NC1178: Impacts of Crop Residue Removal for Biofuel on Soils (formerly NC1017)
Statement of Issues and Justification
The growing importance of renewable energy, specifically as biofuels, is indicative of the insatiable energy demands of the globalizing and growing world economy but hidden in this is the high economic and environmental costs of fossil fuels. In his State-of-the-Union address on 23 Jan. 2007, President Bush enthusiastically proposed to cut U.S. gasoline consumption by up to 20% by 2017 through an increase in ethanol production to 35 billion gallons a year, or a fivefold increase over current ethanol production requirements. There are now 113 North American ethanol plants, with an additional 77 under construction, primarily sited in the Corn Belt of the Midwest. Most of the existing ethanol plants are corn grain-to-ethanol refineries. The long-term goal is to produce 60 billion gallons of ethanol per year to replace 30% of the ground transportation fuel demand of the U.S.To meet these demands, the use of crop residues is being considered as a source of lignocellulosic feedstock for producing ethanol (Wilhelm et al., 2004), although it is not yet a routine practice (Johnson et al., 2004). One ton of corn stover or any other cellulosic biomass could theoretically produce 100 gallons of ethanol. An annual production of 60 billion gallons of ethanol from cellulosic biomass would require 600 million tons of biomass per year. The overall goal for ethanol production is to procure 1 billion tons of biomass for the U.S. (Somerville, 2006) and about 4 billion tons for the world. The current rate of residue production from all crops is about 500 million tons in the U.S. and about 4 billion tons in the world. Some analysts argue that 300 million tons of corn residue could be harvested for meeting the U.S. ethanol demands. Similar to the U.S., 34 straw-burning power plants were being built at the end of 2006 in rural areas in China with a capacity of producing 1.2 million kW of power. The straw-burning power industry is likely to grow in China and associated geographic regions. Of the 600 million tons of straw produced annually, China plans to use 300 million tons to produce energy. India, which produces about 440 million tons of crop residues annually, also has plans to build straw-burning power plants. When crop residue is removed from soil there is increased potential for soil degradation, and erosion resulting in a subsequent decline in soil quality, reduction in crop yield, and decrease in local and regional water quality (Simpson et al., 2008).
Crosson (1984) estimated a monetary loss in the U.S. from soil erosion due to reduced cropland productivity to be 40 million dollars in a given year. In 2001, den Biggelaar et al. (2001) estimated the loss at $55.6 million. Others, Crosson (1986) and Pimentel et al. (1995), have estimated the loss to be over $100 million. The results vary depending on the erosion rates, weather, and crop prices for the years covered in the study. For many soils, continued erosion results in degraded and reduced topsoil thickness. Reduced crop yields occur as root restrictive layers, such as fragipans, subsoil horizons with a large amount of clay, or coarse sand become closer to the soil surface (Langdale et al., 1985).
Offsite damages to the environment caused by soil erosion and subsequent deposition of sediments in the U.S. are considerable (Pimentel, 1992). Deposition of eroded soil materials in surface water bodies such as reservoirs, lakes, rivers, and streams cause a decline in water quality, reduce environmental quality, and decrease the functional life expectancy of reservoirs. Eroded sediments often contain not only soil materials from the organic surface soil which are enriched in nutrients and C, but often include commercial and/or organic (animal waste) fertilizers, pesticides, and agricultural pharmaceuticals (from animal waste).
Excessive removal of crop residues, particularly corn stover, can adversely affect soil quality and exacerbate the problem of soil erosion. Residue removal increases the susceptibility of the soil surface to crusting/increased surface sealing because of rainfall-induced consolidation and abrupt wetting and drying (Or and Ghezzehai, 2002; Blanco-Canqui et al., 2006). Residue mulch increases aggregation (Mochizuki et al., 2008), improves soil hydraulic properties (Strudley et al.2008; Blanco-Canqui et al., 2007) and intercepts raindrops responsible for crust-forming processes such as detachment of soil particles and dispersion of surface aggregates. Crusts are thin soil surface layers about 5 cm thick (USDA-NRCS, 1996), but they are denser and less permeable than the underlying soil layers (Busscher and Bauer, 2003). Because of their greater strength and low permeability, crusts can modify soil surface processes, restricting seedling emergence (Baumhardt et al., 2004), reducing water infiltration and aeration (Wells et al., 2003), and increasing surface runoff (Bajracharya and Lal, 1998) thus inducing a greater level of soil erosion with long-term detrimental effects on plant growth (Maiorana et al., 2001). Stover mulch also reduces the abrupt fluctuations in soil water regimes (Black 1973a). Soils with stover mulch often have greater water content that those without mulch (Shaver et al., 2002). Soil water content is the single most important factor essential to plant growth, evapotranspiration, biological activity, nutrient movement, and other vital soil processes.
Some researchers have estimated that ranges of about 30% (Nelson, 2002), 40% (Kim and Dale, 2004), and 58% (Lindstrom et al., 1979) of the total corn stover production in the U.S. Corn Belt region may be available for biofuel production. These removals are, however, based mainly on the residue requirements to reduce soil erosion risks and not on the needs to moderate soil surface strength or soil C sequestration. Allowable removal rates of corn stover based on the need to reduce soil erosion in the U.S. Corn Belt region are site-specific (Lindstrom et al., 1979; Nelson, 2002; Kim and Dale, 2004). Thus, the quantity of stover that must be retained on the soil to reduce crusting is also likely to depend on site-specific conditions such as tillage and cropping system (Kladivko, 1994), duration and intensity of management (Karlen et al., 1994), soil properties (Gupta et al., 1987), as well as agro-ecosystem and climate (Salinas-Garcia et al., 2001). Knowledge of the threshold levels of stover removal in relation to soil crust strength and water storage is urgently needed to design residue management options for biofuel production while maintaining soil physical quality, reducing risks of pollution of surface water, and sustaining agricultural productivity.
While studies on the interacting effects of traditional tillage systems vs. residue management on soil strength properties are many (Larson et al., 1978; Lindstrom et al., 1979; Kladivko, 1994), changes in crust strength parameters and water content regimes resulting from differential corn stover retention in no-tillage (NT) systems are not well documented (Blanco-Canqui et al., 2006). Moreover, the magnitude of the impacts of crop residue removal on soil crust strength properties can be variable depending on soil textural characteristics and management (Morachan et al., 1972; Black, 1973b; Gupta et al., 1987; Karlen et al., 1994; Shaver et al., 2002). Many of the soils of the North Central region possess silty surface textures which are naturally more susceptible to crusting and physical degradation. Thierfelder et al. (2005) showed that cone index (CI) of crusted soils without vegetative cover was seven times greater than that of those with vegetative cover. In contrast, Karlen et al. (1994) observed that differences in CI and Áb of soils within the surface 5-cm depth after 10 yr of complete removal and double addition of stover mulch annually under NT continuous corn were not significant in Rozetta and Palsgrove silt loams. Decline in soil organic carbon (SOC) concentration and deterioration of soil structural properties as a result of stover removal can be significant. Several studies in the Midwestern U.S. Corn Belt region including those from Iowa (Larson et al., 1972), Indiana (Barber, 1979), Minnesota (Allmaras et al., 2004; Wilts et al., 2004), Wisconsin (Karlen et al., 1994); and Ohio (Blanco-Canqui and Lal, 2007a, b; Blanco-Canqui et al., 2007) have shown that stover removal reduces the SOC concentration. In other regions, reductions in SOC concentration due to stover removal may, however, be small or not significant (Hooker et al., 2005). The SOC concentration is also positively correlated with aggregate stability (McVay et al., 2006).
Previous work by members of NC-1017 has demonstrated the importance of storing soil C (through increasing organic matter) for improving soil quality, including soil physical properties such as improved water holding capacity (Lal, 1999). However, a strong connection between soil erosion and the global C balance has not been well established. There is also a need for developing sound methodology for obtaining a quantitative estimate of the actual distribution of soil C on various eroded and noneroded landscapes in the Midwest. It was recognized during the Kyoto Protocol that net emissions of greenhouse gases, such as CO2 and CH4 could be decreased by either reducing emissions or by increasing the rate of C sequestration in soils. Agricultural soils are one of the largest reservoirs of C, and thus have a great potential to mitigate the increasing concentration of CO2 in the atmosphere (FAO, 2001). Evaluation of the C pool in soils is difficult because of its heterogeneity in time and space (FAO, 2001). The global loss of C because of erosion is estimated to be in the range of 150 to 1500 million tons per year (Lal, 1995; Gregorich et al., 1998; Lal, 2003) but the processes are not well understood. Erosion is a selective process involving detachment and transport of the light soil fraction consisting of soil organic carbon (SOC) and clay (Sharpley, 1985). The fate of eroded soil particles is complex depending on many parameters including soil properties, landscape elements and properties, drainage net and soil management. Many of the soil particles eroded are moved downslope and may remain in the same field or watershed for a considerable length of time. However, this movement results in increased spatial variability of soil properties across the landscape, especially soil organic matter and those elements of environmental concern that are associated with it - carbon and nitrogen (Schumacher et al., 1999).
This proposal outlines a project designed to help better understand changes in soil quality including soil degradation resulting from crop residue removal for biofuel production and animal production systems. It will also provide needed data on the changes in the soil C reservoir related to intensive land use and residue removal for some of the major soils in the North Central region. This study will contribute to our understanding of soil-landscape processes with the potential to provide data that will contribute to improved management of our soil and water resources. We view this approach as a natural progression related to the past research efforts of NC-174 and NC-1017.
Knowledge gained from the proposed research will contribute to a more quantitative understanding of the effects of agroecosystem management (crop residue removal) on global C balance; erosional processes; the amounts and landscape distribution of C and organic matter; and changes in soil quality. The proposed regional research project will provide information that can be used to enable sustainable management of natural resources in different ecosystems, over the varying climates and soil landscapes that occur in the participating states.
The North Central Regional Association of Agricultural Experiment Stations has a current list of research priorities for seven cross-cutting research areas and objectives. The proposed Regional Project addresses four of these objectives for the Natural Resources and the Environment. These include: (1) Understand the ecological processes operating in human, plant and animal communities, (2) Define sustainable principles for resource management, utilization and land use, (3) Identify and apply ecosystem management principles and practices for the utilization and protection of resources, restoration of natural systems and management of rural landscapes, and (4) Assess the relationships of agricultural/forestry practices (primary production) upon soil and water systems and bio-diversity. Our project will contribute to these research priorities.
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