Effects of field-grown transgenic switchgrass carbon inputs on soil organic carbon cycling

Sutie Xu; Sarah L Ottinger; Sean M Schaeffer; Jennifer M DeBruyn; C Neal Stewart Jr; Mitra Mazarei; Sindhu Jagadamma

doi:10.7717/peerj.7887

Effects of field-grown transgenic switchgrass carbon inputs on soil organic carbon cycling

PeerJ. 2019 Oct 16:7:e7887. doi: 10.7717/peerj.7887. eCollection 2019.

Authors

Sutie Xu¹, Sarah L Ottinger¹, Sean M Schaeffer¹, Jennifer M DeBruyn¹, C Neal Stewart Jr^{2

3}, Mitra Mazarei^{2

3}, Sindhu Jagadamma¹

Affiliations

¹ Department of Biosystems Engineering and Soil Science, University of Tennessee, Knoxville, TN, USA.
² Department of Plant Sciences, University of Tennessee, Knoxville, TN, USA.
³ BioEnergy Science Center and Center for Bioenergy Innovations, Oak Ridge National Laboratory, Oak Ridge, TN, USA.

Abstract

Genetic engineering has been used to decrease the lignin content and to change the lignin composition of switchgrass (Panicum virgatum L.) to decrease cell wall recalcitrance to enable more efficient cellulosic biofuel production. Previous greenhouse and field studies showed that downregulation of the gene encoding switchgrass caffeic acid O-methyltransferase (COMT) and overexpression of the switchgrass PvMYB4 (MYB4) gene effectively improved ethanol yield. To understand potential environmental impacts of cultivating these transgenic bioenergy crops in the field, we quantified the effects of field cultivation of transgenic switchgrass on soil organic carbon (SOC) dynamics. Total and active SOC as well as soil respiration were measured in soils grown with two COMT-downregulated transgenic lines (COMT2 and COMT3), three MYB4-overexpressed transgenic lines (L1, L6, and L8), and their corresponding non-transgenic controls. No differences in total SOC, dissolved organic carbon (DOC), and permanganate oxidizable carbon (POXC) were detected between transgenic and non-transgenic treatments for both COMT (10.4-11.1 g kg^-1 for SOC, 60.0-64.8 mg kg^-1 for DOC, and 299-384 mg kg^-1 for POXC) and MYB4 lines (6.89-8.21 g kg^-1 for SOC, 56.0-61.1 mg kg^-1 for DOC, and 177-199 mg kg^-1 for POXC). Soil CO₂-carbon (CO₂-C) production from the COMT2 transgenic line was not significantly different from its non-transgenic control. In contrast, the COMT3 transgenic line had greater soil CO₂-C production than its non-transgenic control (210 vs. 165 µg g^-1) after 72 days of laboratory incubation. Combining the improvement in ethanol yield and biomass production reported in previous studies with negligible change in SOC and soil respiration, COMT2 could be a better biofuel feedstock than COMT3 for environmental conservation and cost-effective biofuel production. On the other hand, MYB4 transgenic line L8 produced more biomass and total ethanol per hectare while it released more CO₂-C than the control (253 vs. 207 µg g^-1). Long-term in situ monitoring of transgenic switchgrass systems using a suite of soil and environmental variables is needed to determine the sustainability of growing genetically modified bioenergy crops.

Keywords: Active carbon; Lignin downregulation; Soil organic carbon; Soil quality; Soil respiration; Transgenic switchgrass.

Grants and funding

This work was funded by the BioEnergy Science Center supported by the Office of Biological and Environmental Research of the DOE Office of Science, and the USDA Hatch grant. There was no additional external funding received for this study. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.