Nutrient acquisition, soil phosphorus partitioning and competition among trees in a lowland tropical rain forest

We hypothesized that dinitrogen (N₂)- and non-N₂-fixing tropical trees would have distinct phosphorus (P) acquisition strategies allowing them to exploit different P sources, reducing competition. We measured root phosphatase activity and arbuscular mycorrhizal (AM) colonization among two N₂- and two non-N₂-fixing seedlings, and grew them alone and in competition with different inorganic and organic P forms to assess potential P partitioning. We found an inverse relationship between root phosphatase activity and AM colonization in field-collected seedlings, indicative of a trade-off in P acquisition strategies. This correlated with the predominantly exploited P sources in the seedling experiment: the N₂ fixer with high N₂ fixation and root phosphatase activity grew best on organic P, whereas the poor N₂ fixer and the two non-N₂ fixers with high AM colonization grew best on inorganic P. When grown in competition, however, AM colonization, root phosphatase activity and N₂ fixation increased in the N₂ fixers, allowing them to outcompete the non-N₂ fixers regardless of P source. Our results indicate that some tropical trees have the capacity to partition soil P, but this does not eliminate interspecific competition. Rather, enhanced P and N acquisition strategies may increase the competitive ability of N₂ fixers relative to non-N₂ fixers.