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 major source Of O-2 over geological timescales. Hence, it is predicted that the rise of plants led to an increase in the O-2 content of the atmosphere from similar to 12 vol %, 570-400 Ma to its present level of similar to 21 vol % 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 are difficult to reconcile with the known persistence of forests, because rising 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 assimilation and increases the production of toxic reactive oxygen species in cells. These effects suppress plant-induced phosphorus weathering and hence organic carbon burial, providing a sensitive negative feedback on O-2. A revised model predicts that this mechanism could have regulated atmospheric O-2 within the range 15-25 vol % for the last 350 million years.