Long-term effects of nitrate on lucerne (Medicago sativa L.) nitrogen fixation is not influenced by the denitrification status of the microsymbiont

Studies on the inhibitory effects of combined nitrogen on biological nitrog en fixation in legume crops have been usually carried out after short-term nitrate treatments at high concentrations. As these treatments are quite di fferent from field conditions, a study was conducted to evaluate the effect s of the continuous presence of nitrate (0, 1, 5 and 10 mM) throughout thre e months on lucerne (Medicago sativa L.). Plants were grown in a greenhouse with perlite as substrate and were inoculated with a denitrifying Sinorhiz obium meliloti strain (102-F-51) and a non-denitrifying strain (102-F-65). During the first 60 days of growth, the highest nitrate treatment resulted in a complete inhibition of the main symbiotic parameters (nodule initiatio n and development and specific nitrogen fixation) in plants inoculated with either strain. However, after 3 months of growth in the presence of nitrat e, this inhibition was partly abolished, with a high number of new function ing nodules being formed. Acetylene reduction activity (ARA) of these plant s was 70% of the control plants. As this process was observed in plants nod ulated with either strain, it is concluded that this was not related to the denitrifying ability of the strain, but is an intrinsic property of the lu cerne nitrogen fixing system. As legume plants usually grow under natural f ield conditions in the continuous presence of nitrate, the ability to use s imultaneously nitrate and atmospheric nitrogen could be of adaptive and agr onomic importance.