Potential Soil Quality Impact of Harvesting Crop Residues for Biofuels

Karlen Douglas L.¹

1USDA Agricultural Research Service (ARS) National Laboratory for Agriculture and the Environment (NLAE)2110 University Boulevard, Ames, IA 50011-3120, United States of America.

E-mail: Doug.Karlen@ars.usda.gov

Recibido: 31/3/11 Aceptado: 14/9/11

Summary

We are in one of the greatest technological, environmental and social transitions since the industrial revolution, as we strive to replace fossil energy with renewable biomass resources. My objectives are to (1) briefly review increased public interest in harvesting crop residues as feedstock for bioenergy, (2) discuss the work soil scientists must do to address those interests, and (3) examine how soil quality assessment can be used to help quantify soil biological, chemical and physical response to this transition. Rising global energy demand, dependence on unstable imports, volatility in price, and increasing public concern regarding fossil fuel combustion effects on global climate change are among the factors leading to an increased interest in development and use of renewable biomass sources for energy production. Although controlling soil erosion by wind and water is no less important than in the past, it is not the only factor that needs to be considered when evaluating the sustainability of land management practices including harvest of crop residues as bioenergy feedstock. The concept of soil quality assessment is reviewed and the Soil Management Assessment Framework (SMAF) is used to illustrate how such assessments can be used for assessing impacts of harvesting crop residue as feedstock for bioenergy production. Preliminary results of the SMAF assessment show that soil organic carbon (SOC) is one of the lower scoring indicators and therefore needs to be monitored closely. Innovative soil and crop management strategies, including a landscape vision are offered as ideas for achieving sustainable food, feed, fiber, and energy production.

Key words: BIOENERGY, SOIL MANAGEMENT ASSESSMENT FRAMEWORK (SMAF), RENEWABLE ENERGY ASSESSMENT PROJECT (REAP)

Resumen

Impacto potencial de la cosecha de residuos para biocombustibles sobre la calidad del suelo

Vivimos una de las mayores transiciones tecnológicas, ambientales y sociales desde la revolución industrial, en que nos esforzamos por sustituir la energía fósil con recursos de biomasa renovable. Mis objetivos son (1) reseñar brevemente el interés público en la recolección de los residuos de cultivos como materia prima para bioenergía, (2) discutir el trabajo que los científicos del suelo deben realizar para hacer frente a esos intereses, y (3) examinar la forma en que la evaluación de la calidad del suelo se puede utilizar para ayudar a cuantificar la respuesta biológica, química y física del suelo a esta transición. La creciente demanda mundial de energía, la dependencia de importaciones inestables, la volatilidad de los precios, y la creciente preocupación pública con respecto al uso de combustibles fósiles y sus efectos sobre el cambio climático mundial, son algunos de los factores que conducen a un mayor interés en el desarrollo y uso de fuentes renovables de biomasa para la producción de energía. Aunque el control de la erosión del suelo por viento y agua no es menos importante que en el pasado, no es el único factor que debe tenerse en cuenta al evaluar la sostenibilidad de las prácticas de manejo del suelo, incluida la recolección de residuos de cultivos como materia prima para la bioenergía. Se revisa aquí el concepto de evaluación de la calidad del suelo, y se usa el marco de evaluación del manejo de suelos (SMAF) para ilustrar cómo estas evaluaciones se pueden utilizar para evaluar los impactos de la recolección de residuos de cultivos como materia prima para la producción de bioenergía. Los resultados preliminares de la evaluación SMAF muestran que el carbono orgánico del suelo (SOC) es uno de los indicadores de puntuación más baja y por lo tanto debe vigilarse estrictamente. Estrategias innovadoras de manejo de suelos y cultivos, incluyendo una visión del paisaje, se ofrecen como ideas para lograr la producción sostenible de alimentos, forrajes, fibra y energía.

Palabras clave: BIOENERGÍA, EVALUACIÓN DEL MANEJO DE SUELOS, EVALUACIÓN DE ENERGÍAS RENOVABLES (REAP)

Introduction

The Global Challenge

A recent report by the US National Academy of Science (National Academy of Science, 2009) stated that world demand for energy has been increasing steadily, especially in developing nations such as China, where prior to the recent global economic recession double-digit increases in economic growth and energy consumption were the norm. It also pointed out that US dependence on foreign oil imports has increased from 40% to 56% since 1990, and that the long-term reliability of traditional sources for those oil inputs is uncertain because of political instability and resource limitations. Coupled with rising concerns regarding the effects of fossil-fuel combustion on global climate change and the volatility of energy prices, development and use of renewable biofuels has received an increasing amount of attention during the past decade.

Many factors and government policies in the US and abroad have been and will continue to influence the energy transition process. Answering questions regarding how we generate, supply, distribute and use energy will involve a complex mix of scientific, technical, economic, social, and political elements that will undoubtedly require decades to complete. Therefore, some have equated this transition to changing the course of a massive ship (National Academy of Science, 2009). One part of this transition is the increased interest in using renewable biomass sources as feedstock for both direct combustion and different types of biofuel. As a result, we in agriculture, especially those who understand soil resources, tillage systems, and other crop production factors, will have a very important role. Our knowledge, skill and ability to recognize and understand the fragile balance among economic, environmental and social factors that will be required to develop, produce, harvest, store, and transport renewable plant biomass to many different types of conversion facilities.

Agricultural scientists are well positioned for this challenge because of experience in developing, producing, harvesting and transporting food, feed and fiber resources to consumers around the globe. We also have the knowledge provided by our mentors (e.g. Larson et al., 1972; Power and Legg, 1978; Elliott et al., 1978; Larson, 1979) and from our own studies (e.g. Karlen et al., 1984) following the first US energy crisis during the 1970s. Collectively, those and many other studies (e.g. Gollany et al., 1991; Mann et al., 2002; Liebig et al., 2005; Moebius-Clune et al., 2008) document effects of soil and crop management on soil erosion, SOC, and productivity that must be recognized to understand the complexity associated with harvesting crop residues and other lignocellulosic materials as feedstock for bioenergy. The studies also show that SOC is one of the most useful indicators available at this time for evaluating soil quality and future productivity (Shukla et al., 2006), and have collectively led to the development of methodologies and databases by Johnson et al. (2006a, 2006b) that were used to estimate the amount of crop residue required to maintain SOC at current levels. Those findings were subsequently used by Wilhelm et al. (2007), who found that, in many cases, the amount of crop residue needed to maintain SOC (and thus soil productivity) is often far greater than the amount of crop residue needed to simply control wind and water erosion (Figure 1).

In the Billion Ton Report (BTR), Perlack et al. (2005) estimated that about 176 million Mg of biomass could currently be harvested from US agricultural lands without negatively impacting current agricultural markets or crop production. With technology advancements, adapted tillage practices, and carefully orchestrated land-use changes, they estimated that amount of harvestable biomass could be increased fivefold within 35 to 40 years. The largest agricultural source of that biomass was from annual crop residues, with corn (Zea mays) stover providing 75% of the total (Nelson, 2002). Stover is most available in the US Corn/Soybean Belt where approximately 21 million ha of corn is cultivated annually, producing an estimated 163 million Mg of total residue (NASS, 2009).

With current collection technology, about 40% of the crop residue (Patterson, 2003; Grant et al., 2006; Shinners and Binversie, 2007) can be collected, but with the development of single-pass harvest technologies it will be mechanically possible to collect more than 70% residue (Shinners et al., 2007). However, from a soils perspective, studies have shown that sustainable residue removal rates are extremely site sensitive, and to maintain SOC, while also preventing erosion, compaction, excessive plant nutrient loss, and other environmental degradation (Figure 2), guidelines and innovative practices will be needed (Nelson, 2002; Wilhelm et al., 2004; Sheehan et al., 2004; Johnson et al., 2006b; Hoskinson et al., 2007).

Having established the need for sustainable feedstock production and harvest strategies, the remainder of this work will focus on our USDA-Agricultural Research Service (ARS) Renewable Energy Assessment Project (REAP) research.

Methodology

Renewable Energy Assessment Project (REAP) Research

The BTR emphasis on crop residues, especially corn stover, raised concern among many soil scientists who feared that without sustainability guidelines excessive removal could degrade soil quality and reduce crop yield (Wilhelm et al., 1986, 2004). As shown in Figure 2, harvesting corn stover for any purpose without offsetting practices will decrease annual carbon input, slowly diminish SOC levels, and thus threaten the soil’s production capacity (Mann et al., 2002; Johnson et al., 2006a). These concerns were accentuated because many soils where corn is produced have artificial drainage, intensive annual tillage, and less diverse plant communities-factors that have already reduced SOC by 30 to 50% when compared to pre-cultivation levels (Schlesinger, 1985).

To address the need for sustainable guidelines regarding harvest of all types of crop residues, the ARS developed a multi-location REAP team in 2006 that involved 14 scientists from nine locations; an effort that now involves more than 25 ARS locations. The research objectives were to:

1. Determine the amount of crop residues (e.g., corn stover, cover crop) that must remain on the land to maintain soil organic carbon and sustain production;

2. Estimate the trade-off between the short-term economic return to growers who harvest crop residues as biofuel or biomass product feedstock versus the long-term benefits to soil, water, and air resources associated with retaining crop residues to build soil organic matter and sequester carbon;

3. Develop robust algorithm(s) to guide the amount of crop residue that can be sustainably harvested as feedstock for biomass ethanol and bio-based products without degrading the soil resource, environmental quality, or productivity; and

4. Develop management strategies (e.g., no tillage) supporting sustainable harvest of residue.

Modify existing or devise new management practices that allow harvest of stover but maintain production level and soil organic carbon through use of cover crops, organic amendment, or other techniques.

Using new and on-going field studies, a Regional Partnership^₁ was developed with university colleagues in six states to help achieve these objectives. Soil quality assessment was chosen to help assess the soil resource impact of harvesting corn stover. The SMAF, developed by Andrews et al. (2004) and used to evaluate chemical, physical and biological responses to various land uses, farming systems and management practices (Karlen et al., 1997, 2006; Liebig et al., 2006; Wienhold et al., 2006; Zobeck et al., 2008; Jokela et al., 2009), was used to evaluate soil quality effects. By focusing on soil quality, the REAP team desired to help change perceptions that crop residues are not important for modern grain production systems.

A common, continuous corn experiment, using no-tillage, three rates of stover harvest [none, high-cut, and low-cut), and four replications was established at each site. Other treatments as appropriate for each location (e.g. cover crops, biochar application, conventional tillage, twin-row high population systems) were also evaluated. To provide a baseline, composite soil samples were collected to a depth of 1 m using increments of 0 to 5, 5 to 15, 15 to 30, 30 to 60 and 60 to 90 cm prior to harvesting any stover. Samples were analyzed for pH, electrical conductivity (EC), total organic carbon (TOC), and several soil fertility indicators using routine soil-test procedures (NCR, 1998).

To illustrate use of the SMAF for soil quality assessment, five measurements that were available from six of the Partnership locations were used for an initial analysis. The SMAF released in 2004 (Andrews et al., 2004) had scoring functions for 11 potential soil quality indicators. Recently, scoring functions for water-filled pore space, soil-test K, and the enzyme â-glucosidase were added (Wienhold et al., 2009; Stott et al., 2010) to the framework. Most studies do not have data for all 14 indicators so a general guideline of using five indicators, with at least one representing biological, chemical, and physical soil properties and processes (Karlen et al., 2007), has been applied to various studies.

Discussion

Data in Table 1 show the results for analyses performed for the 0 to 5 and 5 to 15 cm depth increments. Based on the five indicators, all sites except Mead, NE had an index score of 0.7 to 0.75 for the surface 5 cm and 0.6 to 0.7 for the 5 to 15 cm depth. This means the soils at the sites being used for the field studies are functioning at about 75% of their inherent potential with regard to TOC. The Mead site had index values of 0.58 and 0.57 for the two depths, primarily because TOC values were very low compared to inherent levels associated with the Aksarben silt loam soil. Low TOC at Mead presumably also contributed to the low bulk density score for both depth increments because of the role soil organic matter has in sustaining soil structure. From a fertility or chemical perspective, soil at the Mead site had a good quality.

The amount of data available for this initial SMAF analysis is very limited, so it’s important to be cautious with any interpretations, but overall, it appears that TOC is the factor that needs to be improved the most. At the Florence site, soil-test K in the 5 to 15 cm increment had a score of 0.58 indicating it should be increased, but because of the type and low amount of clay in this soil, this change will not be very feasible until TOC and the associated cation exchange capacity is increased (Karlen et al., 1984; Hunt et al., 1996). The bulk density score for the 5 to 15 cm increment at Florence was also quite low, but this was not unexpected considering soils in this area have very well-defined eluvial (E) horizons that generally require in-row tillage to physically disrupt them on an annual basis (Busscher et al., 1986).

The SMAF analyses support a general hypothesis that to develop sustainable feedstock production strategies, annual carbon (C) input must be accurately estimated. As a starting point, Johnson et al. (2006a) used empirical data and linear regression to correlate C inputs to SOC and proposed minimum source C (MSC) as a term to describe the annual C input needed to ensure no net change in SOC content. Since the initial review, results from several other studies have been added to the database being used for MSC estimates (Johnson et al., 2009). Using above-ground non-grain C inputs, MSC was calculated at 2.5 ± 1.7 Mg C ha^–1 yr^–1 (n = 28) for different crop and tillage practices. This was slightly higher than the original MSC value of 2.2 ± 1.1 Mg C ha^–1 yr^–1 (n = 21).

The new studies confirmed that moldboard plow systems had higher MSC requirements than those with no tillage and agreed with findings by Bayer et al. (2006). Assuming a C concentration of 400 g kg-1 (40%) in corn stover, 6.25 Mg ha^–1 yr^–1 of residue must be left in the field to supply the average MSC (2.5 Mg C ha^–1 yr^–1). This agrees with the value of 6 Mg ha-1 of corn stover reported by Larson et al. (1972) as the amount required for sustaining SOC levels.

In general, there are two basic methods to avoid SOC loss: (1) decrease output (i.e. return all crop residues to the land) or (2) increase input (i.e. increase the amount of organic substrates available to the system). The first, limiting removal of crop residues may be effective for maintaining soil quality but it does not address the reality of meeting needs of an emerging biofuel market that will directly compete for these resources. This emphasizes the importance of quantifying soil quality impact of stover harvest, so that in some areas producers will be able to redirect a portion of their crop residue to these new markets without negatively impacting their long-term productivity and economic viability.

The second method to avoid SOC loss is to increase carbon inputs. This is the ultimate long-term solution to both maintain soil productivity and produce sufficient yields of biomass to support a biofuel industry (Fales et al., 2007; Johnson et al., 2007). The long-term goal must be to maximize the capture and use of light and CO2 available on every unit of arable land and strive to use the resulting crop dry matter in the most appropriate manner. Crabtree and Lewis (2007) estimated that in one hour the sun delivers to the earth the amount of energy used by humans each year. Obviously, not all this energy can be captured in the form of plant biomass, but opportunities exist for improving our efficiency in capturing and using solar energy. To achieve the goal of vastly increased capture of solar radiation and production of reduced C, a series of technologies must be applied. Some of these technologies already exist, but others have yet to be developed.

Existing techniques (i.e., adding cover crops to the crop sequence and using planting times and patterns that maximize solar radiation interception) can be deployed in the near term to reap immediate gains in C input to the system. Other techniques must be refined before deployment (e.g. sensor-driven application of inputs [nutrients and pesticides] based on crop need). Changes in crop genetics and production practices that seem visionary or pioneering today (i.e., developing canopies with structures that result in greater penetration of light to lower leaves, increasing the efficiency of the carbon fixation enzymes, and reduction of photorespiration), will be developed and honed for application in out years (Long et al., 2006). The goal of these efforts is not to increase the demand for existing resources, but to optimize the use of existing resources and increase the sustainable productivity of all resources so that there are sufficient quantities to meet increasing demands for food, feed, fiber, and biofuels.

A Landscape Vision

One approach that could be used to help address several critical issues, including supplying feedstock for bioenergy, decreasing nutrient loss to surface and ground water resources, sequestering carbon, providing improved food and habitat for wildlife, enhancing soil quality, and revitalizing rural communities, is to create a diversified landscape by incorporating annual, perennial, and inter-cropping mixtures into future farming operations. Implementing a landscape-scale vision would also address many concerns regarding productive capacity and the need for increased primary production to support a biofuels industry (e.g., Doornbosch and Steenblik, 2007; Ernsting and Boswell, 2007; Fargione et al., 2008; Searchinger et al., 2008).

To understand the sustainable landscape vision, it is important to recognize that agriculture is more than farms, farmers, and commodity crops {corn, soybean [Glycine max (L.) Merr.], wheat (Triticum aestivum L.), cotton (Gossypium spp.), rice (Oryza sativa L.) and sugarcane (Saccharum spp.)}. Developing lignocellulosic feedstocks and biofuel enterprises within definable watersheds could provide several unique opportunities to more fully integrate economic, environmental, and social aspects of agriculture into integrated systems. By planning to harvest only in areas where the amount of crop residue exceeds that required to maintain soil resources (Lal, 2006) and striving to develop dedicated bioenergy crops, agriculture as a system could help mitigate increased nitrate (NO₃-N) concentrations in streams and groundwater, the need for dredging of sediments, and potential hypoxia problems.

By using biofuel feedstock production as the economic driver, many ecosystem services could be captured through implementation of a landscape management plan that includes establishing woody species as buffers near streams and long-term perennial biomass crops at slightly higher landscape positions (National Academy of Science, 2009). These vegetative buffers could reduce leaching of NO₃-N and runoff of soluble phosphorus (P) while sequestering carbon (C) for several months before being harvested as feedstocks during their dormant period. Slightly higher on the landscape, diverse perennial mixtures of warm season grasses and cool season legumes could produce biomass and store organic carbon in soils. In autumn, these perennials would provide a source of biomass, providing at least three landscape management benefits (biomass production, C sequestration, and water quality). Further up the landscape, a diversified rotation of annual and perennial crops would be used to meet food, feed, and fiber needs. Erosion could be partially mitigated by using cover crops and/or living mulches. Intensive row crop production areas could be established using best management practices (BMPs); if fertilizer recovery was less than desired, there would be substantial buffer areas at lower landscape positions to capture residual nutrients and sediment.

Currently, this landscape vision is conceptual, but calculations based on a recent US study in Iowa suggest that converting just 10% of a watershed from no-tillage corn and soybean to strips of herbaceous perennial plants could decrease water runoff by 49% and sediment export by 96%, while simultaneously increasing native plant, bird, and beneficial insect populations (Helmers and Asbjornsen, 2011). This confirms that understanding complex interactions between economics, soil and crop management decisions, productivity, and environmental consequences can result in agricultural systems that would meet global food, feed, fiber and fuel demands in a truly sustainable manner.

Conclusions

Global development of biofuel production systems appears to be immanent for multiple reasons including increasing demand for finite fossil fuel supplies and rising concern regarding CO2 emissions. Plants, because of their ability to capture and use CO2 and light to produce renewable feedstock and soils because they provide the water and nutrients needed to sustain plant growth provide the foundation for our future. This presentation summarizes my current research activities and thinking with regard to how soil quality and biomass production must be viewed simultaneously to achieve truly sustainable agricultural production systems.

1Funded in part by the North Central Regional Sun Grant Center at South Dakota State University through a grant provided by the US Department of Energy Office of Biomass Programs under award number DE-FC36-05GO85041.

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