Importance of soil hydrology in understanding location and persistence of soil carbon in semiarid loess-paleosol sequence

Our aim is to determine how soil hydrology influences root-zone and deep soil carbon in a loess-paleosol sequence in semiarid southwest Nebraska, USA. Our study site is characterized by large flat to gently sloping plateaus or tables with gullies incising at the table margins. Tables are underlain by thick loess, which contains buried soils characterized by substantial organic carbon (OC) and inorganic carbon (IC). Past climate changes are responsible for the formation and initial preservation of the buried soils (Marin-Spiotta et al., 2014), which formed during wetter climates and then were buried after climatic changes towards drier conditions produced more dust. We investigated the role of soil hydrology in persistence of buried soil OC and accumulation of pedogenic carbonates. We measured the soil water retention curves (SWRCs), for a 4.5 m loess-paleosol sequence, which were used to parameterize a numerical unsaturated flow model, Hydrus 1D. We modeled four different meteorological scenarios from drought to wet conditions. The SWRC measurements showed significant differences between the loess and the paleosols. However, these differences had a small impact on root water uptake. Model results showed that when paleosols were simulated in the root zone as observed, root water uptake decreased by 0.91-1.4 cm/yr compared to a modeled hypothetical profile without paleosols. Additionally, model results showed that wetting rarely reaches depths greater than 1 m, and the annual water flux at 85 cm, the top of a root zone paleosol, was less than 5 cm for all four modeled meteorological scenarios. Around this depth, there is a peak in IC indicating that soil hydrology is an important control on depth of soil carbonate accumulation. Persistent low soil moisture below 100 cm limits interaction of an older paleosol, the Brady Soil, with the modern climate and is a major factor in OC preservation in this soil (233-347 cm). There is also an IC peak at ~350 cm associated with the Brady Soil, and microscopic carbonate morphology observations suggests that it is pedogenic. It is likely that this IC formed when the Brady Soil was at or near the surface and has been preserved. Overall, these results suggest that the carbon and water cycles are tightly linked and change in response to both climate and geomorphology in this semiarid landscape.