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Context Increases in atmospheric carbon dioxide concentration ([CO2 ]) 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, it has become clear that the way precipitation controls elevated CO2 (eCO2 ) effects on grassland biomass is strongly seasonal but no mechanism yet exists to explain these observations. Aims The aims of this study were to determine how seasonal water availability affects aboveground, belowground and total biomass responses of a perennial ryegrass pasture to [CO2 ]. Methods We established the TasFACE2 experiment in a well-fertilised perennial ryegrass (Lolium perenne ) monoculture with four seasonal irrigation schedules and three [CO2 ]. Key results The total biomass production of perennial ryegrass pasture was strongly stimulated by eCO2 , but this extra biomass was preferentially allocated to belowground growth. 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. Conclusions Increases in soil moisture caused by eCO2 contributed to increases in root growth, but root biomass production was also stimulated directly by eCO2 . Restriction of irrigation, therefore, suppressed the belowground response to eCO2 and created a non-linear response of biomass to CO2 concentration. Implications Antagonistic above- and belowground responses mean that the rising [CO2 ] might not increase pasture production in the manner generally predicted.