Abstract

Understanding the implications for ecosystem function of soil biodiversity and processes is the last frontier in terrestrial ecology. Research on this field is lagging behind aboveground studies mainly because soil is such a complex matrix. Some soil processes, such as decomposition and mineralization of organic matter and biogeochemical cycles, have long been recognized as key components of ecosystems. In addition, recent studies in natural ecosystems have revealed that organisms from the rhizosphere—plant pathogens, parasites, herbivores, and mutualists—have a significant impact on natural plant communities (Van der Putten and Peters 1997, Klironomos 2002, De Deyn et al. 2004). The rhizosphere is a hot spot of soil biodiversity driven primarily by plant roots. The exudations of these roots provide nutrients for microbes, and may also attract or repel some organisms (van Tol et al. 2001, Rasmann et al. 2005) . The interactions between plants and rhizosphere organisms can range from mutualistic to pathogenic, including direct competition for resources. In general, nitrogen-fixers and mycorrhizal fungi enhance plant growth and survival, and pathogenic fungi and root-feeders decrease plant fitness. These and other interactions with nonmycorrhizal fungi, rhizosphere bacteria, protozoa, and viruses can also modify the effect of soil-borne pathogens, herbivores, and mutualists in plant populations.