Chronic drought alters extractable concentrations of mineral elements in Mediterranean forest soils

Soil mineral elements play a crucial role in ecosystem productivity and pollution dynamics. Climate models project an increase in drought severity in the Mediterranean Basin in the coming decades, which could lead to changes in the composition and concentrations of mineral elements in soils. These changes can have significant impacts on the fundamental processes of plant-soil cycles. While previous studies have predominantly focused on carbon, nitrogen, and phosphorus, there is a notable lack of research on the biogeochemical responses of other mineral elements to increasing drought. In this study, we investigated the effects of chronic drought (15 years of experimental rainfall exclusion) and seasonal drought (summer period) on the extractable soil concentrations of 17 mineral elements (arsenic (As), calcium (Ca), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), potassium (K), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), sulphur (S), strontium (Sr), vanadium (V) and zinc (Zn)) in a Mediterranean holm oak forest. We also explored the potential biotic and abiotic mechanisms underlying the changes in extractable elemental concentrations under chronic drought conditions. Our findings reveal that soil elemental concentrations varied significantly due to seasonal changes and chronic drought, with soil microclimate, biological activity, and organic matter being the main drivers of this variability. Levels of soil water content primarily explained the observed variations in soil elemental concentrations. Most of the mineral elements (13 out of 17) exhibited higher concentrations during winter-spring (wet seasons) compared to summer-autumn (dry seasons). The chronic drought treatment resulted in K limitation, increasing vegetation vulnerability to drought stress. Conversely, the accumulation of S in soils due to drought may intensify the risk of S losses from the plant-soil system. Under drought conditions, certain trace elements (particularly Mn, V, and Cd) exhibited increased extractability, posing potential risks to plant health and the exportation of these elements into continental waters. Overall, our results suggest that alterations in mineral element concentrations under future drier conditions could promote ecosystem degradation and pollution dispersion in the Mediterranean Basin. Understanding and predicting these changes are essential for effective ecosystem management and mitigating the potential negative impacts on plant health and water quality.