Using Strategic-Level Forest Inventory and Auxiliary Variables to Estimate Soil Organic Carbon on Forest Land in the United States.

Soil organic carbon (SOC) is the largest terrestrial carbon (C) sink on Earth and this pool plays a critical role in ecosystem processes and feedbacks to atmospheric composition and the rate of climate change. Given the cost and time required to measure SOC, and particularly changes in SOC, many of the United Nations Framework Convention on Climate Change signatory nations report estimates of SOC stocks and stock changes using default values from the Intergovernmental Panel on Climate Change or country-specific models. In the United States (US), SOC in forests is monitored by the national forest inventory (NFI) conducted by the Forest Inventory and Analysis (FIA) program within the US Department of Agriculture, Forest Service. The FIA program has been consistently measuring soil attributes as part of the NFI since 2001 and has amassed an extensive inventory of SOC in forest land in the conterminous US and coastal Alaska. That said, the FIA program has been using country-specific predictions of SOC based, in part, upon a model using SOC estimates from the State Soil Geographic (STATSGO) database compiled by the Natural Resources Conservation Service. Estimates obtained from the STATSGO database are averages over large map units and are not expected to provide accurate estimates for specific locations. To improve the accuracy of SOC estimates in forests of the US, NFI SOC observations were used, for the first time, to predict SOC density to a depth of 100 cm for all forested NFI plots. Incorporating soil-forming factors along with observations of SOC into a new estimation framework resulted in a 136 percent (47.96±0.89 Mg·ha^-1) increase in SOC densities nationally. This increase will substantially increase the contribution of the SOC pool in the forest C budget of the US and in future United Nations Framework Convention of Climate Change reports.