Seasonal Hydrologic and Biogeochemical Drivers of Nutrient Availability in Urban Green Spaces

Humans dominate urban nutrient cycles – adding nitrogen (N) and phosphorus (P) through car exhaust, fertilizer over-use, and pet waste. N and P are transported to streams via stormwater runoff, where they create eutrophic conditions and reduced water quality. Urban green spaces, like green stormwater infrastructure (GSI), have potential to mitigate N and P pollution through retention in soils and plants. Nutrient retention is accomplished in urban green spaces through filtration, plant uptake, and biogeochemical processes. These processes are highly variable, due to the influence of varying environmental drivers, such as temperature, rainfall, soil moisture, and soil redox status. This work studies seasonal variation in nutrient mobility and corresponding hydrologic and biogeochemical conditions in urban soils. Ion exchange resins were used to capture the seasonality of soil nutrient availability. Environmental drivers such as soil and air temperature, soil moisture, soil oxygen, and precipitation were also monitored. Sites included a green roof, upland and lowland plots in a constructed wetland, and an urban garden in Milwaukee. Using multiple linear regression models, different environmental drivers predicted nutrient availability across sites. High nitrate pulses occurred in the wetland lowland in the summer following dry conditions, which contrasted low and stable nitrate availability in the wetland upland. Across all upland sites, phosphate mobilization was strongly correlated with precipitation, indicating mobilization of soil phosphorus pools. These patterns indicate varying roles of hydrologic and biogeochemical drivers for N and P availability in urban green spaces. The observed relationships can be used to better understand nutrient retention and dynamics in urban green spaces under variable hydrological and biogeochemical conditions