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The circular nutrient economy: needs and potentials of nutrient recycling


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Food production around the globe rests significantly on industrial fertilizers that make use of virgin phosphorus sources and energy-intensive fixation of nitrogen. At the same the nutrients ending up in the residues of the food system are inefficiently used as fertilizers. When produced in large quantities, organic biomasses such as animal manure may transform into a source of harmful nutrient pollution. This chapter argues for a transition from the utilization of virgin nutrients to nutrient recycling. It specifies criteria for a sustainable transition and provides an overview of the technological, practical and institutional dimensions critical for the change. The transition to the circular nutrient economy calls for policy measures that restrict inefficient nutrient use; generate demand for recycled fertilizers; and provide support for biomass processing before the markets for recovered resources are up and running. Public and private investments, technological developments and institutional shifts can create conditions for profitable and safe production and consumption of recycled nutrients.
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This is a draft chapter. The final version will be available in Handbook of the Circular Economy edited
by Brandão, M., Lazarevic, D., Finnveden, G, forthcoming 2020, Edward Elgar Publishing Ltd. The
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The circular nutrient economy: needs and potentials of nutrient recycling
Helena Valve
Finnish Environment Institute
Latokartanonkaari 11, 00790 Helsinki
Petri Ekholm
Finnish Environment Institute
Latokartanonkaari 11, 00790 Helsinki
Sari Luostarinen
Natural Resource Institute Finland
Tietotie 4, 31600 Jokioinen
Food production around the globe rests significantly on industrial fertilizers that make use of virgin
phosphorus sources and energy-intensive fixation of nitrogen. At the same the nutrients ending up
in the residues of the food system are inefficiently used as fertilizers. When produced in large
quantities, organic biomasses such as animal manure may transform into a source of harmful
nutrient pollution. This chapter argues for a transition from the utilization of virgin nutrients to
nutrient recycling. It specifies criteria for a sustainable transition and provides an overview of the
technological, practical and institutional dimensions critical for the change. The transition to the
circular nutrient economy calls for policy measures that restrict inefficient nutrient use; generate
demand for recycled fertilizers; and provide support for biomass processing before the markets for
recovered resources are up and running. Public and private investments, technological
developments and institutional shifts can create conditions for profitable and safe production and
consumption of recycled nutrients.
Key words: Biomass processing, carbon, fertilizers, organic matter, phosphorus, water protection
Funding information: Academy of Finland, Grant Number: 286368; Strategic Research Council at the
Academy of Finland, Grant Number: 312650.
1. Introduction: from linear nutrient flows to the circular nutrient economy
Phosphorus (P) and nitrogen (N) are the two central macronutrients without which food production
would not be possible. Due to the criticality of nutrients, the production and consumption of
fertilizers have generated complex political economies. Before the takeover of manufactured
mineral fertilizers in 1950s, animal manure counted as a key source of phosphorus and nitrogen.
While this ‘naturally recycled’ (Dawson and Hilton 2011, p. 14) nutrient source still plays an
important role in crop production, the consumption of mineral fertilizers continues to rise, the
demand for P fertilizers expected to be about 2.4 times the 2000 level by 2050 (FAO 2017; Meers
2016, Tilman et al. 2001). This has invoked serious concern about the solidity and sustainability of
food production.
Today, it is impossible to see how the growing populations could be fed without mineral fertilizers.
At the same time it is clear that food production cannot continue to rest on them to the extent it
does today. A transition is needed from the utilisation of virgin nutrients to nutrient recycling,
recovery and reuse. The justifications for a circular nutrient economy are similar to those of circular
economy more generally (see chapters x and y in this volume). On the one hand, it is becoming
increasingly evident how unsustainable current linear systems are. On the other, a shift towards
more circular systems and closed nutrient loops can help to generate added value from waste and
surplus materials while decreasing environmental pollution and degradation.
Mineral fertilizers have many benefits. Most importantly, they have enabled the growth of human
population without the need to use all applicable land area for cultivation (Tilman et al. 2002).
Mineral fertilizers have also practical assets. For example, the products are tailored to meet crop
needs and are easy to transport and use. However, reliance on mineral fertilizers is problematic for
three reasons.
First, the finite reserves of the key raw material non-renewable phosphorus are located
unevenly around the world, with three countries controlling more than 85 per cent of the known
phosphorus reserves (Buckwell and Nadau 2016; Dawson and Hilton 2011; Elser and Bennett 2011;
Sutton et al., 2013). This makes phosphorus availability vulnerable to geopolitics and global crises
(Obersteiner et al. 2013). Significant price increases would threaten food security, especially in low-
income and food-deficient countries.
Second, production of mineral nitrogen fertilizers rests mostly on the use of fossil energy.
Approximately two per cent of world energy use is dedicated to the industrial manufacture of
nitrogen fertilizers (Sutton et al. 2013). Fertilizer production is thus a source of carbon dioxide
emissions. Energy dependence also implies that any rise in energy prices increases fertilizer prices
and threatens food security.
Third, mineral fertilizers alone provide an unsolid basis for further development of food systems
simply because they are mineral, devoid of organic matter. Organic matter and its return to
farmland are essential for soil quality, contributing to higher yields, soil carbon stocks and higher
nutrient use efficiency. However, while nutrients in an organic form, such as farmyard manure, may
produce lower phosphorus losses to the environment than a similar dose of mineral fertilizers, for
example via improving soil structure (Cai et al. 2019; Oldfield et al. 2019), the size of phosphorus
losses is mainly affected by the timing and mode of spreading rather than fertilizer type.
The problems of mineral fertilizers shift attention to organic nutrient sources. This move may seem
as a return in time; to the era before mineral fertilizers with crop cultivation relying on manure
nutrients and biological nitrogen fixation and on clearing of new farmland (e.g. slash and burn)
when the nutrient reserves run out. In the large scale, however, going back in time is impossible.
For one thing, across much of Africa and Latin America, as in parts of South East Asia, the demand
for nutrients cannot be met through nutrient recycling only. Meanwhile, in regions with intensive
livestock production, the spatial reach of nutrient cycles must be extended beyond the local scale to
enable efficient redistribution of nutrients (Sutton et al. 2013).
Nutrient recycling does not reduce environmental degradation or provide economic benefits in any
straightforward fashion. Box one summarizes the key conditions on which recycling can be
sustainable. The challenges in reorganization of material cycles are practical and technological:
efficient recycling requires mobilization of processing technologies that enable nutrients extraction
from waste and surplus biomasses. But since public policies play a key role in the promotion of
technological innovation and market creation, nutrient recycling is significantly also a policy issue.
This chapter aims to give an overview of the dimensions shaping the potentials of nutrient recycling
as a component of a circular economy. It provides an introduction to the potentials, dilemmas and
means of resource recovery. The chapter focuses on critical aspects affecting nutrient recycling in
agriculture, but proposes that nutrient recycling should be seen as a multidimensional, cross-
sectoral phenomenon, see box 1.
Box 1: Sustainable nutrient recycling and reuse
- Increase nutrient use efficiency by exploiting legacy nutrients in soil, using virgin
and recycled nutrients only according to verifiable crop needs and paying attention
to animal diets; inefficient nutrient use causes environmental harm irrespective of
the origin of the nutrients.
- Replace mineral fertilizers with organic ones, but they need not exclude each other:
future fertilizers and fertilization practices may combine the benefits of both.
- Support development of carbon-neutral food systems by returning organic matter
to the soil so that the reserves of organic matter in the soil can be, at minimum,
sustained. The mineralization dynamics in soil need to be studied further to find
means by which this goal can best be promoted.
- Recouple crop and livestock production by investing in manure processing and
nutrient recovery in livestock production to enable nutrient transportation and use
according to the crop needs.
- Minimize losses and ensure the safety of the material loops. Nutrient recycling
cannot happen at the expense of environmental and food safety. Extensive use of
antibiotics in animal husbandry, for example, hampers resource recovery.
- Contribute to the transition towards more sustainable food systems. It is important
to keep in mind that the high phosphorus demand (Metson et al. 2012) and
environmental degradation caused by increased meat consumption can only partly
be compensated by more efficient nutrient recycling.
2. Biomass potentials and the importance of nutrient use efficiency
In principle, the potential for fostering nutrient recycling is huge. For example, out of the 17.5
million tons of phosphorus mined in 2005, only three million tons made it to food. The largest loss
(about eight million tons) was directly from farms through soil leaching and erosion. One million
tons of phosphorus goes with spoiled or wasted food. More than seven million tons of phosphorus
were released to the environment e.g. through animal manure (Cordell et al. 2009). Trapping
phosphorus in human excreta would account for 20 per cent of global fertilizer consumption.
However, making use of human or animal excreta, or any other potentially recyclable nutrient pool,
calls for viable technologies for concentration and fractionation to produce feasible fertilizer
The potentials and prospects of nutrient recycling depend on the qualities and quantities of the
organic biomasses accumulating in specific geographical areas. One way to start the analysis is to
see how the biomass reserves compare in terms of the nutrient recycling potentials they offer. For
example, within the European Union, manure is clearly the largest reserve of recyclable nutrients.
It is estimated to provide for more than 70 per cent of the current total recovered N and P from all
recyclable sources (Buckwell and Nadau 2016). Country-specific estimation focusing on Finland also
points to the significance of manure potentials: out of different biomasses, manure covers 75 per
cent of all recyclable phosphorus in the country (Marttinen et al. 2018).
Of the different biomasses, sewage sludge tends to be rich in P, but it is often tightly bound by iron
or aluminium and thus poorly available for crops. Sewage sludge may also contain contaminants
such as pathogens, pharmaceuticals, harmful organic compounds and heavy metals. While the risks
for soil and crop contamination continue to be debated, some countries already limit the use of
sewage sludge in agriculture (e.g. Germany, Switzerland) and some food processing industries
decline purchasing crops grown with sewage sludge (Marttinen et al. 2018). The restrictions have
turned attention to technologies that enable phosphorus to be recovered from sewage sludge as a
pure chemical, such as phosphoric acid and struvite (Buckwell and Nadau 2016).
The recycling of manure nutrients is impeded by the differentiation of livestock and arable
agriculture meaning that farms have specialized either on animal husbandry or on crop cultivation.
Specialization has led to regional differentiation and nutrient imbalance: Regions with intensive
animal husbandry are coping with high manure phosphorus surplus while the regions dominated
with crop cultivation suffer from a deficiency of organic fertilizers (Jarvie et al. 2015; Marttinen et
al. 2018; Withers et al. 2015). In the conditions of excess manure, manure easily appears as a
surplus biomass that just needs to be located somewhere as cheaply as possible, but the use of
phosphorus in excess to crop uptake results in its accumulation in the soil or hinders decreasing the
level of so-called legacy phosphorus accumulated due to previous high fertilization rates. This
increases the risk of nutrient losses and related environmental problems (see box two). Legacy
phosphorus and its better exploitation is a topic under intensive research (McCrackin et al. 2018).
Furthermore, the differentiation of the production types may affect the reserves of organic matter
in farmland. On areas specialized in crop production and mostly mineral fertilizer use, the loss of
organic matter from field soil occurs at a higher rate than on areas with manure use. For example,
Gerzabek et al. (1998) noted that manure use increased soil organic matter under repeated
application. However, contrary results have also been reported; e.g. Hu et al. (2018) did not notice
any difference in long-term field trials between fertilizing with manure or mineral fertilizers. The
potential carbon sink of farmland and the factors influencing it are also under intensive research.
Figure 1 concretises the problems caused by the geographically differentiated agriculture in the U.S.
(Jarvie et al., 2015). The areas of intensive livestock production and cities are the major ‘hotspots’
of surplus phosphorus production from manure and human waste. On the other hand, the grain
production areas form phosphorus ‘coldspots’ (see Metson et al. 2016) located hundreds of miles
away from the phosphorus hotspots. There is currently ample phosphorus produced in manure and
human waste in the U.S. to satisfy phosphorus demand for crop production. In 2010, the total
phosphorus produced by livestock and humans in the U.S. exceeded the national phosphorus
fertilizer demand by around 1.3-fold. Due to long distances, a large proportion of phosphorus in
animal manures and urban wastes is no longer recycled back to land where the feed grain was
produced. As greater specialization of farming continues, areas of grain production are becoming
almost entirely dependent on inorganic phosphorus fertilizers, rather than recycled phosphorus
from manure and human wastes (Jarvie et al. 2015). This reliance on inorganic phosphorus
fertilizers has implications not only for long-term phosphorus security but also for soil carbon and
soil health.
Figure 1. Distribution of net P production and net P demand (tonnes P) by counties across the
contiguous United States in 2010. Values are derived from the difference between the sum of
livestock P produced plus human P contributions minus inorganic fertilizer P applied. Negative
values indicate areas of net P demand (called as nutrient ‘coldspots by Metson et al. 2016), where
inputs of P are required to support crop production; positive values show the areas of net P
production (nutrient ‘hotspots). The hotspots are located around the beef feedlots in California
and the Texas panhandle, in the dairy farms of Wisconsin and Idaho, in the integrated poultry
operations of Arkansas, Georgia and Maryland, and in the swine farms of North Carolina, Iowa, and
Wisconsin. The coldspots coincide with crop production in the Midwest and Mississippi Valley.
Source: Jarvie et al. (2015).
Since manure, particularly pig and cattle slurry, tend to have high water content, they are expensive
to transport. Even within a single farm, manure is often spread close to the animal shelters and
manure storages rather than transported to more remote fields. Subsequently, the phosphorus
hotspots and coldspots cannot be recoupled without investment in manure processing and nutrient
Where do manure and its nutrients then end up in regions of intensive livestock production and
long distances to crop fields in need of fertilization? In industrialized countries, regulation draws
boundaries between manure spreading counting as nutrient recycling and that which qualifies as
forbidden dumping of the biomass. In the EU, the Nitrates Directive sets thresholds for maximum
amounts of manure-based nitrogen that can be spread to areas classified as Nitrate Vulnerable
Zones (NVZ). Regulatory thresholds and economic incentives also aim to limit phosphorus
fertilization. However, the legal boundaries can sometimes be lax.
Box 2: Environmental impact of low nutrient use efficiency
Since the middle of the twentieth century, environmental flows of phosphorus have accelerated 4-fold,
humanity creating a largely one-way flow of phosphorus from rocks to food system and further to lakes and
oceans, impairing freshwater and coastal marine ecosystems (Childers et al. 2011).
Inputs of phosphorus, whether in the form of mineral fertilizers or manure, exceeding the crop uptake are
built up in soils and gradually lost to water. In aquatic systems, phosphorus boosts the growth of algae,
which renders water turbid, triggers toxic algal blooms and consumes oxygen from near-bottom water layers
as the settling algae are mineralized. Oxygen-depleted marine coastal ‘dead zones’ are becoming larger in
size and coincide spatially with world’s agricultural areas (Rosenberg and Diaz 2008). Such symptoms of
eutrophication have a profound effect on the entire ecosystem. In addition to lowered biodiversity,
eutrophication negatively affects fisheries, water security and recreation.
For example, in Finland the farms not taking part in the voluntary agri-environmental scheme are currently allowed to
apply as much as 65 kg/ha/y of phosphorus and this can also be applied as a five year storage fertilization (325 kg/ha at
a time), if the N limit of the Nitrates Directive is not met first. Still, in policy discourses it is often argued that all manure
and manure nutrients are already being fully recycled since the matter is being spread to fields within the existing
Fertilizer use and legume crops have doubled total annual nitrogen inputs to global terrestrial ecosystems
(Tilman et al. 2002). Nitrogen also boosts eutrophication, especially in marine systems. Excess nitrogen also
poses a threat to groundwater. Because nitrate is not bound to soil particles, it can freely move downward in
the soil horizon. Elevated nitrate concentrations in groundwater can prevent the use of an aquifer as a water
Not all nitrogen lost from agricultural systems enters surface or ground waters but is emitted to air as nitrous
oxide (N2O), a potent greenhouse gas, nitrogen oxides (NOx) or ammonia (NH3). The latter two can be
transformed in the atmosphere so that they compound to climate change and pose a threat for human
health. They can also be transported over long distances and deposited to terrestrial and aquatic systems far
away, accelerating their eutrophication, acidification and loss of biodiversity.
3. Technologies for biomass processing and nutrient recovery
Nutrient deficits in agriculture have traditionally been prevented by reusing the nutrient-rich
manure and leaving the crop residues on the fields. In the regionally and production-wise
differentiated conditions of modern agriculture, however, efficient and sustainable recycling must
make use of technologies that serve nutrient recovery and biomass processing. Processing is a
means to enhance transportability and feasible reuse of the recyclable biomasses on areas of
surplus nutrients and in municipalities and industries with significant quantities of by-products. The
processing enables nutrient recycling through concentrating the nutrients into smaller volumes;
separating them into different fractions; enabling reuse of the organic matter; and controlling risks
related to harmful substances and hygiene (Marttinen et al. 2018).
Accordingly, one technology rarely does the trick alone, and typically technologies are cascaded
together into processing chains producing several different end-products (Buckwell and Nadau
2016; Frear et al. 2018; Marttinen et al. 2018; Meers 2016). Furthermore, since recycled fertilizer
products often differ from mineral fertilizers, the infrastructures and practices related to their
storage and spreading also require development.
There are many different processing technologies currently available (Table 1). Marttinen et al.
(2018, p. 16) differentiate between the following categories:
1) Physical and mechanical technologies separating biomasses into fractions
2) Biological techniques degrading biomasses microbiologically
3) Thermal methods reducing the mass to be processed and concentrating the selected
compounds at elevated temperatures
4) Chemical methods extracting and concentrating biomasses through chemical reactions
The choice of processing technologies significantly affects how biomass turns into resources and
how the end-products can be used, as they vary in the extent they alter the biomass, modify the
nutrients and produce or consume energy. From the fertilization point of view, the changes in the
biomass stability, chemical state of the nutrients and their ratios in the products are critical.
Many of the mentioned technologies in Table 1 are already being used for processing manure,
sewage sludge and municipal and industrial bio-wastes. These include such mature techniques as
mechanical separation, anaerobic digestion, composting and incineration. Some techniques are still
under development with few full-scale applications already in use. These include e.g. membrane
technologies and struvite crystallization. Some techniques require substrates with higher dry matter
content, such as composting and thermal techniques. Some are suitable for only liquid substrates,
such as ammonia stripping and membrane technologies. The idea is that in a processing chain
several types of end-products can be formed and their composition and thus use differs.
Anaerobic digestion in combination with digestate refinement makes a good example of a
potentially effective processing chain. The nutrients are retained in the digestate and only a minor
part of the organic matter is converted to biogas. The digestate contains more soluble nitrogen
than the original feed materials due to mineralization of organic nitrogen during the process. This is
positive for N use efficiency alone, but the digestate can be further tailored into concentrated
recycled fertilizer products transportable to fields in need of such fertilizer products and spreadable
according to soil condition and crop demand. For instance, efficient separation of the digestate into
solid and liquid fractions and additional concentration of the liquid via membrane technologies or
ammonia stripping produce several recycled fertilizer products. The separated solid fraction
contains most of the organic matter and P, while the liquid concentrate from membranes contains
soluble N and P and the liquid fertilizer from ammonia stripping only N (e.g. ammonia sulphate).
The removed water can be used e.g. to adjust the substrate dry matter content for the digester
demand. The overall energy balance of such a processing chain is usually positive and the biogas
produced can be used as heat, electricity or transport fuel further decreasing the climate impact of
the processing chain.
Table 1. Modified from Marttinen et al. 2018. Commercial and developing processing technologies
for nutrient recovery and processing of biomasses.
Aim of processing
Mechanical and physical separation techniques
Separation into a solid
and liquid fraction or
water removal.
Nutrient ratios change
but their availability for
crops does not
Nitrogen mainly found in the
liquid fraction and phosphorus in
the solid fraction
drying and
Some of the nitrogen may
volatilize during the process
(recovery possible), phosphorus
remains in the dried product
Precise separation of nutrients
Biological techniques
Provides a stabilized,
easy-to-use organic soil
Some of the nitrogen evaporates
(recovery possible in closed
systems), phosphorus retained in
the composted biomass, organic
matter as soil improver
Stabilises the biomass
and makes it easier to
thermophilic digestion
provides sanitation;
energy production
Nutrients retained, share of
soluble nitrogen increases,
phosphorus remains available for
crops, organic matter as soil
Thermal techniques
E.g. pyrolysis,
High temperature causes
evaporation of water
reducing the biomass
volume and making it
more transportable;
quality of end-products
depend on the
temperature used;
energy production
High temperature reduces
phosphorus availability for crops
and slows down the
decomposition rate of carbon
added to soil. Nitrogen capture
required already during pre-
drying the biomass.
Incineration removes organic
compounds and phosphorus
retained in the ashes
Chemical techniques
E.g. ammonia
Different aims, e.g.
reconditioning the
biomass and making it
easier to process further
Possible production of an
inorganic nutrient product, the
properties of which are similar to
those in inorganic fertilizers
4. Policies and politics of nutrient recycling
As a policy issue, nutrient recycling and its promotion break established sectoral divisions. In the
conditions in which crop cultivation and animal husbandry have separated practically and
geographically, the generation of more closed cycles of nutrient provision cannot be expected to
happen through agricultural policy measures alone. When the issue is defined in terms of biomass
processing and nutrient recovery, it soon becomes apparent that it also links to energy production,
transportation, waste management and development of food safety. Policies within these sectors
shape the conditions in which the recovery of waste and surplus biomasses can transform into
profitable business and the recovered outcomes replace ‘linear-economy’ commodities.
Radical reorganizations of material circuits cannot be expected to take off spontaneously; without
strong and coherent public steering. Although the transformation of waste and surplus biomasses
into nutrient resources can well be profitable, economization of the activity calls for incentives that
encourage investment in new production modes and processing technologies. In the European
context, Meers (2016, p. 4) calls for development of a third (after crop and animal production)
agro-industrial pillar to be developed in addition to and support of the two existing main pillars of
agricultural activity, namely agro-residue processing and upcycling.
Markets for organic fertilizers are mostly non-existent, but their development can be assisted. In
this, systems of standardization and verification play an important role. The standards ‘should cover
the nutrient content, the maximum level of impurities which could be a threat to health, safety and
environment, and product quality and application techniques (Buckwell and Nadau 2016, p. 13). In
the EU, the fertilizers regulation is being revised to ease the access of organic fertilizers to the EU
single market. The draft regulation defines safety, quality and labelling requirements that all
fertilizing products need to comply.
The safety criteria set restrictions to the use of different biomasses. If they contain residues of
contaminants or antibiotics, for example, the spectrum of valorization pathways narrows down.
This affects the cost-efficiency of resource recovery. Since animal manure is the most substantial
source of recyclable nutrients, promotion of nutrient recycling starts at the animal shelters in which
animals are fed and nurtured (Udikovic-Kolic et al. 2014).
It is also important to recognize that the combination of nutrient recovery with energy recovery
may provide a basis for sound business particularly when the energy i.e. biogas is refined into
transport fuel. However, promotion of biogas production and biogas-based mobility alone is not
enough. The overall sustainability and efficiency of biogas-based nutrient recycling is dependent on
the extent to which investments are made in the refinement of the digestate so that it can be
transported and used where and when needed. In the advancement of new modes of biomass
processing and resource recovery, public research and innovation policies play a critical role.
The fact that promotion of sustainable nutrient recycling requires input from many fields and policy
sectors implies that naming specific nutrient recycling instruments is difficult. For example, all
restrictions designed to prevent nutrient loading and foster nutrient use efficiency can significantly
support nutrient recycling by forcing to search for alternative solutions for biomass treatment and
nutrient management. At the same time too loose fertilization limits may act against a system
change. The multidimensionality of the policy issue implies that the design of an effective policy mix
must engage expertise from various fields.
Nutrients recycling often transforms into a policy issue under the heading of phosphorus policy. In
Japan, a Phosphorus Recycling Promotion Council was established after China imposed high duties
on phosphorus exports (Shiroyama et al. 2015). In U.S and the EU, phosphorus is defined as a
critical raw material (Elser and Bennett 2011; Meers 2016). In Switzerland, recovery and recycling of
phosphorus in the form of inorganic products from all sewage sludge and slaughterhouse waste has
been mandatory since 2016 (Buckwell and Nadau 2016). However, when nutrient recycling is
defined in terms of phosphorus only, there is a risk that the other numerous benefits of organic
fertilizers remain unaddressed in the development of new fertilizer products. The hot spots of
nutrient accumulation, and the problems caused by nutrient pollution, may also gain only limited
2, accessed 5 April 2019.
5. Conclusions
A transformation from linear nutrient flows towards circular nutrient economy can serve numerous
societal and environmental objectives. The replacement of mineral fertilizers with recycled ones has
the potential to increase material and energy efficiency of food production. If the quality of the
recycled fertilizers can be guaranteed, the products can significantly enhance food safety. Reduced
or stagnated growth of mineral fertilizer use, along with more wide-spread circulation of organic
matter, contribute to climate change prevention. This contribution grows if biomass processing is
based on anaerobic digestion and generation of biogas. Moreover, when nutrient recycling is used
to solve challenges with regional nutrient surpluses stemming from intensive livestock production,
the redistribution of manure nutrients can increase nutrient use efficiency and reduce harmful
nutrient loading.
Of different waste and surplus biomasses, manure tends to be the most important source of
recyclable nutrients. With regionally concentrated animal production, increasing the
transportability of manure nutrients is essential and can be achieved via manure processing.
Technological solutions for processing are increasingly available. When placing aims for the circular
nutrient economy, also the recycling of the organic matter should be kept on-board. Furthermore,
the energy balances of the processing chains should be optimized so that energy efficiency goals
are not compromised.
The transformation to nutrient recycling cannot be created, or its benefits attained, without careful
tailoring of policy mixes that restrict inefficient nutrient use; generate demand for recycled
fertilizers; and provide support for biomass processing before the markets for recovered resources
are up and running. The measures required for the shift into nutrient recycling are only achieved via
robust and coherent actions, including significant investments in biomass processing and resource
recovery. Nonetheless, societal and environmental benefits can be expected to exceed the costs of
public resource allocations. Together the technological developments and institutional shifts create
also a breeding ground for profitable business.
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... One crucial but often overlooked element of the regenerative nature of a CE is the return of nutrients to land (e.g., recycling of biowastes and renewable fertilisers derived from them) in ways which maintain soil health and fertility and rebuild natural capital [8,10]. This is vital to avoid (1) nutrient deficits in agricultural land where they are essential for food production and (2) nutrient escape and accumulation as air, water, and soil pollution, with significant consequences for ecosystem (and human) health [11,12]. ...
... Therefore, while the recycling of nutrients must be a key component of a CE more generally, the environmental damage, food system vulnerability, resource insecurity, and market volatility associated with the current patterns of nutrient use and cycling within food systems have led to increasingly insistent calls for the specific development of a circular nutrient economy (CNE) [10,[20][21][22]. A CNE may be broadly defined as "the reduction of nutrient losses-during agricultural production, processing, distribution, and consumption-along with comprehensive recovery of nutrients from organic residuals, for reuse in agricultural production" [23]. ...
... The chemical (blue) nutrient cycle is predominantly linked to residues of energy generation from biomass (normally ashes and digestates) and to technologies for extracting selected nutrients from a waste matrix (e.g., struvite removal from wastewater) [27,28]. There is increasing interest in exploring more opportunities to extract chemicals from organic waste to overcome some of the barriers to their use as secondary nutrient sources [10,29]. However, this risks exacerbating the existing decoupling of nutrient flows from the return of organic matter and carbon to the soil, depleting the soil's health and compromising its role as a carbon sink. ...
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The “circular economy” is an increasingly influential concept linking economic and environmental policy to enable sustainable use of resources. A crucial although often overlooked element of this concept is a circular nutrient economy, which is an economy that achieves the minimization of nutrient losses during the production, processing, distribution, and consumption of food and other products, as well as the comprehensive recovery of nutrients from organic residuals at each of these stages for reuse in agricultural production. There are multiple interconnecting barriers to transitioning from the current linear economic system to a more circular one, requiring strongly directional government policy. This paper uses interpretive policy analysis to review six UK government strategies to assess their strengths and weaknesses in embracing nutrient circularisation. Our analysis highlights the acute underrepresentation of the circular nutrient economy concept in these strategies as well as the potential to reorient the current policy towards its development. We find significant barriers to transition presented by ambiguity in key policy terms and proposals, the use of inappropriate indicators, the lack of a systematic approach to key sustainability objectives, and the presence of a “techno-optimist imaginary” throughout the strategies. We develop these findings to make recommendations to help integrate definitions, objectives, and activities across the policy domains necessary for the operational development of a circular nutrient economy.
... Increasing the use of manure as a processed fertiliser would help reduce the use of inorganic fertilisers, particularly phosphorous, which is an exhaustive resource. Also, the production of nitrogen fertilisers is highly energy-intensive, which underlines the importance of nitrogen recycling (Valve et al., 2020). ...
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Impact bonds have emerged as a policy innovation with the potential to change public policies towards being more outcome-based and pre-emptive – and more effective in answering sustainability challenges. This financial instrument has been used for social problems and only recently for environmental issues. One example is the Nutrient-EIB (environmental impact bond) which is in preparation to be implemented in Southwest Finland. We study involved stakeholders’ framings of the Nutrient-EIB’s potential for solving problems of eutrophication of the Baltic Sea and the lock-in situation of recycled fertiliser development. Based on stakeholder interviews, we identify three interpretative frames: applying the impact investing logic, challenging agri-environmental policy, and extending experimental policy-making. We recognise frictions between the frames regarding the required knowledge base and scale. We discuss how visions of transformative outcomes may challenge each other and what kinds of barriers transformative policy innovations may face.
... The conventional agriculture trend to maximize yields, through intensive cultivation techniques, together with the use of chemical fertilizers, has progressively compromised the fertility of agricultural land (Bonanomi et al., 2020;Saffeullah et al., 2021). Furthermore, the separation of livestock activities from cultivation has reduced the availability of organic matter, such as manure, within farms (Takahashi et al., 2020;Valve et al., 2020). ...
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Composting from organic municipal solid waste (MSW), such as a separate waste collection, is a valid tool for eliminating a considerable amount of waste that would otherwise be destined for landfills and incinerators, thus representing an effective complement to traditional forms of recycling. It allows organic substance to be recovered and reintegrated into the soil, thus preventing erosion phenomena, increasing the biological fertility of the soil and contributing significantly to the restoration of impoverished sites. Modern winegrowing must address the issue of vineyard fertility in the sustainability context. The goal of this study was to assess the advantages of distributing a sustainable product to the vineyard that can achieve vine balance (vegetative and productive equilibrium). In a Vitis vinifera L. Sangiovese cv., vineyard, four soil treatments were applied (three compost rates and a control): municipal solid waste compost (40 tons per hectare – MSW40, 15 tons per hectare – MSW15, 2.5 tons per hectare – MSW2.5), and no compost (CTRL). The vine physiology (leaf gas exchange and water potential) and berry compositions (phenolic and technological maturity) were studied during the 2018–2019 growing seasons in the Sieci area, Italy. The results of this experiment provide some general insights showing that MSW compost options can be expected to reduce water stress, balanced vine performance and provide sustainable recirculation of organic matter. MSW compost is a true agronomic and environmental resource.
... Individual farms struggling with low profitability and manure surpluses may be poorly equipped to take such actions. Efficient manure processing calls for large initial investments that pay off only if the biomass volumes are large enough (Valve et al., 2020). ...
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Farmers are increasingly required to record the actions taken and incidents occurring at farms. They are also expected to use the records to prove that animal welfare and environmental regulations are being followed. Yet, research suggests that record keeping disturbs, rather than supports, farming practices. The policy benefits are also suspected to be scarce. In this paper, we argue that the potential of records to act as useful calculative resources has been overlooked. The Finnish case study shows that recording nutrient use can allow fertilisation practices to be guided by water protection concerns. However, on some farms the requirement to keep nutrient records has been responded in ways that draws a distance between farms and regulatory control. The alignment of farm life and the recording requirements has followed different trajectories from one farm to another. Some farmers contest the science behind the fertilisation limits. Farms may also be poorly equipped to live with the limits that the records make visible. Optimal fertilisation then ceases to be an issue that can be solved by additional calculative support. Instead, record keeping indicates political divisions.
... These wastes known as pig slurry (PS) can be reused in agriculture as organic fertilizer. Its reuse is interesting from both under the economic and an environmental point of view, since it represents an internal resource of rural properties and agro-industries that can be used for application in the soil in the same properties or in properties of the region for grain production, adapting to the nutrients circular economy (Valve et al., 2019). ...
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The nutrients contained in the liquid pig slurry (PS) make this residue a potential organic fertilizer for the cultivation of tree species for the production of wood, which can promote the circular economy. This research aimed to evaluate soil chemical parameters, nutritional status, and development of hybrid eucalyptus clone urograndis from PS application for three consecutive years. The treatments evaluated were without fertilization (C); mineral fertilization (NPK) – 300 kg ha⁻¹ year¹ of the 02-30-10 formulation; fertigation with PS of 200 m³ ha⁻¹ year¹ (FPS200); and fertigation with PS of 400 m³ ha⁻¹ year1 (FPS400). Macronutrients and micronutrients from leaves and soil were evaluated. Tree development was analyzed using stem diameter at breast height (DBH) and total height (Ht). PS treatments resulted in leaves with higher accumulation of K and P and lower accumulation of N, Ca, M, S, Cu, and Fe. The treatments FPS200 and FPS400 presented DBH and Ht equal or superior to the mineral fertilization. Fertilizations with NPK, FPS200, and FPS400 resulted, respectively, in DBH increments 16, 27, and 30% higher than the control. Regarding the chemical attributes of the soil, the use of PS reduced the pH of the soil and increased the levels of P in comparison with the other treatments. The micronutrient values were adequate for fertility and no accumulation of potentially toxic elements at a level considered harmful was observed. The application of PS in eucalyptus hybrid urograndis proved to be an attractive alternative to increase wood production. Keywords: Circular Economy; Eucalyptus hybrid urograndis; Organic fertilization
... Farmers are recommended to apply more manure nutrients in the field than the crop demand. While manure application is challenging to precision nutrient management, it could provide significant soil health benefits compared to inorganic fertilization (Miller et al. 2013;Ramphisa et al. 2020) and could build toward circular nutrient agricultural systems (van Zanten et al. 2019;Valve et al. 2020). ...
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Manure application at the right place, time, and rate is critical for sustainable manure management. A land suitability framework (LSF) was previously developed for identifying suitable areas for manure utilization considering landscape vulnerability. This study aimed to evaluate the developed LSF and provide insights to build a new robust LSF for manure utilization. The study (1) evaluates the potential environmental impacts of manure application compared to inorganic fertilization, (2) evaluates the robustness of the previously developed LSF, and (3) identifies landscape biophysical attributes driving manure nutrient loss. The fertilization scenarios were evaluated using the Soil and Water Assessment Tool (SWAT) in a case study watershed located in Pennsylvania, USA. SWAT‐simulated results showed 80% higher organic nitrogen loss and 44% lower nitrate loss with manure fertilization compared to inorganic fertilization scenarios. The previously developed LSF was a useful screening tool for identifying vulnerable areas for manure application. However, the simulation results indicated limitations in the LSF in representing the combined effect of multiple landscape biophysical attributes. The decision tree model using the landscape biophysical characteristics and nutrient loading simulations implied the LSF can be improved by considering soil biophysical characteristics. The study results provide critical insights toward developing a robust LSF for sustainable manure utilization.
... In practice, we must limit ourselves 'to showing mere glimpses, specific bits and pieces, but pieces that might contribute to breaking open our political imagination about how cases, issues and the political could have been shaped in radically new ways' (Asdal, 2007, p. 319). We make use of our prior expertise (Valve et al., 2020) and parallel analysis of the documents to create a broad understanding of the relationships that might be consequential for the reduction of agricultural nutrient loads in North Savo. ...
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Environmental policies often leave room for case- or region-specific discretion. In this paper, we focus on the transformation of socio-material settings into objects of policy discretion. This move calls for manipulation—ontological work—enabling settings to be connected to policy aspirations. The settings become configured in terms of their professed policy-relevant dimensions. The outcomes affect how environmental liabilities become defined in policy processes. The paper develops a conceptual toolkit to analyse ontological work as it is performed by policy documents. We use the toolkit to analyse three types of policy documents defining how agricultural nutrient loading is to be reduced in the Finnish region of North Savo. The findings show that regulatory decisions and policy recommendations are, to a significant extent, outcomes of ontological work. Environmental liabilities are shaped by the ways ‘unstable junctures’ are brought into being. By these junctures we refer to the points in the configured policy landscapes where choices influential for water protection are, according to the documents, to be made. The documents also generate exclusions that narrow down what liability implies in the unstable junctures. Without a focus on the ontological work and emerging ordering effects, it would have been difficult to show how environmental liabilities became (un)defined in the policy documents. The approach is needed to understand how power is practiced in policy processes and how policy instruments come to have consequences.
Analyses of the future for manufacturing and heavy industries in a climate constrained world many times focus on technological innovations in the early stages of the value chain, assuming few significant changes are plausible, wanted, or necessary throughout the rest of the value chain. Complex questions about competing interests, different ways of organising resource management, production, consumption, and integrating value chains are thus closed down to ones about efficiencies, pay-back times, and primary processing technologies. In this analysis, we move beyond this to identify archetypal pathways that span across value chains in four emissions intensive industries: plastics, steel, pulp and paper, and meat and dairy. The pathways as presented in the present paper were inductively identified in a multi-stage process throughout a four-year European research project. The identified archetypal pathways are i) production and end-use optimisation, ii) electrification with CCU, iii) CCS, iv) circular material flows, and v) diversification of bio-feedstock use. The pathways are at different stages of maturity and furthermore their maturity vary across sectors. The pathways show that decarbonisation is likely to force value chains to cross over traditional boundaries. This implies that an integrated industrial and climate policy must handle both sectoral specificities and commonalities for decarbonised industrial development.
The virtuous reuse in-situ of agri-based organic wastes to produce compost and bio-organic fertilizer in the farm for increasing natural suppressiveness of soil to soil-borne plant pathogens has been a better way for longer time for designing new sustainable cropping systems without using chemical fumigants. Nevertheless, many others co/by-products provided by the biodiesel supply chain (oil-less seed meal/cake/pellet), biopolymer/biocomposite industry (chitin/chitosan) and processing plants for wastewaters depuration (biosolid/sewage sludge) integrated or not with biogas production (anaerobic digestate) are still too under-explored, though of great interest to increase disease suppression of soil. This work aims at covering this lacking of information in the review literature giving recent outcomes on the main effects and potential impacts of these organic inputs on interaction among the beneficial microbial consortia involved in disease suppression with the crop-pathogen systems of major economic impact. The paper first introduces the main sources, characteristics and application fields of the co-products aforementioned. Afterwards, it describes and discusses the main ecological effects of these organic inputs on functioning of disease suppressive soil under the perspective of microbiome disturbance. Then, it analyses and critically discusses the potential impacts of such value-added co-products on soil quality taking into account the main fertility parameters. Finally, how the resulting points of strength and of weakness can drive the progress of a sustainable management of soil for its quality, fertility and plant health are discussed from the perspective of a circular economy approach according to the “Green Deal” policies launched by the European Union. From this review work it turned out that such co-products have great potentiality to increase soil suppression and to improve soil quality as the benefits overall outweigh the drawbacks, though they may give sometime inconsistent disease control. For these reasons, their practical applications as soil amendments are still strongly limited at the field conditions. Keywords: Anaerobic digestate Chitin/chitosan Green Deal Oil-less seed co-product Sewage sludge Soil suppression and soil quality
The circular economy (CE) is currently generating considerable expectations. The concept describes an aspired future but does not provide clear guidance for policy-making. As policy outcomes often rest on initiatives generated in a bottom-up fashion, our attention must be directed to the ways policies are made accessible and interesting to those who might take the initiative. We claim that on-line publicity plays a key role in this. Our findings from a hyperlink analysis focusing on a government funding call for nutrient recycling in Finland show how multiple versions of the policy topic unfold online, as emergent hyperlink clusters prioritize specific agents, material circuits, and policy visions over others. The topic becomes connected with activities and agendas to create path dependencies and to strengthen existing divisions rather than to advocate change. Thus, we argue that CE policy design must recognize the way policy is shaped through online publicity creation.
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Resilient, productive soils are necessary to sustainably intensify agriculture to increase yields while minimizing environmental harm. To conserve and regenerate productive soils, the need to maintain and build soil organic matter (SOM) has received considerable attention. Although SOM is considered key to soil health, its relationship with yield is contested because of local-scale differences in soils, climate, and farming systems. There is a need to quantify this relationship to set a general framework for how soil management could potentially contribute to the goals of sustainable intensification. We developed a quantitative model exploring how SOM relates to crop yield potential of maize and wheat in light of co-varying factors of management, soil type, and climate. We found that yields of these two crops are on average greater with higher concentrations of SOC (soil organic carbon). However, yield increases level off at ∼2% SOC. Nevertheless, approximately two-thirds of the world's cultivated maize and wheat lands currently have SOC contents of less than 2%. Using this regression relationship developed from published empirical data, we then estimated how an increase in SOC concentrations up to regionally specific targets could potentially help reduce reliance on nitrogen (N) fertilizer and help close global yield gaps. Potential N fertilizer reductions associated with increasing SOC amount to 7% and 5% of global N fertilizer inputs across maize and wheat fields, respectively. Potential yield increases of 10±11% (mean±SD) for maize and 23±37% for wheat amount to 32% of the projected yield gap for maize and 60% of that for wheat. Our analysis provides a global-level prediction for relating SOC to crop yields. Further work employing similar approaches to regional and local data, coupled with experimental work to disentangle causative effects of SOC on yield and vice versa, is needed to provide practical prescriptions to incentivize soil management for sustainable intensification.
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All life depends on phosphorus (P), which is why it is an important crop fertilizer. Humans generally consume more P than needed and the excess ends up in sewage systems. Past management of P in fertilizer and human sewage has led to the accumulation of P in soils and sediments of lakes and streams. This accumulation is called “legacy” P because it can leak for decades to downstream lakes and coastal areas where it contributes to environmental problems. We developed a model to understand P dynamics for the entire drainage basin of the Baltic Sea since 1900. This model included a rapid transport pathway that represented sources such as runoff from cropland and a slow pathway that represented leakage from mobile legacy sources. The model suggests that loss from the mobile pool contributes about half of current waterborne inputs to the sea; as a result, it could be difficult to make substantial near-term reductions. However, there are opportunities to meet environmental goals by slowing the accumulation of P in the landscape and by implementing measures that address the rapid transport pathway, such runoff from cropland, and the mobile stores, such as cropland with large soil-P reserves.
Technical Report
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In the transition from a fossil reserve-based to a bio-based economy, it has become a critical challenge to maximally close nutrient cycles and migrate to a more effective and sustainable resource management, both from an economical and an ecological perspective. The production and transport of mineral fertilisers requires significant amounts of fossil energy. For example, the production of reactive ammonium (NH4) through the extraction of unreactive atmospheric nitrogen gas (N2) via the Haber Bosch process amounts to a fossil energy consumption of 35.2-40.5 GJ ton-1 NH4. The total energy consumption is equivalent to ± 2% of world energy use. Hence, the dependency of agriculture on fossil reserve-based mineral fertilisers (especially N, P, and K) must be regarded as a very serious threat to future human food security. Also, estimates of phosphorus reserves are highly uncertain, but based on population growth and future demand for nutrients, it is expected that depletion will occur within 93 to 291 years. In the given perspective, European agriculture needs to progress towards more closed loops concerning nutrients. This can be envisaged both within agricultural systems as well as recycling nutrients as secondary resources for existing chemical (mineral fertilizer) industry. This report was the Starting Paper to initiate discussions between the FG19 experts as selected by EIP-AGRI (European Commission) on 'Nutrient Recycling'. These discussions resulted in the publication of several dedicated papers and an end-report (2017).
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While some local governments push ahead with recycling phosphorus from wastewater, there has been little progress nationwide. In this study. we analyzed the perceptions and interests of each stakeholder in this issue by conducting interviews to assess recycling approaches. In comparison with recycling in other fields, for instance. industrial materials recycling. the stakeholders involved in phosphorus recycling are far more diverse. This makes it more difficult to coordinate their behaviour. Based on interviews with a wide range of stakeholders, we identified seven dimensions that they consider vital for promoting phosphorus recycling: environment, cost, image. transaction stability, distribution, quantity and quality. Key challenges will be to develop technologies to slash costs. form dosho-imu alliances for simultaneous achievement of multiple objectives. and make bargaining arrangements among local governments. fertilizer manufacturers and cement producers to secure stable volumes.
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We make the case that phosphorus (P) is inextricably linked to an increasingly fragile, interconnected and interdependent ‘nexus’ of water, energy, and food security, and should be managed accordingly. While there are many other drivers that influence water, energy, and food security, P plays a unique and under-recognized role within the nexus. The P ‘paradox’ derives from fundamental challenges in meeting water, energy, and food security for a growing global population. We face simultaneous dilemmas of overcoming scarcity of P to sustain terrestrial food and biofuel production; and addressing overabundance of P entering aquatic systems, which impairs water quality and aquatic ecosystems, and threatens water security. Historical success in redistributing rock phosphate as fertilizer, to enable modern feed and food production systems, is a grand societal achievement in overcoming inequality. However, using the USA as the main example, we demonstrate how successes in redistribution of P, and reorganization of farming systems has broken local P cycles, and has inadvertently created instability that threatens resilience within the nexus. Furthermore, recent expansion of the biofuels sector is placing further pressure on P distribution and availability. Despite these challenges, opportunities exist to intensify and expand food and biofuel production through recycling and better management of land and water resources. Ultimately, a strategic approach to sustainable P management can help address the P ‘paradox’, and minimize tradeoffs and catalyze synergies to improve resilience among components of the water, energy, and food security nexus.
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The inefficient use of phosphorus (P) in the food chain is a threat to the global aquatic environment and the health and well-being of citizens, and it is depleting an essential finite natural resource critical for future food security and ecosystem function. We outline a strategic framework of 5R stewardship (Re-align P inputs, Reduce P losses, Recycle P in bioresources, Recover P in wastes, and Redefine P in food systems) to help identify and deliver a range of integrated, cost-effective, and feasible technological innovations to improve P use efficiency in society and reduce Europe's dependence on P imports. Their combined adoption facilitated by interactive policies, co-operation between upstream and downstream stakeholders (researchers, investors, producers, distributors, and consumers), and more harmonized approaches to P accounting would maximize the resource and environmental benefits and help deliver a more competitive, circular, and sustainable European economy. The case of Europe provides a blueprint for global P stewardship.
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Significance The increasing prevalence of antibiotic-resistant bacteria is one of the most serious threats to public health in the 21st century. One route by which resistance genes enter the food system is through amendment of soils with manure from antibiotic-treated animals, which are considered a reservoir of such genes. Previous studies have associated application of pig manure with the dispersal of sulfonamide-resistance genes to soil bacteria. In this study, we found that dairy cow manure amendment enhanced the proliferation of resident antibiotic-resistant bacteria and genes encoding β-lactamases in soil even though the cows from which the manure was derived had not been treated with antibiotics. Our findings provide previously unidentified insight into the mechanism by which amendment with manure enriches antibiotic-resistant bacteria in soil.
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Mineable phosphorus reserves are confined to a handful of countries. Reductions in wastage could free up this resource for low-income, food-deficient countries.
The effects of organic versus conventional farming systems on changes in soil organic carbon (SOC) has long been debated. The effects of such comparisons may depend considerably on the design of the respective systems and climate and soil conditions under which they are performed. Here, we compare a range of arable organic and conventional crop systems at three sites (Jyndevad, Foulum and Flakkebjerg) in Denmark through long-term experiments initiated in 1997. The experimental treatments in the organic farming systems included use of whole-year green manure crops, catch crops and animal manure (as cattle, pig or digested slurry). Data on plant residues and animal manure were used to estimate C inputs to the soil. This was compared with measured changes in topsoil (0–25 cm) SOC content over 4–8 years. During 1997–2004, green manure, catch crops and animal manure enhanced estimated C input by 0.9, 1.0 and 0.7 Mg C ha−1 yr−1 respectively, across all locations. Based on measured SOC changes, green manure enhanced SOC by 0.4 Mg C ha−1 yr−1 and catch crops by 0.2 Mg C ha−1 yr−1, while animal manure by insignificantly 0.1 Mg C ha−1 yr−1. After 2005, advantages of using green manure (grass-clover) on SOC change disappeared, because cuttings of the grass-clover was removed whereas before 2005 they were mulched in the field, albeit there was still a small extra estimated C input of 0.2 Mg C ha−1 yr−1. An estimated higher C input of 0.7 Mg C ha−1 yr−1 with catch crops did not result in significant increase in measured topsoil SOC. From 2005–2008, the first 4 years of comparison between organic and conventional farming at all three sites, organic farming with animal manure had 0.3 Mg C ha−1 yr−1 higher estimated C input, but SOC measurements showed that conventional farming accumulated 0.4 Mg C ha−1 yr−1 more SOC than organic farming. At Foulum from 2005 to 2012, organic farming with animal manure had 0.7 Mg C ha−1 yr−1 more input, and topsoil SOC measurements showed a higher accumulation of 0.4 Mg C ha−1 yr−1 in organic compared with conventional farming. Regressions of changes in topsoil SOC against estimated C inputs showed that 10–20% of C inputs were retained in topsoil SOC over the experimental period. There was no clear indication that belowground C input contributed more to SOC than aboveground C inputs. Despite consistently higher estimated C inputs in organic versus conventional systems, we were not able to detect consistent differences in measured SOC between the systems.