Interspecies competition and N transfer in a tropical grass-legume mixture

Competitiveness of Brachiaria decumbens cv. Basilisk and Stylosanthes guian ensis cv. Minerao was investigated either without root restriction or by se parating their root systems with a fine mesh or a solid barrier in the pres ence or absence of mycorrhiza (Glomus clarum). Nitrogen transfer between th e legume and the grass was assessed with the N-15 isotope dilution techniqu e using a relatively stable N-15-enriched soil derived from a long-term lab elling experiment. During establishment, legume development was severely re stricted by competition from the grass in pots without a root barrier. Howe ver, as the system became N limited, the legume became dominant due to its access to atmospheric Nz which contributed over 80% of the legume N require ments. S. guianensis was highly mycotrophic and inoculation with mycorrhiza favoured rapid establishment even in the treatments with no root barrier. Only in the presence of root barriers, either a mesh or a complete compartm ent separation, was the proportion of N derived from N-2 fixation positivel y affected by the presence of the fungus. No significant direct belowground N transfer from legume to grass was observed during the lifetime of the le gume suggesting that the legume maintains a highly efficient recycling unde r N-limited conditions. However, after cutting the shoot at ground level, t he grass assimilated significant amounts of N derived from decaying legume roots. We conclude that the main pathway of belowground N transfer from S. guianensis to associated B. decumbens occurred via decomposing roots rather than via root exudates or direct mycorrhizal hyphae transfer.