title: Biome-scale nitrogen fixation strategies selected by climatic constraints on nitrogen cycle.
abstract: Dinitrogen fixation by plants (in symbiosis with root bacteria) is a major source of new nitrogen for land ecosystems(1). A long-standing puzzle(2) is that trees capable of nitrogen fixation are abundant in nitrogen-rich tropical forests, but absent or restricted to early successional stages in nitrogen-poor extra-tropical forests. This biome-scale pattern presents an evolutionary paradox(3), given that the physiological cost(4) of nitrogen fixation predicts the opposite pattern: fixers should be out-competed by non-fixers in nitrogen-rich conditions, but competitively superior in nitrogen-poor soils. Here we evaluate whether this paradox can be explained by the existence of different fixation strategies in tropical versus extra-tropical trees: facultative fixers (capable of downregulating fixation(5,6) by sanctioning mutualistic bacteria(7)) are common in the tropics, whereas obligate fixers (less able to downregulate fixation) dominate at higher latitudes. Using a game-theoretic approach, we assess the ecological and evolutionary conditions under which these fixation strategies emerge, and examine their dependence on climate-driven differences in the nitrogen cycle. We show that in the tropics, transient soil nitrogen deficits following disturbance and rapid tree growth favour a facultative strategy and the coexistence of fixers and non-fixers. In contrast, sustained nitrogen deficits following disturbance in extra-tropical forests favour an obligate fixation strategy, and cause fixers to be excluded in late successional stages. We conclude that biome-scale differences in the abundance of nitrogen fixers can be explained by the interaction between individual plant strategies and climatic constraints on the nitrogen cycle over evolutionary time.
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA.|Institute of Plant Sciences and Genetics in Agriculture, The Robert H. Smith Faculty of Agriculture, The Hebrew University of Jerusalem, Israel.
authors.forename: Efrat
authors.initials: E
authors.lastname: Sheffer
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA.|School of Geography, University of Leeds, Leeds, UK.
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Batterman
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Simon A
authors.initials: SA
authors.lastname: Levin
authors.affiliation: Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA.
authors.forename: Lars O
authors.initials: LO
authors.lastname: Hedin