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# Fertilizing potential of nitrogen in human excreta across diverse countries and food systems

Thomas Starck, Fabien Esculier

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Thomas Starck, Fabien Esculier. Fertilizing potential of nitrogen in human excreta across diverse countries and food systems. Environmental Research Letters, 2025, 10.1088/1748-9326/ae0cdf. hal-05295844

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Fertilizing potential of nitrogen in human excreta across diverse countries and food systems

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# Fertilizing Potential of Nitrogen in Human Excreta across diverse countries and food systems

T. STARCK¹*, F. ESCULIER¹²

¹LEESU, Ecole des Ponts, Institut Polytechnique de Paris, Univ Paris Est Creteil, Marne-la-Vallée, France

²METIS, Sorbonne Univ, CNRS, EPHE, Paris, France

E-mail: *thomas.starck@polytechnique.org

April 2025

## Abstract.

Human excreta represent a significant but underutilized source of nitrogen (N) for agriculture. Few studies have quantified the full fertilizing potential of human excreta across national contexts, or under a range of diverse food systems. This study provides the first systematic assessment of the potential contribution of N in human excreta to current cropland fertilization in a wide range of countries, covering 80% of the global population. Using food supply and demographic data, we estimate N in human excreta for each country and compare it to current cropland total N inputs, including synthetic fertilizers, manure, biological fixation, and atmospheric deposition.

Assuming a 100% recovery rate of human excreta to estimate an upper-bound potential, under the current situation, we find that human excreta (37 MtN at the global scale) could provide 15-30% of current cropland N inputs in most countries, 30-50% in sub-Saharan Africa, and over 50% in major agricultural importers (e.g. Japan, Korea, Algeria). In contrast, large agricultural exporters with high external inputs, such as Argentina or the United States, show lower potential (5-10%). For the whole world, the rate is 20%. For most countries, N excreted by humans is also of the same order of magnitude as (and often superior to) livestock manure currently applied to croplands.

In addition to an analysis of the current situation, we assess how improved food system efficiency—via more vegetarian diets, reduced losses and increased manure recycling thanks to better coupling of husbandry and crop cultivation—could affect the potential contribution. Under deeply transformative agro-ecological scenarios, the global contribution of human excreta to N fertilization could exceed 30%.

This work focuses on maximum potentials; future research should focus on realistic recovery rates (<100%) and adjust our results accordingly.

Keywords: Nitrogen, Human Excreta, Fertilization, Food Systems

Submitted to: *Environ. Res. Lett.*
# 1. Introduction

Nitrogen (N) is an essential nutrient for plant growth and animals nutrition. However, the widespread use of reactive nitrogen has profoundly disrupted the global nitrogen cycle, leading to severe environmental impacts. Excess nitrogen contributes to water pollution through eutrophication, air pollution, and biodiversity loss in both aquatic and terrestrial ecosystems [1, 2]. Despite its critical role in food systems, current nitrogen management remains inefficient and unsustainable in many regions.

Human excreta represent a significant but underused nutrient resource in global agriculture. While excreta recycling was not uncommon in traditional systems [3], most nitrogen (N) from human waste is now lost in industrialized countries through centralized sanitation, contributing to environmental pollution via wastewater discharge, among others [4].

Recent studies have explored the fertilizing potential of excreta, particularly urine, given its high N content and relatively low contamination levels in micropollutants or pathogens [5, 6]. These analyses often compare urine-based fertilizers to synthetic Haber-Bosch fertilizers, based on the rationale that they share similar mineral composition and agronomic performance [5].

However focusing solely on synthetic fertilizer substitution at the field scale misses the potential at the food system scale. First, synthetic fertilizers are not the only source of N in agriculture; biological fixation, livestock manure, and atmospheric deposition can also play a major role [7]. In regions where synthetic inputs are low (e.g., parts of sub-Saharan Africa and Southeast Asia) or in farming systems relying on legumes and organic rotations, only focusing on mineral

fertilizers misses the majority of fertilization.

Other studies only looked at the potential at the global scale, for instance estimating that phosphorus in human excreta could replace 20% of the current globabl P demand in agriculture [8]. Yet global estimates can obscure the considerable diversity in national agricultural systems, nutrient inputs, and dietary patterns [9, 10]. The potential contribution of human excreta is fundamentally contextual: it depends on both the quantity of nutrients excreted and the total N inputs in agriculture—factors that vary with diet, food system N use efficiency (NUE) and the quantity of agricultural commodities produced.

A recent study looked at the potential of excreta, pooling human excreta and poorly used livestock manure [11]. The authors studied the potential at the global scale, and also at sub-national (10kmx10km) scale, to identify geographical mismatches; but they did not assess individual countries, and compared excretions to crops nutrients needs recommendations from the early 90s, and not actual current fertilization. Similarly, another study [12] focused on the world's 56 largest cities, to identify hot spots of human excreta and potential mismatches with crops needs, but did not assess the potential in a variety of countries.

This study aims to quantify the maximum theoretical contribution of human excreta to agricultural N inputs across a diverse set of countries. We answer the following question: *What is the potential maximal contribution of N in human excreta to agricultural fertilization, under different contrasted contexts?*

By assuming a 100% recovery and reuse of excreta N, this analysis provides upper-bound estimates of potential contributions under current food systems. We also give upper bound estimates under a range of food systems more efficient than the current one,
based on a set of realistic food system N use efficiency (NUE). Our results can be easily adapted by assuming <100% recovery rates.

All underlying data and methods are publicly available via a GitHub repository, archived on Zenodo, and a more user-friendly interactive website.

## 2. Methods and Sources

### 2.1. *Countries of interest and year of analyses*

As of the time of our analyses, the latest year available for FAOSTAT data was 2022. To reduce potential noise of particular years, we averaged the results over the 5 latest available years, i.e. 2018-2022. All analyses thus refer to the situation circa 2020.

As there are close to 200 countries in the world, we had to apply a selection criteria to keep the analyses reasonably small. We selected all countries with more than 40 million inhabitants. This yielded a total of 35 different countries, totaling 6.2 billion people, or 80% of the 7.8 billion global population in 2018-2022. We also present the results for the entire World.

### 2.2. *Agricultural system limits and N input flows*

Agricultural land is typically divided into two main categories: croplands and permanent pasture (>5 years), following FAO definitions [13] (see Figure 1). Croplands encompass arable land, temporary grasslands (i.e., sown less than five years ago), and permanent crops such as orchards and vineyards. Permanent

pasture, by contrast, are considered permanent if they have not been sown for five years or more.

Among these categories, only croplands are covered by detailed N flow estimates in FAOSTAT. The FAO's standard nutrient balance methodology provides, for each country, annual N inputs to croplands from: synthetic fertilizers, manure, biological N fixation, and atmospheric deposition [13]. Equivalent data are not available for permanent grasslands.

The justification to mainly focus on croplands rests in the fact that they support the large majority of nutrient flows in food production. Globally and in most countries, croplands typically receive typically 90-100% of mineral fertilizers, with only a small share applied to grasslands. A similar pattern holds for manure that is collected and managed by farmers (this does not count manure directly excreted by livestock on pastures, which constituted an internal flow of permanent grasslands). For more details, see Appendix A.1.

Given this imbalance in both data availability and agronomic importance, this study focuses exclusively on croplands. Figure 1 presents an overview of the entire food and excreta system, the boundaries of the cropland subsystem, and the N input flows considered with this system limits.

Accordingly, when evaluating the potential contribution of human excreta, we compare it to the following cropland N inputs:

* synthetic fertilizers

* atmospheric deposition

* livestock manure applied to croplands

* biological fixation. Note that this include both symbiotic fixation (legumes) and asymptotic fixation (rice, sugar cane...). See FAO methodology [15]

<footnote n="2">2 https://github.com/Thomas-Starck/potential-human-excretions-fertilization</footnote>

<footnote n="3">3 https://doi.org/10.5281/zenodo.14591081</footnote>

<footnote n="4">4 https://thomas-starck.github.io/potential-human-excretions-fertilization</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** N flows in the entire food / excreta system. Limits of the "croplands" system (dark yellow, left) and associated input flows are in red, as considered by FAOSTAT cropland nutrient balances [13] (FAO does not consider human excreta). As it is not our main interest here, N losses (volatilization, leaching, denitrification...) and mineralization / immobilization are not represented, out of simplification. For a more complete representation of N flows, see the GRAFS (Generalized Representation of Agro-Food Systems) approach [14].</figcaption></figure>

This is synthesized with the following equation:

$$N_{\text{inputs cropland}} = N_{\text{synth ferti cropland}} + N_{\text{biofix cropland}} + N_{\text{atm dep cropland}} + N_{\text{manure to croplands}} \quad (1)$$

These 4 items are reported, for each country, in FAOSTAT cropland nutrient balances [16]. Note that FAOSTAT does not estimate human excreta N inputs to croplands. While this can locally be non-negligible, at the global scale human excreta are negligible: Chatzimpiros and Harchaoui estimate [17] that only 5% is reused as fertilizers. Using detailed data from waste water treatment plants, Starck et al. (2024) [4] estimated a recycling rate in France of 10%, and found recycling rates of 0-10% in other Western countries such as Germany, Belgium, and

Denmark [18, 19, 20]. For China in 2010, [21] estimated a 5% reuse rate in urban settings and 50% in rural, with a downward trend since the 1980s, suggesting smaller rates for 2020. Consequently, while human excreta inputs to croplands are negligible at the national level in many countries, they can be significant for some countries, especially in rural areas.

N from irrigation is not considered, but can usually be neglected at national levels: for instance, even before the widespread adoption of synthetic fertilizers, irrigation in China during the second half of XXth century contributed only to a few percents of N inputs ([22], appendix R). FAO cropland N balances also do not consider net N mineralization, nor N stabilization.
## 2.3. Estimation of N in human excreta

There is no centralized dataset estimating N excreted by humans for each country. To produce our estimation, we operate in several steps.

First, we begin with national food supply data from the FAO food balance sheets [23], which report per capita protein availability at the retail level for each country. We then convert protein values into nitrogen using the Jones conversion factor (100 g protein = 6.25 g N) [24].

Food supply reflects the quantity of protein available for human consumption after accounting for upstream losses (e.g., post-harvest and processing). As reported by FAO's definitions and methodological notes on food balance sheet [25], the food availability "refers to the quantities of food available for human consumption at the retail level". Losses pre-retail level are thus already subtracted from FAO's reported food supply, as highlighted by FAO's definition of losses for its food balance sheets.

For this, we rely on regional estimates from the World Resources Institute [26], based on FAO data [27], which details wasted and lost calories across the supply chain by world region (Table 1). In line with the FAO's definition of "food supply" as availability at the retail level [25], we focus only on the last two stages—"distribution and market" and "consumption"—when estimating post-retail losses. These vary significantly by region, from 29% in North America to 4% in sub-Saharan Africa. For the analysis at the world scale, we use a population-weighted average, which yields a global food waste and losses of 10%.

After ingestion, we assume that all nitrogen is excreted, with no net storage in the human body [28]. Jönsson et al. [29] found

a mean N storage rate of 2% between 2 and 17 years; this retention during growth would translate into a mere 0.4% total retention over a 80 year life, so we consider or approximation valid.

This is a valid approximation for adults, although it may slightly overestimate excreta in countries with younger populations, where growth-related nitrogen retention is more significant. Below are the equations to compute N excretions per capita:

$$ 
\begin{aligned}
N_{\text{excretion/cap}} &= N_{\text{ingestion/cap}} = \text{protein}_{\text{ingestion/cap}}/6.25 \quad (2) \\
&= N_{\text{food supply/cap}} - N_{\text{food waste/cap}} \\
&= N_{\text{food supply/cap}} \cdot (1 - \%_{\text{waste N supply}})
\end{aligned}
$$

## 2.4. Estimation of potential in current system

As our goal is to assess the maximum potential contribution of human excreta to agriculture, we compare total excreted nitrogen to current nitrogen inputs to croplands (see previous Section), which implies a 100% recovery and reuse rate of excreted nitrogen. While this is unrealistic in practice, the goal here is not to estimate a plausible or achievable recycling rate, which could vary widely depending on the situations and assumptions, but rather to estimate an upper bound of the potential contribution.

Under this hypothesis, the maximum upper bound contribution of human excreta to N inputs to croplands is:

$$ \%_{\text{excr contribution}} = \frac{N_{\text{excretion/cap}} \cdot \text{Population}}{N_{\text{inputs croplands}}} \quad (3) $$

Where $N_{\text{excretion/cap}}$ and $N_{\text{inputs croplands}}$ are estimated with equations 1 and 2
<table><caption>**Table 1.** Food waste and losses for the "distribution and market" and "consumption" stages, by world region, according to [26]. The first 3 columns report the original data, the last one is the weighted average (for the first line: 42% * (7% + 61%) = 29%).</caption><thead><tr><th>World region</th><th>waste and losses across value chain</th><th>% waste and losses due to: distribution and market</th><th>% waste and losses due to: consumption</th><th>waste and losses due to distribution, market, and consumption</th></tr></thead><tbody><tr><td>North America and Oceania</td><td>42%</td><td>7%</td><td>61%</td><td>29%</td></tr><tr><td>Industrialized Asia</td><td>25%</td><td>11%</td><td>46%</td><td>14%</td></tr><tr><td>Europe</td><td>22%</td><td>9%</td><td>52%</td><td>13%</td></tr><tr><td>North Africa, West and Central Asia</td><td>19%</td><td>18%</td><td>34%</td><td>10%</td></tr><tr><td>Latin America</td><td>15%</td><td>17%</td><td>28%</td><td>7%</td></tr><tr><td>South and Southeast Asia</td><td>17%</td><td>15%</td><td>13%</td><td>5%</td></tr><tr><td>Sub-Saharan Africa</td><td>23%</td><td>13%</td><td>5%</td><td>4%</td></tr></tbody></table>

## 2.5. Estimation of potential when the food system is more efficient

Our country-by-country comparisons refer to potentials in the current (wasteful) food systems, and is influenced by countries imports and exports. However it would be interesting to also assess the potential of human excreta with less wasteful food systems, which would mechanically increase the potential, as input needs are diminished.

When there are no imports nor exports, the potential contribution of N in human excreta to total fertilization simply corresponds to the full-chain N use efficiency (NUE) of the food system: out of an input of 100 kgN in the agricultural system, how much is finally ingested (and excreted) by humans. Consequently, if one assesses a range of realistic full-chain food system NUEs, they would also assess the maximum potential contribution of human excreta to fertilization of a closed food system without imports nor exports (for instance, the global food system, or a self-sufficient country).

Chatzimpiros and Harchaoui [17] studied the global food system NUE for the current situation and under a range of scenarios (Figure 3b in their paper and source data of their Figure 2, see Appendix A.3), based on 3 main variables: the share of cropland

production dedicated to feed animals, the recycling rate of manure to croplands, and croplands N use efficiency. While the distribution of vegetal/animal products is not a variable in their model, it is a direct output of the previous 3 variables combinations, and depending on the scenarios it spans from 20% of animal products (a rate similar to current mostly vegetarian India) to 60% (current most carnivorous countries). We use their estimated NUEs range values for our study.

However their N use efficiencies stop at the production stage, before waste and losses: contrary to the previous Section 2.3, this time their losses and waste encompass all of the value chain starting from production, and not just distribution and consumption. At the global scale, this corresponds to a 30% full chain loss rate ([30], based on [27]).

We use and adapt their different scenarios results for the NUE before food losses and waste. Depending on the scenarios, it can be as low as 20% or up to 55% (see Appendix A.3). We report them in Table 3. We then obtain the full chain NUE by applying a further loss rate for food losses and waste. We consider both the current 30% rate or an improved 15% (see Table 3)), coherent with the Sustainable Development Goal to halve current food waste [31], and also the rate for the currently best
performing sub-continent, Latin America ([30], see Table 1).

## 2.6. Normalization

To facilitate the comparison between different countries, we normalize all inputs by the countries populations, in $kgN·cap⁻¹·year⁻¹$. ; this also allows a straightforward comparison with per capita N excretions.

# 3. Results and Discussion

## 3.1. Interpretation of the results

Note that all the percentages presented in the following refer to human excreta as a percentage of N inputs. Let's call it X. This does not automatically translate into a potential contribution of human excreta to current N inputs. Let's call it Y. There can be 2 approaches for this potential: either human excreta *replace* some of the current inputs ($Y_{replace}$) or they are *added to* the current inputs ($Y_{added}$). If one envisions an agricultural system where human excreta *replace* some of the current inputs, then the potential is $Y_{replace} = X$. However, if one envisions a farming system where human excreta are *added to* current inputs, the potential considering this new input is $Y_{added} = X/(1+X)$. If X is already low, then $X/(1+X) \sim X$ and thus $Y_{added} \sim Y_{replace}$. For cropland inputs (Figure 2), this is the case for instance in Ukraine where $Y_{replace} = 10\% \sim 9\% = Y_{added}$; but the approximation does not hold for Japan where $Y_{replace} = 111\% \neq 53\% = Y_{added}$.

It is reasonable to consider that in countries with low N inputs, the aim might be to add human excreta to current inputs. On the contrary, in countries with already high N inputs, the aim could be to simply replace part of the current inputs, or even replace and

diminish current inputs by modifying the food system and diminishing losses, which would even further increase the potential of human excreta.

It is difficult to model a food system change for all the studied countries, so, for each country, we chose to perform an assessment on the potential of human excreta in the *current* food system. We nonetheless explore changes of food system changes on theoretical and generic grounds in Section 3.4. Despite differences in diets, most ingestion/excretions are within a narrow range of physiological values around 3-5 $kgN·cap⁻¹·year⁻¹$. Consequently, the differences in potential are mostly explained by differences in total N inputs to croplands (see Figures below).

## 3.2. Excreta compared to N inputs in croplands

In Figure 2, we compare total human excreta to all N inputs (mineral fertilizers, biofixation, manure and atmospheric deposition) per capita in croplands.

Synthetic fertilization usually represents about 50-80% of total cropland inputs (yellow bars in Figure 2, left). There are a few exceptions below this 50-80% rate: sub-Saharan African countries, reflecting their little synthetic inputs; and Myanmar, Argentina and Brazil, due to their relatively important legume biofixation.

Human excreta tend to represent 15-30% of cropland inputs in most countries from different typologies: rich Western industrialized countries (France, Spain, Germany, UK, Italy), large industrialized countries (China, Russia), and other low and middle incomes countries (India, Iran, Pakistan, Vietnam, Indonesia, Colombia, Bangladesh...).
Potential N Human Excretia

human excreta compared to croplands N inputs

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Left: comparison of N in human excreta (red points) to N inputs to croplands by category (colored bars), in kgN per capita per year. Right: N in human excreta as a share of current total N inputs to croplands (see equation 3)</figcaption></figure>

In sub-Saharan countries (Tanzania, Nigeria, Kenya...), human excreta represent around 30-50% of the current cropland inputs, reflecting the low external N inputs to agroecosystems.

Some agricultural importers can exceed 50%: this is true of rich industrialized Asian countries like South Korea or Japan, reflecting their outsourcing of cropland production, but

also of low and middle income countries, like Democratic Republic of Congo or Algeria.

In the low end, around 5-10%, are agricultural exporters (often of soybean, but also of cereals) and countries with intensive livestock: Argentina, Brazil, USA, or Ukraine. In the case of Argentina, this low rate should be interpreted in context, as the country's crop nutrient balance (cropland nitrogen use
efficiency reaching 80–100% in recent years [16]) suggests a potential depletion of soil nitrogen. The use of human excreta could thus contribute to alleviating this depletion.

Myanmar is a particular case. It has relatively low synthetic fertilizers inputs, around 5-10 kgN·cap⁻¹·year⁻¹, but contrary to sub-Saharan or importers, Myanmar is not characterized by low total N inputs to cropland, experiencing levels similar to Germany or China (around 25 kgN·cap⁻¹·year⁻¹). Its low synthetic fertilizer consumption genuinely reflects a farming system based on biofixation and manure recycling. Contrary to Brazil or Argentina, this legume production is not centered on soybean exported for animal feed, but rather on beans, of which only one third is exported [23]. To a lesser extent, Sudan has a similar situation, with total N inputs around 15 kgN·cap⁻¹·year⁻¹, half of which from biofixation, mainly from groundnuts.

Finally, when considering the world as a whole, human excreta represent an amount of N equal to 20% of current total N input to croplands. The orders of magnitude by country typology are summarized in Table 2.

<table><caption>**Table 2.** Total human excreta compared to current cropland N inputs. See text for countries examples.</caption><thead><tr><th>country typology</th><th>human excreta as % of current cropland N inputs</th></tr></thead><tbody><tr><td>most countries</td><td>15-30%</td></tr><tr><td>agricultural importer</td><td>&gt;50%</td></tr><tr><td>agricultural exporter</td><td>5-10%</td></tr><tr><td>sub-Saharan</td><td>30-50%</td></tr><tr><td>World</td><td>20%</td></tr></tbody></table>

### 3.3. Excreta compared to livestock manure

In the previous section, we compared human excreta to total N inputs to croplands, which

gave a good sense of the potential contribution of human excreta to agricultural production in the current agricultural systems.

We now compare human excreta to a subset of these total inputs, i.e. livestock manure. We find this comparison useful because even though human and livestock excreta are similar from the viewpoint of an agroecosystem, human excreta are treated differently, and often not even considered. While livestock excreta are not systematically collected (Figure A2 left, in Appendix A.2), it would seem absurd to seek to actively *destroy* this collected N; yet, for human excreta, this is considered a standard treatment in wastewater treatment plants [4].

We operate 2 distinct comparisons. First, total human excreta compared to total livestock excreta. This is useful to compare the metabolisms of humans and livestock in the different countries’ agroecosystems. However for most countries, the majority of ruminants excreta are not collected and are simply excreted on permanent grasslands (Figure A2 right, in Appendix A.2), constituting an internal flow into the grasslands system not contributing to net nitrogen export from agroecosystems to food products: this internal flow restores part of the fertility lost following grazing.

For that reason we find it more useful to compare human excreta to the livestock manure that is actually collected and finally spread on croplands. It gives a better sense of the amount of manure actually used to fertilize croplands.

Figure 3 synthesizes the results. On its left pane, the total green columns represent the total N quantity excreted by livestock; their dark green part represents the amount actually collected and spread on croplands, which actually correspond to the green parts
of Figure 2.

At the global scale, N in human excreta would represent about 150% of the N in manure currently spread on croplands.

There is no clear typology to classify countries regarding that metric. That is because it results from the combination of 2 relatively independent factors: the number of livestock per capita (giving the total livestock excreta, light green bars in Figure 3) and the recovery rate of N in livestock excreta (yellow bars in Figure A2 in Appendix A.2). We can nonetheless note that countries with the highest recovery rates, above 50% (Ukraine, Spain, France, Russia, Germany, Italy) tend to have the lowest potentials, around 40-70%. On the contrary, a large herd (Brazil, Argentina, or Sudan) does not necessarily translate in a low human potential, as these herds are made of ruminants on extensive grasslands whose excreta are not collected.

For most low to middle income countries the rate is above or close to 100%, and sometimes even above 200%. This is because these countries tend to not have a high recovery of their livestock excreta.

### 3.4. Potential when the food system is modified

As explained in Section 2.5, when there are no imports nor exports (at the global scale or for self-sufficient countries), the potential of human excreta is simply the food system NUE.

Chaztimpiros and Harchaoui [17], report global NUE before food waste and losses of 21% for the current situation (See Appendix A.3). Further applying the 30% full-chain waste and losses ([30], based on [27]), see Section 2.5) gives an actual full-chain NUE of about 15%. This is consistent with other studies assessing the global full-chain NUE

[33].

For context and comparison, total food system NUEs of published agro-ecological scenarios are around 25-30% [34, 3] (see Appendix A.4 for details). This is lower than the estimated best rate ranges of 30-45% from Table 3. Indeed, in Chatzimirolos and Harchaoui work [17], reaching system NUE around 40% requires major changes in the food system, that even agro-ecological scenarios do not necessarily meet:

* for the highest efficiencies, very small to no poultry and swine production, as all livestock feed is supposed to come from grasslands ($α_{crops} = 0$%), exclusively allocated to ruminants

* mostly vegetarian diets (15-25% animal products, typical of current India for instance)

* high recovery rate of animal manure (80%) not necessarily compatible with free-ranging livestock

* a global cropland NUE of 70% (similar to current France) instead of the current global 50%

From the current 15-20% global NUE rate, it would be thus challenging to reach a 40% NUE value. More probable values for frugal and efficient systems would probably be around 30%, in line with published agro-ecological scenarios, effectively doubling the current potential rate. This gives a sense of the potential contribution of human excreta in a closed system, without imports nor exports, e.g. at the global scale, or for a self-sufficient country.
<table><caption>**Table 3.** Two left column: parameters characterizing the food system in Chatzimpiros and Harchaoui [17] (see details in Appendix A.3). Two middle columns: diets associated with the food system parameters. Three right columns: food system NUE before and after 15-30% food losses and waste, informing on the upper bound of human excreta as a share of N inputs in agriculture: indeed, with 100% recovery of human excreta, these NUE rates also correspond to the share of N inputs that could be covered with human excreta. The reader can adapt the results with a recovery rate they find plausible: for instance, with a 60% recovery rate, a 20% NUE would become 20%*60%=12%.</caption><thead><tr><th colspan="6">Scenarios</th><th colspan="3">Food system NUE:</th></tr><tr><th>parameters:<sup>a</sup><br>β, synth,<br>NUE<sub>crop</sub></th><th>α<sub>crop</sub></th><th>%plant<br>prot</th><th>diet:<sup>c</sup><br>typical<br>of</th><th>before<br>food<br>loss and<br>waste</th><th>after<br>current<br>30%<br>waste</th><th>after<br>improved<br>15%<br>waste</th></tr></thead><tbody><tr><td>B1. β: 35%<br>NUE<sub>crop</sub> 44%<br>synth: yes</td><td>57%<br>30%<br>0%</td><td>62%<br>76%<br>85%</td><td>China<br>India<br>W.Africa</td><td>22%<br>30%<br>37%</td><td>15%<br>21%<br>26%</td><td>19%<br>26%<br>31%</td></tr><tr><td>B4. β: 35%<br>NUE<sub>crop</sub> 44%<br>synth: no</td><td>57%<br>30%<br>0%</td><td>44%<br>57%<br>67%</td><td>E.Europe<br>World<br>N.Africa</td><td>20%<br>25%<br>30%</td><td>14%<br>18%<br>21%</td><td>17%<br>21%<br>26%</td></tr><tr><td>B2. β: 35%<br>NUE<sub>crop</sub> 70%<br>synth: yes</td><td>57%<br>30%<br>0%</td><td>62%<br>76%<br>85%</td><td>China<br>India<br>W.Africa</td><td>32%<br>41%<br>52%</td><td>22%<br>29%<br>36%</td><td>27%<br>35%<br>44%</td></tr><tr><td>B3. β: 80%<br>NUE<sub>crop</sub> 70%<br>synth: yes</td><td>57%<br>30%<br>0%</td><td>62%<br>76%<br>85%</td><td>China<br>India<br>W.Africa</td><td>40%<br>49%<br>57%</td><td>28%<br>34%<br>40%</td><td>34%<br>42%<br>48%</td></tr><tr><td>B5. β: 80%<br>NUE<sub>crop</sub> 70%<br>synth: no</td><td>57%<br>30%<br>0%</td><td>60%<br>72%<br>80%</td><td>China<br>S.Asia<br>E.Africa</td><td>40%<br>47%<br>53%</td><td>28%<br>33%<br>37%</td><td>34%<br>40%<br>45%</td></tr></tbody></table>

<sup>a</sup> Values and parameters from Chatzimpiros and Harchaoui [17]:

* B1-B5 refer to scenarios presented in Appendix A.3, defined based on β, NUE<sub>crop</sub> and the use of synthetic fertilizers.

* β: share of animal manure recovered and spread on crops.

* NUE<sub>crops</sub>: N use efficiency on croplands. Current global value is 44%, best performing countries (without N mining) around 70%.

* synth: use of synthetic fertilizers (i.e, conventional or organic scenario)

* α<sub>crop</sub>: share of cropland production used to feed animals. Current global value is 57%. A value of 0% indicates that all animal feed come from grasslands (and possibly food waste), so there are almost no monogastrics like poultry or swines left, only ruminants

<sup>c</sup> diets, in terms of % of plant protein in total protein food supply, is a consequence of the parameters β, NUE<sub>crops</sub>, synth, and α<sub>crops</sub>, after the model of Chatzimpiros and Harchaoui is run. Typical % plant protein in diet in some world regions: West Africa 85%, East Africa 81%, India 75%, China 61%, World 59%, North Africa 54%, East Europe 45% [32].
human excreta compared to manure excreted and applied to croplands

<figure><img src="image_7.png" /></figure>

**Figure 3.** Left: human excreta (red dots) compared to livestock manure collected and applied to croplands (dark green bars, similar to the green columns in Figure 2), expressed in kgN per capita per year. The total light green columns represent all of the N excreted by livestock. See Figure A2 in Appendix A.2 for more details on manure recovery rate. Right: recovered human excreta as a share of manure applied to croplands. Values cut for greater clarity are: 548% for Turkey, 433% for Japan, 343% for Nigeria, 440% for Ethiopia, 567% for South Africa, 882% for Algeria, 1572% for Egypt, 867% for Dem. Rep. Congo.

based on FAOSTAT data for 2018–2022

### 3.5. *Limitations and comparison to other works*

One limitation of our study is that we applied a common food loss and waste rate across all food groups for the distribution and consumption stage (Section 2.3 and

Table 1). This approach does not account for the significant variation in nitrogen content among different food types. A more accurate approach would involve using food group-specific loss rates, as reported by [26] and [27], which would better reflect the differences in nutrient content and waste patterns among
food categories, to better estimate N intakes and excretions.

As underlined multiple times before, this study only focused on the maximum theoretical agronomic potential of N present in human excreta, assuming a 100% recovery rate. This is because determining a "plausible" recovery rate is subjective judgment, that we did not want to be hidden in the study assumptions. Some may find a 20% global recovery rate already very optimistic, given barriers in regulation, logistics or lack of institutional support [35]; others might argue that there are no biophysical limits to a high recovery of N in excreta, as exemplified by the 80% rates already achieved for non ruminants in some countries (Figure A2. This difference in livestock and human excreta recovery may be rooted in how their respective excreta is framed: part of the farming system for the former, a simple waste for the latter [36].

Our study also presents strengths compared to previous works. Studies estimating the amount of N in human excreta usually only rely on food supply, and do not adjust for subsequent food losses before ingestion, thus inflating their estimation of N in excretions [12, 11]. They also often rely on crop fertilization needs estimated by IFA in 1992, which can be quite different from current actual fertilization rates [12, 11]. Finally, our choice of scale is also original: previous studies assessed potential at the global scale [11, 37], or very locally at a few square kilometers or around cities [12, 11, 38], but usually not at national scales. National studies focused on 1 or few countries [39, 40], and not on a wide range of countries representing 80% of the global population.

## 4. Conclusion

To our knowledge, this is the first study to assess the maximum fertilizing potential of nitrogen in human excreta across such a wide and diverse set of countries, as well as under more efficient food systems.

Our results reveal that in most countries, nitrogen from human excreta is of the same order of magnitude as the nitrogen in livestock manure currently applied to croplands. When compared to total N inputs to croplands, human excreta typically represent 15-30% of cropland fertilization, rising to 30-50% in sub-Saharan African countries. In contrast, large agricultural exporters show lower shares (5-10%), while rates in major importers and countries with limited livestock can exceed 50%. At the global scale, this rate is around 20%.

Importantly, this is also the first study to explore how this potential might evolve under physically plausible, more sustainable food systems. Using recent food system efficiency scenarios, we show that the maximum global potential contribution of excreta could exceed 30% in deeply transformative agro-ecological systems that reduce waste, recycle nutrients more effectively, and improve agronomic efficiency, namely with more vegetarian diets. This shows human excreta could be a non negligible component of circular nutrient strategies for agriculture.

Our analysis deliberately assumes high recovery rates to define upper-bound potentials. However, actual recovery rates will be shaped by context-dependent socio-technical, infrastructural, and behavioral factors. Future work should aim to assess these more realistic recovery rates, allowing to adjust our theoretical recovery rates.
# Appendix A. Additional Information

## Appendix A.1. Share of mineral fertilizers and manure applied to cropland (compared to total Agricultural Land)

In our study, we only focus on croplands for N inputs, while the food system agricultural land is made of both croplands and permanent grasslands. One reason lies in the availability of fertilization data at the country level for our 4 inputs (fertilizers, manure, biofixation, deposition), which is absent for permanent grasslands in FAOSTAT.

Nonetheless, our approximation holds, as for most countries, according to FAO estimates, manure and synthetic fertilization on croplands represents 90-100% of total fertilization on the whole agricultural land, as shown in Figure A1.

While FAOSTAT produces data beyond cropland fertilization for manure and fertilizers at the national scale, such data for biofixation and atmospheric deposition only exist at the global scale, which is why we could not study the whole agricultural land.

## Appendix A.2. Share of livestock manure collected and spread

## Appendix A.3. Physically possible range of Food System N Use Efficiencies (related to potential contribution of human excreta)

In Chatzimpiros and Harchaoui [17], their Figure 3, assesses the global food system N use efficiency for historical values and under a range of scenarios (right). Note that these NUE values stop at the production stage, and consequently do not account for further losses across the supply chain.

Their historical NUE value ends at 21% for the year 2013. Accounting for a further

30% full-chain losses, this yields a full system NUE of 15% for the current situation. They then explore the influence of 3 variables on their NUE:

* β: % of manure recovered to croplands. Their estimated current value is 35%, and they explore scenarios with 80%.

* NUE<sub>crops</sub>: N use efficiencies in croplands only (N<sub>crops removal</sub>/N<sub>crops inputs</sub>). Their estimated current value is 44%, and they explore scenarios with 70%.

* α<sub>crops</sub>: % of cropland production dedicated to livestock feed. Their estimated current value is 57%, and they explore a whole range of values from 0 to 70%.

The combination of β, NUE<sub>crops</sub>, an the allowance of synthetic fertilizers use (conventional vs organic scenarios) defines 5 different scenarios (Table A1). They then explore the variation of NUE (before the 30% food losses) for a range of α<sub>crops</sub> from 0% to 70%. Note that scenario B1 reproduces the current situation for α<sub>crops</sub> = 57%.

<table><caption>**Table A1.** Parameters of Chatzimpiros and Harchaoui scenarios. See text for definitions.</caption><thead><tr><th>scenario</th><th>β</th><th>NUE<sub>crops</sub></th><th>synth fertilizers</th></tr></thead><tbody><tr><td>B1</td><td>35%</td><td>44%</td><td>yes</td></tr><tr><td>B2</td><td>35%</td><td>70%</td><td>yes</td></tr><tr><td>B3</td><td>80%</td><td>70%</td><td>yes</td></tr><tr><td>B4</td><td>35%</td><td>44%</td><td>no</td></tr><tr><td>B5</td><td>80%</td><td>70%</td><td>no</td></tr></tbody></table>

An indirect output of the model, directly influenced by the share of cropland production attributed to livestock feed α<sub>crops</sub>, is the share of animal protein in food supply (from 20% to 60%). Contrary to many agricultural scenarios [34, 3], the more or less vegetarian diets are not an input of the model, but an output.
<figure><img src="image_8.png" /><figcaption>Cropland N fertilization as % of Agricultural Land N fertilization, for...</figcaption></figure>

based on FAOSTAT data for 2018–2022

**Figure A1.** Cropland fertilization as a share of Agricultural Land (i.e., croplands + grasslands) fertilization, for manure (left) and synthetic fertilizers (right). The percentages are calculated by $N_{i \text{ on croplands}}/N_{i \text{ on agricultural land}}$, with *i* standing for manure or for synthetic fertilizers. The rates higher than 100% for synthetic fertilizers are impossible, and reflect uncertainties owing to FAO's sources and methodology. Keeping these uncertainties in mind, rates X lower than 100% indicate that 1-X of the collected manure or synthetic fertilizers are applied to grasslands.
Share of excreted manure collected and applied to agricultural land

<figure><img src="image_9.png" /></figure>

**Figure A2.** Share of excreted manure that is either left on pasture, range and paddock (purple); collected but volatilized indoor (blue); or collected and finally spread (yellow, mainly on croplands but sometimes on grasslands). Numbers inform on the share finally spread (yellow bars). For all livestock (left), non ruminants (middle) and ruminants (right). Figures for all livestock are usually close to figures for ruminants, because these latter represent the majority of livestock excreta, except in some countries

Note that an $\alpha_{crops}=0\%$ is quite extreme, as it implies that all animal feed comes from grasslands, and thus non ruminant production is almost null (they can possibly be fed on food wastes). For that reason, in the next

Section, we also document food system NUEs in published agro-ecological scenarios.
Potential N Human Excretia

Appendix A.4. Potential in published
agro-ecological scenarios

Using a N mass balance, Billen et al (2024) [34] propose an agro-ecological scenario for Europe. They report (in their Table 2) an NUE of 41% for such a system, stopping before food waste and losses. They set these losses equal to 30% (in their supplementary spreadsheet, tab AgroEcol, this is what is called "fraction solid waste" on line 106, set to 0.3 of the apparent human consumption before waste). This results in a full system NUE of 41% * (1 – 30%) = 29%. Note that for current Europe, they find a total NUE of 31% * (1 – 30%) = 22%. This further suggests that current wasteful food systems are around 15-20%, and more frugal systems around 30% or more.

For comparison, in the agro-ecological scenario for the Paris area developed by Esculier 2018 [3] (cf his Figure page 431 of his thesis), the 3.9 kgN·cap⁻¹·year⁻¹ of human intakes are issued from a total of 14.3 kgN·cap⁻¹·year⁻¹ of agricultural inputs in the food system (11.9 biofixation, 1.8 urine-based fertilizers, 0.6 wastewater sludge), resulting in a total NUE of 3.9/14.3 = 27%.
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