Alluvial landform and the occurrence of paleosols in a humid-subtropical climate have an effect on long-term soil organic carbon storage

Abstract

Soil organic carbon (SOC) storage in alluvial depositional zones is important as it serves as a centennial to millennial timescale sink of C. This study examines SOC variability in alluvial bars, floodplains and terraces in a forested, humid-subtropical setting along the Clarks River in the Mississippi River basin (MRB) using replicated soil sampling, characterization and modeling. Soil and depositional profiles range in age from a minimum of 8000 years ago to modern. Normalized (by depth) mean SOC stocks (kg m−2) of the surface soil and buried layers, i.e., buried soils and layers underlying lithologic discontinuities, are highest in the terraces. Normalized mean surface SOC stocks decrease from terraces to floodplains and then to bars; whereas normalized mean buried layer SOC stocks decrease from terraces to bars, and then floodplains. The depth distribution of stocks showed that buried layers accounted for >50% of the total SOC pool, regardless of landform type. A regression tree analysis shows that soil horizon, pH, landform, and magnetic susceptibility were significant predictors of mean SOC content. Notably, the regression tree shows that alkaline pH (>7.9), observed only in buried layers in the terrace, is an important predictor in higher mean SOC. These alkaline soil pH values were associated with paleosol calcite, where the δ13C and 14C from the calcite suggests it formed under C3 vegetation during the drier than modern mid-Holocene hypsithermal, ~8000 years ago. Buried soil and sediment contain the majority of the SOC in this humid-subtropical valley bottom, while landform position and past buried soil-forming environments (paleosols) played an important role in the storage of that SOC. This work reconfirms that there is a long record of a passive, buried SOC pool found deep in our river valley bottoms that holds clues to the past biogeochemical cycling of C on land prior to intensive agriculture and industrialization. The legacy of past Holocene climates and paleosols are directly linked to our current understanding of long-term storage of SOC.