Nitrogen Eutrophication in Chinese Aquatic Ecosystems: Drivers, Impacts, and Mitigation Strategies

Abstract

Nitrogen eutrophication represents a significant environmental challenge in Chinese aquatic ecosystems, exacerbated by rapid agricultural intensification, industrial expansion, and urban development. This review consolidates existing knowledge on the drivers and impacts of nitrogen pollution in Chinese aquatic ecosystems, with a focus on environments such as lakes, rivers, and coastal waters. The primary sources of nitrogen enrichment are excessive fertilizer application, livestock manure discharge, industrial emissions, and untreated industrial and municipal wastewater. These inputs have led to severe ecological consequences, including harmful algal blooms, hypoxia, loss of biodiversity, and deteriorating water quality, threatening ecosystem health and human well-being. The review also examines mitigation strategies implemented in China, encompassing regulatory policies such as the “Zero Growth” fertilizer initiative, as well as technological advancements in wastewater treatment and sustainable farming practices. Case studies highlighting successful interventions, such as lake restoration projects and integrated watershed management, demonstrate the potential for effective nitrogen control. However, persistent challenges remain, including uneven policy enforcement, insufficient public awareness, and gaps in scientific understanding of nitrogen cycling dynamics. This review aims to inform future efforts toward achieving sustainable nitrogen management in China by synthesizing current research and identifying key knowledge gaps. Addressing these issues is crucial for safeguarding China’s aquatic ecosystems and promoting global nutrient stewardship.

1. Introduction

Nitrogen is a vital element for life, playing a crucial role in agricultural productivity and ecosystem function. However, eutrophication caused by excess nitrogen is a significant environmental challenge with far-reaching consequences for aquatic ecosystems worldwide. Excessive nitrogen inputs from agricultural runoff, untreated wastewater, and industrial emissions have led to harmful algal blooms, oxygen depletion, and global biodiversity loss in freshwater and marine environments [1,2]. While this issue affects many regions, China is a hotspot due to its rapid industrialization, dense population, and intensive farming practices. This review focuses on understanding and mitigating nitrogen eutrophication in Chinese freshwater, coastal, and marine ecosystems, examining the causes, consequences, and sustainable solutions specific to the country while offering insights that are relevant to global nutrient stewardship efforts. To achieve this, this review synthesizes findings from the peer-reviewed literature, government reports, and case studies published between 2000 and 2025. Studies were selected based on their relevance to China’s nitrogen eutrophication mechanisms, impacts, and mitigation strategies.

While both nitrogen and phosphorus contribute to eutrophication, recent studies indicate that, in many Chinese aquatic systems, nitrogen inputs dominate due to intensive fertilizer use, livestock density, and industrial activity [2]. Moreover, nitrogen’s mobility through atmospheric deposition and its role in public health risks, such as nitrate contamination in drinking water, distinguish it from phosphorus in terms of urgency and impact [3]. However, the traditional view emphasizes phosphorus’ limitations in freshwater systems [4]. China’s nitrogen-to-phosphorus ratios increasingly reflect nitrogen saturation conditions [5]. These trends, along with substantial legacy nitrogen stores in soil and groundwater [6], warrants a focused synthesis on nitrogen eutrophication and its cascading effects.

In China, nitrogen-based fertilizers have been instrumental in sustaining food production to feed the world’s largest population of over 1.4 billion people. Since the Green Revolution of the mid-20th century, synthetic nitrogen fertilizers have enabled significant increases in crop yields, particularly for staple crops like rice, wheat, and maize [7]. This agricultural success has positioned China as one of the largest producers and consumers of nitrogen fertilizers globally, accounting for approximately 30% of global fertilizer use [8]. However, while nitrogen is indispensable for agriculture, it also presents a paradoxical challenge: its overabundance can lead to severe environmental degradation. In addition to these general factors, current research has started measuring the legacy of previous nitrogen inputs in soils and groundwater. For instance, even if new inputs are drastically reduced, ref [6] predicted that the mobilization of underground nitrogen stocks will continue to feed eutrophication for decades. The necessity of combining input-reduction tactics with actions that stop legacy releases (such as improved denitrification in riparian zones) is highlighted by this soil-groundwater feedback.

Table 1. Soil and Groundwater Nitrogen Legacy: Estimated Release Rates and Timeframes.

Region	Legacy N Store (kg N ha−1)	Annual Release Rate (%)	Estimated Persistence (Years)	Reference
North China Plain	120 kg N ha−1	5%	~20	[6]
Yangtze Basin	90 kg N ha−1	4%	~25	[9]
Bohai Coastal Plain	100 kg N ha−1	6%	~15	[10]

summarizes the legacy of previous nitrogen imports by regional reserves, annual release rates, and anticipated persistence.

The dual role of nitrogen as both a critical resource and a pervasive pollutant highlights the complexity of managing this element sustainably. The average concentrations of nitrogen and phosphorus in lake sediments have increased by 267% and 202%, respectively, since 1850. In the model projections for 2030–2100, nitrogen concentrations in the studied lakes in China may decrease, for example, by 87% in the southern districts and by 19% in the northern districts. However, the phosphorus concentrations will continue to increase by an average of 25% in the Eastern Plain, Yunnan–Guizhou Plateau, and Xinjiang [11]. On one hand, nitrogen supports plant growth, enhances soil fertility, and contributes to food security [12]. On the other hand, excessive nitrogen inputs disrupt ecosystems, leading to water pollution, biodiversity loss, and public health risks [13,14,15]. In China, the imbalance between nitrogen supply and demand has resulted in widespread environmental consequences, with nitrogen eutrophication emerging as a significant concern [2]. The country’s rapid industrialization, urbanization, and intensive farming practices have exacerbated nitrogen losses in the environment, underscoring the urgent need for effective management strategies.

Nitrogen eutrophication refers to enriching water bodies with excessive nitrogen compounds, primarily nitrates and ammonium, leading to ecological imbalances [16]. Nitrogen eutrophication affects freshwater systems, such as lakes and rivers, and marine environments, including estuaries and coastal zones, leading to ecological imbalances and water quality degradation. This process accelerates the growth of algae and aquatic plants, often culminating in harmful algal blooms (HABs), oxygen depletion (hypoxia), and the degradation of marine ecosystems [17,18,19,20]. In China, nitrogen-induced eutrophication is a widespread phenomenon that affects lakes, rivers, estuaries, and coastal zones [2,20,21,22]. For instance, Lake Taihu, one of China’s largest freshwater lakes, has experienced recurring cyanobacterial blooms due to high nitrogen and phosphorus inputs from surrounding agricultural and urban areas [23]. Significant resources have been directed toward pollution control to address such crises, including stricter regulations, wastewater upgrades, and community-led restoration projects. These efforts reflect broader challenges in balancing economic growth with environmental sustainability, as nitrogen pollution remains a persistent threat to water quality and public health.

Anthropogenic activities are the primary drivers of nitrogen eutrophication in China. Fertilizer overuse is a key contributor, as farmers often apply more nitrogen than crops can absorb, resulting in runoff into nearby water systems [16]. Livestock farming further compounds the issue, as animal waste releases large quantities of nitrogen into the environment [24]. Industrial emissions, particularly coal combustion and chemical manufacturing, also contribute to nitrogen deposition through atmospheric pathways [25,26]. Urbanization exacerbates the problem by increasing untreated wastewater discharge, which carries organic nitrogen and other pollutants into waterways. These factors and China’s unique geography and climate create conditions conducive to eutrophication, making it a pressing environmental issue. China stands out as a global hotspot for nitrogen pollution, reflecting its status as the world’s largest producer and consumer of nitrogen fertilizers. The sheer scale of nitrogen use in China and inefficient application practices result in disproportionate environmental impacts. According to recent studies, China accounts for nearly 20% of global reactive nitrogen emissions, far exceeding its share of the global population [9]. This imbalance underscores the urgency of addressing nitrogen pollution within the country.

The socio-economic and environmental stakes are immense. From a food security perspective, nitrogen fertilizers remain crucial for maintaining agricultural productivity. However, the environmental costs, including deteriorating water quality, declining biodiversity, and threats to human health, are increasingly untenable [6,27,28]. Water scarcity is another critical concern, as polluted water resources reduce the availability of clean water for drinking, irrigation, and industrial use. Public health risks arise from exposure to nitrate-contaminated drinking water, which has been linked to diseases such as methemoglobinemia (“blue baby syndrome”) and certain cancers [29]. Furthermore, nitrogen pollution undermines China’s efforts to achieve sustainable development goals, posing challenges to economic growth, social well-being, and environmental protection.

While nitrogen pollution is a global issue, China’s rapid industrialization, dense population, and intensive farming practices make its eutrophication crisis distinct. China’s mitigation strategies must address unique challenges such as smallholder-dominated agriculture and rapid urban sprawl. For example, the European Union’s focus on buffer zones and livestock density caps contrasts with China’s reliance on large-scale policy campaigns like the “Zero Growth” fertilizer initiative. These differences highlight the need for localized solutions that balance global best practices with China’s socio-economic realities. This review examines the causes, consequences, and mitigation strategies specific to China, offering insights for both local policymakers and global nutrient stewardship efforts.

This review aims to (1) examine the biogeochemical and anthropogenic mechanisms driving nitrogen enrichment in Chinese freshwater and marine ecosystems, (2) assess the impact of socio-economic and public health consequences of eutrophication, and (3) evaluate existing policy frameworks and community-led interventions aimed at mitigating nitrogen pollution. This paper seeks to provide evidence-based recommendations for sustainable nitrogen management in China by synthesizing current research and identifying critical knowledge gaps.

2. Study Selection Criteria and Bibliometric Analysis

The bibliometric search was primarily conducted in Web of Science (WoS) and Scopus because of their good metadata standards, comprehensive coverage of environmental science, and compatibility with bibliometric computer software such as VOSviewer 1.6.20. While Google Scholar and CNKI were considered, these platforms have limited export functionality and do not handle long Boolean strings the way Scopus or WoS do, making them unsuitable for large-scale bibliometric mapping. The integration promised strong methodology, replicability, and wide international and regional literature coverage (

Table 2. Search queries for nitrogen eutrophication research in China from WoS and Scopus.

Database	Search Query	Initial Records Retrieved	Final Records Included	Notes
Web of Science	TS = (“nitrogen eutrophication” OR “nitrogen pollution” OR “reactive nitrogen” OR “nitrogen loading”) AND TS = (“harmful algal blooms” OR “water quality” OR “nutrient pollution” OR “aquatic ecosystems” OR “lakes” OR “rivers” OR “coastal waters”)	1453	285	Limited to 2000–2025; China-focused research articles only
Scopus	TITLE-ABS-KEY(“nitrogen eutrophication” OR “nitrogen pollution” OR “reactive nitrogen” OR “nitrogen loading”) AND TITLE-ABS-KEY(“harmful algal blooms” OR “water quality” OR “nutrient pollution” OR “aquatic ecosystems” OR “lakes” OR “rivers” OR “coastal waters”)	2343	959	Limited to 2000–2025; China-focused research articles only

). Exclusion criteria were non-nitrogen subjects, non-China area, or lack of empirical information. Replicable studies with easily accessible datasets were given priority, minimizing bias and maximizing knowledge gaps within nitrogen dynamics and sustainable management.

Comparison of trends in publication and citation (Figure 1) from Scopus and WoS shows the rapid growth of nitrogen eutrophication research in Chinese freshwater environments over the past two decades. In WoS, the figure was relatively low until 2010, when papers began to grow incrementally to a high point of 46 papers in 2022. Citations in WoS show a stark increase after 2015, where more than 1100 citations were received in 2024 alone, signaling an increase in the popularity and influence of current studies. Scopus indicates a similar pattern but with more figures, particularly from 2010 onward. Publishing rose from fewer than 20 per year during the initial half of the 2010s to over 120 articles in 2024, and citations indicated an upward surge, reaching over 4000 in 2024. Both databases indicate a steep increasing trend of research output and impact, but Scopus consistently reported a higher amount of literature and higher citation rates than WoS. Together, these trends not only indicate growing policy and academic prominence for research on nitrogen eutrophication in China but also expanding international interest over the past several years.

Bibliometric characterization of Chinese nitrogen eutrophication studies shows consistent patterns in subjects, journals, authors, institutions, and funding agencies (Tables S1–S5). Environmental science dominates WoS and Scopus, with contributions from water resources, agriculture, and earth sciences, illustrating the multidisciplinary focus of the field. The top journal Science of the Total Environment is helped by high-production Chinese-language titles Huanjing Kexue and Acta Scientiae Circumstantiae. Regular authors are Chinese scientists Bu Hongmei and Zhang Quanfa, and foreign authors like Erik Jeppesen, which shows vigorous domestic–foreign cooperation. Chinese Academy of Sciences institutions top the ranks, with Beijing Normal University, Zhejiang University, and Hohai University coming in second. The National Natural Science Foundation of China funds most of the work, complemented by the National Key Research and Development Program. On the whole, the field is nationally based but with international links, with robust institutional and funding support behind its rapid climb.

VOSviewer-based co-occurrence analysis of author keywords was used to identify thematic structures in nitrogen eutrophication research (Figure 2 and Figure 3). The WoS dataset revealed clusters centered on ecological impacts, with prominent keywords such as nitrogen pollution, harmful algal blooms, eutrophication, and water quality, alongside methodological terms like stable isotopes and denitrification that reflect efforts to trace nitrogen sources and cycling. In contrast, the Scopus analysis highlighted a broader set of drivers and management responses, with frequent keywords related to agriculture (manure, fertilizer, and use) and applied solutions (policy, treatment, restoration). Together, these patterns show that WoS emphasizes ecological processes and environmental monitoring, whereas Scopus captures a more applied and socio-environmental orientation. This evolution signals a proactive shift in Chinese research toward addressing not just immediate ecological threats but also long-term sustainability challenges amid climate change and urbanization. However, the relative underrepresentation of socio-economic terms, such as “cost–benefit analysis” or “community engagement”, suggests a potential gap in translating scientific insights into equitable, actionable strategies, warranting greater emphasis on human-centered dimensions in future studies to enhance real-world impact.

The title and abstract term co-occurrence analysis in the WoS network revealed terms such as nitrate, denitrification, delta N/O isotopes, groundwater, and domestic sewage as hub nodes (Figure 4). These clusters represent nutrient sources, isotopic tracing methods, and biogeochemical processes that control nitrogen dynamics in aquatic systems. New keywords such as risk, human health, and Bayesian modeling point toward an interdisciplinary direction that concerns nitrogen pollution in relation to health and environmental risk assessment. Scopus analysis yielded a more connected and complete network with the following leading denitrification, surface water, water quality, fertilizer, and manure (Figure 5). Compared with WoS, Scopus highlighted higher connectivity with agricultural drivers, watershed management, and systems thinking. Terms such as process, effect, system, and treatment showed greater focus on ecosystem-scale procedures and remediation strategies. Together, the results suggest that WoS contains mechanistic and methodological research, whereas Scopus presents a wider view of agricultural pressures and management actions.

The WoS co-authorship network was dominated by Chinese authors such as Bu Hongmei, Zhang Quanfa, and Meng Wei, with clear domestic collaborations localized within institutions and across regional studies (Figure S1A). Scopus found more international relations with Erik Jeppesen (Denmark) and Lijun Hou (China) as the bridgers (Figure S1B). Institutional networks also showed the same trend (Figure S2). WoS emphasized intense intra-China collaborations with the Chinese Academy of Sciences leadership, while Scopus indicated Chinese university collaborations with international institutions such as Aarhus University and the University of Waterloo. Both of these trends suggest China’s central position in nitrogen research, with WoS reflecting national networks and Scopus showing more global cross-fertilization.

3. Nitrogen Pollution Pathways and Drivers in China

Nitrogen pollution in China is a multifaceted issue driven by the country’s rapid economic growth, intensive agricultural practices, and urbanization. One of the most significant contributors to this problem is agricultural runoff, which stems from the overuse of synthetic fertilizers and improper livestock waste management [30]. The key sources of nitrogen pollution and their impacts are summarized in

Table 3. Summary of significant nitrogen sources in China and their contributions to eutrophication.

Source	Description	Impact
Agricultural Runoff	Overuse of synthetic fertilizers and improper livestock waste management.	Eutrophication, algal blooms, oxygen depletion in aquatic ecosystems, and groundwater contamination.
Animal Manure Mismanagement	Excessive application of manure as fertilizer, particularly in rural areas.	Releases ammonia and organic nitrogen, exacerbating water pollution.
Urban Wastewater	Untreated or inadequately treated sewage from households and businesses.	Fuels eutrophication, contaminates rivers and lakes with nitrogen-rich organic matter.
Industrial Emissions	Release of nitrogen-containing pollutants from factories (e.g., coal combustion, chemical manufacturing).	Acid rain, smog, and atmospheric deposition are affecting soil and water.
Atmospheric Deposition	Nitrogen oxides (NOX) and ammonia (NH3) emissions settle onto land and water through precipitation.	Alters soil chemistry, harms biodiversity, causes soil acidification, and nutrient imbalances.

. China’s farmers rely heavily on nitrogen-based fertilizers to maintain high crop yields and feed its enormous population, which exceeds 1.4 billion people. However, applying these fertilizers often far exceeds what crops can absorb, with surplus nitrogen washing into nearby rivers, lakes, and groundwater during rainfall or irrigation [31]. This runoff contaminates water supplies and contributes to the eutrophication of aquatic ecosystems, leading to algal blooms and oxygen depletion [32]. Compounding the issue is the widespread use of animal manure as fertilizer, particularly in rural areas where chemical fertilizers are expensive or less accessible. While manure can be a valuable resource when appropriately managed, it is often applied excessively without regard for environmental consequences. Additionally, China has one of the highest livestock densities in the world, particularly in regions like the Yangtze River Delta and North China Plain, where pig and poultry farming are concentrated [33,34]. The sheer volume of animal waste generated in these areas is staggering, and much of it is improperly stored or directly discharged into waterways. This releases large quantities of nitrogen compounds, including ammonia and organic nitrogen, further exacerbating water pollution [35]. Given the same volume and water residence time, large and shallow lakes have the same nutrient retention rate as small and deep lakes, but have larger exposure to sediment and higher endogenous loading than small and deep lakes. The presence of a thermocline during summer in deep lakes prevents nutrient recycling from the hypolimnion to the epilimnion. In contrast, in shallow lakes, destratification by turbulence causes the coupling of organic matter decomposition and mineralization to nutrient recycling from the sediment to the overlying water and promotes phytoplankton proliferation [36]. Agricultural runoff and livestock waste threaten China’s water resources, underscoring the need for better farming practices and waste management systems.

Agriculture is the dominant source of nitrogen inputs to Chinese aquatic ecosystems, contributing approximately 60% of total nitrogen pollution. Nitrogen fertilizer consumption in China increased from 22.14 million tonnes in 2000 to a peak of 30.98 million tonnes in 2014, reflecting intensive agricultural practices during this period. Since 2015, consumption has declined to 24.55 million tonnes by 2023, driven by the “Zero Growth” policy aimed at curbing overuse (

Table 4. Nitrogen Fertilizer Use in China (2000–2025) [37].

Year Range	Annual Use (Million Tonnes)	Intensity (kg/ha)	Key Trend/Policy Impact	Reference
2000–2005	22.14–26.64	180–200	Rapid rise post-Green Revolution	FAOSTAT (2025) [37]
2006–2015	27.24–30.98	200–250	Peak due to intensification	FAOSTAT (2025) [37]
2016–2023	24.55–30.39	190–220	Decline via “Zero Growth”	FAOSTAT (2025) [37]
2024–2025	~24.30–24.10 (est.)	~180–200	Continued decline (sustainability focus)

) [37]. For 2024–2025, we project a continued decline to approximately 24.30–24.10 million tonnes, assuming a ~1% annual reduction based on ongoing sustainability efforts. China’s fertilizer intensity averaged 200–250 kg/ha, with hotspots like the North China Plain reaching ~300 kg/ha, significantly exceeding global averages (~100 kg/ha) [38]. Spatially, nitrogen fertilizer application is highly concentrated in the North China Plain, Yangtze River Delta, and Sichuan Basin [39] (Figure 6), regions that also exhibit the highest rates of nitrogen leaching and surface water pollution. These areas overlap significantly with major river basins and eutrophic lakes, highlighting the direct link between agricultural intensity and aquatic nitrogen loading.

Urbanization and industrial activity are equally significant drivers of nitrogen pollution in China, contributing through untreated wastewater discharge, industrial emissions, and atmospheric deposition. As cities expand and populations grow, the demand for clean water and proper sanitation increases, yet many urban areas struggle to meet infrastructure needs. Untreated or inadequately treated sewage from households and businesses often finds its way into rivers and lakes, carrying nitrogen-rich organic matter that fuels eutrophication. Urban wastewater and industrial discharges are significant contributors to coastal ecosystems’ nitrogen pollution. In Shenzhen Bay, rivers and sewage treatment plants were identified as primary sources of dissolved inorganic nitrogen (DIN), with pollution levels exceeding Class IV standards year-round [40]. This highlights the urgent need for improved wastewater treatment infrastructure and stricter regulations on nitrogen discharges. For example, aging sewage systems and insufficient treatment facilities have led to widespread water contamination in rapidly growing metropolitan areas like Beijing, Shanghai, and Guangzhou. A study by [41] found that China’s urban sewage treatment system faces significant challenges, with a 53% lacking rate of sewage pipe networks, a 34% external water infiltration rate, a 54% surplus rate of sewage treatment plants, and a 38% discharge rate of untreated sewage. The engineering efficiency of the system varied widely across provinces and cities, indicating issues like incomplete pipe networks, structural defects, and misconnections of stormwater pipelines. Industrial activities compound this problem, as factories release nitrogen-containing pollutants into the air and water.

Coal combustion, a cornerstone of China’s energy production, emits large quantities of nitrogen oxides (NOX), contributing to acid rain and smog while settling onto land and water bodies [42]. Similarly, ammonia (NH₃) emissions from chemical manufacturing, fertilizer production, and other industrial processes add to the nitrogen load in the environment [43]. These airborne pollutants eventually return to soil and water through precipitation or dry deposition, spreading nitrogen pollution across vast distances and affecting even remote areas [44]. This process, known as atmospheric deposition, is particularly concerning in regions downwind of industrial hubs, where nitrogen inputs can overwhelm local ecosystems. For example, in Lake Dianchi, external nitrogen inputs from riverine inflows and atmospheric deposition accounted for approximately 61% of total nitrogen fluxes during the study period [45]. For instance, studies have shown that forests in southern and eastern China receive some of the highest nitrogen deposition rates globally, with values often exceeding 30 kg N ha⁻¹ yr⁻¹, significantly altering soil chemistry and harming biodiversity [10,46]. This excessive nitrogen input has been linked to soil acidification, nutrient imbalances, and shifts in plant community composition, posing critical challenges for ecosystem sustainability. This highlights the pervasive nature of nitrogen pollution in China, emphasizing the urgent need for integrated solutions that address both point and non-point sources of contamination. Importantly, not all nitrogen from fertilizers follows surface-runoff channels. According to a recent study, up to 30% can volatilize as NH₃, redepositing locally or regionally [14]. Another percentage denitrifies in soils to N₂O or N₂. A comprehensive mitigation approach must include these gaseous losses in national nitrogen budgets. The relative fate of nitrogen from applied fertilizers in Chinese agroecosystems along the volatilization, denitrification, runoff, and absorption pathways is shown in

Table 5. Relative Fate of Applied Nitrogen Fertilizer in Chinese Agroecosystems.

Fate Pathway	Approx. Proportion of Applied N	Environmental Impact	Reference
Crop uptake	40–60%	Supports yield	[16]
Surface runoff	20–30%	Eutrophication of downstream waters	[2]
Ammonia volatilization	10–20%	Particulate Matter formation, regional deposition	[14]
Soil denitrification	5–15%	Emission of N2O (a potent greenhouse gas)	[10]

.

3.1. Effects on Water Quality

Nitrogen eutrophication in China is a complex environmental issue driven by the over-enrichment of water bodies with nitrogen compounds, leading to cascading ecological impacts. At its core, nutrient enrichment occurs when excessive nitrogen enters aquatic systems, stimulating the rapid growth of algae and other primary producers. This process begins with nitrogen-rich runoff from agricultural fields, livestock farms, untreated wastewater, and industrial emissions reaching rivers, lakes, and coastal zones. For example, in regions like the Yangtze River Delta, where agriculture and industry are concentrated, nitrogen flows from fertilized fields and factory discharges eventually reach critical water bodies such as Lake Taihu [23]. Once in these ecosystems, the surplus nitrogen is a fertilizer for aquatic plants and algae, triggering unchecked growth. This initial phase of nutrient enrichment sets the stage for more severe ecological consequences, including HABs and oxygen depletion [47]. The pathways of nitrogen pollution highlight how interconnected human activities are with natural systems, demonstrating that actions taken on land have far-reaching effects on water quality and ecosystem health.

HABs are one of China’s most visible and damaging outcomes of nitrogen eutrophication. These blooms occur when algae grow excessively due to the abundance of nutrients like nitrogen and phosphorus, creating thick mats on the surface of water bodies. In key regions such as Lake Taihu and the Yangtze River Delta, HABs have become a recurring problem, particularly during warmer months when conditions favor algal growth [48]. Lake Taihu, which supplies drinking water to millions of people in nearby cities, has experienced severe cyanobacterial blooms that render the water unsafe for consumption and disrupt local economies reliant on fishing and tourism. Similarly, the Yangtze River Delta, a densely populated and economically vital area, faces frequent HABs that degrade water quality and harm biodiversity. These blooms block sunlight from reaching underwater plants and release toxins that pose risks to aquatic life, wildlife, and humans. The prevalence of HABs underscores the urgent need to address nitrogen pollution at its source, as their frequency and intensity are directly linked to the continued input of nitrogen into these ecosystems.

Oxygen depletion is another critical consequence of nitrogen eutrophication, particularly in coastal areas like the Bohai Sea. When algal blooms die off, the decomposing organic matter consumes large amounts of dissolved oxygen in the water, creating hypoxic or “dead zones” where most marine life cannot survive [49]. The Bohai Sea, a semi-enclosed bay surrounded by heavily industrialized and agricultural regions, is especially vulnerable to this phenomenon. Excess nitrogen from upstream sources, including agricultural runoff and untreated wastewater, flows into the sea, fueling algal blooms that deplete oxygen levels [50,51]. These dead zones devastate fisheries, reduce biodiversity, and disrupt food webs, posing significant challenges to the environment and local communities that depend on marine resources. In the central Bohai Sea [51], agricultural nitrogen loading and rising temperatures were identified as primary drivers of deoxygenation. Nutrient imbalances and stratification promoted harmful algal blooms (HABs) and hypoxic conditions, with future projections suggesting continued risks of bottom water hypoxia in summer. The formation of dead zones highlights the interconnectedness of terrestrial and aquatic ecosystems, as nitrogen pollution originating on land can travel long distances before manifesting as oxygen depletion in coastal waters.

Positive feedback loops further exacerbate nitrogen eutrophication, creating self-reinforcing cycles that make recovery increasingly difficult. For instance, once a water body becomes enriched with nitrogen, the resulting algal blooms and oxygen depletion alter the ecosystem in ways that perpetuate the problem. Decomposing algae release additional nutrients into the water, fueling future blooms [47,52]. Additionally, oxygen-depleted environments favor certain bacteria that convert nitrogen compounds into forms that are even more readily available to algae, accelerating the cycle. Internal nutrient cycling, such as sediment phosphorus release driven by seasonal nitrogen limitation, can further amplify HABs and prolong their duration [53]. In Lake Dianchi, internal positive feedback mechanisms were found to promote water quality improvement during recovery phases, highlighting the complex interplay between external and internal nitrogen fluxes [45]. In Lake Chaohu, for example, sediment at the bottom of the lake stores accumulated nitrogen and phosphorus, which can be re-released into the water column under certain conditions, prolonging eutrophication [54]. These feedback mechanisms make it harder to restore affected ecosystems, as simply reducing nitrogen inputs may not immediately reverse the damage. Understanding these dynamics is crucial for designing effective interventions that break the cycle of eutrophication.

The pathways through which nitrogen moves from its sources to key water bodies illustrate the complexity of the issue and the importance of addressing multiple contributors simultaneously (Figure 7). Agricultural activities, such as excessive fertilizer use and improper manure management, are significant sources of nitrogen pollution in inland water bodies like Lake Taihu. Runoff from these activities carries nitrogen into tributaries that feed into larger lakes and rivers, amplifying the impact over time. Similarly, untreated wastewater from rapidly growing urban centers introduces organic nitrogen and ammonia into aquatic systems, contributing to nutrient enrichment. Industrial emissions add another layer of complexity, as nitrogen oxides and ammonia released into the atmosphere settle onto land and water through atmospheric deposition, affecting even remote areas. Nitrogen pollution originates from upstream agricultural runoff, urban wastewater discharge, and industrial activities along the coastline in coastal regions like the Bohai Sea. Based on nitrate δ15N (6.1–8.9 ‰) and δ18O (0–5.7 ‰) values, the primary nitrogen sources in Xiangshan Bay were identified as domestic sewage and agricultural runoff, influenced by seasonal agricultural activities and monsoon-driven Changjiang Diluted Water (CDW) dynamics. Nitrate levels peaked in autumn (53.1 ± 1.6 μmol/L), due to intensive agricultural activity and substantial CDW intrusion driven by the northeast monsoon [55].

Tracing these pathways makes it clear that nitrogen eutrophication is not an isolated issue but a systemic challenge requiring coordinated efforts across sectors and regions. Addressing this problem will demand targeted strategies to reduce nitrogen inputs at their source while also mitigating the impacts already felt in vulnerable ecosystems.

Nitrogen eutrophication has profoundly altered China’s ecosystems, leaving lasting scars on aquatic and terrestrial environments. The most visible impacts are in freshwater lakes and coastal zones, where excessive nitrogen pollution disrupts the natural balance and leads to severe ecological consequences. HABs, oxygen depletion, and fish kills have become common phenomena, particularly in regions like Lake Taihu, Lake Poyang, Dianchi Lake, the Bohai Sea, and the Yangtze River Delta [20,56,57,58]. These areas, critical for drinking water, fishing, and biodiversity, are now under immense pressure due to human activities that release large amounts of nitrogen into the environment. Beyond aquatic systems, nitrogen pollution also affects terrestrial ecosystems by causing soil acidification, reducing plant diversity, and allowing for invasive species to take over. Together, these changes threaten native wildlife, degrade habitats, and compromise water quality, creating challenges for nature and people.

3.2. Impact of Harmful Algal Blooms

HABs caused by excess nitrogen have become a recurring pollutant issue in China’s freshwater bodies, especially in Chinese lakes and rivers [59,60,61]. These blooms occur when excessive nitrogen and other nutrients enter the water, fueling the rapid growth of algae. The process begins with nutrient enrichment, often from agricultural runoff laden with nitrogen-based fertilizers, untreated wastewater, and industrial discharges. Rain or irrigation washes these nutrients into nearby streams, rivers, and eventually larger water bodies, creating an environment conducive to algal proliferation [62]. The results indicate that Chla is a key indicator of water eutrophication, with both nutrients being related to its trophic state. Furthermore, TP plays a more significant role than TN, particularly in hypereutrophic Chla conditions. With the increase in TN/TP, Chla concentration is decreased, which may be the co-limitation of TN and TP. These three values essentially explained the increment of eutrophication Chla, although the relative effects differed across nutrients, with changes in the TN total contributing 5.2% of the increment in Chla, while the TP change contributed 272.0% of the increment in Chla. At spatial scales, logarithm-transformed response ratio (lnRR) of Chla indicated that the lake shifts from N limitation (64.0% > 28.0%) in oligo-mesotrophic to P limitation (170.0% > 99.0%) in eutrophic and co-limitation (66.0% and 46.0%) by N & P in hypereutrophic [63]. Under normal conditions, nutrients support balanced aquatic ecosystems, but when nitrogen concentrations exceed natural thresholds, algae grow uncontrollably, forming dense blooms that dominate the ecosystem.

In China’s largest freshwater lakes, the proliferation of algal blooms has resulted in significant ecological and environmental challenges. These blooms, characterized by excessive cyanobacteria and other phytoplankton growth, have rendered the water visibly green and viscous, severely compromising its quality. The presence of such blooms not only poses risks to aquatic ecosystems but also renders the water unsuitable for both human consumption and recreational activities. Local communities rely heavily on this lake for their water supply, but treatment plants struggle to remove toxins and unpleasant odors from the water during bloom outbreaks. Similarly, Dianchi Lake in southwestern China has faced decades of pollution-driven degradation [64]. Once a pristine body of water, it is now plagued by dense algal mats that block sunlight, suffocate aquatic plants, and harm fish populations. These blooms damage the ecosystem and hurt local economies that depend on tourism and fishing. The decomposition of algal blooms further exacerbates the problem. When algae die, they sink to the bottom of the water body and are broken down by bacteria. This decomposition process consumes vast amounts of dissolved oxygen, creating hypoxic or anoxic conditions. In extreme cases, these oxygen-depleted zones become “dead zones,” where most aquatic life cannot survive. Oxygen depletion has a domino effect on the ecosystem, causing fish and other organisms to perish or migrate to areas with better conditions. Even species tolerant of low-oxygen environments may struggle if the dead zone expands or persists over long periods. The situation is equally dire in coastal zones such as the Bohai Sea and Yellow Sea, where oxygen depletion and fish kills have devastated marine life [65].

Excess nitrogen from agricultural runoff, untreated wastewater, and industrial discharges also fuels algal blooms in marine environments. Similarly, nutrient-rich inputs in coastal zones such as the Bohai Sea lead to harmful algal blooms (HABs) that deplete oxygen levels, creating “dead zones” where most marine life cannot survive [51]. These phenomena highlight the interconnectedness of terrestrial and aquatic ecosystems, as nitrogen pollution originating inland ultimately affects downstream waters. The decomposition of algal blooms further exacerbates the problem, consuming dissolved oxygen and causing cascading ecological impacts across marine habitats. For example, nutrient-laden runoff from inland regions flows into estuaries and seas, which fuels marine algal blooms [55]. Coastal dead zones, such as those found in the Bohai Sea and Yellow Sea, illustrate how terrestrial pollution manifests in oceanic environments. Here, oxygen depletion leads to mass die-offs of marine organisms, devastating fisheries, and coral reefs that are crucial for biodiversity [66]. As the dead algae break down, they consume oxygen in the water, creating “dead zones” where most marine organisms cannot survive (Figure 8). The persistence of algal blooms underscores the complex feedback loops created by nitrogen enrichment. High nutrient levels promote rapid algal growth, leading to oxygen depletion and habitat degradation. As these conditions worsen, the ecosystem becomes less capable of recovering, even if nutrient inputs are reduced. This cycle of degradation demonstrates how nitrogen-driven eutrophication fundamentally alters the structure and function of aquatic ecosystems, making them more vulnerable to further disturbances. Understanding the relationship between nitrogen pollution and harmful algal blooms is essential for addressing their ecological impacts. By focusing on the mechanisms through which nitrogen fuels eutrophication, it becomes clear that controlling nutrient inputs is critical to mitigating the proliferation of HABs. Strategies to reduce nitrogen runoff from agriculture, improve wastewater treatment, and restore degraded ecosystems can help break the bloom formation and decomposition cycle. Such efforts are necessary to protect the health and stability of aquatic environments threatened by nitrogen-driven ecological imbalances.

4. Socio-Economic Consequences and Human Health Risks of Nitrogen Eutrophication in China

Nitrogen eutrophication has become a pressing environmental issue in China, driven by human activities such as intensive agriculture, industrial emissions, and urbanization. Addressing this problem requires a multifaceted approach that combines changes in agricultural practices, improvements in wastewater management, effective policies, innovative technologies, and community involvement. By tackling nitrogen pollution at its source and mitigating its impacts, China can work toward restoring its ecosystems and safeguarding water resources for future generations.

4.1. Agricultural Practices: Reducing Nitrogen Losses from Farms

Agriculture is one of China’s significant contributors to nitrogen pollution, primarily through excessive fertilizer use and inefficient farming practices [50,68]. To address this, precision agriculture offers a promising solution by helping farmers apply fertilizers more accurately and efficiently. Precision agriculture uses tools like soil testing, GPS mapping, and sensors to determine how much fertilizer is needed for each part of the field [69]. This ensures that crops receive only the nutrients they need, reducing the amount of excess nitrogen that can wash into nearby rivers and lakes. For example, in regions like the North China Plain, where over-fertilization is common, precision agriculture has been shown to cut nitrogen losses significantly while maintaining or even improving crop yields. Thus, optimizing irrigation practices is another critical step in reducing nitrogen runoff. Excessive or poorly timed irrigation exacerbates nutrient leaching, as water carries unused fertilizers from fields into adjacent water bodies. By adopting advanced irrigation techniques such as drip irrigation or sprinkler systems, farmers can minimize water wastage and ensure that fertilizers remain within the root zones of crops [2]. These systems deliver water directly to plant roots in controlled amounts, reducing the likelihood of nutrient runoff. Studies conducted in China have demonstrated that switching from flood irrigation to drip irrigation conserves water and reduces nitrogen losses [70,71,72]. Such innovations highlight the potential for technology-driven solutions to mitigate agricultural pollution effectively.

In addition to technological advancements, integrating crop rotation and cover cropping into farming systems can further reduce nitrogen losses. Crop rotation involves alternating different types of crops in the same field across growing seasons, which helps maintain soil fertility and prevents nutrient buildup. Leguminous crops, such as soybeans and clover, are particularly effective because they fix atmospheric nitrogen in the soil, reducing the need for synthetic fertilizers [73]. Cover crops, planted during off-seasons, act as a protective layer over the soil, preventing erosion and capturing residual nitrogen that might otherwise leach into groundwater or surface water. Farmers have successfully implemented these practices in provinces like Heilongjiang, where large-scale grain production dominates, to improve soil health while cutting nitrogen runoff [74].

Few studies have measured the detrimental effects of algal blooms in Lake Taihu on public health, even though their existence is widely known. According to recent risk evaluations, the incidence of liver cancer in neighboring cities may increase by 4–10% because of long-term exposure to microcystin in drinking water [17,61]. By comparing interventions to these health-risk estimates, policymakers can prioritize those that prevent the most serious illnesses and premature deaths, therefore reducing the overall disease burden. Moreover, establishing vegetative buffer zones along field edges is another practical measure to intercept nitrogen before it enters waterways. Buffer strips consist of grasses, shrubs, or trees planted between agricultural fields and water bodies. These plants absorb excess nutrients from runoff, acting as a natural filter. Research has shown that well-maintained buffer zones can reduce massive nitrogen loads in nearby streams [75]. Governments can enhance their efforts to combat eutrophication and preserve aquatic biodiversity by promoting policies that incentivize the adoption of buffer zones.

4.2. Wastewater Management: Cleaning Up Urban and Industrial Pollution

Untreated or poorly treated wastewater is another major source of nitrogen pollution in China, particularly in rapidly growing cities and industrial zones. To combat this, upgrading wastewater treatment plants to include denitrification processes is essential [76]. Denitrification removes nitrogen compounds from wastewater by converting them into harmless gases that can be released into the air. While many treatment plants in China already use basic filtration systems, adding advanced denitrification technology can significantly reduce nitrogen discharges into rivers and lakes. In addition to harming ecological and human health, eutrophication degrades ecosystem services; since 2010, Lake Poyang’s fisheries yields have decreased by 25%, costing local people an estimated USD 50 million yearly [4]. Multi-sectoral investment in nutrient reduction can be stimulated by valuing these losses economically.

Recycling nitrogen-rich sludge from wastewater treatment plants is another innovative solution. Instead of disposing of this sludge as waste, it can be processed and reused as fertilizer for agriculture. This approach reduces nitrogen pollution and provides farmers with a sustainable alternative to synthetic fertilizers. In the Yangtze River Delta, pilot projects have demonstrated the potential of recycling sludge to support crop production while minimizing environmental harm [77]. By scaling up these efforts, China can turn a significant pollutant into a valuable resource, creating a circular economy that benefits farmers and ecosystems. A critical step is improving household wastewater management systems, especially in densely populated urban areas. Many households in China still rely on outdated septic tanks or improperly connected drainage systems, which allow for untreated wastewater to seep into groundwater or flow directly into nearby waterways [78]. Replacing these systems with modern, efficient alternatives can drastically reduce nitrogen leakage. These units use biological processes to break down nitrogen compounds before releasing treated water into municipal systems [76]. In rural and peri-urban regions, source-separated sanitation systems offer a promising avenue for reducing nitrogen discharges. These systems, including urine-diverting dry toilets and composting latrines, enable the recovery of nitrogen from human excreta for agricultural reuse, thereby closing nutrient loops [79,80]. Pilot programs in the Yangtze River Basin have demonstrated that such ecological sanitation systems can cut nitrogen pollution by over 30% while enhancing soil fertility [68]. However, social acceptability, infrastructure investment, and maintenance remain key barriers to scaling adoption in rural China. Policy support and targeted education campaigns will be essential to promote uptake and ensure health safeguards.

Industrial facilities also need to adopt stricter measures to prevent nitrogen discharges. Factories producing chemicals, textiles, and food products often release high concentrations of nitrogen-rich effluents into rivers and lakes [81,82]. Installing advanced treatment systems, such as membrane bioreactors or reverse osmosis units, can help industries meet stricter discharge standards. These technologies not only remove nitrogen compounds but also reduce overall water consumption by enabling treated water to be reused within industrial processes. Beyond individual factories, improving industrial wastewater infrastructure at a broader scale is essential. Many industrial zones lack centralized treatment facilities capable of handling nitrogen pollution effectively. Developing shared wastewater treatment plants equipped with denitrification capabilities could address this gap, ensuring that all factories within a zone comply with environmental standards. Such infrastructure investments would create a more systematic approach to managing industrial discharges and reducing their cumulative impact on water bodies [83]. Stormwater management also plays a critical role in mitigating industrial nitrogen pollution. During heavy rainfall, stormwater runoff from industrial sites often carries nitrogen-rich sediments and chemicals into nearby rivers and lakes. Implementing green infrastructure solutions, such as permeable pavements, rain gardens, and retention ponds, can help filter out nitrogen-rich sediments and protect aquatic ecosystems [84]. These methods mimic natural processes, allowing for nitrogen to be absorbed by plants or broken down by microorganisms, thereby minimizing its environmental impact.

Finally, real-time monitoring systems can play a vital role in managing wastewater pollution. Installing sensors at key points in wastewater treatment plants and along river networks allows for authorities to track nitrogen levels continuously. This data can be used to identify pollution hotspots, predict algal blooms, and optimize treatment processes. For instance, remote sensing technologies combined with machine learning algorithms have been successfully used in several parts of the world to monitor water quality and guide intervention strategies [85]. Adopting similar tools in multiple cities in China could enhance the effectiveness of nitrogen management efforts.

5. Case Studies: Lessons from Lake Dianchi and Shenzhen Bay

Lake Dianchi in Yunnan Province and Shenzhen Bay in Guangdong Province offer valuable lessons for managing nitrogen pollution in Chinese aquatic ecosystems. Lake Dianchi, one of China’s most polluted lakes, has faced severe water quality issues for decades due to excessive nitrogen inputs. Between 2002 and 2018, total nitrogen (TN) concentrations in Lake Dianchi showed significant improvements, dropping from over 2.0 mg L⁻¹ (Grade V by China’s water quality standards) to below 1.5 mg L⁻¹ (Grade IV) in recent years [45]. This reduction was primarily driven by mitigation measures such as wastewater treatment plants (WWTPs), which reduced nitrogen loadings by approximately 13,775 tons per year in 2018, up from 3140 tons per year in 2005. Transboundary water transfer projects and pollution interception measures significantly reduced nitrogen inflows. Despite these achievements, legacy nitrogen stored in lake sediments continues to drive internal nutrient cycling, with sediment-water exchanges accounting for approximately 39% of total nitrogen fluxes during the study period [45].

Shenzhen Bay, a coastal estuary in southern China, provides a complementary example of how rapid urbanization and industrialization can degrade water quality. Over the past 40 years, nitrogen and phosphorus pollution in Shenzhen Bay increased significantly, with seasonal variations showing higher nitrogen concentrations during the wet season due to urban runoff and riverine inflows [40]. Mitigation efforts, including stricter wastewater treatment standards and mangrove restoration, have led to measurable reductions in nitrogen levels. For instance, nitrogen concentrations decreased by approximately 25% between 2010 and 2020, with peak reductions observed during the dry season when tidal mixing was less pronounced. However, unforeseen nitrate accumulation occurred in some areas, highlighting the complexity of nutrient cycling in estuarine environments. Advanced monitoring tools, such as UV–visible spectrophotometry, revealed that nitrogen removal rates varied seasonally, with denitrification rates ranging from 0.5 to 2.0 g N m⁻² month⁻¹ depending on temperature and oxygen availability [40].

Together, these case studies reveal shared challenges and unique insights into nitrogen pollution dynamics. Human activities heavily influence Lake Dianchi and Shenzhen Bay, but their responses to mitigation efforts differ due to geographic and hydrological factors. For instance, Lake Dianchi’s shallow depth and limited water exchange make it particularly vulnerable to pollution, while Shenzhen Bay’s tidal mixing offers some natural resilience. Despite these differences, both ecosystems highlight the importance of reducing external nitrogen inputs, addressing legacy pollution, and adopting advanced monitoring tools to guide decision-making. Quantitative analysis shows that WWTPs and pollution interception measures reduced nitrogen loadings by over 50% in Lake Dianchi, while stricter regulations and mangrove restoration in Shenzhen Bay contributed to a 25% reduction in nitrogen concentrations over a decade. By integrating lessons from these case studies, policymakers can develop more effective strategies to restore water quality and protect aquatic ecosystems across China.

6. Strengthening Policies and Community Engagement to Address Nitrogen Pollution

Government policies are pivotal in addressing nitrogen pollution, particularly in mitigating its adverse effects on water bodies. In recent years, China has introduced several landmark initiatives to curb excessive nitrogen use and improve environmental quality. One such initiative is the Zero Growth Action on Fertilizer for chemical fertilizers, launched in 2015, which seeks to halt the rise in fertilizer consumption by promoting efficient application methods and sustainable agricultural practices (Figure 9). This policy has already shown promising results, with several provinces reporting reductions in fertilizer use without negatively impacting food production [86]. The success of this initiative highlights the importance of setting clear targets and providing incentives to encourage farmers to transition away from over-reliance on synthetic fertilizers. The quantitative results, enforcement issues, and important takeaways from China’s flagship nitrogen-management programs and similar foreign directives are compiled in

Table 6. Summary of Key Nitrogen Management Policies: Outcomes, Enforcement Challenges, and Lessons Learned.

Policy/Case Study	Outcomes & Metrics	Challenges	Lessons Learned	References
Zero Growth Fertilizer Initiative (2015-)	Provinces such as Heilongjiang and Shandong achieved 5–10% reductions in total fertilizer applied with no yield loss.	Limited uptake by smallholders; uneven enforcement in remote counties.	Clear targets + financial incentives drive adoption-but extension support must reach smallholders.	[86,88]
“Water Ten”National Action Plan (2015-)	15% average reduction in nitrogen + phosphorus loads in key river basins by 2020; >80% of major WWTPs upgraded.	Persistent gaps in rural sewage networks; local governments under-budgeted for long-term monitoring.	Multi-sector coordination (agriculture, industry, urban) succeeds with both penalties and subsidies, but rural roll-out lags.	[41,88]
Lake Taihu Restoration (Integrated Watershed Mgmt.)	Peak-season cyanobacterial blooms down ~30% (2015–2022); dissolved O2 recoveries of ~1 mg/L in summer.	Upstream non-point agricultural runoff remained high, conflicting mandates among the five prefecture governments.	Combining dredging with buffer-zone reforestation yields better results than either alone.	[23,52,89]
U.S. Clean Water Act (CWA)—Chesapeake Bay TMDL Program	By 2022, nitrogen loads had decreased by approximately 25% compared to pre-TMDL levels; improvements in water clarity and aquatic habitat recovery were documented in tributaries like the Patuxent River and James River.	Coordination challenges across six states and D.C.; funding disparities and political will vary by jurisdiction.	Science-based regulatory enforcement combined with multi-jurisdictional cooperation, adaptive management, and public participation is effective; binding pollution caps ensure accountability.	[90,91]
India: Soil Health Card Scheme (SHCS) under NMSA/SM-SHF	Contributed improve nitrogen use efficiency and reduced ammonia volatilization; improved crop yields and reduced input costs for farmers in Punjab, Haryana, and Andhra Pradesh.	Disparities in implementation across states are due to differences in extension capacity, infrastructure, and farmer awareness.	Data-driven decision support tools (like SHCS and NCU) can be scaled effectively in smallholder-dominated systems; tailored nutrient recommendations improve efficiency and sustainability.	[92,93,94]
EU Nitrates Directive (1991)—Denmark & Poland	Denmark initially met the 50 mg NO3/L standard by 2000, but later struggled; Poland remains non-compliant in many zones.	Denmark’s extremely high livestock density; Poland’s low social capital and financial constraints.	Buffer strips and livestock density caps can work, but require strong governance capacity and social buy-in.	[95,96]

.

Complementing this effort is the National Action Plan for Water Pollution Control, also known as the “Water Ten Plan” which was introduced in 2015 and focused on improving water quality across the country. This plan establishes specific goals for reducing nitrogen and phosphorus levels in critical water bodies [88]. It enforces stricter regulations on industrial discharges and agricultural runoff, holding polluters accountable for their contributions to water degradation. By coordinating national, provincial, and local actions, the plan adopts a complete approach to tackling nitrogen pollution, ensuring that all sectors, such as agriculture, industry, and urban development, contribute to cleaner waterways. Enforcement mechanisms, including fines for violations and subsidies for adopting environmentally friendly practices, further support these policies. Halving nitrogen pollution is crucial for achieving SDGs. The reactive nitrogen (Nr) pollution could be roughly halved by managed urban development in China by 2050, with NH₃, NOx, and N₂O atmospheric emissions declining by 44%, 30% and 33%, respectively, and Nr to water bodies by 53%. An investment of approximately US$61 billion in waste treatment, land consolidation, and livestock relocation yields an overall benefit of US$245 billion [9].

However, challenges persist, particularly in rural areas where limited resources and enforcement capacity hinder progress [97]. Strengthening governance and allocating adequate resources will be essential to achieving the desired outcomes. On Heilongjiang pilot farms, decision-support technologies that adjust fertilizer rates based on site-specific soil tests have reduced nitrogen application by 15–20% without affecting yield [70]. Farmer training and subsidized soil-testing services will be necessary to scale such precision-ag methods.

Although emerging membrane bioreactor (MBR) systems can remove 85–95% of the total nitrogen from industrial effluent, they come with a yearly operating and maintenance cost of approximately USD 100,000 and a capital investment of about USD 2 million per million liters per day [76]. With capital expenses of about USD 1 million per Million Liters Per Day (MLD) and operating and maintenance costs of about USD 80,000 annually, anammox processes, which directly convert ammonia into nitrogen gas, achieve removal efficiencies of 60–80% [77]. At roughly USD 200,000 per MLD in construction expenditure and USD 20,000 per MLD yearly for operation and maintenance, constructed wetlands deliver more modest removal, typically 40–60%, but at far cheaper costs [32]. Therefore, public–private financing methods may be essential to facilitate the broader use of intermediate-cost and high-performance nitrogen removal technology throughout China’s industrial zones.

Globally, other countries have adopted diverse strategies to address nitrogen pollution, offering valuable lessons for China. In Europe, the European Union’s Nitrates Directive has been instrumental in regulating nitrate levels in water bodies since 1991 [98]. This directive designates “nitrate vulnerable zones” where agricultural practices are closely monitored and regulated. Farmers in these zones must adhere to guidelines such as reducing fertilizer application rates and implementing buffer strips along waterways to prevent runoff. The implementation of these measures varies significantly across Member States. Research observations from [95] have shown that Denmark has historically pioneered by setting ambitious targets and implementing comprehensive aquatic environmental plans even before the directive was established. However, recent years have shown a shift as Denmark struggles with meeting Water Framework Directive requirements due to high livestock density and governance challenges. Poland faces difficulties primarily due to internal pressures such as low social capital, financial constraints, and fragmented farming systems, which hinder effective compliance. The Polish government’s reluctance to fully designate its territory as a nitrate-vulnerable zone highlights the ongoing challenges in aligning domestic interests with EU environmental goals.

In Italy, studies have highlighted the impact of derogations from the Nitrates Directive, allowing for certain regions like Lombardy to apply higher levels of nitrogen fertilizers. The reintroduction of stricter limits on nitrogen use showed immediate effects, reducing nitrate loads entering rivers from alluvial aquifers, demonstrating that regulatory adjustments can swiftly mitigate contamination when effectively enforced [96]. This situation underscores the potential benefits and complications arising from temporary relaxations of stringent environmental policies. These variations in implementation performance among EU countries illustrate the complexity of addressing nitrate pollution through a universal approach.

The U.S. Clean Water Act (CWA), enacted (rewritten) in 1972, remains one of the most comprehensive global legislative frameworks for water quality protection [99]. Its overarching goal is to “restore and maintain the chemical, physical, and biological integrity of the Nation’s waters”. Under the CWA, the Environmental Protection Agency (EPA) regulates discharges of pollutants into U.S. waters and sets water quality standards for all contaminants in surface waters. One of the most significant mechanisms under the CWA is the Total Maximum Daily Load (TMDL) program, which establishes limits on nutrient inputs—particularly nitrogen and phosphorus—for impaired water bodies. A TMDL specifies the maximum amount of a pollutant that a water body can receive while still meeting water quality standards and allocates this load among different sources, including point and non-point sources. Implemented in 2010, the Chesapeake Bay TMDL is one of the largest and most complex ever developed in the U.S. It covers six states (Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia) and the District of Columbia [91]. The program sets binding pollution caps for nitrogen (185.9 million pounds/year), phosphorus (12.5 million pounds/year), and sediment (6.1 billion pounds/year) across the entire watershed [84]. Reduction targets were phased in over time, with milestones set every two years to ensure progress toward full implementation by 2025. By 2018, total nitrogen concentrations in Chesapeake Bay had decreased by an average of 25% compared to the mid-1980s across all monitored stations, with notable improvements observed in tributaries such as the Patuxent River and James River [90]. The success of the Chesapeake Bay Program demonstrates the effectiveness of science-based regulatory enforcement combined with multi-jurisdictional cooperation, adaptive management, and public participation. In addition to regulatory tools, the CWA emphasizes non-point source pollution control through voluntary farmer programs and financial incentives. The Environmental Quality Incentives Program (EQIP), administered by the USDA, provides technical and financial assistance to agricultural producers to implement conservation practices such as cover crops, buffer strips, and nutrient management plans. EQIP has allocated over $2 billion annually since 2020 to support sustainable agriculture, with a growing share directed toward nutrient reduction strategies [100]. The U.S. experience highlights the importance of long-term monitoring, adaptive management, and public participation—elements that remain underdeveloped in many Chinese river basins. Additionally, the CWA emphasizes non-point source pollution control through voluntary farmer programs and financial incentives, such as the USDA’s Environmental Quality Incentives Program (EQIP). These mechanisms may offer applicable models for China, where smallholder-dominated farming systems require tailored engagement strategies rather than top-down mandates.

India’s agrarian policies have increasingly focused on reducing nitrogen (N) pollution, driven by the dual goals of ensuring food security and mitigating environmental degradation caused by excessive use of nitrogen fertilizers. Overuse has led to significant losses of reactive nitrogen, causing air and water pollution, soil degradation, and contributing to climate change through nitrous oxide emissions. Several policy interventions and scientific strategies have been proposed and implemented to address these challenges [93].

One key policy mechanism is the Nutrient-Based Subsidy policy, introduced in 2010, which aims to promote balanced use of fertilizers—particularly N, phosphorus, and potassium- by subsidizing based on nutrient content rather than the product itself. This was intended to reduce over-reliance on nitrogenous fertilizers. However, its effectiveness has been limited due to continued imbalanced fertilizer use, especially in cereal-dominated systems like rice and wheat [101].

The Soil Health Card Scheme, launched in 2015, provides farmers with information on soil nutrient levels and recommends site-specific nutrient applications. This initiative supports better nitrogen management by identifying deficiencies such as those in nitrogen, zinc, and boron, and guiding optimal fertilizer use. While this scheme improves awareness, adoption remains uneven across regions [93]. Another notable effort is the promotion of Neem-Coated Urea (NCU), a government mandate since 2015, which acts as a natural nitrification inhibitor, slowing the conversion of ammonium to nitrate and thereby reducing leaching and gaseous losses. Studies suggest that combining neem coating with urease inhibitors could improve nitrogen use efficiency and reduce ammonia volatilization [94]. Furthermore, the National Mission for Sustainable Agriculture, under India’s Climate-Smart Agriculture framework, promotes integrated nutrient management and conservation practices, such as crop diversification and legume integration. Rotating cereals with legumes enhances biological nitrogen fixation, thereby reducing dependency on synthetic fertilizers [92]. Despite these measures, data from FAO and the Fertiliser Association of India indicate that India’s nitrogen surplus per hectare and associated nitrous oxide emissions have risen steadily since the Green Revolution. In 2021, fertilizer N application exceeded 9 million metric tons, with NUE remaining below 30%, underscoring the need to strengthen policy implementation and farmer adoption [93]. To meet the Sustainable Development Goals (SDGs), particularly SDG 13 (climate action), SDG 2 (zero hunger), and SDG 6 (clean water), India must integrate cross-sectoral policies that simultaneously enhance productivity and reduce its nitrogen footprint. Strengthening monitoring mechanisms, incentivizing efficient technologies, and fostering international collaboration are essential steps toward achieving this balance.

Local governments and non-governmental organizations (NGOs) also play a vital role in fostering stakeholder collaboration. Watershed management committees, for example, bring together farmers, businesses, and community leaders to develop shared strategies for protecting water resources [102]. These partnerships emphasize collective responsibility and long-term planning, ensuring that everyone contributes to solving the nitrogen pollution problem. China is taking meaningful steps toward mitigating nitrogen pollution by combining strong policies with technological innovations, improved agricultural practices, better wastewater management, and active community engagement [23]. Each strategy addresses a different aspect of the issue, working together to create a comprehensive framework for reducing nitrogen pollution and restoring ecological balance.

7. Future Considerations

Addressing nitrogen eutrophication in China requires a multifaceted approach integrating advanced scientific tools, interdisciplinary collaboration, and innovative policy frameworks. To bridge the gaps identified in this review, future research should focus on several key areas:

Real-time monitoring of nitrogen pollution is essential for identifying hotspots, tracking trends, and prioritizing interventions. Remote sensing technologies, such as hyperspectral imaging and satellite-based systems, can provide spatially explicit data on nitrogen concentrations in water bodies. Machine learning algorithms trained on historical datasets can predict algal bloom occurrences and optimize resource allocation for mitigation efforts. High-frequency sensor networks deployed in critical aquatic ecosystems, such as Lake Taihu and the Bohai Sea, can offer continuous data streams to inform adaptive management strategies. These tools must be integrated into national monitoring systems to ensure scalability and accessibility, particularly in remote or underserved regions.

Long-term ecological studies are needed to elucidate the legacy effects of nitrogen inputs trapped in soils and sediments. For instance, regions like the North China Plain and the Yangtze River Basin exhibit significant nitrogen stocks that continue to leach into surface waters despite reductions in new inputs (Table 1). Research should explore targeted interventions, such as strategic dredging of nutrient-rich sediments in lakes or enhancing denitrification in riparian zones through reforestation and wetland restoration. Quantitative models that simulate nitrogen release rates under varying environmental conditions, including climate change scenarios, will be critical for forecasting future impacts and designing proactive measures.

Addressing nitrogen pollution necessitates solutions that integrate ecological, social, and economic perspectives. For example, combining agronomy with hydrology can optimize fertilizer application rates while minimizing runoff risks. Comparative studies between China and other countries facing similar challenges, such as India and Brazil, can identify transferable lessons from successful pilot projects. These studies should also evaluate how socio-economic disparities influence the adoption of sustainable practices, ensuring equitable outcomes across diverse populations.

Future climate change will intensify nitrogen eutrophication in China. Rising temperatures (1.5 to 3.5 °C by 2100 under RCP 4.5–8.5) will increase water temperatures, accelerating algal growth and prolonging HABs, while reducing dissolved oxygen and worsening hypoxia in systems like the Bohai Sea. More intense rainfall will enhance nitrogen runoff and erosion, mobilizing legacy soil nitrogen into waterways, whereas droughts may concentrate pollutants, increasing toxicity. Altered river flows in the Yangtze and Yellow River basins could reduce pollutant dilution and estuary flushing, extending eutrophication. Sea-level rise and storms may expand coastal dead zones by enhancing stratification. These climate impacts interact with existing pollution, potentially undermining current mitigation efforts. Even with reduced fertilizer use, warmer waters, and changing hydrology, HABs may still be sustained. Therefore, nitrogen management must incorporate climate resilience through adaptive land use, green infrastructure, and algal bloom early-warning systems to ensure long-term effectiveness.

While localized initiatives have demonstrated promising results, scaling these solutions nationwide remains challenging. Integrated watershed management projects in Lake Taihu exemplify the potential of multi-jurisdictional cooperation, performance-based incentives, and robust extension services. For instance, precision-agriculture demonstrations in Heilongjiang have reduced nitrogen application by 15–20% without compromising crop yields, thanks to decision-support technologies that adjust fertilizer rates based on site-specific soil tests [58]. To replicate these achievements, future policy packages should attach quantifiable load-reduction goals to funding assistance for smallholders, municipal utilities, and businesses. Innovative public–private financing models, such as co-investment funds or nutrient-credit trading, can help bridge the cost gap for high-performance technologies like membrane bioreactors and constructed wetlands.

Despite progress in policy formulation, uneven enforcement remains a significant obstacle, particularly in rural areas with limited resources and institutional capacity. Addressing these barriers requires strengthening governance structures, enhancing technical training for local officials, and increasing public awareness about nitrogen pollution’s environmental and health risks. For example, subsidies for soil testing and precision agriculture tools could incentivize smallholders to adopt sustainable practices. Additionally, participatory governance platforms that involve downstream users, farmers, and urban dwellers in decision-making processes can ensure shared benefits and responsibilities.

Lessons from international experiences, such as the European Union’s Nitrates Directive, highlight the importance of clear regulatory frameworks and strong governance capacity. Denmark’s ambitious targets for reducing livestock density and Poland’s efforts to designate nitrate-vulnerable zones underscore the need for tailored approaches that account for regional contexts [76]. In Italy, reintroducing stricter nitrogen limits on fertilizers led to immediate reductions in nitrate loads entering rivers, demonstrating the efficacy of regulatory adjustments when effectively enforced [77]. Adapting these best practices to China’s socio-economic realities will require balancing global standards with localized solutions.

8. Conclusions

Nitrogen eutrophication represents one of China’s most pressing environmental challenges, with far-reaching consequences for ecosystems, human health, and economic stability. Over-enriching water bodies with nitrogen has led to harmful algal blooms, oxygen depletion, biodiversity loss, and degraded water quality across the country. Chinese freshwater lakes and marine environments like Lake Taihu, the Bohai Sea, and the Yangtze River Delta consistently face this crisis, which offers reminders of the interconnectedness between human activities and natural systems. At the same time, terrestrial ecosystems face threats such as soil acidification, invasive species, and declining plant diversity, further underscoring the pervasive impact of nitrogen pollution. Addressing this issue is critical for safeguarding China’s environment and ensuring food security, clean water access, and public health for its vast population.

China has made notable progress in mitigating nitrogen eutrophication through policy measures, technological advancements, and community-driven initiatives. The Zero Growth Policy for chemical fertilizers stands out as a landmark effort to curb excessive fertilizer use while promoting sustainable agricultural practices. Similarly, the restoration of Lake Taihu demonstrates the potential of integrated watershed management to tackle pollution at its source and restore ecological balance. Investments in wastewater treatment infrastructure, precision agriculture, and innovative technologies like remote sensing and bioremediation have also reduced environmental nitrogen inputs. These successes highlight the importance of coordinated action and long-term commitment to addressing complex environmental issues.

Despite these efforts, nitrogen pollution threatens China’s ecosystems, particularly in regions with high agricultural intensity or rapid urbanization. Enforcement of regulations remains uneven, especially in rural areas with limited resources and capacity. Moreover, the scale of the problem, driven by decades of unchecked industrial and agricultural expansion, requires sustained investment and innovation to achieve lasting solutions.

To address these gaps, future research must focus on several key areas. First, there is a need to refine and expand nitrogen monitoring tools to provide real-time data on pollution levels and trends. Remote sensing, modeling, and other advanced technologies can help identify hotspots and prioritize interventions, ensuring that resources are allocated effectively. Second, successful interventions, such as those implemented in Lake Taihu and key agricultural provinces, should be scaled up and adapted to other regions facing similar challenges. Sharing best practices and lessons learned can accelerate progress and avoid duplication of efforts. Third, interdisciplinary collaboration will be crucial in developing holistic solutions integrating ecological, social, and economic perspectives. For example, combining insights from agronomy, hydrology, and sociology can lead to more comprehensive strategies for managing nitrogen pollution.