Is urbanization only skin deep? Deep soil cores in an arid urban ecosystem

Background/Question/Methods

Urbanization can significantly alter soils. Construction usually replaces native soils with construction fill, and urban management of yards includes replacing native with nonnative vegetation and addition of water and nutrients. Compacted, nutrient rich soils can have negative effects on downstream ecosystems by contributing excess runoff and nutrient pollution. Previous research has focused on urban surface soils, but deep soil profiles are useful for understanding soil nutrient storage and transformations. We asked: how does urbanization alter surface and deep soil properties? Do deep soil properties differ between urban land cover types (mesic vs xeric) and native desert cover? We collected 25 1m deep soil cores in the Phoenix, AZ metropolitan area from three land cover types: mesic residential, xeric residential, and native desert. We measured soil physical and hydraulic properties (soil texture, bulk density, soil moisture, soil organic content, and saturated infiltration rate), extractable nutrient content (nitrate (NO₃), ammonium (NH₄), phosphate (PO₄)) and chloride (Cl^-).  

Results/Conclusions

In contrast to other urban research, we found that infiltration rates were significantly higher in urban soils than desert soils. Within urban sites, infiltration was higher in xeric than mesic sites. Infiltration rates across sites were related to soil texture, especially sand and gravel content. NO₃ and NH₄ concentrations in desert surface soils were roughly equal, whereas N was found predominantly as NO₃ in urban soils. Although mesic yards are more likely to be fertilized, surface [NO₃] was higher in xeric sites. Surface [NO₃] were two orders of magnitude greater in urban than desert soils. Regression analysis indicated that urban soil [NO₃] is related to soil moisture and organic matter content, suggesting that [NO₃] in mesic yards may be low due to denitrification. Deep core [NO₃] profiles in desert sites suggested accumulation of NO₃ at depth. In mesic soils, [NO₃] is highest in surface soils and declines with depth, indicating biological removal. [NO₃] profiles in xeric sites were inconsistant, with some resembling native desert soils, and others resembling mesic sites. Due to high infiltration, low moisture and organic matter at most xeric sites, lack of accumulation in soil profiles may be due to hydrologic rather than biological mechanisms.