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

Abstract

 Competitiveness of Brachiaria decumbens cv. Basilisk and Stylosanthes guianensis cv. Minerão was investigated either without root restriction or by separating their root systems with a fine mesh or a solid barrier in the presence or absence of mycorrhiza (Glomus clarum). Nitrogen transfer between the legume and the grass was assessed with the ¹⁵N isotope dilution technique using a relatively stable ¹⁵N-enriched soil derived from a long-term labelling experiment. During establishment, legume development was severely restricted by competition from the grass in pots without a root barrier. However, as the system became N limited, the legume became dominant due to its access to atmospheric N₂ which contributed over 80% of the legume N requirements. 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 compartment separation, was the proportion of N derived from N₂ fixation positively affected by the presence of the fungus. No significant direct belowground N transfer from legume to grass was observed during the lifetime of the legume suggesting that the legume maintains a highly efficient recycling under N-limited conditions. However, after cutting the shoot at ground level, the 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.