It is clear that a wide variety of plant species as well as plant cultivars accumulate perchlorate mostly in their leaves, followed by fruits, stems, branches, and roots. Overall, the amount of perchlorate found within plants depends on (1) the concentration of perchlorate in the growth media, (2) the duration of plant growth, (3) the plant species, (3) the presence of competing ions in solution, (4) the amount of water transpired, (5) the extent of phytodegradation within the plant, (6) the extent of exudation from the plant, and (7) the portion of the plant analyzed. Evidence suggests that transpiration plays a key role in perchlorate delivery to plant roots, where perchlorate is actively taken up by the nitrate ion transporter through cotransport with protons.
The most promising conditions for perchlorate phytoremediation are (1) using plants within which perchlorate is fully phytodegraded to chloride or (2) stimulating rhizodegradation under anaerobic conditions by low nitrate availability and high concentration of electron donors. Of most concern to human health are produce in which mostly leaves are consumed.
More research is needed in order to understand the extent of perchlorate metabolism within produce and the extent to which edible modified stems and roots (e.g., tubers) accumulate perchlorate, both pre- and postharvest.
Additional research utilizing isotopic labeling to determine the extent of translocation, phytodegradation, and exudation of environmentally relevant levels of perchlorate and its metabolites should also be conducted.
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Perchlorate in Higher Plants 123
C H A P T E R T H R E E
Sugarcane for Bioethanol: Soil and Environmental Issues
Alfred E. Hartemink*
Contents
1. Introduction 127
2. Changes in Soil Chemical Properties 128
2.1. Data sources and types 128
2.2. Monitoring over time 129
2.3. Samples from different land-use systems 133
2.4. Soil organic matter dynamics 137
2.5. Leaching, denitrification, and inorganic fertilizers 140
2.6. Nutrient balances 143
3. Changes in Soil Physical Properties 147
3.1. Compaction and aggregate stability 147
3.2. Soil erosion 149
4. Changes in Soil Biological Properties 151
4.1. Macrofauna 152
4.2. Microbes 153
5. Environmental Issues 154
5.1. Herbicides and pesticides 156
5.2. Inorganic fertilizers 158
5.3. Air and water quality 160
6. Discussion and Conclusions 161
6.1. Sugarcane for bioethanol 161
6.2. Effects on the soil 162
6.3. Effects on air and water 164
6.4. Sugarcane yields 166
6.5. The potential for precision farming 169
Acknowledgments 172
References 172
Advances in Agronomy,Volume 99 #2008 Elsevier Inc.
ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00403-3 All rights reserved.
*ISRIC - World Soil Information, 6700 AJ Wageningen, The Netherlands
125
Abstract
Cultivation of sugarcane for bioethanol is increasing and the area under sugar- cane is expanding. Much of the sugar for bioethanol comes from large planta- tions where it is grown with relatively high inputs. Sugarcane puts a high demands on the soil because of the use of heavy machinery and because large amounts of nutrients are removed with the harvest; biocides and inorganic fertilizers introduce risks of groundwater contamination, eutrophication of surface waters, soil pollution, and acidification. This chapter reviews the effect of commercial sugarcane production on soil chemical, physical, and biological properties using data from the main producing areas. Although variation is considerable, soil organic C decreased in most soils under sugarcane and, also, soil acidification is common as a result of the use of N fertilizers. Increased bulk densities, lower water infiltration rates, and lower aggregate stability occur in mechanized systems. There is some evidence for high leaching losses of fertilizer nutrients as well as herbicides and pesticides; eutrophication of surface waters occurs in high-input systems. Soil erosion is a problem on newly planted land in many parts of the world. Trash or green harvesting overcomes many of the problems. It is concluded that sugarcane cultivation can substan- tially contribute to the supply of renewable energy, but that improved crop husbandry and precision farming principles are needed to sustain and improve the resource base on which production depends.
1. Introduction
Bioenergy is energy from biofuels. Biofuel is produced directly or indirectly from biomass such as wood, charcoal, bioethanol, biodiesel, biogas (methane), or biohydrogen (FAO, 2006). It is big business. Demand for biofuels is surging because of the rise in crude oil prices and the global search for renewable energy (Valdes, 2007) and global biofuel production tripled between 2000 and 2007. Currently, the most important biofuel crops are corn, rapeseed, soybean, sugarcane, and oil palm whereas suitable trees for bioenergy production include eucalyptus, poplar, and willow.
Biofuel production itself needs fossil energy. Currently, agriculture accounts for about 15% of the global energy demands (fertilizers, transport etc.) but it is estimated that agriculture can produce half to several times the current global energy demand (Smeetset al., 2007).
The environmental impact of the shift toward growing crops for energy is still to be assessed. It is a complex matter with economic interests and other factors interacting on several scales. For example, the cultivation of biofuel crops is competing with food crops and may drive up commodity prices (UNEP, 2007)—over the last few years, world food prices have increased because of market demand for corn, wheat, and soybean. There
126 Alfred E. Hartemink
is further concern that the expansion of biofuel crops takes place at the expense of rainforest and has negative effects on biodiversity and the environment.
Sugarcane as a biofuel crop has much expanded in the last decade, yielding anhydrous ethanol (gasoline additive) and hydrated ethanol by fermentation and distillation of sugarcane juice and molasses (Gunkel et al., 2007; Pessoaet al., 2005). By-products are bagasse and vinasse (stillage or dunder), which is the liquid waste sometimes used for fertigation pur- poses. Bagasse, a by-product of both sugar and ethanol production, can be burned to generate electricity or be used for the production of biodegrad- able plastic. It provides most of the fuel for steam and electricity for sugar mills in Australia and Brazil.
One hectare of sugarcane land with a yield of 82 t ha1produces about 7000 liter of ethanol. Brazil currently produces about 31% of the global production and it is the largest producer, consumer, and exporter of ethanol for fuel (Andrietta et al., 2007). The industry employs more than one million people (Pessoa et al., 2005). The value of the sugar and ethanol industry reached $8 billion in 2006, some 17% of Brazil’s agricultural output (Valdes, 2007).
Between 1990 and 2005, global average sugarcane yields increased from 61 to 65 Mg ha1(http://faostat.fao.org). In 1990, global production was 1050 million Mg and in 2005, production of sugarcane was 1225 million Mg. Much is grown on large plantations but in some countries sugarcane is grown by smallholders, for example in Thailand where there are more than 100,000 farmers growing sugarcane (Sthiannopkao et al., 2006). In Brazil, less than 20% of the sugarcane is produced on small farms; most is grown in the southeast with over 60% of the production in the Sa˜o Pula district (FAO, 2004). In some countries, sugarcane is the main source of revenue and in Mauritius, sugarcane occupies 90% of the arable land (Ng Kee Kwonget al., 1999). Globally, the area harvested increased by 2.6 million ha in the period 1990–2005; the largest expansion was in India and Brazil. It is expected that the area under sugarcane in Brazil will expand by 3 million ha over the next 5 years whereas the area under sugarcane in China is forecast to rise by 5% or more than 100,000 ha year1. Brazil has a long tradition of growing sugarcane. In sixteenth century, it was the world’s major supplier of sugar (Courtenay, 1980). In 1975, the area under sugarcane in Brazil was 1.9 million ha (de Resendeet al., 2006), now there is about 6.2 million ha under sugarcane in Brazil compared to 21 million ha soybean and 14 million ha corn. Other big sugarcane producers are India (4.2 million ha), China (1.4 million ha), Thailand (1.1 million ha), and Pakistan (0.9 million ha) whereas the sugarcane areas in Australia, Cuba, Indonesia, Mexico, and South Africa cover some 0.5–0.6 million ha in each country. In the United States, there are about 170,000 ha in Louisiana and 167,000 ha in Florida.
Against the trend, the area under sugarcane in Hawaii has decreased from
Sugarcane for Bioethanol: Soil and Environmental Issues 127
about 100,000 ha in the 1930s to 6000 ha in 2007, and also the area under sugarcane in Cuba has been more than halved in the last 15 years.
Traditionally, sugarcane was harvested manually; the senescent leaves (trash) and stalks were removed by people using big knives. Green harvest- ing was common in Brazil up to 1940s (de Resendeet al., 2006), but the large volume of trash makes manual harvesting difficult (Boddey et al., 2003). As labor shortages developed, it became common practice to burn of the dead leaves prior to the harvest (preharvest burning). In the last two decades, preharvest burning has been replaced by mechanical green- or trash harvesting by cutter-chopper-loader harvesters that leave the trash on the field. Most of the sugarcane in Australia and parts of the West Indies is now arvested like this (Graham et al., 2002a). Up to the 1960s, Australian sugarcane was harvested manually but a decade later, following severe labor shortages, nearly all sugarcane was harvested mechanically (Brennan et al., 1997). Currently, about 30% of the Brazilian sugarcane is green- harvested, the rest is harvested manually with preharvest burning. All sugar- cane in the United States is mechanically harvested but over 90% of the fields are burned after the green harvesting, to get rid of the trash blanket.
Sugarcane is grown as a ratoon crop: the whole above ground biomass is harvested each year and harvests may continue for a number of years (ratoons). Yields decline with ratooning and, after some years, the land is ploughed and new sugarcane is planted. Much of the world sugarcane is grown with a high degree of mechanization. Also, large amounts of biomass are annually removed with the harvest and herbicides and pesticides are used extensively. Irrigation and large amounts of inorganic fertilizers are often required for high yields. As a consequence, soil properties are likely to change under sugarcane cultivation and the high biocide inputs may affect the environment. Environmental concerns and policies are key factors affecting the future of sugarcane production (Valdes, 2007). There is a also risk that the sugar industry is expanding on marginal lands where the costs or preventing or repairing environmental damage may be high (Arthington et al., 1997). This chapter reviews the main soil and environ- mental issues under continuous sugarcane cultivation. Most of this work predates the surge of sugarcane production for bioethanol but the results are very relevant for the new situation.