The use of silvopastoral systems with tree legumes is a viable alternative to recover and develop pastures, as they add N to the system influencing pasture growth. This study hypothesized that the herbage and litter of signalgrass (Urochloa decumbens Stapf) is affected by legume trees in the pasture. Treatments were composed of (1) signalgrass + Mimosa caesalpiniifolia Benth.; (2) signalgrass + Gliricidia sepium Jacq.; and (3) signalgrass monoculture. The 3-year experiment followed a randomized complete block design with three replications. Tree legumes were planted in double rows (15 × 1 × 0.5 m), in 1 ha paddocks. Litter samples were taken in five distance points (0, 1.8, 3.7, 5.6 and 7.5 m) perpendicular to tree legume rows. Signalgrass was taller at longer distances from the trees (P < 0.05). Signalgrass height differed between treatments, with taller signalgrass found in pastures mixed with G. sepium (15.6 cm) compared to M. caesalpiniifolia (9 cm) (P < 0.05). Herbage N content decreased with increasing distance from tree rows (P < 0.05). Litter N content followed a similar pattern, ranging from 23 g/kg under the trees to 12 g/kg at 7.5 m away from tree rows. Signalgrass did not grow under the tree crown (0–1.8 m), especially when intercropped with M. caesalpiniifolia. The findings of this study suggest that the type of legume trees used in the silvopastoral system has the potential to modify the pattern of grass growth and content of N in pasture litter.

Interactive effects of elevated atmospheric CO2 and soil N availability on N2 fixation and biomass production were examined using Gliricidia sepium, a tropical leguminous tree species. Our objective was to determine if elevated CO2 alters the inhibitory effects of soil N on N2 fixation, and whether the response of Gliricidia to elevated CO2 was a function of N source originating from either substrate N fertilizer or N2 fixation. We hypothesized that CO2 enrichment would ameliorate the inhibitory effects of N fertilization on seedling nodulation and N2 fixation through increased C partitioning to nodules. Seedlings were grown from seed for 100 d in growth chambers at either 350 or 700 μmol mol−1 CO2. Seedlings were inoculated with Rhizobium spp. and grown either with 0, 1 or 10 mM N fertilizer. The δ15N isotope-dilution technique was used to determine N source partitioning between N2 fixation and inorganic N fertilizer uptake. The addition of 10 mM N fertilizer significantly reduced nodule number and mass, specific nitrogenase activity, the specific rate of N2 fixation, and the proportion of plant N derived from N2 fixation. Elevated CO2, however, strongly ameliorated the inhibitory effects of N fertilization, indicating that increased C availability for nodule activity may partially offset the inhibition of N2 fixation caused by substrate N, as nodule sugar concentrations were stimulated with CO2 enrichment. This study clearly shows that elevated CO2 enhanced plant productivity and net N content of Gliricidia tree seedlings by stimulating N2 fixation. In addition, seedling biomass production was greatly enhanced by elevated CO2, regardless of whether plant N was derived from the substrate or from the atmosphere. We conclude from this study that CO2 enrichment mitigates the inhibitory effects of substrate N on nodule initiation and development and specific N2 fixation, either through increased C allocation to nodule production and activity, or through increased N demand by the plant for biomass production. This experiment with Gliricidia provides evidence for a positive feedback between increased atmospheric CO2 concentrations, C allocation to symbiotic N2-fixing bacteria, and plant C and N accumulation that may occur when N2-fixing plants are grown under conditions where substrate N may typically inhibit N2 fixation.

Evaluations of toxicity, antifeedant, growth-regulatory activity of the methanol extract of Gliricidia sepium leaves were carried out against Dysdercus koenigii Fabr. (Heteroptera: Pyrrhocoridae), Achaea Janata Linn, and Spodoptera litura Fab., (both Lepidoptera: Noctuidae). Topical application of 100-300 μg of the extract to different stages of the test insects resulted in dose-dependent effects including larval-pupal intermediates and deformed adults. The highest tested dosage (300 μg/ μl) resulted in reduced survival rate and morphological abnormalities. At certain doses a strong antifeedant activity was evident against the lepidopteran insects.