Vegetation effects on soil pore structure and hydraulic properties in volcanic ash soils of the high Andes

Abstract

Soil hydraulic properties control the provision of hydrological services. Vegetationand topography influence these properties by altering soil structure and porosity. Theunderlying mechanisms are not yet fully understood for the high Andean region. Inthis study, we examined how vegetation and topographic attributes are related to soilhydraulic properties and soil pore structure in young volcanic ash soils, and furthercorrelated them to soil texture, organic carbon, and root characteristics to explainthese relationships. In a 0.7 km2study site located in the Andean páramo of northernEcuador, we measured soil water retention, saturated hydraulic conductivity, bulkdensity (BD), and pore size distribution parameters on eight soil profiles with con-trasting vegetation types (cushion-forming plants vs. tussock grasses) and topo-graphic positions (summit vs. hillslope). We observed significant differences in soilhydraulic properties and soil pore structure in the uppermost horizons by vegetationtype, whereas topography had a minor effect. In the A horizons, we found higherwater retention at saturation and field capacity (10%–14%), higher total availablewater (8%–15%), and higher saturated hydraulic conductivity (4–12 times) undercushion-forming plants compared to tussock grasses. The elevated values under cush-ion plants were attributed to the presence of larger pores, lower soil BD, and highersoil organic carbon content as a result of coarser root systems. Total available waterwas generally high (0.34–0.40 cm3cm 3), and locally not associated with any soilproperty. The higher water retention in soils under cushion vegetation can enhancesoil water storage for plants and the regulation of water flows during prolonged rainfall events. The saturated hydraulic conductivity of the surface horizons is highcompared to rainfall intensities resulting in high infiltration capacity; whilst its declinewith depth reveals the potential for generation of subsurface stormflow, especiallybelow cushion-forming plants. Our findings highlight that soil hydraulic properties dif-fer among vegetation types, and show the significance of vegetation types for soilhydrology