Hydrology of a Semiarid Loess-Paleosol Sequence, and Implications for Buried Soil Connection to the Modern Climate, Plant-Available Moisture, and Loess Tableland Persistence

Soil hydrology provides important background for understanding the fate of organic carbon (OC) buried by geomorphic processes as well as the influence of runoff, infiltration, and plant root uptake on long-term erosion and landscape evolution. We modeled the hydrology of a 4.5-m loess-paleosol sequence on an eroding tableland in the U.S. central Great Plains using Hydrus 1D, a numerical unsaturated flow model, parameterized with high resolution measurements of the soil water retention and hydraulic conductivity curves, which were distinct for the loess and paleosols. We hypothesized that (a) the connection of paleosols to modern climate depends on their burial depth, (b) paleosols in the root zone would have broader pore-size distributions than unweathered loess, and (c) this broader pore-size distribution increased root water uptake and made vegetation more resilient to drought, increasing the stability of loess tablelands despite high erodibility and high local relief. Four years with varying total annual precipitation were simulated for the observed profile and two hypothetical profiles, one without paleosols and another with a shallow, strongly developed paleosol. In these simulations, soil moisture in shallow paleosols responds quickly to precipitation while a deeply buried paleosol is largely disconnected from the modern climate, contributing to buried OC preservation. Contrary to our expectation, the presence of paleosols did not increase root uptake relative to unweathered loess in either wet or dry years. The unweathered coarse loess we studied may have an optimal pore-size distribution for root uptake, providing an alternative hypothesis for why highly erodible loess tablelands persist. Plain Language Summary In many semi-arid or previously glaciated regions, silt-sized, wind-deposited dust accumulates forming loess deposits. Often, deposition occurred during colder and drier climates, and slowed in warmer and wetter climates, allowing soils to develop. Landforms in southwestern Nebraska are underlain by thick loess with buried soils that contain significant amounts of organic carbon (OC). They have persisted for millennia but are being eroded by gullies. Modern climate change may increase erosion reducing the area where available cropland is found and possibly releasing the carbon stored in buried soils to the atmosphere. To understand how these landscapes may respond to climate change, we used measured soil hydraulic properties of loess and buried soils to parameterize a model of water flow and storage, including runoff, infiltration, and root uptake. Moisture in shallow buried soils responds quickly to precipitation and droughts, but deeper soils are disconnected from the modern climate, helping to preserve OC. For wet and dry years, the buried soils did not increase root water uptake compared to the loess, because this loess may have optimal hydraulic properties for water availability. This characteristic of the loess, rather than soil development, may enhance vegetation growth, protecting the loess tablelands from rapid erosion.