Changing controls of soil water retention in an alpine catchment: Integrating sedimentological and pedological processes

Abstract

Soil’s hydrological and ecological functions depend largely on soil water retention, which is affected by both structural and compositional soil properties. Studies have shown that alpine soils derived from sediments with different origins in the northeast Qinghai-Tibetan Plateau differ significantly in terms of particle size distribution, bulk density and soil organic carbon content. However, the variation pattern and its drivers of soil water retention in this region remain unclear. In this study, soil water retention characteristics (saturated soil water capacity, field capacity, wilting point and plant available water capacity) from different geomorphic settings (i.e., hillslopes, alluvial fans and floodplains) of an alpine catchment in the western part of the Qilian Mountains were analyzed in relation to sediment types and soil properties. Results show that soil water retention varies significantly with sediment types. Soils with an aeolian origin have a higher capacity to retain water compared with soils developed on fluvial sediments. Variations in soil water retention are mainly controlled by particle size distribution for fluvial sediments and by soil organic carbon content for aeolian sediments. The accumulation of soil organic carbon enhances plant available water capacity for sandy fluvial sediments but not for silty fluvial and aeolian sediments. The critical eco-hydrological functions of the mattic surface soil horizon in relation to water retention, runoff regulation and erosion control were highlighted for the alpine ecosystems. By deciphering sedimentological and pedological controls of soil water retention, this study improves the understanding of the evolution and functioning of alpine soils, which has further implications for other sedimentary ecosystems.