Dinitrogen fixation by legume shade trees and direct transfer of fixed N to associated cacao in a tropical agroforestry system.

Natural abundance of (15)N (δ (15)N) was determined in bulk soil, rhizospheric soil and vegetation in an organically managed cacao (Theobroma cacao L.) plantation with Inga edulis Mart. legume trees (inga) as the principal shade for studying the nitrogen (N) cycle in the system. Cacao without contact with legumes in an adjacent plantation was used as the reference for N2 fixation and direct N transfer calculations. Bulk and rhizospheric soils contained 72 and 20%, respectively, of whole- system N. No vegetation effect on δ (15)N in rhizospheric soil was detected, probably due to the high native soil N pool. Fine roots of the cacaos associated with inga contained ∼35% of N fixed from the atmosphere (Nf) out of the total N. Leaves of all species had significantly higher δ (15)N than fine roots. Twenty percent of system Nf was found in cacao suggesting direct N transfer from inga via a common mycelial network of mycorrhizal fungi or recycling of N-rich root exudates of inga. Inga had accumulated 98 kg [Nf] ha(-1) during the 14-year history of the plantation. The conservative estimate of current N2 fixation rate was 41 kg [Nf] ha(-1) year(-1) based on inga biomass only and 50 kg [Nf] ha(-1) year(-1) based on inga and associated trees.

Transfer of nitrogen from a tropical legume tree to an associated fodder grass via root exudation and common mycelial networks.

Symbiotic dinitrogen fixation by legume trees represents a substantial N input in agroforestry systems, which may benefit the associated crops. Applying (15)N labelling, we studied N transfer via common mycelial networks (CMN) and root exudation from the legume tree Gliricidia sepium to the associated fodder grass Dichantium aristatum. The plants were grown in greenhouse in shared pots in full interaction (treatment FI) or with their root systems separated with a fine mesh that allowed N transfer via CMN only (treatment MY). Tree root exudation was measured separately with hydroponics. Nitrogen transfer estimates were based on the isotopic signature of N (delta(15)N) transferred from the donor. We obtained a range for estimates by calculating transfer with delta(15)N of tree roots and exudates. Nitrogen transfer was 3.7-14.0 and 0.7-2.5% of grass total N in treatments FI and MY, respectively. Root delta(15)N gave the lower and exudate delta(15)N the higher estimates. Transfer in FI probably occurred mainly via root exudation. Transfer in MY correlated negatively with grass root N concentration, implying that it was driven by source-sink relationships between the plants. The range of transfer estimates, depending on source delta(15)N applied, indicates the need of understanding the transfer mechanisms as a basis for reliable estimates.

Applying a universal scaling model to vascular allometry in a single-stemmed, monopodially branching deciduous tree (Attim's model).

West, Brown and Enquist (1999a) modeled vascular plants as a continuously branching hierarchical network of connected links (basic structural units) that ends in a terminal unit, the leaf petiole, at the highest link order (WBE model). We applied the WBE model to study architecture and scaling between links of the water transport system from lateral roots to leafy lateral branches and petioles in Populus deltoides Bartr. ex Marsh. trees growing in an agroforestry system (open-grown trees) and in a dense plantation (stand-grown trees). The architecture of P. deltoides violates two WBE model assumptions: (1) the radii of links formed in a branching point are unequal; and (2) there is no terminal unit situated at the end of a hierarchical network, rather, petioles are situated at any link order greater than 1. Link cross sections were taken at various link orders and morphological levels in roots and shoots of open-grown trees and shoots of stand-grown trees. Scaling of link radii was area-preserving. From roots to branches, vessel diameters were scaled with link order in accordance with a 1/6-power, as predicted by the WBE model indicating general vessel tapering. However, analysis of the data at the morphological level showed that vessel radius decreased intermittently with morphological level rather than continuously between successive link orders. Estimation of total water conductive area in a link is based on conducting area and petiole radius in the WBE model. The estimation failed in P. deltoides, probably because petioles are not a terminal unit. Biomass of stand-grown trees scaled with stem basal radius according to the 3/8-power predicted by the WBE model. Thus, the WBE model adequately described vascular allometry and biomass at the whole-tree level in P. deltoides despite violation of Assumption 1, but failed in predictions where the leaf petiole was used as a terminal unit.

Effects of nitrogen source and defoliation on growth and biological dinitrogen fixation of Gliricidia sepium seedlings.

Effects of four N sources and two defoliation treatments on growth and nitrogenase activity of Gliricidia sepium (Jacq.) Walp seedlings were studied in a greenhouse. All nutrients were supplied in irrigation water to the sterile growing medium. The N sources were: (1) 100 mg l(-1) of N supplied as NO(3) (-) (high-NO(3) (-)), (2) 50 mg l(-1) of N supplied as NO(3) (-) and inoculation with Rhizobium spp. medium-NO(3) (-)), (3)100 mg l(-1) of N supplied as NH(4)NO(3), and (4) inoculation with Rhizobium spp without mineral N (N(2)). At 35 weeks after sowing, mean total biomass was 130.5, 50.5, 22.9 and 17.4 g seedling(-1) in the NH(4)NO(3), N(2), medium-NO(3) (-) and high-NO(3) (-) treatments, respectively. The root/shoot ratio was high in all of the N treatments (1.73-2.77) because the seedlings had big taproots. The medium-NO(3) (-) treatment completely inhibited nodulation, whereas seedlings in the N(2) treatment were profusely nodulated. At 32 weeks after sowing, groups of seedlings in the N(2) and high-NO(3) (-) treatments were subjected to 50 or 100% defoliation. Closed-chamber acetylene reduction assays of intact root systems were conducted to compare nitrogenase activity at 7, 14 and 28 days after defoliation (DAD). At 7 and 14 DAD, nitrogenase activity of completely and partially defoliated seedlings was about 10 and 60%, respectively, of that of undefoliated controls. At 28 DAD, nitrogenase activity of completely defoliated seedlings was twice the predefoliation value, whereas nitrogenase activity of partially defoliated seedlings was only 87% of the predefoliation value. Recovery of nitrogenase activity was strongly correlated with foliage regrowth in the completely defoliated seedlings, but not in the partially defoliated seedlings. Abundant belowground C and N reserves in the large taproot probably contributed to the rapid recovery from defoliation. Accumulation of belowground biomass may also improve defoliation tolerance of mature trees.

Influence of forage harvesting regimes on dynamics of biological dinitrogen fixation of a tropical woody legume.

Effects of three forage harvesting regimes-total removal of foliage and branches once (T-12) or twice a year (T-6) and 50% removal every 2 months (P-2)-on growth and biological dinitrogen fixation of Gliricidia sepium (Jacq.) Walp were studied under subhumid tropical conditions in Guadeloupe, French Antilles. Gliricidia sepium was grown in association with the perennial C(4) grass Dichantium aristatum (Poir) C.E. Hubbard in a two-storied fodder production system. The medium-term effects of pruning on N(2) fixation were assessed by the (15)N natural abundance method. Gmelina arborea Roxb. was used as the non-fixing reference. The trees in the T-12 regime followed the natural phenological cycle, and flowering and podfilling at the beginning of the dry season reduced both foliage and nodule biomass. The T-6 regime impeded flowering, and only a few flowers, on older branches, were produced in the P-2 regime. In trees in the T-12, T-6, and P-2 regimes, fixed N comprised 54-87, 54-92, and 60-87%, respectively, of the total N in aboveground biomass, depending on sampling date. Total annual accumulation of N in harvestable aboveground biomass was highest in trees in the T-6 regime at 313 kg ha(-1), of which 204 kg ha(-1) of N was fixed from the atmosphere. In all treatments, about 70% of the N exported per year from the plot in the fodder harvest came from N(2) fixation. Thus, N(2) fixation makes an important contribution to the N economy of the G. sepium-D. aristatum forage production system, and greatly reduces the need for fertilizer application.