RHIZOSPHERE CARBON FLOW IN TREES, IN COMPARISON WITH ANNUAL PLANTS - THE IMPORTANCE OF ROOT EXUDATION AND ITS IMPACT ON MICROBIAL ACTIVITY AND NUTRIENT AVAILABILITY

The release of organic substances from roots is a key process influenc ing nutrient availability in the rhizosphere. Rhizodeposition, includi ng root exudation can influence plant growth directly by making cation s available for uptake through processes such as chelation or indirect ly by influencing soil microbial activity. It is important to gain kno wledge about the range of compounds released and the factors influenci ng their release, to understand their effects on the microbial communi ty and enable development of techniques to enhance microbial activity. The increasing growth of trees in various land use systems is coupled with a limited knowledge of the interactions between nutrient availab ility and tree growth. This highlights the need for a greater understa nding of factors affecting nutrient availability in these systems. The purpose of this paper is to review the various strategies which are u sed to measure rhizodeposition by plants and demonstrate that root exu dates are an important component of carbon loss from plants and that t hey may have a more important role in nutrient acquisition and plant g rowth than previously thought. The paper will discuss the character of carbon loss from trees in comparison to annual plants and discuss the increasing evidence of the importance of non-nutrient components of r oot exudates as host specific recognition signals. The factors affecti ng exudate release and the impact of these compounds on nutrient avail ability will be discussed. The Limitations of previous studies of rhiz odeposition and root exudation through omission of a mycorrhizal compo nent, and the need for further research in this neglected area, will b e highlighted. Manipulation of plant-microbial interactions is discuss ed in relation to improving or maintaining plant growth in sustainable systems.