Aridity and reduced soil micronutrient availability in global drylands

Drylands cover more than 40% of the terrestrial surface, and their global extent and socioecological importance will increase in the future due to the forecasted increases in aridity driven by climate change. Despite the essential role of metallic micronutrients in life chemistry and ecosystem functioning, it is virtually unknown how their bioavailability changes along aridity gradients at the global scale. Here, we analysed soil total and available copper, iron, manganese and zinc in 143 drylands from all continents, except Antarctica, covering a broad range of aridity and soil conditions. We found that total and available micronutrient concentrations in dryland soils were low compared with averages commonly found in soils of natural and agricultural ecosystems globally. Aridity negatively affected the availability of all micronutrients evaluated, mainly indirectly by increasing soil pH and decreasing soil organic matter. Remarkably, the available Fe:Zn ratio decreased exponentially as the aridity increased, pointing to stoichiometric alterations. Our findings suggest that increased aridity conditions due to climate change will limit the availability of essential micronutrients for organisms, particularly iron and zinc, which together with other adverse effects (for example, reduced water availability) may pose serious threats to key ecological processes and services, such as food production, in drylands worldwide.