Effect of Early Mycorrhizal Infection on Nodulation and Nitrogen Fixation in Trifolium subterraneum L

Three experiments are described. Rapid establishment of vesicular-arbuscular mycorrhizas in roots of T. subterraneum cv. Mt Barker, using natural soil inoculum, was associated with improved nodulation, increased nitrogenase activity per plant (nmol C2H2 reduced per plant per hour) and increased nodule efficiency on the basis of nodule volume (nmol C2H2 reduced per mm� nodule per hour). In two experiments (on soil low in nutrients), this increase occurred before any positive growth response to mycorrhizal infection was apparent. In all experiments, mycorrhizal roots had a higher phosphorus concentration (%P) than did non-mycorrhizal roots. This difference, which was evident before any differences in total plant phosphorus were detected, was not accompanied by an increase in nodule phosphorus concentration, so that differences in nodule efficiency could not be attributed to differences in this parameter. In the third experiment (on soil with higher nutrient levels), establishment of mycorrhizas was also accompanied by increased growth, phosphorus and nitrogen contents within a 35-day experimental period. Phosphorus inflow into roots (moles P per cm root per second) was higher in mycorrhizal plants. Delay in formation of mycorrhizas (by reduction in the amount of inoculum in soil) was accompanied by lower inflow, and delay in both the establishment of high root phosphorus concentration and in the onset of enhanced nodulation and nitrogenase activity.