Interactions among nitrogen fixation and soil phosphorus acquisition strategies in lowland tropical rain forests

Background/Question/Methods

Symbiotic dinitrogen (N₂) fixation – a process that allows some plants to overcome nitrogen (N) limitation by converting atmospheric N₂ into bioavailable forms – is an energetically expensive mutualism that requires carbon and phosphorus (P) from host plants to support their microbial symbionts. Ecological theory suggests that N₂ fixers – and N₂ fixation – should have a competitive advantage in low N-high P environments, but be outcompeted – or down-regulated – in high N-low P environments. Yet, N₂ fixers are relatively rare in N-poor temperate forests, but abundant and active in N-rich lowland tropical forests. Previous research addressing this paradigm hypothesized that N₂ fixers have the capacity to acquire more soil P than non-N₂ fixers because they can invest fixed N₂ in extracellular phosphatase enzymes, giving N₂ fixers a competitive advantage in acquiring this often-limiting nutrient. We tested this hypothesis, as well as the potential ability for N₂ fixers to host more arbuscular mycorrhizal (AM) fungi than non-N₂ fixers, by comparing enzyme activities and AM colonization between active N₂ fixers and non-N₂ fixers in two lowland rainforest sites that compose a soil P gradient in Costa Rica.

Results/Conclusions

At both lowland rainforest sites, phosphatase enzyme activity and AM colonization were significantly greater on the roots of N₂ fixers than non-N₂ fixers. Also, AM colonization was strongly and positively related to both N₂ fixation rates and nodule biomass, and the nature of the relationships was similar between both lowland rainforest sites. This indicates a potential link between the two mutualistic strategies of acquiring soil N and P, though the mechanism enabling N₂ fixers to host more AM fungi than non-N₂ fixers remains unclear. Overall, our results show a consistent set of N and P interactions across a range of plant species in two functional groups (i.e., N₂ fixers and non-N₂ fixers), and across a large gradient of soil total and extractable P that comprise the two lowland rainforest sites. Our results also suggest that the enhanced P acquisition strategies provide N₂ fixers with a competitive advantage in nutrient acquisition across a wide range of soil nutrient conditions found in the tropics, and perhaps contributing to their relatively high abundance in tropical forests.