Humans have released around 3 × 10^11 tons of carbon since the industrial revolution.
Agriculture, particularly modern industrial agriculture, releases massive amounts of carbon from the soil which then forms CO2. Since the dawn of agriculture, 8% of the world's soil carbon (1 × 10^11 tons of carbon) has been "lost" - released to form atmospheric CO2 (or oceanic carbon).
This puts the agricultural contribution to carbon emissions at close to one third of the overall human emissions.
CO2 taken up by plants during photosynthesis is largely returned to the atmosphere when those plants are consumed. Thus, photosynthesis as involved in industrial agriculture has only short term effects and essentially balances out.
Thus, agriculture does not offset carbon emissions. Instead, it is a significant contribution to it. If agricultural practices were changed to sequester carbon instead of releasing it, there would still be a net release of carbon from human activities (though it would be very significantly reduced compared to current levels).