Effect of Transgenic Switchgrass Plants Down-Regulating COMT on Soil Chemistry and Microbiology.

Cell wall lignin poses a major challenge in the use of switchgrass for biofuels.  Low-lignin transgenic switchgrass plants were produced via down-regulation of caffeic acid O-methyltransferase (COMT), a lignin biosynthetic enzyme. In order for these transgenic plants to be planted on larger scales, it is critical to evaluate potential environmental impacts.  In particular, altered plant chemistry of lignin down-regulated plants may alter the soil environment resulting and impact soil functions (e.g. carbon cycling).  The objective of this study was to evaluate effects of COMT-down-regulated switchgrass plants on soil chemistry and microbiology.  Over a two year field trial, we sampled shallow (0-15 cm) and deep (15-30 cm) soils where two independent transgenic lines (COMT2 and COMT3) and their non-transgenic controls were growing.  Over this period, soil pH ranged from 6 to 7 and soil organic matter content ranged from 2.5% to 4.0 %, but we did not observe any significant differences in soil pH or organic matter content between transgenic and control plant plots.  We also examined the concentrations of 19 soil elements, but did not see a significant difference in elemental composition. There was also no significant effect on soil respiration, suggesting no overall effect on microbial activity.  An analysis of soil bacterial communities via high throughput 16S rRNA gene amplicon sequencing revealed no effects of transgenic plants on bacterial diversity or richness.  Composition of the bacterial communities varied by season, but overall they were not different under transgenic plants compared to controls.  Our results indicate that over the short term (two years), lignin down-regulation in switchgrass does not have an adverse effect on their soils.