Stormwater Green Infrastructure (SWGI) are used to help municipalities reduce pollutant loads, receiving large amounts of stormwater that is detained for treatment before being released. The Birch Effect is a phenomenon observed when dry soils are exposed to moisture, evidenced by a release of N and CO2. Microbial activity underlies this effect, and in SWGI microbial activity causes denitrification, used to reduce nutrient loading. Despite the importance of these wet-dry dynamics, they remain poorly characterized in SWGI features. This study explores how decentralized and centralized SWGI experience the Birch Effect, imposing drought and wet conditions on mesocosms built from soils from extended dry ponds and rain gardens in Montgomery County, MD. Soils are characterized by total C and N, pH, organic matter, and initial microbial biomass. Two treatments are used that vary the duration of drought (precipitation once a week vs. twice a month). Microbial biomass and denitrification potential are measured before and after the experiment. Respiration rates and N2O fluxes are measured to assess microbial activity and nutrient processing.
Results/Conclusions
By comparing bioretention and extended dry ponds, this study brings an understanding of how these different types of practices react to different precipitation regimes. Mesocosm collected after incubations found that rain garden soils had a higher average % SOM content. The extended detention pond soil showed an increase in CO2production per gram of soil C. After two weeks of water additions, both the rain garden and extended detention pond soil showed a decrease in CO2 produced per gram of soil C, or a decrease in the birch effect. The mesocosms exposed to water only once in two weeks did not show a change after initial wet up as well. Soils exposed to extended dry periods have been found to have a lag period in the spike in CO2 production per gram of soil C ,as well as bacterial growth. A lag time may explain why soils did not show in increase in CO2 production per gram of soil C after the second addition.
SWGI is being implemented to help communities become more resilient in the face of more intense and variable precipitation events. Little is known however, about how these structures react to these events, and what the appropriate maintenance actions should be. Studies like this hope to inform managers on what transformations occur in these structures over time, and will provide guidance on how to properly maintain them.