Assessing the impact of associated vegetation and soil properties on water and solute fluxes along the soil profile depth in the high Ecuadorian Andes

The high tropical Andes ecosystem, known as páramo, provides important hydrological services to densely populated areas in the Andean region. In order to sustainably manage these services, it is crucial to understand the biotic and abiotic processes controlling water and solutes. However, there is still a knowledge gap regarding the role played by soil-vegetation interactions in controlling soil-water processes and resulting water and solute fluxes. Therefore, we compare the soil hydrological processes and geochemical fluxes in two soil profiles under different vegetation types (cushion plants - CU vs. tussock grass - TU) in the páramo of northern Ecuador. We further relate these processes and fluxes to soil properties along the soil profile.

Soil hydrological processes were daily modeled in the period of Jan/2019- Mar/2021 and further solute fluxes were estimated biweekly. The water flux in the soil profiles was simulated by calibrating and validating the 1D HYDRUS model based on observed soil moisture at three depths. The resulting fluxes were further independently validated by field-measured water fluxes. Soil solution was collected biweekly at different soil horizons, analyzed in the field and lab for anions, cations, DOC and Si. Finally, solute concentrations were verified by ionic balance and used to estimate solute fluxes.

Our results suggest that soil hydrological processes under CU are controlled by high evapotranspiration (77 ± 5%) as a result of higher water content available at the uppermost soil layer produced by large air-entry head and large total available water associated to lowest bulk density and highest saturated hydraulic conductivity (K_SAT). This results in lower water fluxes as well as lower deep drainage. Contrastingly, TU profile is dominated by higher water fluxes and deep drainage (55 ± 10%) resulting from lower evapotranspiration related to lower air-entry head at the uppermost layer along with a uniform K_SAT with depth. On the other hand, the impact of vegetation type on concentration and solute fluxes was mainly restricted to the uppermost horizon. Solute fluxes were significantly higher under CU for HCO₃ (~13 mg 15days^-1) and especially for non-particulate organic carbon (NPOC) fluxes (~21 mg 15 days-1) compared to TU (8 and ~1 mg 15days^-1 for HCO₃ and NPOC, respectively). These fluxes can be related to higher total organic carbon (TOC) under CU in the A horizon. The highest NPOC flux can also be related to the highest K_SAT. On the contrary, in the same uppermost horizon, under TU fluxes of NO₃, Ca and Mg were significantly higher compared to CU.

This contribution provides a case study that illustrates the existence of associations between vegetation types and soil properties with water and solute fluxes along the soil profile depth in the Ecuadorian high Andes.