Long-term effects of CO2 enrichment and temperature increase on the carbon balance of a temperate grass sward

Perennial ryegrass swards were grown in large containers ona soil, at two N fertilizer supplies and were exposed duringtwo years in highly ventilated plastic tunnels to elevated (700µl l^–1 CO₂) or ambient atmospheric CO₂ concentrationat outdoor temperature and to a 3C increase in air temperaturein elevated CO₂. The irrigation was adjusted to obtain a soilwater deficit during summer. The daily net C assimilation wasincreased in elevated CO₂ by 29 and 36% at the low and highN supplies, respectively. Canopies grown in elevated CO₂ for14 to 27 months photosynthetized significantly less rapidly,in both elevated and normal CO₂ concentrations, than their counterpartsdeveloped in ambient CO₂ but the magnitude of this effect wassmall (–8% to –13%). Elevated CO₂ resulted in alarge increase in the fructan concentration in the pseudostemsand laminae (+46% and +189%, respectively). In elevated CO₂,the hexose and sucrose pool increased by 28% in the laminae,whereas it did not vary significantly in the pseudo-stems. A3C temperature increase in elevated CO₂ did not affect significantlythe average WSC concentrations in the pseudostems and laminae.The elevated CO₂ effects on the net C assimilation and on thenocturnal shoot respiration were greater in summer than in spring.On average, a 35% increase in the below-ground respiration wasmeasured in elevated CO₂. At the high N supply, a 3C increasein air temperature led to a decline in the below-ground respirationdue to a low soil moisture. The below-ground carbon storagewas increased by 32% and 96% in elevated CO₂ at the low andhigh N supplies, respectively, with no significant increasedtemperature effect. The role for the below-ground carbon storageof CO₂-induced changes in the root fraction of the grass andof temperature-induced changes in the moisture content of thesoil are discussed. Key words: Climate change, grassland, gas exchange, carbohydrates, carbon cycle