ENVIRONMENTAL IMPACTS OF CONVERSION OF CROPLAND TO BIOMASS PRODUCTION

Proc., BIOENERGY '96 - The Seventh National Bioenergy Conference: Partnerships to Develop and Apply Biomass Technologies, September 15-20, 1996, Nashville, Tennessee.

A study was initiated to determine the effects of conversion of row crop land to biomass production on runoff quality and quantity. Treatments were: 1) remain in row crop (no-till corn); 2) convert to short rotation woody crop (SRWC) production with sweetgum (Liquidambar styraciflua L.) planted in a 1.5 m by 3 m spacing maintaining complete weed control; 3) convert to SRWC with a tall fescue (Festuca eliator L.) cover crop planted in a 2.4 m strip centered between rows of trees to reduce erosion; and 4) convert to switchgrass (Panicum virgatum L.) as a biomass energy crop. Plots within a block similar in size (approximately 0.45 ha in block 1 and 0.20 ha in block 2), slope, soils, topographic position, recent land use history, etc. Although switchgrass plots eroded more early in the growing season, erosion was low once it became well established. Conversely, plots where trees were grown with no cover continued to erode throughout the growing season. These results indicate that growing short-rotation intensively cultured hardwoods with complete weed control will provide little erosion relief in agricultural fields, at least during the first growing season. Planting switchgrass for bioenergy production, however, does protect the soil. Nutrient runoff was related to fertilization.

Keywords: short-rotation woody crops, switchgrass, water quality, erosion

INTRODUCTION

Atmospheric CO₂ concentration has increased considerably since pre-industrial revolution levels (Watson et al., 1992) and is expected to double within the next century unless intervention is forthcoming (King et al., 1992). This increase in CO₂ may be driving an anthropogenic increase in global temperatures (Massman 1995). A continuation of this increase could cause adverse changes in the natural environment, including alterations in both the natural (McGuire and Joyce 1995) and social (IPCC 1990) environments. Much of the increased atmospheric CO is the result of 2 burning fossil fuels for energy production (Rotty and Marland 1986).

Concern over this increase in atmospheric CO₂ is prompting many utilities to consider alternative energy sources derived from biomass. Bioenergy (energy derived from biomass) production would create a closed cycle of CO₂, resulting in no net increase in atmospheric CO concentrations (Hall et al., 1990, Wright et al., 1992, OTA 1993). It is estimated that wood will provide 4 percent of domestic energy production by the year 2010 (U.S. Department of Energy 1986). A wood conversion facility may consume as much as 1 million dry tons per year (Ranney et al., 1987). At a production of 4 dry tons per acre per year (Graham and Downing 1995), this means that a single facility could require 250,000 acres of short-rotation woody crops (SRWC) for a land base. This land base is most likely to be derived from conversion of agricultural land to bioenergy crops (Graham and Downing 1995).

If large amounts of land are converted from cropland to bioenergy crops, environmental impacts upon runoff water quantity, timing, and quality in the affected region will be substantial. Conversion of cropland to short rotation woody crops is predicted to result in reductions in soil erosion, reductions in nitrate, phosphorus, pesticides and herbicides in runoff and groundwater (Pimentel and Krummel 1987, Hohenstein and Wright 1994, Ranney and Mann, 1994). In addition, benefits in terms of reduced greenhouse gas emissions ( CO₂) are expected from both the sequestration of carbon on site as well as the replacement of non-renewable fossil fuels by a renewable source of energy (Hall et al., 1990, Wright et al., 1992, Hohenstein and Wright 1994). However, some of these benefits have yet to be demonstrated, and none have been quantified for soils in the western part of the Tennessee Valley, where the conversion of cropland to woody crops is considered the most viable economically.

Pimentel and Krummel (1987) estimate the erosion from short rotation woody crops to be an order of magnitude less than that from row crops, while hayland (or switchgrass) erosion is an order of magnitude less than that from woody crops. The decrease in erosion with SRWC might be greater if a cover crop were used to stabilize soils during the first two growing seasons (Ranney and Mann 1994). In addition, one of the largest impacts of cropland conversion could be upon the quantity of runoff. Woody crops have a larger leaf area than annual crops and maintain that leaf area for a considerably longer portion of the year. This factor, plus their deeper rooting depth, could result in substantially greater evapotranspiration and less potential for runoff and leaching.

There is a need for better information about the environmental impacts of conversion of crop land to bioenergy production. To our knowledge, no direct comparisons of environmental impacts of typical agriculture with those from bioenergy crops have been conducted. Ranney and Mann (1994) suggest that a critical examination is required in regard to the effects of such a conversion on erosion, water quality, chemical and fertilizer fates, wildlife habitat and biodiversity. The objective of this study is to allow such a critical examination.

The specific objectives of this project are to determine the effects of conversion of row crop land to bioenergy crop production on soil erosion, surface hydrology, and nutrient and chemical runoff.

This site was chosen to represent the limestone valleys of the Tennessee Valley region. The soils are moderately to severely eroded Decatur silty clay loam, undulating phase. The slopes average 2.5 to 3%. The area has been under cultivation for at least the last 15 years, and represents a large portion of the sites available for conversion to bioenergy crops in the Tennessee Valley.

The study consists of two blocks of four plots each. Plots within a block are matched for similar sizes, slopes, soils, topographic position, recent land use history, etc., and are approximately 0.45 ha in block 1 and 0.20 ha in block 2.

Earthen berms (approximately 0.5 m high) were constructed around each plot to direct all runoff water through an H-flume at the downslope end of four plots each in two blocks. Each flume is equipped with a flow meter and an automatic sampler for collection of runoff samples. Each flow meter continuously records water level in the flume and flow rate during each runoff event. The sampler is programmed to collect 500 ml of the runoff water every 4000 l of flow. Runoff samples are sent to TVA lab in Muscle Shoals, and analyzed for sediment load and nutrient concentration.

Four treatments per block are being evaluated: a control plot (remaining under no-till corn as representative agricultural crop), a SRWC (short rotation woody crop) plot without cover crop (planted with sweetgum on a 1.5- by 3-m spacing with all competing vegetation controlled), a SRWC plot with cover crop (planted with sweetgum with an 2.4-m strip of tall fescue between rows), and switchgrass (converted to switchgrass for biomass production). Tree plots were subsoiled during December of 1995 to a depth of 18 inches. Cover crops were established by no-till drill in January of 1995. Sweetgum seedlings were planted during February and March of 1995. Dead and dying seedlings were replaced as soon as possible and final survival exceeded 98%.

Plot 5 (planted with corn, block 2) showed a tendency to erode to a much greater extent than any other plot. Since baseline data were unavailable for use as a covariate, the decision was made to take measures to reduce the erosion in this plot. The first attempt was the placement of crushed rock in the plot to prevent erosion. This was deemed unacceptable and the decision was made to convert this plot to switchgrass in the hopes that switchgrass would result in the least erosion of any treatment, and these effects would be confounded in a conservative way. Corn was killed in both blocks, and corn and switchgrass planted again in both blocks near the end of May. In addition, erosion control fabric and sandbags were placed in plot 5 in order to reduce sediment runoff.

Runoff Quantity

Plots with trees tended to have lower total runoff amounts than other crop systems, this trend being more pronounced during the early part of the first growing season (Table1). The subsoil troughs apparently increased water infiltration in the tree plots, decreasing runoff amounts.

There was a tendency for switchgrass plots to runoff more water than the corn plots (Table 1). This phenomenon might be partially explained by differences in corn and switchgrass biomass growth. During the first growing season, corn produced 10,432 kg ha of biomass, while switchgrass produced only 4,446 kg ^{- 1} ha ^{- 1} of biomass. Increased transpirational demand associated with the increase in biomass production may be partially responsible for the reduction in runoff of corn over switchgrass; however, the lack of baseline information makes it impossible to remove the effect of site difference and erosion control effort from the effect of the treatment.

Sediment Loss

Sediment loss was greatest early in the growing season for the corn and switchgrass plots. As these stands became established, however, sediment loss was reduced dramatically (Table 2). Sediment loss remained low during the winter months in the corn plots, probably due to the high residual stubble left on the corn fields after harvest in no-till systems. The high erosion of the corn and switchgrass plots at the beginning of the year, then, appear to be an artifact of the plot establishment activities.

Erosion on plots with trees only, conversely, tended to increase as the year progressed (Table 2). These plots were maintained in as nearly a weed-free state as possible (weed cover less than 5%), with the trees providing the only cover, and the only protection from erosion. Crown closure on these plots were only approximately 35% by the end of the growing season, and provided little such protection. After leaf-off for winter these plots had even less protection from erosion. Erosion in these plots is expected to be reduced in subsequent years as the stand develops.

Using a cover crop between tree rows provided increased protection from erosion. Trees with cover crop was consistently among the lowest of the treatments in sediment loss (Table 2). This decrease in erosion was obtained without a noticeable loss of productivity by the trees (data not shown), care having been taken not to allow competing vegetation encroach into the tree rows.

(kg ha ) -1 Month Corn Switchgrass Trees - Trees + a b May 49 101 14 11 July 7 15 18 2 September 1 0 16 1 November 4 6 25 4 January 8 21 49 8 Trees with no cover crop. a Trees with cover crop. b

Nutrient Loss

Nutrient loss is illustrated by nitrate (Table 3). Corn and switchgrass plots showed considerably higher losses of nitrate early in the growing season than tree plots. Later on, nitrate loss in these plots was reduced to levels similar to tree plots. Similar results were obtained with NH -N and P losses, and are explained by fertilization regimen. 3 Both corn and switchgrass plots were fertilized the first year, while fertilization of the tree plots was delayed until the second year.

ha ) -1 Month Corn Switchgrass Trees - Trees + a b May 275 477 1 14 July 4 44 9 2 September 1 0 11 3 November 5 12 1 0 January 6 0 2 0 Trees with no cover crop. a Trees with cover crop. b

Bioenergy crops have been suggested as an environmentally sensitive alternative crop for farmers wishing to provide increased protection against erosion. This study shows that not all bioenergy crops are equally beneficial during the first year after establishment. Switchgrass does provide erosion control, once established. However, growing trees may not provide the initial protection previously expected. If erosion protection is required, the use of either switchgrass or trees with cover crops is recommended. Trees can be successfully grown with a cover crop between the rows if care is taken to keep the tree row itself free from weeds. Underplanting of leguminous cover crops into sycamore (Platanus occidentalis L.) plantations during the second growing season has been shown to increase tree growth during subsequent years (Haines et al., 1978). The current study indicates that the use of cover crops during the establishment phase is a viable alternative for SRWC production. More research is needed into which cover crop best reduces erosion in SRWC plantations, while causing the least growth reduction.

1. Graham, R.L., and M.E. Downing. 1995. Potential supply and cost of biomass from energy crops in the TVA region. ORNL-6858. Environmental Sciences Division Publication No. 4306. Oak Ridge National Laboratory, Oak Ridge, Tennessee. 41 p.
2. Hall, D.O., H.E. Mynick and R.H. Williams. 1990. Carbon Sequestration Versus Fossil Fuel Substitution-Alternative Roles for Biomass in Coping with Greenhouse Warming. PU/CEES Report No. 255. Center for Energy and Environmental Studies, Princeton University, Princeton, New Jersey.
3. Haines, S.G., L.W. Haines and G. White. 1978. Leguminous plants increase sycamore growth in northern Alabama. Soil Sci. Amer. J. 42:130-132.
4. Hohenstein, W.G., and L.L. Wright. 1994. Biomass energy production in the United States: an overview. Biomass and Bioenergy 6:161-173.
5. IPCC. 1990. Climate change, the IPCC scientific assessment. J.T Houghton, G.J. Jenkins and J.J. Ephraums (eds.). Cambridge University Press, Cambridge. 364 p
6. King, A.W., W.R. Emanuel and W.M. Post. 1992. Predicting future concentrations of atmospheric CO with global carbon cycle models: the 2 importance of simulating historical changes. Environ. Manage. 16:91-108.
7. Massman, W. 1995. Climate and climate modeling. p 3-8 In: L.A. Joyce (ed.) Productivity of America's forests and climate change. Gen. Tech. Rep. RM-271. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado. 70 p.
8. McGuire, A.D., and L.A. Joyce. 1995. Responses of net primary productivity to changes in CO₂ and Climate. p 9-45 In: L.A. Joyce (ed.) Productivity of America's forests and climate change. Gen. Tech. Rep. RM-271. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado. 70 p.
9. Pimentel, D., and J. Krummel. 1987. Biomass energy and soil erosion: assessment and resource cost. Biomass 14:15-38.
10. Ranney, J.W., and L.K Mann. 1994. Environmental considerations in energy crop production. Biomass and Bioenergy 6:211-228.
11. Ranney, J.W., L.L. Wright and P.A. Layton. 1987. Hardwood energy crops:the technology of intensive culture. J. For. 85(9):17-26.
12. Rotty, R.M., and G. Marland. 1986. Production of CO from fossil fuel burning 2 by fuel type, 1860-1982. Report NDP-006. Carbon Dioxide Information Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
13. U.S. Congress, Office of Technology Assessment (OTA). 1993. Potential environmental impacts of bioenergy crop production - background paper. OTA-BP-E-118. U.S. Gov. Print. Off., Washington, D.C.
14. Watson, R.T., L.G.M. Filho, E. Sanhueza and others. 1992. Greenhouse gases: sources and sinks. p. 25-46 In: J.T. Houghton et al. (eds.) Climate Change 1992: the Supplementary Report to the IPCC Scientific Assessment. Cambridge University Press, Cambridge.
15. Wright, L.L., R.L. Graham, A. Turhollow and B. English. 1992. Opportunities to mitigate carbon dioxide buildup using short rotation woody crops. In: R.N. Sampson and D. Hair (eds.) Forests and Global Warming, Vol. 1. American Forestry Association, Washington, D.C.