Investigating the relationship between hydrology and biogeochemistry in semi-arid urban green infrastructure

Tyler Rockhill

Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

Disproportionate population growth and urbanization in semi-arid and arid regions has led to alterations in the water, carbon (C), and nitrogen (N) cycles (Gallo et al. 2014), prompting demands for mitigation strategies.  Green Infrastructure (GI) is one of the methods used in urban storm water mitigation that delays and attenuates stormwater runoff by storing water in vegetated depressions.  In the Southwest these depressions, also called bioswales, also have the potential to act as biogeochemical hot spots, encouraging nutrient cycling, infiltration, plant growth, and microbial activity (McClain et al. 2003).  The influx of water to GI initiates a combined physical and microbial process that result in increased CO2 efflux and Nitrogen mineralization known as the Birch Effect. This study examines GI in Tucson, AZ through inducing an artificial precipitation regime and determining how soil properties, GI design, and biogeochemical characteristics influence the response. In natural systems it has been shown that soil moisture, soil properties, organic matter, length of dry period, nutrients such as Carbon and Nitrogen, and microbial biomass influence soil respiration and nitrogen mineralization [Wang et al. 2015, McClain et al. 2003, Gallo et a. 2014, McIntyre et al. 2009].  However, soils in manmade GI are inherently different, and the water chemistry, quantity, and sediment entering the GI are also developed differently than in a natural system.  The purpose of this study is to examine the Birch effect in urban GI due to wetting in a similar manner as natural ephemeral streams.  Additionally we seek to determine how soil and nutrient properties and precipitation regime affect the amplitude of the response.  The results of this study lend insight into how GI functions in an urban stormwater setting, which can be used to influence the design characteristics and spatial distribution of GI.  Specifically GI can be used to mitigate many of the issues associated with Urban Stream Syndrome (USS) such as flashier hydrography response, increased nutrient and contaminant concentrations, increased erosion, altered channel morphology and reduced biodiversity (Meyers et al. 2005). Knowledge about the nonlinear nature of biogeochemical hot spot reactions could be utilized to improve urban storm water quality and mitigate USS.

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