With population increase, urban sprawl and the growing interest in the use of biofoels, significant pressures are occurring on some of the highest quality agricultural soils in many nations. Growth of grain and oilseed crops such as barley, corn, soybean and wheat have been an important part of the agricultural economy for years and the continuous increases in demand and prices have led farmers to apply highly intensive agricultural management practices, with the aim of increasing crop productivity. Tillage, crop rotation, fallows, changes in plant cultivars and pesticide application are often used with broadacre field crops, and all these practices influence the surrounding environment (Mozafar et al., 2000; Carter and Campbell, 2006).
Fertilizer use represents a common agricultural management practice, but a growing body of evidence has demonstrated an array of negative impacts on ecosystems from their use. No matter which form of fertilizer is applied (organic or mineral), conventional farming generates large N and P surpluses, which can lead to N leaching through the soil profile and P losses in runoff (Brady and Weil, 2002). Not only is there a high financial cost to farmers associated with this loss, but the phenomenon also resulted in soil contamination. In addition, excess fertilizer inputs can be a major threat to aquatic ecosystems through surface and groundwater degradation (Kirchmann and Thorvaldsson, 2000). Recently, fertilizer runoff from agricultural fields was emphasized among the causes of excessive cyanobacterial growth and increasing of potentially harmful blooms leading to restricted access to lakes.
Low-input agricultural systems have gained attention in many Industrialized countries due to increased interest in the conservation of natural resources, reduction of environmental degradation, and the escalating costs of fertilizers. Conventional farming systems using lower application rates of fertilizers and pesticides have been developed, but are used only minimally in North American grain production, perhaps due to insufficient understanding of agricultural soils dynamics (Ryan and Graham, 2002).
Numerous biological, chemical and physical factors influence soil quality. Among them, rhizosphere microbial communities have been shown to directly affect soil fertility by carrying out essential processes that contribute to nutrient cycling, and enhancing soil structure and plant growth and health (Mader et al, 2002; Wu et al, 2005; Miransari et al, 2007; St-Arnaud and Vujanovic, 2007). The extent to which these communities interact is thus of great importance and involves phenomena such as hormone production, enhancement of nutrient availability, and decrease of root diseases. Arbuscular mycorrhizal symbioses have been shown to benefit growth of many field crops in large part due to the extensive hyphal network development in soil, more efficient exploitation of nutrients, and enhanced plant uptake (Smith and Read, 1997). AM symbiosis also increases resistance to biotic and abiotic stresses and reduces disease incidence, representing a key component of sustainable agriculture (St-Arnaud and Vujanovic, 2007; Subramanian and Charest, 1999; Aliasgarzad et al., 2006). Appropriate management of mycorrhizae in agriculture should ultimately result in a substantial reduction in chemical use and production costs.
Soils generally contain indigenous AM fungi that colonize plant roots (Covacevich et al., 1995). The growth enhancement and P uptake of plants colonized by AM fungi is a well-known process (Pfleger and Linderman, 1996; Schweiger and Jakobsen, 1999; Jeffries et al., 2003). Not all plants are dependent on mycorrhizal associations (Azcón and Ocampo, 1981; Trouvelot et al., 1982; Hetrick et al., 1993); however, most increase in yield following inoculation with AM fungi (Jakobsen and Nielsen, 1983; Baon et al., 1992; Talukdar and Germida, 1994; Xavier and Germida, 1997; Al-Karaki et al., 1998) particularly in low-P soils (Thompson, 1990; Rubio et al., 2003). With the current tendency for reduced use of agrochemicals, research is being directed at crop yield improvement and yield sustainability. The efficient use of AM fungi may allow for the attainment of acceptable yield levels with minimum fertilizer dose, while also reducing costs and environmental pollution risk (Covacevich et al., 2007). This is a promising approach for obtaining high yields with low fertilizer inputs in order to support sustainable agricultural systems.
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