Contribution of Leaf Litter to Nutrient Export during Winter Months in an Urban Residential Watershed

Abstract

Identification of nonpoint sources of nitrogen (N) and phosphorus (P) in urban systems is imperative to improving water quality and better managing eutrophication. Winter contributions and sources of annual N and P loads from urban watersheds are poorly characterized in northern cities because monitoring is often limited to warm-weather periods. To determine the winter export of N and P, we monitored stormwater outflow in a residential watershed in Saint Paul, Minnesota during 2012-2014. Our data demonstrate that winter melt events contribute a high percentage of annual N and P export (50%). We hypothesized that overwintering leaf litter that is not removed by fall street sweeping could be an important source to winter loads of N and P. We estimated contributions of this source by studying decomposition in lawns, street gutters, and catch basins during two winters. Rates of mass and N loss were negligible during both winters. However, P was quickly solubilized from decomposing leaves. Using mass balances and estimates of P leaching losses, we estimated that leaf litter could contribute 80% of winter total dissolved phosphorus (TDP) loading in this watershed (∼40% of annual TDP loading). Our work indicates that urban trees adjacent to streets likely represent a major source of P pollution in northern cities. Management that targets important winter sources such as tree leaves could be highly effective for reducing P loading and may mitigate eutrophication in urban lakes and streams in developed cities.

Contrasting nitrogen and phosphorus budgets in urban watersheds and implications for managing urban water pollution

“…High road densities promoted runoff and losses during snowmelt periods (SI Appendix, Table S4). The annual storm drain exports presented here were approximately double warm season estimates of nutrient output for these watersheds (37), demonstrating the importance of wintertime and/or snowmelt processes in nutrient export from northern urban watersheds (38,39). Storm drain exports were highest in watersheds with high road density and no remnant surface waters (Phalen Creek and East Kittsondale) and lowest in watersheds with lakes and wetlands (SAP, Trout Brook West Branch, and Trout Brook Outlet) (37), which are important features for retaining both N and P (40,41).…”

Stormwater management network effectiveness and implications for urban watershed function: A critical review

“…During construction, low infiltration capacity through compaction of soils can lead to decreased functionality of infiltration SCMs (Line & White, ), and increased fertilization to establish vegetation can lead to increased dissolved nutrient export (Bedan & Clausen, ; Line & White, ). Over time, short‐ and long‐term changes in vegetation in SCMs, riparian zones, and other forested urban areas can be a sink for nitrogen through uptake and denitrification (Bettez & Groffman, ) but a source of phosphorus via leaf litter decomposition (Bratt et al, ; Selbig, ). These studies highlight the importance of matching stormwater controls with pollutant sources.…”

It Is Not Easy Being Green: Recognizing Unintended Consequences of Green Stormwater Infrastructure

“…While many regions are facing increased and intensifying periods of drought [82][83][84][85], the current trends in many other parts of the world are for increases in both the frequencies and magnitudes of not only extreme storm events but storms of all sizes [86]. Loading of urban stormwater contaminants, such as phosphorus, are expected to increase in colder climates because of increased winter rains [3,87]. Although loading is expected to decrease in arid climates from decreased rainfall and increased evaporation, concentrations in stormwater will be higher and continue to pose a risk [88].…”

“…Trees also produce organic matter, however, in the form of leaves, twigs, pollen, seeds, fruits, and nuts, and some forms are nutrient rich. Runoff encountering urban tree litter transports much of this organic matter and associated nutrients to storm drains [62,87], although this may not represent a major fraction of the nutrient load in all watersheds [64]. Thus, for the stormwater benefits of trees to be fully realized, the organic matter and nutrients from falling leaves and other debris must also be considered in tree placement strategies and street sweeping plans.…”