FIELD-MEASUREMENTS OF NITROGEN-FIXATION IN LEGUMINOUS TREES USED IN AGROFORESTRY SYSTEMS - INFLUENCE OF N-15-LABELING APPROACHES AND REFERENCE TREES

Appropriate N-15-labeling methods are crucial for estimating N-2-fixat ion in trees used in agroforestry systems. A 4-year field experiment w as conducted on an Alfisol in Southwestern Nigeria to compare the esti mates of N-2 fixed in Leucaena leucocephala, using two non-N-2-fixing leguminous trees, Senna siamea and S. spectabilis, as reference plants and three different methods of introducing N-15 into soil. The atom % N-15 uptake pattern (as reflected in the leaves) was identical in bot h N-2- and non-N-2-fixing tree species irrespective of the N-15-applic ation method. There was a significant decline in atom % N-15 excess in the leaves of L. leucocephala (from 0.266 to 0.039), S. siamea (0.625 to 0.121), and S. spectabilis (from 0.683 to 0.118) from the first sa mpling 12 months after planting and the second sampling 18 months afte r sampling. From the second harvest in 1991 until the end of the exper iment (fifth) harvest in 1993, however, the atom N-15% excess decline in leaves of the three species was less pronounced and depended on the method of N-15 application. In those plants to which the tracer was a pplied once at planting, the N-15 decline was steady between the secon d and the last prunings. In the split-application treatment, the atom N-15% excess increased slightly at the third pruning and decreased dur ing the subsequent two prunings. The reference tree and the method of N-15 application influenced the estimated proportion of N derived from atmospheric N-2 by L. leucocephala, calculated as 73 and 64%, corresp onding to 119 and 98 kg N ha(-1) of N-2 fixed per 6 months, when S. sp ectabilis and S. siamea were used as reference trees, respectively. Th e approach by which N-15-labeled fertilizer was applied to the soil in three splits gave slightly higher estimates of N derived from the atm osphere but this was of little agronomic significance because total N- 2 fixed was similar for all methods.