Jim Cook and Jan Beyea
National Audubon Society
Concerns about global climate change and air quality are driving increased interest in biomass and other energy sources that are potentially CO2-neutral and less polluting. Large-scale bioenergy development could bring significant environmental benefits -- or equally significant damages -- depending on the specifics. In particular, the land requirements for biomass production could be immense.
Various entities in the United States have done cost-supply assessments, environmental impact assessments, life cycle analyses and externality impact assessments for biomass crops and other potential biomass energy feedstocks. Most of these efforts have focused on perennial herbaceous crops and fast-growing woody crops. Simultaneously, various national and regional groups of bioenergy stakeholders have issued consensus recommendations and guidelines for sustainable bioenergy development.
It is a consistent conclusion from these efforts that displacing annual agricultural crops with native perennial biomass crops would very likely help restore natural ecosystem functions in worked landscapes, and help to preserve natural biodiversity. Conversely, if biomass crops displace more natural land cover -- such as forests and wetlands -- there would very likely be a loss of ecosystem functions and reduced biodiversity.
Concerns about global climate change and air quality are driving increased interest in energy sources that are potentially CO2-neutral and less polluting (Rubin et al., 1992; Brower et al., 1993; US Congress OTA, 1995). Four such alternatives are being seriously considered for large scale implementation: biomass, wind, solar and geothermal. Because biomass can readily supply base load electrical power and be converted to liquid transportation fuels, it will likely be a key part of the solution.
The US Department of Energy (DOE) Biofuels Feedstock Development Program (BFDP) has explored a wide variety of annual and perennial plant species -- 34 herbaceous species and 125 tree species -- as potential biomass crops (Tolbert and Schiller, 1996). US DOE BFDP efforts have focused in recent years on switchgrass (Panicum virgatum) -- a perennial grass native to the US prairie -- and several fast-growing woody crops -- hybrid poplar (Populus spp.), willow (Salix spp.), sweetgum (Liquidambar styraciflua), sycamore (Platanus occidentalis) and maple (Acerspp.) -- as model species for testing at larger scales. The US paper industry is also exploring fast-growing woody crops as an environmentally sound fiber resource (Malcolm, 1994).
Large scale biomass energy development could bring significant environmental benefits -- or equally significant damages -- depending on the path taken. Sustainable bioenergy development could 1) reduce net greenhouse gas emissions, 2) improve air quality and reduce acid deposition, 3) reduce landfilling, 4) reduce agricultural chemical runoff, and 5) improve habitat for native wildlife. Conversely, inappropriate bioenergy development could do great environmental damage. In particular, the land requirements for biomass production could be immense. The nature and extent of the impacts of these changes in land use will depend on the specifics.
Various entities in the United States have done cost-supply assessments (Graham and Downing, 1995; Graham et al., ???; Walsh and Graham, ???), environmental impact assessments (Cook et al., 1991; Miles and Miles, 1992; Perlack et al., 1992; Antares, 1993; US Congress OTA, 1993; Tolbert and Downing, 1995; Tolbert and Schiller, 1996), life cycle analyses (DynCorp EENSP, 1995) and externality impact assessments (Fang and Galen, 1994; Swezey et al., 1994) for biomass crops and other potential biomass energy feedstocks. Most of these efforts have focused on perennial herbaceous crops and fast-growing woody crops. These efforts have addressed such issues as energy and greenhouse-gas budgets, soil health and erosion, surface water and groundwater pollution, biodiversity and landscape ecology, and emissions from conversion facilities. Other studies have explored biomass production as an alternative to the Conservation Reserve Program for environmentally sensitive cropland (US GAO, 1995; Walsh et al., ???).
Simultaneously, a wide range of bioenergy stakeholders -- farmers, utilities, fuel producers, environmental NGOs and government agencies -- have convened workshops and roundtables to share concerns and engage in a process of joint fact finding, negotiation and consensus building. These efforts have resulted in recommendations and guidelines for developing and implementing bioenergy technologies in ways that are economically viable, socially beneficial and ecologically sustainable (Beyea et al., 1992; National Biofuels Roundtable, 1994; CONEG Governors' Biomass Policy Roundtable, 1995; Southeast Bioenergy Roundtable, 1996).
Global Climate Change
The risk of eventual climate changes due to anthropogenic greenhouse gas emissions has been cited by the US EPA Science Advisory Board as one of the highest priority issues facing the Agency (US EPA, Science Advisory Board, 1990). Increasing energy efficiency and displacing fossil fuels with renewable energy are two of the leading options for reducing emissions of CO2, the principal greenhouse gas. The main US sources of anthropogenic CO2 emissions -- accounting for about two thirds of the US total -- are fossil-based power generation and transportation. Biomass will likely play key roles in reducing CO2 emissions in both of these sectors, because it can readily supply base load electrical power and be converted to fluid transportation fuels. Biomass can also displace fossil fuels indirectly as durable products that replace products made from such energy-intensive materials as steel, plastics and aluminum (Schlamadinger and Marland, 1996).
Generally speaking, the effectiveness of biomass in reducing CO2 emissions from fossil fuels depends on two main factors: 1) the net effective greenhouse gas flux for the overall biomass production-use cycle and 2) the relative efficiency of the biomass conversion or end-use process (Williams, 1985; Hall et al., 1991; Overend, 1996; Schlamadinger and Marland, 1996; Williams and Larson, 1996). Although conversion and end-use efficiencies for biomass energy feedstocks are currently lower than those for fossil fuels, these may be transient symptoms of technological immaturity and small-scale implementation. Even now there are exceptions -- biomass can be co- fired in large and efficient coal-fired electrical power plants with minimal modifications and efficiency penalty (Overend, 1996). For the longer term, new technologies -- such as pre-drying, new combustion technologies, gasification, gas turbines and combined cycle systems -- promise even greater efficiencies (Bryden et al., 1994; Overend, 1996; Williams and Larson, 1996).
The net effective greenhouse-gas flux for a particular biomass feedstock depends primarily on two characteristics of the biomass production-use cycle: 1) the net greenhouse gas flux, and 2) the order and timing of the component source and sink terms (Schlamadinger and Marland, 1996; Marland et al., 1997). Fluxes of CO2 and other greenhouse gases for bioenergy systems involve several sources and sinks. The principle ones are 1) CO2 fixation during biomass growth, 2) changes in the organic matter content of the soil, and 3) CO2 emissions during biomass conversion and/or use. Other emissions for bioenergy systems include 1) CO2 emissions from fossil-fueled equipment used to manage, harvest, process and transport biomass, 2) CO2 emissions from fossil energy used in the production of fertilizers and pesticides, and 3) N2O emissions from nitrogen- fertilized soil (Perlack et al., 1992; Wright et al., 1992; DynCorp EENSP, 1995; Turnbull and Boman, 1995; Schlamadinger and Marland, 1996).
Changes in the Organic Matter Content of the Soil
Generally speaking, the conversion of land from natural cover to intensive annual crop production progressively decreases the organic matter content of the soil. The major factors are 1) decreased detrital inputs and 2) increased erosional and metabolic losses caused by increased soil temperature and aeration. For organic-rich soils, this loss of organic matter can result in obvious subsidence. However, we will assume for this analysis that most cropland has already lost the most labile component of its soil carbon, and that ongoing losses are therefore minimal.
Conversely, the conversion of land from intensive annual crop production to perennial herbaceous species progressively increases the soils' organic matter content. For example, the conversion of land from annual crops (cotton, wheat and corn) to native perennial grasses (as part of the Conservation Reserve Program) added an average of 1.1 Mg C per ha per yr to the soil (Gebhart et al., in press). Bransby and coworkers obtained similar results for the conversion of land from annual crops to switchgrass (Bransby et al., 1996).
The conversion of land from annual crops to fast-growing woody crops added an average of ca. 1-2 Mg C per ha per yr over the course of the rotation, although there was a transient loss of soil carbon from increased erosion and mineralization until canopy closure at ca. 6 years (Hansen, 1993; also see Harmon et al., 1990). Other workers have failed to find the expected increase (Grigal and Berguson, 1996). Additional studies of soil carbon changes are planned for larger scale (12-120 ha) hybrid poplar plantings near Alexandria, Minnesota, USA (Tolbert and Downing, 1995).
Projected yields of harvestable biomass on good agricultural sites are 15-20 Mg dry biomass per ha per yr for perennial herbaceous crops and 10-15 Mg dry biomass per ha per yr for woody crops (Perlack et al., 1992). Assigning the net increase in soil carbon to the harvested biomass crop -- as a negative component of its net carbon flux -- yields 60-70 Kg C per Mg dry biomass for perennial herbaceous crops and 0-200 Kg C per Mg dry biomass for woody crops.
Fossil Energy Inputs
The major fossil energy inputs for biomass crop production are fertilizers (mostly nitrogen) and fuel (for planting, management and harvesting). Nitrogen fertilizers are made from natural gas.
Shapouri and coworkers estimated that fossil energy inputs for corn production currently average 2.3 GJ per Mg -- with 0.9 GJ per Mg as nitrogen fertilizer and 0.5 GJ per Mg as fuel (Shapouri et al., 1995). Lorenz and Morris estimated a current average of 2.8 GJ per Mg -- with 1.2 GJ per Mg as nitrogen fertilizer and 0.3 GJ per Mg as fuel (Lorenz and Morris, 1995). These estimates are equivalent to 30-40 Kg C per Mg based on the CO2 emissions from the mix of fossil feedstocks used. Including most of the stover with the harvest reduces the estimate to ca. 20 Kg C per Mg.
Projected fossil inputs for perennial crop production are considerably lower -- 0.72 GJ per Mg for switchgrass and 0.48 GJ per Mg for hybrid poplar (Perlack et al., 1992). Although projected fuel requirements are 0.30 GJ per Mg for both crops, switchgrass is projected to require more nitrogen fertilizer than hybrid poplar -- 0.34 GJ per Mg vs. 0.16 GJ per Mg. These projections are equivalent to 12 Kg C per Mg for switchgrass and 8.3 Kg C per Mg for hybrid poplar based on the CO2 emissions from the mix of fossil feedstocks used.
Order and Timing of CO2 Fixation and Emission
Woody crops sequester CO2 during growth, serving as a transient carbon sink. Taking the carbon content of dry wood to be ca. 540 Kg C per Mg, assuming linear tree growth and using a discount rate of 3% per year for past CO2 uptake (Marland et al., 1997), this adds 30 Kg C per Mg for a three year rotation (e.g., willow) and 90 Kg C per Mg for a 10 year rotation (e.g., hybrid poplar).
Although the estimated net effective CO2 flux for the corn production-use cycle is positive -- 20 to 40 Kg C per Mg dry biomass -- it is relatively small compared to the total carbon content of the corn (ca. 400 Kg C per Mg. In contrast, the estimated net effective CO2 fluxes for perennial biomass crops are actually negative -- -80 to -70 Kg C per Mg for switchgrass, -220 to -20 Kg C per Mg for willow, and -280 to -80 Kg C per Mg for poplar. Indeed -- especially for woody crops -- the reduction in anthropogenic climate forcing via transient CO2 sequestration during growth and CO2 fixation as soil organic matter may be appreciable compared to that from direct fossil fuel displacement.
Air Quality and Acid Precipitation
Biomass feedstocks contain little sulfur compared with oil and coal, and varying amounts of nitrogen. Uncontrolled SOX emissions from biomass combustion are negligible compared to uncontrolled SOX emissions from coal and oil combustion, but uncontrolled NOX emissions can be comparable -- and are dependent on the conversion process and nitrogen content of the biomass (Antares, 1993). NOX emissions comprise fuel-bound NOX and thermal NOX. Generally, wood contains less nitrogen (i.e., protein) than perennial herbaceous crops or crop residues. Fluidized bed boilers generate less thermal NOX than grate-fired boilers or gasifier-based boilers and gas turbines because of their lower and more uniform temperatures.
Land Use Change
The ecological effects of growing large quantities of biomass for energy -- the effects on wildlife habitat and biodiversity, on soil fertility and erosion, and on water quality -- will depend on the specifics. The ecological implications of this land use change would very likely be positive -- as long as perennial biomass crops displaced annual agricultural crops. However, the ecological implications of displacing more natural land cover (such as forests and wetlands) with energy crops would very likely be negative (Cook et al., 1991; Miles and Miles, 1992; US Congress OTA, 1993; Tolbert and Downing, 1995; Tolbert and Schiller, 1996).
Soil Erosion and Water Quality
It has been projected that displacing annual crops with perennial biomass crops would reduce runoff -- decreasing soil erosion and improving water quality (Perlack et al., 1992; US Congress OTA, 1993). Even so, runoff during crop establishment could be comparable to or greater than that from annual row crops, especially for tree crops treated with herbicides to suppress competing vegetation.
Steady state infiltration rates do appear to increase with tree crop age, with comparable rates for one year old sycamore, soybean and corn (Bandaranayake et al., in press). First year data from a series of switchgrass and tree crop trials in the Southeastern USA (Joslin and Schoenholtz, in press) show little difference between the perennial crops and annual crops (corn and cotton) -- although runoff from one of the cottonwood plots began to decrease dramatically by Spring of the second year (Thornton et al., in press; Green et al., in press). Although cover crops (winter rye grass, tall fescue, crimson clover and interstate sericea) do appear to reduce first year erosion in sweetgum, they also inhibit tree growth (Malik et al., in press).
Displacing annual crops with perennial biomass crops would significantly reduce net pesticide use -- and could also reduce net fertilizer use, depending on which biomass crops were deployed and what agricultural uses they displaced (Perlack et al., 1992; US Congress OTA, 1993).
Habitat and Biodiversity
Displacing annual agricultural crops with perennial biomass crops could also improve habitat for native wildlife -- especially if native crop species were used in ecologically appropriate locations. Perennial energy crops could also be integrated with annual crops as buffers around remnant natural areas -- perennial herbaceous crops around grassland remnants and woody crops around forest remnants -- and as filter strips along streams. The introduction of such crops in worked landscapes could improve wildlife habitat, preserve natural biological diversity and restore natural ecosystem functions -- and simultaneously diversify the income mix of landowners.
Results from field research in hardwood plantations (Beyea et al., 1994; Christian et al., in press; Hanowski et al., in press; Hoffman, in preparation; Christian et al., in preparation) support the hypothesis that replacing row crops with native woody biomass crops (or hybrids with a native parent) in formerly forested regions will help increase populations of some forest-dependent bird species whose habitat has been -- and continues to be -- eliminated and fragmented by human activities. In particular, our results support the recommendation that such woody crops be sited to surround and fill gaps between remaining forest fragments, buffering them from cleared areas, reducing habitat fragmentation and increasing the availability of valuable forest-interior habitat.
Initial results from our field research in large switchgrass plantings (Beyea et al., 1994; Hoffman et al., 1995) support an analogous possibility -- that native perennial grasses grown as energy crops in former grasslands may provide suitable habitat for some prairie-dependent bird species. This could be a lucky break for grassland songbirds, many which are in very serious condition (with declines of 90-95% being not uncommon).
Efforts are underway at the U.S. Department of Agriculture to evaluate biomass crops as an alternative to Conservation Reserve Program (CRP) set-asides for controlling soil erosion and chemical runoff. Although farmers have commonly planted perennial grasses (and trees) on CRP set-aside lands, harvesting has not been permitted. It appears that biomass crops can provide many of the wildlife habitat benefits of CRP management if they are managed and harvested appropriately.
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