Urbanization Alters Soil Microbial Functioning in the Sonoran Desert

Cities can transform ecosystems in multiple ways, through modification of land use and land cover and through exposure to altered physical, chemical, and biological conditions characteristic of urban environments. We compared the multiple impacts of urbanization on microbial carbon (C) and nutrient cycling in ecosystems across Phoenix, Arizona, one of the fastest growing metropolitan areas in the USA. Land-use/land-cover change from desert to managed ecosystems altered soil microbial functioning, primarily through changes in organic matter supply. Although residential xeriscapes often feature native plants and patchy structure like deserts, spatial heterogeneity in soil biogeochemical cycling was not tightly linked to plant canopies. Grassy lawns exhibited higher nitrogen (N) and phosphorus demand by microorganisms than other landscape types, suggesting that high C quality may effectively sequester these nutrients during periods between fertilization events. Soils in native desert remnants exposed to the urban environment had higher organic matter content, but supported lower activities of extracellular peroxidase enzymes compared to outlying deserts. Experimental N enrichment of desert systems decreased peroxidase activities to a similar extent, suggesting that protected desert remnants within the city are receiving elevated N loads that are altering biogeochemical functioning. Although some microbial processes were spatially homogenized in urban desert remnants, resource islands associated with plants remain the dominant organizing factor for most soil properties. The extent to which native desert preserves within the city functionally resemble managed xeriscapes and lawns suggests that these remnant ecosystems are being 'domesticated' by exposure to the urban environment.