LONG-TERM EFFECTS OF CO2 ENRICHMENT AND TEMPERATURE INCREASE ON A TEMPERATE GRASS SWARD .1. PRODUCTIVITY AND WATER-USE

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 mu L L(-1) [CO2]) or ambie nt atmospheric CO2 concentration at outdoor temperature and to a 3 deg rees C increase in air temperature in elevated CO2. These swards were either fully irrigated (kept at field capacity) in each climatic condi tion (W+), or received the same amount of water in the three climate t reatments (W-). In the latter case, the irrigation was adjusted to obt ain a soil water deficit during summer and drainage in winter. Using a lysimeter approach, the evapotranspiration, the soil water balance, t he 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 du ring the growing season: a small, and sometime not significant effect (+6%, on average) was obtained in spring and in autumn, while the summ er growth response was stronger (+48%, on average). In elevated CO2, t he temperature increase effect on the annual above-ground dry-matter y ield 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 grow ing season and increased drainage by 9% during winter. A supplemental 3 degrees C in elevated CO2 reduced the drainage by 29-34%, whereas th e evapotranspiration was increased by 8 and 63% during the growing sea son and in winter, respectively. During the growing season, the soil m oisture content at W- and at the high N supply declined gradually in t he control climate, down to 20-30% of the water holding capacity at th e last cut (September) before rewatering. This decline was partly alle viated under elevated CO2 in 1993, but not in 1994, and was enhanced a t +3 degrees C in elevated CO2. The water use efficiency of the grass sward increased in elevated CO2, on average, by 17 to 30% with no sign ificant interaction with N supply or with the soil water deficit. The temperature increase effect on the annual mean of the water use effici ency was not significant. Highly significant multiple regression model s show that elevated CO2 effect on the dry-matter yield increased with air temperatures above 14.5 degrees C and was promoted by a larger so il moisture in elevated compared to ambient CO2. The rate of change in relative dry-matter yield at +3 degrees C in elevated CO2 became nega tive for air temperatures above 18.5 degrees C and was reduced by a lo wer soil moisture at the increased air temperature. Therefore, the alt ered climatic conditions acted both directly on the productivity and o n the water use of the grass swards and, indirectly, through changes i n the soil moisture content.