You are correct: To get a net reduction of atmospheric carbon dioxide, you need to increase total plant mass or prevent plants from decomposing/being eaten. As

You are correct: To get a net reduction of atmospheric carbon dioxide, you need to increase total plant mass or prevent plants from decomposing/being eaten. As far as I know, the majority of net oxygen production happens through algae, because they die and sink to the bottom of the ocean, permanently removing carbon from the carbon cycle.