Biomass yield and energy balance of three perennial crops for energy use in the semi-arid Mediterranean environment
Introduction
Increasing biomass use is one of the key tools proposed by the European Community to reduce its dependence on imported oil and oil products, thus improving the security of energy supply in the medium and long term (European Biofuels Technology Platform, 2008). Moreover, biomass use on a global scale could contribute to improving the environment, given that biomass sources are ‘carbon neutral’ since the carbon they emitted into the atmosphere when burned is offset by the carbon that plants absorb from the atmosphere whilst growing (Royal Society, 2008).
Several biomass feedstocks for energy can readily be produced in the EU, such as those from arable crops currently grown for food: sugar, starch and oil crops, forestry or domestic waste and marine biomass. However, it is also possible to increase the production of dedicated crops, the ‘energy crops’, “that are bred or selected to produce biomass with specific traits that favour their use as an energy vector” (European Biofuels Technology Platform, 2008).
One of the most promising sources of biomass are lignocellulosic crops that can be used for the production of heat and electricity by means of direct combustion or the production of biofuel and biogas from pyrolysis and gasification these are already mature technologies. The production of so called ‘second generation’ biofuels, like BTL-biomass to liquid, SNG-gas-synthetic gas, bio hydrogen, and in particular 2nd generation bioethanol which can be derived from a feedstock rich in cellulose and hemicelluloses, could open up new frontiers for lignocellulosic crops (Yang and Wyman, 2008).
Research carried out in recent years in the Mediterranean environment, where the constraints are low water availability and high temperatures during summer, have indicated giant reed, Miscanthus spp. and cardoon are among the most promising species for energy and cellulose production (Lunnan, 1997, Foti and Cosentino, 2001, Anatoly and Pereira, 2002, Cosentino et al., 2005, Cosentino et al., 2007, Cosentino et al., 2008, Christian and Haase, 2001, FAIR3 CT96-2028, 2000, Jones and Walsh, 2001, Lewandowski et al., 2000). These unimproved perennial species produce considerable amounts of lignocellulosic biomass; they are either native to the Mediterranean area (cardoon), naturalised in these environments (giant reed) or again have good adaptation capacity (Miscanthus).
As a first consideration, the use of energy crops presupposes a close scrutiny of the energy accumulated and used in their production because according to Lewandowski and Schmidt (2006) “only crops that yield significantly more energy than is required to grow them are suitable energy crops”.
Energy balance was a much-debated topic in the early 1970s during the first world energy crisis (Pimentel et al., 1973). It has also been widely discussed recently mainly due to environmental emergencies and the high prices of fossil fuels.
Indeed, there is growing interest in considering the energy balance of crop production, since energy parameters may be used as indicators of environmental effects and the sustainability of plant production (Girardin et al., 2000, Hülsbergen et al., 2001). Energy balance is an adequate instrument to identify an efficient energy crop (Boehmel et al., 2008); it has been performed on different energy crops, but a direct comparison of these results is very difficult because of the various methodologies used. Therefore, the aim of this study is to evaluate the yield and the energy balance of three species under the Mediterranean climate, using consistent methods so that the amount of energy used for irrigation and nitrogen application may be compared.
Section snippets
Agronomic techniques
The field experiment was carried out in a 5-year period from 2002 to 2007 at Enna (Sicily, 550 m a.s.l., 37°23′N Lat, 14°21′E Long) in a typical Xerorthents sandy soil (USDA, 1975). The 1.20 m deep soil had the following properties: field capacity of 20.9% of dry weight at −0.03 MPa and wilting point of 10.6% of dry weight at −1.5 MPa, and apparent volumetric mass 1.2 kg m−3.
Three different perennial lignocellulosic species were compared: Arundo donax L. (local clone Fondachello, Cosentino et al.,
Meteorology
The meteorological data recorded throughout the growing season highlighted the typical characteristics of the semi-arid Mediterranean climate with a mild, rainy winter and hot dry summer (Fig. 1). During the summers, when A. donax and M. x giganteus grew, rainfall was almost absent. There were some exceptions in 2005 and 2006, with 198 and 115 mm in the period May–September. Summer temperatures generally exceeded 30 °C with maximum temperatures over 40 °C in July–August. Winter temperatures rarely
Discussion
After the establishment year, the yield pattern highlights that the two gramineous species produced at least until the fifth year (Angelini et al., 2005, Ercoli et al., 1999). A different behaviour was found in C. cardunculus, whose above ground dry matter yield tended to decrease from the first year onward until an almost complete disappearance. This is most likely due to water distribution, soil pathogens accumulation and aging of the crop, which confirms the findings of Gherbin et al. (2001)
Conclusions
All three crops (A. donax, M. x giganteus and C. cardunculus) seemed suited for cultivation in the Sicilian interior environment, which is particularly hot and dry from June to September, provided that there was a certain amount of water for irrigation.
The results demonstrate the high yield of useable energy of the three crops in different agronomic conditions, with minimum or no energy inputs.
In particular, A. donax performed better than the other two crops in terms of yield, net energy yield
Acknowledgements
The research was partly supported by the Italian Ministry of Agriculture in the framework of the project “Sustainable innovative techniques for energy and non food crops” (TISEN).
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