the potential for floodplains to sustain biomass feedstock production systems

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Rapid industrialization, especially in developing coun-tries, has led to a steep rise in the demand of fossil fuel, up to a point where 80% of the primary energy consumption in the world is derived from fossil fuel [1]. Energy consumption in the world is expected to rise by 40% from 2007 to 2030, with an estimated 77% increase in the demand for fossil fuel over the same period of time [201]. Fossil fuel combustion has been identified as the primary source of anthropogenic climate change [202]. In addition to this, dependence on foreign oil has been linked with geopolitical complexi-ties and conflicts. Due to the progressive depletion of fossil fuel and volatile fuel prices, the goals for energy independence and global climate change have led towards a push for biomass-based alternative renewable energy development. The US Energy Independence and Security Act RFS2 was framed with the desire to reduce the dependence on foreign oil and triple the current

biofuel production by 2022, from the current 45 bil-lion l to 136 billion l per year, with an anticipated production of up to 80 billion l per year primarily from advanced biofuels [203]. Similar frameworks have been developed for the medium- and long-term energy secu-rity in the EU by way of increased use of biomass and bioenergy [204].

It has been documented that the production of bio-fuels from first-generation feedstocks can have a net negative impact on the environment [205] and climate [2]. Other research findings show that the production of biofuel from corn or other food crops is unsustainable in the long run, as artificial shortages in food supply and subsequent impacts could destabilize the global econ-omy [206]. While only a fraction of the US agricultural output is channeled for energy production [207], the vast majority is in the form of corn grain, and it is evident that diverting common agricultural products to biofuel

The potential for floodplains to sustain biomass feedstock production systems

Sougata Bardhan & Shibu Jose*Production of biofuels from corn or other food crops is considered unsustainable in the long term since it creates artificial shortages in food supply, increases in food price, and subsequent socioeconomic and environmental concerns. Second-generation biofuels, however, have shown promise, with improvement in technologies for converting cellulosic feedstocks into liquid transportation fuels. The development of biomass feedstock production systems and advanced biofuel refineries in floodplains and marginal lands can generate up to 45 t of biomass and 14,000 l of advanced biofuel per hectare per year, achieving considerable offset in dependence on fossil fuel. Promising biomass species for floodplains include short-rotation trees such as poplar and willow, perennial grasses such as Miscanthus and switchgrass, and annuals such as high-biomass sorghum. However, river floodplains are often susceptible to flooding and drought events, partly due to the impact of climate change on hydrological cycles and human interventions such as the construction of dams and levees. In the USA, floodplain biofuel production systems could generate up to 30% of renewable biofuels by 2022 and provide additional benefits such as carbon sequestration, GHG reduction and ecosystem sustainability. However, successful implementation will depend on the social adaptability and economic viability of such systems.

Review

The Center for Agroforestry, School of Natural Resources, 203 Anheuser-Busch Natural Resources Building, University of Missouri, Columbia, MO 65211, USA *Author for correspondence: Tel.: +1 573 882 0240; Fax: +1 573 882 1977; E-mail: [email protected]

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production has had a significant impact on scarce arable land and water resources [3,4], as well as the supply chain economy. For example, corn prices jumped from US$3.40 a bushel in 2007 to $4 per bushel in 2008 [5], and have been going over

$7 per bushel on speculation [208] with prices at $6.3 for the first quarter of 2012 [209]. In recent years, the rapidly increasing use of corn, sugarcane and sunflower seeds for production of bioethanol, or the use of soybeans and various other oil crops for production of biodiesel, has been met with mounting criticism [5], based on the fact that the resource allocation for biofuel production com-petes directly with food production. It is important to note that further investigation to accurately quantify the impacts of such systems are ongoing and those results will be critical for future understanding.

While corn-based biofuel systems have been criti-cized for small energy ratio [6,7], restored mixed prairie ecosystems with minimal external inputs have been criticized for their low productivity [8,9]. Second-generation biofuels, specifically cellulosic biofuel pro-duction, have a smaller GHG footprint than corn-based or first-generation feedstocks [10]. These lignocellulosic feedstocks, which could promote large-scale energy pro-duction, include crop residues, perennial grasses and short-rotation woody crops (SRWC). When managed properly, the high productivity of perennial grasses and SRWC and their relatively high tolerance to soil constraints make them ideal feedstocks for biofuel pro-duction. While a conversion efficiency of 313 l of cel-lulosic biofuel per dry ton has been envisioned [11], an increase in the productivity of feedstocks per unit land area would be essential for large-scale impact on biofuel production [12].

Expansion of biofuel production could lead to sub-stantial GHG reduction benefits [13]; these feedstocks can be grown on marginal lands traditionally not used for foodgrain or fiber production [14]. Utilization of marginal agricultural lands will not add to the biofuel carbon debt and such a biofuel production system would be more sustainable than biofuel produced through land clearing [2]. Wang et al., through life cycle ana lysis, demonstrated that corn ethanol reduced emissions by 19–52%, while cellulosic ethanol could decrease emis-sions by as much as 86% [15]. Campbell et al. quantified that approximately 56–60 million ha (140–150 million acres) of marginal agricultural land could be tapped into biomass feedstock production within the USA [16]. Feedstock production from such a conversion could result in 0.22–0.35 billion Mg of total output or approximately 68 billion l to 110 billion l of liquid drop-in fuel.

Floodplains: prospects & challengesFloodplain–river ecosystems are unique and dynamic landscapes where hydrologic variations interact to create various ecosystem services such as recreation, wildlife habitat and aesthetic appeal [17,18]. Floodplains form a transition between aquatic and terrestrial environments where exchange of carbon and nutrients between the river channel and the floodplain is facilitated by the connecting water [19]. A natural floodplain (Figure 1) promotes biodiversity and native vegetation/wildlife, maintains water quality, prevents floods and enhances soil fertility and productivity. Unfortunately, human development has altered floodplain–river ecosystems through the creation of dams and levees, and separated the different natural variables that regulated these sys-tems [20]. In a pristine system, the flood–pulse concept controls the adaptation of floodplain biota, which in turn regulates the overall biodiversity in a floodplain [20]. Construction of levees and dams modifies the natu-ral flood–pulse interaction, preventing the exchange of nutrients between the floodplain and the river channel, and fundamentally destroys the natural and ecological structures of floodplains [21].

Floodplains and surrounding areas in river valleys are essential for livelihood of the native population, but their use has been challenged by frequent flooding and drought events that cause large loss of property, land degradation and famines. Due to the global climate change impacts, it is expected that in the near future flooding events will intensify in various parts of the globe [22–24]. Pall et al. used a complex hydrometeoro-logical model and predicted a 20–90% increase in flood risk from anthropogenic GHG emissions for England and Wales [25]. In recent years, developing countries have experienced huge losses (~$35 billion per year) via natural disasters [26], but at the same time disas-ter damages in developed countries have resulted in comparable suffering. Flooding and drought events can impact livelihood along river corridors through crop loss, land degradation and displacement. Agricultural crops are often severely damaged by flooding and the soil ecosystem is negatively impacted by deposition of large amount of sand and silt. In many instances, large-scale sand deposition in fertile agricultural lands has rendered them less productive and in certain cases completely useless.

Large-scale flooding with widespread damaging impact on agriculture has been reported from around the world in recent years. Some examples include large-scale flooding in the Negro River Basin in Argentinean Patagonia [27], Okavango Delta in Botswana [28] and Red River flooding of 2009 along the Red River, in North Dakota and Minnesota in the USA, and Manitoba in Canada [210]. Annual large floods during the growing

Key term

Liquid drop-in fuel: Alternative fuel that can be blended with gasoline and used in the current gasoline infrastructure of pipelines, pumps and engines.

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season have resulted in lower wheat yields in northern India [29]. In May 2011, 53000 ha of Missouri farmland were inundated by flood waters released by an inten-tional levee breach, in order to prevent levee breach along populated areas. Although the 2011 flood was an aberration, the severity of flooding has increased in the St Louis (MO, USA) region over the years due to the increase in flood stages on the Mississippi and Missouri rivers (Figure 2) [30]. Previous studies have documented that the Mississippi River flood stage has increased significantly in the last 160 years [30,31]. Myers and White reported that construction of arti-ficial levees caused the increase in flood stages on the Mississippi River in 1973 and that climate change did not have a major influence in the rising flood stages of the river [32]. Frequent flooding in recent years has resulted in crop loss as well as destruction of prop-erty and landscape along the Mississippi and Missouri River corridor. Increased flood stages, frequent flood-ing and the resulting loss of agricultural production have caused large-scale economic loss and hardship for farming communities around the world.

Major floods have been reported every 10 years, while smaller ones can occur every alternate year in India [29]. However, Milly et al. reported frequent incidence of high-intensity floods related to climate change models and the likelihood that the trend will continue to grow [22]. Feyen et al. also predicted that large-scale flooding with serious economic impact will be prevalent as a result of climate change [33].

Similarly, many floodplain regions around the world also experience regular droughts that affect agricultural production and crop yields. For example, the 2011 drought in the USA severely affected agricultural pro-duction in the midwestern states of Texas, Oklahoma, Kansas, Missouri and Illinois [211]. The Amazon region in South America also faced scorching drought in 2010, which resulted in larger than normal carbon emissions from tree deaths [34]. The El Niño and La Niña effects on rainfall patterns and subsequent impacts [35,36] have become lasting and intense in the last few decades in many parts of the world [212]. Studies relying on global circulation models have predicted increased frequency and severity of droughts in the Amazonia [37]. Drought

Left flood plain Central channel Right flood plain

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Figure 1. Typical cross-section of a river–floodplain ecosystem.



can severely impact photosynthesis and other physiologi-cal processes and, thereby, growth and biomass accu-mulation. Compared with flooding, drought can result in higher seedling mortality [38]; however, the response is dependent on individual species. Establishment of drought-tolerant species that can withstand flooding and vice versa would be a key for a sustainable biomass and biofuel cropping system in the river floodplains. Breeding and introduction of perennial crops with higher yields, drought and flood tolerance, and other improved characteristics in marginal lands to increase the overall production base for advanced biofuel feedstocks, will be critical, while also complementing the most robust and diverse food, feed and biomaterials production systems.

Floodplain biomass productionVery little information exists about the potential of establishing biomass feedstock systems in floodplain landscapes. Incorporation of the biomass species that are flood tolerant or mixed cropping of woody species with agricultural crops in an agroforestry setting can provide protection against total loss of crops and revenue due to flood events [39]. Agricultural practices consistent with periodic inundation include pasture, timber harvesting

and the cultivation of flood-tolerant crops. Reconnected floodplains can be restored to natural habitats, and at the same time can remain in private ownership and productive agricul-ture [40]. A broader variety of crops

could be cultivated on reconnected lands with projected longer recurrence intervals for inundation. In addition to traditional commodity benefits from crops, floodplains also provide a broad range of ecosystem services, several of which have the potential to provide revenue to land-owners. Research should explore potential markets for these ecosystem services such as carbon sequestration, nutrient filtration, groundwater recharge and recreation. Public sources of funding, such as the US Department of Agriculture’s Wetlands Reserve Program, can compensate landowners for socially valuable ecosystem services, such as wildlife habitat and open space, which may be dif-ficult to capture through markets. Other programs such as the Conservation Reserve Program have promoted the widespread planting of herbaceous and woody feedstock species on marginal lands. While current policies do not allow harvesting of biomass from such lands, the potential for harvesting biomass without compromising the con-servation values of such land needs to be considered. The compensation from the Conservation Reserve Program can be adjusted to account for the financial benefit derived by landowners from the harvest of such feedstocks.

River floodplains have the potential to develop and sustain regional advanced biofuel production systems because of their capacity to generate large-scale biomass production and support rural economy through ancil-lary industries for processing and transportation. For example, the Mississippi/Missouri river corridor has large acreages of both marginal (over 40 million ha) and productive land with a variety of soil types, surplus sur-face- and ground-water, favorable climate and growing seasons, and the largest biomass production potential in the nation [213]. It can also sustain barge traffic, which would reduce transportation costs for biomass and bio-fuels considerably. Previous studies have found that by utilizing marginal lands resources, for example riparian and upland buffer strips, brownfield sites and marginal agricultural land, it would be possible to produce enough feedstocks to meet 22% of the energy requirement for the US state of Nebraska from its current contribution of 2% [41]. Such research is encouraging because it is essential that an advanced biofuel production system should complement agricultural food production and not compete with it.

In theory, biomass energy can be produced in all areas that are currently growing food crops. However, con-verting large segments of agricultural land into biomass production systems could be detrimental in the long run due to leakage effects of food shortage and higher crop prices. Such systems will most likely result in GHG emission issues with a minimal energy ratio. Ideally, a sustainable biomass production system would involve dedicated energy crops with specific breeding and trait development for their use as an energy vector [6]. Such

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Figure 2. Flood stages of the Missouri River at Hermann in the last 130 years showing gradually increasing flood stages and larger floods.Data taken from [221].

Key term

River floodplains: Land adjoining rivers subjected to flooding and inundation. Such areas are characterized by high fertility status and support diverse ecosystem.



crops should be rich in lignocellulosic content and should be usable for combined heat and power or for conversion to liquid drop-in fuel, an alternative to the conventional jet fuel. Several such crops that have been cultivated and evaluated around the world include SRWC such as pop-lar (Populus spp.) and willow (Salix spp.), and perennial grasses such as switchgrass (Panicum spp.) and Miscanthus (Miscanthus spp.). An annual crop, sorghum (Sorghum spp.), has shown tremendous potential for achieving high biomass yields in trials conducted in the USA and other parts of the world. The suitability of these plant species for biomass cultivation has been validated in assessments from the US Department of Energy [214] and in inde-pendent studies such as the Battelle Report [215]. Since these biomass species are flood tolerant, with periods of prolonged inundation, the only issue would be a delayed harvesting when the water level recedes. On the con-trary, with traditional agricultural crops/practices, there is often a complete crop failure. It is important to note that the higher moisture content in the biomass following the inundation may decrease the feedstock quality and increase the cost of transportation. Storage of biomass with high moisture content is not recommended and drying would be essential. Although the crops are flood tolerant, prolonged inundation could thus increase the cost of biomass production through higher transporta-tion cost, reduced feedstock quality and the added cost of the drying process.

Some of the important factors for consideration in a sustainable biomass production system are selection of species based on local climate and soil type, management system, maintaining year-round supply of biomass and harvest strategies. For example, by altering harvest time it would be possible to enhance nutrient cycling with reduced nutrient content in the biomass [42,43]. Delayed harvesting has other associated benefits such as reduced nitrogen and sulfur oxide emissions in the atmosphere [42], since more of these elements are left in the field with the leaf fall. Delayed harvesting has also been documented as avoiding the need for artificial drying of biomass material and, thus, reducing the final cost of biomass production. Harvest cycles, whether 2, 3 or 4 year, are also critical in maximizing the efficiency of biomass production in a particular location. In the USA, Europe and other tem-perate regions a 3- or 4-year cycle is preferred for SRWC [44], whereas in the tropical and subtropical regions, 2- or 3-year cycles may be followed. Mantineo et al. observed that, along with differences in the energy ratio among different crops, by managing the harvest cycle one can alter the energy ratio of a given crop [45]. The energy ratios for biomass production from poplar, willow, Miscanthus, switchgrass and sorghum are much higher than that of ethanol production from first-generation biofuel crops. When compared with the ethanol production equivalent,

the energy ratio for switchgrass or sweet sorghum ethanol production is much higher than that of corn and wheat, with less GHG emissions (Figure 3). In general, for ethanol crops the conversion process is more energy consuming due to the need for distillation.

Floodplain biomass speciesWhile frequent flooding can result in reduced productiv-ity, arrested development, increased mortality and loss of ecological functions [46], some species are better adapted to flooding than others [47] and, thus, could be produc-tive even with prolonged hydric stress. Species that are flood tolerant undergo physiological changes through the formation of hypertrophied lenticels [48], which permits the exchange of dissolved gases in the flood water. In this section, we will review some of the most promising biomass species – herbaceous and woody – that exhibit moderate to high degrees of flood tolerance (Table 1). While there is very little research information available about the suitability of these species in floodplains, their physiological traits and production data from agricultural and forestry trials offer promise for their use as floodplain biomass production systems.

� WillowWillow plantations have been employed for soil con-servation, ornamental planting and windbreaks for centuries. Willow is a promising biomass crop due to high biomass potential, vegetative propagation, broad genetic base, short breeding cycle, ability to resprout after multiple harvests and sustainability through low chemical and pesticide requirement [40]. Willow root systems are robust and extensive, support at least three to five harvest cycles and help in mitigating soil erosion. Willow biomass can be easily used to generate energy without a negative carbon footprint [40]. Willow spe-cies are generally flood tolerant [49,50], with some docu-mented to survive up to 150 days in partial waterlogged conditions [51], making them suitable to river floodplain areas that are subjected to frequent annual inundation. Willow species can maintain substantial root and shoot growth in inundated conditions, as well as with a declin-ing water table, through elongation of roots [52], making them an excellent biomass crop in floodplains that may be subjected to flooding.

Willow production systems in floodplain soils may also have the added benefit of phytoremediating heavy metals, as river waters are frequently laden with arsenic and other heavy metals [53,54]. Stolarski et al. reported that economically feasible willow biomass production could be achieved through pole cuttings planted in marginal soils [55]. The experimental plots yielded bio-mass of 12.1–19.6 Mg ha-1 year-1, while in comparison annual biomass production from natural forests was only



4.2 Mg ha-1 year -1. The author further pointed out that while the natural forests cutting age was between 30 and 40 years, short rotation willow provided up to three- to five-times more biomass in ten-times shorter rotation compared with natural forests. First-rotation commercial harvest of willow biomass crops yielded an average of 7.5 Mg ha-1 year -1, while there was potential for increased yield through improved weed control, breeding new vari-eties, optimizing nutrient management and optimizing planting density [56].

� PoplarPoplar, a genus of deciduous flowering plants with 25–35 species, is one of the most important feedstocks for bio-fuel production, as poplar exhibit some of the essential bioenergy plantation qualities such as rapid growth, with maturity in 6 years, reaching heights of 20–25 m and 25 cm in diameter. Poplar usually thrives in ripar-ian areas and can maintain efficient productivity even under saturated conditions [57–59]. Du et al. observed that some flood-tolerant poplar clones demonstrated better

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Figure 3. Energy ratio and GHG emissions from biomass and ethanol production for relevant floodplain biomass species based on published literature. (A) Energy ratio; (B) GHG emissions. Gasoline used for reference. No data for GHG emissions shown for Miscanthus and switchgrass biomass.Data taken from [107,113].



ratio of 43:1 [63]. Thus, short-rotation poplar can be even more energy efficient through breeding pro-grams to increase biomass production. The full DNA sequence of western balsam poplar (Poplar srichocarpa) was decoded in 2006, thereby opening the possibility of genetic improvement of poplar hybrids for biomass production. Poplar cultivation for biomass production can follow different management practices – they can be coppiced and noncoppiced and have been shown to grow well in both marginal and agricultural land, as well as on landfill sites. Poplar species survive and perform well in river floodplains where they are subjected to moving water table and episodic flooding events throughout the year [64,65].

� MiscanthusInitially introduced in the USA and Europe as an orna-mental plant, Miscanthus generated interest as a source of renewable biomass energy, primarily due to its C

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mechanisms to avoid oxygen scarcity and maintained a high level of photosynthesis through efficient water use and regulating the water potential and stomatal closures [60]. Amlin and Rood observed that both cottonwood and willow can withstand waterlogging up to a certain extent, while willow is relatively more tolerant [49]. Cao and Conner observed that the degree of flood tolerance is dependent on the type of species within each genus [61], which has also been confirmed by Amlin and Rood [49]. Therefore, careful selection would be critical for establishment of poplar species in floodplains that are frequently inundated. Poplar also has a high energy-in and energy-out ratio and, thus, large carbon mitigation potential. Production levels in hybrid poplar have been reported to correspond well and beyond the average agricultural crop yield, which is estimated to be in the order of 20 Mg ha-1 year-1 [62]. Production and distribu-tion of 1 ton oven dry poplar chips consumes 432 MJ of non renewable energy, and generates an output-to-input

Table 1. Example of yield potential for various biomass crops under different irrigation and fertilizer treatments in the USA and Europe.

Soil/planting Irrigated Fertilized Rotation(years)

Yield (oven dry Mg ha-1 year-1)

Study† Ref.

Willow

Double row + + 4 27 Adegbidi et al. [99]

Double row + + 4 30 Stolarski et al. [100]

SRCW - + 3 15.2 Boehmel et al. [101]

Pole cutting ++ - 3 11–14.6 Tworkoski et al. [102]

Poplar

SRIP + + 3–4 12.4 Deckmyn et al. [103]

SRIP + + 3–4 22.45 Deckmyn et al. [103]

SRIP + + 2 20.15 Paris et al. [104]

SRIP + + 6 12 Pearson et al. [105]

SRIP + + 3 22 Guo and Zhang [106]

Miscanthus

Field + + Second year 17 Behnke et al. [107]

Field - + 3 19.2 Gauder et al. [108]

Field (early harvest) + + 3–4 28 Strullu et al. [109]

Field + + 3–4 18.1 Boehmel et al. [101]

Switchgrass

Field + + 3 17.8 Kering et al. [110]

Field - - 4 8.6 Knoll et al. [111]

Field - + 3–4 14.1 Boehmel et al. [101]

Field + + 4 20.5 West and Kincer [112]

Photoperiod sorghum

Field (19-cm row) - + Annual 32.89 Snider et al. [114]



Field - + Annual 30.13 Rocateli et al. [71]†Highest yields are reported for each study.+: Plots were irrigated or fertilized; -: Plots were not irrigated or fertilized; ++: Excess water in certain period. SRCW: Short-rotation coppice willow; SRIP: Short-rotation intensive poplar.



photosynthesis mechanism, with an ability to produce high biomass with a wide range of adaptability [66]. Miscanthus, which originated from east Asia, is a rhizom-atous perennial crop and can be grown for up to 20 years following establishment at one location [66]. Miscanthus has a high photosynthetic rate and captures and converts the incident solar radiation efficiently. Miscanthus yields significantly higher biomass than comparable C

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and, thus, qualifies as a promising bioenergy crop. By changing the planting density, previous stud-

ies have shown that higher yields can be obtained by increasing the plant density. Being a rhizhomatious plant, Miscanthus allocates a significant portion of the captured carbon into the root, thereby resulting in better carbon sequestration compared with other crops. The Miscanthus genotype with the greatest biomass potential to date is giant miscanthus (Miscanthus x giganteus), a sterile hybrid of Miscanthus sacchariflorus and Miscanthus sinensis. While there are concerns about invasiveness of giant miscanthus, it is unable to produce seeds. However, M. sacchariflorus and M. sinensis have escaped cultivation and can be invasive. Beale and Long found that irrigated Miscanthus had a higher yield potential compared with non-irrigated practices [67]. Miscanthus may thus be suit-able for floodplain areas with intermediate flooding, as they seem to perform well with irrigation.

� SorghumSorghum (Sorghum bicolor Moench.) is a resilient crop that grows well in a wide range of climates, and is con-sidered a promising bioenergy crop. One of the most important characteristics of sweet sorghum that can enhance its acceptability as a biomass crop is its ability to maintain high rates of photosynthesis under poor soil nutrient conditions, including soil nitrogen levels [68]. Orchard and Jessop did not observe any difference in the biomass yield of the main stem in sorghum because of waterlogging [69]. Although sorghum is typically well known as a drought-tolerant crop, the impact of flooding on sorghum growth and yield is not well documented. Screening trials to test different sorghum cultivars for flood tolerance would be critical for recommendation of sorghum as biomass crops in lowland floodplains.

In the USA, the focus of the sorghum breeding pro-grams has been on stem sugar yield in low-fertility soils [70]. The biomass sorghum, or photoperiod-sensitive sor-ghum (which remains in vegetative stage for most of the growing season), is best suited for high biomass produc-tion [71]. With the availability of the sorghum genome sequence [72], and the recent release of a sorghum con-sensus genetic linkage map that includes major-effect genes, it is feasible to associate individual genes or clusters of genes with specific quantitative trait loci [73], which greatly enhance breeding efforts. Furthermore, selection

for biomass yield under nonlimiting soil moisture con-ditions may indirectly improve productivity under water-stressed environments.

� SwitchgrassSwitchgrass shows promise as an economical and effi-cient source of cellulosic biofuel. Until recently, switch-grass breeding focused primarily on enhancing the nutritional value of forage for livestock with emphasis on higher leaf quality and ratio compared with stem [74]. Conversely, desired biomass qualities are high cellulose and low ash content, resulting in high energy conversion efficiency. Switchgrass is broadly adapted to different soil and climatic conditions, and grows well in partially water stressed to flooded conditions, as well as in mar-ginal soils. Two distinct ecotypes of switchgrass have been identified, namely lowland tetraploids and upland octaploids [75]. The lowland ecotypes have been found to be more prevalent in wetter conditions [76].

Results from a greenhouse experiment revealed that lowland switchgrass clones produced 40% more total biomass and were 40% taller under flooded conditions as compared with a control [77]. McGraw and Houx found that switchgrass was the second-most flood tolerant out of ten grass species and 15 legumes screened as part of their experiment [216]. Knapp found that irrigating switchgrass in wet years augmented the biomass yields more than other grasses, implying the ability of switch-grass to utilize excess water and increase productivity [78]. In light of these results, selection and establishment of the lowland switchgrass ecotypes in floodplains can be a very successful proposition where flooding limits conventional agricultural productivity.

Environmental, economic & social sustainabilityAccording to the Report of the World Commission on Environment and Development, sustainable develop-ment is one that “meets the needs of the present without compromising the ability of future generations to meet their own needs” and should encompass environmental, social and economic dimensions [79]. Sustainability of biofuel feedstock production systems will ultimately depend on all three aspects – environmental, economic and social attributes. The economics of increasing food prices and biomass production have been competing in the last decade, and by 2011 approximately 40% of US corn was diverted for ethanol production. Recent studies have suggested that a 10% replacement of fossil fuel with first-generation biofuels in the USA, Canada and the EU will utilize 30–70% of the cropped area [80,81].

Large-scale biofuel production systems will cause major impact on the social, environmental and eco-nomic aspects of farming systems around the world. The fact that land is a finite resource limits the overall



impact of the bioenergy utilization to a small fraction of global energy consumption. Approximately 0.8 EJ biomass energy was produced from first-generation bio-fuels in 2005 and with a potential for 15 EJ by 2050, it is more likely that land use constraints and the ‘food versus fuel’ debate will dominate biofuel development and implementation in the near future [217]. Even with an anticipated 24-EJ bioenergy production from second-generation lignocellulosic technologies, Doornbosch and Steenblik predict that offsetting 20% of liquid fuels will most likely not be fulfilled in reality [217]. However, it is aptly clear that first-generation biofuel and biofuel feed-stocks produced in agriculturally productive land will not be economically viable or environmentally sustainable. Hence, use of marginal and less-productive lands to grow high yielding cellulosic biomass for biofuel production would be necessary for long-term success.

Environmental health is one of the principal aspects of sustainable feedstock production and includes main-taining soil and water quality at the watershed scale, reduction in GHG emissions and restoration of degraded landscapes. Converting frequently flooded marginal cropland along the river corridor to flood-tolerant peren-nial biomass crops, such as poplar and willow, could offer a number of environmental benefits. In addition to providing wildlife habitat [82,83] and sequestering carbon [84–86], biomass crops can positively impact soil micro-flora and microfauna. Felten and Emmerling found posi-tive influence of intensive Miscanthus cultivation on the diversity and activity of earthworm communities in the soil [87].

Kohn [88] and Jones et al. [89] reported that non-point source pollu-tion is minimal in a forested landscape compared with intensive agricultural areas. Monocropping and grazing are two of the most important factors resulting in pollution of surface- and ground-waters [90]. Biomass feedstock production systems including SRWC and perennial grasses will signifi-cantly alter the physical and chemi-cal conditions of soil in comparison with row cropping and would result in reduced surface run-off, wind ero-sion, and transport of pollutants and other non-point source pollution. Strategically placed tree and grass buffers have been shown to remove 80% nitrate from being discharged as non-point source pollution [91,92]. Allen et al. showed that 300–500-foot wide tree buffers can help protect the levees and reduce flood damage along

the corridor [93]. Lastly, incorporation of multiple species means preservation of biodiversity. Studies have shown that agroforestry practices can preserve biodiversity in human-dominated landscapes [94].

One very important focus in the development of bio-fuels is the desire to mitigate GHG emissions from fossil fuels. Establishment of biomass feedstock production systems should therefore consider the effects of land use change. Gopalkrishnan et al. suggested that reduction in GHG emissions could be achieved through improved crop and soil management practices, including crop choice, intensity of inputs, harvesting strategy and tilling practices [41]. Qin et al. found net primary production of switchgrass and Miscanthus to be much higher (622 and 1546 g C m-2 year-1, respectively) compared with the net primary production of food crops (600 g C m-2 year-1) in China [95]. They also reported that soil organic carbon in both biofuel ecosystems compared equally with soil carbon levels of grasslands in China. Forested ecosystems are a significant sink for carbon around the world and cultivation of SRWC for biomass production can result in a significant increase in carbon sequestration potential of agricultural practices. Fast-growing poplar species can accumulate rapid biomass growth over a short period of time and could have an effective role in carbon seques-tration through agriculture and forestry [96]. Fang et al. observed that poplar species had a greater contribution in carbon sequestration and reduction of GHG emissions in comparison with other tree species in China [97]. They also observed that management practices can signifi-cantly influence the total amount of carbon sequestered

Residue Woody biomass

Switchgrass Foragesorghum

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Figure 4. Yield and delivery data for five biomass energy materials.Data taken from [98].



and suggested that with a higher plant density it would be possible to sequester more carbon in such systems.

Another important attribute of successful biomass production systems is continuous year-long supply of biomass at economically viable rates. Based on informa-tion from the US Department of Energy, delivered cost for each dry ton of biomass to their respective conver-sion facilities should be in the range of $40–50. Current production costs of biomass (per dry ton) for dedicated energy crops are much higher and vary between $50 and $100 depending on type of crop, management practices and climatic region. A comparison between yield and delivered cost of a dry ton of biomass to a conversion facil-ity shows that bioenergy sorghum is by far the most effi-cient in terms of yield and delivery cost ratio (Figure 4) [98]. McCutchen et al. also suggested that important criteria to reduce the delivery cost of biomass to the conversion plant would be crops with high tonnage, and an efficient production and delivery chain.

Expansion and innovation of agricultural practices in rural areas could be used as an avenue for rural eco-nomic development. A total output of approximately 68–110 billion l of liquid drop-in biofuels will result in direct economic impact of an estimated $60–100 billion in the USA alone. Using similar calculations as Hodur and Leistritz, the multiplier effect of such a biofuel pro-duction system would result in an indirect impact of $150–250 billion annually [218]. The net result would, thus, be an economic impact of up to $350 billion annu-ally spent domestically and a lesser dependence on for-eign oil. Such biofuel production systems will promote rural economic benefits through employment generation and ancillary industries coupled with environmental and social benefits.

Future perspectiveSecond-generation biomass feedstock production sys-tems in river floodplains and marginal lands can be an excellent alternative to food crop-based biofuel produc-tion. Biomass species such as poplar, willow, Miscanthus, switchgrass and high-biomass sorghum have shown great potential as biomass feedstocks. The use of liquid drop-in fuels is expected to rise in the near future as major com-panies in the USA are seeking to purchase large volumes of advanced biofuels. For example, FedEx has publicly committed to replace 30% of its annual fuel use with biofuels by 2030 [219]. The US Navy, Air Force and Army have all set ambitious targets for adoption of renewable, domestically produced biofuels [220]. The Navy plans to launch the ‘Great Green Fleet’, a fighting force of ships, submarines and planes powered entirely by biofuels, with testing from 2012 and for the fleet to be operational by 2016. The Air Force hopes to get half of the fuel it uses for domestic flights from alternative renewable sources by

2016. However, the development of a sustainable, repli-cable feedstock system and scalable supply chain has not been on pace with the technology development; the result being a bottleneck in which the technology cannot be deployed until the feedstock production, processing and logistics system is in place.

To achieve dedicated and continuous feedstock sup-ply, advanced rural biofuel refineries (ARBRs) need to be developed that will create ‘market pull’ for locally pro-duced biomass feedstock. Such facilities are scalable to meet the demands of advanced biofuel developers, while achieving economic viability through the development of a variety of co-products and biomaterials. Current market pull (created by ARBRs and existing biofuel and biopower producers) calls for improving productivity and expanding acreage of biofuel crops. ARBRs can supply commercial quantities (150,000 Mg year-1 or more) of densified biomass (pellets), concentrated sugar (syrup), SRWC chips and baled perennial crops to major bio-refinery ‘hubs’, to accomplish the large-scale commercial production of advanced biofuels. The influence of such production systems on the environment and society will depend on numerous factors that range from the poli-cies that govern biofuel production, cultural and social practices, and the demand for biomass and associated trade practices.

Along with managing supply-chain logistics, future research should focus on breeding and crop improvement, optimizing management practices and socio economic adaptability. Future research should also examine the agronomic and economic feasibility of large-scale deploy-ment of biofuel feedstocks on hydrologically connected floodplains. Best management practices should be devel-oped for site preparation, weed and pest control, fertilizer recommendations and irrigation management. Innovative production systems, such as agroforestry, that combine woody and herbaceous feedstock species offer great promise for floodplains and need to be explored further.

AcknowledgementsThe authors would like to thank the anonymous referees for helpful comments on an earlier version of the manuscript. Discussions with S Flick, P Nelson, B Hood, S Sachdev and M Gold were helpful in elaborating on the floodplains for biomass production concept.

Financial & competing interests disclosureThe authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or finan-cial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert t estimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.



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Executive summary

Floodplain biomass production � Floodplains have the potential to accommodate large-scale biomass production systems that would provide a sustainable supply of

biomass for the emerging advanced liquid drop-in biofuels industry. � While effort should be undertaken to include as much marginal and degraded land as possible for biomass production, proper emphasis

should also be placed on improving the stress (both biotic and abiotic) tolerance of selected biomass species so that their maximum production potential can be achieved.

Floodplain biomass species � The most promising biomass species suitable for river floodplains include poplar, willow, Miscanthus, switchgrass and sorghum.

Economic, social & environmental aspects of biomass production � The biofuel industry could generate approximately US$350 billion annually in the USA. Efficient delivery and supply-chain development is

critical to maintain the cost of biofuels at the threshold for economic sustainability. � Social, economic and environmental sustainability should be a priority for establishment of such biofuel production systems.



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