Foreword to the ‘Rhizosphere 2004’ papers in this issue of the Journal

It is now a century since Hiltner first formulated the concept of the rhizosphere and introduced the term to the vocabulary of soil science. The concept has attracted interest from diverse fields, each with its own operational definition. Initially only of interest to soil microbiologists, it was then taken up by soil chemists, later physicists, and is now again an important theme in soil microbiology with its new molecular tools. One of the major hurdles that research on the rhizosphere has had to overcome is the difficulty in sampling such an inherently inaccessible zone. Often sampling modifies the system, and when gradients are steep and relations non-linear, important effects can be overlooked if sampling averages across too large a space. Major advances in understanding have been made when sampling and measurements are miniaturized and when mathematical models are used alongside experimental approaches. Studies of the rhizosphere require old boundaries between the traditional subdisciplines of soil science to be rethought. Plants and microorganisms are not passive sinks for nutrients and pollutants; they modify their environment and influence their exposures to both essential and potentially toxic elements. Soil is increasingly recognized to be a heterogeneous and dynamic medium across all spatial and temporal scales applied to it. Experimental and modelling approaches that ignore this are bound to be inadequate. Good ideas are rarely isolated, and when the Journal was approached by the organisers of the conference, Rhizosphere: Perspectives and Challenges – A Tribute to Lorentz Hiltner, held in Munich in September 2004, we had already decided to publish an issue on the rhizosphere to mark Hiltner's centenary. This issue gives a selection of the communications presented at that conference. We hope it provides a representative sample of current work on the rhizosphere that is within the scope of the Journal, and indicates future research needs and priorities. Further communications from the Conference appear in Biology and Fertility of Soils, Environmental Microbiology, FEMS Microbiology Ecology, The New Phytologist, Journal of Plant Nutrition & Soil Science, Journal of Environmental Quality and Plant and Soil.