GREEN INFRASTRUCTURE PRESENCE AND MAINTENANCE AFFECTS SOIL HYDROLOGIC AND BIOGEOCHEMICAL PROPERTIES

Green infrastructure is being implemented in many cities to ameliorate water quantity, water quality, heat island and other environmental impacts of urbanization. While these systems have been designed and investigated from a theoretical perspective there are fewer on the ground studies of how these systems function in the environment over time and as affected by human maintenance of these systems. We have conducted widespread characterization of bioswale systems in the semi-arid city of Tucson, Arizona. In these studies we have characterized soil organic matter, biogeochemical response, and soil hydraulic properties. Furthermore we have investigated how design and maintenance of these systems impacts their function. Our results are as follows. First, green infrastructure does cause increased soil organic matter, 8-10% content versus 1-2% in nearby untreated soils, and does so even over long time scales that are unlikely to be due solely to installation practices. Second, the increased soil organic matter also results in an increased respiration response when these soils are rewetted. Third, the green infrastructure soils have increased water holding capacity, 40-60% in green infrastructure soils versus 25-35% in untreated soils. Fourth, green infrastructure soils have higher hydraulic conductivity and lower bulk density than nearby untreated soils. Fifth, the effect on hydraulic conductivity is uneven. Hydraulic conductivity is often lower in soils with little to no mulch as fine sediments appear to clog the surface. Additionally maintenance practices have an effect on infiltration rates. Somewhat unexpectedly soils that are maintained closer to standard landscape aesthetic standards led to lower infiltration rates versus systems that were described as poorly maintained. The likely cause of this effect of maintenance is that maintenance recommendations consist of practices that result in reduced soil organic matter and this reduction likely leads to lower infiltration rates. These results of this research indicate that modifications to soil biogeochemistry and hydrology by green infrastructure are important and have a variety of interactions with design and maintenance of these systems.