not easy stuff (as I am but a humble journalist), but what I have understood so far is that the oxygen production by photosynthesis is a netto neutral affair, in that all the oxygen produced by trees shrubs and plankton is consumed by the breakdown of these organisms. So there must be other oxygen sources and if I understand Watson/Lenton well then one of these sources is the chemical weathering of rocks by plantroots that releases oxygen in the atmosphere and the burial of marine life in geologic time that once produced oxygen but which was buried without reclaiming the produced oxygen back. Bu~t I would be very happy to see on the Wiki page how you interpret this. Thanks 

 Theo Richel (www.richel.org/resume)