The role of land plants, phosphorus weathering and fire in the rise and regulation of atmospheric oxygen

The evolution of vascular plants and their spread across the land surface, beginning similar to 420 Ma, progressively increased the rate of weathering of phosphorus from rocks. This phosphorus supply promoted terrestrial and marine productivity and the burial of organic carbon, which has been the ma jor source Of O-2 over geological timescales. Hence, it is predicted that t he rise of plants led to an increase in the O-2 content of the atmosphere f rom similar to 12 vol %, 570-400 Ma to its present level of similar to 21 v ol % by similar to 340 Ma. Previous modelling studies suggest that O-2 then rose to similar to 35 vol % similar to 300 Ma. Such high concentrations ar e difficult to reconcile with the known persistence of forests, because ris ing O-2 increases the frequency and intensity of vegetation fires, tending to decrease biomass and cause ecological shifts toward faster regenerating ecosystems. Rising O-2 also directly inhibits C3 photosynthetic carbon assi milation and increases the production of toxic reactive oxygen species in c ells. These effects suppress plant-induced phosphorus weathering and hence organic carbon burial, providing a sensitive negative feedback on O-2. A re vised model predicts that this mechanism could have regulated atmospheric O -2 within the range 15-25 vol % for the last 350 million years.