Call for cities as bright spots of resource management

With over 700 cities committed1 to achieving net-zero carbon goals by 2050, urban areas have become central in combatting climate change and achieving carbon sustainability2. Just as for carbon, urban populations are the motor driving the anthropogenic phosphorus cycle through their demand for phosphorus-intensive agricultural production. Yet, urban phosphorus management is currently insufficient and locked into a pattern of high phosphorus consumption and high waste. Cities receive phosphorus as food imports and concentrate emissions of phosphorus from waste/residue streams to the aquatic environment (Fig. 1a). A lack of coordinated efforts within and across cities place city dwellers at risk of food price fluctuations and water pollution associated with poor phosphorus management.

Fig. 1: Transformation to a net-zero phosphorus city.
figure 1

Typical phosphorus flows in cities currently depicted in (a), in contrast to a net-zero phosphorus city depicted in (b), and the key transformational pathways to transition to more sustainable urban centres between the two panels. These include (i) minimising unproductive phosphorus outputs from the city to protect waterways, (ii) developing infrastructure to couple waste streams to agricultural phosphorus demand, and (iii) transitioning to healthy diets and consumption behaviours with low phosphorus footprints (the phosphorus footprint is here defined as the phosphorus in fertiliser required to produce a unit of a food/non-food product). This should be done by developing a coordinated approach with the net-zero carbon movement, to maximise synergies and innovation with an experimental approach to adapt to local conditions. This figure has been designed using resources from Flaticon.com.

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Here, we propose the ‘net-zero phosphorus cities’ concept and highlight the key flows that must be addressed to connect urban centres within circular phosphorus economies designed to deliver benefits that resonate far beyond individual city boundaries. Inter-city coordinated action on sustainable phosphorus management is an opportunity to deliver on global sustainability ambitions, including multiple UN Sustainable Development Goals (SDGs) related to food security and water pollution through more sustainable production and consumption (Table 1)3,4. We present opportunities to build on existing plans and frameworks put in place by cities within net-zero carbon initiatives, and in doing so embrace a systems approach deemed necessary for a transition to sustainable cities5. City mayors and municipal entities (e.g., departments tasked with city planning, green infrastructure, solid and liquid waste management, and public procurement) can use the proposed framework to become leaders in urban phosphorus stewardship.

Table 1 Link between sustainable development goals (SDG) targets and phosphorus (P) management.
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The global phosphorus challenge

Phosphorus is an essential component of fertilisers and is part of all organic matter. Globally, around 85% of marketable mined phosphorus is processed to make mineral phosphorus fertilisers to satisfy crop demands; a further 10% is consumed in animal feed supplements, while the remaining 5% is used in diverse chemical industry processes including detergent and battery production, and metallurgy6. Phosphorus is geopolitically scarce; 85% of world phosphate rock reserves are in just five countries, notably Morocco and Western-Sahara, China, and Russia7. Export tariffs and bans (e.g., China in 2008 and 20228) and wars (e.g., Russia’s invasion of Ukraine affecting energy and subsequently fertiliser prices9), in addition to physical and logistical constraints, affect phosphorus resource availability, which in turn contributes to food price fluctuations. Long-term reliable access to phosphorus (which is stored in rocks, organic materials, and soils) is an essential part of a sustainable food system. All farmers require phosphorus, although some farmers require more phosphorus than others to achieve desirable yields because of local soil characteristics (e.g., those high in iron, aluminium, or calcium, and/or with a history of under-fertilisation), and not all farmers have the same physical or financial access to mineral phosphorus10,11. Over the last 60 years, the average amount of mineral phosphorus fertiliser required to produce food for one person, over one year, has risen by 38%, mainly due to the growing consumption of animal products12. For instance, between 1961 and 2013 the amount of phosphorus lost to support the average Chinese diet increased from <1 kg to ~5 kg of phosphorus per year per person13. Urban residents often have higher meat consumption and higher food waste production than their rural counterparts because of higher income, making them central in efforts to decouple wealth from unsustainable consumption patterns. Increases in animal product consumption, and changes in farm practices, have not only increased the use of mineral phosphorus fertilisers but also increased net losses of phosphorus to soil and water along the food production chain.

Currently, ~14 million tonnes of phosphorus are lost to global aquatic ecosystems every year14 causing a multitude of problems. Phosphorus is lost through runoff and erosion from fields and areas with livestock, as well as from human settlements with insufficient capacity to treat organic waste (especially human excreta and other wastewaters). Just as phosphorus and nitrogen can be used to boost crop growth, an excess of these nutrients in waterways (called eutrophication) can boost aquatic plant growth, which can cause problematic algal blooms and associated bottom-water hypoxia15. Eutrophication is estimated to cost the US economy over 2.2 billion dollars each year in losses associated with, for example, a reduction in biodiversity, recreational opportunities, and lakefront property value16. In addition, algal blooms in eutrophic waters are more likely to be dominated by cyanobacteria which are harmful to humans and animals. In 2014, the city of Toledo in the U.S. temporarily lost access to all drinking water because of cyanobacteria contamination17. The presence of hypoxic zones in lakes and coastal waters (prominent examples include the Gulf of Mexico18 and the Baltic Sea19) reduces fish and shellfish production, affecting livelihoods, food security, and ecological integrity. To avoid further damages, losses of phosphorus (and nitrogen) need to be addressed along the entire food production chain20,21, and treating phosphorus-containing residues from cities is a key piece of the puzzle.

Simultaneous and urgent action is required to address both issues of phosphorus scarcity for food security and phosphorus excesses for water quality3,22. More sustainable phosphorus management options23,24 fall into three broad categories, (1) increase efficiency and decrease waste/losses throughout the food system, (2) increase recycling of organic waste high in phosphorus (e.g., excreta and food and crop waste), and (3) decrease demand (e.g., change human diets, animal diets and reliance on particular species, and plant cultivars). The technology and know-how to make significant progress toward more sustainable phosphorus use in cities are already available25,26. However, a lack of awareness, acceptance, and governance currently holds back the uptake of phosphorus recycling approaches and effective use throughout the food system.

Carbon vs phosphorus cycling and the role of cities

The momentum gained on net-zero carbon initiatives may offer an opportunity to deliver also on sustainable phosphorus management. However, fundamental differences exist between ‘net-zero’ concepts for phosphorus and carbon because these elements cycle differently.

  1. 1.

    Whilst major pathways for problematic anthropogenic carbon emissions accumulate in the atmosphere, those for phosphorus flow from land to aquatic ecosystems. For carbon, local actions that reduce atmospheric emissions can have a large global impact through addressing the drivers of climate change. For instance, decarbonizing transportation and fuel consumption in any city’s territory is beneficial to the entire planet. For phosphorus, regional actions including reducing emissions from wastewater to aquatic ecosystems, are required to deliver local impacts including the provision of clean drinking water. This is because the effects of carbon emissions are moderated by atmospheric processes, and the benefits of reduction are on global climate systems. For phosphorus, the impacts of emissions are moderated by the sensitivity of the receiving lake, reservoir, river, or coastal ecosystem, and the benefits of emissions reductions appear along the transport pathway. For carbon, local actions anywhere are impactful, which means that some cap-and-trade or offset system could be meaningful in principle. Global, or even national, cap-and-trade or offset schemes are likely ineffective for phosphorus pollution. Coordinated actions among all phosphorus emitters within a watershed, including cities, are needed to protect the water quality of receiving water bodies. To address the food security angle of phosphorus management, even more coordination is needed. Global actions, such as reducing excess fertilizer demand, are likely required for meaningful local impacts, for example, ensuring access to affordable fertilisers to increase yields10,27.

  2. 2.

    Atmospheric carbon emissions, in the context of net-zero, are a ‘pollutant’ to reduce, whilst phosphorus is a resource to be better managed and conserved. The largest urban sources of carbon emissions are related to energy, meaning that substituting fossil fuels for renewable energy achieves a large part of a net-zero target. There is no substitute for phosphorus; it is an integral part of all organic matter which means it is impossible to have zero use on farms and zero outputs as residues from cities. For carbon, the discussion can (at least, theoretically) go from net-zero to zero. For phosphorus, the discussion must be around net-zero, where unavoidable urban and rural outputs are recycled back to where they are needed. Cities cannot only be concerned with reducing phosphorus emissions, they must also engage meaningfully with the food security angle of the phosphorus challenge. This means taking actions that will increase circularity, returning urban phosphorus ‘waste’ to food production, and decrease demand throughout the food system by reducing consumption and waste of products with a high phosphorus-demand.

  3. 3.

    City governments may feel they have less power to affect phosphorus than carbon. Although all sectors that must be mobilised to achieve net-zero carbon cities also touch phosphorus (Tables 1 and 2), the relative importance of each sector for phosphorus management is different. This means that different departments within city governments may need to be more mobilized, or different actors brought in to create meaningful change. For carbon, city governments often have some centralised power over sectors that can result in drastic emission reductions. For example, re-designing cities to limit cars and favour collective and active transport28 is in the purview of a small number of entities, even if it is individual residents that decide to drive or walk. Similarly, a city (or a public company) may be an energy provider (i.e., electricity or gas), meaning that they have power to change production methods, create different building codes or purchase orders, and decrease waste in a given infrastructure network. For phosphorus, wastewater, the management of green spaces (e.g., lawns, parkland, and agricultural lands within a city), and organic solid wastes are relatively more important than for carbon29. Changes to the infrastructure around these issues, in particular wastewater, can require long-term planning and centralized investment led by city governments. Importantly, capturing phosphorus before it enters a local waterway via recovery in wastewater, green spaces, or solid waste does not ensure desirable outcomes for food security or water quality. Recovery needs to be done in tandem with policies and practices that ensure it is recycled to produce food. Similarly, and as mentioned already in point two, coordinated actions are required across a watershed to deliver water quality benefits. City action plans need to embrace the fact that transformation must occur on multiple fronts, beyond administrative city boundaries, to ensure coupled carbon and phosphorus net-zero goals can be set, and, met30.

    Table 2 Questions to ensure urban decarbonisation includes phosphorus (P) sustainability as a way of meeting multiple Sustainable Development Goals (SDG).
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The concept of urban metabolism is helpful to describe and quantify how cities simultaneously concentrate resource flows (e.g., carbon, phosphorus, nitrogen, and others) on the landscape, whilst also having a large resource footprint beyond their physical boundaries31,32,33. One can track the amounts of resources entering a city, transformed within its boundary, and exported. In addition, one can account for the indirect use of energy or resources associated with direct flows, including, energy consumption, environmental footprints, and life-cycle assessments34. For carbon, there is a well-established greenhouse gas inventory framework35, but cities do not always include both emissions within and outside their boundaries. This makes comparisons between cities difficult and can obfuscate real efforts through greenwashing36,37.

In addition to describing flows, it is essential to consider networks of drivers (both social and natural drivers) for resource flows to find effective governance solutions31,38,39. Taking a systems approach is necessary to design urban social and physical infrastructures that account for how urban environments are dependent upon, and affect, multiple resources at multiple scales40. This is, in part, why we advocate for phosphorus, building on existing net-zero carbon approaches which are already integrated into diverse planning schemes.

Defining ‘net-zero phosphorus’ for cities

The ‘net-zero phosphorus city’ concept aims to improve phosphorus sustainability across product value-chains and underlying social norms. In practice, a net-zero phosphorus city will minimise inputs and outputs (particularly losses) of phosphorus from the city, recycle those outputs that cannot be eliminated, and support the efficient use of phosphorus, including the use of recycled phosphorus fertilisers on farms (Fig. 1b). In the context of the net-zero phosphorus concept, a city is defined as the administrative boundaries of densely populated areas, as we envision the framework being used by mayors and their municipal staff. Instead of reinventing the wheel for each interlinked sustainability priority, we suggest an expansion and alteration of existing net-zero carbon initiatives to explicitly tackle phosphorus. The power of net-zero carbon lies in the coordinated efforts of many cities which is exemplified by an urban session at COP 2641. Phosphorus management requires the same concerted effort, where cities learn from each other and collectively use their influence, and capital, to increase phosphorus recycling and reduce pollution along the food production and consumption chain.

We envision an iterative process where phosphorus flows are carefully monitored to allow for adaptive management. As mentioned in the previous section, mapping the resource use impacts of a city’s consumption patterns is complex due to their embeddedness within larger-scale infrastructural, social, and ecological systems42. To be effective, all net-zero phosphorus cities need a consistent monitoring approach to collect accurate and comparable data on phosphorus flows. Data should include the phosphorus content and footprint of food and non-food products, and phosphorus residues flowing in and out of cities. Urban supply chain footprinting would allow for the identification of dependencies and key interactions, for example, import pathways for fertilisers and loss pathways for products that incur a high phosphorus footprint.

The net-zero phosphorus concept embraces the idea that solutions need to account for the specific social, ecological, and technological context of a city43,44. Therefore ‘placed-based’ experimentation and collaboration will be necessary to achieve transformational goals. Understanding city-specific motivators and barriers to current phosphorus use and recycling are necessary to meet sanitation and food production objectives45. These may not be named ‘phosphorus issues’, but through a systems perspective one can identify how phosphorus is linked to existing priorities (e.g., Tables 1 and 2) and challenges. For example, poor road infrastructure may limit the capacity to move organic waste to recycle phosphorus46. We draw on lessons not only from the net-zero carbon movement but also on cities as living labs47, where investments in physical and social infrastructure are dynamic and reflexive. Importantly, experiments should be designed as learning spaces. To fully utilise the ‘power’ of the city to affect change beyond its borders, individual citizens need to be engaged in ways that affect social norms and behaviours on a wide spectrum of issues. To do so, learning must be explicitly and intentionally designed in experimental settings48.

In summary, the net-zero city concept can be used to motivate strategies designed to better integrate phosphorus management within climate change adaptation and mitigation plans49. Doing so will deliver wider benefits including supporting circular economies50 whilst contributing towards achieving multiple SDGs. Below we highlight the three major phosphorus flows that a net-zero phosphorus city would transform.

Minimise loss flows out of the city

Net-zero phosphorus cities would identify and control existing residue streams to reduce losses and deliver multiple benefits associated with ecosystem restoration. Managing these flows falls within the first of the three categories to address the phosphorus challenge: increase efficiency and decrease waste/losses. Actions may be centred on impacted ecosystems, accounting for the first difference between carbon and phosphorus cycling mentioned previously. For example, improving water quality in small urban waters (e.g., parkland ponds) to reduce human health risks for users, create new eco-tourism jobs, or increase shoreline property values, may be achieved through sub-urban groups targeting local emissions reduction actions. However, ensuring the delivery of emissions reduction programmes for transboundary lakes or marine ecosystems, for example, to support food production or freshwater and marine biodiversity protection, requires coordinated actions across multiple cities, and countries within multi-lateral Strategic Action Programmes, or conventions15.

Globally, human excreta and other organic residues contain about 3.3 Mt and 4.7 Mt phosphorus year-1, respectively, of which 1.1 Mt is recycled and the remainder is lost to landfill or discharged to surface waters14. With most people located in cities, a large proportion of these losses are concentrated in urban areas. In low-income countries, only 8% of wastewater undergoes treatment, supporting the often-cited approximation that, globally, over 80% of all wastewater is discharged without treatment51. Phosphorus discharge to rivers is likely to increase by 70% by 2050 without concerted efforts from global cities to treat human excreta52.

Up-scaling urban actions to deliver emissions reduction from local to transboundary scales is challenging. A net-zero phosphorus cities initiative may help to support coordination and collaboration across borders required to deliver on some of the world’s most challenging ecosystem restoration programmes. For example, the establishment of the International Commission for the Protection of the Danube River recognises that the Danube River carries waste from 27 cities on its path to the Black Sea. International cooperation has been vital in achieving reductions in urban wastewater and agricultural phosphorus discharges to the Black Sea, although further coordinated efforts are necessary to ensure ecosystem protection53.

Maximise recycling flows from the city to agricultural lands

A net-zero phosphorus city must not only collect concentrated phosphorus waste but also ensure productive reuse. Managing these flows falls within the second of the three categories to address the phosphorus challenge: increase recycling of organic waste high in phosphorus. Closing the phosphorus cycle requires linking urban areas to agricultural land outside cities, embracing the second difference between carbon and phosphorus cycling – phosphorus cannot be viewed as a pollutant to decrease but as a resource. Regional city collaborations could facilitate coordinated reuse to match agricultural needs, and global net-zero collaborations could provide an opportunity to learn from others in terms of technology implementation and mechanisms for social acceptability.

Instead of discharging phosphorus to waterways, ensuring the safe, and source-separated, collection of organic residue streams is necessary. This would involve changes in sanitation infrastructure, from toilet design to treatment technologies. In addition, unavoidable food and landscaping residues should be diverted from landfills and recycled. Many cities are located within agricultural regions where phosphorus derived from urban organic residues could be effectively recycled54,55. Around 50% of human urine in 56 of the world’s largest cities would need to travel less than 50 km to contribute to food production54. For Beijing (the 8th largest city in the world), 95% of the phosphorus in human urine could be recycled within 21 km. Key barriers, here, include insufficient transport logistics and ensuring market opportunities for fertilisers derived from urban organic residues56,57, in a fertiliser market dominated by mineral-derived fertilisers. The economic value of recycled fertilisers can be maximised by selecting methods to process organic materials that produce additional co-benefits, such as renewable energy from biogas production and recovery of other nutrients58. Currently, examples of small-scale biodigester programmes can be found in more than 50 countries across Africa, Asia and South America, as a way of advancing agricultural productivity, renewable energy use, and residue management59.

There is no ‘one solution fits all’ for the treatment technologies or recycling arrangements for cities; experimentation and testing will be required. Importantly, concerns about safety related to the re-use of phosphorus from urban organic residues, in particular human excreta, remain a barrier in many places. Multiple waste treatment technologies are available that address human health concerns and are appropriate for upscaling60. Citizen participation and education through the expansion of phosphorus-sensitive urban green spaces (e.g., community gardens properly using recycled phosphorus), and climate-smart buildings with phosphorus recovery sanitation, could help influence consumer preferences for phosphorus management in imported food and alternative waste infrastructure.

Minimise losses before food flows into the city

Net-zero phosphorus cities would accelerate the implementation of programmes that minimise the need for phosphorus in food production and support environmentally and socially sustainable farming practices globally. Managing these flows falls within the third of the three categories to address the phosphorus challenge: decrease demand (change human diets, animal diets and reliance on particular species, and plant cultivars), but also supports effective actions in the two other categories. As powerhouses of consumption cities drive food systems through the sum of individual, company, and community choices. From product selection to acceptable production methods, these choices have an impact on phosphorus use. Here cross-city collaborations would be a space for peer-to-peer learning on how to effectively facilitate behaviour change for residents and companies within cities to reduce upstream losses of phosphorus. This pertains to the third difference between carbon and phosphorus cycling – that city governments must engage with sectors where they have less centralized power, and thus where a systems approach to coordination will be needed.

Supporting low-animal product diets is an essential action to transforming the phosphorus cycle, even if it is challenging61. For example, if by 2030 Beijing residents adopted a healthy low-meat diet, as recommended in the EAT-Lancet report, losses of phosphorus related to supplying the city with food could be reduced by over 80%. If residents, in addition to eating less meat, consumed only products produced using phosphorus efficient practices (e.g., a 50% reduction in fertiliser, crop, and food losses along the food chain) then, overall, three times less phosphorus would be released to the environment. More specifically, food production to support Beijing’s population today releases food 81 Gg of P (multiplying the phosphorus footprint per capita in China13 and the population in Beijing62); but this would be reduced to 30 Gg of phosphorus released to produce food for the city in 2030 despite population growth (calculated by matching EAT-Lancet food category amounts per capita as phosphorus63 to phosphorus footprint per food category in China13 and halving these amounts, and then multiplying by projected 2030 city population64).

Changing food purchasing and consumption behaviours (as well as waste behaviours) at the individual level is extremely challenging as it is a habitual behaviour that is also culturally, socially, and financially embedded65,66,67. Still, cultural norms have changed and as such can, and will, change again. For example, and in relation to carbon cycling, holiday air travel in Sweden went from being considered a luxury, to fairly recently being viewed as an integral part of mobility, to now being disrupted by holidays that stay on the ground due to consumer concerns over climate change68.

Experiential learning69 (learning by doing and being as opposed to increasing knowledge) can support resident behaviour changes. Instead of being told to eat less meat, people should experience nutritious and tasty low-animal product meals and be part of creating these meals. Registration for catered city events, including conferences and festivals, could specify catering requirements which put environmentally friendly meals front and centre, as well as alternative sanitation facilities. Similarly, public procurement and meal preparation for schools, hospitals, and other government-supported institutions could increase exposure/uptake of foods that support diets with low phosphorus footprints, reduced food waste, and organic waste recycling. Menus can be co-created with users and can be part of larger sustainability initiatives and living labs. Community gardens and other forms of urban agriculture may also be a fruitful space for learning and experimentation with regard to nutrient cycling and sustainable food production70,71,72.

Overall, urban purchasing should support farmers who produce food with a low phosphorus footprint and utilise recycled phosphorus products in ways that minimise damage to ecosystems and optimise fertiliser use to reduce reliance on mined sources. Collecting data on human activity and preference will be key to re-adjusting behaviour-change campaigns and technology options in the city and provide feedback to food producers about acceptance related to urban-derived fertilisers.

Where do we go from here?

The role of cities in multi-level governance has been highlighted in plans for climate change mitigation73, and phosphorus sustainability46. However, fully leveraging the power of what constitutes a city is an ongoing academic and practical challenge. Although cities concentrate resource flows on the landscape and represent a powerhouse of consumption, and thus resource use outside of city boundaries, cities are not a singular entity. They encompass a collection of individual resident and company decisions, while also remaining dependent on the individual decisions of producers outside the city, and regional and national policies and plans. Given the pressing nature of the climate crisis, the phosphorus challenge, and other crossed planetary boundaries74, urgent action is required to test how cities can act to meet local and global goals. For example, the Our Phosphorus Futures Report calls for governments to consider the ‘50:50:50’ Goal; a 50% reduction in global phosphorus pollution and a 50% increase in the recycling of phosphorus lost in residues and wastes, by 205022.

‘Green shoots’ of progress for phosphorus sustainability exist in some cities but are siloed to a particular issue. For instance, national mandates for phosphorus recovery from wastewater treatment plants over a certain size in Germany, Switzerland, and Austria are enabling many cities to invest in recovery technologies from incinerated sewage ash75. Many states in the U.S. have banned the use of phosphorus fertilisers on lawns, which has helped cities reduce losses to local waterbodies29. In isolation, such measures will fail to protect waterbodies sufficiently, and importantly do not ensure food systems are more phosphorus secure.

Transforming cities to net-zero phosphorus will require nothing short of an overhaul of sanitation systems, the relationship of cities to the hinterlands that produce their food, and how residents view their actions as consumers and infrastructure users. We call for mayors to take bold actions by adding a phosphorus lens to the momentum they are already creating to tackle urban and global sustainability challenges through net-zero carbon initiatives and SDG reporting schemes. We suggest they;

  1. 1.

    Review existing plans to explicitly identify areas of overlap with phosphorus (as shown in Table 2),

  2. 2.

    Create specific phosphorus performance indicators that can be monitored for each of these overlaps,

  3. 3.

    Develop the capacity to monitor these indicators because sustainable transformation is an iterative process and monitoring to course-correct is essential,

  4. 4.

    Enact policy changes and on-the-ground experimentation to re-invent cities as producers of sustainable phosphorus fertilisers and conscientious circular economy consumers.

By adding a net-zero phosphorus approach to their endeavours, cities will be better able to ensure residents have access to affordable and reliable food and clean waterways, and the benefits that these deliver.

The United Nations Framework Convention on Climate Change is pioneering the ‘Race to Zero’76 global campaign to rally leadership and support from businesses, cities, regions and investors to achieve net-zero carbon emissions by 2050. Currently, over 1000 cities, 5000 businesses, 400 investors and 1000 education institutions have joined this alliance, covering nearly 25% of global CO2 emissions and over 50% of gross domestic production. A similar campaign could be used to establish a net-zero phosphorus cities network. This would support coordinated actions and knowledge exchange between cities and the diverse actors that are required to improve urban phosphorus sustainability and achieve relevant SDGs. We call for action, not only from individual cities and mayors, but also for similar collaboration, perhaps led by a relevant United Nations body, to establish a special task force on net-zero phosphorus for cities, to coordinate such a network, and to ensure priority actions align with SDG targets (Tables 1 and 2).