The Origins of Deep Podzolized Carbon in Coastal Plain Soils As Revealed By Soil Morphology and Biogeochemistry.

Podzolization is a widespread and important component of soil formation and carbon cycling in the southeastern United States Coastal Plain. This process is extensively studied in relatively shallow Bh horizons (shallower than one-meter) where the frequency and duration of near surface saturation exerts strong control over near surface podzolized carbon (NSPC). Coastal Plain soils however can also contain deep podzolized carbon (DPC) well below a depth of one-meter and often beyond the reach of traditional two-meter assessments. While DPC is now recognized regionally as extensive (covering up to 3 million ha) and a significant soil carbon pool (containing approximately one billion metric tons of carbon), the hydrologic processes that underpin its formation are not known.

To better constrain the relationship between hydrology and DPC, we sample, describe, and analyze deep soil profiles (up to 5-meters) along a catena in north central Florida with a gradient in near surface saturation. While horizons containing NSPC tend to lighten with depth, horizons containing DPC darken with depth. Carbon concentration follows these morphologic indicators, decreasing by up to 1.3-fold in horizons with NSPC and increasing by nearly 7-fold in horizons with DPC. We also find that horizons overlying NSPC are more strongly elluviated than those overlying DPC, suggesting that NSPC is vertically translocated while DPC may be laterally translocated. Our work distinguishes two different types of podzolized carbon that can exist on the same Coastal Plain landform and even within the same soil profile. We show that while NSPC is strongly influenced by near surface hydrology, DPC appears to be influenced by deep soil hydrology that translocates carbon laterally. We suggest that minor modifications to the way we currently describe horizons with podzolized carbon could facilitate a more comprehensive and accurate understanding of soil on the Coastal Plain landscape.