Seasonal changes in chemical composition and leaf proportion of elephantgrass and energycane biomass

Highlights

• Elephantgrass cell wall constituents increased until late summer then plateaued.

• Energycane cell wall constituents peaked in late summer then tended to decrease thereafter.

• Late season harvest was preferred for elephantgrass as biomass feedstock quality increased.

• Compositional characteristics of ‘UF1′ elephantgrass were similar to ‘Merkeron’.

Abstract

Changes in chemical composition of warm-season perennial grasses during the growing season affect conversion of biomass to biofuels, thus influencing choice of harvest date. The objective was to quantify these changes for three candidate bioenergy grasses in the USA Gulf Coast region during two growing seasons and relate them to optimal harvest management. Grasses included two elephantgrass [Pennisetum purpureum Schum.; synonym Cenchrus purpureus (Schumach.) Morrone] entries, ‘Merkeron’ and breeding line UF1, and the energycane (Saccharum spp. hybrid) cultivar ‘L79-1002’. Quantification of cell wall constituents and mineral composition of above-ground biomass occurred monthly throughout the growing season. With the exception of hemicellulose, elephantgrass cell wall constituents (cellulose, lignin, neutral detergent fiber and acid detergent fiber) increased from early in the growing season until late summer and either remained relatively constant (UF1) or increased slightly (Merkeron) during the remainder of the season. In contrast, concentrations of energycane cell wall constituents peaked in late summer and decreased during the remainder of the growing season. Nitrogen, P, and ash concentrations decreased with increasing maturity for all grass entries, and they were much greater in leaf than in stem. Elephantgrass leaf, particularly of UF1, contributed less to total biomass harvested than energycane leaf. Likewise, the proportion of total ash harvested that was in the leaf fraction was greater for energycane than for elephantgrass when harvest occurred late in the growing season. Thus, delayed harvest until late in the season was generally a superior management strategy for elephantgass because it resulted in biomass with greater cell wall constituent concentrations, lesser leaf percentage, and lesser concentrations of N and ash, all of which may provide advantages in some conversion processes. In contrast, greater accumulation of extractives by energycane and greater lignin concentration in elephantgrasses late in the growing season may reduce efficiency of some conversion methods.

Introduction

Dependence on imported fossil fuels has created political and economic challenges for many countries, and combustion of fossil fuels is associated with climate change (Kim and Day, 2011, Parrish and Fike, 2005). Lignocellulose represents an alternative energy production system; a potential cellulose-to-liquid fuel bioenergy (Anderson and Akin, 2008, Carroll and Somerville, 2009). Perennial C4 grasses are a promising source of lignocellulose for conversion to biofuel in the Southeast USA (Knoll et al., 2012, Na et al., 2015a). Elephantgrass [or napiergrass; Pennisetum purpureum Schum.; synonym Cenchrus purpureus (Schumach.) Morrone], and energycane (Saccharum spp. hybrid) have documented high biomass production (Morais et al., 2012, Na et al., 2015b, Woodard and Prine, 1993) and are considered two of the most promising dedicated energy crops in tropical and subtropical environments (Fedenko et al., 2013).

While high levels of lignocellulose are desirable for biofuel production, high N and/or ash concentrations in biomass may reduce the efficiency of thermochemical conversion to fuel (Shahandeh et al., 2011). Thus, characterizing the chemical properties of biomass is important. The detergent fiber analyses, which were originally proposed for forages (Van Soest et al., 1991), provide estimates of cell wall constituents including cellulose, hemicellulose, and lignin. They can be useful indicators of cellulosic biomass quality because the plant cell wall is the primary energy source for ruminant microorganisms (Guretzky et al., 2011, Jung and Lamb, 2004), and in the ruminant animal these microbes face barriers limiting access to structural carbohydrates that are similar to those that occur in bioconversion to ethanol (Lorenz et al., 2009a). Further, it has been shown that cellulose and hemicellulose concentrations, estimated from detergent fiber analysis, are correlated with theoretical ethanol yield potential (r = 0.91 and 0.51, respectively; Lorenz et al., 2009b).

For a relatively low value crop such as perennial grasses for biofuel, it is essential to minimize N input and increase efficiency of N cycling (Erickson et al., 2012, Erisman et al., 2010). Previous research has shown that concentrations of N, P, and K in harvested plant material are dependent on harvest date (Adler et al., 2006, Heaton et al., 2009, Kering et al., 2012), and nutrient removal in harvested biomass is strongly affected by harvest date and frequency (Na et al., 2014). Changes in plant-part proportion as the season progresses, specifically decreasing amount of leaf (Woodard et al., 1993), affect biomass nutrient concentration and inform harvest timing decisions.

Most biomass plant-part and nutrient composition data for perennial grasses are from experiments harvested once at the end of the growing season (Adler et al., 2006, Fedenko et al., 2013, Guretzky et al., 2011). Thus, there are limited data available that describe compositional changes of perennial grasses throughout the growing season. This information, along with seasonal patterns of biomass accumulation (Na et al., 2015a), is valuable for identifying optimum harvest dates and frequencies. The objectives of this experiment were to quantify seasonal changes in i) fiber and nutrient composition of elephantgrass and energycane biomass and ii) leaf proportion and the effect of inclusion of leaf on composition of harvested biomass.