Field symbiotic fixation using 15N : II. Vicia faba L. and Pisum sativum L.

In Portugal, results on the amount of N2-fixed and efficiency of grain legumes-rhizobia symbiosis under field conditions are scarce. A two-years experiment in two Orthic Luvisols of Elvas (ENMP and CV), with uninoculated and inoculated fababeans and peas, was set in randomized blocks, replicated five times. Barley was used as control crop. At sowing, 20 kg N ha−1 as 15(NH4)2SO4 4.8 atom% 15N excess were applied in each microplot. Plants were harvested at physiological maturity. In the 1st year experiment, dry-matter, N yield and fixation in both crops were higher at CV soil, probably due to its higher fertility, namely the higher Mo content. In fababean,% Ndfa varied from 65% at ENMP soil to 90% at CV, corresponding to N2-fixed of 80 and 125 kg N ha−1; as to pea,% Ndfa ranged from 35% at ENMP to >85% at CV, corresponding 22 and 107 kg N2-fixed ha−1. Sites and years statistically affected crops, mainly by the deficient Mo level at ENMP, and drought stress in the 2nd year. Inoculation did not significantly affect fixation of fababean, but significantly affected pea. Values of% Ndfa were >60%, for both crops, at both sites, but due to the hydric stress in the 2nd year, N2-fixed was lower than in the 1st year: an average of 70 kg N ha−1 by fababean, and 37 to 55 kg N ha−1, respectively by uninoculated and inoculated peas at CV. At ENMP, pea poorly fixed N2, and negative values for% Ndfa in the pea straw were found.