The carbon cycle. The carbon plants are made of was taken from the air in the first place. If all we ever did was burn plants which regrew using the CO2 in the atmosphere, the total carbon in the system would be the same.

Where we have a problem is taking carbon trapped under the earth, burning it, and adding carbon to the system continually. That's why fossil fuels are such a problem.

That said, growing a bunch of plants on land that displaces natural ecosystems using mechanized farming equipment and fertilizers and then burning them is not exactly an environmental panacea, and I think most studies have shown it to be more of a farm subsidy that's a bit of a wash environmentally.