Soil moisture dynamics of calcareous grassland under elevated CO2

Water relations of nutrient-poor calcareous grassland under long-term CO2 e nrichment were investigated. Understanding CO2 effects on soil moisture is critical because productivity in these grasslands is water limited. In gene ral, leaf conductance was reduced at elevated CO2, but responses strongly d epended on date and species. Evapotranspiration (measured as H2O gas exchan ge) revealed only small, non-significant reductions at elevated CO2, indica ting that leaf conductance effects were strongly buffered by leaf boundary layer and canopy conductance (leaf area index was not or only marginally in creased under elevated CO2). However, these minute and non-significant resp onses of water vapour loss accumulated over time and resulted in significan tly higher soil moisture in CO2-enriched plots (gravimetric spot measuremen ts and continuous readings using a network of time-domain reflectometry pro bes). Differences strongly depended on date, with the smallest effects when soil moisture was very high (after heavy precipitation) and effects were l argest at intermediate soil moisture. Elevated CO2 also affected diurnal so il moisture courses and rewetting of soils after precipitation. We conclude that ecosystem-level controls of the water balance (including soil feedbac ks) overshadow by far the physiological effects observed at the leaf level. Indirect effects of CO2 enrichment mediated by trends in soil moisture wil l have far-ranging consequences on plant species composition, soil bacteria l and faunal activity as well as on soil physical structure and may indirec tly also affect hydrology and trace gas emissions and atmospheric chemistry .