Nitrogen and phosphorus interact to control tropical symbiotic N2 fixation: a test in Inga punctata

Symbiotic di-nitrogen (N-2) fixation is a critical biogeochemical process in tropical forests, yet it remains unresolved how fixation is controlled by the availability of soil nitrogen and phosphorus, two nutrients often considered limiting in terrestrial ecosystems. We examine whether individual N-2-fixing trees can overcome nitrogen and phosphorus constraints by employing different strategies of nutrient acquisition and use: N-2 fixation, phosphatase exudation, mycorrhizal symbiosis and changes in root-shoot ratio or tissue stoichiometry. We grew a common and widespread N-2 fixer, Inga punctata, in a full factorial nitrogen and phosphorus addition experiment (each nutrient at three levels) and evaluated whether trees adjusted their strategies of nutrient acquisition to overcome limitation. N-2 fixation was controlled by nitrogen availability in phosphorus-sufficient soils, but both fixation and plant growth were constrained by phosphorus in the unamended native phosphorus-poor soils. Despite the investment in both extracellular phosphatases and mycorrhizal symbionts, plants were unable to overcome phosphorus limitation. Our findings support the hypotheses that: (i) N-2 fixation is proximately controlled by nitrogen availability, consistent with a facultative fixation strategy, and (ii) N-2 fixation and N-2 fixer biomass growth are ultimately constrained by soil phosphorus. We found no support for the hypothesis that fixers can overcome phosphorus limitation by trading fixed N-2 for soil phosphorus. Synthesis. This study provides new knowledge about how nitrogen and phosphorus interact to regulate tropical N-2 fixation by examining a suite of strategies that plants may employ to overcome nutrient limitation. These findings, focused at the organismal level, have broader implications for biogeochemical controls at the ecosystem level in tropical forests.