A new review in the Agronomy Journal suggests that crop residue removal from corn, wheat, and grain sorghum may not be the most earth-friendly source of biomass for cellulosic ethanol production.
The review found that removal of more than 50% of crop residue can have negative consequences on soil structure, reduce soil organic carbon sequestration, increase water erosion, and reduce nutrient cycling and crop production, particularly in erodible and sloping soils.
While most research is focused on the conversion of cellulosic feeedstocks into ethanol and increasing production of biomass, the impacts of growing energy crops and the removal of crop residue on soil and environmental quality have received less attention.
"Crop residue removal can make no-till soils a source rather than a sink of atmospheric carbon," says Kansas State University researcher Humberto Blanco, even at rates less than 50%.
Residue removal at rates of less than 25% can cause loss of sediment in runoff relative to soils without residue removal. To avoid the negative impacts on soil, perhaps only a small fraction of residue might be available for removal. This small amount of crop residues is not economically feasible nor logistically possible.
An alternative to crop residue removal is growing warm season grasses and short-rotation woody crops as dedicated energy crops. These crops can provide a wide of range of ecosystems services over crop residue removal. Available data indicate that herbaceous and woody plants can improve soil characteristics, reduce soil water and wind erosion, filter pollutants in runoff, sequester soil organic carbon, reduce net emissions of greenhouse gases, and improve wildlife habitat and diversity.
Whereas crop residue removal reduces carbon concentration, dedicated energy crops can increase soil organic carbon concentration while providing biofuel feedstock. Because of their deep root systems, warm season grasses also promote long-term carbon sequestration in deeper soil profile unlike row crops.
Growing dedicated energy crops in marginal and abandoned lands instead of prime agricultural fields will further benefit the soil and environment. Warm season grasses can grow in nutrient-depleted, compacted, poorly drained, acid, and eroded soils.
Herbaceous and woody energy crops cannot replace natural forest and native prairie lands, but well-managed dedicated energy crops may provide a myriad of benefits to soil and environment while supplying much needed feedstocks for cellulosic ethanol production. Developing the next generation of biofuels will not only require new technologies to transform it into fuel, but new agricultural methods for growing it.
Citation: Humberto Blanco-Canqui, 'Energy Crops and Their Implications on Soil and Environment', Agronomy Journal, March-April 2010, 102:403-419; doi: 10.2134/agronj2009.0333
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