1. Introduction
Human activities are now the primary cause of environmental change on a worldwide scale during the Anthropocene epoch [
1]. Two of the three human-induced environmental changes that are outpacing safe planetary bounds in the Earth-system process are the biogeochemical nitrogen cycle (N-cycle) and climate change, which constitute a serious threat to the planet’s ability to support itself in the future [
2], [
3]. To produce fertilizers, the synthetic ammonia industries (Haber-Bosch process) have played a crucial role. As a result, our population has increased from 1.6 billion in 1900 to over 8.0 billion in 2023. Despite these benefits, there is a disadvantage in that excessive fertilizer use and environmental nitrate pollution have led to eutrophication of surface waters, groundwater nitrate pollution, and high nitrous oxide (N
2O) emissions [
4], [
5]. However, the anaerobic ammonium oxidation (anammox) microbial mechanism may help the earth system maintain or regain the equilibrium in the N-cycle.
For many years, it was believed that the only process removing fixed nitrogen from the atmosphere was heterotrophic denitrification, which converts bioavailable nitrogen to dinitrogen gas (N
2) [
6]. Anammox, which fills a critical gap in the environmental N-cycle, has challenged this knowledge [
7], [
8]. Autotrophic planctomycete bacteria that are capable of oxidizing ammonium anoxically into N
2 and nitrite reduction cause anammox. Anammox, which oxidizes ammonium using nitrite, does not produce N
2O, in contrast to other nitrogen transformation processes (such as nitrification and denitrification) [
9], [
10], [
11]. The potentially widespread occurrence of anammox bacteria was reflected by the low half-saturation constant for substrates (
Ks (NH
4+) ≤ 7 μmol·L
−1 [
12];
Ks (NO
2−) < 5 μmol·L
−1 (
Brocadia) and 0.2-0.3 μmol·L
−1 (
Kuenenia) [
13]), and thereby play an important role in the global biogeochemical N-cycle [
14], [
15], [
16], [
17], [
18]. Additionally, anammox can remove nitrite and ammonium from the environment at vanishingly low quantities since it has a stronger affinity for substrate than its rivals (nitrite-oxidizing bacteria and denitrifiers).
Anammox use in wastewater treatment in designed systems not only makes it easier to remove nitrogen, but can also transform energy-guzzling systems into energy-saving wastewater treatment facilities [
19]. This is because, in contrast to typical nitrification-denitrification processes, anammox bacteria may grow anaerobically, and their autotrophic capacity ultimately requires less aeration and no carbon supply. As a result, the anammox process has a great deal of potential to play a significant role in nitrogen removal and the reduction of the greenhouse gas N
2O. Anammox may also aid in achieving some more general goals related to environmental sustainability. In engineered systems for high-strength ammonium wastewater treatment, such as sidestream treatment of municipal wastewater, landfill leachate, tannery wastewater, and potato wastewater, anammox has been widely used as a key biotechnology to date [
20], [
21], [
22], [
23], [
24]. The use of anammox technology in a natural aquatic habitat is not well understood, nevertheless.
The International Union for Conservation of Nature (IUCN) defines nature-based solutions (NbSs) as activities that protect, sustainably manage and restore natural or modified ecosystems, address societal challenges (such as climate change, food and water security, and natural disasters) effectively and adaptively, and simultaneously benefit human well-being and biodiversity. Wetlands are frequently viewed as examples of natural solutions that can help society in a variety of ways, including social, economic, and environmental [
25]. We have studied anammox in wetland ecosystems on a local, regional, national, and international scale for the past 25 years. We also successfully designed and enhanced the Anammox Hotspot Wetland Project, which has been successfully running for more than ten years. Based on this knowledge, we proposed a biogeoengineering method to design and construct anammox hotspots, using NbS and taking planetary boundaries as the design criterion, to achieve water purification, reduction of greenhouse gas emission, improvement of biodiversity, and optimal local ecological and socio-economic benefits.
2. Hotspots of anammox at various oxic-anoxic interfaces
Hotspots of biogeochemical processes are frequently seen where two nearby ecosystems converge [
26]. The electron transport in oxidative and reducing states is the fundamental component of the N-cycle. The majority of findings from several studies conducted over the past 25 years support the idea that the oxic-anoxic boundary in many aquatic ecosystems represents an important anammox hotspot. Because of its high redox potential, the oxic-anoxic interface has the potential to be a hub for microbially mediated N-cycle processes.
Both oxidized (NO
2−/NO
3−) and reduced (NH
4+) nitrogen molecules must be nearby for the biological anammox reaction to proceed. Because it offers a region where both aerobic and anaerobic N-cycle processes coexist, the oxic-anoxic transition zone is exceptional (
Fig. 1). Through ammonium oxidation to nitrite and nitrite oxidation to nitrate, respectively, aerobic processes like nitrification can produce the necessary nitrite and ammonium needed to fuel anammox. Anammox has been found to tolerate O
2 concentrations from sub-micromolar to micromolar levels, whereas aerobic nitrification has recently been shown to occur at remarkably low oxygen concentrations of only a few nanomolar, or even when oxygen is undetectable [
27], [
28], suggesting that both processes have the potential to overlap at the oxic-anoxic interface. Additionally, essential coupling activities on the ammonium and nitrite supplies for anammox are anaerobic processes such as dissimilatory nitrate reduction to ammonium (DNRA) and nitrate reduction to nitrite [
29], [
30], [
31]. Remineralization of organic nitrogen, which is produced when bacteria break down organic material and release ammonium, is another important source of ammonium [
30]. In the oxic-anoxic transition zone, it has been demonstrated that each of the aforementioned processes is closely connected with anammox [
32]. Anammox activity has been quantified and its distribution across the oxic-anoxic transition zone using methodological approaches using
15N-labelled stable isotope addition experiments (
15NH
4+ and
15NO
2−), in combination with molecular tools targeting specific anammox genes (
hzo and
hzs).
Due to the poor solubility of oxygen in water and the variable diffusion coefficients between water and soils/sediments, oxic-anoxic interfaces frequently arise in aquatic environments. Large organic matter input and inadequate regional ventilation in marine oxygen minimum zones (OMZs) maintain the low oxygen conditions, which then draw down oxygen concentrations and create a distinct oxic-anoxic transition zone [
33]. Oxic-anoxic gradients create ideal conditions for the anammox process, which is discussed below, to take place in the rhizosphere of aquatic plants, wetland riparian zones, surface sediments, terrestrial subsurface saturated-unsaturated horizons, granules sludge and biofilms in wastewater treatment, and marine OMZs.
2.1. Soil-air interfaces (surface of paddy soils)
The surface soil of paddy fields is explicitly referred to as the soil-air interfaces in this context since anammox does not exist in soil from dry land. According to data from Food and Agriculture Organization Statistics (FAOSTAT) 2020, rice is one of the most significant food crops, providing food for more than 50% of the world’s population in an area of around 1.9 × 10
6 km
2. The paddy fields’ top soils receive occasional irrigation and drainage. Water replaces the gaseous phase in the soil pores after floods. A wide variety of oxic-anoxic transition zones may form above the plow pan because the oxygen diffusion time in the water is four orders of magnitude lower than that in the air [
34]. As a result, oxygen may be depleted from the bulk topsoil within 24 hours. Within these horizons, an anammox rate of (2.55 ± 0.30) nmol·g
−1·h
−1 (measured in N) may be observed, contributing to significantly higher N-loss than that in the subsurface ((1.01 ± 0.50) nmol·g
−1·h
−1) [
10].
2.2. Soil-root interfaces (rhizosphere)
Because anammox does not occur in soil from dry land, the term “soil-root interfaces” here explicitly refers to the rhizosphere of aquatic plants [
17], [
35], [
36]. One of the most dynamic interfaces on earth is the rhizosphere, which is the biologically active region of the soil around plant roots in aquatic plants [
37]. In small zones next to roots and rhizomes in otherwise oxygen-poor zones in bulk soils, the rhizosphere creates an oxidized microenvironment for aquatic plants to grow in [
38], [
39], [
40]. As a result, rhizosphere oxygenation might provide a redox gradient between the rhizosphere and bulk soils, enabling anammox to flourish in that particular niche. Experimental evidence revealed that anammox in rhizosphere soils contributed to 31%-41% N
2 production with rates ranging from 0.33 to 0.64 nmol·g
−1·h
−1, whereas the non-rhizosphere anammox bacteria contributed to only 2%-3% N
2 production with lower activities of 0.08-0.26 nmol·g
−1·h
−1 [
35].
2.3. Soil-water interfaces (riparian zone)
The transitional boundary areas between terrestrial and aquatic ecosystems are known as riparian zones, and they are one of the most prevalent wetland kinds. These zones are crucial for controlling interactions at the landscape level in three dimensions. According to Naiman and Décamps [
26] and McClain et al. [
41], natural riparian zones are among the most diverse, dynamic, and complex ecosystems in terrestrial systems. The biogeochemical hotspots of the N-cycle in riparian zones, with high rates of ammonium oxidation, nitrate reduction, oxygen consumption, and N-loss, have also been the subject of numerous research [
42], [
43], [
44]. Riparian zones are regularly subjected to fluctuating anoxic/oxic conditions as a result of changes in water level, which creates strong redox driving forces. Anammox bacteria are a hotspot at the land-freshwater interfaces, where they make up a small proportion of the microbial community (0.01%-0.07% of the overall bacterial population) but have a significant impact on N-loss (contributing 11%-35%) [
11].
2.4. Soils-groundwater ecotones (groundwater system)
According to Aeschbach-Hertig and Gleeson [
45] and Taylor et al. [
46], aquifers are typically characterized by long water residence durations (1400 years on average globally), relatively sluggish water exchange rates, and low dissolved organic matter levels. Aquifers are a perfect home for slow-growing autotrophic bacteria because of this. Aquifers are an excellent possible niche environment for the growth of anammox bacteria because they grow slowly, doubling in 2.1-20 days [
47], and are competitive in environments with low water exchange rates [
48].
Because of the soil pores and plant roots, unsaturated surface soil is typically aerobic. In contrast, due to prolonged flooding and low exchange fluxes, saturated soils below the groundwater table are typically anoxic [
49], [
50]. Worldwide NH
4+ and NO
3− contamination of surface soils and rivers is rising as a result of the development of new agricultural techniques to meet the rising global need for food [
51], [
52], [
53]. According to Sun et al. [
54], Zhang et al. [
53], Prosser and Nicol [
55], Lehtovirta-Morley et al. [
56], and Tourna et al. [
57], NH
4+ in soils is partially adsorbed by clay minerals and partially oxidized to NO
2− by ammonia oxidation archaea (AOA) and bacteria (AOB). However, NO
3− enters the groundwater aquifer under leaching. As a result, certain aquifers have significant NO
3− contents [
58], [
59]. At the groundwater table, oxidized and reduced nitrogen can interact and move between saturated and unsaturated soils. Redox gradient zones may offer a good habitat for N-cycling microbes in these oxic-anoxic interfaces between saturated-unsaturated horizons [
58], [
60], [
61], [
62]. Therefore, in the subsurface ecotone of soil and groundwater, significant anammox processes may occur ((3.38 ± 1.82) mmol·m
−2·d
−1), accounting for 77.9% ± 15.1% of the total N-loss [
63].
2.5. Water-sediment interfaces (rivers and lakes)
The oxygen concentration in natural waters can remain high and only drop with increasing water depth due to the reoxygenation of water bodies and oxygen production by aquatic plants and algae [
64]. In the sediment-water interface, additional physical, chemical, and biological processes (such as migration, transformation, adsorption/desorption, diffusion, burial, and biological disturbance, as well as the decomposition of plant biomass) may take place that control the exchange and transport of nutrients between the sediment and the water body [
65], [
66], [
67]. According to Di Toro and Connolly [
64] and Di Toro and Fitzpatrick [
68], active oxygen respiration by heterotrophs in the underlying water layers is fueled by the buildup of organic matter in sediments, which causes a considerable depletion of oxygen in surface sediments. Anammox activity is significantly connected with organic matter content, which provides the ammonium needed to feed anammox (through organic matter remineralization) [
69]. These circumstances not only support anoxic zones for anammox to thrive. Additionally, nitrification, which produces the required nitrite for anammox in permeable sediments, is somewhat associated with anammox activity [
18]. Subsequently, these conditions promote anammox activity in river ((28.3 ± 20.5) μmol·m
−2·h
−1, 14.7% ± 7.9% of total N-loss) and lake sediments ((70.2 ± 44.4) μmol·m
−2·h
−1, 19.2% ± 7.3% of total N-loss) [
17], and even contributed up to 58% of
in situ N
2 production in permeable river sediments [
18].
2.6. Oxic-anoxic zones of stratified basins
Anammox was originally identified in the marine environment in sediments in 2002 [
70], and it was later found in the anoxic water column [
7], [
8]. Over the following ten years, it was discovered that anammox was persistent and widespread in a variety of aquatic ecosystems, including marine sediments [
71], [
72], sea ice [
73], and OMZs [
14], [
74]. Overall, OMZs account for 20%-50% of fixed N removal in the worldwide ocean, although making up only 1% of its volume [
75], with anammox playing a significant part. Anammox activity increases at the oxic-anoxic transition zone in OMZs and other stratified basins and lakes before decreasing as a function of depth [
32], [
76]. In these systems, anammox in OMZs, such as in the Peruvian/Chilean upwelling zones, is primarily driven by the export of organic matter from surface waters [
30]. The oxic-anoxic transition zone remineralizes sinking phytoplankton material to produce ammonium, which in turn powers anammox. Additionally, anammox is maintained in the water column of the OMZ by ammonium produced by benthic processes (DNRA and benthic organic nitrogen remineralization) [
30]. While nitrate reduction to nitrite and nitrification [
31], [
74] maintain the necessary nitrite driving anammox. Aside from OMZs, anammox has also been demonstrated to be active in the oxic-anoxic zone of stratified basins, including freshwater lakes like Lake Tanganyika [
32], [
77] and the Black Sea [
78], [
79]. The sulfur cycle is also closely related to anammox activity and can increase anammox rates in coastal OMZs [
80].
2.7. Microzones in biofilms and sludges
The anammox process is frequently used in industrial, municipal, and field-scale wastewater treatment plants (WWTPs), where anammox hotspots are found in granular sludge [
81], [
82], [
83] and thick biofilms [
84], [
85], [
86]. Anammox and nitrifying bacteria coexist in this environment, with the latter giving anammox the essential nitrite [
87]. It is interesting to note that anammox may reside in the anoxic interior of flocs and granules, whereas nitrifiers might predominately exist at the outer rim or exist in suspension where oxygen is present [
88], [
89]. As anammox bacteria are shielded from oxygen and given access to their substrates, the transformation of loose sludge flocs into dense biofilm or granules may encourage their development and retention in WWTPs [
90]. In wastewater systems enriched in anammox bacteria, the production of anammox biofilms and granules is the key active component and determining factor of nitrogen removal [
85], [
91], [
92].
3. How can we maximize anammox capacity?
We believe that figuring out how to direct anammox activity to assist nature-based biogeoengineering initiatives is a crucial next step given our understanding of how it thrives in the environment. The main benefit of anammox is that, unlike traditional nitrification-denitrification, it can convert surplus reactive nitrogen to N
2 gas without generating N
2O, a powerful greenhouse gas. The anammox pathway also avoids the energy-intensive aeration and organic carbon needed in the traditional nitrification-denitrification process by using NO
2− instead of O
2 to oxidize NH
4+ and reduce NO
3− at the same time. Several natural biogeoengineering techniques based on oxic-anoxic interfaces can be created to boost the anammox hotspots, offering a fresh method for controlling Earth’s N biogeochemistry. The three requirements for NbSs must be met in the design and implementation of these strategies: benefits for society, the economy, and nature all occur at once; they are transdisciplinary, combining ecology, economics, sociology, engineering, and environmental sciences; and they endure throughout time [
93].
3.1. Riparian systems
The particular anammox rate per unit area in these zones continues to be the highest, ranging from 7 to 20 nmol·g
−1·h
−1 discovered in Baiyangdian Lake, even though the area of riparian zone (i.e., 1-100 m width) system only accounts for a small portion of the aquatic ecosystem [
11]. To boost the anammox process, it may be helpful to increase the area covered by riparian systems. Based on studies into natural anammox hotspots in Baiyangdian, artificial anammox hotspots have been created and are being used in the Shijiuyang Wetland, Jiaxing, China (250 000 m
3·d
−1) (
Fig. 2(a)). Shijiuyang Wetland has built plant-bed/ditch systems with numerous land-water interfaces, which increases the prevalence of anammox hotspots, in contrast to wetlands with horizontal or vertical flows (
Fig. 2(b)). Additionally, the sediment and soil create a variety of oxic-anoxic interfaces, on which unique anammox hotspots were promoted, as a result of periodically raising the water level variation in the riparian zones. Here, anammox bacterial abundances and activities were about 1-2 orders of magnitude higher than those in open water. The varying water level may also encourage the recurrent flow of water through the reedbed’s plant root system, which greatly increases anammox activity in the rhizosphere (
Fig. 2(c)). Anammox was predicted to contribute 30%-40% of the N-loss in rhizosphere soils, compared to only 4%-37% in non-rhizosphere soils [
35]. According to Wang et al. [
94], the anammox in these hotspots of the riparian zone of the Shijiuyang Wetland removed roughly 43.0% of the fixed nitrogen while reducing the
in situ N
2O emission flux by about 27.1% in comparison to the open water.
The Anammox Hotspot Wetland Project had a positive impact on biodiversity. Since the wetland was built in 2008, there has been significant improvement in the animal and plant diversity in the Shijiuyang Wetland project (
Fig. 2(d)) [
95]. Two hundred and seventy-three volunteer aquatic and terrestrial plant species, as well as 16 typical aquatic plant species, were discovered over the three-year monitoring period in the wetland system. The marsh is now home to predators like weasels and snakes as well as waterfowl like egrets, rabbits, and wild ducks, increasing animal diversity (
Fig. 2(a)). Furthermore, there are significantly fewer mosquitoes around the wetland project. For more than 15 years, the wetland has been kept running steadily without becoming clogged. This can be attributed to the adoption of a new water diversion method—root channel (
Fig. 2(c)). Under the situation of a 20 cm head loss, our field measurements revealed high hydraulic conductivity (in both the root and field systems), reaching up to 17 m
3·m
−2·d
−1. The root channel can also filter out a lot of insect eggs, which might account for the decline in mosquito populations. The ecological functions and stability of the wetland ecosystem have also been significantly enhanced.
The Shijiuyang Wetland project’s economic benefit analysis revealed that while some initial investment is required for engineering construction, wetland operation and maintenance, and reed harvesting, a reduction in water treatment costs, income from selling reeds, and an increase in house prices relative to a nearby city area are all highly advantageous. Based on three-year average data, the cost-effectiveness ratio (CER), which considers benefits to society, the economy, and the environment all at once, was −19.45 USD·m
−3. Several seminatural wetlands have been built in the Jiaxing region, taking into account the anammox hotspots, based on the Shijiuyang Wetland’s success. The Jiaxing Wetland Engineering Group currently treats roughly 1.90 × 10
6 t of water each day on 1000 ha (1 ha = 10
4 m
2) of land (
Fig. 3). It has won numerous accolades, including the Dubai International Model Award for Improving Human Settlement Environment, and has grown to be a significant demonstration base for environmental education.
3.2. Paddy field ecosystems
In the terrestrial anthropogenic system, agriculture is the main cause of N
2O emissions, with paddy field ecosystems playing a significant role [
96]. Important anammox hotspots are the sporadic oxic-anoxic soil layers above the plowing layer and the rhizosphere soils. The variation in water level with the rice fields' growing season must be considered to improve the anammox process in the rice paddy field system. Although the anammox process occurs frequently and persistently in paddy soils, its existence necessitates moist soil conditions. When the soil moisture content in dryland soil exceeds 29%, the process can be initiated [
97]. The presence of water provides anoxic conditions and promotes the mass transfer of substrate NH
4+ and NO
x−.
Anammox should preferably be weakened in rice fields, but only to eliminate excess nitrogen, since agricultural systems need to contain nitrogen to prevent N-loss, in contrast to aquatic ecosystems, which have a surplus of nitrogen. Therefore, it is important to lessen the water level fluctuations in the rhizospheric horizons that encourage anammox hotspots. As a result of less anammox hotspot being created during periods of flooding or drying, there is less nitrogen fertilizer loss for rice (
Fig. 4(a)). However, excessive fertilization is difficult to prevent since it frequently causes substantial non-point source pollution because a lot of nitrogen pollutants infiltrate associated farmland streams through runoff and leaching [
98]. The N
2O emissions from rivers increased fourfold, according to a recent study of the global N
2O budget, with streams being responsible for the great bulk of the increased N
2O emissions [
96], [
98]. The increased N
2O discharge in global river ecosystems is most likely caused by this. As a result, the stream’s extra ammonia needs to be cleaned up and eliminated. Enhancing riparian anammox hotspot technology is one of the best ways to deal with this issue. The best way to achieve anammox augmentation in streams is to control water level changes (
Fig. 4(b)). When fully operational, an ecological gate valve or solar-powered windmill might be utilized to control water level fluctuations based on the site’s topography and water level gradient, thereby enhancing the anammox hotspot in the stream’s riparian zone.
3.3. Groundwater system
Aquifers can also harbor anammox hotspots [
63]. The production of drinking water, agricultural irrigation, and the security of the world's food supply are all dependent on groundwater, which is the greatest accessible freshwater resource [
45], [
46]. Due to the extensive use of N-rich fertilizers, groundwater within aquifers is becoming contaminated with inorganic NO
x on a global scale, endangering both human and environmental health. The global occurrence of anammox in aquifers, where anammox was found to be activated in saturated and inactivated in unsaturated soil horizons, contributed 36.8%-79.5% to N-loss in saturated soil horizons, the remainder being due to denitrification which has traditionally been considered the main pathway for removal of N-pollutants from aquifers [
60], [
62], [
63], [
99]. Numerous investigations have demonstrated that DNRA can tightly couple with anammox and contributes a significant portion of the NH
4+ required to drive anammox activity. Anammox has been shown to contribute to N-loss through soil and bioreactor systems, aquatic water column investigations, and soil-DNRA coupling [
29], [
32], [
100], [
101], [
102]. Currently, there is a lot of nitrate pollution in the groundwater, which provides a good substrate for this coupling and encourages the development of anammox in the groundwater system.
Anammox bacteria will become more active in unsaturated soil above the groundwater table as a result of groundwater recharge. Although there was no evidence of anammox activity in the unsaturated soil strata, it might become active if it came into touch with increasing groundwater [
102]. It was discovered that the global occurrence of anammox in aquifers alternated between saturated and unsaturated soil strata. The anammox hotspot rate ((3.38 ± 1.82) mmol·m
−2·d
−1) consistently maximized in horizons of saturated-unsaturated soils, where the potential rates of anammox were even higher than those of denitrification, accounting for 77.9% ± 15.1% of the total N-loss [
63]. According to Aeschbach-Hertig and Gleeson [
45] and Taylor et al. [
46], more than one-third of all renewable water resources are currently recharged from groundwater around the world. More anammox bacteria may reactivate in deep unsaturated soils with increased groundwater storage, which would increase their contribution to N removal in the global groundwater system. The South to North Water Diversion Project, the world’s largest and longest water diversion project that benefits the most people and areas, was launched by China in 2015. Previous studies in Baiyangdian, the largest freshwater lake in the north that receives 112 t of water per year from the South to North Water Diversion Project, have shown that the contribution to N-loss via anammox increased significantly with the water level, which was accompanied by a significant reduction in N
2O emission (∼39.3% ± 10.6%) since N-loss by anammox does not cause N
2O emissions. By promoting the anammox peak horizon at the soil-groundwater interface and shifting from denitrification to the more environmentally friendly anammox pathway to reduce greenhouse gas emissions, groundwater recharge, and intermittent groundwater fluctuation can play a significant role in the terrestrial N-cycle.
4. Conclusions and perspectives
Since the industrial revolution, a new age known as the Anthropocene has formed, during which human activity has taken over as the primary driver of environmental change on a global scale, pushing the N-cycle over its safe operating limits. Although the manufacture of synthetic ammonia increased global food security, excessive N-fertilizer use may have several long-lasting impacts on the planet’s ecosystems. From the perspective of earth system engineering, global issues demand the coordination of worldwide research. Based on the discovery of an anammox hotspot capable of oxidizing ammonium anoxically and reducing nitrate into N2 without N2O by-product, this study offers a new perspective on geoengineering for climate change, biodiversity loss, social-environmental services, and efficient use of water resources. This is the first large-scale natural solution using an anammox hotspot to assure clean drinking water, control climate, encourage animal and plant diversity, and enhance sustainable environmental quality that we are aware of. We also show that while there are significant environmental and ecological benefits from healthy wetland systems, they can also generate financial gain. Two of the biggest global issues of this century—climate change and the N-cycle—can be addressed through projects that manage, safeguard, and restore ecosystems.
Acknowledgments
The authors gratefully acknowledge Ting Yang, Fanping Zheng, Bin Yu, Tongyu Li, Huawei Pan, Xumin Wang, Jiayi Jiang, Libo Sun, Gawhar Armanbek, and Longbin Yu for their kind assistances on collecting and discussing about anammox data from literatures, and Weidong Wang on providing photos. The authors also gratefully acknowledge Xiaotang Ju and Xiaoyuan Yan for participating the initial discussion. This research is financially supported by the National Natural Science Foundation of China (91851204 and 42021005) and the Special project of eco-environmental technology for peak carbon dioxide emissions and carbon neutrality (RCEES-TDZ-2021-20). The authors Guibing Zhu and Shanyun Wang gratefully acknowledges the Program of the Youth Innovation Promotion Association of Chinese Academy of Sciences.
Compliance with ethics guidelines
Guibing Zhu, Bangrui Lan, Shuci Liu, Cameron M. Callbeck, Shanyun Wang, Liping Jiang, Asheesh Kumar Yadav, Jan Vymazal, Mike S.M. Jetten, Ganlin Zhang, and Yongguan Zhu declare that they have no conflict of interest or financial conflicts to disclose.
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