Estimating Forest Soil Organic Carbon in the United States Using National Forest Inventory Data.

The Forest Inventory and Analysis (FIA) program of the USDA Forest Service monitors organic carbon stocks in forest soil profiles as part of a national forest inventory (NFI) program mandated by the US Congress. We sample surficial soil profiles (surface litter and 0-20 cm) with standardized protocols, and we have amassed an extensive inventory of forest soil organic carbon (SOC) in the conterminous US and coastal Alaska since 2001. Here we describe a new modeling framework to replace previous submissions to the United Nations Framework Convention on Climate Change (UNFCCC) and report profile-informed estimates of forest soil carbon density (Mg∙ha^-1) obtained from the NFI and used in negotiations for the Paris Conference of Parties scheduled for December, 2015. We extended the NFI estimates to a depth of 30 cm, aligning them with IPCC good practice guidance, and 100 cm for direct comparison with the current model predictions. This was accomplished using generalized soil carbon profiles extracted from the International Soil Carbon Network’s database for forested landscapes. Plot-level estimates were expanded to the broader landscape using auxiliary information (e.g., forest type, soil order, and climate data) in RandomForests. Initial comparisons between the new profile-informed estimates of mineral SOC and current models suggest that the US has been underestimating the contribution of mineral SOC in the Nation’s forest C budget across all regions and forest types. This is particularly the case in the Western US where the new NFI estimates are more than 76 percent larger (29.35±17.47 Mg∙ha^-1) than current model predictions.