Response of biomass and nitrogen yield of white clover to radiation and atmospheric CO₂ concentration

The objectives of the present study were to test (i) whether the effect of season-long CO₂ enrichment on plant dry matter production of white clover (Trifolium repens cv. Karina) depends on the temperature or can solely be explained by changes in radiation use efficiency, and (ii) whether the atmospheric CO₂ concentration affects the relationship between tissue %N and plant biomass. Plants were grown in pots with adequate nutrient and water supply and were exposed to ambient and above ambient CO₂ concentrations (approximately +80 ppm, +160 ppm, +280 ppm) in open-top chambers for two seasons. Nitrogen fertilizer was given only before the experiment started to promote N₂ fixation. Plants were clipped to a height of 5 cm, when the canopy had reached a height of about 20 cm and when the CO₂ effect had not been diminished due to self-shading of the leaves. Photon exposure (400–700 nm) measured above the canopy was linearly related to the above ground biomass, the leaf area index and the nitrogen yield (r² > 0.94). The slopes of the curves depended on the CO₂ concentration. Since most of the radiation (>90%) was absorbed by the foliage, the slopes were used to calculate the CO₂ effect on the radiation use efficiency of biomass production, which is shown to increase curvilinearly between 380 and 660 ppm CO₂ from 2.7 g MJ⁻¹ to 3.9 g MJ⁻¹. CO₂ enrichment increased above ground biomass by increasing the leaf number, the individual leaf weight and the leaf area; specific leaf weight was not affected. The relative CO₂ response varied between harvests; there was a slight but not significant positive relationship with mean daytime temperature. At the beginning of the season, plant nitrogen concentration in the above ground biomass was decreased by CO₂ enrichment. However, at later growth stages, when the plants depended solely on N₂ fixation, nitrogen concentration was found to be increased when the nitrogen concentration value was adjusted for the decrease due to the higher biomass of the plants exposed to elevated CO₂. (author)