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.