Background/Question/Methods Many semi-arid grassland communities of the southwestern United States have experienced an increase in woody plant density in the last two centuries. Two of the many woody plants which have increased in these ecosystems are the nitrogen fixing legumes Prosopis glandulosa and Acacia farnesiana. Several studies have reported that woody nitrogen fixing plants influence soil fertility below their canopies, and it is recognized that each plant species in a community may differ in its influence on soil characteristics. However, few comparisons have been made between the soil chemical and biological properties of these two species and their associated intercanopy spaces. In this study, soil chemical (soil organic carbon, and soil nitrogen), and biological properties (number of bacteria, number of fungi, and microbial biomass carbon) were evaluated in three P. glandulosa communities and three A. farnesiana communities located in Bexar County in central Texas. Comparisons of soil properties were analyzed between tree species in two distinct positions – canopy vs. intercanopy. Results/Conclusions Results indicate that P. glandulosa and A. farnesiana trees uniquely influence soil chemical and biological properties, resulting in an overall increase in soil fertility. We found that soils beneath the canopies of these two species effectively accumulated organic carbon, soil nitrogen, and exhibited higher microbial populations compared with the intercanopy spaces. However, P. glandulosa communities exhibited higher soil nitrogen content, soil microbial biomass carbon, and numbers of bacteria and fungi under its canopy than A. farnesiana; while soil organic carbon was higher underneath A. farnesiana compared with P. glandulosa. This difference could be related to the rate of decomposition underneath the canopy of these species. It seems that the rate of carbon mineralization is slower underneath A. farnesiana trees than under P. glandulosa. Although several factors could be involved in this reduced rate of decomposition in the A. farnesiana canopy microhabitat, smaller microbial populations appear more likely associated with this tendency and these reduced microbial populations may be related to the chemical composition of the leaf litter produced by this species. Considering that litter chemistry has a direct influence on microbial metabolic activity, which in turn may affect pools and fluxes of nutrients, the variation in nutrient concentrations of P. glandulosa and A. farnesiana leaves may give a valid, yet undetermined explanation for the effects that those two species have on soil fertility.