The symbionts made me do it: legumes are not hardwired for high nitrogen concentrations but incorporate more nitrogen when inoculated

Abstract

High tissue nitrogen (N) concentrations in N-fixing legumes may be driven by an evolutionary commitment to a high N strategy, by higher N availability from fixation, or by some other cause. To disentangle these hypotheses, we asked two questions: are legumes hardwired to have high N concentrations? Aside from delivering fixed N, how does inoculation affect legume N concentrations? In order to understand drivers of plant stoichiometry, we subjected four herbaceous legume species to nine levels of N fertilization in a glasshouse. Half of the individuals were inoculated with crushed nodules, whereas the other half remained uninoculated and could not fix N. Across four legume species, we found that tissue stoichiometry and nutrient content were more plastic than has been described for any other plant species. In addition, inoculated plants had higher tissue N concentrations than N fixation activity alone can explain. Rather than being hardwired for high N or phosphorus (P) demand, the legumes we examined were highly flexible in their nutrient allocation. Understanding the drivers of legume N concentrations is essential to understanding the role of N fixers in community- and ecosystem-level processes.