Pedogenic and Spatial Characteristics of a Massive and Understudied Soil Carbon Pool.

The Atlantic Coastal Plain contains Spodosols that can have massive carbon accumulations belowthe Bh horizon. These carbon-rich subsoil layers are often designated as B’h horizons, implying that their formation is a result of pedogenic translocation and illuviation of carbon. However, since the Atlantic Coastal Plain is a depositional landform, an alternate explanation has been made that the massive carbon accumulations formed as a result of burial events. We evaluate these competing hypotheses by conducting a regional meta-analysis of physical and chemical soil properties, and soil profile descriptions from the Florida Soil Characterization Program (n≈1300 profiles and n≈8000 horizons). Additionally, we use geospatial analyses to make a first order estimate of the spatial distribution of these subsoil layers, and their potential contribution to regional soil carbon budgets.

Our findings suggest that these deep subsoil layers formed as a result of pedogenic translocation and illuviation of carbon rather than deposition and burial. The sand size distribution (% mass) in all profiles containing carbon-rich subsoil horizons (n=53) is indistinguishable throughout the entire profile. Further, we find that sodium-pyrophosphate extractable carbon constitutes approximately 70% of total carbon in both Bh horizons and in carbon-rich layers below Bh horizons, but only 25% in A horizons. This is significant, because Spodosol A and Bh horizons can be distinguished by their relative proportion of total carbon that is sodium-pyrophosphate extractable (a lower ratio exists in A horizons where relatively more carbon is plant detritus and less carbon is metal associated). Beyond elucidating pedogenic mechanisms, we reveal that these subsoil horizons are distributed predictably across the United States Atlantic Coastal Plain (in close association with ancient shorelines dating as early as the mid-Pliocene), and demonstrate that they are significantly underestimated components of terrestrial carbon budgets, containing as much carbon on a per hectare basis as the top 1 meter of permafrost soil.