Wheat Residue Composition, Decomposition and Management

Chapter 3 – Residue Management, No. 16, Winter 1990

Don Wysocki

The basis of most conservation tillage systems is management of crop residues. Maintaining crop residues at or near the soil surface protects soil from the erosive force of wind or water. Ideally, the goal of residue management for erosion control is to keep a sufficient amount at or near the soil surface to protect against erosion, but still be able to carry on planting and other operations. Factors that effect the amount of residue maintained over time include type, composition and initial amount of residue and type and number of tillage operations. If crop residues are burned, buried or decompose before the critical erosion period, there may be an insufficient amount to provide adequate erosion protection. Residue management and study of the interactions between residue, erosion control, diseases weeds, nutrients and soil water and temperature has been the focus of much STEEP research. This article describes some of the work that STEEP researchers Harold Collins and Clyde Douglas have done on residue composition and decomposition, and discusses some consideration for management.

Collins and Douglas are respectively, soil microbiologist and soil scientist with the USDA-ARS at the Columbia Plateau Conservation Research Center at Pendleton, OR. Collins was formerly a graduate student in the Agronomy and Soils Department at Washington State University and performed his research at Pullman, WA. He is continuing microbiological studies at Pendleton. Douglas conducted his studies at Pendleton, OR.

Residue Composition

Chemical and physical composition of residue can have significant influence on residue behavior and decomposition, which may influence management decisions. Wheat residue is comprised of various plant parts that differ slightly in chemical makeup. To study residue composition, Collins sampled 21 field sites each during two harvest seasons at the USDA-ARS Conservation Farm near Pullman, WA. These sites ranged in yields from 20 to 90 bu/acre. The variety was Daws and all sites were no-till plantings. At each site Collins harvested a 20 x 1-foot strip of complete plants clipped at the soil surface. Plants were dried, separated into five components (leaf blades, leaf sheath, stem, chaff and grain) and weighed. Physical composition of wheat plants is given in Table 1. Note the differences in residue production and composition of components. This is most likely a function of environmental conditions of each growing season.

Leaf blades, leaf sheaths, stems and chaff were analyzed for total carbon, total nitrogen, total non-structural carbohydrates (TNSC) and water soluble carbon (Table 2). Residue is approximately 40 percent carbon and the remainder is other constituents, including oxygen, hydrogen, phosphorus, calcium, magnesium, nitrogen, potassium and sulfur. Chemical composition of residue has an influence on the rate of residue decomposition. Soluble carbon, TNSC and carbon-nitrogen ratio (C: N) are indicators of the relative rate at which residue will decompose. Higher values for soluble carbon and TNSC and lower C :N ratio indicate a more rapid rate of decomposition. From this, it can be inferred that the relative rate of decomposition of the various components would be leaf blades > leaf sheaths > chaff > stem.

Table 1. Physical composition by weight of Daws winter wheat plants and residue (modified from Collins 1987).

Table 2. Chemical composition of Daws winter wheat residue (modified from Collins 1987).

Decomposition and Breakdown

The process of residue breakdown and decomposition is an interaction between physical, chemical and microbial processes. Of these processes, microbial decomposition is the most significant. Briefly let’s describe physical and chemical processes and then address microbial decomposition. Physically, residue is broken down by wind, snow, freezing, wetting and drying, animal activity, or tillage and mechanical chopping. Basically, physical processes break residue into smaller segments and expose internal area to water and microorganisms, An example would be rupturing of a straw stalk by freezing of trapped water. Chemical breakdown of residue occurs chiefly from wetting and leaching. As precipitation wets dried residue, soluble materials dissolve and leach into the soil. This includes small amounts of carbon, nitrogen, phosphorus, potassium and other elements, Successive wetting and drying probably enhances the leaching process. Leaching can account for a substantial removal of elements such as potassium, but the total effect of chemical or physical breakdown is small in comparison to microbial decomposition.

Microbial Decomposition

Microbial processes account for a multitude of important reactions in the soil. Decomposition is just one of the these. Several different organisms are responsible for the decomposition process, Broadly, these are bacteria, fungi and actinomycetes. Microbial decomposition of wheat residue is influenced by several environmental characteristics. These include temperature, soil and residue water content, aeration, soil fertility and soil pH. In the dryland grain-producing region of the PNW, temperature and water are the factors that most often limit residue decomposition. Decomposition ceases or proceeds very slowly at soil temperatures below 35°F or as soil water content approaches the wilting point. In the field this means that decomposition occurs most rapidly in the late spring and early summer, as soils warm but are yet moist from winter and spring precipitation. In late summer and fall, soil drying can slow the rate of residue decomposition. During late fall and winter, soil temperatures again limit decomposition. Factors such as size of residue, placement and mixing of residue within the soil and, as previously stated, residue composition also affect decomposition. The rate of decomposition is most rapid when residue is broken into small fragments and well mixed with soil.

Decomposition in the laboratory is typically studied by incubating known amounts of residue and measuring carbon dioxide evolution over time. Microbes convert carbon in residue to gaseous carbon dioxide. In the field, decomposition has been studied by placing known amounts of residue in, on or above the soil and measuring weight loss over time.

Collins strung 10 g (approximately 0.35 ounces) bundles of wheat residue together with monofilament line and placed them on the mineral soil surface or on top of existing residue in October. The rotation was annual crop, no-till winter wheat. He recorded the weight loss of these bundles periodically over the next year. Douglas placed 20 g (approximately 0.7 ounces, this approximated an application rate of 3,800 lb/acre) samples of three different residues in fiberglass mesh bags (18 mesh/inch). Bags of each residue were then placed in three positions: (1) on a metal frame above the soil with a board between the soil and the bag, (2) tied to the soil surface or (3) buried 6 inches in the soil. These placements approximated standing stubble, chopped or matted residue and incorporated residue. The rotation was winter wheat-spring green peas. He recorded weight loss of the bags periodically over the 2-year rotation.

Weight of residue remaining after given intervals of time from these two studies is presented in Table 3. Results from the two studies differ because of the differences in methods, residue composition and environmental conditions between locations. Both studies show that after 1 year residue at or above the surface retained about 70 to 80 percent of its original weight. Douglas points out that in his experiment the decomposition rate for the above surface placement is probably slower than standing stubble because residue in bags has less air space and exposure. Residue that was buried retained about 35 percent of its original weight after 1 year and 15 percent after 2 years. The obvious contrast in decomposition between surface and incorporated residue results from differences in environmental conditions. Residue at the surface is subject to more frequent drying caused by air movement and sunlight, Conversely, buried residue is in contact with soil which moderates drying and serves as a source of inoculum and nutrients for microbes.

Table 3. Weight of wheat residue remaining under different soil placement after successive time intervals (modified from Collins 1987 and Douglas et al. 1980).

The three residues that Douglas and co-workers selected differed in nitrogen and sulfur content. Residue 1, 2 and 3 contained respectively 0.78, 0.49 and 0.19 percent nitrogen and 0.10, 0.05 and 0.02 percent sulfur. A comparison of residue weight after given time intervals for three placements of these residues is presented in Table 4. As suspected, residues with higher amounts of nitrogen tended to decompose more quickly. When placed above ground residue 1 decomposed more rapidly, while residues 2 and 3 decomposed at about an equal rate. When placed on the surface, residue 1 decomposed most rapidly followed by residue 2 and then residue 3. Decomposition rates of buried residues did not vary as much as the above or on surface placements. This is probably because microorganisms had access to soil nitrogen and therefore nitrogen was not so limiting in the low nitrogen residues.

A question that is frequently asked is “how much nitrogen is tied up in the decomposition of wheat residue?” This question is difficult to answer because it depends both on the rate of decomposition and the nitrogen content of the residue. Nitrogen and other nutrients continuously cycle through the tissue of soil microorganisms during the decomposition processes. When C:N ratios of residue exceed about 3040:1, microbes will use inorganic soil nitrogen in the decomposition process. This process called “immobilization” ties up nitrogen in microbial tissue making it unavailable for plants. Immobilized nitrogen becomes available when microbial tissue decomposes and nitrogen is released as ammonia. This process is known as “mineralization.”

Table 4. Weight remaining of three different wheat residues under different soil placements after successive time intervals (modified from Douglas et al. 1980).

Table 5. Weight and nitrogen content of three buried wheat residues remaining at successive time interval (modified from Douglas et al. 1980).

During his study Douglas measured nitrogen content of residue at the end of each time period. For ease of understanding let’s evaluate this information on an acre basis. The estimated initial residue application rate in this study was 3,800 pounds per acre. Using this initial weight and the measured nitrogen content, residue weight and nitrogen content in pounds/acre are presented in Table 5. The numbers have been rounded for simplicity. An increase in nitrogen over time indicates that soil nitrogen has been immobilized. This occurred in residues 2 and 3 midway during the exposure period. The difference in initial nitrogen content and final nitrogen content is the amount of nitrogen that was released from the residue. Respectively these residues released 15, 10 and 2 pounds/acre nitrogen. This nitrogen would likely be used for microbial growth because the C :N ratio of the residue at the end of the last period was still greater than 30:1

Residue Management

Crop residues have both beneficial and adverse effects in crop production systems. Adverse impacts include difficulty with tillage and seeding operations, increased incidence of soilborne disease, reduced nutrient availability and inferior weed control. In contrast, beneficial effects include improved soil and water conservation, soil conditioning and organic matter maintenance or improvement, improved soil biological activity and fertility. Most of the disadvantages associated with crop residues involve problems with plant response, while benefits are generally soil related.

Beneficial and adverse effects should be considered in developing residue management strategies. Developing residue management systems involves decisions about short-term plant responses and long-term soil benefits. Ideally, residue management strategies should seek to minimize adverse effects of residue on plants and maximize the long-term benefits to soil.

Basically, the options for managing residue are burning or tillage and to a limited extent crop rotation (this controls how much residue is produced). Burning as a management tool is an extreme option and generally not recommended. Burning eliminates both the beneficial and adverse influences of residue. Plant response problems are alleviated when residue is burned, but all of the soil improving benefits are also lost. Also plant nutrients such as nitrogen are lost to the atmosphere.

If burning is the only option, practices such as burning in combination with tillage, partial field burns, and season and timing of burn should be considered. Incorporating residue into the soil before burning will retain a portion of the residue. Burning only problem areas within fields can reduce problems while maintaining residue on other parts. Fall burning is more devastating than spring burning because of higher burn temperatures and evaporative water loss from bare fields over winter.

Tillage is the most common residue management technique. The amount and type of residue and the types of tillage tools available are critical to residue management. Basically, three systems of residue management have evolved around groups of equipment. These are leaving residue at the surface (no-tillage), shallow incorporation (minimum tillage) and deep incorporation (moldboard plowing). Deep incorporation of wheat residue lessens the risk of many of the problems associated with residue, but long-term soil benefits are also less. The erosion control benefits of surface and shallow residue are well documented. In addition, surface and shallow incorporated residue affect soil characteristics in the first few inches of soil. Long-term, repeated additions of residue at or near the soil surface improves organic matter content, structural stability and soil tilth in this critical zone. Soil in this position has the greatest influence on erodibility, infiltration and evaporation. Soil improvements in the surface few inches will likely have greater benefit than similar improvements deeper in the profile.

Residue management is an important aspect of a conservation farming system. Short-term plant responses and long-term soil benefits must be considered together to develop a balanced residue management strategy

References

Collins, H. P. 1987. Decomposition and interactions of surface applied wheat residue components. Ph. D. dissertation Washington State University, Pullman.

Douglas, C. L., R. R. Allmaras, P. E. Rasmussen, R. E. Ramig and N. C. Roger. 1980. Wheat straw composition and placement effects on decomposition in dryland agriculture of the Pacific Northwest. Soil Sci. Sot. Am. J. 44:833-837.