Plant roots release both oxygen and carbon to the surrounding soil. While oxygen leads to less production of methane (a greenhouse gas), carbon often has the opposite effect. We investigated these processes in two plant species, S. patens and S. americanus. We found that S. patens roots produce more carbon and less oxygen than S. americanus. Additionally, the S. patens pool of root-associated carbon compounds was more dominated by compound types known to lead to higher methane production.