Moister soils at elevated CO 2 stimulate root biomass production but suppress aboveground biomass production in a temperate pasture

Increases in atmospheric CO 2 concentration ([CO 2 ]) drive increases in biomass production via impacts on photosynthesis and water use. In grasslands, the scale of this stimulation is related to soil water availability. Recently, seasonal influences on how precipitation controls elevated CO2 (eCO 2 ) effects on grassland biomass production have emerged. We established the TasFACE2 experiment in a well-fertilised perennial ryegrass (Lolium perenne) monoculture with four seasonal irrigation schedules to determine how seasonal water availability affects aboveground, belowground and total biomass responses to three CO 2 concentrations. While total biomass production was strongly stimulated by eCO 2 , this effect was almost entirely driven by belowground responses. The relationship between soil water content and aboveground biomass varied seasonally but there was a strong positive relationship between soil water content and root biomass production in all seasons. Increases in soil moisture caused by eCO 2 , therefore, contributed to increases in root growth. However, root biomass production was also stimulated directly by eCO 2 regardless of eCO 2 impacts on soil moisture, further increasing the biomass allocation belowground. Restriction of irrigation, therefore, suppressed the belowground response to eCO 2 and created a non-linear response of biomass to CO 2 concentration. Thus, total biomass responses to eCO 2 cannot be predicted from aboveground measurements alone.