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Crop Residue Use: Evaluating Trade-offs

David Huggins, Chad Kruger, and Hans Kok

Conversion of crop residue biomass to bio-energy has captured the interest of farmers, governmental representatives and the public. In part, this has occurred as reported on-farm inventories of biomass indicate a large quantity of available feed-stocks (crop residues) that could be used to produce bio-energy through, for example, cellulosic fermentation or pyrolysis. These crop residues, however, are not “trash”, and play a critical role in many conservation farming systems and in farm resource sustainability. Crop residues directly or indirectly regulate carbon storage and nutrient cycling, snow capture and soil water conservation, soil erosion, aggregation and water infiltration, surface temperature characteristics, wind turbulence and desiccation, the life cycles of many organisms both beneficial and problematic, and the application of farming practices. 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. The large quantities of surface residues could be harvested for bioenergy or bioproducts thereby providing another income stream that would be welcomed by farmers. But what are the trade-offs? Will removal of crop residues further decrease soil organic matter and degrade the soil resource base? What are potential impacts on crop rotation, fertilizer and pesticide use and other farming practices? Will farmers be trading long-term sustainability for short-term gain? These questions need to be answered in a balanced evaluation of bio-energy production from crop residues. In order to contribute toward this goal, we assessed trade-offs associated with different crop residue management options, particularly impacts on soil carbon sequestration, crop nutrient removal, soil erosion and energy production. This assessment is derived from field-scale research conducted at the Cook Agronomy Farm. In particular, we were interested in evaluating the site-specific impacts of various practices, recognizing that different portions of a field may be more or less suited to different crop residue uses. To date our evaluation has included: (1) residue removal over the course of a crop rotation and potential energy production; (2) influence of tillage alternatives and residue removal on soil erosion rates and soil carbon sequestration; (3) influence of tillage alternatives and residue removal on the Soil Conditioning Index; and (4) amount of major nutrients (N, P, S, K) removed in crop residues. Preliminary outcomes of this analysis are: (1) energy production from baled straw averaged 2000 kW hours per acre; (2) residue removal during cereal portions of rotation would like decrease soil organic matter across whole field; (3) The Soil Condition Index remained positive when residue was baled and removed under continuous no-tillage; (4) nutrients removed per acre ranged from 9 to 34 lbs N, 3 to 13 lbs P2O5, 2 to 8 lbs of S and 20 to 77 lbs of K.

Cook Agronomy Farm, Pullman, WA
Assessing Tradeoffs: Bioenergy, SOC, SCI Nutrient Removal

assessing tradeoffs

Baled residue produced on average 803 kWhr/ha of energy (about 2000 kW hours/ac) but reduced the return of C in remaining residue to amounts below critical levels needed to maintain soil organic matter.