C02-C equivalent for diesel and gasoline respectively (Lal et al., 1998). However, relative to other sources of CO2, these sources are small. Estimated CO2 emission directly from ag­ricultural energy use in the USA in 2001 is only 2% of total CO2 emissions (USDA, 2004). Similarly, UK statistics sug­gest that emissions due to use of agricultural fossil fuel and lime accounted for less than 1 % of total CO2 emissions in the UK (MAFF, 2000).

3.1.1.7 Environmental consequences of changes in farm size and structure
One of the changes in farm structure over the last 50 years has been the increase in sizes of fields and farms and the simplification (in the number of products per farm) of crop­ping systems. In Europe changes in farm sizes are often as­sociated with other changes in agricultural practice, which in turn can have environmental impacts. The fine grained nature of traditional European landscapes, with small fields separated by hedges, trees, walls and ditches and with small seminatural areas between fields, has become coarser with the loss of many of the traditional boundary features that are often the key to the success of indigenous plants, in­vertebrates, mammals and birds. (Roschewitz et al, 2005; Herzog et al., 2006)
     Intensification of production in eastern Europe dur­ing the socialist era has resulted in greater negative envi­ronmental effects than has occurred in western Europe. Although crop yields were increased, politically driven, cen­tral management has resulted in greater erosion, salination and chemical pollution (Bouma et al., 1998). Changes since 1990 are now endeavoring to limit adverse side effects from agriculture.

3.1.1.8 Environmental consequences of growing more bioenergy crops
One incentive for the use of biofuels and biomass crops is their replacement of fossil fuels. While any burning of fos­sil fuels (without sequestration) contributes to increases in carbon dioxide in the atmosphere, power produced from bioenergy appears neutral at the point of use as the carbon in the bioenergy crops came from the atmosphere. However, much of NAE agriculture is energy intensive and the emis­sions saved by use of biofuels and biomass crops is signifi­cantly reduced by the fossil fuels used directly (e.g., running farm machinery) or indirectly (energy used in the production of fertilizer and agrochemicals) during the production of the crop. There are some estimates that the current production of biofuels (maize-based ethanol) is actually carbon nega­tive in that it takes more fossil fuel to produce biofuel than the petroleum it is intended to replace (e.g., Pimentel and Patzek, 2005) though the consensus seems to be that there is a positive net carbon balance in the production and use of biofuels (e.g., Farrell et al., 2006; Worldwatch, 2006).
     Two concerns associated with the expansion of biofuel and biomass production are that there is likely to be com­petition for land between requirements to grow crops for food or for bioenergy, with associated impacts of food prices and that there could be pressure to put uncropped land into energy crop productions, especially highly erodible lands, wetlands, buffer areas and mature forests. Many of these ar­eas are currently providing environmental benefit and their

loss would increase environmental impacts. Production of energy crops with irrigation would put increasing demands on water use. Putting or returning uncropped lands into ag­ricultural production may (depending upon the clearing and agricultural systems used) also release the carbon in biomass and soil organic carbon into the atmosphere.
     The prospects for greater production of biofuels with­out greater effects on the environment rely on a second generation of biofuel sources. It is expected that in the rela­tively near future that it will be possible to produce ethanol from the non-starch and non-sugar components of plants, expanding the amount of carbon that can be converted from food crops and making non-food plants suitable for biofuel production (Gray et al., 2006; Tilman et al., 2006). How­ever, agricultural practices will have to assure that sufficient plant materials remain in the soil to maintain soil health and soil organic carbon and maintain other benefits (e.g., Lal and Pimentel, 2007). Losses of soil organic carbon would tend to negate benefits from use of non-fossil fuels.
     Future developments may also entail breeding of food crop varieties and non-food plants specifically to increase their utility for energy production. Non-food crops may in­clude hardwood species such as poplar and willow, switch-grass and even algae. It should be noted that ethanol and biodiesel are not the only prospects for second generation fuels. Butanol can also be produced by (bacterial) fermenta­tion of sugars and may have significant advantages over eth­anol as a gasoline replacement (Ramsey and Yang, 2004). Biogas may also be produced from plant materials.

3.1.2 Environmental consequences of changes in animal production

3.1.2.1 Environmental impacts of differing animal husbandry systems
There are three distinct animal production systems in the NAE (Seré et al., 1996): grazing, mixed farming and indus­trial systems. Each has potential environmental impacts, especially the latter. The increased specialization that has occurred in the last 50 years has resulted in many areas in separation of production into "crop production areas" and "animal production areas". As a result the number of mixed farms has declined.
     Grazing systems feed animals mostly on native grass­land, with little or no amounts of other plant material and rarely including imported inputs, resulting in low calorific output per unit land area (Jahnke, 1982). However, if too many livestock are kept on the grazed area, the desirable forage plants may be reduced too severely, creating oppor­tunity for invasive species.
     Mixed farming systems integrate livestock and crop ac­tivities and have traditionally been the dominant approach to agriculture. By-products (crop-residues, manure) from one enterprise can serve as inputs for the other, resulting in environmentally friendly systems. Thus, the detrimen­tal environmental effects from fertilizers can be minimized by efficient use and recycling of nitrogen and phospho­rus. However, even in mixed farming systems, animal by­products can cause environmental damage, if they are not recycled efficiently. The shift from haymaking to silaging for feeding grassland-based cattle in mixed (and intensive)