Trace Metal Accumulation In Manuretreated Soils

Because some soils can have fertility levels that are out of balance (Table 33.6), animal manures have historically been applied to soils as a fertilizer and to improve the soil's physicochemical properties [51,52]. A large portion of the approximately 10 million Mg of broiler litter (a mixture of manure, wasted feed, feather, and bedding materials such as wood shavings) produced annually in the southeastern U.S. is applied to hay field, pasture, and row crops [53]. After applications of poultry litter to low-fertility soils (Table 33.7), monthly monitoring of trace metal concentrations in topsoils shows that levels do not change dramatically within 1 year after application. Although manures contain essential micronutrients and organic matter, their high trace metal concentration is recognized as a significant source for trace metal accumulation in soils [7,10,14,26].

Brink et al. [54] reported that application of swine effluent (annual mean of 10 ha cm) to Bermuda grass pasture added annual averages of 0.6 kg ha-1 Cu, 2.2 kg ha-1 Fe, 0.3 kg ha-1 Mn, and 0.86 kg ha-1 Zn to the soil. Evers [55] also reported application of 9 Mg ha-1 broiler litter added averages of 5.85 kg ha-1 Zn, 5.0 kg ha-1 Fe, 9.4 kg ha-1 Cu, and 6.55 kg ha-1 Mn to soil. In another study, it was reported [56] that high Cd levels in soils and vegetables across Sydney, Australia, were due to repeated applications of poultry manures. Animal manure applications to agricultural lands in England and Wales accounted for 25 to 45% of the total annual Cu and Zn inputs [7].

Kingery et al. [26] examined long-term application (15 to 28 years) of poultry litter on nutrient levels in several Ultisols in Alabama. They reported that topsoil Cu and Zn concentrations (2.5 and 10 mg kg-1) in poultry litter-treated soils were higher than topsoil Cu and Zn concentrations (0.75 and 2.2 mg kg-1, respectively) measured in control soils (no history of manure application). Nicholson et al. [7] modeled potential annual trace metal loadings in soil treated with 250 kg N ha-1 yr-1 from various manure sources. The quantities of trace metals were based on the manure dry matter and typical metal concentrations in manures. Significant quantities of Cu and Zn were added to soil through the application of swine (2.2 and 1.6 kg ha-1), poultry (1.1 and 0.5 kg ha-1), and cattle (1.0 and 0.3 kg ha-1) manures, respectively. These researchers noted that the modeled trace metal loading rates may be higher in areas where Cu- and Zn-enriched swine and poultry manures have been applied for many years [7].

Many studies have suggested additional reasons for trace metal accumulation in manure-treated soils. These reasons include repeated applications of manure to the same field and a low trace metal crop nutrient requirement. Repeated manure applications to the same fields occur because manure transportation costs are high and land availability for manure disposal is limited. Frequently, animal manures are not transported very far, and they are often applied close to the production facility. Land available for manure application continues to decline because of high real estate prices, encroachment of urban sprawl, zoning restriction, and exhaustion of the soil's nutrition assimilation capacity. Expecting some animal producers to locate more land available for manure application may not be economically feasible. Unless directed to cease by a restrictive nutrient management plan, some fields will probably continue to receive trace metal-enriched animal manures.

The low trace metal requirement by crops is also an important reason for their accumulation. Researchers have conducted experiments to determine the sufficiency level of trace metals utilized by agronomic crops (Table 33.4). Data in this table exemplify that the sufficiency range of trace metals in crop tissue is much smaller than for macronutrients (Table 33.4). For example, the nutrient sufficiency range for trace metals like Co and Mo are orders of magnitude smaller than for macronutrients like N and K. Through field experiments, researchers have measured the amount of trace metals removed by harvesting crops. For example, Bermuda grass (Cynodon dactylon L.), legumes, and other temperate annual and perennials grasses are commonly used in southern and southeastern U.S. pasture systems to assimilate nutrients contained in animal manures [51,54,55,57,58]. The grass can remove 0.009 and 0.218 kg of Cu and Zn, respectively, per 7.3 Mg/ha yield of above-ground biomass [59].

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