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 N-2 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-15 dilution methods correlated well (r = 0.87-0.93) in defining an increasing pattern of N-2 fixation with time in ail species, with the highest increment rate between sowing and 8 months. The percent N-2 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 N-2 fixed at 12 months after planting. In both the proportion and amount of N-2 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 N-2 fixation, with soil being the predominant source. This study evidenced the benefit of relying on N-2-fixing woody legumes as sustainable N-supplying sources to soil-crop systems.