COS 111-7
Interactions among nitrogen fixation and soil phosphorus acquisition strategies in lowland tropical rain forests

Thursday, August 14, 2014: 3:40 PM
311/312, Sacramento Convention Center
Megan K. Nasto, Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT
Silvia Alvarez-Clare, Department of Ecosystem and Conservation Sciences, The University of Montana, Missoula, MT
Ylva Lekberg, Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT
Benjamin W. Sullivan, Department of Ecosystem and Conservation Sciences, The University of Montana, Missoula, MT
Alan R. Townsend, INSTAAR and Department of Ecology and Evolutionary Biology, University of Colorado at Boulder, Boulder, CO
Cory C. Cleveland, Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT
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-Nfixers 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.