Warming and Elevated CO2 Interact to Alter Seasonality and Reduce Variability of Soil Water in a Semiarid Grassland

Global changes that alter soil water availability may have profound effects on semiarid ecosystems. Although both elevated CO₂ (eCO₂) and warming can alter water availability, often in opposite ways, few studies have measured their combined influence on the amount, timing, and temporal variability of soil water. Here, we ask how free air CO₂ enrichment (to 600 ppmv) and infrared warming (+?1.5 °C day, +?3 °C night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. We found that eCO₂ and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming on soil water largely offset positive effects of eCO₂. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with eCO₂. These interactions constrained the combined effect of eCO₂ and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, eCO₂ increased soil water under drier conditions, and warming decreased soil water under wetter conditions. By increasing soil water under dry conditions, eCO₂ also reduced temporal variability in soil water. These temporal patterns explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with eCO₂ plus warming in autumn. They also suggest that eCO₂ and warming may favor plant species that grow in autumn, including winter annuals and C₃ graminoids, and species able to remain active under the dry conditions moderated by eCO₂.