Seed meals are byproducts of oilseed processing. Oil is extracted from cottonseed and canola seed for cooking purposes, although recent interest in using these oils for production of biofuels has increased. A hexane extraction method (Zeigler et al., 1982) is used to remove oil from the seeds leaving meal and soapstock as byproducts. Soapstock is an undesirable chemical compound removed by a chemical reaction during caustic refining (Kuk et al., 2005). Soapstock consists of low-quality fatty acids (Davis et al., 2002). Processors add it to meal because it increases nutritional value for animal feed and provides a means of disposal (Kuk et al., 2005).
Seed meals have the potential to be suitable organic fertilizers. They are relatively high in nutrients and easily soluble compounds that make them biodegradable and a source of macro- and micronutrients. Cottonseed meal and canola meal generally contain 6% to 7% N, 2% phosphorus, and 2% potassium and have a pH of 6.0 to 6.5 (Mitchell, 2008) with a carbon-to-nitrogen (C/N) ratio of 8:1 (Gale et al., 2006). Such a low C/N ratio suggests that seed meals will behave as a nutrient source rather than immobilize soil N through microbial activity. Information concerning mineralization rates of cottonseed meal and canola meal is limited. Snyder et al. (2009) showed that mineralization of Brassicaceae seed meals ranged from 30% to 81% in a 96-d period. Greater N mineralization occurred in meal-amended plots than control plots and mineralization rates for meals adequately supplied N for plant growth (Snyder et al., 2009). Gale et al. (2006) examined decomposition and availability of N released from manure, compost, and specialty products (pelleted organic fertilizer, feather meal, and canola meal) under field and laboratory conditions to determine accuracy of mineralization prediction models based on C/N ratios. Specialty products with C/N ratios of 4 to 8 decomposed 76% and released an average of 78% of the plant-available N from the products in 70 d, whereas broiler litter with higher C/N ratios of 8 to 10 released only 40% of plant-available N in 70 d (Gale et al., 2006). Kelderer et al. (2008) sought to improve use of commonly used organic fertilizers by determining N mineralization rates. Mineralization rates for castor (Ricinus communis L.) seed meal and three seed cakes were tested. After 14 d of incubation, 16% of N mineralized from castor seed meal, whereas 3% mineralized from seed cakes of undefined seed sources (Rigen Plus; Europa Trading s.r.l.), 13% mineralized from seed cakes consisting of soya (Glycine max L.) and maize (Zea mays L.) (Fertilvegetal; Delta concimi s.a.s.), and 22% mineralized from seed cakes of unidentified seed types (Ecolverdepiu; Sala s.r.l.). After 60 d of incubation, 27% of N mineralized from castor seed meal and 20% from unidentified seed cakes (Rigen Plus), 21% mineralized from the seed cakes containing soya and maize, and 30% mineralized from unidentified seed cakes (Ecolverdepiu). Many synthetic fertilizers such as ammonium nitrate rapidly release plant-available N as they dissolve in water. In contrast, seed meal mineralization occurs with microbial degradation; thus, mineralization from seed meals is slower than N release from ammonium nitrate and other synthetic highly soluble fertilizers. Kelderer et al. (2008) found that soil N increased 75% after 14 d and 71% after 60 d with ammonium nitrate.
Rate of N mineralization from seed meals depends on several factors including organic composition of the residue, soil temperature and water content, drying and rewetting events, and soil characteristics (Cabrera et al., 2005). Snyder et al. (2010) conducted a study using 15N-labeled Brassicaceae seed meals to investigate the production of biologically active glucosinolate degradation products that might reduce microbial activity thereby reducing N mineralization. They found that Brassicaceae seed meals can be used to increase inorganic N in soil and that glucosinolate degradation products have different short-term effects on the microbially mediated soil N cycle.
Determination of decomposition rates for cottonseed meal and canola meal are needed to establish application rates. The objective of this study was to determine mineralization of NH4-N and NO3-N from three mineral soils amended with cottonseed or canola meal.
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