Determination of the amount of N fixed by faba beans and its residual effect as green manure on a succeeding potato crop using 15N techniques in the field

A study was conducted under field conditions to assess the amount of N fixed by faba beans and the residual effect of the N incorporated into the soil with the residues of the harvest on a succeeding potato crop using the isotope dilution method. In the first part of the experiment, Vicia faba cv. Vica was inoculated with Rhizobium leguminosarum bv. viceae, strain CIAT-339; a treatment without innoculation was also included. As the reference plant, Triticum vulgare cv. Icata was used. In the second part, non-labelled faba bean plant residues were chopped to a length of less than 5 cm and incorporated into the soil after harvesting the green pods. Forty-five days after incorporating of these residues, Solanum tuberosum cv. Pardopastusa was sown in all the plots. As reference, those plots were used that had been under fallow in the first part of the experiment. The study demonstrated the existence of indigenous Rhizobium strains that fixed 64% of the plant N from the atmosphere. Inoculation increased the fixation efficiency to 90% (172 kg N/ha). When whole residues of faba bean were incorporated as green manure, they contributed nearly 120 kg N/ha to the soil, constituting a net input (100 kg N/ha) of atmospheric N to the system; when only the roots were used, a net depletion in soil N occurred. Potato used only 40% of the fertilizer N, indicating that a high proportion was immobilized or lost from the system. Potato used approximately 20% of the N applied as faba bean green mature. This would reduce the fertilizer N to be used nearly 30 kg/ha, decreasing the risk of contamination of waters with the N lost from the fertilizer. (author). 16 refs, 4 tabs