Long-term effects of CO2enrichment and temperature increase on the carbon balance of a temperate grass sward

Abstract

Perennial ryegrass swards were grown in large containers on a soil, at two N fertilizer supplies and were exposed during two years in highly ventilated plastic tunnels to elevated (700 μl l−1 [CO2) or ambient atmospheric CO2 concentration at outdoor temperature and to a 3°C increase in air temperature in elevated CO2. The irrigation was adjusted to obtain a soil water deficit during summer. The daily net C assimilation was increased in elevated CO2 by 29 and 36% at the low and high N supplies, respectively. Canopies grown in elevated CO2 for 14 to 27 months photosynthetized significantly less rapidly, in both elevated and normal CO2 concentrations, than their counterparts developed in ambient CO2 but the magnitude of this effect was small (−8% to −13%). Elevated CO2 resulted in a large increase in the fructan concentration in the pseudostems and 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. A 3°C temperature increase in elevated CO2 did not affect significantly the average WSC concentrations in the pseudostems and laminae. The elevated CO2 effects on the net C assimilation and on the nocturnal shoot respiration were greater in summer than in spring. On average, a 35% increase in the below-ground respiration was measured in elevated CO2. At the high N supply, a 3°C increase in air temperature led to a decline in the below-ground respiration due to a low soil moisture. The below-ground carbon storage was increased by 32% and 96% in elevated CO2 at the low and high N supplies, respectively, with no significant increased temperature effect. The role for the below-ground carbon storage of CO2-induced changes in the root fraction of the grass and of temperature-induced changes in the moisture content of the soil are discussed.