Nitrogen-cycling process rates across urban ecosystems

Abstract

Nitrogen (N) pollution of freshwater, estuarine, and marine ecosystems is widespread and has numerous environmental and economic impacts. A portion of this excess N comes from urban watersheds comprised of natural and engineered ecosystems which can alter downstream N export. Studies of urban N cycling have focused on either specific ecosystems or on watershed-scale mass balances. Comparisons of specific N transformations across ecosystems are required to contextualize rates from individual studies. Here we reviewed urban N cycling in terrestrial, aquatic, and engineered ecosystems, and compared N processing in these urban ecosystem types to native reference ecosystems. We found that net N mineralization and net nitrification rates were enhanced in urban forests and riparian zones relative to reference ecosystems. Denitrification was highly variable across urban ecosystem types, but no significant differences were found between urban and reference denitrification rates. When focusing on urban streams, ammonium uptake was more rapid than nitrate uptake in urban streams. Additionally, reduction of stormwater runoff coupled with potential decreases in N concentration suggests that green infrastructure may reduce downstream N export. Despite multiple environmental stressors in urban environments, ecosystems within urban watersheds can process and transform N at rates similar to or higher than reference ecosystems.

Seeing the light: urban stream restoration affects stream metabolism and nitrate uptake via changes in canopy cover

“…A review of N cycling across urban ecosystems found that various N spiraling metrics, which quantify different aspects of N cycling within streams (Stream Solute Workshop ), were as high or higher in urban streams compared to reference streams (Reisinger et al. ). This result shows that in spite of multiple stressors associated with the urban stream syndrome, urban streams remain capable of high rates of N uptake and removal.…”

“…Despite multiple physical, chemical, and biological stressors (Walsh et al 2005, Wenger et al 2009), urban streams remain capable of providing high rates of various ecosystem functions, even in unrestored states (Reisinger et al 2016). Nitrogen (N) cycling, in particular, represents a critical function of stream ecosystems across a range of human land uses (Mulholland et al 2008).…”

“…Understanding the capacity of urban streams to retain and remove nutrients prior to downstream export, and strategies to maximize these nutrient retention processes, is vital to reduce nutrient pollution of sensitive receiving water bodies. A review of N cycling across urban ecosystems found that various N spiraling metrics, which quantify different aspects of N cycling within streams (Stream Solute Workshop 1990), were as high or higher in urban streams compared to reference streams (Reisinger et al 2016). This result shows that in spite of multiple stressors associated with the urban stream syndrome, urban streams remain capable of high rates of N uptake and removal.…”

Recovery and resilience of urban stream metabolism following Superstorm Sandy and other floods

“…In fact, a recent review found that stream nitrogen uptake rates were similar between reference and urban streams across a range of urban settings (Reisinger et al. ), and urban stream networks are dynamic transformers of materials and energy, particularly during baseflow (Kaushal et al. , b).…”

“…These physicochemical and biological changes to urban streams are speculated to result in reductions in stream ecosystem functions such as metabolism and nutrient uptake, despite a limited amount of empirical evidence (Walsh et al 2005, Wenger et al 2009). In fact, a recent review found that stream nitrogen uptake rates were similar between reference and urban streams across a range of urban settings (Reisinger et al 2016), and urban stream networks are dynamic transformers of materials and energy, particularly during baseflow (Kaushal et al 2014a, b).…”