Biological nitrogen fixation by fababean, pea and chickpea, under field conditions, estimated by the 15N isotope dilution technique

In southern Europe, information on the biological nitrogen (N-2) fixation capacity of grain legumes in the field is scarce. At two locations in Portugal on Haplic Luvisols, N-2 fixation by fababean, pea and chickpea was evaluated over two consecutive years by the N-15 isotope dilution technique, using barley as the control crop. Crop growth and N-2 fixation varied between sites and years. The legumes derived more than 60% of their N from the atmosphere under favourable soil conditions, but the proportion was reduced vu hen the availability of soil moisture and molybdenum (Mo) were constrained. Annual rates of N-2 fixation by uninoculated fababean varied from 76 to 125 kg N ha(-1), with a regular rainfall distribution during the growth cycle (the first year experiment), and from 55 to 72 kg N ha(-1) under drought stress (the second year experiment). Annual rates of N-2 fixation by uninoculated peas varied from 31 to 107 kg N ha(-1) with regular precipitation, and from 4 to 37 kg N ha(-1) under drought stress. Chickpea was tested only in a drought year, when it fixed from 19 to 24 kg N ha(-1). Mean values for N derived from the atmosphere by chickpea varied from more than 70% at one location to less than 45% at another, where Mo was later found to be deficient. Inoculation with Rhizobium leguminosarum did not significantly affect the N-2 fixed by fababean in the drought year, whereas peas fixed 50% or more N after inoculation. Inoculation with Bradyrhyzobium cicer did not improve fixation by chickpea. Based on the N economy of the shoots, it appeared that any N addition to the soil in harvest residues varied with crop and location. (C) 1999 Elsevier Science B.V. All rights reserved.