The renewable energy-intensive global scenario described in Section 1.2 calls for some 400 million hectares of biomass plantations by the second quarter of the 21st century. If this magnitude of biomass is used, the questions raised are whether the net energy balances are sufficiently favorable to justify the effort, whether high biomass yields can be sustained over wide areas and long periods, and whether such plantations are environmentally acceptable (Johanson et al., 1993).
Achieving high plantation yields requires energy inputs, especially for fertilizers and harvesting and hauling the biomass. The energy content of harvested biomass, however, is typically 10-15 times greater than the energy inputs.
However, whether such high yields can be achieved year after year is questionable. The question is critical because essential nutrients are removed from a site at harvest; if these nutrients are not replenished, soil fertility and yields will decline over time. Fortunately, replenishment is feasible with good management. Twigs and leaves, the parts of the plant in which nutrients tend to concentrate, should be left at the plantation site at harvest, and the mineral nutrients recovered as ash at energy conversion facilities should be returned to the plantation soils. Nitrogen losses can be restored through the application of chemical fertilizers; make-up requirements can be kept low by choosing species that are especially efficient in the use of nutrients. Alternatively, plantations can be made nitrogen self-sufficient by growing nitrogen-fixing species, perhaps intermixed with other species. In the future, it will be possible to reduce nutrient inputs by matching nutrient applications to a plant's cyclic needs.
Intensive planting and harvesting activities can also increase erosion, leading to productivity declines. Erosion risks for annual energy crops would be similar to those for annual food crops, and so the cultivation of such crops should be avoided on erodible lands. For crops such as trees and perennial grasses, average erosion rates are low because planting is so infrequent, typically once every 10-20 years.
An environmental drawback of plantations is that they support far fewer species than natural forests. Accordingly, it is proposed here that plantations be established not on areas now occupied by natural forests but instead on deforested and otherwise degraded lands in developing countries and on excess agricultural lands in industrialized countries. Moreover, a certain percentage of land should be maintained in a natural state as sanctuary for birds and other fauna, to help control pest populations. In short, plantations would actually improve the status quo with regard to biological diversity.
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