In this paper we report results on the decomposition in soil of woody trunk material from poplar (Populus tremula×Populus alba) trees with genetic modifications to lignin biosynthesis grown for 4 years in a field trial. Lengths of trunks were salvaged following the premature termination of the trial as a result of serious damage to the trees by protestors against the release of genetically modified plants. The decomposition in soils of sections of trunk from trees with antisense transgenes for two enzymes in the monolignol pathway, cinnamyl alcohol dehydrogenase and caffeic acid O-methyl transferase (two lines of each), and material from unmodified trees were determined during laboratory incubation for 552 days. Although total CO2 production from soil amended with trunk material was 2.0- to 4.3-times greater (P<0.010) than that from unamended soils during the first 77 days of incubation, no significant differences between modified or unmodified plants were detected for either total CO2 production over 77 days or total mass loss from the trunk material over 552 days. Addition of the plant materials significantly increased the soil microbial biomass, but the effects of the different genetic modifications on biomass were not consistent or in most cases not significant. We conclude that environmental variability during growth in the field has a greater influence on future wood decomposition than modifications to lignin biosynthesis.