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Managing excess nutrients remains a major obstacle to improving ecosystem service benefits of urban waters. To inform more ecologically based landscape nutrient management, we compared watershed inputs, outputs, and retention for nitrogen (N) and phosphorus (P) in seven subwatersheds of the Mississippi River in St. Paul, Minnesota. Lawn fertilizer and pet waste dominated N and P inputs, respectively, underscoring the importance of household actions in influencing urban watershed nutrient budgets. Watersheds retained only 22% of net P inputs versus 80% of net N inputs (watershed area-weighted averages, where net inputs equal inputs minus biomass removal) despite relatively low P inputs. In contrast to many nonurban watersheds that exhibit high P retention, these urban watersheds have high street density that enhanced transport of P-rich materials from landscapes to stormwater. High P exports in storm drainage networks and yard waste resulted in net P losses in some watersheds. Comparisons of the N/P stoichiometry of net inputs versus storm drain exports implicated denitrification or leaching to groundwater as a likely fate for retained N. Thus, these urban watersheds exported high quantities of N and P, but via contrasting pathways: P was exported primarily via stormwater runoff, contributing to surface water degradation, whereas N losses additionally contribute to groundwater pollution. Consequently, N management and P management require different strategies, with N management focusing on reducing watershed inputs and P management also focusing on reducing P movement from vegetated landscapes to streets and storm drains.

As cities grow, lakes are often assumed to suffer from increasing non‐point pollution. Many waterbodies have become more eutrophic in recent decades, as expected—but many others became less eutrophic, especially in urban/suburban areas. What policies, practices, and ecosystem processes have helped some lakes stay stable or become less eutrophic even in a growing city? Identifying and understanding success stories are important to continue protecting these lakes and improving other urban/suburban lakes. We found one such success story when we examined water‐quality trends over the past 25 years (1998–2022) in Lake Washington, a well‐studied large lake in the Seattle metro area. The watershed population grew rapidly during that time (34% from 2000 to 2020), yet Lake Washington became substantially less eutrophic and indicators of development impacts stabilized or decreased. Chlorophyll concentrations during the main spring bloom decreased sharply (−25% per decade), and water clarity and near‐bottom dissolved oxygen both increased (8.5% and 17% per decade, respectively). Alkalinity and specific conductance had increased during the 1970s–1990s, but in recent decades, they held stable. Peak winter/spring nitrogen and phosphorus concentrations decreased (−4.9% and −5.6% per decade, respectively), indicating decreased watershed inputs. The type of development during this time was likely a key contributor: we found no net loss of forest area and little increase in developed land area (4.7% from 2001 to 2021). Instead of expanding into new areas, redevelopment increased density on already‐developed land and likely drove improvements in stormwater treatment and other environmental protections. Future work comparing stream watersheds could help discern which specific aspects of redevelopment helped reduce nutrients and other impacts. However, nutrient reductions were not the only factors controlling the lake's trophic state; chlorophyll decreased much more strongly than phosphorus did. Lake Washington is a complex ecosystem governed not only by water chemistry but also by interactions with physical and biological factors such as stratification, warming, phytoplankton community shifts, or food‐web interactions. A better understanding of all these factors is essential to provide sound scientific guidance and ensure that Lake Washington and other lakes can thrive in a growing city.

Studies in both terrestrial and aquatic ecosystems have documented the potential importance of consumers on ecosystemlevel nutrient dynamics. This is especially true when aggregations of organisms create biogeochemical hotspots through nutrient consumption, assimilation, and remineralization via excretion and egestion. Here, we focused on aggregations of humans in cities to examine how diet and waste management interact to drive nitrogen- (N) and phosphorus- (P) fluxes into nutrient pollution, inert forms, and nutrient recycling. We constructed six diet patterns (five US-based and one developing nation) to examine N- and P-consumption and excretion, and explored their implications for human health. Next, we constructed six waste-management patterns (three US and three for developing nations) to model how decisions at household and city scales determine the eventual fates of N and P. When compared to the US Recommended Daily Intake, all US diet patterns exceeded N and P requirements. Other than the “enriched CO2 environment scenario” diet, the typical US omnivore had the greatest excess (37% N and 62% P). Notably, P from food additives could account for all of the excess P found in US omnivore and vegetarian diets. Across all waste-management approaches, a greater proportion of P was stored or recycled (0 to > 100% more P than N) and a greater proportion of N was released as effluent (20 to > 100% more N than P) resulting in pollution enriched with N and a recycling stream enriched with P. In developing nations, 60% of N and 50% of P from excreta entered the environment as pollution because of a lack of sanitation infrastructure. Our study demonstrates a novel addition to modeling sustainable scenarios for urban N- and P-budgets by linking human diets and waste management through socio-ecological systems.

Many urban waterways suffer from excess nitrogen (N) and phosphorus (P), feeding algal blooms, which cause lower water clarity and oxygen levels, bad odor and taste, and the loss of desirable species. Nutrient movement from land to water is likely to be influenced by urban vegetation, but there are few empirical studies addressing this. In this study, we examined whether or not urban trees can reduce nutrient leaching to groundwater, an important nutrient export pathway that has received less attention than stormwater. We characterized leaching beneath 33 trees of 14 species, and seven open turfgrass areas, across three city parks in Saint Paul, Minnesota, USA. We installed lysimeters at 60 cm depth to collect soil water approximately biweekly from July 2011 through October 2013, except during winter and drought periods, measured dissolved organic carbon (C), N, and P in soil water, and modeled water fluxes using the BROOK90 hydrologic model. We also measured soil nutrient pools (bulk C and N, KCl‐extractable inorganic N, Brays‐P), tree tissue nutrient concentrations (C, N, and P of green leaves, leaf litter, and roots), and canopy size parameters (leaf biomass, leaf area index) to explore correlations with nutrient leaching. Trees had similar or lower N leaching than turfgrass in 2012 but higher N leaching in 2013; trees reduced P leaching compared with turfgrass in both 2012 and 2013, with lower leaching under deciduous than evergreen trees. Scaling up our measurements to an urban subwatershed of the Mississippi River (

Studies in both terrestrial and aquatic ecosystems have documented the potential importance of consumers on ecosystem‐level nutrient dynamics. This is especially true when aggregations of organisms create biogeochemical hotspots through nutrient consumption, assimilation, and remineralization via excretion and egestion. Here, we focused on aggregations of humans in cities to examine how diet and waste management interact to drive nitrogen‐ (N) and phosphorus‐ (P) fluxes into nutrient pollution, inert forms, and nutrient recycling. We constructed six diet patterns (five US‐based and one developing nation) to examine N‐ and P‐consumption and excretion, and explored their implications for human health. Next, we constructed six waste‐management patterns (three US and three for developing nations) to model how decisions at household and city scales determine the eventual fates of N and P. When compared to the US Recommended Daily Intake, all US diet patterns exceeded N and P requirements. Other than the “enriched CO 2 environment scenario” diet, the typical US omnivore had the greatest excess (37% N and 62% P). Notably, P from food additives could account for all of the excess P found in US omnivore and vegetarian diets. Across all waste‐management approaches, a greater proportion of P was stored or recycled (0 to > 100% more P than N) and a greater proportion of N was released as effluent (20 to > 100% more N than P) resulting in pollution enriched with N and a recycling stream enriched with P. In developing nations, 60% of N and 50% of P from excreta entered the environment as pollution because of a lack of sanitation infrastructure. Our study demonstrates a novel addition to modeling sustainable scenarios for urban N‐ and P‐budgets by linking human diets and waste management through socio‐ecological systems.

Urban nutrient sustainability faces challenges of both too much and too little: Excess nutrient loading to the environment can degrade ecosystem functions and impact human health, while at the same time depleting nonrenewable nutrient sources and moving nutrients into unrecoverable pools. Most studies and efforts to date have focused on source reduction, identifying and reducing the largest drivers of carbon (C), nitrogen (N), and phosphorus (P) consumption. However, this addresses only one aspect of urban nutrient cycling; processes that transport, transform, or retain nutrients also determine their eventual fate as pollution, inert storage, or recycling. The first chapter examined C, N, and P output fluxes from ∼2,700 households in the Twin Cities metropolitan area (Minneapolis-Saint Paul, Minnesota, USA), and tracked these fluxes through various transformations in the waste streams to their eventual fates. We found few opportunities to redirect pollutant fluxes to either inert storage or recycling; reducing household nutrient pollution must rely primarily on reducing consumption. High pollution fluxes were driven not only by household nutrient outputs, but also by waste-management practices (e.g. septic vs. sewer) and spatial considerations. In contrast, we found substantial opportunities to increase household N and P recycling by ten-fold, which could potentially exceed household inputs of N and P in food. To complement this study of opportunities for improving nutrient waste management, the second and third chapters examined opportunities to manage the biophysical environment - specifically, the urban forest - to reduce nutrient pollution. We focused on the role of urban trees driving N and P movement from land to water, both leaching to groundwater and loading to stormwater. In the second chapter, we compared nutrient leaching under 33 trees of 14 species, as well as open turfgrass areas, and explored correlations with soil nutrient pools and plant functional traits. Trees had similar or lower N leaching than turfgrass in 2012 but higher N leaching in 2013; trees reduced P leaching compared with turfgrass in both 2012 and 2013, deciduous trees more than evergreens. Scaling up our measurements to the Capitol Region Watershed (∼17,400 ha), we estimated that trees reduced P leaching to groundwater by 533 kg in 2012 and 1201 kg in 2013. Removing the same amounts of P with stormwater infrastructure would cost $2.2 million and $5.0 million per year, respectively. In the third chapter, we measured tree litter nutrient inputs to street gutters, which can ultimately contribute to stormwater loading, under four species of boulevard trees. Differences among tree species in the total amount of nutrients in the street gutters were driven primarily by interspecific differences in the mass of litter dropped, which were much greater than differences in litter chemistry. In developing management recommendations, we found that tree phenology is a more important consideration than litter chemistry. Cleaning up spring and autumn pulses of tree litter shortly after they fall has substantial potential to reduce nutrient inputs to stormwater; for autumn litterfall, we estimated that doing so could remove 219.0-274.4 kg N km-2 and 14.2-20.6 kg P km-2. Because of the wide variation in species' litterfall timing, achieving this goal is likely to require adjusting both boulevard tree selection and litter cleanup strategies.

Eutrophication of urban surface waters from excess nitrogen (N) and phosphorus (P) inputs remains a major issue in water quality management. Although much research has focused on understanding loading of nutrients from storm events, there has been little research to understand the contribution of baseflow, the water moving through storm drains between rainfall events. We investigated the relative contributions of baseflow versus stormflow for loading of water and nutrients (various forms of N and P) by the storm drain network in six urban sub-watersheds in St. Paul, MN, USA. Across sites, baseflow made substantial contributions to warm season (May–October) water yields (27–66 % across sites), total N yields (31–68 %), and total P yields (7–32 %). These results show that while P was predominantly delivered by stormflow, N loading was similar between baseflow and stormflow. We found that baseflow was dominated by groundwater inputs, likely caused by interception of shallow groundwater by storm drains, but also that variability in N and P among sites was related in part to the connectivity of the storm drains to upstream lakes and wetlands in some watersheds. The substantial loading by groundwater-dominated baseflow, especially for N, implies that N management may require a broader focus on N source reduction, perhaps through improved land management, in order to prevent contamination of shallow groundwater via infiltration.

Leaf litter may be an important source of nutrients to stormwater and ultimately contribute to eutrophication of surface waters associated with urbanization. Thus, understanding decomposition and nutrient release from leaf litter that falls on impervious surfaces is important for stormwater management. However, few studies have examined leaf litter decomposition in the unique urban environment of the street gutter. We compared decomposition of leaf litter of five street tree species in a parking lot gutter in St. Paul, Minnesota, USA. In contrast to our expectations, comparisons with past studies revealed that litter decomposed more rapidly in the gutter than in nearby natural areas. And decomposition rates were as rapid as those measured in other urban settings (forests and streams), with most species losing 80 % of their initial mass after 1 year. Litter of most species had retained more than half of its initial N and P after 1 year. However, in contrast to N, litter P dynamics largely were uncoupled from litter mass dynamics, with litter P increasing and decreasing unpredictably over the year. Short-term (24 h) laboratory studies revealed that litter had the potential to lose a high fraction of its initial P, with high variation among species (from 27 to 88 %), and a smaller fraction of its initial N (<10 %) via leaching. Thus, street tree species may differ in their potential contributions to nutrients that are released during decomposition. Our results suggest that careful selection of street tree species and timely removal of litterfall have significant potential to reduce nutrient fluxes from streets to storm drains, particularly for P.