A quantitative review comparing the yields of two candidate C4 perennial biomass crops in relation to nitrogen, temperature and water
Introduction
In the United States (US) and the European Union (EU), biomass energy research has investigated both woody and herbaceous crops grown for energy. These biomass energy crops have included tree species (Salix, Populus spp.), traditional row crops (Zea mays, Triticum spp.) and perennial rhizomatous grasses (PRGs) (Arundo, Panicum, Miscanthus). Of these, the last group has the most characteristics favorable to biomass energy production [1], [2], [3].
An ideal biomass crop is characterized by attributes that allow it to provide high-energy output for little input. A biomass crop must require only low investments of energy (fossil fuels) and money if it is to be environmentally and commercially viable while still providing clean, cost-effective fuel [4], [5]. PRGs, especially those using the C4 photosynthetic pathway, typically use nutrients, water, and solar radiation more efficiently than other plants [6]. The perennial rhizome system allows nutrients to be cycled seasonally between the above- and below ground portions of the plant, thus minimizing external additions of fertilizer [7]. If the crop is harvested after senescence, and associated nutrient translocation, has occurred, the resultant fuel will have a low mineral content and therefore release little pollution when combusted [8]. As a perennial, the crop requires only one planting and related tillage, reducing costs and fossil fuel use, as well as soil erosion [9], [10], [11]. Such physiological aspects allow PRGs to provide clean fuel and more dry matter per unit input than can other potential biomass crop options [3], [12]. Finally, the crop would ideally use the C4 photosynthetic pathway. Biomass crops are essentially used to collect and store solar energy for later release, and for all but the coldest environments, C4 photosynthesis is an important factor in maximizing the efficiency of conversion of intercepted solar radiation into stored biomass energy (radiation use efficiency, or RUE) [6]. By concentrating carbon dioxide around Rubisco, C4 plants largely eliminate photorespiration [13]. This allows them to use more of the available light energy as well as convert it to stored energy more efficiently than plants with the C3 pathway [14]. The gain in RUE conferred by C4 photosynthesis can be as much as 40% over conversion rates in C3 plants [15].
Over the past few decades, C4-PRGs have been investigated on both sides of the Atlantic. US research has focused on the native prairie grass, switchgrass (P. virgatum L.) [16], [17], [19], while EU programs have considered Miscanthus × giganteus, a hybrid species from Asia [18]. Both of these grasses have additional characteristics that favor their use as biomass crops over other PRGs (Table 1).
Though P. virgatum has traditionally been of interest as a warm-season forage crop, the US Department of Energy (DOE) chose to examine it as a model species for biomass energy production [19]. These trials have led to the development of high yielding cultivars such as “Alamo” that can achieve annual yields of [20]. P. virgatum responds strongly to nitrogen (N) fertilizer, and is often drought tolerant [21], [22], [23]. It can effectively sequester carbon in the soil, and provides excellent cover for wildlife [24], [25], [26]. As a native species, its use as a biomass crop is considered more environmentally acceptable than the introduction of an exotic species for the same purpose. This, however, is somewhat undermined by the fact that the improved cultivars may have a very different genetic make up than local populations and, as fertile open-pollinating crops, could disrupt native populations [27].
By contrast, M. × giganteus has been investigated in several countries throughout the EU, largely as part of the Miscanthus Productivity Network. The main objective of this network was to generate information on the potential of M. × giganteus as a non-food crop in the EU [18]. Among other results, projects within the network found M. × giganteus yields can reach under good growing conditions [28]. The crop can photosynthesize well at low temperatures [29], and attain high yields with little N fertilizer [30]. Like P. virgatum it has been shown to be a good tool for carbon sequestration and soil quality improvement, as well as wildlife cover [31], [32]. Though an exotic species, M. × giganteus is a sterile triploid hybrid of M. sinensis and M. sacchariflorus and is incapable of producing viable pollen or spreading by seed [33]. It has the disadvantage that planting must be from rhizome fragments or young plants, but the advantage that it cannot become invasive nor transfer genes to wild populations.
While agronomic development programs have been established to explore the biomass crop potential of P. virgatum in the US, and M. × giganteus in the EU, to date there have been no direct comparisons of mature stands of these two crops reported in the peer-reviewed literature. How might these crops compare when grown side by side? Are some environments more favorable to P. virgatum production, and others to M. × giganteus? This study addresses the following questions:
- (1)
How do the yields of P. virgatum and M. × giganteus compare?
- (2)
What are the major factors influencing M. × giganteus and P. virgatum yields?
Section snippets
Methods
Peer-reviewed journal articles were surveyed using the Science Citation Index ExpandedTM (Thomson-ISI, Philadelphia, PA, USA) and SilverPlatter (Ovid Technologies, New York, NY, USA) electronic databases. P. virgatum is the subject of many articles on forage production and quality, but the cultural practices between forage and biomass crop production can be very different. For this reason, only studies of P. virgatum produced with practices typical of those used in biomass crop production were
Results
Overall, M. × giganteus was found to yield more biomass per ha than P. virgatum (p>0.004), representing a two-fold difference, with means of 22.4(±4.1) and , respectively (Table 2). Comparison of the raw yield data suggests this relationship is consistent over each variable of interest (Fig. 2). An overall analysis of variance indicated that only precipitation (p<0.0001) and N fertilizer (p=0.0003) seem to influence yield. GDD did not significantly affect yield in either
Discussion
Results of this analysis indicate that:
- (1)
Miscanthus × giganteus produces more biomass per unit area and per unit input than does P. virgatum; and
- (2)
Miscanthus × giganteus yields are most strongly influenced by water, while those of P. virgatum are more strongly controlled by N; GDD did not significantly affect yield in either crop.
Miscanthus × giganteus, on average, produced more biomass than P. virgatum (Table 2) when considered over a range of growing conditions, representing more than a
Acknowledgements
The Illinois Council on Food and Agriculture Research substantially funded this work under project ILLU-15-0271. The authors wish to thank Pat B. Morgan, Shawna L. Naidu, Carl J. Bernacchi, Charles P. Chen, Richard J. Webster, Xinguang Zhu and Victoria E. Wittig for critical reviews of the manuscript, and M. Katherine Ciccodicola and Janel K. Woods for their help in gathering articles.
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