Sustainable Crop Removal: Maintaining Soil Quality (E3079)
Agricultural biomass, the aboveground plant material that is not grain, is being looked to as a promising renewable energy source for heat and power, ethanol, syngas and bio-oil.
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Agricultural biomass, the aboveground plant material that is not grain, is being looked to as a promising renewable energy source for heat and power, ethanol, syngas and bio-oil. When used for these purposes biomass is often referred to being a cellulosic bioenergy feedstock. Currently of particular interest is the use of crop residue—the biomass left on the field after grain harvest. The primary crop residue being considered in the Corn Belt of the United States is corn stover. Perennial grasses such as switchgrass (Panicum virgatum) and Miscanthus giganteus are also being researched for their potential as cellulosic bioenergy feedstocks.
Production of agricultural biomass for bioenergy offers many potential environmental benefits, but we cannot afford to overlook the production and environmental costs associated with wide-scale removal of biomass from the land. We need to ask how much crop biomass is needed to protect and maintain the soil resource, and correspondingly, how much can be harvested as renewable fuel?
Crop residue protects the soil from erosion and adds organic matter. In addition, as Table 1 demonstrates, it provides many other valuable benefits to society, called ecosystem services. Removing too much crop residue will limit these services and affect soil quality. This could have a negative economic impact—lower yields and/or augmented use of external inputs, such as fertilizer, to compensate for decreased soil quality. In addition to field-level effects, unsustainable removal of crop biomass would have negative impacts on a global scale (Table 1). As a society, we need to determine what tradeoffs in ecosystem services we will allow in the emerging bioenergy economy and support research efforts that investigate these issues.
Simple estimates of the amount of stover needed to maintain current soil organic matter levels can be made using our knowledge of current soil organic matter levels, microbial decay, and the amount of aboveground crop residue and roots needed to replace soil organic matter. Residue removal consistently reduces soil organic matter levels across sites, but impacts of residue removal on other soil quality parameters and crop production are site-specific. How residue removal affects soil quality depends on factors such as soil texture, drainage, slope, duration of residue management, rate of residue removal, tillage and cropping system, application of fertilizers, use of organic amendments and climate. Therefore, scientists advise a cautious approach to harvesting agricultural biomass for energy until research provides answers and guidance to the critical questions of how much biomass to harvest, when to harvest and where.