The response of simulated swards of perennial ryegrass and white clover to enriched atmospheric C0₂: Interaction with nitrogen and photosynthetic photon flux density

Abstract

This paper reports the growth response to elevated CO 2 concentration of artificial swards of clover (Trifolium repens L. cv. Grasslands Pitau), ryegrass (Lolium perenne L. cv. Grasslands Ruanui), and mixed clover -ryegrass. The growth rate of the swards was varied by changing nitrogen nutrition and photosynthetic photon flux density (PPFD). We examined whether the response to elevated CO 2 was the same in a dense, established sward as previously reported for isolated plants. Artificial swards of clover and ryegrass were grown separately or together in 15 cm 2 pots. The swards were established in a glasshouse and then transferred to 3 controlled environment rooms at 20 D C day/15 D C night, with a 12 h photoperiod. After 3 weeks in the controlled environment rooms, all combinations of the following treatments were imposed: 340, 600, or 900 cm 3 C02/m 3 ; 185 or 460 p.mollm 2 per s PPFD; and 20 or 200 ppm nitrogen. The swards were harvested 4 times at II -15-day intervals beginning 117 days after sowing. At harvest they were cut to 2.5 cm above the soil surface. Mean growth rate was constant over the final 2 harvests. Over all treatments the growth rate of clover was stimulated by 5211,70 at the elevated C02 concentrations, ryegrass growth rate by up to 3911,70, and the mixed swards by 32070. Growth rate for the mixed sward was higher (0.231 g/ day per pot) than for the pure swards (0.185 g/day per pot clover; 0.175 glday per pot ryegrass). There were no significant interactions for the final 2 harvests between sward type and C02 concentration, although the response of clover to elevated C02 concentrations tended to be greater than that of ryegrass. There were significant interactions of nitrogen nutrition and PPFD with CO 2 concentration and these are discussed. These responses to increased CO2 concentration were similar to reported results on spaced plants. The results are interpreted in terms of the balance between the supply of carbohydrate from photosynthesis and the utilisation of that carbohydrate during growth. Some implications for the breeding of grass and clover are discussed.

Direct effects of increasing carbon dioxide on pasture plants and communities

“…At subsequent cuts a response of clover to CO 2 was not evident, whereas ryegrass showed increasing gains in productivity until the final regrowth period. These results are in contrast to those recorded by Hardacre et al (1986). In their mini-swards, with defoliation, clover growth was more strongly enhanced by CO 2 than ryegrass growth.…”

“…The response to CO 2 is generally stronger at higher Photon Flux Densities (PPFDs). An exception appears to be the response of ryegrass in the experiment of Hardacre et al (1986), which was found to have a stronger enhancement of growth rate at the lower PPFD. However, the "high" PPFD treatment in this experiment was only 460 ~ mol!…”

“…values of 7.0 for ryegrass and 3.5 for clover (Brougham 1958), with a much greater relative shortfall for the clover component. In contrast, Hardacre et al (1986) defoliated when 90% of the PPFD was intercepted at the soil surface. At this point, the swards must have been approaching their optimal LAI which Brougham (1956) has suggested occurs when 95% of incident light is intercepted.…”

“…Clover proportion increased. Overdieck & Reining (1986) Hardacre et al (1986) CO 2 increased survival of Marks & Strain (1989) C 3 in drought and increased competitiveness over C 4 in both droughted and well-watered conditions. C 3 increased in competitiveness at high CO 2 .…”

Elevated CO2 and temperature effects on soil carbon and nitrogen cycling in ryegrass/white clover turves of an Endoaquept soil

“…In a recently established mixed sward on an artificial medium, elevated CO2 markedly stimulated the growth of ryegrass and white clover (Hardacre et al, 1986). In contrast, elevated CO2 had a comparatively small effect on plant yields in our older pasture communities, and then mainly at the higher temperatures.…”