Some amount of carbon is sequestered by plants getting buried when they die, though not as much as in the past because we have well- slated decay organisms in the biosphere. Some makes its way into the sea and gets stuck there. And a decent chunk is bound up as carbonate minerals from the weathering of the Earth (one of the future biosphere death states is when this process outstrips CO2 production due to the increase in the sun's luminosity about 700 million years from now).