Organic Acid Exudation Rates. from N₂-fixing tropical legume evolution: a contributor to enhanced weathering through the Cenozoic?

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 atmospheric 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 CO₂. Here we hypothesize that the increasing abundance of N₂-fixing legumes in tropical forests amplified silicate weathering rates by increased input of fixed 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 likely driven by their high demand for phosphorus and micronutrients required for N₂-fixation and nodule formation.