Recycled Nitrogen for Regenerative Agriculture: A Review of Agronomic and Environmental Impacts of Circular Nutrient Sources

Abstract

Global agriculture faces the twin challenges of meeting rising food demand while minimizing environmental impacts, necessitating transformative approaches to nutrient management. Recycled nitrogen fertilizers (RNFs), derived from diverse organic and waste sources such as urine, manure, compost, digestate, biosolids, and struvite, offer a groundbreaking pathway to close nutrient loops, reduce reliance on synthetic inputs, and foster regenerative agroecosystems. This comprehensive review synthesizes peer-reviewed studies published over the last two decades, selected based on relevance, study quality, and applicability to agronomic and environmental outcomes. Unlike earlier reviews that focus on individual RNF types, this work provides a novel cross-sectoral synthesis linking agronomic performance, environmental trade-offs, and socio-economic feasibility within the regenerative agriculture framework. Using a structured analytical framework, we critically assess RNF technologies and applications across agronomic efficacy, ecological implications, economic viability, and socio-regulatory landscapes. Despite promising benefits, including enhanced soil health, greenhouse gas mitigation, and alignment with circular economy principles, widespread RNF adoption remains constrained by logistical complexities, variable nutrient quality, regulatory uncertainties, and social acceptance challenges. By integrating multidisciplinary evidence and identifying system-level synergies and bottlenecks, this review advances a unified understanding of how RNFs can be strategically scaled in regenerative agricultural systems. Key knowledge gaps and integrated research and policy strategies are identified to unlock the full potential of RNFs. Embracing recycled nitrogen within tailored, context-sensitive frameworks has the potential to revolutionize sustainable agriculture, delivering resilient food systems, restoring ecosystem services, and advancing global climate goals.

1. Introduction

The intensification of modern agriculture over the past century has been closely tied to the widespread use of synthetic nitrogen (N) fertilizers [1]. These fertilizers have played a critical role in boosting crop yields and ensuring food security for a growing global population [2,3]. However, this success has come at a significant environmental cost. The production, transportation, and excessive application of synthetic N fertilizers are major contributors to greenhouse gas (GHG) emissions [4], nitrate leaching into water bodies [5], soil acidification [6], and disruption of global nitrogen cycles [7]. Globally, 40–68% of the applied N fertilizer is lost to the environment in many developing regions, contributing to ecological instability [8]. Globally, nitrogen fertilizer use has surged from 11.4 Tg N in 1961 to 108 Tg N in 2021 [9]. However, fertilizer use often exceeds the nitrogen demand of crops, with approximately half of the applied nitrogen being lost to the environment [10]. In particular, reactive nitrogen loss from agriculture is now recognized as a leading cause of water and air pollution, biodiversity loss, and climate change [11,12]. Solely agriculture is responsible for approximately 60% of global N₂O emissions, which further causes global warming and has an overall adverse effect on different agricultural production systems [8].

In parallel, a vast amount of nitrogen-rich waste is generated from human and animal excreta, food processing residues, municipal biosolids, and organic waste streams [13,14,15]. These materials often contain substantial quantities of nitrogen in various forms such as organic, ammoniacal, or mineral, but are frequently underutilized or disposed of in environmentally damaging ways. As global efforts intensify to develop circular economies and reduce dependency on finite resources, the concept of recycled nitrogen fertilizers (RNFs), derived from biological or waste-origin sources, has gained increasing attention as a promising strategy for sustainable and regenerative agriculture [16,17].

Recycled nitrogen sources encompass a diverse array of materials including urine-derived fertilizers (UDF), compost, animal manure, digestate from anaerobic digestion, biosolids from wastewater treatment, struvite recovered from liquid waste, and blends or co-products of these materials. Each of these sources has distinct chemical compositions, nutrient release patterns, and agronomic potential, as well as varying risks such as the presence of heavy metals, pathogens, or micropollutants [18,19,20]. Furthermore, their performance in the field is influenced by factors such as processing method, application timing, soil type, crop species, and climatic conditions [21,22].

The potential benefits of using RNFs are multifold. Agronomically, they can provide slow-release nitrogen and improve soil structure, organic matter, and microbial health [23,24,25]. Ecologically, they reduce reliance on fossil fuel-derived fertilizers [2], lower GHG emissions [26], and help mitigate nutrient losses to water systems [27]. Economically and socially, they may offer localized nutrient solutions, especially in low-input or smallholder farming systems, and align with global sustainability goals such as the United Nations’ Sustainable Development Goals (UN SDGs) [28,29].

Despite growing interest, the adoption of RNFs at scale remains limited. One major barrier is the lack of synthesized knowledge regarding the comparative agronomic efficacy and ecological impacts of these recycled inputs. Previous reviews and studies often focus on single RNF types or specific outcomes, provide limited quantitative comparisons, and rarely integrate agronomic, environmental, economic, and social dimensions. These shortcomings create a fragmented understanding of RNF performance and applicability. While several studies have explored individual RNFs or specific outcomes (e.g., yield or N leaching), a comprehensive, integrative review that systematically evaluates how RNFs perform across agroecological and environmental metrics is still missing. Moreover, the role of RNFs within regenerative agriculture—a system that emphasizes soil health, biodiversity, and circularity—remains underexplored in the literature. The core scientific issues addressed in this review are (i) how different RNFs compare in their nitrogen composition and release patterns, (ii) their relative agronomic performance and contribution to crop productivity, (iii) their environmental impacts including GHG emissions and nutrient losses, and (iv) socio-economic, regulatory, and practical factors influencing adoption in diverse agricultural contexts. By explicitly highlighting these knowledge gaps, this review seeks to provide a structured synthesis that addresses limitations in existing literature and guides future research and policy development. This review aims to critically evaluate the feasibility of recycled nitrogen fertilizers as sustainable inputs for regenerative agriculture. For this narrative review, literature was consulted from major scientific databases including Web of Science and Scopus, using relevant keywords related to recycled nitrogen fertilizers, regenerative agriculture, and circular nutrient management. Studies were selected based on thematic relevance and scientific significance rather than formal systematic screening criteria, to provide a comprehensive synthesis of current knowledge across agronomic, environmental, economic, and social dimensions. The study is focused on seven major categories of RNFs: urine-derived fertilizers (UDF), compost, manure, digestate, biosolids, struvite, and mixed-source formulations. The review explores their agronomic performance (e.g., yield, N use efficiency, soil fertility), ecological impacts (e.g., GHG emissions, nutrient losses, soil health), and practical feasibility (e.g., availability, cost, risks, and regulation). The specific objectives are to: (1) Characterize the major types and sources of RNFs and their nitrogen composition, (2) Synthesize findings on crop productivity, nitrogen availability, and soil quality outcomes, (3) Evaluate the environmental trade-offs, including pollution risks and climate impacts, (4) Assess social, economic, and regulatory factors influencing their adoption, and (5) Identify knowledge gaps and future research needs for RNFs in regenerative systems. The underlying hypothesis is that appropriately processed and applied RNFs can match or exceed the agronomic benefits of synthetic fertilizers while minimizing environmental harms. This hypothesis is grounded in studies showing that RNFs can provide controlled nitrogen release synchronized with crop demand, improve soil structure and microbial activity, and reduce losses of reactive nitrogen through leaching or gaseous emissions, thereby supporting both agronomic productivity and environmental sustainability. Although the fundamental properties and nutrient dynamics of recycled nitrogen fertilizers (RNFs) such as urine-derived fertilizers, compost, manure, digestate, biosolids, and struvite are well documented, the novelty of this review lies in its integrative and cross-sectoral analysis. It connects recent scientific advances with practical applications within the framework of regenerative and circular agriculture, emphasizing comparative agronomic performance, environmental trade-offs, and socio-economic feasibility across RNF types. Unlike previous reviews, this study synthesizes emerging findings from agronomy, environmental science, and policy to highlight synergies, limitations, and pathways for scaling RNFs in regenerative systems.

This review is structured to provide a comprehensive and accessible synthesis of recycled nitrogen fertilizers (RNFs) for a diverse audience of stakeholders. It begins with an overview of the major RNF types and sources, followed by assessments of their agronomic performance, environmental impacts, practical feasibility, and regulatory and social considerations. The review concludes by identifying knowledge gaps and future research priorities. By consolidating evidence across disciplines and regions, this review informs both scientific inquiry and practical decision-making for researchers, farmers, policymakers, and industry stakeholders seeking to implement sustainable nitrogen management strategies in regenerative agricultural systems.

2. Overview of Recycled Nitrogen Fertilizers

2.1. Urine-Derived Fertilizers (UDF)

Approximately 65% of nitrogen used in human systems remains unrecovered, with substantial losses occurring through wastewater discharge and the disposal of biosolids [30]. Although various strategies exist to capture and reuse these nutrients, such as applying treated sewage sludge or extracting ammonia from livestock effluents, urine diversion and recycling have gained increasing attention as a particularly energy-efficient approach. This is largely because most nitrogen ingested by humans is excreted in urine, making it a concentrated and accessible source for fertilizer production [31,32]. Human urine accounts for nearly 80–90% of the nitrogen excreted by the human body and contains nitrogen primarily in the form of urea, which is rapidly hydrolyzed into ammonium and then subject to volatilization and nitrification [33,34]. UDFs aim to recover this nitrogen in usable forms through various processing techniques. Common UDFs include stored/alkalized urine (stabilized against ammonia loss), nitrified urine (via biological treatment), struvite precipitation (magnesium ammonium phosphate), and evaporated or concentrated urine products [35]. These fertilizers are typically low in organic matter but rich in mineral nitrogen, offering rapid plant availability [19]. UDFs are gaining attention in circular sanitation systems and urban nutrient recovery efforts. However, their adoption is constrained by concerns over pharmaceutical residues, odor, public acceptance, and lack of commercial-scale infrastructure [36,37]. For example, in the production of UDF, technologies like activated carbon sorption and advanced oxidation have been shown to effectively lower pharmaceutical residues [38]. However, these treatments add complexity and expense to the process, and as a result, targeted removal of pharmaceuticals is not routinely implemented in all UDF applications [35].

2.2. Compost

Compost is produced through the aerobic decomposition of organic waste, including green waste, food scraps, and agricultural residues [39]. Composting is a widely used method for processing organic wastes, including manure and sludge, by blending them with carbon-rich materials. This mixture is carefully adjusted to provide adequate aeration, moisture, and nutrient balance, creating optimal conditions for microbial activity and the breakdown of organic matter [40]. When nitrogen-rich feedstocks (e.g., sewage sludge, food waste) are included, composts can contribute to nitrogen supply, although much of it is in organic forms with slow mineralization rates [41,42]. Sewage sludge contains valuable nutrients like nitrogen and phosphorus, which accumulate through nitrification and denitrification during wastewater treatment [43]. These nutrients make sludge a potentially beneficial fertilizer. When co-composted with materials such as sawdust, sludge undergoes microbial transformation led by aerobic thermophilic organisms. These microbes metabolize organic matter, producing heat that can reach 55–70 °C, sufficient to destroy many pathogens, particularly Enterobacteriaceae. The end product is a stabilized, humus-like material that is low in odor and largely free of human pathogens, making it suitable for agricultural use [44]. Although compost typically contains lower nitrogen levels than synthetic fertilizers, it significantly enhances soil organic matter, structure, microbial health, and long-term fertility [45]. Moreover, composting improves nutrient stability and reduces risks associated with pathogen transmission [44]. Aerobic composting yields a nutrient-enriched soil amendment that is generally free from pathogens, weed seeds, parasites, and insect larvae. Because of these qualities, compost is widely applied as a soil conditioner, particularly on organic farms [40]. However, the slow release of nitrogen may limit their use in high-demand cropping systems unless supplemented.

2.3. Manure

Livestock manure remains one of the oldest and most widespread RNFs, available in raw, semi-treated (e.g., dried, separated), or processed (e.g., pelletized) forms [46,47]. The nitrogen content and availability vary depending on species, feed, handling, and storage. Cattle and chicken manure are the main sources of animal manure across the world. In an analysis of global data, it has been shown that a steady increase in manure nitrogen production, globally, occurred between 1998 and 2014 to 131 Tg N yr⁻¹ [48]. Globally, cattle were the leading contributors to manure-derived nitrogen in 2014, accounting for approximately 43.7% of total production. In contrast, goats and sheep collectively generated around one-third of global manure nitrogen during the same period [49]. Manures contain both readily available ammonium and organic nitrogen, and often supply other essential nutrients (P, K, micronutrients). It has been observed that an increase in crop yields of approximately 8.5–14.2 Mg ha⁻¹ follows fertilization with manure without any increase in chemical nitrogen fertilizer [50]. Challenges include variable composition, nitrogen losses through volatilization, and the potential for pathogen or antibiotic residue contamination. Applying manure to agricultural soils can influence soil characteristics, though these impacts often take time to become noticeable [51]. Over the long term, manure use has been shown to enhance soil nutrient availability, benefiting crop production years after initial application [52]. The primary cause of soil acidification is the overuse of synthetic fertilizers [53]. Typically, manure influences soil pH due to its ash alkalinity content [54]. While the alkaline nature of manure tends to raise soil pH after application, the nitrification of nitrogen within the manure can produce protons, which may lead to a decrease in pH [55]. Best practices in manure management, including composting or precision application, can improve its efficiency and environmental safety. According to a meta-analysis, soil organic carbon (SOC) sequestration rates are notably greater when cover crops are used compared to systems without cover crops [56]. In the rice-wheat cropping systems of the Indo-Gangetic plains, the use of farmyard manure, rice straw, and nitrogen fertilizers helped maintain SOC levels comparable to those found in uncultivated soils [57]. Besides supplying carbon, manure applications also contribute various nutrients essential for crop uptake.

2.4. Digestate

Digestate is a by-product of anaerobic digestion (AD) of organic materials such as manure, food waste, or crop residues. It exists in solid and liquid fractions, both of which may be used as fertilizers. The nitrogen in digestate is partially mineralized (mostly ammonium), making it more plant-available than raw organic matter [58]. Previous studies have explored the application of AD as a fertilizer in soil cultivation. Research indicates that the liquid fraction of AD can effectively fertilize vegetables and cereal crops when derived from poultry litter, whereas digestate from dairy waste is more suitable for leguminous plants due to the distinct nutrient profiles of these digestate types [59]. Studies have indicated that the impact of AD on soil characteristics largely depends on the type of feedstock used for digestion and the application rate of the digestate [60]. Furthermore, AD has been found to enhance cucumber quality grown in sandy-loam soils by boosting levels of phenols and flavonoids [61]. Long-term use of AD has also been associated with an increase in soil microbial biomass [62]. Digestate is often valued for its high pH, reduced odor, and improved hygienic status compared to raw manure. Still, its application must be managed carefully to prevent ammonia losses in soils. One common issue with using AD as a fertilizer is the nutrient partitioning between its liquid and solid fractions. Typically, the liquid portion is richer in mineral nitrogen, whereas the solid fraction contains higher levels of phosphorus [63]. This separation allows to produce fertilizers tailored to specific needs rather than broad applications, which could potentially restrict the market for AD-based products. Additionally, the high moisture content of AD poses challenges for storage and transportation, increasing associated costs [58]. Moreover, due to its semi-liquid consistency, untreated AD may not be suitable for use directly as a liquid fertilizer [64].

2.5. Biosolids

Biosolids are nutrient-dense organic byproducts produced through the processing of domestic sewage at wastewater treatment facilities [65]. After stabilization and pathogen reduction, they may be classified as Class A (safe for unrestricted use) or Class B (restricted use, further treatment needed) [66]. Biosolids contain a high level of organic matter, making them valuable for improving soil health. Their application promotes the recycling of key plant nutrients such as nitrogen, phosphorus, potassium, sulfur, copper, and zinc. Beyond nutrient supply, using biosolids on land can help prevent soil erosion, aid in land reclamation, and enhance forestry areas [67]. However, biosolids may also carry undesirable substances that pose environmental risks. These include potentially toxic trace elements (PTEs) that can accumulate in soils and enter the food chain, along with the risk of contaminating surface and groundwater. Therefore, careful evaluation of the advantages and possible environmental impacts of biosolid application is essential [66]. In regions with strong quality control and advanced treatment, biosolids can be safe and effective RNFs. After standard anaerobic digestion, biosolids generally consist of about 50% organic matter. They inherently possess fertilizer properties and can be utilized accordingly. The total nitrogen content in biosolids varies depending on the treatment method but usually ranges between 1 and 6% [68], while phosphorus, potassium, and sulfur contents also differ depending on processing steps such as composting, dewatering, or thermal treatment. For example, dewatering processes can concentrate nitrogen and phosphorus but may reduce ammonium availability, whereas composting biosolids often leads to organic nitrogen stabilization and gradual mineralization, enhancing long-term N availability to crops [65,69]. Thermal treatments can reduce pathogen load and stabilize organic matter but may also cause some nutrient losses, particularly nitrogen volatilization [70]. Therefore, understanding how different biosolid treatments alter nutrient composition is critical for determining application rates and optimizing agronomic performance. In agricultural settings, biosolids are typically applied at rates designed to meet crop nitrogen requirements. Additionally, their use enhances soil quality by adding organic matter with each application [66].

2.6. Struvite

Struvite is a crystalline fertilizer that can be recovered from urine, manure digestate, or wastewater streams through chemical precipitation. It contains nitrogen, phosphorus, and magnesium in plant-available forms and is characterized by slow, steady nutrient release [71]. As a recovered mineral fertilizer, struvite offers consistent composition, low pathogen risk, and high agronomic efficiency. Struvite crystals can be applied directly to fields for crop production or lawn maintenance. Compared to using liquid manure as fertilizer, struvite offers several benefits as a solid fertilizer, including easier handling, lower transportation costs, reduced storage volume, and elimination of odor and pathogen concerns [72]. It performs effectively as a fertilizer across a wide range of soil pH values [73]. Struvite usually forms stable white crystals with an orthorhombic pyramidal structure. When these crystals aggregate into pellets, struvite is marketed as a slow-release fertilizer with a nutrient composition of approximately 5.7% nitrogen, 28.9% phosphorus, 0% potassium, and 9.9% magnesium [59]. Struvite releases nutrients gradually in the environment, aligning well with crop nutrient uptake rates. Its low solubility helps prevent common issues linked to fertilization, such as crop damage and nutrient runoff [74]. Research has demonstrated that vegetable growth improves more rapidly with struvite application compared to organic fertilizers and compost [75]. Additionally, the magnesium content in struvite makes it a suitable fertilizer alternative for certain crops, such as beetroot [76]. A key benefit of slow-release fertilizers is their ability to prevent root burn by gradually supplying nutrients. Such fertilizers are especially suitable for locations where fertilization occurs infrequently, such as pastures and forests, where applications may happen less than once per year or every few years. Research indicates that using slow phosphorus-release fertilizers like struvite enhances both the green and dry biomass of legumes [77]. The effectiveness of struvite arises from the interaction of its inorganic components (N, P, and Mg) in forming stable complexes with varying solubility [78]. However, the recovery of struvite requires specialized technology and is often limited to centralized facilities. Economic feasibility is still a barrier for widespread agricultural use.

2.7. Mixed-Source Fertilizers and Co-Products

These fertilizers are blends of two or more recycled materials, often designed to balance nutrient ratios or improve stability and handling. Examples include composted manure, co-digested food, green waste, and urine-stripped digestate products. Fresh manure is composed mostly of water (over 80%) and has relatively low nutrient density, making transportation costly and limiting how far it can be economically moved for fertilizer use [79]. This often results in heavy manure application on fields close to feedlots, which increases soil nitrate (NO₃⁻-N) concentrations and raises the risk of nitrate leaching and environmental harm [80]. Additionally, fresh manure emits a strong odor and may carry viable weed seeds that can spread to uncontaminated cropland. Composting the manure is one approach to mitigate these issues. Composting significantly decreases the moisture content of manure, from approximately 80% in fresh manure down to around 20–25%, which lowers the volume needing transport and thus cuts transportation costs. Additionally, the composting process diminishes the viability of weed seeds present in manure [81,82]. Compared to fresh manure, composted manure emits less odor and has improved physical characteristics, such as a loose, crumbly texture with consistent particle size. However, composting may also lead to some reduction in its fertilizer value [83]. While water removal concentrates nutrients in compost, these nutrients may become less accessible to plants. The quality of compost derived from various waste sources can differ significantly, largely depending on the compost’s degree of maturity. Consequently, it is important to assess maturity indicators in manure compost practically and to develop appropriate quality standards for animal manure compost. Some immature composts may contain high levels of free ammonia, specific organic acids, or other water-soluble substances, which are known to inhibit seed germination and root growth [84]. Mixed-source RNFs offer flexibility in nutrient formulation and may address shortcomings of individual materials. However, their properties can be variable, and consistent quality control is essential. Research on the interactions among blended RNFs, including how their combined use may enhance or reduce nutrient availability and crop response, is still limited. A summary of the comparison between RNFs has been provided in

Table 1. Summary comparison of recycled nitrogen fertilizers (RNFs).

RNF Type	Primary N Forms	Typical N Content (%)	C/N Ratio	Release Rate	Key Benefits	Key Challenges	References
UDF	Urea, NH4+, NO3−	0.5–1.5 (variable)	Low	Rapid	High N availability, urban recycling, lower heavy metal content	Odor, micropollutants, infrastructure	[85,86]
Compost	Organic N	0.5–2.0	High	Slow	Soil health, OM increase	Low N availability, variability	[82]
Manure	NH4+, Organic N	1–6 (species dependent)	Med	Moderate	Readily available, on-farm resource	Odor, leaching/volatilization risk	[51]
Digestate	NH4+, Organic N	2–6	Low-Med	Moderate-Rapid	High availability, stabilized material	Logistics, excess NH4+, uneven quality	[87,88]
Biosolids	Organic N, NH4+	2–6	Med	Moderate	Waste reuse, long-term N supply	Heavy metals, contaminants, regulations	[89,90]
Struvite	NH4+ (fixed)	5–6 (as N)	N/A	Slow	Clean, consistent, P and Mg included	Cost, tech-intensive recovery	[91]
Mixed Sources	Variable	Variable	Variable	Variable	Balanced nutrients, flexibility	Quality control, unknown interactions	[83,92]

.

3. Agronomic Performance

3.1. Nitrogen Availability and Release Dynamics

A key determinant of fertilizer efficacy in achieving high and sustainable crop yields is the form and timing of nitrogen availability. While synthetic fertilizers are engineered to deliver highly soluble and uniform nitrogen doses, RNFs vary widely in composition, mineralization rate, and agronomic behavior due to the biological origin and processing intensity of the source material. RNFs span a spectrum from highly soluble mineral nitrogen, like ammonium and nitrate, in urine-derived fertilizers [86] or digestate [72], to organic nitrogen forms that require microbial decomposition, as in compost or raw manure [93].

Urine-derived fertilizers, particularly when biologically nitrified or chemically stabilized, can deliver nitrogen in plant-available forms (ammonium, nitrate) that are immediately accessible to crops [85,94]. The mineral N fraction in these products allows for rapid uptake, supporting early-season growth and reducing the lag between application and nutrient assimilation [95]. However, the high reactivity of these forms also presents a risk for nitrogen loss via ammonia volatilization or nitrate leaching, especially when applied without incorporation into the soil or under wet conditions [96]. Acidification or incorporation methods are therefore critical for improving N retention [97]. In contrast, compost and well-aged manure contain nitrogen primarily in organic forms, which must undergo microbial mineralization to be plant-available [98]. This slow-release behavior can be advantageous for maintaining residual fertility over time, particularly in perennial or long-season cropping systems [99]. However, it also means that nitrogen availability may lag behind peak crop demand, potentially requiring supplementation with faster-releasing fertilizers. Digestate, as a by-product of anaerobic digestion, offers a middle ground. Typically, about 50–70% of the nitrogen in digestate is present as ammonium, making it more immediately available than raw organic amendments [87,100]. Yet, digestate’s effectiveness also depends on storage conditions, feedstock composition, and whether solid–liquid Separation has occurred [101]. Rapid ammonia volatilization can still occur if digestate is surface-applied and left exposed to the atmosphere [102]. Struvite, though mineral in form, has a unique slow-release dynamic [103]. Its crystalline structure dissolves gradually in the soil, making nitrogen and phosphorus available over time [95]. This property reduces the risk of nutrient losses but also means that peak crop demands may not be fully met if struvite is the sole N source. Thus, it is often best used as a base or starter fertilizer in combination with faster-releasing products. Therefore, nitrogen release patterns from RNFs must be carefully aligned with crop needs and application manner. Matching these dynamics requires understanding not only the fertilizer’s chemistry but also its interaction with the soil–plant–microbe system.

The release and availability of nitrogen from RNFs are strongly influenced by soil properties, microbial communities, and environmental conditions. For example, clay soils with high cation exchange capacity may retain ammonium more effectively than sandy soils, while soil pH, temperature, and moisture regulate microbial mineralization rates [104,105]. Diverse microbial communities mediate the transformation of organic nitrogen into plant-available forms, and their activity can be altered by soil management, prior cropping history, or co-application of other amendments [106,107]. Environmental factors such as rainfall, temperature fluctuations, and aeration further modulate nitrogen dynamics, affecting both the rate of release and the risk of losses [108,109]. Therefore, optimizing RNF use requires integrating knowledge of fertilizer chemistry with site-specific soil and environmental characteristics to synchronize nutrient availability with crop demand.

3.2. Crop Yield Response

Yield is the most tangible and widely reported agronomic indicator of fertilizer effectiveness. Numerous studies have evaluated RNFs across various cropping systems, often comparing their performance with synthetic nitrogen sources such as urea or ammonium nitrate. Urine-derived fertilizers have demonstrated high yield potential in both controlled and field conditions [110,111]. When applied at equivalent nitrogen rates and managed to minimize losses (e.g., through incorporation or acidification), UDFs can match or slightly exceed urea in cereals like wheat, maize, and barley [112]. For example, studies in Belgium and Nepal have shown comparable crop yields from nitrified urine and synthetic urea when application was timed appropriately [113,114]. In vegetable systems, where nitrogen demand is often intense, UDFs can also perform well, although odor and public perception may limit their broader adoption [115]. Manure and digestate have long histories of use in cereal and forage production. When applied at agronomically appropriate rates, they can achieve yields similar to those from synthetic fertilizers, especially in systems with healthy microbial communities and sufficient soil moisture to promote nitrogen mineralization [116,117]. Digestate, in particular, tends to outperform raw manure in terms of yield due to the higher fraction of mineral nitrogen [118,119]. However, high variability in nutrient content and potential logistical constraints (e.g., transport, odor) can reduce consistency across fields or regions. Compost, while excellent for improving soil quality, typically falls short as a standalone nitrogen source for high-yield systems. Its nitrogen is released slowly and may not be sufficient to meet crop demand during critical growth stages unless paired with a supplemental source [120]. Still, compost plays a crucial role in organic farming systems where synthetic inputs are prohibited and long-term fertility is prioritized over short-term yield maximization [121]. Biosolids have shown variable yield responses depending on the stabilization level and processing method. Class A biosolids, which are more pathogen-free and nutrient-stabilized, tend to perform better and are more acceptable for food crop use [122]. In long-term trials, biosolids can maintain yield parity with synthetic fertilizers, particularly in rotational systems where organic matter replenishment and slow nutrient release are valued [123,124]. Struvite excels in scenarios where phosphorus limitation is the main constraint, such as low-P soils or organic systems without conventional P inputs [18]. Its nitrogen content, though modest, can support early growth when supplemented [125]. Yields under struvite-based fertilization tend to be high when struvite is part of a nutrient-balanced strategy. Mixed-source fertilizers, which blend complementary materials (e.g., compost + struvite), can be tailored to achieve both nutrient supply and soil health goals [126]. While yield results vary depending on formulation, such products offer a flexible path to optimizing multiple agronomic and environmental objectives.

3.3. Nitrogen Use Efficiency (NUE)

NUE is a key metric that reflects how efficiently plants use applied nitrogen, often calculated as the ratio of nitrogen uptake to nitrogen input [127]. High NUE implies reduced nitrogen losses and improved input efficiency, goals aligned with both economic and ecological sustainability [128]. Among RNFs, UDF and digestate consistently show moderate to high NUE, especially when managed to reduce ammonia loss [19,129]. Nitrified urine, in particular, has a nitrate-rich profile that matches plant uptake and reduces gaseous losses [37]. However, the absence of organic matter limits its soil-retentive capacity, meaning it may not contribute to long-term fertility or carbon sequestration [110]. Manure and compost, while less efficient in the short term due to slow nitrogen release, can contribute to long-term NUE by building soil organic matter and microbial biomass, which improve soil nitrogen retention and turnover [130,131]. These materials are also less prone to leaching under certain conditions, as their organic N is more tightly held in the soil matrix until mineralized [132]. Struvite’s slow-release nature lends itself to controlled, season-long availability, which can improve NUE in leaching-prone systems or sandy soils [133]. However, its efficacy depends on correct placement (e.g., banding near roots) and compatibility with the crop’s growth curve [103]. Overall, in addition to agronomic practices like land management to enhance nutrient cycling, achieving high NUE with RNFs often depends on blending materials with varying nitrogen-release characteristics.

3.4. Soil Fertility and Organic Matter Contributions

Beyond immediate nutrient delivery, RNFs play a critical role in supporting soil health, especially in regenerative agriculture frameworks that prioritize soil structure, microbial diversity, and carbon sequestration [134]. Compost, manure, and biosolids are particularly valuable for their high organic matter content, which contributes to improved soil structure, enhanced water-holding capacity, and more stable aggregates [90,135,136]. They also introduce microbial biomass and functional groups that can stimulate biological nitrogen cycling and support beneficial soil fauna [137,138]. In degraded or sandy soils, repeated applications of these materials have been shown to increase total soil nitrogen and reduce bulk density [139,140]. Digestate, although lower in organic matter than compost, can still improve microbial activity due to its residual carbon content and high nitrogen concentration [141]. When applied with cover crops or in reduced tillage systems, digestate can foster synergistic effects on soil fertility [142,143]. UDF and struvite, by contrast, provide little to no organic matter. While they can efficiently deliver nitrogen, their contribution to long-term soil health is limited unless co-applied with organic amendments [111,144]. As such, they may be better suited to integrated fertility plans that include cover cropping, residue management, or periodic compost addition. Soil fertility improvements from RNFs are not just chemical but biological and physical, reinforcing the multifunctionality of these fertilizers in agroecosystems that aim to regenerate productivity. While synthetic fertilizers offer precision and consistency, recycled nitrogen fertilizers, when properly managed, can deliver comparable crop yields and support long-term soil health. RNFs like UDF and digestate show the greatest promise for high-input systems, while compost and manure are valuable in systems emphasizing organic matter buildup. Matching the nitrogen release dynamics of RNFs with crop needs is critical to maximize agronomic efficiency. A summary of agronimc performance of RNFs has been provided in

Table 2. Agronomic performance of major RNFs compared to synthetic fertilizer.

RNF Type	N Availability	Yield Potential	NUE Potential	Soil Fertility Impact	Agronomic Notes	References
UDF	High (urea, NH4+, NO3−)	Comparable to urea with good management	Medium–High (if incorporated/acidified)	Low (little OM)	Rapid uptake, good starter fertilizer, but risk of N loss	[110,113]
Compost	Low–Moderate (organic N)	Often lower unless combined with mineral N	Low initially, improves long-term NUE	High (builds SOM, microbes)	Best for soil improvement and organic systems	[45,136]
Manure	Moderate (NH4+ + organic N)	Competitive at medium–high rates	Medium (depends on handling)	High (variable, but boosts OM and microbes)	Widely used; timing and storage critical
Digestate	Moderate–High (mostly NH4+)	Often comparable to synthetic N	Medium–High	Moderate (stimulates microbes, less OM)	Good balance of immediacy and residual effects	[129]
Biosolids	Moderate (stabilized N)	Comparable in long-term use	Medium	Moderate–High (micronutrients, OM)	Regulatory limits apply; good for low-input rotations	[124]
Struvite	Low–Moderate (NH4+)	High in nutrient-balanced strategies	Medium–High	Low (mineral fertilizer)	Ideal for P-limited soils; slow-release N source	[144]
Mixed Sources	Variable	Blended to meet needs	Variable	Variable (depends on components)	Customizable; offers synergistic benefits	[82,136]

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4. Environmental Considerations

4.1. Greenhouse Gas (GHG) Emissions

The environmental footprint of nitrogen fertilization extends beyond the farm gate, influencing atmospheric chemistry, water quality, soil health, and ecosystem stability. Synthetic nitrogen fertilizers are known contributors to greenhouse gas (GHG) emissions, nitrate leaching, and eutrophication. GHG emissions from fertilizer use primarily involve nitrous oxide (N₂O), a potent greenhouse gas released during nitrification and denitrification processes in soil [145]. Additional emissions may occur during fertilizer production, transportation, and field application. UDF, particularly when acidified or nitrified, tends to produce lower GHG emissions than synthetic urea [146]. Studies show that direct N₂O emissions from UDF-treated plots are often reduced due to a more synchronized N release and lower conversion of ammonium to nitrate under optimized conditions [147]. However, high ammonia (NH₃) volatilization from unincorporated urine or alkaline soils can indirectly contribute to N₂O formation elsewhere (indirect emissions) [148]. Compost and manure typically result in lower short-term N₂O emissions compared to synthetic fertilizers due to their slow nitrogen mineralization [99]. However, when applied in excess or under wet, anaerobic conditions, emissions can spike, especially if decomposition leads to elevated microbial activity [149]. Digestate is often linked with higher direct N₂O and NH₃ emissions, particularly when surface-applied, due to its high ammonium content and rapid mineralization [150]. Emissions can be mitigated through injection, acidification, or use of nitrification inhibitors [151,152]. Biosolids show variable GHG profiles. Stabilized biosolids tend to have moderate N₂O emissions [153], but the presence of residual organic matter and inconsistent nitrogen forms can result in unpredictable emission patterns [154]. Struvite, due to its low solubility and gradual N release, has been shown to reduce N₂O emissions compared to fast-release mineral fertilizers. It is also less prone to volatilization, making it attractive for systems with high leaching or emission risks [133]. In general, RNFs with slower or more controlled N release (e.g., compost, struvite) tend to exhibit lower direct GHG emissions, whereas those with high ammonium content (e.g., digestate, UDF) require careful management to minimize losses.

4.2. Nitrogen Loss Pathways: Leaching and Volatilization

Nitrogen losses not only reduce fertilizer efficiency but also pose serious environmental risks, particularly through nitrate leaching into groundwater and ammonia volatilization into the atmosphere. UDF, particularly in urea or ammonium form, can be highly vulnerable to volatilization when surface-applied without incorporation or acidification [155]. However, the nitrate-rich formulations (e.g., nitrified urine) are more susceptible to leaching, especially in coarse-textured soils and high rainfall conditions [96]. Manure and digestate often contain a mix of organic and inorganic N. Ammonium-N can volatilize quickly if not incorporated, while the organic fraction mineralizes slowly, contributing to residual soil N and potential late-season leaching [156]. Compost, with its high carbon-to-nitrogen (C:N) ratio and stabilized organic N, typically shows low leaching and volatilization losses, making it one of the most environmentally benign RNFs in this regard [157]. Biosolids, depending on treatment level, often release N slowly, reducing leaching but potentially contributing to longer-term accumulation and runoff if applied frequently or in excess [90]. Struvite’s crystalline structure limits solubility, leading to minimal leaching and volatilization losses [158]. This makes it particularly suitable for sandy soils or areas with high rainfall [103]. In summary, the form of nitrogen, application method, soil type, and climatic conditions play critical roles in determining the extent of N losses. Incorporation, timing, and blending strategies can significantly reduce volatilization and leaching from RNFs.

4.3. Heavy Metals, Pathogens, and Contaminants

One environmental concern associated with RNFs is the potential accumulation of non-nutritive substances, including heavy metals (e.g., Cd, Pb, Zn, Cu), pathogens, and emerging contaminants like pharmaceuticals, hormones, and microplastics. Biosolids are subject to the most scrutiny, as they can contain elevated levels of trace metals, antibiotic residues, and microplastics [159]. Although Class A biosolids undergo pathogen reduction and stabilization, concerns remain about long-term soil accumulation and bioavailability of contaminants [160]. UDF is generally considered low in heavy metals but may carry trace levels of pharmaceuticals or personal care products [161]. Research shows that most of these compounds degrade in soil or are present at concentrations far below risk thresholds, but more long-term fate studies are needed. Manure and digestate can contain veterinary antibiotics, hormones, and resistant bacteria, especially from intensive livestock operations [162]. Composting and anaerobic digestion reduce many of these risks, but residuals may still persist, depending on process efficiency and storage conditions [163]. Compost, especially when sourced from mixed municipal waste, may contain microplastics and metal residues, although high-quality compost made from clean green or food waste has minimal risk [164,165]. Struvite, being chemically precipitated, is usually highly pure and free from biological or chemical contaminants, assuming a well-controlled recovery process [166]. While most RNFs can be safely used under regulated application rates, repeated long-term use in the same fields may lead to elemental accumulation. Therefore, monitoring and risk assessment are essential, especially for biosolids and manure-derived fertilizers. While some RNFs pose risks related to emissions or contamination, they also offer substantial environmental benefits, including improved nutrient cycling, reduced fossil dependency, and enhanced soil health. The key to maximizing these benefits while minimizing risks lies in appropriate sourcing, treatment, and application strategies. When integrated with good agricultural practices, RNFs can play a pivotal role in climate mitigation, pollution control, and the transition toward a circular bioeconomy. A summary of environmental performance has been provided in

Table 3. Environmental performance comparison of RNFs.

RNF Type	GHG Emissions (N2O, NH3)	N Losses (Leaching, Volatilization)	Contaminant Risk	Notes	References
UDF	Moderate–Low (if managed)	High (if surface-applied); Low with incorporation	Low–Moderate (pharma traces)	Needs pH/stabilization	[147,155,161]
Compost	Low	Very Low	Low (if clean feedstock)	Best for soil C enhancement	[157,165]
Manure	Variable	Moderate–High	Moderate–High (pathogens, antibiotics)	Composting helps	[135,163]
Digestate	Moderate–High	High volatilization unless injected	Moderate (depends on source)	High moisture, odor issues	[102,150]
Biosolids	Moderate	Low–Moderate	High (metals, pharma, plastics)	Regulations essential	[123,154]
Struvite	Very Low	Very Low	Very Low	Limited availability	[18,166]
Mixed Sources	Variable	Variable	Depends on inputs	Requires quality control	[37,136]

Note: The classification of Contaminant Risk (Low, Moderate, High) is based on a qualitative synthesis of literature-reported levels of heavy metals, pathogens, pharmaceuticals, and microplastics associated with each RNF type.

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5. Economic and Practical Feasibility

5.1. Production and Processing Costs

The cost of producing RNFs varies significantly depending on the source material, treatment method, and technology level. Manure and compost are relatively low-cost due to their on-farm availability and minimal processing needs, especially in smallholder systems. However, composting still requires labor, space, and time, and nutrient losses during the process may reduce overall N efficiency. Digestate from anaerobic digestion is often a by-product of energy production and may have low marginal costs, though post-treatment (e.g., solid–liquid separation or ammonia stripping) adds operational expenses. Biosolids typically incur high costs for transport, stabilization, and regulatory compliance—often borne by municipalities or treatment plants, not farmers directly. Urine-derived fertilizers (UDFs) involve advanced treatment technologies (e.g., struvite precipitation, nitrification, distillation, pasteurization), requiring investment in collection systems and decentralized or centralized processing units. These technologies are still emerging and have limited cost-efficiency at scale. Struvite, while offering high nutrient density and ease of application, is expensive to recover due to the need for precise chemical conditions and reactor technologies. Mixed-source RNFs, such as blends of digestate and compost or biosolids and struvite, may optimize nutrient ratios but increase complexity in sourcing, blending, and standardization. Overall, the cost per kg of plant-available N in RNFs can be higher than synthetic urea, unless supported by subsidies, waste management offsets, or energy co-benefits (e.g., biogas) [167]. However, when externalities like avoided emissions, landfill diversion, or wastewater treatment costs are accounted for, RNFs may be economically favorable in full-cost accounting frameworks. RNFs lower expenses related to waste management and the manufacturing of synthetic fertilizers by converting waste materials into useful agricultural resources. It is also important to note that these production and processing costs vary regionally; a detailed comparison between Global North and Global South contexts is provided in Section 5.5.

5.2. Transportation and Application Challenges

One of the most significant constraints to the practical use of RNFs is their bulky nature and variable nutrient content, which complicate storage, transport, and field application. Many RNFs (e.g., raw manure, digestate, compost) have low nutrient density and high moisture content, requiring large volumes to meet crop N demand [168]. Transporting RNFs over long distances, especially from urban waste treatment facilities to rural farms, can be costly and carbon-intensive, limiting their feasibility beyond local or peri-urban regions [169]. Solid RNFs (e.g., struvite) are easier to store but may require specialized spreading equipment or uniform granulation for consistent field application. Liquid RNFs (e.g., urine, digestate) demand storage tanks, pumps, and injection equipment to avoid ammonia volatilization and runoff. Odor, pathogen concerns, and public nuisance risks (e.g., near residential areas) may restrict when and where these materials can be applied. These logistical hurdles mean that RNFs are most practical in localized nutrient loops (e.g., urban farms, peri-urban horticulture, integrated livestock systems), but are less competitive in large-scale, industrial monocultures without infrastructural adaptation.

5.3. Compatibility with Existing Farming Systems

Adoption of RNFs is influenced by how well they integrate into farmers’ existing routines, machinery, and management strategies. Timing and synchronization of nutrient release with crop demand is crucial, as organic RNFs often release N more slowly or unpredictably than mineral fertilizers [103,124]. RNFs that lack standardization (e.g., variable N content, uneven moisture) require farmers to test, calibrate, or supplement with other sources, adding labor and risk. Many smallholder or organic farms already use compost or manure effectively, but conventional systems may resist switching unless performance and cost are equivalent. Additionally, mechanization gaps are critical. Farms without compost spreaders, injectors, or digestate handling infrastructure are unlikely to adopt RNFs without support or incentives. In regions where custom application services, cooperative models, or municipal-farmer partnerships exist, integration is more feasible. Public–private initiatives, such as urine collection pilots or biosolids reuse programs, can also help bridge these gaps and support broader integration.

5.4. Economic Incentives, Market Structures, and Externalities

For RNFs to become economically viable, they must be supported by enabling financial mechanisms and robust market structures. Incentives such as subsidies or carbon credits that reward nutrient recycling, soil carbon sequestration, or avoided GHG emissions can help make RNFs more competitive. In practice, these measures are applied through government programs that provide direct payments or tax benefits to farmers using RNFs, through carbon trading schemes that assign monetary value to reduced emissions, or via certification programs that allow recycled fertilizers to access premium markets. At the same time, applying true-cost accounting to synthetic fertilizers (factoring in their energy use, water pollution, and public health impacts) would further highlight the relative affordability of RNFs. Circular bioeconomy models, such as biogas facilities that generate both energy and nutrient-rich digestate, offer promising examples of how multiple revenue streams can enhance feasibility. Despite these opportunities, RNFs often lack mature markets. Pricing, quality standards, and labeling protocols are limited or nonexistent, increasing the risk for both producers and end users. Variability in the quality and consistency of recycled nitrogen fertilizers (RNFs) often hampers their competitiveness against synthetic fertilizers, which provide more reliable performance and uniformity. Furthermore, economic challenges—including substantial initial investment requirements and limited acceptance among farmers—remain significant obstacles to the widespread adoption and market success of RNFs [170,171]. To overcome these barriers, a coordinated economic framework is needed, one that integrates waste management, agricultural systems, and climate policy to support the scaling of RNFs.

5.5. Feasibility in Global South vs. Global North Contexts

The feasibility of RNFs varies widely depending on the regional context. In the Global South, they present a low-cost, locally available alternative to expensive synthetic imports. However, adoption is often constrained by limited infrastructure, technical capacity, and access to adequate treatment systems. In contrast, the Global North benefits from advanced nutrient recovery technologies, yet faces different barriers such as regulatory constraints, public skepticism, and resistance within established fertilizer markets. Despite these challenges, innovative approaches like ecological sanitation, community-scale composting, and urban–rural partnerships offer adaptable models that can enhance feasibility across diverse settings when properly tailored. Ultimately, the economic and practical viability of RNFs is shaped by a complex set of interrelated factors, including production and processing costs, transportation logistics, farming system compatibility, policy support, and market development. While RNFs can be both feasible and cost-effective within localized or integrated systems, broader adoption in mainstream agriculture will require sustained investment in infrastructure, targeted economic incentives, and strong institutional coordination. Without these foundational supports, RNFs are likely to remain niche solutions, underutilized despite their significant sustainability potential.

6. Regulatory and Social Acceptance

6.1. Regulatory Frameworks

Regulations surrounding the use of RNFs vary widely by region, reflecting differences in environmental standards, public health priorities, agricultural policies, and waste management strategies. Many countries have strict regulations regarding the application of organic wastes such as biosolids, manure, and digestate, primarily to protect against pathogen transfer, heavy metal contamination, and nutrient overloading. For instance, the U.S. Environmental Protection Agency governs biosolids use, setting limits for pathogen reduction, metal concentrations, and land application rates [172]. Similarly, the European Union’s Sewage Sludge Directive and the Nitrates Directive regulate nutrient management and pollution risks [173,174]. Lack of standardized certification for many RNFs (especially newer ones like urine-derived fertilizers or struvite) poses a barrier to market acceptance and regulatory approval. This absence leads to uncertainty over product consistency, efficacy, and safety. Application restrictions on timing, rates, and methods, intended to limit nutrient runoff, leaching, and emissions, may disproportionately affect RNFs due to their nutrient release variability. For example, surface application of liquid digestate may be restricted during wet seasons, limiting its practical use. One significant regulatory hurdle is that many RNFs originate as waste or by-products, complicating their classification as fertilizers. This legal ambiguity affects their marketing, liability, and acceptance. Clear pathways for waste-to-fertilizer conversion are still evolving in many jurisdictions. Some regions have begun developing adaptive regulatory frameworks that allow for experimental permits, accelerated approvals, or differentiated rules for emerging RNFs, balancing innovation incentives with risk management.

6.2. Social Acceptance and Perception

Public and farmer perceptions play a crucial role in the adoption of RNFs, especially those derived from human waste or municipal biosolids. Odor is frequently cited as a primary barrier to acceptance, particularly with manure, digestate, and biosolids. These sensory impacts can reduce community support and lead to opposition from neighbors or local governments. Consumers often express apprehension about the use of fertilizers derived from human waste (e.g., urine) or treated sewage sludge due to concerns about pathogens, pharmaceuticals, or chemical contaminants entering the food chain. These fears, whether scientifically justified or not, can slow adoption. Adoption depends heavily on farmers’ trust in the reliability, nutrient value, and safety of RNFs. Many farmers are reluctant to deviate from synthetic fertilizers without clear evidence and extension support demonstrating RNFs’ benefits and risks. In some societies, cultural taboos around human excreta or animal waste may limit acceptance of certain RNFs, necessitating culturally sensitive outreach and education. Transparent communication about treatment processes, safety protocols, and environmental benefits is vital. Successful pilot projects, demonstration farms, and participatory approaches can enhance social buy-in. Increasing consumer demand for sustainable and organic products creates a market pull for circular fertilizers, but certification schemes (e.g., organic labeling) often have strict limitations on certain RNFs, restricting market access.

6.3. Institutional and Policy Support

The successful adoption of recycled nitrogen fertilizers (RNFs) depends on a supportive enabling environment shaped by coordinated efforts from multiple stakeholders. Government incentives and subsidies, such as financial support through tax breaks or carbon credits, can encourage investment in RNF technologies and infrastructure. Equally important are research and extension services, where public funding advances the efficacy, safety, and application methods of RNFs while providing essential training programs for farmers. Public–private partnerships further strengthen this framework by fostering collaboration among municipalities, agribusinesses, and farmers to develop reliable supply chains, sharing both costs and risks. Finally, integrating policies across nutrient management, waste treatment, climate, and agriculture ensures regulatory consistency and helps eliminate barriers to RNF adoption.

6.4. Case Studies and Lessons Learned

Several successful examples demonstrate how regulatory and social challenges can be overcome. Sweden’s urine diversion and recycling programs have combined stringent treatment, clear regulatory frameworks, and public education to build acceptance. In Europe, the reuse of biosolids and digestate on agricultural land was spearheaded by countries like the Netherlands and Germany, whose early legislation laid the groundwork for systems that now integrate regulatory compliance, farmer incentives, and efficient logistics. Pilot projects in Nepal and Kenya have shown promising acceptance of urine-based fertilizers through community involvement and targeted outreach [112,175]. The regulatory and social dimensions of recycled nitrogen fertilizer adoption are as important as their technical and economic factors. Without clear, science-based regulations and proactive efforts to build trust and acceptance among farmers and the public, RNFs will struggle to achieve mainstream uptake. Integrated policy frameworks, transparent communication, and inclusive stakeholder engagement are essential to transform RNFs from niche solutions to cornerstone tools in sustainable, circular agriculture.

6.5. Regional Perspectives

Europe: Leading in regulatory harmonization and innovation uptake, with multiple countries integrating recovered fertilizers into conventional markets. Strong policy incentives, advanced waste infrastructure, and public awareness underpin this trend.

North America: Focused on biosolids and manure management; innovative urine diversion projects exist but remain niche. Regulatory complexity and fragmented policies slow broader adoption.

Asia: High population density and limited arable land create urgent nutrient recycling needs. Japan leads in urine recycling policies; China promotes digestate use alongside biogas development. However, widespread adoption is limited by inconsistent standards and farmer awareness gaps.

Africa and Latin America: Resource constraints and weak regulations hamper RNF adoption. Nonetheless, localized projects on urine fertilization and composting are gaining traction with non-governmental organizations (NGOs) and community support, showing promise for improving food security and sanitation.

7. Knowledge Gaps and Research Needs

Despite growing interest and promising advances in RNFs, significant knowledge gaps and research challenges remain. Addressing these gaps is essential for improving the effectiveness, safety, and adoption of RNFs across diverse agricultural systems and environmental contexts. This section identifies priority areas for future research, spanning fundamental science, technology development, environmental assessment, and socio-economic integration.

7.1. Agronomic Performance and Nutrient Dynamics

Long-term field trials: While many short-term studies document yield responses to RNFs, long-term, multi-site trials are needed to understand their sustained agronomic performance, nutrient release patterns, and effects on soil fertility over multiple cropping cycles.

Nutrient synchronization and availability: Research should focus on improving prediction models for N mineralization and availability from heterogeneous RNFs, accounting for variability in composition, environmental conditions, and crop uptake dynamics.

Crop- and soil-specific responses: More studies are required to elucidate how different crops and soil types influence RNF efficiency, especially under marginal soils or stress conditions (drought, salinity).

7.2. Environmental Fate and Impact

Greenhouse gas emissions: There is a need for comprehensive, standardized measurement protocols to quantify GHG emissions (N₂O, CH₄) from diverse RNFs under realistic field conditions, including direct and indirect emission pathways.

Nutrient losses and pollution: A Better understanding of leaching, runoff, and volatilization processes from RNFs in varied climates and soils is critical to developing mitigation strategies.

Contaminants and risk assessment: Systematic evaluation of emerging contaminants—pharmaceutical residues, microplastics, heavy metals—in RNFs and their accumulation in soils, crops, and water bodies is urgently needed to assess long-term risks. Priority pollutants that warrant immediate attention include heavy metals (e.g., Cd, Pb, Hg), pharmaceutical and personal care products (e.g., antibiotics, hormones), and microplastics, as these have been most frequently detected in recycled nutrient streams and pose the greatest risks to soil and food safety.

Soil microbiome interactions: Research on how RNFs influence soil microbial communities, nutrient cycling processes, and soil health will help optimize their use for ecosystem services.

7.3. Technology Development and Optimization

Cost-effective treatment technologies: Innovations are needed to reduce the costs and complexity of nutrient recovery and stabilization technologies (e.g., urine treatment, struvite precipitation) to enable decentralized and scalable systems.

Formulation and delivery: Development of standardized, user-friendly RNF formulations (granules, pellets, liquid concentrates) tailored for different cropping systems will facilitate adoption.

Storage and handling: Improved methods for safe, odorless storage and transport of liquid RNFs, especially in warm climates and smallholder contexts, are needed.

Integration with precision agriculture: Combining RNFs with sensor-based nutrient management and variable rate application technologies can improve nutrient use efficiency and reduce environmental impacts.

7.4. Socio-Economic and Policy Research

Adoption barriers and incentives: Social science research is needed to understand farmer perceptions, behavioral drivers, and barriers to RNF use in different socio-cultural and economic contexts.

Market development: Studies on supply chains, pricing models, and certification systems for RNFs will support market integration and farmer trust.

Regulatory frameworks: Comparative policy analysis can identify best practices and gaps in existing regulations to inform adaptive governance of RNFs.

Life-cycle assessment and externalities: Holistic evaluations of RNFs considering energy use, emissions, economic costs, and social impacts will guide sustainable deployment strategies.

7.5. Knowledge Exchange and Capacity Building

Extension and education: Developing targeted extension programs, farmer training, and public awareness campaigns will be vital to increasing knowledge and acceptance of RNFs.

Interdisciplinary collaboration: Encouraging cross-sectoral collaboration between agronomists, engineers, economists, sociologists, and policymakers will accelerate innovation and problem-solving.

Data sharing and standardization: Establishing open-access databases and harmonized protocols for RNF characterization, performance, and impacts will improve comparability and knowledge synthesis. Due to the diversity of RNF types, scales of application, and regional contexts, comprehensive quantitative cost–benefit comparisons are currently limited. While this review synthesizes qualitative assessments and illustrative examples of costs, transportation, and infrastructure requirements, future studies employing modeling approaches or region-specific case analyses will be essential to provide robust quantitative evaluations. Also, future meta-analyses or experimental studies could strengthen quantitative assessments.

8. Conclusions

Recycled nitrogen fertilizers (RNFs) offer a promising pathway to enhance nutrient circularity, reduce environmental impacts, and support regenerative agriculture amid growing pressures on global nitrogen resources. This review synthesized current knowledge on diverse RNF types—including urine-derived fertilizers, compost, manure, digestate, biosolids, struvite, and mixed sources—highlighting their agronomic performance, environmental implications, economic feasibility, and socio-regulatory dimensions. RNFs provide clear benefits such as improved soil health, lower greenhouse gas emissions, and alignment with circular economy goals, but their large-scale adoption remains limited by challenges in cost, logistics, quality assurance, and public perception. Particularly, large initial investments are often required for infrastructure and technologies such as urine collection systems, anaerobic digesters, composting facilities, struvite precipitation units, storage tanks, and application equipment. Policy and financial support are especially needed in areas involving decentralized collection, nutrient recovery processing, and integration with existing farming systems to enable adoption at scale. Addressing these constraints through integrated research, technology innovation, and adaptive policy frameworks is essential to realize their potential. Future priorities include long-term field studies, development of cost-effective nutrient recovery systems, and stronger coordination among policymakers, researchers, and practitioners to ensure safe, trusted, and scalable implementation. Integrating RNFs into context-sensitive nutrient management strategies can accelerate the transition toward sustainable, climate-smart, and resilient food systems that advance both agricultural productivity and environmental stewardship.