The mechanistic links between nitrogen (N) availability and investment in plant phosphorus (P) acquisition have important implications for plant growth, species distributions and responses to CO2 fertilization under global change, especially in P-poor tropical ecosystems. Currently, it is unclear whether investment in strategies that enhance plant P acquisition (arbuscular mycorrhizal, AM, colonization and/or root phosphatase activity, RPA) are determined primarily by phylogeny, or whether these strategies differ between N2-fixing legumes and non-fixing plants as a result of differing N availability. We hypothesized that plant N status, which can vary widely independent of N fixation, correlates with investment in P acquisition, because: 1) N and P concentrations scale in plant tissue indicative of coupled demand and 2) plants with more N may have more resources available to allocate to acquisition strategies. To test this, we compared seedlings of eight tropical tree species from three families (including three N2-fixing and one non-fixing legume) that represented almost the full range of foliar N observed in tropical trees. Seedlings were grown in field soil for four months under greenhouse conditions.
Results/Conclusions
Neither foliar N nor P concentrations correlated with investment in P acquisition. Across all species, we found an inverse relationship between investment in AM colonization and RPA, but this trade-off was unrelated to foliar N or P and did not differ between functional types (i.e. N2-fixers versus non-fixers). Instead, this trade-off depended on relative growth rate, with faster growing plants investing proportionally more in AM. Within legumes (family Fababceae), two strategies were evident that were unrelated to fixation status. High-fixing Inga and non-fixing Dialium displayed high foliar N and P concentrations and greater proportional investment in RPA versus AM, while two lower-fixing Ormosia species were characterized by lower foliar nutrient concentrations and proportionally more investment in AM. We conclude that investment in P acquisition strategies in tropical trees is not dependent on foliar N or the ability to fix N. Instead, our results suggest that investment is controlled, in part, by resource trade-offs related to plant growth. Finally, our observation of highly variable strategies among closely related species cautions again the use of simple functional groupings to draw conclusions about nutrient acquisition in tropical trees.