Interactions among plants, symbiotic N-fixing bacteria, and arbuscular mycorrhizal fungi in tropical rain forest: Results from a theoretical model

Background/Question/Methods

Biogeochemical theory posits that net primary production (NPP) in tropical forests is predominantly phosphorus (P) limited, a hypothesis that is consistent with both long-term losses of P via weathering and erosion, and increases in nitrogen (N) availability via N fixation through time. Despite some evidence for P (and little evidence for N) limitation, symbiotic N-fixing trees are surprisingly abundant in tropical forests. Efforts to resolve this paradox suggest that symbiotic N₂ fixation may indirectly enhance P acquisition by stimulating phosphatase enzyme production and/or enhancing mycorrhizal fungal colonization. To address this hypothesis, we developed a theoretical model to investigate interactions between plants (legumes), symbiotic N₂-fixing bacteria (Rhizobia), and arbuscular mycorrhizal fungi (AM), and parameterized the model with data from a lowland tropical rain forest site in Costa Rica to address the following questions: (1) Under what environmental conditions do the carbon (C) benefits of obtaining nutrients via symbiosis with both arbuscular mycorrhizae and rhizobia outweigh the C costs of associating with Rhizobia alone?; and (2) how sensitive are these predictions to model parameters and environmental conditions? We hypothesized that the NPP of trees with both N₂-fixing and AM symbionts would exceed rates of NPP of trees without symbionts. 

Results/Conclusions

Our initial results suggest that under most environmental conditions – and absent large inputs of exogenous N, tropical plant productivity is commonly P limited. NPP of plants in symbioses with either N₂-fixing bacteria or AM fungi was higher than NPP of plants without symbionts. Surprisingly, plants with both symbiotic N₂-fixing bacteria and AM achieved lower NPP rates than plants with AM alone, suggesting that in the long term, maintaining both symbionts doesn’t enhance NPP. However, during early succession or during phases of ecosystem development when N availability is relatively low, there could be a short-term benefit to having both symbionts. In addition, the model results suggest that tropical NPP is most sensitive to plant stoichiometry, the fraction of the plant that is lost via death and litterfall, and N resorption rates. By contrast, rates of N fixation are most sensitive to the bioavailability of P, Michaelis-Menten kinetics of plant P uptake, and P resorption. These results imply that symbiosis with both N₂-fixing bacteria and AM may be beneficial when nutrient availability is highly heterogeneous, providing selective pressures for plants to associate with both AM and N₂-fixing bacteria even though this strategy does not appear to maximize NPP.