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In the central highlands of Mexico, mesquite (Prosopis spp.) and huizache (Acacia tortuosa), N-2 fixing trees or shrubs, are dominating the vegetation and are considered to be used in a reforestation program to prevent erosion. We investigated how the two species affected soil characteristics and dynamics of C and N. Soil was sampled under the canopy of mesquite (MES treatment) and huizache trees (HUI treatment), outside their canopy (OUT treatment) and from fields cultivated with maize (ARA treatment) at three different sites. Production of CO2, and dynamics of inorganic N (NH4+, NO3-) were monitored in an aerobic incubation. The microbial biomass C was significantly greater in the HUI than in the MES treatment and both values were greater than in the OUT and ARA treatments. The microbial biomass N showed the same results, except that there was no significant difference between the MES and HUI treatment. The number of bacteria, actinomycetes and fungi was largest in the HUI treatment. The production of CO? was similar in the HUI and MES treatments and significantly greater than in the OUT treatment and ARA treatment. The production of NO3- was similar in the HUI and MES treatments and five times greater than in the ARA treatment. The production of NO3- in the latter was two times greater than in the OUT treatment. It was found that both mesquite and huizache created islands of fertility with larger soil microbial biomass C and N, C and N mineralisation and organic and total N compared to the areas between the woody trees or sites cultivated with maize and beans for more than 14 years. (C) 2002 Elsevier Science B.V All rights reserved.