Abstract
AbstractCell wall recalcitrance poses a major challenge on cellulosic biofuel production from feedstocks such as switchgrass (Panicum virgatum L.). As lignin is a known contributor of recalcitrance, transgenic switchgrass plants with altered lignin have been produced by downregulation of caffeic acid O‐methyltransferase (COMT). Field trials of COMT‐downregulated plants previously demonstrated improved ethanol conversion with no adverse agronomic effects. However, the rhizosphere impacts of altering lignin in plants are unknown. We hypothesized that changing plant lignin composition may affect residue degradation in soils, ultimately altering soil processes. The objective of this study was to evaluate effects of two independent lines of COMT‐downregulated switchgrass plants on soils in terms of chemistry, microbiology, and carbon cycling when grown in the field. Over the first two years of establishment, we observed no significant differences between transgenic and control plants in terms of soil pH or the total concentrations of 19 elements. An analysis of soil bacterial communities via high‐throughput 16S rRNA gene amplicon sequencing revealed no effects of transgenic plants on bacterial diversity, richness, or community composition. We also did not observe a change in the capacity for soil carbon storage: There was no significant effect on soil respiration or soil organic matter. After five years of establishment, δ13C of plant roots, leaves, and soils was measured and an isotopic mixing model used to estimate that 11.2 to 14.5% of soil carbon originated from switchgrass. Switchgrass‐contributed carbon was not significantly different between transgenic and control plants. Overall, our results indicate that over the short term (two and five years), lignin modification in switchgrass through manipulation of COMT expression does not have an adverse effect on soils in terms of total elemental composition, bacterial community structure and diversity, and capacity for carbon storage.
Highlights
Switchgrass (Panicum virgatum L.) is a promising bioenergy crop due to high biomass production with relatively low agronomic inputs
While transgenic bioenergy crops hold a great deal of promise from an energy production standpoint, it is unknown whether there maybe trade-offs when grown in the field
Studies on Bacillus thuringiensis (Bt) and herbicide-tolerant (HT) food crops have generally revealed no major differences in soil microbial community structure, enzyme activities, residue decomposition rates, and other aspects of soil functioning (Griffiths et al, 2007; Lupwayi et al, 2007; Liu et al, 2008; Yanni et al, 2010; Fang et al, 2012; Dohrmann et al, 2013; Kolseth et al, 2015)
Summary
Switchgrass (Panicum virgatum L.) is a promising bioenergy crop due to high biomass production with relatively low agronomic inputs. Plants with altered cell walls might affect soils directly through altered plant nutrient use and/or changes in residue and root exudate composition, altering decomposition rates and nutrient turnover (Motavalli et al, 2004; Kolseth et al, 2015). Unlike the Bt and HT modifications, downregulating lignin pathways is carried out with the specific goal of changing the recalcitrance of plant tissues. It remains unknown what affect the altered root and residue composition has on rhizosphere processes, with respect to soil carbon storage
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