic carbon measurements are used to account for model uncertainty related to imperfections in the structural relationships represented in CENTURY.

This is joint work with Stephen M. Ogle and Keith Paustian, Natural Resources Ecology Laboratory, Colorado State University