ReviewLong-term productivity of lowland and upland switchgrass cytotypes as affected by cutting frequency
Introduction
Much attention has been recently paid in switchgrass as potential bioenergy crop. The main reason behind this interest is the ability of switchgrass to produce high biomass yields under low input techniques (Vogel et al., 2002). This characteristic is due to the high resistance of switchgrass to pests and diseases as well as high tolerance to severe water stress conditions, salinity and marginal soils (McLauglin and Kszos, 2005). Moreover, with respect to other perennial energy crops (e.g. giant reed and miscanthus), switchgrass has a number of significant advantages that make this crop very attractive to farmers. For example, the equipment needed for growing, harvesting and storage is very familiar to farmers; also, switchgrass, being seedable, is much more economic to be planted than other perennial crops being propagated by rhizomes (e.g. miscanthus and giant reed). Nonetheless, switchgrass pays a significant lower biomass yield to other competing annual and perennial energy crops (Lewandowski et al., 2003, Monti et al., 2005); for example, fibre sorghum or giant reed can produce up to twofold or threefold biomass yields than switchgrass under similar environmental conditions (Lewandowski et al., 2003). As a result, whether or not switchgrass will be the winner crop for energy will mostly depend on technical advances toward achieving a higher productivity.
In this view, the research on switchgrass has been intense over the last decades, covering genotype screening (Taliaferro, 2002, Monti et al., 2004), establishment technique (Elbersen et al., 1999, Monti et al., 2001, McLauglin and Kszos, 2005), nitrogen and water management (McLauglin and Kszos, 2005 and references therein), harvest time (Venturi et al., 2004) and environmental and economic impacts (Fazio et al., 2007, Monti et al., 2007). Overall, it resulted that nitrogen and water are major factors affecting biomass yield, but they also radically contribute to energy consumption, environmental charge and management costs (McLauglin and Kszos, 2005, Monti et al., 2007). As such, Parrish et al. (2003) pointed out that, in a single-cut system, 50 kg ha−1 year−1 of N are adequate to maintain satisfactory switchgrass yields, though Muir et al. (2001) reported that the optimal nitrogen dose is more than twofold. These results were also corroborated by other findings (McLauglin and Kszos, 2005 for a review). Water is also a major determinant of switchgrass yield (McLaughlin et al., 1997), however, a widespread opinion is that current levels of water consumption by agriculture is not sustainable into the next future as more water resources will be needed for human, municipal, industrial needs, or eventually for food crops (Condon et al., 2004, Ciais et al., 2005). Therefore improving productivity of energy crops by increasing water supply seems generally not practicable.
Another possible way to improve switchgrass productivity, that has not probably received adequate attention, is the optimization of cut management. Switchgrass is conventionally harvested once per year after the killing-frost, however repeated harvests over the growing season could increase the total biomass yield by exploiting the re-growth capacity of the crop, providing that additional biomass yields will offset the extra harvest costs (Keeney and De Luca, 1992, Vogel et al., 2002).
At our knowledge, no experiences on the effect of cutting frequency on switchgrass yield have been conducted in Europe so far. Also, the US studies mostly refer to forage yields, that require different quality characteristics of the feedstocks than those needed for biofuels (Stroup et al., 2003, Sarath et al., 2007). Nonetheless, researches specifically addressed to energy end-use showed counteracting results, with genotypes reaching the highest yields under either cutting systems (Madakadze et al., 1999, Vogel et al., 2002, Thomason et al., 2004, Fike et al., 2006a). Again, a number of studies did not distinguish between upland and lowland cytotypes (Madakadze et al., 1999, Sanderson et al., 1999, Vogel et al., 2002), two main switchgrass groups being expected to respond very differently to harvest management because of their diverse morphological characteristics (Porter, 1966), ploidy level, habitat preference, input requirements (Stroup et al., 2003) and cycle length (Hultquist et al., 1996). In short, upland cytotypes are characterized by a shorter growth cycle, faster growth and higher photosynthetic rates than lowland types (Wullschleger et al., 1996), which allow to conclude the primary period of vegetative growth early in the summer. In a recent study Fike et al. (2006a) found upland cytotypes to produce 38% more dry biomass when cut twice per year, while the yield increase of lowland cyto types was trivial. The simultaneous presence of upland and lowland types having succeeding harvest times in the same farm might be strategic to allow a regular round-year biomass supply to conversion plants. Moreover, a higher harvest flexibility could help farmers in reducing risks associated with fluctuations of crop and bioenergy markets, as well as weather patterns.
Alike productivity, the quality of biomass can be significantly affected by cut management. For example, comparing single and double-cut systems, Reynolds et al. (2000) found that the nitrogen content of summer-harvested biomass was about threefold than in late autumn-harvested biomass. Again, in red canary grass, a very similar crop to switchgrass, alkali and chlorine, which are released during combustion causing fouling and corrosion problems in boilers, declined by a factor of 2–6 by delaying the harvest after the killing-frost (Burvall, 1997). Moreover, ash fusion temperature increased by leaf loss and leaching of alkali from 1070 °C to 1400 °C (Burvall, 1997). This was likely due to the incomplete mineral translocation from the above ground organs to rhizomes during summertime, as well as the higher leaf component of still green biomass (Sanderson et al., 1999, Sanderson et al., 1996). The calorific value of biomass was also found to decrease by 0.2 MJ kg−1 with every 10 g kg−1 increase in ash content (Jenkins et al., 1998). Finally, ash content can also drastically change among cytotypes. For example, comparing 18 switchgrass cytotypes (Monti et al., 2005) showed the ash content to be typically higher in upland than lowland cytotypes and negatively related to biomass yield.
Therefore, the objectives of this study were to assess not only the long-term biomass yield of upland and lowland cytotypes of switchgrass in response to a different harvest management, but also to determine the quality of raw materials.
Section snippets
Management and measurements
The field trial was conducted at the experimental farm of the University of Bologna (Lat. 44°25′, Long. 11°28′, 80 m a.s.l.) in the period 2002–2006. According to soil taxonomy (USDA, 1999) soil was classified as Udifluventic Haplustepts fine silty, mixed, superactive, mesic. Soil physical–chemical analysis resulted in pH 7.7; 42%, 34% and 24% of sand, silt and clay contents, respectively; very rich in exchangeable K (115 mg kg−1); 1.6% organic matter and 0.03 and 1.5 Mpa of field capacity and
Weather data
Summer (June–August) average air temperature was about 4.5 °C warmer in 2003 (27.7 °C) than 2004 and 2005, while 2006 showed intermediate values (25.2 °C) very close to the local long-term average temperature (24.5 °C). Within the same period, rainfall (Fig. 1) was also clearly lower in 2003 (only 45 L m−2) than following three years (147, 180, 131 L m−2, respectively) and long-term local average rainfall (141 L m−2). As for the total seasonal rainfall (April–October), 2003 and 2006 registered the lowest
Discussion
Understanding a proper harvest management is necessary to maximize long-term biomass yields of switchgrass, while maintaining moisture and ashes low in biofuels. Perennial crops like switchgrass are conventionally harvested once per year in autumn or after killing-frost. Alternatively, a diversified and more flexible harvest management such as a two-cut system could provide a longer round-year supply of biomass to conversion plants, while allowing farmers to easy dry the summer-harvested
Conclusions
Harvesting switchgrass twice per year would have several advantages from time-management point of view. However, though it first appeared worthwhile, especially with U upland cytotypes, it does not offset additional harvest costs due to the too low re-growth biomass yield, both of upland and lowland cytotypes. Furthermore, starting from the third productive year, the two-cut system sensibly decreased the plant vigour in both cytotypes. Also, summer and re-growth harvested biomass showed a
Acknowledgement
This research was carried out in the framework of the EU Project “Bioenergy Chains from perennial crops in southern Europe”.
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