Journal of Arid Environments (1999) 42: 61–70 Article No. jare.1999.0503 Available online at http://www.idealibrary.com on Variation in soil micro-organisms and nutrients underneath and outside the canopy of Adesmia bedwellii (Papilionaceae) shrubs in arid coastal Chile following drought and above average rainfall Lorgio E. Aguilera*-, Julio R. GutieH rrez* & Peter L. Meserve? * Departamento de BiologıH a, Universidad de La Serena, Casilla 599, La Serena, Chile ? Department of Biological Sciences, Northern IIlinois University, DeKalb, IL 60115, U.S.A. (Received 29 September 1998, accepted 15 February 1999) Micro-organisms and nutrients were determined in soil samples (top 20 cm) collected underneath and outside the canopy of Adesmia bedwellii in mid fall (April 1997), after 8 months of drought, and in mid spring (October 1997) after heavy winter rains associated with the 1997-ENSO in a Chilean coastal desert site. Bacteria and fungi were two orders of magnitude higher in mid spring, but no seasonal effects were detected for chemical parameters. Micro-organisms, organic matter and nitrogen content were significantly high- er underneath shrub canopies, especially in mid fall. Micro-organism abund- ances were positively correlated with nitrogen levels and soil moisture. ( 1999 Academic Press Keywords: soil fertility; micro-organisms; micorrhizae; fertility island; arid zone; Chile Introduction In arid environments nutrients are scarce and patchily distributed, with greater amounts under shrub canopy generating ‘fertility islands’ (sensu GarcıH a-Moya & McKell, 1970). Micro-organisms associated with these fertility islands are important for plant growth, since they favor the assimilation of nutrients (Davison, 1988), produce hormones that promote growth (Denarie et al., 1992), fix nitrogen (Farnsworth et al., 1977), suppress pathogens (Shippers et al., 1987) and permit the dissolution of minerals (Nakas & Klein, 1980). The abundance of micro-organisms in soil varies spatially as well as temporarily, and this pattern is related to temporal and spatial variations in the quantity and quality of nutrients (Nedwell & Gray, 1987; Wardle, 1992). Micro-organisms respond to nitrogen (Jenkins et al., 1988; Wardle, 1992), organic matter (Hussey et al., 1985; Jenkins et al., - (E-mail: laguiler@elqui.cic.userena.cl). 0140}1963/99/050061#10 $30.00/0 ( 1999 Academic Press