Abstract
Fossil and phylogenetic evidence indicates legume-rich modern tropical forests replaced Late Cretaceous palm-dominated tropical forests across four continents during the early Cenozoic (58–42 Ma). Tropical legume trees can transform ecosystems via their ability to fix dinitrogen (N2) and higher leaf N compared with non-legumes (35–65%), but it is unclear how their evolutionary rise contributed to silicate weathering, the long-term sink for atmospheric carbon dioxide (CO2). Here we hypothesize that the increasing abundance of N2-fixing legumes in tropical forests amplified silicate weathering rates by increased input of fixed nitrogen (N) to terrestrial ecosystems via interrelated mechanisms including increasing microbial respiration and soil acidification, and stimulating forest net primary productivity. We suggest the high CO2 early Cenozoic atmosphere further amplified legume weathering. Evolution of legumes with high weathering rates was probably driven by their high demand for phosphorus and micronutrients required for N2-fixation and nodule formation.
Highlights
Biogeochemical weathering of silicate rocks is a key process in the carbon cycle that acts as a long-term sink of atmospheric carbon dioxide (CO2) [1]
During the transient climate warming event across the Palaeocene–Eocene thermal maximum (PETM; 55.8 Ma) that is linked to a rise in atmospheric CO2 and continental weathering regimes [76], the abundance of fossilized leguminous leaf specimens increased to 73% and declined to 21% post-PETM in the Bighorn Basin, USA [23]
Fossils and molecular dating suggest that a worldwide shift from palm-dominated communities to ‘modern’ tropical forests occurred early in the Cenozoic and involved the development of N2-fixing legume-rich and symbiotically diverse communities
Summary
Biogeochemical weathering of silicate rocks (e.g. basalt, andesite, dunite) is a key process in the carbon cycle that acts as a long-term sink of atmospheric carbon dioxide (CO2) [1]. An increased proportion of legume fossil leaves recovered from 56 Ma old strata correlate with intensification of insect damage This is a pattern consistent with the influx of fresh, fixed nitrogen (N) into the ecosystem [13]. The symbiotic status of their nearest living relatives (electronic supplementary material, figure S2), evidence of increased insect damage in the fossil record in likely response to high foliar N and molecular clock dating appear to indicate that N2-fixation and diverse mycorrhizal symbioses had evolved in legumes by the early Cenozoic. We review the rise of N2-fixing legume-rich tropical forests early in the Cenozoic and propose a new testable hypothesis for how the evolution of this biome may have strengthened the long-term carbon cycle feedbacks that helped shape Earth’s CO2 and climate history in the Cenozoic
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