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

Abstract

Perennial ryegrass swards were grown in large containers on a soil, at two N fertilizer supplies, and were exposed over 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. These swards were either fully irrigated (kept at field capacity) in each climatic condition (W+), or received the same amount of water in the three climate treatments (W-). In the latter case, the irrigation was adjusted to obtain a soil water deficit during summer and drainage in winter. Using a lysimeter approach, the evapotranspiration, the soil water balance, the productivity (dry-matter yield) and the water use efficiency of the grass swards were measured. During both years, elevated CO₂ increased the annual above-ground drymatter yield of the W- swards, by 19% at N- and by 14% at N+. Elevated CO₂ modified yield to a variable extent during the growing season: a small, and sometime not significant effect (+6%, on average) was obtained in spring and in autumn, while the summer growth response was stronger (+48%, on average). In elevated CO₂, the temperature increase effect on the annual above-ground dry-matter yield was not significant, due to a gain in dry-matter yield in spring and in autumn which was compensated for by a lower summer productivity. Elevated CO₂ slightly reduced the evapotranspiration during the growing season and increased drainage by 9% during winter. A supplemental 3°C in elevated CO₂ reduced the drainage by 29–34%, whereas the evapotranspiration was increased by 8 and 63% during the growing season and in winter, respectively. During the growing season, the soil moisture content at W- and at the high N supply declined gradually in the control climate, down to 20–30% of the water holding capacity at the last cut (September) before rewatering. This decline was partly alleviated under elevated CO₂ in 1993, but not in 1994, and was enhanced at +3°C in elevated CO₂. The water use efficiency of the grass sward increased in elevated CO₂, on average, by 17 to 30% with no significant interaction with N supply or with the soil water deficit. The temperature increase effect on the annual mean of the water use efficiency was not significant. Highly significant multiple regression models show that elevated CO₂ effect on the dry-matter yield increased with air temperatures above 14.5°C and was promoted by a larger soil moisture in elevated compared to ambient CO₂. The rate of change in relative dry-matter yield at +3°C in elevated CO₂ became negative for air temperatures above 18.5°C and was reduced by a lower soil moisture at the increased air temperature. Therefore, the altered climatic conditions acted both directly on the productivity and on the water use of the grass swards and, indirectly, through changes in the soil moisture content.