Mechanical pruning of trees is becoming increasingly common in commercial pecan orchards. This practice generates considerable biomass. For example, in Doña Ana County, N.M., an estimated 25,500 tons of pecan pruning wood is produced annually (Cabral, 2005). Pruning wood is currently burned as an economic means of disposal. However, the New Mexico Environment Department and the U.S. Environmental Protection Agency are concerned with this practice and may restrain, prohibit, or otherwise control burning in the future (New Mexico Air Quality Bureau, 2003). Other disposal methods of pecan pruning wood such as mulching, composting, and the use of pecan wood as firewood are not economically feasible. The use of pecan wood chips as mulch around young, nonproducing pecan trees has been practiced and proven beneficial (Foshee et al., 1996, 1999; Smith et al., 2000). Nevertheless, mulching wood chips around productive trees interferes with field operations and especially harvesting (Foshee et al., 1996). Composting the wood would require even more input by producers for a product that has low economic value. Most of the pruning wood is too small in diameter to be sold as firewood. A disposal method for pecan pruning wood is needed that is economically viable and environmentally acceptable. Chipping and incorporation of pecan wood chips into the soil may be an alternative disposal method.
Although numerous studies have examined the effects of manure, biosolids, and other organic amendments to soil, few published reports exist that have examined the effect of wood chip incorporation on soil nutrient availability. Soil incorporation of woody residues has received limited agricultural attention in the past because of their high C:N ratio and the risk of nitrogen (N) immobilization. Recent interest in wood chip incorporation is the result of environmental rather than agricultural concerns.
As an alternative to burning, almond pruning wood has been chipped and incorporated into soil in areas of the San Joaquin Valley, Calif., for more than 14 years (Holtz, 1999). The practice has gained acceptance because the waste is eliminated and can serve as a source of soil organic matter.
The addition of almond wood chips at a rate of one-third part wood chips to two-thirds part soil to almond trees planted in the San Joaquin Valley initially reduced leaf petiole and soil nitrate levels in the first year, did not significantly reduce leaf petiole nitrate levels by the second year, and significantly increased both leaf petiole and soil nitrate levels by the third year (Holtz et al., 2004). The initial immobilization of soil inorganic N in the first year could be partially explained by the high wood chip application rate that was ≈410,000 kg·ha−1 or one-third of soil weight. However, despite the initial N immobilization when almond wood chips were first added to the soil, as the wood chips fully decomposed, they eventually returned nutrients to the soil.
Sanborn et al. (2004), while studying the effect of wood chip incorporation on the rehabilitation of older landings and roads constructed on fine-textured soils in British Columbia, Canada, found that the wood chip treatment, compared with subsoiling and shallow tillage, resulted in the highest 3-year growth rates of hybrid white spruce; leaf tissue analysis also indicated that macro- and micronutrient concentrations were adequate. Based on these promising early results, the authors recommended silviculturists to use chipped wood wastes in the rehabilitation of disturbed lands.
Nitrogen mineralization is the conversion of organic N to mineral forms that are available for plant uptake, whereas N immobilization is the conversion of inorganic N to organic forms that are generally unavailable for plant uptake (Krishna, 2002; Paul and Clark, 1989). The mineralization and immobilization of N and other inorganic nutrients in soil are of considerable importance to plant nutrition. Nitrogen mineralization and immobilization are largely determined by the C:N ratio of the substrate organic matter (Paul and Juma, 1981; Van Veen et al., 1984). At optimum environmental conditions, if the incorporated organic matter has a high N content (low C:N ratio) such as in fresh, green plant materials, the N content of the substrate is usually sufficient for microbial metabolism and the excess N is excreted to the soil. On the other hand, if the incorporated organic matter has a low N content (high C:N ratio) such as in straw and wood, insufficient N exists in the substrate for microbial metabolism and microorganisms must use N from the soil (Coyne, 1999; Herrmann, 2003). Absorption of inorganic N, and possibly other nutrients, from soil by microorganisms competes with plants for inorganic N and may result in N deficiency of plants growing in the soil.
Assuming optimum environmental conditions exist, the extent and length of the N immobilization process will depend on the C:N ratio of the organic substrate, the quantity as well as physical size of the organic substrate added to the soil, and the resistance of the organic substrate to microbial attack that is a function of lignin, waxes, and fat present in the organic substrate (Alexander, 1998; Havlin et al., 1999). The C:N ratio is a good but not absolute indicator of N mineralization–immobilization turnover. Coyne (1999) indicated that some organic materials can have high C:N ratios and still mineralize N because the effective C:N ratio of their tissue is lower as a result of their high lignin content. Lignin is insoluble, hard to degrade, and has no direct effect on the metabolism of other substrates.
If soil incorporation of pecan wood can be shown not to interfere with harvest or compete with trees for nutrients, growers would be more likely to adopt chipping and soil incorporation as an alternative to burning. However, the influence of pecan wood chip incorporation must be viewed over several years and with repeated applications for a clear picture of the beneficial or detrimental effects of this disposal method. The purpose of this study was to determine the beneficial and detrimental effects of pecan wood incorporation on soil nutrient availability over several application rates and numbers of applications.
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