Incorporating soil ecological knowledge into green infrastructure design: ecosystem services and rain gardens in a semi-arid city

Background/Question/Methods

As rates of urbanization continue to rise and a greater proportion of the population lives in urban and suburban areas, the provision of ecological services becomes increasingly important to for the sustainability and resilience of cities. Soils play a primary role in the healthy functioning of ecosystems that provide supporting, provisioning, regulating, preserving, and cultural ecosystem services. Soils in urban ecosytems are highly heterogeneous, and are highly modified in cities, reflecting both direct and indirect effects.  Soils can be manipulated as key control points to affect the function of urban ecosystems, drawing on approaches from green design to result in designer and novel ecosystems. Here I discuss one such approach, rain garden basins in and around Tucson, AZ, that have been installed in the urban landscape with the purpose of providing various ecosystem services to local residents and the greater urban ecosystem. This study was conducted in summer 2010, and expands on previous research on soil quality of rain basins in the Tucson urban ecosystem. This study compares soil properties, microbial and microfaunal communities and function, and ecosystem functions (C and N flux rates) in basins with different management strategies with the goals of (1) assessing the mechanisms by which green infrastructure alters and improves urban soils, and (2) determine best management practices for providing ecosystem services in desert and semi-arid cities. 

Results/Conclusions

Soils in arid rain gardens have higher organic matter contents than both native and urban soils, and correspondingly, greater microbial function and abundance, a more complex soil food web, and higher rates of N-mineralization.  The specific local scale management of these rain gardens has important implications for the degree to which soil quality is improved and ecosystem services are provided. The inclusion of mulch in the garden design is a critical design element that incorporates soil ecological knowledge to improve ecosystem function from a restorative perspective.  Using mulch in the basin construction doubles the percent soil organic matter content in the basins and helps increase soil water content.  Basins with mulch support greater microbial biomass and have a 20% increase in microbial activity.  Importantly, these alterations to soil C pools and microbial function reduce the rate of N-mineralization by 15%, suggesting that the incorporation of mulch helps to immobilize mineral N.  The incorporation of soil ecological knowledge in these small scale green infrastructure installations have a cascading effect to the provision of ecosystem services in the urban environment.