Subsurface accumulation of CaCO3 and Cl− from groundwater under black locust and poplar plantations

Abstract

When conditions are similar, more water evaporates from forest plantations than herbaceous vegetation, thereby affecting hydrological fluxes and ion transport in the soil. The vertical distribution of CaCO3 and Cl− ions shifts due to afforestation. The effect of groundwater depth and clay content were studied in the Great Hungarian Plain where forest area has been increasing for decades by analyzing soil and groundwater samples from stands of black locust (Robinia pseudoacacia, 11 plots) and poplar (Populus spp., 11 plots). All study sites contained one herbaceous (control) and one or more forested plots. CaCO3 and Cl− ions accumulated in the soil profile in greater quantities under tree cover than in the controls. The scale of this process largely depended on the species and on soil and ion properties. Under black locust, Cl− accumulated between 1.3 and 6.3 m, with a maximum difference of 0.3 pCl unit (pCl is Cl− activity, the negative of the logarithm to base 10 of the concentration of the chloride ion, determined using an ion-selective electrode, it is a dimensionless quantity.), while the difference in CaCO3 accumulation was at most 3.5% in some layers, compared to control plots. This result may be explained by the difference in the mobility of Ca+ and Cl− ions. Different mechanisms were noticeable under poplar plantations due to their higher water uptake: Cl− accumulation was detected below 0.9 m to the groundwater with a maximum difference of 0.5 pCl units, while CaCO3 accumulation was continuous at depths of 2.3–6.8 m with a maximum difference of 8.4%, compared to the controls. With increasing clay content, there was a discernible effect on CaCO3 and Cl− accumulation under black locust, but not observed under poplars. These differences were explained by the differences in water uptake mechanisms and root patterns of the two species and the different mobility of Ca2+ and Cl− ions.