Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions II. Nitrogen Fixation and Nitrate Uptake

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Abstract
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 harvested at intervals over an d88 d growht period. The swards received a nutrient solution daily, which was either High (220 mg l−1) or Low (10 mg l−1) in nitrate N. The nitrate was labelled with the 15N isotope. An acetylene reduction assay was carried out on each sward just prior to harvest. Rates of acetylene reduction agreed qualitatively with the l5N analyses but absolute values did not match (assuming a 4:1 C2H4:N2 ratio) and errors in the acetylene assay are discussed. In the Low-N swards clover relied almost entirely on symbiotically fixed N2, fixing more than ten times as much as the High-N clover plants. In the Low-N treatment the grass was N-deficient despite obtaining much more nitrate per unit root dry weight than clover. In the High-N swards, however, clover took up more nitrate per unit root weight than grass. The High-N clover plants also fixed some N2 and maintained a higher total-N content than grass throughout the period. There was no evidence of transfer of symbiotically fixed N from the clover to the grass in either treatment.