Quantifying Urban Bioswale Nitrogen Cycling in the Soil, Gas, and Plant Phases

Bioswales are a common feature of urban green infrastructure plans for stormwater management. Despite this fact, the nitrogen (N) cycle in bioswales remains poorly quantified, especially during dry weather in the soil, gas, and plant phases. To quantify the nitrogen cycle across seven bioswale sites located in the Bronx, New York City, we measured rates of ammonium and nitrate production in bioswale soils. We also measured soil nitrous oxide gas emissions and plant foliar nitrogen. We found that all mineralized nitrogen underwent nitrification, indicating that the soils were nitrogen-rich, particularly during summer months when nitrogen cycling rates increase, as indicated by higher levels of ammonium in the soil. In comparison to mineralization (0 to 110 g N m(-2) y(-1)), the amounts of nitrogen uptake by the plants (0 to 5 g N m(-2) y(-1)) and of nitrogen in gas emissions from the soils (1 to 10 g N m(-2) y(-1)) were low, although nitrous oxide gas emissions increased in the summer. The bioswales' greatest influx of nitrogen was via stormwater (84 to 591 g N m(-2) y(-1)). These findings indicate that bioswale plants receive overabundant nitrogen from stormwater runoff. However, soils currently used for bioswales contain organic matter contributing to the urban nitrogen load. Thus, bioswale designs should use less nitrogen rich soils and minimize fertilization for lower nitrogen runoff.