Estimations of rooting depths and sources of plant-available water (PAW) in flatland petrocalcic soils under different land uses

Abstract

Plant-available water (PAW) is one of the most important issues in productive landscapes around the world. Besides climatic conditions, it depends on several factors, among them, land uses, the rooting and water table depths, and the presence of soil heterogeneities (e.g., petrocalcic horizons). In the present work, PAW evolution was assessed over 441 days in a vertical 1D domain of a monitored Pampean petrocalcic soil under two different land uses and contrasted water demand (i.e., natural and afforested grasslands). For this purpose, rooting depths were estimated using a process-based numerical model of the aquifer-soil-plant-atmosphere continuum constrained by both in situ measured atmospheric demand and water table fluctuations. This model was calibrated against soil water content measurements at different depths and finally, as validation, computed transpiration rates were compared with the sap flow measurements. Modeling results, and calibrated rooting depths, suggest that petrocalcic horizons induce a limited hydraulic connection between the shallow soil portions and the phreatic aquifer. Thus, water storage is enhanced above the petrocalcic horizon in the natural grassland plot particularly during the periods of low atmospheric demand, whereas it is immediately consumed by forest to soil wilting point below the afforested grassland (hydric stress). The numerical methodology followed here appears as an alternative to estimate the forest groundwater usage in afforested grasslands, a key issue in soil salinization research in great plain areas.