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Journal of Integrative Environmental Sciences
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Urban nitrogen budgets: flows and stock changes
of potentially polluting nitrogen compounds in
cities and their surroundings – a review
Wilfried Winiwarter , Barbara Amon , Zhaohai Bai , Andrzej Greinert , Katrin
Kaltenegger , Lin Ma , Sylwia Myszograj , Markus Schneidergruber , Monika
Suchowski-Kisielewicz , Lisa Wolf , Lin Zhang & Feng Zhou
To cite this article: Wilfried Winiwarter , Barbara Amon , Zhaohai Bai , Andrzej Greinert ,
Katrin Kaltenegger , Lin Ma , Sylwia Myszograj , Markus Schneidergruber , Monika Suchowski-
Kisielewicz , Lisa Wolf , Lin Zhang & Feng Zhou (2020) Urban nitrogen budgets: flows and stock
changes of potentially polluting nitrogen compounds in cities and their surroundings – a review,
Journal of Integrative Environmental Sciences, 17:1, 57-71, DOI: 10.1080/1943815X.2020.1841241
To link to this article: https://doi.org/10.1080/1943815X.2020.1841241
© 2020 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Published online: 15 Dec 2020.
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Urban nitrogen budgets: ows and stock changes of
potentially polluting nitrogen compounds in cities and their
surroundings – a review
, Barbara Amon
, Zhaohai Bai
, Andrzej Greinert
, Lin Ma
, Sylwia Myszograj
, Markus Schneidergruber
, Lisa Wolf
, Lin Zhang
and Feng Zhou
International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria;
Institute of Environmental
Engineering, University of Zielona Góra, Zielona Góra, Poland;
Leibniz Institute for Agricultural Engineering
and Bioeconomy (ATB), Potsdam, Germany;
Center for Agricultural Resources Research, Chinese Academy
of Sciences, Shijiazhuang, Hebei, China;
brainbows, Vienna, Austria;
E.C.O. Institute of Ecology, Klagenfurt,
Laboratory for Climate and Ocean–Atmosphere Studies, Department of Atmospheric and Oceanic
Sciences, School of Physics, Peking University, Beijing, China;
College of Urban and Environmental Sciences,
Peking University, Beijing, China
Concepts of material ow and mass consistency of nitrogen com-
pounds have been used to elucidate nitrogen’s fate in an urban
environment. While reactive nitrogen commonly is associated to
agriculture and hence to large areas, here we have compiled scien-
tic literature on nitrogen budget approaches in cities, following
the central role cities have in anthropogenic activities generally.
This included studies that specically dealt with individual sectors
as well as budgets covering all inputs and outputs to and from a city
across all sectors and media. In the available data set, a clear focus
on Asian cities was noted, making full use of limited information
and thus enable to quantitatively describe a local pollution situa-
tion. Time series comparisons helped to identify trends, but com-
parison between cities was hampered by a lack of harmonized
methodologies. Some standardization, or at least improved refer-
ence to relevant standardized data collection along international
norms was considered helpful. Analysis of results available pointed
to the following aspects that would reveal additional benchmarks
for urban nitrogen budgets: analysing the share of nitrogen that is
recycled or reused, separating largely independent sets of nitrogen
ows specically between food nitrogen streams and fossil fuel
combustion-related ows, and estimating the stock changes for
the whole domain or within individual pools.
Received 16 December 2019
Accepted 12 October 2020
Nitrogen budget; urban
chain; nitrogen cascade;
material ﬂow analysis
On a global scale, human activities greatly accelerated the cycle of reactive nitrogen
compounds – reactive nitrogen (Nr) representing all chemical forms of N except for the
dominant stable form of molecular N
. At currently about twice its natural level (Fowler
CONTACT Wilfried Winiwarter email@example.com International Institute for Applied Systems Analysis (IIASA),
A-2361 Laxenburg, Austria
JOURNAL OF INTEGRATIVE ENVIRONMENTAL SCIENCES
2020, VOL. 17, NO. 1, 57–71
© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
et al. 2013), environmental impacts of Nr to air, water, soils, climate and biodiversity have
been observed and investigated at dierent scales (Sutton et al. 2011; Erisman et al. 2013).
As nutrients to boost plant production for human consumption are a key element to the
increase observed, analysis logically focused on agriculture as one key sector, hence
predominantly covering area sources. Administrative regions such as countries that
provide plentiful statistical data therefore are considered adequate to investigate the
fate of nitrogen compounds, e.g. in the form of national nitrogen budgets (Leip et al.
2011). On a global scale, the nitrogen cycle is believed to be one of the very few conrmed
examples that human activities already now have exceeded the “planetary boundary” of
a safe and sustainable earth (Rockström et al. 2009; Steen et al. 2015).
Anthropogenic activities focus on cities. With, globally, 55% of the population living in
cities, a number that is expected to increase to 68% by 2050 (UN 2018), cities provide an
immense opportunity for interventions in malfunctioning systems on a global scale.
Already now, about 80% of GDP is generated in cities (Grübler and Fisk 2012).
Interventions on an urban scale have proven to be more ecient and less time-
consuming than similar action on a national or country level, which often require
legislative procedures and involvement of a large number of stakeholders. Hence also
several policy processes that previously had been under the authority of national govern-
ments have started their urban initiatives and show potential towards signicant progress
(Kuramochi et al. 2019).
Here we hypothesize that not only their high population density make cities a clear
target for successful environmental remedies (in a combination of enhanced impacts and
the associated political strength) also in relation to Nr. We argue that, further to that,
urban lifestyle, urban practices and production patterns directly inuence the local
nitrogen cycle, so that action in cities and around cities, in the peri-urban space, can
lead to immediate benets that a large share of the population prot from. While Nr and
the “nitrogen cascade” (Galloway et al. 2003) typically are associated with agricultural
activity (e.g., Galloway et al. 2008), reports occasionally point to predominant impacts
from urban sources, e.g. on urban air pollution related to Nr (Pan et al. 2016). Moreover,
urban and peri-urban agriculture may specically aect the environment (Zhao et al.
2017). The role of cities for a future global food system (and their relationship to green-
house gases) has just recently been emphasized (Pradhan et al. 2020).
In order to understand the level of support given by science to tackle potential impacts
related to urban Nr ows, we investigated the scientic literature for their use of nitrogen
budget approaches to cities. Specically, we searched for the terms “urban” in combina-
tion with either “nitrogen budgets” or “nitrogen balance”. Both Google Scholar (titles
only) and Scopus (title, keywords and abstract) were used. From the resulting overview of
papers, we selected those that specically attempted to quantify physical ows of Nr
compounds in a clear urban setting (areas within city administrative boundaries or peri-
urban regions with high population density). Only physical ows were covered, indirect
eects or footprints were not part of this analysis. We noted that a sizable number of
studies focused on specic sectors or environmental media only so that we had to analyse
them separately from the investigations of full budgets that addressed interactions
between all sectors. The set of publications was extended by relevant articles cited in
these papers, as well as by articles referencing them. Specic attention was given to
identifying issues that could characterize Nr ows or make them comparable between
58 W. WINIWARTER ET AL.
individual studies. All of this material allowed us to assess which approaches these studies
have in common, what results deserve particular attention and in which areas further
specic development of urban nitrogen budgets are needed.
This paper is structured as follows. Evidence and examples of the importance of
sectoral nitrogen budgets on urban scales are shown in section 2, while section 3 reports
on full urban nitrogen budgets. These two sections provide an overview of the breadth of
approaches and the diversity of concepts. In section 4 we identify common elements from
these studies and conclude in section 5.
Sectoral views on nitrogen inputs and outputs
Nitrogen budgets use material ow approaches to constrain the amount of Nr by
quantifying ows in and out of a system boundary, the sources and sinks within the
boundary and stock changes. The method allows to accommodate analysis of individual
sectors, or of environmental media that commonly oer storage and transport functions
for Nr. For the latter, observations along homogeneous physical media (soil, water,
atmosphere) are available, for the former budget studies on specic sectors of anthro-
pogenic activities have been made (food system or waste and wastewater). Dierences in
system boundaries, substrates considered, and sectors covered provide considerable
challenges to compare results of published studies.
Soil: Soil nitrogen budgets have been a classical instrument in agronomy, hence their
application on urban agricultural areas was merely a logical extension. Hou et al. (2012)
studied smallholder farms in peri-urban Beijing, and they observed N surplus about three
times as high in vegetable systems (nearly 1600 kg N ha
) compared to orchards or
cereal production. While reported variability between farmers was high and quantity
clearly depended on practices, the relevance of cropping type and especially of quantity
of vegetable production was shown clearly. A totally dierent system, application of
mineral fertilizers on private gardens and lawns, was investigated by Law et al. (2004)
for Baltimore. Again characterized by huge variability, average application rates of almost
100 kg N ha
were determined for systems that would not target for production.
Analyses also were available under N decit conditions (Tadesse et al. 2019) in two peri-
urban regions in Ethiopia. Livestock-oriented mixed farms had higher income, obtained
external sources of N and hence 4–5 times higher N input, but a poor nitrogen use
eciency (NUE) compared to crop-oriented mixed farms. High-input systems, at least in
relative terms, have been observed in urban agriculture in Ghana (Werner et al. 2019),
where the widespread use of wastewater for irrigation led to nitrate leaching losses as
high as 200 kg N ha
Water: With water being an essential transport medium for Nr, budgets on riverine
N were available also on an urban scale. Barles and Lestel (2007) focused on a historical
situation, nineteenth-century Paris, and early quantication attempts of nitrogen losses
along rivers. These authors also described eorts to reclaim urban nitrogen for agronomic
purposes, in a situation where nitrogen was a scarce resource. Dierences between
measured upstream and downstream concentrations of nitrogen compounds have
been used to identify the urban contributions to the pollution of the Po river near the
city of Turin, Italy, and to estimate the phenomena responsible (Genon and Marchese
2007). Extending such an approach to the total upstream area led to consideration of
JOURNAL OF INTEGRATIVE ENVIRONMENTAL SCIENCES 59
watersheds, a logical spatial delineation when working with rivers and water bodies as
transport media. Watersheds provided the basis for inverse modelling by Divers et al.
(2013) to quantify sewage leakage in an urban watershed of Pittsburgh, PA, at rates
between 6 and 14 kg N ha
, based on biweekly water sampling and measurements.
Urban farming impacts on water quality were studied by Cameira et al. (2014) for
vegetable gardens in Lisbon (Portugal), with mean observed drainage concentrations of
almost 300 mg L
and N accumulation rates in lower soil depth found close to
100 kg ha
. A combination of dedicated measurements and an inventory approach
was taken to assess the N budget of a small watershed in the city of Changchun (Jilin
province, North-Eastern China). The study area, agricultural land constituting a major
source of drinking water for the city, received close to 190 kg N ha
as inputs, and
outputs were about 100 kg N ha
(including N contained in product, leaching and
denitrication), indicating a considerable N accumulation in soil due to human activities
(Song and Liu 2013). Human impacts were also quantied to a large water body next to
a city, the Lake Pontchartrain next to New Orleans (LA), by Turner et al. (2002). While still
nitrogen limited, the authors nd a 10-fold increase in nitrogen loading due to anthro-
pogenic activities, most of all by water advection. N loads could triple easily when ood
protection gates were being opened to divert Mississippi river water into the lake, or
water diversion projects for wetland protection were implemented, demonstrating the
environmental trade-os encountered in urban environments. In a statistical analysis of
measurements of ionic component (ammonia, nitrate, nitrite) from rivers in Moscow,
Russia, Laryushkin-Zheleznyi and Novikov (2005) identied unimodal distributions for
nitrite, but often bimodal distribution for other N forms. They used correlations between
the respective components to develop a kinetic model describing nitrication and deni-
trication processes, suggesting maximum admissible levels of total ionic N of 3.5 mg L
of 1.3 mg L
to prevent assimilation of N species by aquatic organisms.
Atmosphere: Nitrogen budgets seemed to play much less of a role for the other
important transport medium, the atmosphere. Urban airshed and air quality modelling
provided a detailed account of a number of trace constituents, their distribution, conver-
sion, and deposition to the surface, but the atmospheric reality was considered much less
appropriate to take advantage of specically limiting nitrogen. Instead, other elements of
conversion took a role, like the occurrences of urban ozone pollution formed by photo-
chemical reactions of nitrogen monoxide and nitrogen dioxide with volatile organic
compounds (Lu et al. 2018), or the formation of particulate matter (secondary inorganic
aerosols) by combination of ammonia, sulphur dioxide and nitrogen oxides, or their
reaction products (Stokstad 2014). The lifetimes of Nr and its products in the atmosphere
are about a few weeks (Fowler et al. 2013), and thus urban or peri-urban Nr emissions can
be transported outside and aect air quality over a broader region. Source apportionment
studies (see the overview by Viana et al. 2008) allowed to obtain a more direct link
between the release of gaseous compounds and observed atmospheric concentrations
of particulate matter. Using a source attribution method, Zhang et al. (2015) found that
reduced and oxidized nitrogen emissions together contribute about 22% of the heavy
wintertime particulate matter in Beijing.
Food: With reactive nitrogen being a key element of protein, and protein being
essential in human food, observing the fate of reactive nitrogen along the food chain
seemed plausible. Forkes (2007) investigated the urban ow of food and N contained
60 W. WINIWARTER ET AL.
within for the city of Toronto, Canada. Quantifying ows for 3 years within a 15-year
period, the author found improvements on a very low level in reclamation of N due to
organic waste diversion eorts (from below one permille in 1990 to 4.7% and 2.3% in the
2000s). N in the food system behaved basically very linearly. More decisive changes
observed were improvements of the wastewater treatment system, which eectively
removed N from waste streams and converted a growing fraction (more than 40% of
inputs in the food system in the last year observed, 2004) into molecular N
. A much more
dynamic situation than for Toronto was described by Ma et al. (2014), who analysed
impacts of urban expansion on nitrogen and phosphorus ows in the food system of
Beijing over a 30-year period (1978–2008). Using a combination of statistical databases,
surveys and the NUFER model (nutrient ow in the food system, environment and
resource), these authors captured the greatly increased food imports to Beijing metropo-
litan area as a consequence of a rapid increase in the number of temporary migrants.
While input of N to the Beijing food system increased from 180 to 281 Gg during the
observation period, the share of N in food and feed imports doubled from 31% to 63%,
and losses of ammonia and N
O to air and of N to groundwater and surface waters
increased by a factor of about 3. Overproportionate increase in pollution was interpreted
as lack of treatment of waste streams – about 52% of N input accumulated as wastes (in
crop residues, animal excreta, and human excreta and household wastes). This pointed to
abatement potential for N pollution, e.g. from livestock production in high intensive peri-
urban agriculture. Wei et al. (2018) showed for Beijing that the success of existing and
future polices relied on optimizing spatial management of new livestock production
systems. They concluded to focus on optimizing livestock diet and on-farm manure
management in industrial livestock production systems typical for peri-urban regions.
Waste and Wastewater: Due to its function of N burying and removal (conversion to
by nitrication), the waste and wastewater sector takes a central role in the
urban N cycle. Municipal waste treatment plants have a great impact on the reduction of
Nr and the control of direct emission to air, soil and water. Yet, little research was found
that would comprehensively allow to trace the fate of N compounds. Instead, just the
emissions of N
O and NH
during waste treatment and storage were commonly reported.
emissions occurred mainly in the composting and storage process. N
O was formed
at every stage of waste management, from treatment to landlling. According to Beck-
Friis et al. (2001) about 98% of N emissions from waste were NH
and 2% were N
O. Emissions can vary widely (two orders of magnitude), in general increasing with longer
treatment times (Clemens and Cuhls 2003). In landlls, leachate was considered a source
O when recirculated (Lee et al. 2002). As a source of N
O emissions to the atmo-
sphere and nitrate release to water bodies, wastewater treatment plants were more
relevant than landlls (Svoboda et al. 2006), even while highly dynamic and depending
on operational conditions (Arnell 2016). Plants that achieved high levels of nitrogen
removal generally emitted less N
O (Law et al. 2012). Both the nitrication step (oxidation
of ammonium/ammonia to nitrite) and denitrication of nitrite or nitrate to molecular N
contributed to N
O formation. Removal of N compounds was never complete (Wen et al.
2018). Due to its relatively high solubility in water, N
O could be retained in the aqueous
phase and denitried downstream of a wastewater treatment plant, unless aeration
O to be stripped to the atmosphere (Kampschreur et al. 2009). Remaining
N from waste water treatment plants generally was rst treated (composting, liming,
JOURNAL OF INTEGRATIVE ENVIRONMENTAL SCIENCES 61
and/or anaerobic digestion) before being recycled back to agricultural soils via direct land
application. Recovery to ammonium sulphate or ammonium nitrate guided towards
production of nitrogen fertilizer with positive environmental impacts (Shaddel et al. 2019).
Evaluating complete urban ows
Bringing together information from individual sectors or media, a city can be investigated
as a whole. Urban systems have features that resemble an organism: materials enter, they
are converted, and again leave the city boundary. Conceptualizing a city as a metabolism
(Wolman 1965; Kennedy et al. 2007) facilitates material ow analysis to be performed. The
approach allows adopting biological principles and denominating the investigated situa-
tion an “ecosystem”.
A simple example for such an approach has been presented by Svirejeva-Hopkins et al.
(2011), who developed an urban N budget for the city of Paris and its urbanized
surroundings for the year 2006. Largely independently, fossil fuel-related emissions to
the atmosphere, and urban food consumption including wastewater treatment formed
two separate chains of N ows. While food imports into the city constituted the largest
N ow, just over half of that amount was being converted into molecular N
treatment plants. The largest impact on the environment, by quantity, came from gaseous
emissions from fossil fuel combustion. Hence, reducing road trac NO
identied having the largest potential to reduce negative impacts of Nr. Enabling recy-
cling of N in wastewater treatment would have the largest overall impact towards circular
Recycling of N (and P) also was the focus of a study on Bangkok Province, Thailand
(Faerge et al. 2001), purely based on aggregated statistics and literature values for
N contents of goods. Huge quantities of N discharged into the nearby river (and the
sea), constituting 92% of inputs, kept recycling rates at a low 7%, for an overall balanced
situation of N ows in and out of the city in the year 1996. According to the authors,
improving the ecological nutrient cycle could be possible by operating wastewater and
night soil (human excreta) treatment plants, which would allow to return more N to food
production, but even with 50% of households connected the resulting recycling rates
would remain at a meagre 11%.
A comprehensive accounting of ows of N in and out of city boundaries, largely based
on locally collected data, was performed for the larger Phoenix, AZ, area (Baker et al.
2001), an arid region that prevented intensive agricultural activities and hence the exact
choice of the geographical boundaries was not decisive. These authors dierentiated nine
“subsystems” and estimated ows individually, taking advantage of, either, studies avail-
able for the local situation, or downscaling from a US dataset. While ows between the
subsystems were assessed, results would not allow for an additional validation level.
Inputs into the system were considered much larger than outputs, and despite of
providing just one-term data (authors seemed to implicitly assume constant conditions
over time, using data from the 1990s), considerable amounts of accumulation of N (for
a single year, 44% of inputs in agricultural area, and 25% in urban area) have been
Further studies on urban N budgets have been performed for Chinese cities. Gu et al.
(2009, 2012) investigated N budgets of Hangzhou and Shanghai, respectively, both cities
62 W. WINIWARTER ET AL.
in the vicinity of the Yangtze river delta. Using concepts of a “greater urban area”, these
authors attempted to cover both the built-up city itself as well as the peri-urban agricul-
tural belt. In the earlier work on Hangzhou (Gu et al. 2009), the concept very much drew
from that on the Phoenix area, with some adaptations to the now 10 “subsystems”. River
systems contributed distinctively to N ows in this humid environment. Accumulation, as
the dierence between inputs and outputs, here was found somewhat lower at about
17.5%. Balances were calculated for two distinctive years, 1980 and 2004, with roughly
a doubling of N ows in that period. For the analysis of Shanghai (Gu et al. 2012), the
approach was rened in both dimensions. Largely the same subsystems (now 13) were
used to create a model framework (“CHANS”), which here allowed for the development of
a whole time series extending over 53 years, based on the coupling of specic information
for any given year (from statistical yearbooks) and time-invariant coecients describing
the transfers. During that period, a nine-fold increase of Nr inputs was observed. However,
no accumulation was identied, by contrast, throughout the whole time series outputs
The urban ecosystem of Zhengzhou (Henan province, China) was chosen as investiga-
tion area by Zhao et al. (2018). For the base year 2014, their approach investigated nine
subsystems, with fossil fuel combustion-related NOx emissions in industry and the
“human” subsystem (including transportation) covering 60% of all N inputs. With inputs
about 10% larger than outputs (but quite variable within subsystems), an accumulation
Zhang et al. (2016) analysed in detail the ows entering and leaving the city of Beijing,
China, and they further pushed on improving the methodology towards applying
a consistent and proven modelling framework. Exclusively focusing on inputs and out-
puts, these authors performed network modelling of N ows to and from the external
environment and between 16 source sectors (in this approach termed “nodes”). Transfer
between nodes used statistical information as well as transfer coecients. These coe-
cients, compiled from available literature and presented in full detail, were maintained
over time, assuming processes remained unchanged. The approach exposed signicant
dierences and strong temporal trends over the observation period 1996–2012. It dis-
tinguished direct and integral (consisting of direct and indirect) ows, with integral ows
having clearly higher values. Over the 16-year period, N ows changed markedly, and
while farming and farmland played a central role in the early phase, this was largely
replaced by transport as a new central node. All direct ows together increased by 24%
over time (or 1.33% per year; input ows even less, 18% over the entire period – roughly
1% per year), as fertilizer input to farmland decreased massively compensating much of
the increase in the transport sector. Data presented did not allow to analyse accumulation
of Nr. A follow-up analysis of the Beijing situation (Zhang et al. 2018) identied structural
dependencies between the network’s nodes in order to devise paths of impacts, but still
did not further resolve their quantities. Partly the same author team performed a factor
decomposition analysis for Beijing N ows (Zhang et al. 2020) and detected increases over
time of energy- and food-related N, while N in fertilizer and animal feed decreased.
Specically, the increase in NO
emissions from transport to almost 80% of energy-
related N input in 2015 (or about 40% of total N input into the city) represented a most
decisive change. These authors demonstrated some key achievements that urban nitro-
gen budgets were able to provide: trends and temporal development of Nr ows over
JOURNAL OF INTEGRATIVE ENVIRONMENTAL SCIENCES 63
time, also relative to each other, were isolated and priorities (or change in priorities) of
mitigation were identied. Factor decomposition helped trace reasons for change. The full
potential of such budgets could be further extended from here, once having evaluated
the potential oered by the available literature.
Discussion: from individual case studies to a generalized description of
urban nitrogen ows
The individual studies discussed above, implemented for city areas and their surround-
ings, demonstrated the importance given and the wealth of results available, also for
urban situations that were typically not characterized by the prime source of N release,
agriculture. Specically considering urban situations was deemed possible, and pinpoint-
ing the ows individually allowed to focus attention to areas of relevance and to sectors
undergoing changes. The original hypothesis underlying this study, that cities are relevant
entities for nitrogen pollution as well as mitigation eorts, has proven valid.
Available literature primarily focused on the unique situation of individual cities.
Approaches have been developed according to the respective requirements of
a specic city and possibly also a particular sector. Hence, results were often not easily
comparable. However, such a comparison is essential to identify common features and to
Hence, a few of the studies devised ways that allowed at least on a limited level to nd
similarities. For those approaches that used the identical method for two or more base
years, temporal trends were used as indicators whether or not environmental relevance of
Nr was increasing. Both sectoral as well as complete budgets delivered such trends.
Dynamics in some cases, specically for East Asian cities, were so strong that, beyond
a signal of the general economic development, making further conclusions became
dicult. Another way of normalizing the results from dierent cities took advantage of
the population numbers. Specic ows per capita were derived by Zhang et al. (2016,
2020) and compared to the results of other available literature. Interpretation, however,
was limited due to methodological dierences. Here harmonization of approaches should
be able to provide additional benets to understand and interpret results.
Normalization of results also has proven eective on an area scale. Several of the
sectoral balances (see above) provided results as loss (or accumulation) per area. In
agronomy, the unit of “kg N/ha” is widely used; therefore, a comparison between
N surplus due to irrigation water of 200 kg N/ha (Werner et al. 2019) and loss as
a consequence of leaking sewage (Divers et al. 2013) became meaningful despite of
describing very dierent sectors and activities.
It is worth noting that several approaches made use of a rather limited dataset.
Quantication of an urban budget based alone on ows in and out of an urban domain
required rather little information from the city itself (see, e.g., Faerge et al. 2001). Such
approaches left outcomes less robust, especially when it comes to comparing cities. Full
input/output matrices for subsectors, as implemented in several of the urban N budgets
presented, clearly was an advantage, as also ows between these sectors could be
evaluated. As, e.g., shown by Zhao et al. (2018), accumulation in every single sector
could be derived. If, however, accumulation merely was assessed as the dierence
between inows and outows, it remained a mathematical entity. Stock changes are
64 W. WINIWARTER ET AL.
physically possible, both accumulation and depletion of material. Understanding dicul-
ties in data acquisitions, quantifying such material eects is deemed extremely valuable in
this context, helping validating results. In a situation of outputs continuing to be larger
than inputs over a full 50-year time series (as reported by Gu et al. 2012), data consistency
is at stake. It does not need measuring pool changes to pinpoint a potential data problem.
Closing the balances is a central purpose of any material ow analysis. A stock-ow
approach not only allows to assess an overall N balance but balances for each pool
individually. What is true for the overall balance also counts on the level of each pool
(“sector”, “subsystem” or “node”): accounting for all ows, sources, sinks and stock
changes in a pool should total zero. In practice, considerable variations in such accounts
will provide hints to processes not so well understood. Here biophysical explanations of
these processes, collecting and using parameters specic for the respective situation
rather than default values will be benecial. Also, interpretation of results from
a mechanistic view, i.e. including the possible underlying mechanisms, can be very help-
ful. Environmental concentrations of nitrogen loads (air pollutant or water pollutant
concentrations) are frequently measured and collected, and such results are available
for comparison with statistical inputs and nitrogen contents. Comparison is possible, but
rarely has been done in the literature investigated, and should include considerations of
ows as much as of stock changes and retention of compounds in environmental media.
Filling data gaps and investigating variations in sectoral analyses can strongly benet
from interaction with stakeholders. Expertise on the respective processes, as indicated
above, will be needed and that may require the input of experts beyond scientists.
Including policymakers, NGOs, industry and other interest groups may provide such
expertise. Using “round table discussions” – a method widely used in urban planning
(Sperling 1999; Wiese-von Ofen 2001) – these stakeholders may reect and discuss the
impact of an N budget along the given content, substance and range. In environmental
policy and the management of protected areas, “Good Governance” also means devel-
oping solutions that involve those people that are aected. Practice proves such
approaches to be useful (Cash et al. 2003; Borrini-Feyerabend et al. 2013; Amon et al.
2014; Jungmeier et al. 2019), as stakeholders were able to take joint ownership of the
In order to further analyse urban nitrogen budgets, from the available material we noted
specically three aspects of general relevance. They may have been identied and studied
previously, but not systematically or consistently and hence results are available only in
part of the studies. These aspects (see Figure 1) may be considered an essential set of
parameters to be developed in such approaches. The rst is the “linearity” of N throughput
in cities – or, if put inversely, the recycling rate. Even with processes only partially under-
stood, that parameter may guide to minimize N waste in cities, and to optimize their use
(see Forkes 2007). Also denitrication (as in wastewater treatment) is a way to reduce
pollution, nevertheless it needs to be seen as the destruction of Nr as an otherwise valuable
resource. Next, any possibility to identify stratied ows (disentangle ows that are
unconnected) needs to be pursued. As evident in the analysis by Svirejeva-Hopkins et al.
(2011), fuel combustion-related nitrogen oxide formation in cities may be largely indepen-
dent of the food chain extending from production to the waste streams. In environmental
consequences, like soil acidication or even atmospheric particle formation, interactions
will certainly occur, but any possible separation should be identied and investigated, as
JOURNAL OF INTEGRATIVE ENVIRONMENTAL SCIENCES 65
also intercomparisons may then be simplied and dierentiated mitigation measures can
be taken. The notion of a “nitrogen cascade” reects such a straightforward ow pattern in
urban surroundings better than it represents the interfacing web of nitrogen transforma-
tions it is usually taken for (Galloway et al. 2003). Finally, the question of accumulation of
matter (in a respective pool) becomes relevant when describing the situation of Nr. Beyond
the question discussed above whether stock changes in the respective pools may also
indicate data problems, excess Nr or depletion of Nr both will aect environmental
performance of pools and have consequences on ows. While it may be dicult to quantify
pools, assessing pool changes can provide otherwise missing information, valuable when
comparing between dierent situations (cities, years).
There is a long way from conceptual statements of intervention possibilities (see,
e.g., Sutton et al. 2013) to quantiable and robust methods to implement measures
that minimize the losses of nitrogen in practice on an urban level. A few relevant
successful approaches have been made available in the scientic literature already in
the past. A focus on urban areas seems to provide very useful and practical results. As
is quite typical for N related issues, sectoral exclusive views too often take the risk of
identifying one-sided solutions considering benets for a specic sector only. This
becomes evident from some of the trends identied, e.g. by the observed decrease in
animal feed (Zhang et al. 2020), which just pushes the problem outside city regions.
Hence, further coupling of this tool with other scales (province or country) may
Figure 1. Concept of Urban N budget. Key N ﬂows are denoted as arrows, open arrows are inputs, solid
arrows represent outputs (products or pollutants). The rectangular boxes enclosed in dashed lines
represent some important pools containing Nr in a city, which itself is here represented as the elliptical
shape. Relevant for detailed evaluation are (see text) the extent of recycling, the stratiﬁcation of ﬂows
(extent to which output ﬂows can be directly traced back to inputs), and the sub-budgets within each
of the pools.
66 W. WINIWARTER ET AL.
introduce new and necessary perspectives – implying also to consider “footprints” or
eects a city may have on areas outside.
Conclusions and way forward
With N compounds in the centre of many environmental hazards, sustainable develop-
ment and the achievement of the United Nations Sustainable Development Goals can be
greatly advanced by improving and harmonizing the understanding of the environmental
behaviour of N compounds.
The concept of urban nitrogen cycles to describe the environmental situation in
city environments has not been used extensively in the scientic literature, but a few
very relevant studies are available, which each refer to one specic city only. Such
work has been focusing in part on specic sectors, but we also noticed seven cities
for which full budgets are available, ve of which are in Asia. The availability of these
studies conrms our hypothesis of urban activities being relevant also for the
N cycle. Moreover, as urban nitrogen budgets can be derived also with incomplete
data sets and complementing from budget considerations, this approach seems
useful also when data access is limited.
Results found in the literature, whether sectoral or full budgets, are promising as they
identify key aspects on priorities, trends and future directions of N related environmental
issues, whether this is air pollution, water pollution or impacts on biodiversity. They are
useful for the respective application, but do not allow for easy comparison due to
dierences in the system boundaries set, the sectoral pools used to classify N ows to,
and the chosen methodologies. Comparability between cities of dierent makeup needs
standardization. Flows (and stock changes) per area or per population may serve as rst
proxies. More thorough development of indicators is possible from the experience pre-
sented with the selected studies. Specically, we identify the following promising
approaches to characterize urban N budgets: (1) the rate of N recycling indicates the
extent of N wasted, (2) in an urban situation, N ows may be stratied in part and dealt
with independently to simplify balancing, and (3) pool changes and especially accumula-
tions pointing to potential future release sites.
Taking up from international programmes originally devised on a national scale
(e.g. greenhouse gas inventories) and implementing existing information on the
processes of N release and its environmental conversions can push further to
harmonize available results. Directly comparing cities that dier in their urban fabric
using a common method can be a good testing ground to develop benchmarks.
Resulting information to stakeholders and to urban environmental policy will sup-
port abatement eorts for improved air and water quality, reduce climate impacts
and ultimately prevent shifts of N compounds between environmental
Authors declare that they are not aware of any conicts of interests.
JOURNAL OF INTEGRATIVE ENVIRONMENTAL SCIENCES 67
This publication contributes to UNCNET, a project funded under the JPI Urban Europe/China
collaboration, project numbers UMO-2018/29/Z/ST10/02986 (NCN, Poland),  (NSFC,
China) and  (FFG, Austria). WW wishes to acknowledge support received from the Chinese
Academy of Sciences via the President’s International Fellowship Initiative [2019VCA0017].
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