Global potential nitrogen recovery from anaerobic digestion of agricultural residues

Meeting the anticipated 50% increase in global food demand by 2050 requires a crucial reassessment of agricultural practices, particularly in terms of nitrogen fertilizers inputs. This study analyzes the technical potential of nitrogen recovery from livestock manure and crop residues, bringing attention to the often-overlooked resource of digestate derived from anaerobic digestion. Our analysis highlights the significant capacity of the anaerobic digestion process, yielding approximately 234 ± 5 million metric tons (Mt) of nitrogen annually, sourced 93% from livestock manure and 7% from crop residues. Additionally, we estimated that substituting synthetic nitrogen with nitrogen from anaerobic digestion has the potential to reduce greenhouse gas emissions by 70% (185 Mt CO2-eq yr−1). Lastly, 2.5 billion people could be sustained by crops grown using nitrogen from anaerobic digestion of manure and crop residues rather than synthetic nitrogen fertilizers. Although agricultural residues have double the technical potential of current synthetic nitrogen fertilizer production, 30% of croplands encounter difficulties in satisfying their nitrogen needs solely through crop residues and anaerobic digestion manure. This deficiency primarily results from inefficient reuse attributed to geographical mismatches between crop and livestock systems. This underscores the urgent need to reconnect livestock and cropping systems and facilitate the transport and reuse of manure in crop production. In conclusion, the mobilization of these large amounts of nitrogen from livestock manure and crop residues will require to overcome the nitrogen from anaerobic digestion green premium with incentives and subsidies.


Introduction
The projected 50% increase in global food demand by 2050 [1] requires a critical reevaluation of agricultural practices, specifically regarding nutrient inputs [2].Historically, nitrogen has been a limiting factor in agricultural productivity, requiring conversion from the inert nitrogen form in the atmosphere to the reactive form usable by organisms through nitrogen fertilizers [3,4].Currently, the Haber-Bosch process [5] has significantly enhanced agricultural productivity by enabling the industrial production of ammonia (NH 3 ) based fertilizers, or synthetic nitrogen fertilizers [6].Synthetic nitrogen fertilizers, essential for modern agriculture, produce food that feed half of the global population [7], contribute significantly to environmental issues, emitting approximately 1.01 gigatons of carbon dioxide equivalent annually, constituting 1.3% of global greenhouse gas (GHG) emissions [8].One-third of these emissions result from direct carbon dioxide emissions during ammonia production driven by energy inputs [3,9], while the remaining two-thirds stem from nitrous oxide-a GHG 300 times more powerful than carbon dioxide (CO 2 )-produced during microbial conversion of excess nitrogen-based fertilizers in the soil [10][11][12].Additional unintended environmental consequences from synthetic nitrogen fertilizers include groundwater contamination [13,14], water eutrophication [15], air pollution [13,16], and stratospheric ozone depletion [17].Recent studies advocate for net-zero GHG emissions in fertilizer production, highlighting the urgent need for sustainable approaches to address rising agricultural nitrogen demand [3,7,9,18], emphasizing alternative pathways to decarbonize the nitrogen fertilizer sector and meet future demand projections [8].
The adoption of circular economy models, such as anaerobic digestion, provide an opportunity to convert organic waste, including livestock manure and crops residues, into valuable resources [19][20][21].Anaerobic digestion is a bioenergy technology employed for the breakdown of biomass feedstocks, such as organic waste and livestock manure, yielding two primary products, namely biogas and digestate [22,23].Biogas, made of approximately 60% methane and 40% carbon dioxide, can be harnessed for local heating or for combined heat and power generation [24,25].On the other hand, digestatea nutrient-rich substance-finds use as a valuable fertilizer [8,26].Digestate, when applied in liquid and solid forms, provides a substantial amount of nitrogen [26,27].Finally, captured CO 2 can be sequestered to generate carbon dioxide removal to offset hard-to-abate GHG emissions from sectors like agriculture, chemical industry, steel, and cement production [28][29][30][31].Moreover, anaerobic digestion shows potential for enhancing farm financial sustainability through energy sales and reducing fertilizer costs [32].Nonetheless, conducting a thorough cost-benefit analysis is essential to assess potential cost savings.This analysis should account for factors such as the availability, quality, environmental impact, and costs associated with the transportation and application of biogas and digestate as fertilizers.Consequently, this technology creates a circular economy model, where organic waste transforms into renewable energy and fertilizer, diminishing dependence on fossil fuels, reducing waste and emissions, and enriching soil fertility [32].Previous work has quantified the global biomethane and carbon dioxide removal achievable from anaerobic digestion [29,33].A total of 1.5 Gt of CO 2 per year can be captured and a total of 10 700 TWh of biomethane per year can be produced from anaerobic digestion worldwide, potentially meeting one third of natural gas demand [29,33].However, the potential of nitrogen recovery from agricultural residues and its role to offset synthetic nitrogen fertilizers has not been estimated at high resolution worldwide.
Here, we quantify the total potential of nitrogen derived from digestate generated through anaerobic digestion of crop residues and livestock manure worldwide.We quantify at the pixel level of 10 km resolution the different contributions to nitrogen potential between manure and crop residues.First, we quantify availability of crop residues and livestock manure.Second, we estimate the amount of digestate recoverable from anaerobic digestion.Third, we quantify nitrogen content from digestate.Fourth, we conduct a comparison between nitrogen from anaerobic digestion and synthetic nitrogen across various aspects and quantify the role of nitrogen from anaerobic digestion to reduce dependance on synthetic nitrogen.We provide an analysis of the potential GHG emission reductions achievable through the implementation of nitrogen from anaerobic digestion, considering the emissions associated with the Haber-Bosch process used in synthetic nitrogen production.Finally, we estimate the population that can be sustainably fed using both nitrogen from anaerobic digestion and synthetic nitrogen.
This research sheds light on the global technical potential of nitrogen from anaerobic digestion, offering insights into its role as a sustainable alternative to synthetic nitrogen.In this study, we not only estimate the amount of nitrogen that could be recovered from crop residues and livestock manure through anaerobic digestion, but we also highlight its regional availability at a 10 km pixel resolution.We consider crop residues and livestock manure that could be sustainably used for anaerobic digestion, while considering that a fraction of biomass feedstock should remain in the soil to preserve soil organic carbon [29,33].By providing a comprehensive comparison and analysis at a pixel level worldwide, we contribute to the ongoing conversations on sustainable agriculture and the urgent need to reduce the environmental footprint of nitrogen fertilizers.

Assessment of agricultural waste and residues availability
We utilized a global spatially detailed agricultural waste and residues biomass of crop residues and livestock manure dataset at a 10 km resolution [29].This dataset provides information on the quantity of livestock manure and crop residues available for sustainable agricultural use through the anaerobic digestion process.It includes the fraction of reusable waste biomass and excludes the portion that needs to be left in-situ to preserve soil organic carbon.Unlike biomass plantations cultivated for bioenergy, the use of livestock manure and crop residues does not necessitate the expansion of land or water use and does not contribute to additional environmental impacts [34].A summary table with the description of the datasets used, their units, and references is shown in supplementary table 1.
Feng and Rosa (2024) assessed the quantity of manure produced in weight, denoted as Z (million metric tons, t), by each of eight livestock species (i) by multiplying the number of animals of each species, H i (head), by the quantity of manure produced per animal, I i (t/head/year), and the availability factor AF i (%) [29].
H i density was derived from a dataset with a 10 km resolution, encompassing data on eight primary livestock species, namely cattle, buffaloes, horses, sheep, goats, pigs, chickens, and ducks [35].AF i is the proportion of manure that can feasibly and sustainably be collected and processed through an anaerobic digestion facility.This value varies, with a range of 0.1 for horse and goat, to 0.8 for pigs (supplementary table 2) [29,36].I i also varies depending on the livestock species as it is shown in supplementary table 2 [29,[36][37][38][39].These estimates were taken from previous studies covering main farming areas such as Europe, North America, and South America [29].
Feng and Rosa (2024) assessed the quantity of crop residues available for anaerobic digestion.This study considers a total of 42 crops across 9 groups, including cereals, roots, beans, fruit trees, vegetables, oil crops, sugar, fiber, and coffee.These crops account for nearly 100% of food crops reported by the United Nations Food and Agriculture Organization [40].To determine the amount of crop residues available in weight, Y j (t), we calculated it by multiplying the crop production O j (t) by the straw-to-grain ratio of each crop, G j (-), and the sustainable removal rate, RR j (%) [29]. ( A global gridded agricultural production dataset, with a resolution of 10 km, served as the basis for determining crop production (O j ) [40].The straw-tograin ratio (G j ) accounts for the weight ratio of straw to grain and varies for each specific crop [29,[41][42][43].This ratio ranges from 0.26 to 2.40 for each specific crop [29,[41][42][43].RR j represents the percentage of crop residues that can be collected from the ground without significantly impacting soil carbon content (supplementary table 3).This rate ranges from 30% to 70% (supplementary table 3) [44].
It is important to note that a portion of the straw and livestock manure must be left on the ground to prevent soil erosion and maintain soil fertility [44][45][46].Feng and Rosa (2024) incorporated this factor by introducing AF i and RR j , both of which represent the amount of waste biomass that can be sustainably collected as feedstock for the anaerobic digestion process.Lastly, the datasets used to derive the biomass availability in Feng and Rosa (2024) might introduce uncertainty to the result of this article.For example, the average values used such as the manure production rate (I i ), the availability factor (AF i ), the straw-to-grain ratio (G j ), and the sustainable removal rate (RR j ) can vary at a regional level.The use of the average value for these parameters was justified by the absence of available data incorporating regional factors.

Assessment of nitrogen from livestock manure and crops residues
To quantify the nitrogen potential from anaerobic digestion, (B), per pixel, (p), we multiplied the amount by weight of livestock manure (Z p ) and crop residues (Y p ) by the anaerobic digestion efficiency (e) and respective nitrogen contents (NC).NC m indicates the nitrogen content of livestock manure, while NC c represents the nitrogen content of crop residues.Lastly, we summed nitrogen in crop residues and manure to determine the total technical potential of nitrogen per pixel worldwide (B p ).
As 90% to 95% of the total mass of waste biomass fed into the anaerobic digestion process is transformed into digestate, and the remaining 5% to 10% is converted into biogas [47], we used the term anaerobic digestion efficiency (e) to describe the conversion of waste biomass into digestate.Subsequently, we calculated the amount of nitrogen potential for the respective feedstocks.
To quantify nitrogen content in crop residues (NC c ), as there is limited research available for the 42 crops classes used in the estimation of biomass availability, we relied on the nitrogen content (%) by dry weight basis from prominent agricultural crop residues, namely corn, rice, soybean, and wheat [48] (supplementary table 4).The nitrogen content in corn, rice, soybean, and wheat, used to calculate the potential for nitrogen in crop residues, was estimated from respective samples that were air-dried, milled, and sieved to 2 millimeters [48].Conversely, for manure, we consider the nitrogen content (NC m ) by dry weight basis from recent studies focusing on major livestock manure sources [49] (supplementary table 5).Since the raster datasets (Y p and Z p ) contained the total amount of waste biomass of crop residues from 42 crops and of livestock manure from 8 species, we decided that the most accurate approach would be to average the values of nitrogen content for crops and for livestock species.We calculated nitrogen estimates for both crop residues and livestock manure by averaging the nitrogen content values from the respective sources, using the midpoint between the lowest and highest values.As shown in supplementary tables 4 and 5, nitrogen content (NC m ) from manure is roughly an order of magnitude larger than the nitrogen content (NC c ) from crop residues.

Assessment of potential to offset synthetic nitrogen fertilizers
We compared the geospatial availability of nitrogen from crop residues and manure of anaerobic digestion with 2020 demand for synthetic nitrogen fertilizers to quantify the potential offset at a spatial resolution of 10 km worldwide.We derived our estimation of 2020 synthetic nitrogen demand for fertilizers from a recent study that covers 21 crop classes for the year 2020 [50].The 21 crops classes include barley, cassava, cotton, fruit, groundnut, maize, millet, oil palm, potato, rapeseed, rice, rye, sorghum, soybean, sugar beet, sugarcane, sweet potato, vegetables, wheat, sunflower, and other crops (representing all remaining crops) [50].The global current synthetic nitrogen demand from the geospatial dataset is calculated to be 98.9 Mt yr −1 [50], representing the most up-to-date geospatial data available for assessing global-scale synthetic nitrogen demand.To assess the potential offset per pixel (p) achievable by nitrogen from manure and crop residues (U p ), we divided the nitrogen from manure and crop residues of anaerobic digestion per pixel (B p ) by synthetic nitrogen use per pixel (C p ).This method provides insights into the extent to which nitrogen from manure and crop residues of anaerobic digestion can offset synthetic nitrogen fertilizer usage at the pixel level worldwide.
Our assumption aims to quantify the technical potential of nitrogen derived from digestate generated through anaerobic digestion of crop residues and livestock manure.However, the overall nitrogen content in digestate may not precisely match the nitrogen accessible to plants throughout the growing period.In fact, a significant portion of digestate nitrogen might be lost through ammonia volatilization or be tied up in organic compounds that plants cannot immediately utilize [51,52].

Assessment of GHG emission reductions
The Haber-Bosch process is the traditional method for producing ammonia (NH 3 ), the pillar of all synthetic nitrogen fertilizers.On a global scale, this process primarily relies on natural gas (70%) and coal (26%) as feedstocks, while oil and electricity contribute to less than 4% of the overall production [3].The choice of feedstock and the availability of process energy are pivotal factors influencing the location and method of ammonia production [53].The prevalence of inexpensive natural gas in the United States, Middle East, and Russia is a significant factor contributing to their dominant roles in ammonia production through natural gas-based plants [3].In contrast, China heavily relies on its abundant coal reserves, constituting approximately 85% of its production [3].It is assumed that most countries use natural gas as the primary feedstock, except for China (combining 85% coal and 15% natural gas), the United States, South Africa, and Indonesia (utilizing a mixture of 20% coal and 80% natural gas) [3].In terms of the raw materials used, the natural gas-based process consumes approximately 0.79 t of natural gas (or 0.59 t of fossil carbon) to produce one t of N [3].On the other hand, the coal-based process requires about 1.06 t of coal to produce one t of N [3].Through stoichiometric calculations, it can be deduced that the standard NH 3 production processes using natural gas and coal emit approximately 2.2 t CO 2 per t of N and 3.9 t CO 2 per t of N, respectively.We used these values to estimate the emission factors, EF k ( t C t N ) for each country (k).To facilitate its use and conduct a pixel-level analysis, we created a raster dataset containing emission factors, EF k,p ( t C t N ) per country (k) and per pixel (p).Given that the emission factor was calculated on a country level, all the pixels in a specific country are constant, assuming that emission factors are approximately similar in each pixel within each country.
To determine the reduction in GHG emissions, we first computed for each pixel (p) the residual amount of synthetic nitrogen that must be produced following the implementation of nitrogen from anaerobic digestion in the supply chain (DC p ), which is computed as the difference between synthetic nitrogen fertilizers use (C p ) and nitrogen potential from anaerobic digestion (B p ).When estimating the GHG emission reductions resulting from substituting synthetic nitrogen fertilizer with digestate nitrogen, we solely account for the decrease in emissions from reduced use of the Haber-Bosch process.We do not include emissions linked with the increased utilization of digestate nitrogen, such as those from raw material production, construction of anaerobic digester facilities, and digestate transportation [54].
Subsequently, we multiplied DC p by the countryspecific emission factor, EF k,p ( t C t N ) per country (k) and per pixel (p), and the conversion factor to convert one mole of carbon to one of CO 2 .Lastly, we found the amount of GHG emission saved (SEM p ) subtracting the remaining emissions following the implementation of nitrogen from anaerobic digestion (BEM p ) by the emission produced by synthetic nitrogen production (CEM p ).
When the difference between synthetic nitrogen (C p ) and nitrogen from manure and crop residues of anaerobic digestion (B p ) was greater than 0: When the difference between synthetic nitrogen (C p ) per pixel (p) and nitrogen from anaerobic digestion (B p ) per pixel (p) was smaller than 0, complete emission reduction can be achieved.

Assessment of people fed
For all countries, we estimated the amount of people fed by synthetic nitrogen.Then, we estimated the number of people fed by nitrogen of anaerobic digestion from synthetic nitrogen demand per pixel worldwide.To determine the amount of nitrogen from manure and crop residues of anaerobic digestion that can be utilized locally, we measured the nitrogen usage from manure and crop residues of anaerobic digestion per pixel by comparing it to the demand for synthetic nitrogen.If nitrogen from anaerobic digestion pixel exceeds the demand for synthetic nitrogen, we would limit the nitrogen from anaerobic digestion at the demand level.Conversely, if a pixel has more synthetic nitrogen compared to nitrogen from manure and crop residues of anaerobic digestion, nitrogen from anaerobic digestion becomes the limiting factor, and this is the value we use to estimate the population that can be fed.
To determine the number of people fed per country (Pc) with synthetic nitrogen and the number of people fed by nitrogen from manure and crop residues of anaerobic digestion (Pi) per country (k), we employed the approach initially developed by Rosa and Gabrielli in 2022.The weight of synthetic nitrogen (C k ) and nitrogen from anaerobic digestion (B k ) per country is multiplied by the fraction of nitrogen lost from farm to fork, ρ (%), and the countryspecific nitrogen use efficiency, σ k (%), representing the ratio between nitrogen inputs and outputs.Finally, we divide by the daily nitrogen intakes in diets (γ k ) [7].
Population fed by synthetic nitrogen: Population fed by nitrogen from manure and crop residues of anaerobic digestion: Our food systems exhibit significant inefficiencies [28].It is assumed that approximately 42.5% of nitrogen is lost from farm to fork [55,56].These estimates encompass the quantification of production and losses of digestible protein throughout the global food system, including harvesting, post-harvest stages, non-food uses, processing and packaging, distribution, and consumption waste.The daily nitrogen intakes in diets (γ k ), specific to each country, were derived from per capita daily protein intakes as reported by the Food and Agriculture Organization of the United Nations for the year 2019 [57].This derivation considers that approximately 18% of protein content consists of nitrogen, a crucial building block for amino acids required in protein production [58].Lastly, the nitrogen use efficiency ( σ k ) reflects the relationship between nitrogen inputs and outputs in terms of nitrogen.For instance, a nitrogen use efficiency of 40% indicates that 40% of the nitrogen inputs are converted into nitrogen in the form of crops [59].

Nitrogen production from anaerobic digestion
By utilizing crop residues and livestock manure as feedstock, the anaerobic digestion process can yield approximately 234 ± 5 million metric tons (Mt) yr −1 of nitrogen worldwide.Approximately 218 ± 5 Mt yr −1 of nitrogen can be derived from manure and 16 ± 0.04 Mt yr −1 of nitrogen can be sourced from crop residues.The regions that show more than 500 t of N potential per year per pixel are shown in figure 1.

Potential of nitrogen from anaerobic digestion to offset synthetic nitrogen
We conduct an analysis of the potential of nitrogen from anaerobic digestion to reduce the utilization of synthetic nitrogen fertilizers at a resolution of 10 km.The primary objective is to gain insights into geographical regions that possess the potential to transition away from using synthetic nitrogen fertilizers  and embrace nitrogen from anaerobic digestion as a viable alternative.
When aggregating data on a country level, the availability of nitrogen from agricultural residues may seem sufficient.However, as shown in figure 3, at a pixel-level resolution, the outcome is different, highlighting the geographical disconnect between crop cultivation and livestock farming.Currently, 70% of synthetic nitrogen can be met locally using nitrogen produced from crop residues and livestock manure.Figure 3(a) provide a clear visual representation of the insufficiency of nitrogen from anaerobic digestion in meeting current nitrogen demands.Several regions have the potential to offset 0% to 50% of synthetic nitrogen fertilizers locally, per pixel, with nitrogen from crop residues and livestock manure (figure 3).
Figure 3(b) indicates that crop residues offer a relatively minimal contribution to reducing dependence  on synthetic nitrogen compared to manure.The only regions where crop residues prove sufficient to meet the demand for synthetic nitrogen are in southern Brazil, northern Argentina, Indonesia, and scattered regions in central Africa.

GHG emission savings
Figure 4 illustrates the GHG savings from substituting synthetic nitrogen fertilizers with nitrogen from crop residues and manure for anaerobic digestion.Specifically, figure 4 presents the emissions generated before and after the adoption of nitrogen from the anaerobic digestion process.
Figure 5(a) shows the regions where the most substantial emission reductions from production of synthetic nitrogen fertilizers, ranging from 0 to 1100 t of CO 2-eq yr −1 per pixel, would occur, highlighting areas with significant potential for emissions reduction.
The remaining areas have limited potential to reduce emissions, likely due to scarce agricultural activities or low nitrogen potential from the anaerobic digestion process.Lastly, livestock manure alone can reduce 173 Mt of CO 2-eq yr −1 , while crop residues alone can contribute to a reduction of 30 Mt of CO 2-eq yr −1 .

People fed
Here, we estimated the total number of individuals being fed with both nitrogen from the anaerobic digestion process and synthetic nitrogen.
Figure 6 shows that approximately 70% of the people fed by synthetic nitrogen could be fed by nitrogen from crop residues and manure of anaerobic digestion today.The technical potential of nitrogen from the anaerobic digestion process can feed 2.5 billion people, while synthetic nitrogen today feeds 3.6 billion people.The United States has a nitrogen from the anaerobic digestion process to synthetic nitrogen ratio of approximately 59% (figure 6).This ratio implies the potential to feed 220 million people with nitrogen from crop residues and manure, compared to the nearly 372 million currently supported by synthetic nitrogen.The United States is a large food exporter and some of the food grown in the country is used to feed people abroad (figure 6).India and China can feed 386 and 361 million people, compared to the approximately 590 and 460 million people fed using synthetic nitrogen today (figure 6).Indonesia, Brazil, and Russia can feed nearly 170, 146, and 97 million people by using nitrogen fertilizers from anaerobic digestion, compared to the 300, 187, 116 million people fed by synthetic nitrogen (figure 6).

Discussion
Synthetic nitrogen fertilizers production is a major contributor to global GHG emissions, constituting 14% of agricultural and 1.3% of global emissions [8].Considering the crucial role of fertilizers in global food security, urgent actions are needed for decarbonization [7].With an expected manufacturing capacity of 15 Mt per year of low-carbon ammonia projected by 2030 [60], the fertilizer sector is actively working on decarbonization to address environmental goals and reduce supply chain shock from reliance on fossil fuels.
Globally, anaerobic digestion of crop residues and manure can capture 1.5 Gt of CO 2 per year and produce 10 700 TWh of biomethane, offsetting 29% of natural gas supply [29,33].This study shows that anaerobic digestion can also yield approximately 234 ± 5 Mt of nitrogen per year, with 93% from livestock manure and 7% from crop residues.In contrast, in 2022, the total use of synthetic nitrogen was 107.7 Mt yr −1 [12].Regions with higher potential for sustainable nitrogen from anaerobic digestion are characterized by intense agricultural activity and can benefit from implementing a circular model with anaerobic digestion to enhance crop production.
Our findings reveal a substantial availability of nitrogen from agricultural residues compared to current synthetic nitrogen use.However, when examined at a regional level, a clear geographical disconnect emerges between regions with abundant manure and crop cultivation (figure 3).Our results indicate that 30% of croplands face challenges in meeting their nitrogen requirements solely through crop residues and manure recycling using anaerobic digestion (figure 3).The adoption of synthetic nitrogen fertilizers has resulted in a relocation of highly productive crops to fertile regions and livestock farming to less fertile ones, creating a barrier between where most nitrogen from manure is produced and where it is needed [61][62][63][64].Notable regions exhibiting this disparity include the Midwest of the United States, southern Canada, eastern Spain, northwestern France, northwestern India, Borneo in Indonesia, northeastern China, southern Australia (figure 3).The recoupling of crop and livestock production can enhance the reuse of manure, reduce the need for synthetic nitrogen fertilizers, and facilitate GHG emission abatement [65,66].Overcoming this mismatch requires incentives and subsidies to mobilize nitrogen from livestock systems to crop systems through the anaerobic digestion process.A potential solution could be the implementation of an environmental credit system to subsidize farmers who actively adopt such environmentally friendly measures [65].

Challenges to large-scale adoption of anaerobic digestion
Traditionally considered a waste, digestate from anaerobic digestion is gaining attention for its nutrient-rich composition of nitrogen, but also phosphorus and potassium [67].However, there are several challenges that limit its deployment.Addressing these obstacles is crucial for nitrogen production from anaerobic digestion.These include high transportation costs [68], primarily attributed to the liquid state of the majority of digestate, continuous yearround digestate production in contrast to fertilizers application occurring only once or twice a year [69], unintended GHG emissions from digestate handling [70], and mismatch between nitrogen-to-phosphorus ratio in organic fertilizer and crop uptake [71].The application of digestate according to the nitrogen requirements of corn leads to the buildup of phosphorus, along with other ions and salt in the soil, due to the typically lower nitrogen-to-phosphorus ratio in manure or compost compared to the nitrogen-tophosphorus uptake ratio of corn [71].The disparity between nitrogen and phosphorus levels in organic fertilizers compared to crop uptake poses a significant obstacle.If left unaddressed, this imbalance could lead to the phenomenon known as pollution swapping [71].
While digestate can be directly applied to land, it can be separated into solid and liquid components.This separation serves multiple purposes, including facilitating its utilization and reducing transportation costs [68] improving marketability to accommodate year-round nitrogen production from anaerobic digestion [69], and minimizing unintended GHG emissions [70].The solid-liquid separation is usually conducted by a screw press or a decanter centrifuge [67].Following separation, the solid portion (approximately 10% to 20%) of digestate can be used as fertilizer or animal bedding, or it can be processed through composting or drying before sale, enhancing its marketability [72].The liquid fraction, comprising the largest share of digestate (approximately 80% to 90%), can also be directly applied to the land via fertigation, which is the application of fertilizers through the irrigation system [73].However, it is often subjected to further treatment to reduce transportation costs.One approach involves harnessing the heat generated by the gaseous byproduct of the digester to evaporate the liquid, reducing the volume of the product.This method can result in a significant volume reduction of approximately 50%, leading to equivalent savings in transportation costs [68,73].Additional treatments aimed at reducing transportation costs and enhancing nitrogen concentration for marketability include ammonia stripping, reverse osmosis, and biological treatments [68,74].These methods not only generate a marketable product but also serve as a solution to pollution swapping caused by the mismatch between the nitrogento-phosphorus ratio in organic fertilizer and crop uptake [72].Furthermore, new methods are currently being studied to achieve ammonia recovery from waste biomass using electrochemical methods [74].At present, while these approaches can cut transportation expenses, there is an economic green premium that hinder the initial investment in such technologies.Ongoing research and development efforts hold the promise of reducing technology costs and creating more efficient products in the future.
Currently, the global emissions related to crop residues and manure management in absence of the anaerobic digestion process correspond to 1% (0.23 Gt of CO 2-eq yr −1 ) and 8% (1.34 Gt of CO 2-eq yr −1 ) of the total emissions from agricultural systems [8].However, it has been proven that anaerobic digestion has the potential to reduce GHG emissions from crop residues and manure management [75].A comparison reveals that, in isolation, anaerobic digestion achieves a 25% reduction in methane emissions from digestate storage, while solid-liquid separation achieves a 46% reduction of methane emissions, and the combination of anaerobic digestion followed by solid-liquid separation achieves of 68% methane emission reduction [76].While anaerobic digesters can minimize methane emissions, certain GHG emissions can persist within anaerobic digestion systems [77].These include fugitive emissions arising from the digester itself, crop residues and manure temporarily stored before being used as a feedstock to the digestor, and digestate disposal [76].Despite a decrease in methane emissions, there is an observed increase in emissions of ammonia and nitrous oxide [76,77].

Conclusion
We conducted an analysis to assess the technical potential of nitrogen derived from anaerobic digestion, shedding light on the underutilized resource of digestate.Our estimates reveal that the anaerobic digestion process has the capacity to generate approximately 234 ± 5 Mt of nitrogen yr −1 , with 93% of this potential coming from livestock manure.These findings bear implications for climate mitigation and food security.Although the technical potential of nitrogen from anaerobic digestion of manure and crop residues appears to be the double of today's total production of synthetic nitrogen, several regions cannot completely fulfill their nitrogen demand highlighting a geographical disconnect between crop and livestock systems.Logistic, economic, and social challenges make nitrogen production from anaerobic digestion more expensive than business-as-usual Haber-Bosch fertilizer production.Therefore, subsidies and incentives will be crucial to overcoming the nitrogen green premium, enabling digestate to emerge as a valuable nitrogen resource.

Figure 1 .
Figure 1.Geospatial distribution of nitrogen production potential from crop residues and manure.(a) displays the total amount of nitrogen derived from crop residues and manure.(b) focuses on the nitrogen obtained from crop residues and manure separately, shedding light on nitrogen production potential at a regional level.

Figure 2 .
Figure 2. Country-specific technical potential of nitrogen from crop residues and manure.The figure illustrates the distribution of nitrogen production potential in each country highlighting the breakdown between crop residues and manure.

Figure 3 .
Figure 3. Geographical distribution of nitrogen potential from anaerobic digestion to reduce synthetic nitrogen.(a) Illustrates the potential impact of nitrogen from both crop residues and manure of anaerobic digestion on reducing the use of synthetic nitrogen.(b) Focuses on comparing the nitrogen obtained from crop residues and manure separately to synthetic nitrogen.

Figure 4 .
Figure 4. Quantitative representation of GHG emissions abatement per country.Figure compares the emission released from synthetic nitrogen use to the remaining emissions following the use of nitrogen from the anaerobic digestion process.

Figure 5 .
Figure 5. GHG emissions abatement from using nitrogen from the anaerobic digestion process.(a) Delineates the reduction in GHG emissions achieved through the utilization of nitrogen from both crop residues and manure.(b) Shows the reduction in GHG emissions obtained by implementing nitrogen from crop residues (figure (b) on the left) and from manure (figure (b) on the right).

Figure 6 .
Figure 6.Impact of nitrogen from the anaerobic digestion process of agricultural residues and synthetic nitrogen on global food security.