Effect of nitrogen supply on the grass and clover components of simulated mixed swards grown under favourable environmental conditions. I. Carbon assimilation and utilization

Simulated mixed swards of Perennial Ryegrass (Lolium perenne L.) cv. S23 and White clover (Trifolium repens L.) cv. S100 were grown from seed under a constant 20 °C day/15 °C night temperature regime and their growth and carbon economy examined. The swards received a nutrient solution daily, which contained either High (220 mg l−1) or Low (10 mg l−1) nitrate N. Rates of canopy photosynthesis and respiration, and final dry matter yields were similar in the two treatments although the proportions of grass and clover differed greatly. The Low-N swards were made up largely of clover. The grass plants in these swards had high root: shoot ratios and low relative photosynthetic rates – both signs of N deficiency – and were clearly unable to compete with the vigorously growing Low-N clover plants. These had higher relative growth rates and dry matter yields than their High-N counterparts. In the High-N swards clover contributed around 50 per cent to the sward dry weight throughout the measurement period despite having a smaller proportion of its dry weight in photosynthetic tissue (laminae) than grass over much of it. The latter was compensated for, initially by a higher specific leaf area than grass, and later by a higher photosynthetic rate per unit leaf weight. The results are discussed in relation to observed declines in the clover content of swards after the addition of nitrogen fertilizer in the field.