The Evolution of Soil Hydrological and Physical Properties under the Impact of Mineral Weathering and Organic Matter Sequestration

Abstract:

Soil water characteristic (SWC) curve is important for modeling unsaturated flow and reactive transport. SWC also likely influences mineral weathering by affecting water-mineral interactions in the critical zone. The depth profiles of SWC could form as a result of long-term chemical weathering and physical erosion processes. We measured SWCs on samples collected from soil pit and drill cores which captured the weathering profiles from ground surface to slightly weathered bedrock (~7m depth) on a hillslope in Laurel Preserve in West Chester, PA. The weathering profile represents the co-evolution of hydrological and physical properties with varied degrees of weathering and organic matter (OM) content. A WP4C Dewpoint Potentiameter was used to obtain the SWCs (matric potential range: -300 ~ -1.5MPa) for original and OM-removed samples, where OM was removed by heating samples to 350℃ for 24 h. From the WP4C measurement, soil samples from shallower depths (≤44cm) retained more water at a given matric potential, with water contents at shallower depths generally ~2-3 times more than that for saprolite samples from deeper depths at similar matric potentials, indicating that soil water retention increased with degree of weathering. The SWCs measured by WP4C were used to estimate the specific surface area (SSA) using the Tuller and Or (TO) model (Tuller and Or, 2005) , which is based on the assumption that water forms a film on micropore surfaces at the dry end (matric potential ≤ -1.5MPa). The SSA estimation from the TO model followed the same trend as SSA measured with the N₂-BET method. However, the estimated SSA was greater than BET-based SSA, indicating possible underestimation of SSA by the N₂-BET method. The comparison of SWC and SSA between original and OM-removed samples indicated the presence of a threshold depth of ~40cm. At depths shallower than 40cm, especially at A horizon (OM ≥ 9.5Wt%), OM removal significantly decreased estimated SSA and water content at similar matric potentials, while at deeper depths (OM ≤ 2.7Wt%), the OM-removal treatment resulted in either no change or a slight increase in SSA. The possible existence of a threshold depth indicates that the interactions between OM-mineral sorption and water-mineral interaction differ with depth and are particularly strong within OM-rich surface soils.