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 influencing nutrient availability in the rhizosphere. Rhizodeposition, including root exudation can influence plant growth directly by making cations available for uptake through processes such as chelation or indirectly by influencing soil microbial activity. It is important to gain knowledge about the range of compounds released and the factors influencing their release, to understand their effects on the microbial community 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 availability and tree growth. This highlights the need for a greater understanding of factors affecting nutrient availability in these systems. The purpose of this paper is to review the various strategies which are used to measure rhizodeposition by plants and demonstrate that root exudates are an important component of carbon loss from plants and that they may have a more important role in nutrient acquisition and plant growth 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 root exudates as host specific recognition signals. The factors affecting exudate release and the impact of these compounds on nutrient availability will be discussed. The limitations of previous studies of rhizodeposition and root exudation through omission of a mycorrhizal component, and the need for further research in this neglected area, will be highlighted. Manipulation of plant-microbial interactions is discussed in relation to improving or maintaining plant growth in sustainable systems.