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Purpose Cities are hotspots of consumption of matter, energy, and water and hotspots of production of wastes, which are also secondary resources. Nutrients such as phosphorus are hardly extracted and recycled from these wastes, except from sewage sludge. This paper discusses a concept for the recycling of P from a great variety of urban wastes (phyto-P-mining). Materials and methods Phyto-P-mining is based on the plant extraction of P from waste materials, which were used to produce planting substrates. They are intended for the greening of urban structures, which were de-vegetated during urbanization or which were not intended to be vegetated before (secondary urban green). After the newly established plants have extracted P, their biomass can be used to produce bioenergy (biogas, wood) or compost. Phosphorus could then be recycled from digestion residues and ashes or directly from compost. Results and discussion Phyto-P-mining is based on otherwise wasted nutrients and on the greening of a high number of not yet vegetated plots, including public or private plazas, sidewalks, roofs, and fallows. Greening is a major goal for urban planning, as functioning soil-vegetation-complexes provide ecosystem services such as climate regulation, dust absorption, wind brake, or aesthetic improvement. Especially in the dense inner city quarters, where vegetation is rare, new green improves public health and well-being. However, due to the lack of available horizontal but the high abundance of vertical structures like walls and facades in city centers, vertical green will be very important for phyto-P-mining. It can efficiently extract P from wastes due to its high ratio of biomass to ground area. Like the vertical areas, the vertical greens are often private properties. Although private greening is primarily conducted for social and cultural reasons, direct market benefits such as bioenergy or fertilizers may reduce costs for the greening. This will foster private urban greening to the benefit of the community and also the recycling of nutrients from urban resources. Conclusions Phyto-P-mining based on secondary urban green will reestablish soil functions and natural cycling mechanisms in artificial urban systems. The approach has a great potential (i) to improve the urban living environments and to deliver benefits such as (ii) the recycling of phosphorus and other nutrients from urban wastes for the application in urban or rural agri- or horticulture and (iii) the ethically and ecologically sound production of bioenergy.
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SOILS AND SEDIMENTS IN URBAN AND MINING AREAS
Letter to the editors: Phyto-P-miningsecondary urban
green recycles phosphorus from soils constructed of urban wastes
Thomas Nehls &Christophe Schwartz &Kye-Hoon John Kim &
Martin Kaupenjohann &Gerd Wessolek &Jean-Louis Morel
Received: 7 March 2014 /Accepted: 6 November 2014
#Springer-Verlag Berlin Heidelberg 2014
Abstract
Purpose Cities are hotspots of consumption of matter, energy,
and water and hotspots of production of wastes, which are also
secondary resources. Nutrients such as phosphorus are hardly
extracted and recycled from these wastes, except from sewage
sludge. This paper discusses a concept for the recycling of P
from a great variety of urban wastes (phyto-P-mining).
Materials and methods Phyto-P-mining is based on the plant
extraction of P from waste materials, which were used to
produce planting substrates. They are intended for the green-
ing of urban structures, which were de-vegetated during ur-
banization or which were not intended to be vegetated before
(secondary urban green). After the newly established plants
have extracted P, their biomass can be used to produce
bioenergy (biogas, wood) or compost. Phosphorus could then
be recycled from digestion residues and ashes or directly from
compost.
Results and discussion Phyto-P-mining is based on otherwise
wasted nutrients and on the greening of a high number of not
yet vegetated plots, including public or private plazas, side-
walks, roofs, and fallows. Greening is a major goal for urban
planning, as functioning soil-vegetation-complexes provide
ecosystem services such as climate regulation, dust absorp-
tion, wind brake, or aesthetic improvement. Especially in the
dense inner city quarters, where vegetation is rare, new green
improves public health and well-being. However, due to the
lack of available horizontal but the high abundance of vertical
structures like walls and facades in city centers, vertical green
will be very important for phyto-P-mining. It can efficiently
extract P from wastes due to its high ratio of biomass to
ground area. Like the vertical areas, the vertical greens are
often private properties. Although private greening is primar-
ily conducted for social and cultural reasons, direct market
benefits such as bioenergy or fertilizers may reduce costs for
the greening. This will foster private urban greening to the
benefit of the community and also the recycling of nutrients
from urban resources.
Conclusions Phyto-P-mining based on secondary urban green
will reestablish soil functions and natural cycling mechanisms
in artificial urban systems. The approach has a great potential
(i) to improve the urban living environments and to deliver
benefits such as (ii) the recycling of phosphorus and other
nutrients from urban wastes for the application in urban or
rural agri- or horticulture and (iii) the ethically and ecologi-
cally sound production of bioenergy.
Keywords Constructed soils .Ecosystem services .
Phosphorus recycling .Secondary urban green .Vertical green
1 Introduction
In the phyto-P-mining approach (Fig. 1), plants are utilized
to extract phosphorus from planting substrates, which are
newly constructed from urban wastes. In the following, the
prerequisites and boundary conditions for the concept are
Responsible editor: Rongliang Qiu
T. Neh ls (*):G. Wessolek
Institute of Ecology, Chair for Soil Conservation, Technische
Universitaet Berlin, Ernst-Reuter-Platz 1, 10587 Berlin, Germany
e-mail: thomas.nehls@tu-berlin.de
C. Schwartz :J.<L. Morel
Laboratoire Sols et Environnement, Université de Lorraine, UMR
1120, 54500 Vandœuvre-lès-Nancy, France
C. Schwartz :J.<L. Morel
Laboratoire Sols et Environnement, INRA, UMR 1120,
54500 Vandœuvre-lès-Nancy, France
K.<H. J. Kim
Department of Environmental Horticulture, University of Seoul,
Seoul, Republic of Korea
M. Kaupenjohann
Institute of Ecology, Chair for Soil Science, Technische Universität
Berlin, Ernst-Reuter-Platz 1, 10587 Berlin, Germany
J Soils Sediments
DOI 10.1007/s11368-014-1023-0
discussed exemplarily for the city of Berlin, then the concept
itself is introduced and discussed.
1.1 Limited availability of phosphorus from geogenic sources
Recent agriculture mostly relies on mineral P fertilization. The
availability of rock-phosphate-derived fertilizers is predicted
to strongly decrease in the course of the next 50 to 300 years
(Cordell et al. 2009; Van Kauwenbergh et al. 2013). The
predicted period probably depends on the intention and focus
of the authors. Rock phosphates are of geostrategic impor-
tance as almost 80 % (year 2013) of them are mined in only
five countries: China (43%), USA (14%), Morocco (13%),
Russia (6%), and Jordan (3%) (USGS 2014). Two strategies
have been discussed to approach this shortage: (i) to improve
the availability of P, which is already accumulated in agricul-
tural top soils (Stutter et al. 2012), and (ii) to improve the
efficiency and the recycling of P from fertilizers, manures,
composts, and other wastes. There is a high recycling poten-
tial, as the global phosphorus cycle (e.g., UNEP 2011)isnota
cycle at all. Closed cycles exist in natural ecosystems, but only
partly in forestry and agriculture. Urban areas are the final
parts in the food supply chain. From here on, P is lost mainly
to dumpsites, river sediments, and the oceans, except for P in
sewage sludge. Sewage sludge as a soil amendment (10 % in
Poland to 70 % in the UK) remains relevant for the recycling
of P and other nutrients (Sartorius 2012). However, its direct
application will probably be replaced by engineering ap-
proaches to extract P from the sludge or its ashes (e.g.,
Adam et al. 2007; Scheidig et al. 2009). Techniques like
FIX-PHOS and ASH DEC are expected to be profitable
between 2015 and 2030 (Sartorius 2012). Finally, all other
organic and inorganic urban waste materials should be exam-
ined for their P resource potential. It would be a successful
adaptation of natural recycling concepts to use these waste
materials to manufacture plant substrates and to utilize
plants for P extraction (phyto-P-mining).
1.2 Availability of phosphorus from urban sources
As areas of concentrated import and consumption of goods,
food, and energy, cities produce large amounts of waste. Some
of these wastes can be used as mineral constituents and
organic matter for the construction of planting substrates. In
Germany, both are produced in remarkable amounts
(g capita
1
a
1
with 80,585,700 inhabitants in Germany):
concrete, bricks, tiles and ceramics (6.5×10
5
), track ballast
(2.7× 10
4
), soil material and stones (1.4×10
6
), or street
Fig. 1 The phyto-P-mining concept integrated into existing matter, en-
ergy fluxes, and infrastructures (gray arrows and objects). Waste materials
are used as planting substrates for a high number of secondary urban
greens. The concept offers opportunities for biogas production and sub-
sequent urban internal cycling as well as export of nutrients back to
agricultural land (new matter and energy fluxes in green)
J Soils Sediments
sweeping (5.0×10
2
), and organic material such as green
wastes (6.6×10
4
), biodegradable kitchen wastes (8.5×10
3
),
and organic household wastes (4.9×10
4
). The latter are col-
lected separately in a special bin and usually composted, but
nowadays are increasingly used for biogas production (all data
for the year 2012, not contaminated materials, Statistisches
Bundesamt 2014).
In order to construct plant substrates from such wastes, the
physical and chemical properties of the pure materials as well
as the properties of its mixtures need to be known. Some
materials like compost (e.g., Deportes et al. 1995)orbricks
(e.g., Nehls et al. 2013) have already been studied. During the
last decade, Séré et al. (2008,2010)andRokiaetal.(2014)
systematically and comprehensively investigated the con-
struction of soils from urban waste materials. Based on feasi-
bility and pedological criteria, they selected 25 binary and
tertiary mixtures out of 220 theoretically possible mixtures
(not considering the different mixing ratios) from eleven dif-
ferent components including nine waste materials (green
waste, compost, street sweeping, sewage sludge, paper mill
sludge, bricks, track ballast, rubble, concrete) and two soil
materials. The study demonstrated that the investigated waste
materials are appropriate to construct plant substrates.
Mixtures of brick particles and compost are already in practi-
cal use as green roof substrates.
The total P contents of the mentioned waste materials range
from 0.7 to 21 g kg
1
for bricks and compost, while the
available amounts (extracted according to Olsen) range from
0.03 to 1.1 g kg
1
for bricks and green wastes, respectively
(Rokia et al. 2014). So eight of the abovementioned nine
waste materials (sewage sludge excluded) could substitute a
share of 18 % (m/m) of the 1.2×10
11
g of imported mineral
fertilizer-P, which are applied to agricultural areas per year in
Germany (7×10
3
gPha
1
a
1
).
1.3 Lack of soil, lack of urban green
Soils are sealed (Morel et al. 2014) and vegetation is regularly
missing in dense inner city quarters, often showing fractions
of impervious surfaces of 60 % and more (Shuster et al. 2005)
For instance in Berlin, Germany, the highest impervious frac-
tion of 69 % is measured in Friedrichshain-Kreuzberg, an
inner city quarter, where the provision of green spaces per
capita is the lowest (4.5 m
2
), while one of the lowest soil
sealing fractions, 28 %, is measured in Spandau, a quarter with
30 m
2
of green space per capita (Umweltatlas Berlin, online).
Pervious pavements are one strategy to partially reestablish
the soils filter and storage function (Nehls et al. 2008).
However, these areas should also be greened as vegetation
fulfills several additional provisioning, regulating, supporting,
and cultural functions. For public health and well-being, the
regulating ecosystem services such as cooling by shade and
transpiration, and purification and moistening of the urban air
are of the highest relevance for inner city quarters. Important
as well are cultural services like aesthetic reinforcement of the
living surrounding, enhancement of the spiritual dimension, or
recreation of city dwellers. Green structures are preferred over
non-vegetated structures, positively co-notated and perceived
as beautiful,relaxing and restorative (White and
Gatersleben 2011), with the positive effects being more pro-
nounced with increasing species richness (Fuller et al. 2007).
The provisioning ecosystem services of green can be the
production of food, fodder (for livestock farming, bees), cut
flowers, biomaterials, medicines, and bioenergy.
However, there are also ecosystem disservices of plants,
which are the more important the more inhabitants are affect-
ed: shading of flats by trees, spreading of allergens, smells,
annoying leaves or insect secretions on the windshields of
cars, provision of habitats for annoying singing-early-in-the-
morning-especially-at-weekends-birds, but also for rats, spi-
ders, and beetles. Depending on the design, urban green can
also decrease the subjective perception of security (refreshing
to read: Lyytimaki and Sipila 2009). These disservices should
be seriously considered for the design and management of
secondary urban green.
As the positiveimpact of urban green on human health and
well-being is widely accepted (e.g., Tzoulas et al. 2007), it
should be the aim of municipalities and inhabitants to increase
the number of green spaces. In the following, we will discuss
promising ways to reach that goal as a prerequisite to integrate
phyto-P-mining.
1.4 New forms of urban green
Green areas such as parks are of limited spatial effectiveness
concerning ecosystem services, e.g., climate regulation
(Bowler et al. 2010). For the benefits of green, its abundance
and visibility in the direct residential environment is crucial.
Facade greening could be such a form of green. However, it is
not as widespread as the proven ecosystem services would
suggest (Ottele et al. 2010). So, often inner city inhabitants
stand in front of their apartment blocks, greened roofs may be
invisibly 20 m above and the next green park is some 100 m
away. City planning should promote a high number of green
spots, even small ones, which can form a city-wide green
cover rather than to support only some parks or nature
patches, which add only some green dots to the gray city
fabric.
Obviously, especially former not vegetated surfaces should
be greened (secondary green), as then the highest gain of
ecosystem services can be achieved. Thus, it must be the goal
to green paved surfaces, whether horizontal or vertical. As
mentioned above, existing soils may have to be de-sealed,
assessed for their suitability to grow plants (Moir and
Thornton 1989;Kahle2000;Abeletal.2014) and probably
ameliorated which all together can be costly. Instead,
J Soils Sediments
constructed planting substrates in containers could be used
(see below). New designs for such small-scale, horizontal, or
vertical greening are needed as well as techniques for its
irrigation and maintenance.
This small-scale greening can be privately financed which
would be interesting as the financial resources of municipal-
ities are usually limited. Additionally, most of the
abovementioned spaces, especially the verticals, are private
or rented properties and have long been outside the focus and
beyond the direct reach of top-down urban planning. In this
case, the whole private sector, inhabitants, companies, com-
munities, associations, etc. should be included in the greening
process (urban green 2.0). Thus, it would be possible to make
people directly responsible for their own residential environ-
ment (Francis and Lorimer 2011). There are strong hints that
city dwellers are willing to take that responsibility, as indicat-
ed by the renaissances of urban gardening and urban agricul-
ture, or movements like guerilla gardening. This points to the
rediscovered lust of city dwellers to experience and to learn
how to cultivate plants in their living environment (Bendt
et al. 2013). Often, this formerly suspiciously observed gar-
dening activities are organized community based (Rosol
2010). Inhabitants should be provided with the participatory
right to establish green in their direct living environment and
with a reliable planning framework, which secures their in-
vestments (Francis and Lorimer 2011). Urban green 2.0 would
guarantee that greening activities in the anthropocentric city
are in accordance with the cityzensneeds (see ecosystem
disservices). It will develop a great variety of adapted greening
strategies and designs (Dallimer et al. 2012), depending not
only on the availability of space, water, and nutrients but also
on regional or individual taste, financial resources, and needs
of the neighborhood. One of these needs is the production of
food for subsistence or the market. This importance will rise,
as the world population will grow, especially in urban areas
(Colding and Barthel 2013). The concurrence about land,
water, and fertilizers for biofuel production will increase the
pressure to value urban space and resources for food produc-
tion (Godfray et al. 2010). Urban green can provide fibers, cut
flowers, medicine, and biomass for fuel or energy production.
For example, Springer (2012) determined the biomass
production of a city in the southern plains, USA to be 0.8 to
1.9×10
7
gDMha
1
a
-1
of planted land from lawn dethatching
and mowing, leaf raking, and tree pruning. For facade green,
no such data is available yet; therefore, it is estimated in the
following based on data on biomass and available vertical
areas for the example of Berlin, Germany. The typical vertical
green species Boston ivy vine or Japanese creeper
(Parthenocissus tricuspedata) produces organic dry matter
(oDM) in amounts of 0.7 to 2.1×10
6
goDMha
1
a
1
of
vertical area (Bartfelder and Köhler 1987). The potentially
available vertical area of a city (facades, walls) can be esti-
mated by remote sensing. For instance, Nehls (2010)
estimated a gable wall area/ground area ratio of 0.27 ha ha
1
for the inner city quarters of Berlin (8000 ha ground area),
which results in a total vertical area of 2160 ha. Thus, the
potential to produce green biomass in the inner city of Berlin
would be up to 4.5× 10
9
ga
1
or up to 0.6× 10
6
goDMha
1
a
1
based on ground area. Note, that this would be only the
production from gable walls without windows. In our opin-
ion, the potential of urban areas to produce biomass is not
adequately investigated and discussed yet, especially as
these spaces are unproductive.Therefore, such biomass
production would not be in concurrence to food production
or nature preservation (see the debate on food or biofuel). In
the following chapter, it will be discussed if the ecological
prerequisites, especially the availability of planting sub-
strates for such a productivity-oriented greening, are given
in cities.
However, it must also be accepted that urban gardeners
follow also goals different from production of food or bio-
mass. They seek meaningfulfree time activity, recreation,
education, communication, and the pleasure of working out-
side (Bendt et al. 2013). For instance, in the well-reported
Prinzessinnengarten(prinzessinengarten.net), an urban
community garden project in Berlin, Germany, more than
1500 people grow plants at less than 0.6 ha. Obviously, the
yield-oriented production or subsistence is the minor goal
there.
1.5 Availability of space, water, and nutrients in urban areas
The ecological prerequisites for sustainable urban greening
are space, water, and nutrients. They have to be reliably
available, and in most cases, they are. Usually, the needed
resources are even wasted or ignored: for the space, it is waste
land, fallows, brown fields, yards, roofs, terraces, balconies,
facades, or other vertical structures. While fallows and brown-
fields may only be available for interim use, the vertical
structures are available on a long-term perspective.
Water is often wasted in urban areas as waste water or
rainwater runoff, and it is available in excess in case of raising
groundwater tables (Gobel et al. 2004). Even in semi-arid and
arid regions, it is possible to collect and store rain water from
sealed surfaces to green at least small oasis patches. In the
tropics and subtropics, the use of waste water can be of higher
relevance (Pescod 1992) than in temperate regions, where
urban greening can contribute to surface runoff reduction. In
terms of rainwater, the temporal shift in availability and de-
mand must be buffered by storages like cisterns. Knowledge
on water demands of green roofs and vertical greens, which is
needed to dimension such storages, is hardly available.
The next prerequisite for a sustainable urban green are
nutrients, preferably provided by a soil or plant substrate.
They are also available and are wasted if the city is seen as
whole. However, in the highly sealed inner city quarters where
J Soils Sediments
the secondary greening should be installed, there is a lack of
soils. Using fertilizers or soil material from outside the city is
not an option, as it would just enlarge the ecological footprint
of the city. However, cities and their surroundings are areas of
constant construction activity, suburbanization, and urban
sprawl. Thus, soil material from construction activities could
be used inside the cities. Their availability depends on legis-
lative regulations, soil properties, their contamination status,
and the market. Purpose-designed soil substitutes could be
produced from locally available waste, the citiessecondary
resources. However, in Berlin, with the beginning of biogas
production from organic wastes in 2013, the availability of
organic wastes for compost production decreased from 6.6 ×
10
10
ga
1
to only 1.0 × 10
10
ga
1
. The produced compost
goes almost completely to rural agriculture (personal
communication Kristian Kijewski, Berliner Stadtreinigung
GmbH, 21.10.2014). For the Prinzessinnengarten in Berlin,
starting compost production was the crucial and initial step for
the garden. It is situated on paved and probably contaminated
ground, so the plants are cultivated in containers. Today, the
gardeners carefully select residues of organic food in the
neighborhood and produce organiccompost for themselves
and for the market. Due to mineralization, nutrients, and
carbon are lost from compost (Teemusk and Mander 2007).
Growing substrates which copy natural soils in quantities of
organic and inorganic constituents are more sustainable as
reported for green roof substrates (Czemiel Berndtsson et al.
2009).
So, there is a need to produce new planting substrates or to
ameliorate existing soils especially suited for food production
or non-food greening purposes. The phyto-P-mining concept,
introduced in the following, brings together all the aforemen-
tioned aspects.
2 The phyto-P-mining concept
For phyto-P-mining (Fig. 1), plants are grown to extract
phosphorus from planting substrates, which were newly con-
structed from waste materials. Phyto-P-mining is not a single
purpose concept but must be understood as a surplus benefit
of the above discussed secondary urban greening strategy. It is
especially suitable to exploit P from materials with compara-
ble low P concentrations. In terms of maximum P extraction,
the use of P hyperaccumulators, in other words, species and
plant communities with high P extraction and luxury con-
sumption like Brassica napus would be preferable.
However, the plant or plant community should be chosen
according to the primary function of the vegetation such as
climatic regulation (shade, cooling by transpiration), food
production (contaminant exclusion), ornamental purposes, or
suitability to architecture as well as site-specific conditions
such as climate, light, and water availability. For phyto-P-
mining, vertical green seems to be promising as it can reach
higher biomass to ground area (rooted volume) ratiosand thus
higher extractions than horizontal green (see below).
The concepts to use the extracted P could be transferred
from phytoremediation and hyperaccumulation of heavy
metals by plants (e.g., Schwartz et al. 2003). To make the
process profitable, Li et al. (2003) discuss not only to exploit
the target metal but also to use the biomass for bioenergy
production. Thus, the biomass produced from urban green,
which was installed for other purposes (e.g., ornamental),
becomes a resource itself. In the case of phyto-P-mining, the
final use of the biomass should be the production of biogas or
wood pellets. Both would guide matter and energy into
established flow chains and existing infrastructures. In
Germany, there were 7500 biogas plants already installed in
2012 and another 400 prognosticated for 2014 (Biogas e.V.
2013). They are then delivering about 5 % of the electricity
demand of the country.
In the following, we estimate the bioenergy production of
two model species and demonstrate their potential to contrib-
ute to P recycling, although both are rather used for other
purposes than biogas production. The two species having a
comparable high P uptake are rape (B. napus) for horizontal
urban green and hop (Humulus lupulus) for vertical green.
Due to harvest and storage losses, we assume0.85 g g
1
of the
above groundbiomass of the modelspecies to be converted to
biogas. The organic dry matter yield for rape is around 3 to4×
10
6
goDMha
1
a
1
, while for 4000 hop plants per hectare, it
is 5.5 to 6.5×10
6
goDMha
1
a
1
(Heetkamp 2011). From
rape and hop biomass, methane (given in standard liters, L
N
,at
air pressure of 1.01 10
5
Pa and air temperature of 273 K)
can be produced in amounts of 262 and 212 L
N
kg
oDM
1
(Petersson et al. 2007;Heetkamp2011). For hop, with a usual
inter- and intra-row spacing of 1.5 mand a height of theplants
of 7 m, the virtual vertical greened area is 44.1 m
2
m
2
ground
area, thus the yield per vertical area is about 1.2 to
1.5 10
2
gm
2
. In order to use this biomass sustainably, adapted
business models and harvesting strategies have to be
developed.
With P concentrations in the biomass of 5 to 7× 10
3
gg
1
and3to4×10
3
gg
1
for rape and hop, respectively, both extract
about 2 g P m
2
a
1
. For hop, grown in containers, one can
expect much higher extraction rates. Assuming a planting sub-
strate container of 1 m
3
per plant (7 m high, 1.5×10
3
goDM),
hop would extract up to 6 g P m
2
a
1
and thus up to three times
the amount of horizontal plants, as discussed above.
The accruing amounts of biogas digestate (AD) and its P
contents are intensively discussed topics in the bioenergy
community. Mokry and Kluge (2009) found that the amount
of AD is usually about 75 % (m/m) of the input for fresh green
plant materials. The total P content of the digestate is usually
1.5 to 2× 10
3
gg
1
of which 60 to 70 % are water soluble.
J Soils Sediments
Zirkler et al. (2014) and Moller and Muller (2012)foundP
accumulations of 16 to 25 % in the AD compared to the plant
biomass, which is proportional to the dry mass reduction
through digestion, while the availability of P was 20 to 47 %
lower than that of the undigested material. Such data is not yet
available for H. lupulus or other climbers like Hedera helix,
Parthenocissus tricuspidata,andFallopia baldschuanica.
The direct application of ADs as manures in agriculture is
already practice (Mokry and Kluge 2009). Alternatively, P
and other nutrients could be extracted and concentrated from
the residues. Such techniques (Kuroda et al. 2012; Campos
et al. 2014) are nowadays investigated due to rising amounts
of digestate. The digestates of urban biomass could be used as
fertilizers in (urban) agriculture and thus contribute to close
nutrient cycles inside cities and between cities and their rural
surroundings. Thus, not only the production of planting sub-
strate but also the production of food and other plant products
as well as the production of energy and fertilizers offer the
possibility to enhance the economic benefits of phyto-P-
mining.
3 Conclusions and outlook
The phyto-P-mining concept, that means waiving of geogenic
P resources for urban greening, biomass, and food production
(depending on the local needs and preferences), and the ex-
traction of P from waste materials, which are seen as second-
ary resources, may provide a model approach to foster the
sustainably use of P resources in urban areas. The main point
of the concept is to base urban green on urban resources.
The approach is small-scale oriented and includes
traditional forms of urban agriculture as well as new
concepts. Small-scale, private sector-based urban green-
ing (urban green 2.0) would influence neighborhoods
both in terms of ecological inventory as well as their
awareness towards food production, food security, food
quality, and the environment in general. As also public
spaces are influenced by private greening initiatives, it
would improve community building and political activ-
itiesin short: participation. However, the approach
may contain business aspects as well. It must be inves-
tigated, for whom and under which conditions urban
green could be designed and managed in a profitable
way. Production of biomass, bioenergy, and finally fer-
tilizers might be important parts of the greening-
connected value chain.
However, the concept as depicted in this letter also raises
some questions of interest for soil science and urban planning.
Analyzing and understanding a complex biogeochemical and
physical construct like a soil and describing its functions is
quite different from predicting characteristics of newly con-
structed soils. This experimental soil science or soil
engineering will contribute to our general understanding of
soils. More specifically, first the feasibility of the phyto-P-
mining concept must be investigated in terms of nutrient
contents and availability from pure waste materials and mix-
tures. This should be done preferably by applying traditional
chemical laboratory methods as well as plant experiments in
pots but also on the field-scale. Scientific questions in that
context concern medium and long-term availability of all
relevant nutrients, not only P, nutrient leaching from the
substrate by seepage water (eutrophication risk), and possible
contamination of food and biomass (Deportes et al. 1995).
Second, the substrates should be physically analyzed for self-
compression, the development of structure (e.g., by biological
activity) and pore system characteristics (e.g., water availabil-
ity, air capacity), rooting, and erosion aspects.
The socioeconomic dimension of the approachincludes the
practical, organizational, and financial aspects. It is necessary
to check the feasibility of the concept for a set of individual
geographical and economical circumstances. Thereby,
practice-oriented experiments under participation of urban
gardeners would be helpful.
Acknowledgments We thank the French-German academic exchange
program PROCOPE (DAAD and CAMPUS FRANCE), the German
Science Foundation (DFG, FOR 1736), and the French ADEME-
SITERRE program for financial support.
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... Thus, a viable strategy for reducing the demand of natural soil is the construction of purposedesigned Technosols, preferably composed of locally available, recycled waste materials (Majsztrik et al. 2011). Previous studies have shown the use of technogenic materials to develop diverse fertile substrates and construct Technosols using recycled materials like rubber, paper-mill sludge (Rokia et al. 2014), biochar, bricks (Nehls et al. 2013), concrete, and excavation waste (Pruvost 2018;Deeb et al. 2019;Prado et al. 2020), which can lower considerably the natural soil consumption (Flores-Ramírez 2018), besides diminishing urban waste disposal problems (Séré et al. 2008;Yakovlev et al. 2013;Rokia et al. 2014;Nehls et al. 2015). ...
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Abstract Purpose: The sustainable use and management of global soils is one of the greatest challenges for the future. In the urban ecosystem, soils play an essential role with their functions and ecosystem services. However, they are still poorly taken into consideration to enhance the sustainable development of urban ecosystems. This paper proposes a categorization of soils of urbanized areas, i.e., areas strongly affected by human activities, according to their ecosystem services. Materials and methods : Focus is put first on ecosystem services provided by non-urban soils. Then, the characteristics and number of services provided by soil groups of urbanized areas and their importance are given for each soil group. Results and discussion: Soils of urbanized areas are here defined as SUITMAs, because they include soils of urban, sensu stricto, industrial, traffic, mining, and military areas. This definition refers to a large number of soil types of strongly anthropized areas. SUITMAs were organized in four soil groups, i.e., (1) pseudo-natural soils, (2) vegetated engineered soils, (3) dumping site soils, and (4) sealed soils. For each soil group, examples for ecosystem services were given, evaluated, and ranked. Conclusions : This proposal contributes to foster the dialogue between urban spatial planning and soil scientists to improve both soil science in the city and recognition of SUITMAs regarding their role for the sustainable development of urban ecosystems and, in particular, to enhance multifunctional soils in urban areas.
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We analyse environmental learning in public-access community gardens ('PAC-gardens') in Berlin, representing public green spaces that are collectively managed by civil society groups. Through extensive fieldwork, and drawing upon social theories of learning, we describe learning communities in four PAC-gardens and analyse factors that influence participation and boundary interaction, that is when experiences brought in from the outside encounter socially defined competences. Results show that these PAC-gardens have self-generated social and physical structures, which to different degrees inhibit or facilitate boundary interactions, whereas skills of individuals to put those to work, in combination with the quality of the surrounding neighbourhoods, can be ascribed for creating broader participation and greater diversity in the content of learning about local sustainability. Identified learning streams included learning about gardening and local ecological conditions; about urban politics, and about social entrepreneurship. We discuss results in relation to environmental learning that combats the generational amnesia in cities about our dependence on nature, where PAC-gardens clearly distinguish themselves from more closed forms of urban gardening such as allotment gardens and gated community gardens. We conclude that PAC-gardens that intertwine gardening with social, political and economic practices can create broader and more heterogeneous learning about social-ecological conditions, and help develop sense-of-place in degraded neighbourhoods.