Inhibition of N-2 fixation by white clover (Trifolium repens L.) at low concentrations of NO3- in flowing solution culture

The impact of sustained low external concentrations of NO3- (0, 10, 100 and 1000 mmol m(-3)) on plant growth and the relative acquisition of N through N-2 fixation and NO3- uptake by established, nodulated white clover (Trifolium repens L. cv. Blanca) was studied over 28 days in flowing solution culture. Nitrogen fixation was measured by N difference and N-15 dilution methods. Plants supplied with NO3- achieved higher relative growth rates ((x) over bar=0.091 d(-1)) compared with 'control' plants dependent on N-2 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 NO3-. The severity of inhibition of N-2 fixation increased with increasing NO concentration and 3 with time. The total amounts of N-2 fixed per plant between days 0-7 after supplying 10,100 and 1000 mmol m(-3) NO3-, respectively, were 37-39, 28-30 and 0-13%, of the total N acquired. Between days 7-28 the proportional contributions of N-2 fixation to total N acquisition declined to 3, 0.5 and 0%, respectively, in these treatments. The corresponding mean specific rates of N-2 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 NO3- plants. There was no evidence of a transitory increase in N-2 fixation following the addition of NO3-, even at the lowest supply concentration.