Soil moisture dynamics of calcareous grassland under elevated CO₂

Water relations of nutrient-poor calcareous grassland under long-term CO₂ enrichment were investigated. Understanding CO₂ effects on soil moisture is critical because productivity in these grasslands is water limited. In general, leaf conductance was reduced at elevated CO₂, but responses strongly depended on date and species. Evapotranspiration (measured as H₂O gas exchange) revealed only small, non-significant reductions at elevated CO₂, indicating that leaf conductance effects were strongly buffered by leaf boundary layer and canopy conductance (leaf area index was not or only marginally increased under elevated CO₂). However, these minute and non-significant responses of water vapour loss accumulated over time and resulted in significantly higher soil moisture in CO₂-enriched plots (gravimetric spot measurements and continuous readings using a network of time-domain reflectometry probes). Differences strongly depended on date, with the smallest effects when soil moisture was very high (after heavy precipitation) and effects were largest at intermediate soil moisture. Elevated CO₂ also affected diurnal soil moisture courses and rewetting of soils after precipitation. We conclude that ecosystem-level controls of the water balance (including soil feedbacks) overshadow by far the physiological effects observed at the leaf level. Indirect effects of CO₂ enrichment mediated by trends in soil moisture will have far-ranging consequences on plant species composition, soil bacterial and faunal activity as well as on soil physical structure and may indirectly also affect hydrology and trace gas emissions and atmospheric chemistry.