Interactions between climate and soil hydrology influence preservation and location of carbon pools in a loess-paleosol sequence in semi-arid Great Plains, USA

Abstract:

The objective of this study is to determine if the presence of paleosols alter water infiltration through thick loess in semi-arid south-central Nebraska to better understand potential hydrological impacts on geomorphic processes, plant growth, and belowground carbon storage. This landscape is characterized by loess-paleosol sequences, and in particular, the Brady Soil, a 1-m thick paleosol found up to 6 m deep that contains significant carbon pools. We hypothesized that finer textured paleosols would have different hydraulic properties than the surrounding loess, which could influence water flow through the profile. This altered redistribution could have implications for plant growth, landscape stability in aeolian systems and location of carbon pools. The numerical model, Hydrus 1D, was used to simulate water flow and root uptake through a 4.5 m loess-paleosol sequence using measured soil hydraulic parameters (Fig 1). We ran the model for two wetter years, a dry year, and an average year to understand the impact of differing meteorological conditions. We also ran the yearly scenarios on a second experimental soil profile, in which all paleosols were removed (Fig 1). The paleosols had distinctly different hydraulic properties compared to the loess and model results confirmed that the layering of these different hydraulic properties slows water redistribution compared to the profile without paleosols (Fig 2). This altered redistribution likely influences the depth at which pedogenic carbonates form and hence, the location of inorganic carbon pools. Model results from both profiles show that meteorological conditions are the primary control on volumetric water content (VWC) at depths < 2 m. As a result, it is unlikely that landscape stability and plant growth are substantially affected by the presence of the paleosols. Paleosol removal had only minor impacts on total root uptake. At depths > 2 m, soil hydraulic properties control the generally low VWC because infiltration events rarely cause wetting deeper than 2 m. These results indicate that in the modern climate, persistent low VWC at depth contributes to the preservation of carbon stocks in the Brady Soil, when it is buried by thick loess. In sum, interactions between climate and soil hydrology influence preservation and location of carbon pools in this system.