The Role of Arbuscular Mycorrhizas in Organic Farming

Abstract

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in natural and agricultural ecosystems. AMF enhance uptake of nutrients by plants, particularly phosphorus (P), and may also improve plant drought avoidance and disease control. AMF may also be necessary for the long-term sustainability of ecosystems, particularly due to their role in the maintenance of soil structure, and plant community structure and diversity. In agricultural systems, high colonisation of roots by AMF is favoured by the absence of mineral fertilisers that supply readily soluble P, minimal soil disturbance, avoidance of non-host plants and bare fallows, and, perhaps, a high degree of plant diversity and minimal use of biocides. Colonisation by AMF is often higher on organic farms than conventional farms and there is some evidence of an increase in species diversity of AMF on organic farms. These differences appear primarily due to the lack of fertilisers containing readily soluble P on organic farms. Yet high colonisation by AMF is not an inevitable outcome of organic farm management and colonisation may be limited on some organic farms by high rates of tillage or residual high soil available P. There is some indication that organic farms can develop a community of AMF with an increased capacity to enhance plant P uptake. However, AMF do not substitute for fertiliser inputs as the nutrients taken up by the fungi primarily originate from the finite pool of soil available nutrients and their removal in farm products must be matched by inputs from off-farm sources. Indeed, high colonisation by AMF may be considered an indicator of low soil available P. As AMF depend on host photosynthate for energy, high levels of colonisation may reduce plant growth under some environmental or farm management conditions. For instance, monocultures, high soil fertility, and high rates of tillage may stimulate development of less beneficial communities of AMF The need for inoculants containing AMF on organic farms is unknown, but they may prove beneficial if bare fallows or weed-free crops of non-hosts are regularly included in the rotation. Overall, the high abundance of AMF that often results from organic management suggests an important role for the fungi in the functioning of organic farming systems.