Points, patches, and regions: Scaling soil biogeochemical patterns in an urbanized arid ecosystem

Cities are rapidly growing throughout the world and are altering biologic processes in many regions, with global consequences. Urbanization in the Phoenix, USA metropolitan region has dramatically altered regional ecosystem patterns, but little is known about how these changes have influenced soil organic matter, total nitrogen, and the distribution of nitrogen stable isotopes. Because urban development is a phenomenon occurring at multiple scales, ecological consequences of urbanization will likely differ between individual patches and the entire metropolitan region. To investigate such changes we conducted spatially explicit surveys including three dominant land-use types in this region: native desert, agriculture, and mesic residential. These data were combined for analysis with previously collected samples from a synoptic regional survey. A landscape scaling approach was implemented to compare the dependence of soil variability on the sampled extent and the uncertainty associated with scaling from points to patches, land-use types, and the Phoenix metropolitan region. The multiple-scale analysis of soil properties showed that variation in total soil nitrogen, soil organic matter, and δ⁵N content of soils differed between patch and regional scales. The majority of variation in the urbanized patch types was exhibited between patches while for the native desert the majority of variation was observed within individual patches. These differences show the impact of urbanization on the scaling relations of ecosystem components. Overall, urbanization in this region appears to have increased soil organic matter by 44%, total nitrogen by 48%, and has elevated δ¹⁵N by 21%.