We conducted a glasshouse mesocosm study that combined (13)C isotope techniques with wild-type and transgenic aspen (Populus tremuloides) in order to examine how altered lignin biosynthesis affects plant production and soil carbon formation. Our transgenic aspen lines expressed low stem lignin concentration but normal cellulose concentration, low lignin stem concentration with high cellulose concentration or an increased stem syringyl to guaiacyl lignin ratio. Large differences in stem lignin concentration observed across lines were not observed in leaves or fine roots. Nonetheless, low lignin lines accumulated 15-17% less root C and 33-43% less new soil C than the control line. Compared with the control line, transformed aspen expressing high syringyl lignin accumulated 30% less total plant C - a result of greatly reduced total leaf area - and 70% less new soil C. These findings suggest that altered stem lignin biosynthesis in Populus may have little effect on the chemistry of fine roots or leaves, but can still have large effects on plant growth, biomass partitioning and soil C formation.