August 22, 2007 | General

Bioenergy And Soil Carbon Sequestration

BioCycle August 2007, Vol. 48, No. 8, p. 23
David Huggins, Hans Kok and Chad Kruger

A commentary in the March 2007 issue of BioCycle initiated a discussion about how to achieve a balanced approach to atmospheric carbon management and renewable fuels development from biomass. The authors, Mark Fuchs and Chery Sullivan of the Washington State Department of Ecology, are part of a group of researchers, policy makers and practitioners in Washington State who are addressing how to value the diverse set of carbon sources derived from organic waste “resources” for fuels, energy and sequestering carbon while creating a renewable, regionally-based sustainable bioeconomy. While they are focusing on Washington State, the topics on the table cross state and even country borders.
This month’s Climate Change Connections column continues our look at issues being addressed in Washington State. It presents a scientifically-based assessment of potential tradeoffs between using crop residues as feedstocks to produce bioenergy versus using crop residues for sequestering soil carbon.
What are the current roles of crop residue in agriculture, and how will harvesting them as feedstocks for bioenergy production impact this role? Exporting too much residue off the farm will significantly reduce sustainability of agricultural production and the quality of air, water, soil and biological resources. To contribute to better public understanding, we assessed the consequences associated with different crop residue management options using field-scale research at the Washington State University Cook Agronomy Farm near Pullman. In particular, we evaluated the impacts of residue removal for energy production on soil carbon sequestration, crop nutrient removal, and soil erosion. This column summarizes the impacts of residue removal on soil carbon sequestration. In a follow-up article we will present a summary of our analysis on the impacts of residue removal on the supply of major crop nutrients.
In the dryland cropping region of the Inland Pacific Northwest, winter wheat yields often exceed 100 bu/ac and associated crop residues following harvest can be in excess of 10,000 lbs/ac, equivalent to about 400 gal/ac of ethanol with current technology. Crop residues are composed of about 40 to 45 percent carbon, therefore, 10,000 lbs/ac of residues contain about 4,000 to 4,500 lbs/ac of C. Considering that carbon inputs of about 2,000 lb/ac will sustain current levels of soil carbon in typical agricultural lands, it appears at first glance that residue removal for bioenergy is an attractive option with little downside.
As is often the case, however, the actual situation is more complicated and turns out that if this information is used by itself, several questionable assumptions have to be accepted, notably that:
1) Current soil carbon levels are optimal or at least adequate and sufficient for sustained agricultural production and fulfillment of ecosystem services;
2) Production levels of crop residues are similar for a given field every year; and
3) Production levels of crop residues are uniform within the same field in any given year.
The first assumption is inaccurate for most dryland agricultural soils around the world, including the Inland Pacific Northwest. Reliance on mechanical cultivation (e.g. moldboard plowing, disking, etc.) coupled with reduced organic carbon inputs led to exponential declines in soil organic matter (which is 58% C) following the conversion of native prairies to agricultural production. Current soil carbon levels are about 50 percent of the original levels found under native prairies. Therefore, soil management practices have contributed to atmospheric carbon increases.
In the dryland cropping regions of the Pacific Northwest, severe soil erosion, which preferentially removes soil organic materials from farm fields, also contributed to declining soil carbon with eroded soils often averaging only 25 percent of native soil carbon levels. The loss of soil organic matter has seriously degraded the native productivity of many soils by reducing water infiltration, water holding capacity, nutrient supplying power and effective rooting depth. Degraded soils require increased inputs of nutrients, water and pesticides to maintain crop yields and often are more vulnerable to further degradation under normal weather extremes.
Advances in cultural practices, such as no-till seeding technology, have enabled farmers to slow carbon losses and in some cases to actually regain some soil carbon. Reversal of long-term negative trends in soil organic carbon, however, have not only relied on reduced tillage but also on returning all crop residues to the soil. While reaching native levels of soil carbon is an unrealistic goal for most farms, continued increases in soil carbon of degraded soils will improve agricultural productivity, decrease the reliance on external farm inputs such as energy-intensive synthetic fertilizers and pesticides, and result in removing carbon dioxide (one pound of stored soil carbon is equivalent to 3.67 pounds of carbon dioxide) from the atmosphere, thereby contributing to the mitigation of greenhouse gas emissions.
The next assumption that production levels of crop residues are similar for a given field every year is also inaccurate. Basically, it is difficult to raise high-yielding crops like winter wheat on the same field each year in the dryland regions of the Pacific Northwest due to limitations of available water and to increasing pressures from various weeds, diseases and insects. Therefore, crop rotation including fallow periods is used to break-up cycles of disease and weeds. Rotating crops aids in pest management and enables soil to store more water, which enhances the production of winter wheat, the primary cash crop.
Common crop rotations of the region consist of winter wheat grown once every two or three years with the remaining years consisting of rotation crops or fallow. In contrast to winter wheat, rotational crops such as spring barley, grain legumes and canola produce much less crop residue than winter wheat. The quantities are insufficient to maintain soil carbon levels.
Fallow periods reduce the total amount of residue available over the crop rotation. Consequently, to maintain soil carbon levels in a given field, the relatively high amounts of residue produced during winter wheat rotation must compensate for fallow periods and rotational crops that produce little residue during the remainder of the rotation. Therefore, the amount of winter wheat residue that is actually available for removal is much lower than what it seems at first glance.
The last assumption that production levels of crop residues are uniform within the same field in any given year creates a special concern for decision-making in the Pacific Northwest due to high variability of field topography and soil types in our region. It is common for a single field to have three to four times the variability in yield potential in a given year due to differences in soil carbon levels, slope and aspect. This means that uniform management strategies applied to a single field will have dramatically different impacts in different parts of the field. Therefore, it may be reasonable to remove substantial residues from one part of a field without creating detrimental impacts, while removing any residue from another part of the field may be extremely detrimental.
One field study on residue removal at the WSU Cook Farm looked at the amount of energy available in the collectable crop residues for a winter wheat-spring pea rotation in kilowatt hours/hectare. While the average for the field was about 800 kWhr/ha, there were many places in the field where it was probably not even economical to remove the crop residues because the energy benefit would not be high enough. Another factor to evaluate is the impact on soil carbon levels after the removal of residues for energy production. In a study, we estimated that it took between 2,000 and 2,500 mg carbon/ha just to maintain soil carbon levels over the course of the rotation. In that case, removal of crop residues for energy production did not leave sufficient residues to maintain soil carbon levels, much less increase them.
Are there any sustainable options for producing bioenergy from cropland? We think there are, and we have two quick principles that we think need to be utilized in making these decisions.
First, use advanced biophysical modeling tools such as the examples provided here to make good, science-based decisions for residue removal. Simple biomass estimates are useful for inventorying the potential biomass available in a region, but they do not provide the necessary detail for understanding the consequences of management decisions in a given field.
Second, perennial crops can substitute for – and even enhance – the agro-ecological functions of crop residues while providing the biomass needed for energy production. Long-term, sustainable strategies for utilizing biomass as an energy feedstock will require a commitment to developing viable perennial grass biomass crops.
David Huggins and Hans Kok are in the Crop and Soil Sciences Department at Washington State University in Pullman, Washington. Chad Kruger is at the Center for Sustaining Agriculture and Natural Resources in Wenatchee, Washington.

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