Relation between soil health, wave-like fluctuations in microbial populations, and soil-borne plant disease management Ariena H.C. van Bruggen 1 , Alexander M. Semenov 2 , Anne D. van Diepeningen 1 , Oscar J. de Vos 1 and Wim J. Blok 1 1 Biological Farming Systems, Wageningen University, Marijkeweg 22, 6709 PG Wageningen the Netherlands; (Fax: +31-317-468213; E-mail: ariena.vanbruggen@wur.nl); 2 Department of Microbiology, Biological Faculty, Moscow State University, 119899 Moscow, Russia; Key words: biological control, disease management, harmonic fluctuations, resilience, soil health, soil-borne pathogens Abstract A healthy soil is often defined as a stable soil system with high levels of biological diversity and activity, internal nutrient cycling, and resilience to disturbance. This implies that microbial fluctuations after a disturbance would dampen more quickly in a healthy than in a chronically damaged and biologically impoverished soil. Soil could be disturbed by various processes, for example addition of a nutrient source, tillage, or drying-rewetting. As a result of any disturbance, the numbers of hetero- trophic bacteria and of individual species start to oscillate, both in time and space. The oscillations appear as moving waves along the path of a moving nutrient source such as a root tip. The phase and period for different trophic groups and species of bacteria may be shifted indicating that suc- cession occurs. DGGE, Biolog and FAME analysis of subsequent populations in oscillation have confirmed that there is a cyclic succession in microbial communities. Microbial diversity oscillates in opposite direction from oscillations in microbial populations. In a healthy soil, the amplitudes of these oscillations will be small, but the background levels of microbial diversity and activity are high, so that soil-borne diseases will face more competitors and antagonists. However, soil-borne pathogens and antagonists alike will fluctuate in time and space as a result of growing plant roots and other disturbances, and the periods and phases of the oscillations may vary. As a consequence, biological control by members of a single trophic group or species may never be complete, as pathogens will encounter varying populations of the biocontrol agent on the root surface. A mixture of different trophic groups may provide more complete biological control because peaks of different trophic groups occur at subsequent locations along a root. Alternatively, regular addition of soil organic matter may increase background levels of microbial activity, increase nutrient cycling, lower the concentrations of easily available nutrient sources, increase microbial diversity, and enhance natural disease suppression. Abbreviations: BCA – biocontrol agent; CFUs – colony forming units; DGGE – denaturing gradient gel electrophoresis; FAME – fatty acid methyl esters; GFP – green fluorescent protein Introduction Health is a necessary condition for the survival of individual living organisms, communities, ecosystems, and for nature in general. Ecologists have long recognized that the state of health of terrestrial, edaphic and aquatic ecosystems is important. However, it is not so easy to define, let alone measure, ecosystem health. This is also true for soil, which is considered as a living system, where European Journal of Plant Pathology (2006) 115:105–122 Ó Springer 2006 DOI 10.1007/s10658-005-1250-8