Estimating biological N-2 fixation by a tropical legume tree using the non-nodulating phenophase as the reference in the N-15 natural abundance method

The non-nodulating phenophase of a legume tree was tested as a non-N-2-fixi ng reference in application of the N-15 natural abundance method for estima ting the N-2 fixation. We applied this method to study the effects of three pruning intensities, complete pruning every 6 months (T-6), ca. 50% prunin g every 3 months (P-3) and intact control (C), on N-2 fixation in Erythrina lanceolata (Papilionaceae) planted as shade and support trees for vanilla (Vanilla planifolia) in a subhumid tropical site in Quepos, Costa Rica. We measured nodulation and N2 fixation for 12 months. The trees under the C re gime nodulated abundantly during the rainy season vegetative growth but did not nodulate during the dry season and flowering. A linear regression (r(2 ) = 0.76) was observed between the nodule biomass and ON values under the C regime, suggesting quite a stable specific N-2 fixation rate. The stable d elta N-15 values throughout the year in the non-N-2-fixing Morus nigra (Mor aceae) growing on the same soil indicated that the temporal variation in th e plant available soil N-15 was low. We used the intercept term of the regr ession (delta N-15 = 3.5 when nodule biomass was 0) as the non-N-2-fixing d elta N-15 value when estimating the percentage of N fixed from atmosphere o ut of tree total N. The percentage varied from 0 during the driest period t o 53% during rainy season. Pruning reduced rainy season nodulation under th e T-6 and P-3 regimes almost to nil, and the delta N-15 values were high. O ur results suggest that the conditions for using the non-nodulating phenoph ase as the non-N-2-fixing reference required in the N-15 natural abundance method were fulfilled. The C trees showed a clear phenological cycle in N-2 fixation, while pruning severely disturbed the N-2 fixation under the T-6 and P-3 regimes, indicating that E. lanceolata is better suited for agrofor estry systems in which the trees are not managed by periodic prunings. (C) 2001 Elsevier Science Ltd. All rights reserved.