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Straw Removal for Industrial Uses: Implications for Direct Seed Systems
Bill Pan, Soil Scientist, Washington State University, Pullman; Bill McKean, Pulp
and Paper Engineer, University of Washington, Seattle; Dennis Roe, USDA NRCS, Pullman

It is a recurring theme of the industrialized world: as raw materials and natural resources begin to limit industrial production and processing, efficient and more effective alternative materials and resources are identified. For example, plastics have supplemented steel in manufacturing; fiber optics and satellite networking have replaced copper wiring in communications. As we enter the 21st century, wood fiber supplies are projected to become limiting (Bruenner, 1994) and crop straw may become an increasingly valuable commodity and fiber source in U.S. manufacturing. At the same time, growers are being confronted with important decisions about straw management. The options are numerous (burn, till, harvest, direct seed) and each option has a different set of agronomic and environmental consequences. Our objective for this paper is to review i) crop straw production in Washington, ii) fiber needs of the pulp and paper industry, iii) fiber quality of crop straw grown in WA, and iv) implications of straw removal for agronomic systems.

Straw utilization alternatives to field-burning are needed to improve the sustainability of wheat and Kentucky bluegrass seed production in the Pacific Northwest (PNW). Increasing restrictions on post-harvest residue straw burning have resulted from public concerns over air quality. Management systems that minimize reliance on burning are needed. Straw removal can improve bluegrass seed production and no-till direct seeded cereal based systems, but viable economic outlets for the harvested straw are limited, resulting in the accumulation of unused stacks of straw by producers. At the same time, a major challenge to fiber-based manufacturing industries of the 21st century is the optimized utilization of plant/crop based renewable resources. Energy efficient, environmentally sound and economically viable industries may increasingly rely on multiple uses of biomass produced in agricultural systems. Development of economic markets for straw will provide on-farm income and allow growers to offset costs of post-harvest straw.

Annual crop augmentation of traditional fiber supplies to the pulp and paper industry will require a large and reliable source of raw material. Cereal and bluegrass seed production in the PNW provides that stable source of straw upon which evolving manufacturing alternatives can be based. The 2 million+ acres of wheat and 70,000 acres of Kentucky bluegrass in WA state, producing 1 to 5 tons of residue per acre, represents a tremendous potential fiber source. Much of this residue will need to be returned for soil conservation and to build soil quality. Nevertheless, partial straw harvesting in the high production areas is feasible, particularly when coupled with soil conserving agronomic practices and with recycling of some of the straw C in the processing byproduct. This focus on high production areas may result in both regional and within-field selective harvesting, e.g. the irrigated and annual cropping zones, and perhaps site specific-harvesting gently sloping sideslopes, toeslopes and non-eroded summits within dryland wheat fields.

Straw chemical composition and cellular morphology that affect papermaking is greatly influenced by genetics and agronomic management (Jacobs et al., 2000). For example, fiber length is positively related to stem height in wheat, suggesting that taller cultivars may provide better paper making fiber than dwarf cultivars. Since Kentucky bluegrass cultivars vary greatly in stem height, it would be interesting to determine if a similar relationship is applicable. . Preliminary comparison of bluegrass from E. WA shows that pulp has similar fiber length and is superior in properties to rye and wheat straw. Furthermore, agronomic management practices such as irrigation alter fiber length, and it is of interest to determine the extent to which papermaking properties can be improved through agronomic management and/or selective regional straw collection. An important key will be to identify unique niches that can be filled in paper and cardboard making that will produce superior products. Grass fibers somewhat resemble hardwood fibers but have a much wider length distribution. Their major role may be to provide filler and smooth surface properties for printing. Straw internodal sections exhibit the best papermaking properties. However, grass crop residues generally contain considerable amounts of fine morphological structures which limit pulp and paper and solid board operating rates and product properties (Perhaym, 1989). Furthermore, the silica content is about ten times higher than occurs in wood and wood residues, which poses problems with liquor combustion for energy and for treating waste process materials (Watson, 1997). But, our recent results suggest that simple straw preprocessing steps can remove much of these silica rich and fine morphological structures from crop straw and improve process behavior and product properties to hardwood quality (Jacobs, 1999).

Nutrient loss occurs on fields when grain and straw residues are removed, therefore nutrient additions are required to sustain soil quality. The economic value of the carbon compounds and the ash minerals, including P, K, Ca, Mg and micronutrients should be considered in the analysis of straw harvesting. Recent characterization of these components found in WA wheat straw will be discussed in this presentation. Land application of the spent liquor from the pulping process can be used to help recycle and replenish nutrients removed with straw harvesting (Pan et al., 1996). The by-product from pulping wheat straw is an important issue to address. The spent liquor by-product from chemical pulping could be rich in nutrients such as N, C, Ca, K, Si and S depending on the pulping chemistry selected. Conversely, the black liquor of other pulping processes (e.g. soda pulping) could have deleterious effects on agricultural land. Preliminary results suggest pulping byproducts have beneficial effects on plant growth and soil structure (Mahoney, 1998). Wong used a K-based pulping process of annual crop straw to yield both paper and a fertilizer by-product (Paul, 1995). Xie et al.(1994) incubated a clay soil with urea-N fertilizer plus NH4-lignosulfonate. Soil amendments must not be toxic to plants or interfere with microbial metabolism. The application of spent liquor to agricultural land has the potential to improve soil aggregation, and organic matter content. Lignin, a principal component of pulping black liquor, is also the most resistant to decomposition, therefore plants containing high amounts of lignin decompose more slowly. Lignin is an important precursor in the formation of soil humic substances (Paul and Clark, 1989). Its effects may decrease soil erodibility and potentially improve crop productivity.

In summary, prospects for straw utilization in papermaking provide potential solutions to environmental and production issues facing the paper and agricultural industries. Selective harvesting should concentrate on high production regions. Coupling straw harvesting with direct seeding and spent liquor recycling should be considered to sustain soil quality. Cooperation between the industries, researchers and government agencies is needed to integrate these systems.

References

Bruenner, R. 1994. Fiber supply crisis in the Pacific NW, TAPPI Pacific Section Seminar, Seattle, WA.

Jacobs, R. 1999. The papermaking properties of Washington State Wheat Straw. Ph.D. Dissertation, University of Washington, Seattle, WA.

Jacobs, R. S., B. Miller, W. L. Pan, R. A. Allan, W. T. McKean. 2000. Pacific Northwest Wheat Straw: Glance at the Fiber Morphology. Pulping Conference Proceedings, TAPPI Press, Atlanta, pp. 237-242. (accepted for publication in TAPPI Journal)

Mahoney, M. 1998. Effect of ammonium sulfite spent liquor on soil properties and spring wheat growth. M.S. Thesis, Washington State University, Pullman, WA.

Pan, W. L., R. S. Jacobs, B. C. Miller, R. A. Allan, W. S. Fuller, and W. T. McKean. 1996. Prospects for utilizing wheat straw from the Pacific Northwestern United States in Paper Production. U.S.-Egypt Manufacturing Technologies Workshop, sponsored by NSF, Egypt NRC, December 6-9, 1996.

Paul, D. 1995. Growth and diversity of non-wood pulping projects worldwide. Pp.100-107. In Proc. Of the 3rd tropical pulp and paper conference. Forest Research Institute, Malaysia.

Paul, E.A. and F.E. Clark. 1989. Soil microbiology and biochemistry. Academic Press, CA

Perhaym, D.A. 1989. Comparative economic & other factors affecting the viability of non-wood plant fiber bleached chemical pulp mills, TAPPI Pulping Conference Proceedings, p. 407.

Watson, P and A.. Garner. 1997. The opportunities for producing pulp from agricultural residues in Alberta: A review of non-wood pulping technologies, Pulp and paper research institute of Canada, Miscellaneous Report, MR 355, May 1997.

Xie, R.J., A.F. MacKenzie, L. P. O'Halloran, and J.W. Fyles. 1994. Concurrent transformation of lignosulfonate carbon and urea nitrogen in clay soil. Soil Sci. Soc. Am. J. 58: 824-828.