Pecan is one of the few native North American plant species to have been developed into a significant agricultural crop. Pecans are commercially produced across a wide geographic range in 14 states within the United States, generating a crop valued at $369 million [U.S. Department of Agriculture (USDA), 2010]. Aside from the United States, pecans are also produced in Mexico, Brazil, Peru, Argentina, Australia, South Africa, Israel, and Egypt. Georgia lies outside the tree's native range but annually produces more pecans than any other state, comprising nearly 30% of the U.S. pecan crop annually (USDA, 2010).
The bottomland soils on which pecans grow in their native range vary considerably from those on upland sites where the trees are cultivated and grown commercially in the southeastern United States. The loamy bottomland ridges to which pecans are adapted typically consist of well-drained soils high in organic matter (Sparks, 2005). By contrast, upland soils of the southeastern United States are relatively shallow and are comparatively low in organic matter.
Most studies of pecan orchard soils have focused primarily on soil mineral nutrition status and their effects on pecan production. Studies regarding the biological parameters of pecan orchard soils are limited. Biological indicators of soil quality such as microbial biomass and enzyme activity are sensitive to changes in agricultural management practices (Doran et al., 1987). Shading of soil and high detritus inputs in the form of leaf and litter fall may reduce soil temperatures and increase soil moisture in orchard systems. Thus, microbial biomass and nutrient cycling processes are affected by the altered microclimate in orchard and agroforestry systems as compared with forest ecosystems or agroecosystems (Lee and Jose, 2003).
In the early years of the Southeastern pecan industry, legumes and cow manure, provided by cattle grazing the orchards, were commonly used to provide fertilizer N for pecan trees. After World War II, pecan producers began using synthetic fertilizer on a large scale as a result of the low cost and ease of application (White et al., 1982). As air-blast sprayers became available and fungicide and insecticide applications became more routine, grazing cattle in the orchards was discontinued by most growers. Recent instability in fuel and fertilizer prices have led pecan producers to once again consider the use of various legumes such as crimson clover as an orchard floor cover to supplement the pecan tree's N requirements. In 2005, clover (Trifolium sp.) was used in only 15% of surveyed pecan orchards in Georgia, but by 2008, nearly half of all pecan orchards surveyed used clover as an orchard floor cover (Wells, 2009). Smith et al. (1996) suggested that a combination of crimson clover and hairy vetch (Vicia villosa Roth) could be used as a cover crop to meet the N requirement of pecan.
Early studies of pecan by Blackmon (1936) showed that animal manure can lead to high yields and serve as an excellent source of plant nutrients. Georgia poultry farmers produce over two million tons of poultry litter (manure) annually (Ritz and Merka, 2009) in the production of meat and eggs, creating a need for disposal. Proper application of litter to the land as a soil amendment is an appropriate use for the waste product. The organic material and nutrients found in poultry litter are beneficial byproducts that have proven useful in amending agricultural soils (Mitchell and Tu, 2005).
Doran and Parkin (1994) defined soil quality as “the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health.” Soil quality is generally related to the physical, chemical, and biological factors on a particular site. Characterization of soil quality requires the selection of indicators sensitive to changing agricultural practices (Doran et al., 1994). Soil microorganisms are vital to the biogeochemical cycling of organic and inorganic nutrients in the soil and maintenance of soil quality. Microbial activity in the rhizosphere is a major factor that determines the availability of nutrients to plants and has a significant effect on plant health and productivity. Enzymes produced by soil microorganisms and plants mineralize organically bound nutrients and are often used as indicators of soil quality (Doran and Parkin, 1994).
Mycorrhizal relationships are often considered the most significant form of plant–microbe symbiosis (Smith and Read, 1997). Ectomychorrhizal fungi form a particular, extracellular morphological complex with the roots of many temperate forest trees, including pecan (Woodruff, 1933). However, the arbuscular mycorrhizal (AM) fungi form the most widespread symbiotic relationship between plants and fungi in nature. Over 80% of plant species can form AM. The extensive hyphal network created by AM can improve soil aggregation (Miller and Jastrow, 1992). Organic compounds produced by mycorrhizal hyphae may bind soil microaggregates into macroaggregates (Tisdall, 1994). Mycorrhizae have been shown to favor soil conditions of high organic matter and low fertility (Miller and Jastrow, 1992).
Research on mycorrhizal relationships in pecan are limited. Woodruff (1933) described several forms of ectomycorrhizae on pecan roots in Georgia. Marx and Bryan (1969) found that Scleroderma bovista formed ectomychorrhizae on pecan roots and inhibited pathogenic fungi associated with feeder-root necrosis of pecan. Sharpe and Marx (1986) demonstrated enhanced growth and mineral nutrient content of pecan seedlings by inoculation of trees with Pisolithus tinctorius ectomycorrhizae. Arbuscular mychorrhizae have not been documented from pecan roots; however, both ectomycorrhizae and AM occur in southeastern U.S. pecan orchards and thrive under similar conditions (Hayman, 1982; Mader et al., 2000; Marx and Bryan, 1969; Woodruff, 1933).
Soil amendment with organic materials such as poultry litter and/or legumes such as clover should improve southeastern U.S. pecan orchard soils, thus enhancing their sustainability. There are few studies regarding the activity of soil quality biological indicators in pecan orchards and none examining the effects of clover and poultry litter on soil quality biological indicators in southeastern U.S. pecan orchards. Therefore, the purpose of this study was to examine the effect of poultry litter application and the use of crimson clover as a cool-season cover crop on soil chemistry and soil quality biological indicators, including MIP, MBC, and phosphatase activity in a southeastern U.S. Coastal Plain pecan orchard system.
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