One hundred soil samples were collected along a 100-m transect at 1-m intervals from a wastewater-irrigated field in Bakersfield, California, and from an adjacent control field that has been irrigated with groundwater. The treated field received municipal wastewater effluents and has been in production for the past 70 years. The soils in these 2 fields were first sampled at the end of the growing season, and sampled again at the beginning of the following growing season. The physical and chemical properties of the soils were determined by measurements of 29 parameters describing the ability of soils to hold, accept, and release water, nutrients, and pollutants to plants, downstream surface water bodies, and sub-soil. The data were analyzed for field spatial variability. Along the selected transect, the fields exhibited inherent spatial interdependence at distances from 10-15 m, depending on the parameter considered. A subset of the parameters were selected for soil quality evaluation by employing principal component and factor analyses procedures which ranked these parameters according to their contributions to the variance of the data. The soil quality of the field might be evaluated by considering 5 of the 29 parameters, such as biomass production, available water capacity, Mg, P, and Zn contents. The diversity of the soil microbial community was examined by assaying the BiOLOG substrate metabolism patterns, fatty acid methyl ester (FAME) profiles, the PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) based 16S/18S rDNA probes, and the hybridization of 16S/23S probes. Principal component analysis and hierarchical cluster analysis were employed to analyze the data. It was found that the diversity of the bacterial community in the wastewater-irrigated field was significantly enhanced, while diversity of the eukarya and archaea communities of the treated soil were not significantly different from those of the control soils. The FAME profiles, however, showed a temporal variation of the microbial communities in soils from the end of one growing season to the beginning of the following growing season. The findings from each assay complemented one another. The proteobacteria groups, especially the alpha proteobacteria, accounted for most of the changes of diversity in the bacterial community, and were the single most important factor in the shift of microbial diversity in the soils. The dominant fungal species in the soils shifted when the soil was left fallow between growing seasons. Compared to the wastewater treatment, the differences caused by spatial variability of the fields were not significant. The composition of the microbial community remained unchanged across the landscape of soils receiving the same treatment. The ability of soil to support crop production was not adversely affected by long-term continuous application of municipal wastewater.