Arbuscular mycorrhizal fungi and organic farming

Symbiotic associations between arbuscular mycorrhizal fungi and plant roots are widespread in the natural environment and can provide a range of benefits to the host plant. These include improved nutrition, enhanced resistance to soil-borne pests and disease, improved resistance to drought, tolerance of heavy metals and better soil structure. Many agricultural crops are mycorrhizal and there is widespread if equivocal evidence that crop plants benefit from the arbuscular mycorrhizal (AM) association in the same way. However, many agricultural practices including use of fertilisers and biocides, tillage, monocultures and the growing of non-mycorrhizal crops are detrimental to arbuscular mycorrhizal fungi (AMF). As a result, agroecosystems are impoverished in AMF and may not provide the full range of benefits to the crop. Organic farming systems may be less detrimental to AMF because they exclude the use of water-soluble fertilisers and most biocides and generally have diverse rotations. The evidence available suggests that this leads to increased AMF inoculum in soils, greater crop colonisation and enhanced nutrient uptake. AMF might therefore be able to substitute for reduced fertiliser and biocide inputs in organic systems, though there is little evidence for increased yield resulting from high rates of AMF colonisation in organic systems. This review examines the benefits that the AM association can have for agroecosystems and how farm management practices influence the AM association. Management options that may be employed to increase the benefits that AMF can bring to this type of farming system, such as changes to the rotation and careful use of tillage, are discussed. (c) 2005 Elsevier B.V. All rights reserved.