Time course of biological nitrogen fixation, nitrogen absorption and biomass accumulation in three woody legumes

In order to determine how dependent a woody legume can be upon potential N sources, the time course of N2 fixation and N absorption was studied on Gliricidia sepium cv. ILG50, Leucaena leucocephala cv. K28, and Albizia lebbeck in a screenhouse for 16 months using Senna siamea as a reference tree. Dry matter and N yield in all four species increased steadily with time. The numbers and dry weight of nodules also increased with tree age. N-difference and N dilution methods correlated well (r = 0.87–0.93) in defining an increasing pattern of N2 fixation with time in all species, with the highest increment rate between sowing and 8 months. The percent N2 fixed ranged from 17.9 to 74%, 27.7 to 71.9% and 43.6 to 83.6%, equivalent to 191 to 3385 mg, 321 to 2863 mg and 533 to 6419 mg N in Leucaena, Gliricidia and Albizia, respectively, for the period between 4 and 16 months. In contrast to others, G. sepium had its peak of amount of N2 fixed at 12 months after planting. In both the proportion and amount of N2 fixed over time, A. lebbeck was superior to L. leucocephala and G. sepium which were similar. The proportion of plant N derived from absorption (soil+fertilizer) over time decreased inversely to the increase in symbiotic N2 fixation, with soil being the predominant source. This study evidenced the benefit of relying on N2-fixing woody legumes as sustainable N-supplying sources to soil-crop systems