Seasonal soil nutrient dynamics in urban green spaces

Urbanization degrades surface waters through increased pollutant accumulation in urban watersheds and their transport by stormwater runoff. Green Stormwater Infrastructure (GSI) offers an opportunity to mitigate such non-point source pollution through processes such as filtration, plant uptake, and biogeochemical transformations. While GSI can reduce pollutant loads by runoff volume reduction, the efficiency of GSI to reduce stormwater pollutant concentrations, especially dissolved nutrients like Nitrogen (N) and Phosphorus (P), is less understood. GSI pollutant removal efficiency can vary substantially between sites and between storm events. Therefore, there is a need to better characterize soil N and P dynamics in GSI systems. Toward this end, we studied the seasonal variation and hydrological controls on nutrient availability in urban soils over a six-month period in three urban green spaces: an urban garden, a green roof, and a constructed wetland. Ion exchange resins (IERs) were buried for 2-week intervals (May to Sept 2019) to monitor in-situ nutrient availability. IER extractant was measured for nitrate, phosphate, ammonium, and metals. In addition, soil moisture and oxygen were recorded at 5-minute intervals. Sites with a larger drainage area than surface area showed higher metal concentrations associated with stormwater, confirming their function as points of pollutant accumulation. Nutrient availability varied with seasonal patterns of soil moisture, driven by rainfall variability and soil hydrology associated with landscape position, interception, and evapotranspiration. The role of seasonal soil moisture patterns in nutrient dynamics was confirmed by low oxygen availability. These results demonstrate variability in GSI soil nutrient processing across sites and seasons, driven in part by soil hydrology, indicating the ecohydrology of these engineered plant-soil systems may be an important control on stormwater treatment efficiency. The data also further underscore the complexity of hydro-biogeochemical function in spatially heterogeneous urban landscapes. This knowledge is important in order to develop GSI design and maintenance strategies that optimize nutrient removal and retention in an effort to mitigate urban surface water degradation from urbanization.