Contrasting effects of aridity and seasonality on global salinization

Soil salinization is the most diffuse form of soil degradation in drylands, where it represents a rising threat to crop production and ecosystem functioning. While aridity is thought to be the main driver of salt accumulation, the role played by other forms of climatic forcing, such as rainfall seasonality and synchronicity between precipitation and atmospheric water demand, remains uncertain. Here, we use a combination of global climatic data, soil observations and ecohydrological models to show that average precipitation and seasonality have contrasting impacts on soil salinization. Aridity enhances salinization by lowering soil moisture and suppressing leaching events. By contrast, rainfall seasonality can reduce salt accumulation in the soil by boosting percolation during the wet season, when salt removal efficiency is greatest. Consequently, salt removal is more effective in regions where seasonality is coupled with strong asynchronicity between water supply and demand, such as in Mediterranean climates. As a result, neglecting the interplay of aridity, seasonality and asynchronicity may lead to inaccurate assessments of the impacts of climate on global soil salinization.