Inhibition of N₂ fixation by white clover (Trifolium repens L.) at low concentrations of $NO_3^ - $ in flowing solution culture

The impact of sustained low external concentrations of $NO_3^ - $ (0,10,100 and 1000 mmol m⁻³) on plant growth and the relative acquisition of N through N₂ fixation and $NO_3^ - $ uptake by established, nodulated white clover (Trifolium repens L. cv. Bianca) was studied over 28 days in flowing solution culture. Nitrogen fixation was measured by N difference and ¹⁵N dilution methods. Plants supplied with $NO_3^ -$ achieved higher relative growth rates (x̅ = 0.091 d⁻¹) compared with 'control' plants dependent on N₂ fixation (0.073 d⁻¹). Nitrate plants showed progressive increases in shoot : root d.w. ratios from 4 to 6.5-7.6 between days 0-28, compared with 5.1 on day 28 for control plants. Increases in both nodule d.w. and numbers per plant were inhibited after day seven at all concentrations of $NO_3^ - $. The severity of inhibition of N₂ fixation increased with increasing $NO_3^ - $ concentration and with time. The total amounts of N₂ fixed per plant between days 0-7 after supplying 10,100 and 1000 mmol m⁻³ $NO_3^ - $ respectively, were 37-39, 28-30 and 0-13%, of the total N acquired. Between days 7-28 the proportional contributions of N₂ fixation to total N acquisition declined to 3,0.5 and 0%, respectively, in these treatments. The corresponding mean specific rates of N₂ fixation between days 0-7 were, respectively, 5.4, 3.2, and 2.0 mmol N d⁻¹ g⁻¹ nodule d.w., compared with 7.9 mmol N d⁻¹ g⁻¹ nodule d.w. for zero $NO_3^ - $ plants. There was no evidence of a transitory increase in N₂ fixation following the addition of $NO_3^ - $ even at the lowest supply concentration.