Impact of Drought on Chemical Composition and Sugar Yields From Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Miscanthus, a Tall Fescue Mixture, and Switchgrass

Abstract

Environmental factors like drought impact the quality of biomass entering a bioconversion process. Drought often reduces the sugar content in lignocellulosic biomass, which could have economic impacts, particularly when compounded with losses in dry biomass yield; however, the effects on conversion efficiency are not completely understood. This study investigated how drought may impact biomass composition and sugar yields from dilute-acid pretreatment and enzymatic hydrolysis of Miscanthus, a tall fescue mixture, and switchgrass from Nebraska, Missouri, and Oklahoma, respectively, grown as part of Regional Feedstock Partnership field trials. Samples were grown and harvested in 2010 during non-drought conditions and in 2012 during extreme drought conditions. Non-structural glucose and proline were significantly greater in 2012 compared with 2010 for Miscanthus, which suggests drought stress occurred. Structural glucan and xylan were significantly decreased in 2012 for Miscanthus; however, reactivity and sugar yields from dilute-acid pretreatment and enzymatic hydrolysis were significantly greater in 2012 compared with 2010, suggesting that although structural sugars may decrease during drought conditions, sugar yields and reactivity may increase. For the tall fescue mixture, proline was greater, and structural sugars were lower in 2012, indicating drought stress, but minimal differences were observed in the conversion experiments. Few differences were observed for switchgrass composition and reactivity between years. The observed patterns are likely because of site-specific climatic conditions combined with the tolerance each species may have to drought. As drought occurrence and severity have increased, it is necessary to understand drought impacts to mitigate risks to future bioenergy industry growth.

Drought Impacts on Bioenergy Supply System Risk and Biomass Composition

“…It must also be considered how drought impacts the formation of certain degradation products that decrease conversion efficiencies though inhibition of enzymes during enzymatic hydrolysis and microorganisms during the fermentation step in a biochemical conversion process. For Miscanthus pretreated with dilute acid, enzymatic and fermentation inhibitors did not increase, however, this was likely a result of the dilute-acid pretreatment temperatures being lower than those required to form inhibitors [34]. In contrast, fermentation inhibitors were increased in drought stressed switchgrass in a study by Ong et al [32] where the switchgrass was Drought Impacts on Bioenergy Supply System Risk and Biomass Composition DOI: http://dx.doi.org/10.5772/intechopen.85295 chemically pretreated using ammonia fiber expansion (AFEX).…”

“…Complicating reductions in crop yields, plants experience compositional changes during drought; increased extractive components, including soluble sugars, and decreased structural sugars were reported for important potential bioenergy crops like switchgrass, Miscanthus, mixed grasses, and corn stover [26,[32][33][34]. Studies have even observed reduced lignification in some cases possibly resulting from decreased plant growth as well as changes in lignin component distribution in plant cells impacting cell wall degradability [33][34][35]. These compositional changes can greatly impact yield of bioenergy conversion products from these biomass resources.…”

“…In addition, compositional changes in response to drought in the Drought Impacts on Bioenergy Supply System Risk and Biomass Composition DOI: http://dx.doi.org/10.5772/intechopen.85295 literature are mixed. A number of studies report hemicellulose and lignin contents decreasing after drought treatments [33][34][35]40], and in contrast other studies report hemicellulose and lignin contents remaining unchanged or increasing under drought conditions [33,39,41]. These differences are not completely understood; however, studies have suggested they arise from differences in drought severity and timing [33,34,39], genetics [35], and species specific differences [31,42].…”

“…In reality, there are many other considerations regarding biomass composition that can affect the pretreatment, enzymatic hydrolysis, and fermentation steps that are necessary to convert biomass to products in biochemical conversion. Hoover et al [34] reported that Miscanthus carbohydrate yields from dilute-acid pretreatment and enzymatic hydrolysis were actually higher in drought affected plants compared to those grown in a non-drought year, which was hypothesized to be a result of higher extractable glucose and lower lignin contents. It is thought that reduced lignin content, observed in some drought-stressed plants, can decrease recalcitrance by creating better access to cell wall carbohydrates and increasing conversion efficiency, but changes in lignin distribution in tissues may also play a role in cell wall degradability in water stressed plants [35,43].…”

“…The increase in carbohydrate yields is not isolated to dilute-acid pretreatment and enzymatic hydrolysis, as drought-stressed Miscanthus had increased carbohydrate yields in mild-alkali pretreatment and enzymatic saccharification [39] and after a mild hot water pretreatment and saccharification in nutrient rich environments [28]; in both studies this trend was either less pronounced or not present for leaves when compared to stems. A tall fescue mixture also had few significant increases in carbohydrate conversion yields, thought to be a result of less severe drought growing conditions [34]. A recent report documented increased extractability of pectin components in the cell wall ultrastructure of loblolly pine in response to low soil moisture [44].…”