Urban Base Cations: Fate, Transport, and Effect on Nitrogen Cycling Genes in Green Stormwater Infrastructure

Urbanization has significantly changed the natural cycling of base cations, primarily due to the use of deicer salts, such as sodium chloride, and chemical weathering of limestone and concrete infrastructure. An unintended consequence is the accumulation of salt across urban settings. These base cations, transported mainly by stormwater runoff, accumulate in soils, streams, lakes, and groundwater, posing risks to soil health, water quality, and ecosystems. To mitigate stormwater runoff and pollutants, cities have adopted stormwater best management practices, shifting from traditional gray to hybrid gray-green infrastructure. As a result, salt-laden runoff is increasingly routed into green stormwater infrastructure (GSI), where salts can accumulate. This accumulation may deteriorate soil structure, inhibit infiltration, increase erosion, hinder plant growth, alter biogeochemical cycles, and reduce microbial biomass, activity, and diversity. Elevated salt content in the soil is an acknowledged concern that impedes the GSI soil’s capacity to fulfill key functions such as infiltrating and evapotranspiring stormwater runoff and treating excess nitrogen (N). However, long-term salt accumulation and its potential to cause salinization and sodification remain uncertain. This dissertation investigated the question through three objectives. Objective 1 demonstrated that GSI soils retain base cation and can accumulate them at levels that may lead to salinization and sodification. Objective 2 examined how soil texture, age of GSI soils, and winter temperatures influence salt parameters and base cation distribution in GSI compared to natural soils. These factors were found to explain the spatial variability of base cations, with relationships differing over time and among individual base cations. Objective 3 shifted focus to N cycling, finding that the composition and diversity of N cycling genes had low dissimilarity across GSI types and catchment areas. However, fines percentage and the concentration of salts potassium (K) and sodium (Na) were significant factors influencing the composition and diversity of N cycling, with these relationships varying among and within the metabolic pathways. Overall, this work identifies critical factors driving salt and nutrient dynamics in GSI soils. The findings offer a foundation for developing effective strategies to manage long-term salt impacts, supporting the sustainability and performance of GSI in urban stormwater management.