Nitrogen is one of the primary chemical components in coal, and its geochemical role in coal is crucial for tracing its origins, transformation, and enrichment processes1. Organic nitrogen in coal primarily originates from proteins, amino acids, chlorophyll, and other components of coal-forming plants and fungi, which were fixed during the extended geological history2. Meanwhile, precise measurements of nitrogen isotopes can unveil the evolutionary history of nitrogen in coal and provide invaluable information about ancient climates, environments, and biology3. However, nitrogen oxides (NOx) produced from coal combustion have emerged as one of the primary contributors to environmental issues4,5. Therefore, from the perspective of the global nitrogen cycle, in-depth research into the isotopic composition characteristics and speciation control mechanisms of nitrogen in coal is of great significance for understanding nitrogen evolution during coal formation, tracing modern atmospheric pollution, and identifying the sources of acid rain6.

The key scientific issues in this study concerning nitrogen isotopes (δ15N) in coal are mainly manifested in three aspects: Firstly, there has been relatively little research on nitrogen isotopes (δ15N) in coal7,8,9,10,11, and their geological and geochemical characteristics remain unclear12. Meanwhile, the mechanism underlying the regional variation of δ15N values in coal worldwide has not been clearly elucidated. The δ15N values of coal in China range from + 1.4 to + 5.1‰13, which is consistent with the range (Australian lignite to sub-anthracite: + 0.3 to + 3.7‰14; German anthracite: + 2.7 to + 3.7‰15; American anthracite: + 2.2 to + 5.4‰16) of δ15N values in coal from other regions of the world. However, domestic studies have indicated the existence of negative δ15N values in coal17. Whether this variation is an inherent characteristic of Chinese coal requires further investigation. Secondly, the formation of nitrogen in coal and its isotopic composition may influence the generation of δ15N values during coal combustion18. Therefore, understanding the isotopic composition of nitrogen in coal and its correlation with δ15N values is of great significance in environmental protection and will contribute to enhancing burner performance and aiding the development of combustion modeling. Lastly, existing research suggests that the composition of nitrogen isotopes (δ15N) in coal is the result of the combined effects of multiple factors, including petrographic characteristics, coal-forming epochs, and depositional environments7,13,19. However, the specific mechanisms through which some of these factors influence the isotopic composition of nitrogen in coal remain unclear.

In terms of measurement methods, although early methods such as pyrolysis, combustion, the Kjeldahl-Rittenberg method, and the combined use of Elemental Analyzer-Conflo IV-Isotope Ratio Mass Spectrometry (EA-Conflo IV-IRMS) provided nitrogen isotope information, they suffered from issues like destructiveness, cumbersome operations, and time-consuming processes20,21. With technological advancements, scholars at home and abroad have explored methods such as Nuclear Magnetic Resonance (NMR), Infrared Spectroscopy (IR), X-ray Photoelectron Spectroscopy (XPS), and X-ray Absorption Near Edge Structure (XANES) for the determination of nitrogen species22,23,24,25,26. Among them, XPS has become a mainstream technique in related research due to its high sensitivity, chemical state resolution, and non-destructiveness. The Elemental Analyzer/Isotope Ratio Mass Spectrometry (EA/IRMS), on the other hand, can comprehensively determine the content of all elements in coal samples and provide accurate nitrogen isotope data22,23,24,25,26.

In view of this, this study utilizes XPS in combination with EA/IRMS and Vario EL III to investigate the nitrogen isotope content, composition, and speciation distribution in coal samples. These findings are then compared with previous research results from Permian coal-bearing strata in the southern margin of the north China Craton Basin. The objectives of this study are to enrich the domestic data on nitrogen isotopes in coal, verify whether the difference in δ15N values between China and other countries is an inherent characteristic, examine the factors influencing the nitrogen isotope ratios and morphological compositions in coal, and determine the specific mechanisms affecting the nitrogen isotope composition in coal.

Results

Coal quality

The results of the proximate and ultimate analyses of the 16 coal samples studied are presented in Table 1. The moisture content ranged from 1.309 to 1.591%. According to the Chinese national standard for moisture (MT/850-2000, ultra-low moisture coal ≤ 6.00%), the 16 coal samples in this study are classified as ultra-low moisture coals. The ash content ranged between 26.776 and 36.517%. Based on the Chinese national standard (GB/T 15,224.1–2018, which categorizes coals as low-ash (10.00% < Ad ≤ 20.00%), medium-ash (20.00% < Ad ≤ 30.00%), and high-ash (30.00% < Ad ≤ 40.00%)), all samples in this study, except for samples 8-2, 8-5, and 8-6 which are medium-ash coals, are classified as high-ash coals. The volatile matter yield ranged from 38.299 to 56.348%. According to the Chinese national standard (MT/849-2000, which categorizes coals as low volatile matter (10.00% < Vdaf ≤ 20.00%), medium volatile matter (20.00% < Vdaf ≤ 28.00%), medium–high volatile matter (28.00% < Vdaf ≤ 37.00%), high volatile matter (37.00% < Vdaf ≤ 50.00%), and extremely high volatile matter (> 50.00%)), all samples in this study, except for sample 9–7 which is an extremely high volatile matter coal, are classified as high volatile matter coals. Based on the classification standards for fixed carbon according to China’s national standard for coal classification (GB/T 5751-2009), FCdaf in this study (ranging from 43.652 to 65.259%), the main body consists of medium-fixed carbon coal (FCdaf 50 –70%), with only individual samples (9-7) being low-fixed carbon coal. Specifically, Sample 9-7 (FCdaf = 43.652%) exhibits the lowest fixed carbon content due to its extremely high volatile matter content (56.348%), but they are still at a medium level overall. This indicates that the studied coal samples are characterized by high volatile matter content, and the fixed carbon content is jointly constrained by both volatile matter and ash content, aligning with the typical properties of high-volatile bituminous coal. Table 2 lists the proportions of organo-petrographic constituents and minerals in the coal. Vitrinite is the most abundant component, accounting for over 80% of the entire organo-petrographic constituents.

Table 1 Results of industrial and elemental analyses of 16 coal samples (%).
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Table 2 Maceral content (%) of 16 coal samples.
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Nitrogen forms

According to the results presented in Table 3 and Fig. 1, the nitrogen forms in the study area can be classified into four types: N-5, N-6, N-Q, and N-X. Among them, N-5 dominates the N 1 s XPS analysis results of all coal samples, with N-6 and N-Q having lower proportions than N-5. Pyridinic nitrogen (N-6) is relatively abundant and present in all coal samples, occupying the secondary peak position in the N 1 s XPS analysis results (except for coal sample 8), which aligns with previous research findings26. In contrast, the proportion of quaternary nitrogen (N-Q) is extremely low and even absent in the N 1 s XPS analysis results of some coal samples. A considerable amount of N-X exists in the eight coal samples of this study due to the long-term exposure of coal to air, leading to the oxidation of pyridinic nitrogen located at the edges of the coal molecular structure27. Additionally, the research indicates that there is no inorganic nitrogen present in the sample coals used in the experiment, or its content is too small to be detected by XPS. The schematic diagram of the N-5 functional group is shown in Fig. 3, where pyrrolic nitrogen is bonded to one hydrogen atom and two carbon atoms in a five-membered ring28. Furthermore, under thermal treatment, the N–H bond of pyrrolic nitrogen breaks, causing the five-membered ring to expand and open, followed by the combination of pyrrolic nitrogen with another carbon atom to form a six-membered ring nitrogen29.

Table 3 Results of XPS n1s spectra and nitrogen isotope ratios (‰) of the studied coals.
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Fig. 1
figure 1

Results of N 1 s XPS analysis of coal samples.

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The schematic diagram of the functional groups for N-6 and N-Q is shown in Fig. 2, where N-6 corresponds to bonding with two carbon atoms30,31. Furthermore, there is a significant relationship between the presence of N-Q in coal and the dry ash-free basis carbon content (Cdaf). When the Cdaf of coal is below 90%, N-Q primarily exists in the form of protonated pyridinic-like nitrogen structures containing oxygen functional groups16,30(Fig. 2). These structures gradually disappear and convert into pyridinic nitrogen during the deprotonation process during bituminization. When the pyrolysis temperature reaches 1100 °C, N-Q completely decomposes32, leaving N-6 as the residual nitrogen in coal, which is oxidized to nitrogen oxides (N-X) under aerobic conditions. When Cdaf exceeds 90%, N-Q mainly exists in the form of graphitized quaternary nitrogen, where nitrogen atoms are directly bonded within graphene layers. In this study, the Cdaf range is from 83.122 to 83.974, corresponding to protonated quaternary nitrogen. As shown in Table 3, the N 1 s XPS spectra of coal samples 8 and 9-5, 9-7 do not exhibit N-Q peaks, indicating that quaternary nitrogen has fully deprotonated and converted into pyridinic nitrogen. The N-X peak in coal sample 9 is not prominent, and its presence can be attributed to the oxidation of the coal sample in air, categorized as nitrogen oxides33. Due to the loss of oxygen-containing functional groups during thermal treatment, N-Q can convert to N-6. Kelemen et al.34 found that the sum of N-6 and N-Q remains constant during pyrolysis. In this study, the relative contents of N-5 and N-6 in coal samples with different carbon contents remain relatively stable (Fig. 3), which is consistent with Thomas’ suggestion35. Therefore, it can be inferred that the N-Q peak corresponds to “protonated” quaternary nitrogen in the coal samples.

Fig. 2
figure 2

Schematic representation of the functionality of the main nitrogen forms in coal.

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Fig. 3
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Ratio of N-5 and N-6 to Cdaf content in the studied coals.

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Nitrogen isotope

Previous studies have indicated no systematic variation between δ15N values and Cdaf content16, a conclusion that is corroborated in this study (Fig. 3a). In this research, a significant negative correlation was observed between the δ15N values and Nadf content of coal (Fig. 4b). The nitrogen isotope δ15N in coal may depend on the composition of its macerals36. This study revealed that the macerals, vitrinite and inertinite, exhibit a certain degree of correlation with the δ15N values of coal (Fig. 5). Specifically, vitrinite shows a clear negative correlation with δ15N, while inertinite displays a significant positive correlation37. The characteristics of liptinite remain unclear, which is consistent with previous research findings38,39. Figure 6 illustrates the proportional relationships between the N-5, N-6, and N-6 + N-Q components in coal and their corresponding δ15N values. A strong correlation exists between N-6 and δ15N values, with δ15N values decreasing as the pyridinic nitrogen content increases. N-5 also exhibits a certain negative correlation with nitrogen isotope δ15N. Additionally, the N-Q peak in the coal samples in this study was identified as quaternary nitrogen with a protonated pyridine structure (Fig. 2). Although there is a difference in binding energy positions between pyridinic nitrogen and protonated quaternary nitrogen, both belong to pyridine-structured nitrogen in terms of molecular structure. Based on this, nitrogen in coal is classified into pyrrolic-structured nitrogen and pyridine-structured nitrogen. The cross-plot of δ15N values against pyrrolic-structured nitrogen (N-5) and pyridine-structured nitrogen (N-6 + N-Q) reveals significant correlations (Fig. 6). Previous studies have also pointed out clear linear relationships between N-5, N-6, N-6 + N-Q, and δ15N values26.

Fig. 4
figure 4

Ratio of Cdaf and Ndaf to δ15N content in the studied coals.

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Fig. 5
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Ratio of microcomposition to δ15N values for the studied coals.

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Fig. 6
figure 6

Ratio of N-5, N-6 and N-6 + N-Q to δ15N values for the studied coals.

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Discussion

Nitrogen isotope δ 15N content

In 2018, predecessors conducted a study on the δ15N values of 141 coal samples from China, discovering that these samples exhibited positive δ15N values (+ 1.4 to + 5.1‰, with an average of + 3.5‰), which are consistent with the δ15N value range of coals from foreign countries13. However, the coal samples collected this time from the coal-bearing strata in the eastern part of the north China Craton Basin showed a range of δ15N values that included negative values (− 1.9 to + 3.15‰)26. This aligns with previous research findings from the same basin but does not conform to the typical δ15N value range of Chinese coals. Further analysis revealed that the δ15N values of Coal No. 8 were all positive and greater than those of Coal No. 9. Combining Fig. 12d with factors such as sedimentary environment, diagenesis, and microbial alteration, it was found that Coal No. 8, which is overlying limestone and underlying sandstone, is in a marine sedimentary environment. High salinity and hypoxic conditions may have promoted denitrification, leading to enrichment of 15N in residual nitrate or organic nitrogen and resulting in a positive trend in δ15N values40. In contrast, Coal No. 9, which is overlying sandstone and underlying mudstone, is in a terrestrial peat environment. During the early stage of diagenesis, nitrogen fixation was dominant, with nitrogen-fixing microorganisms converting atmospheric N2 into organic nitrogen accompanied by slight fractionation41, resulting in negative δ15N values for some samples. Additionally, the limestone overlying Coal No. 8 enhanced the supply of nitrate in pore water, further promoting denitrification42, while the mudstone underlying Coal No. 9 may have restricted the input of nitrate from seawater, reducing the contribution of denitrification43,44. Meanwhile, modern peat, as a precursor to coal, exhibits a broad range of δ15N values45,46,47,48,49, with high-latitude peat (such as northern peatlands) having very low δ15N values (− 5 to + 2‰)48. Limited microbial activity and preferential preservation of organic nitrogen in high-latitude regions may also affect the δ15N values of coal48. The eastern part of the north China Craton Basin is located in a high-latitude region, and the δ15N value range of its coal samples is generally lower than the typical positive range of Chinese coal samples. Combining previous research on microbial alteration49 and thermal evolution34 provides a reasonable explanation for the δ15N value range of − 1.9 to + 3.15‰ in the coals of the north China Craton Basin, including the occurrence of negative values and their rationality for being lower than the typical δ15N values of Chinese coals.

Factors influencing nitrogen isotope composition and their geographical variation

Elemental factors

Based on the research by Kelemen et al.34 there is generally a correlation between the nitrogen isotope ratio (δ15N) and carbon content in coal, where an increase in carbon content often leads to a decrease in the δ15N value. In this study, due to the minimal variation in carbon content (Cadf) among the coal samples, the correlation between δ15N values and carbon content was not significant (see Fig. 4a). However, Fig. 4a does show that the carbon content of Coal No. 9 is higher than that of Coal No. 8, but its nitrogen isotope ratio (δ15N) is lower than that of Coal No. 8 samples, a phenomenon consistent with the reports by Kelemen et al.34 The influence of nitrogen content (Nadf) in coal on the nitrogen isotope δ15N manifests as higher δ15N values in low-nitrogen coals. This is because in high-nitrogen coal samples, the conversion process of nitrogen is more significant, with N-Q converting to N-6 or N-X, leading to more negative δ15N values (see Fig. 4b)50. Conversely, in low-nitrogen coal samples, the N-Q content is higher, and most remains in its protonated form with minimal conversion51. Therefore, this study observed a clear negative correlation between the δ15N value of coal and its nitrogen content (Nadf) (see Fig. 4b), which aligns with previous research. Figure 4c and e demonstrate the influence of hydrogen content (Hadf) and oxygen content (Oadf) on the nitrogen isotope δ15N. There is a significant positive correlation between hydrogen content and δ15N, while oxygen content exhibits a certain negative correlation with δ15N. Under reducing conditions (such as low oxygen or high hydrogen environments), the degree of nitrogen oxidation in coal is lower, and the nitrogen isotope ratio typically remains at a higher positive value52. Therefore, coal samples with high hydrogen content are more likely to maintain a reducing environment, thereby reducing the formation of oxidized nitrogen species and resulting in higher δ15N values53. This is consistent with the impact of hydrogen and oxygen content on the nitrogen isotope ratio shown in Fig. 4c and e. Additionally, low-sulfur coal samples may undergo less oxidation, leading to relatively higher nitrogen isotope ratios in coal54 (Fig. 4d), as less oxidation helps retain lighter isotopic forms of nitrogen (such as N-5).

Maceral content factors

The macerals in coal primarily include vitrinite, inertinite, and liptinite55. The composition, structure, and chemical properties of these macerals have significant impacts on the chemical characteristics, pyrolysis behavior, and nitrogen isotope ratios (δ15N) of coal56. Vitrinite mainly originates from plant lignocellulosic tissue and is rich in nitrogen derived from plant proteins with low δ15N values (− 5 to + 5‰), thus retaining lighter nitrogen isotopes57. Inertinite, on the other hand, is often formed in oxidizing environments or under fire conditions, and its precursor materials may have undergone denitrification, leading to enrichment of 15N (i.e., higher δ15N values). For example, predecessors found that the δ15N values of inertinite are 3–5‰ higher than those of vitrinite, which may be related to microbial denitrification in oxidizing environments16. During diagenesis, vitrinite forms in an oxidizing environment at the peatification stage and may undergo more intense microbial activity, resulting in higher δ15N values58. As the coal rank increases, vitrinite may release 14N-containing volatiles during pyrolysis, further increasing the δ15N value, while the structure of inertinite is stable, with minimal changes in its δ15N value59. Additionally, in oxidizing depositional environments, inertinite develops with higher δ15N values (e.g., δ15N values of inertinite in modern peat can reach + 8‰60). In contrast, in reducing depositional environments, vitrinite dominates with lower δ15N values (averaging + 2 to + 4‰), which may be related to the inhibition of denitrification under anoxic conditions61. As for the correlation between liptinite and nitrogen isotope δ15N, the characteristics fluctuate greatly, and there are relatively few relevant studies. This study found a certain negative correlation between vitrinite and nitrogen isotope δ15N (see Fig. 5a). Combined with the analysis in Table 2, it can be seen that Coal No. 9 has a higher vitrinite content, while the inertinite content is lower than that of Coal No. 8. Furthermore, there is a significant positive correlation between inertinite and nitrogen isotope δ15N (see Fig. 5b), which is consistent with previous research conclusions60,61. In summary, it can be confirmed that the macerals in coal, especially vitrinite and inertinite, have a significant impact on the nitrogen isotope ratios (δ15N) of coal. At the same time, combining previous studies, this study quantified the correlation between macerals and nitrogen isotope δ15N. That is, vitrinite usually corresponds to lower δ15N values, while inertinite corresponds to higher δ15N values.

Geographical differences

Studying the nitrogen isotope composition in the Carboniferous and Permian strata of the north China Craton Basin, an important oil and gas resource region in China, is crucial for deepening our understanding of the geological evolution, oil and gas genesis, and source area characteristics of this region62. Although both Huainan and Hebei are located within the tectonic setting of the north China Craton Basin, their geographical differences (Hebei in the eastern part of the basin and Huainan at the southern edge) provide unique perspectives for studying coal quality characteristics and nitrogen isotopes. Therefore, this study adopts the research results of Ding et al.26 and conducts a detailed analysis of nitrogen isotopes in Late Carboniferous coal samples from the Huainan area, providing a solid theoretical foundation and reference framework for the comparative analysis in this paper. Combined with paleogeographic reconstruction maps (see Fig. 7), it is found that the Huainan area was more prone to shallow marine to coastal facies deposition in the paleoenvironment, while Hebei exhibited a coastal to littoral plain environment profoundly influenced by arid climatic conditions. These paleoenvironmental differences are directly related to the heterogeneity of the original coal-forming environments, which in turn affected various aspects such as depositional processes, coal maturity, and key factors influencing nitrogen cycling. Ultimately, these factors led to differences in coal quality characteristics and nitrogen isotope contents in different regions within the north China Craton Basin.

Fig. 7
figure 7

Carboniferous Permian paleogeography of northern China. (1- Palaeolands; 2- Intermountain basins; 3- Uplands and alluvial plains; 4- Riparian and coastal plains; 5- Riparian; 6- Shallow sea and riparian; 7- Direction of sea incursion; 8- Late Early Permian to Early Late Permian arid climate-controlled areas; 9- Humid climate control area; 10-δ15N content).

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Generally, coals with lower maturity often contain higher levels of volatiles and moisture, which may result in higher ash yields63. The ash content of coal samples from the southern margin of the north China Craton Basin does not exceed 30%, while the ash content of coal samples from the eastern part of the basin is generally higher than 30% (Fig. 8), classifying them as high-ash coals. This indicates that the maturity of coal samples from the eastern part of the north China Craton Basin is lower than that of those from the southern margin, with a relatively lower degree of thermal evolution and weaker nitrogen isotope fractionation effect, leading to correspondingly lower δ15N values. The δ15N data obtained in this study are consistent with previous research conclusions34. High-ash coals are typically formed in proximal, high-energy environments, while low-ash coals are more often formed in reducing environments, aligning with the paleogeographic reconstruction map in Fig. 7. Therefore, combining δ15N values with ash content is of great significance for paleogeographic reconstruction. In addition, regarding the trend of ash content with burial depth, coal samples from both the eastern part and the southern margin of the basin exhibit a consistent trend. In summary, although these two regions have different ash contents, they are located within the same craton basin and exhibit similar characteristics in terms of ash content variation, highlighting the controlling role of regional geological conditions on the ash distribution pattern in coal seams.

Fig. 8
figure 8

Ash content changes in coal from different regions of the North China Craton Basin.

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The vitrinite-inertinite ratio (V/I) in coal, as a key parameter in coal petrology, can reveal the metamorphic grade and the characteristics of the coal-forming environment of coal seams64. Generally, coal seams with high V/I values indicate a relatively high content of vitrinite, reflecting a relatively humid and reducing environment with high water coverage during the coal-forming process65. This may lead to the preferential conversion or loss of lighter nitrogen isotopes (such as δ14N) through microbial-mediated nitrification–denitrification processes, resulting in an increase in the δ15N content in the remaining coal. Such coal seams have a lower degree of metamorphism and are in the initial stage of coalification. Conversely, coal seams with low V/I values indicate a higher content of inertinite and a dry, oxidizing coal-forming environment with limited water coverage, leading to a decrease in δ15N content and reflecting the degree of water coverage and redox conditions in the coal-forming swamp64. The V/I values of coal samples from the eastern part of the north China Craton Basin are greater than 4, indicating that the coal seams in the east are in a deep, stagnant, and reducing environment; most coal samples from the southern margin of the north China Craton Basin have V/I values between 1.5 and 3, indicating moderate water coverage and a weakly reducing environment (Fig. 9). The environments of these two regions are consistent with the paleogeographic reconstruction map (Fig. 7), and the V/I content shows a consistent trend with burial depth. In summary, the metamorphic grade and coal-forming environmental characteristics of coal seams significantly affect the nitrogen isotope δ15N in coal. This study reflects these characteristics through the V/I ratio, indicating that the δ15N ratio of coal seams in the eastern part of the north China Craton Basin is lower than that at the southern margin.

Fig. 9
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Plot of mirror inert ratio content and trend of change in coal in different areas of the North China Craton Basin.

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In this study, correlation analyses were conducted between Cdaf, Ndaf, vitrinite, inertinite, exinite, N-5, N-6, and N-6 + N-Q with nitrogen isotope δ15N (Figs. 4, 5, 6). These correlation values were compared with corresponding values from the southern margin of the north China Craton Basin (Fig. 10), revealing significant differences between the two. Notably, the Permian and Carboniferous, as two distinct coal-forming periods, differ in terms of the original coal-forming environment, degree of coalification, inertinite content, and coal seam age66,67,68,69,70,71: During the Carboniferous, a warm and humid climate, coupled with lush vegetation, facilitated the formation of swamps and peatlands conducive to coal formation, resulting in a relatively high degree of coalification and inertinite content, which led to a relatively low nitrogen content. In contrast, the Permian period was characterized by arid conditions and sparse vegetation, leading to lower degrees of coalification and inertinite content but higher nitrogen content. These differences collectively influenced the distribution of δ15N, showing that a humid and anoxic environment, combined with high coalification, contributed to the enrichment of 15N in Carboniferous coals, while arid and oxidizing conditions, along with low inertinite content, resulted in isotopic depletion in Permian coals. This finding provides critical geochemical indicators for paleoenvironment reconstruction and studies on nitrogen cycling in coal seams, emphasizing the importance of comprehensively considering coal-forming periods and tectonic backgrounds in cross-regional comparisons. Furthermore, despite geographical and geological differences between the coal samples in this study and Permian coal samples from the southern margin of the north China Craton Basin, there is a certain similarity in nitrogen speciation, with N-5 accounting for more than 50% in both, followed by N-6 (Fig. 11). This result suggests that future research could combine molecular isotope techniques (such as CSIA) to further explore the δ15N fractionation mechanisms of specific nitrogen species.

Fig. 10
figure 10

Heat map of correlation between elements and δ15N of coal samples from different areas of the North China Craton Basin.

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Fig. 11
figure 11

Morphological share of elemental N in different areas of the North China Craton Basin.

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Experimental methods and preparation

Samples preparation

The coal samples collected in this study are located at the north Shaft of Dongpang Mine in Neiqiu County, Xingtai City, Hebei Province. Dongpang Mine is situated in the eastern landmass of the north China Craton Basin, in the southwestern part of Neiqiu County, Hebei Province. The mine area stretches approximately 8 km north to south and 5 km east to west, covering an area of 40.76 km2 (Fig. 12). The coal-bearing strata belong to the Carboniferous-Permian period, rich in coal resources with well-developed faults and folds. The north Shaft of Dongpang Mine is located in the west of the Dongpang Mine field, approximately 10 km southwest of Neiqiu County, Xingtai City (Fig. 12). The total thickness of the coal seams ranges from 5.88 to 7.6 m, with a coal seam dip angle of 10° to 14°. Within the explored depth range, there are a total of 18 coal seams with a combined thickness of 18.95 m. Among them, six seams are minable or locally minable, with a total thickness of 12.74 m. In accordance with Chinese National Standard GB/T482-2008, this study sampled from coal seams #8 and #9 in the Upper Carboniferous Taiyuan Formation. Samples were collected from locations completely unaffected by magma and from magmatic intrusion areas to ensure the representativeness, accuracy, and reproducibility of the obtained samples. Based on research requirements, continuous stratigraphic or interval sampling strategies were selected to ensure that the rock samples could reflect vertical variations in rock characteristics72. The coal samples were labeled as 8-1, 8-2, 8-3, 8-4, 8-5, 8-6, 9-1, 9-2, 9-3, 9-4, 9-5, 9-6, 9-7, 9-8, 9-9, and 9-10. Each coal sample weighed approximately 1 kg and was stored in plastic bags to minimize contamination and oxidation. In the laboratory, the coal samples were mechanically crushed and sieved to the required particle size for experimentation. To eliminate the influence of external moisture on coal spontaneous combustion experiments, an appropriate mass of coal samples was placed in a vacuum drying oven, with the oven temperature set to 40 °C, and the coal samples were dried in the same vacuum environment for 24 h. After drying, the coal samples were placed in self-sealing bags to prevent contact with water vapor and oxygen in the air, which could affect the experimental results73.

Fig. 12
figure 12

Geographic location map (A), tectonic outline map (B), sampling location map (C) and sampling bar chart (D) of the study area.

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Experimental methods

Coal analysis

An approximate analysis of moisture, ash, and volatile matter content was conducted on coal samples crushed to < 212 μm (accurately weighed at 10.00 ± 0.1 mg) in accordance with ASTM standards D3173-23(2023), D3174(2019), and D3175-20(2020). In accordance with the GB/T 212–2008 standard, in the condition of a dry-ash-free basis, the fixed carbon content is calculated using the formula:

$${\text{FC}}_{{{\text{daf}}}} = 100{\text{\% }} - {\text{V}}_{{{\text{daf}}}}$$
(1)

The analysis process is outlined as follows: The coal samples were placed in pre-dried ceramic crucibles. The moisture content was determined by drying to constant weight at 105 °C for 4 h. The volatile matter content was obtained by heating in a muffle furnace at 900 °C for 7 min in an airtight environment. The ash content was determined by oxidative ignition at 750 °C for 6 h. All thermal treatment steps were conducted under strictly controlled conditions within a nitrogen-filled glovebox, with each sample independently measured in triplicate, and the results were reported as averages to ensure a relative standard deviation below 2.5%. Additionally, ultimate analysis, was conducted using a Vario EL III elemental analyzer (Elementar Analysensysteme GmbH, Hanau, Germany) at Instruments’ Center for Physical Science of USTC, with the combustion tube and reduction tube temperatures set at 1150 °C and 850 °C, respectively. Helium was used as the carrier gas at a flow rate of 230 mL/min. The instrument was calibrated with sulfanilamide, and the sample weight was controlled at 20 ± 2 mg to determine the C, H, S, N, and O contents. According to the standard methods of the International Committee for Coal and Organic Petrology (ICCP)74,75, the coal samples were mixed with epoxy resin at a mass ratio of 1:4 and supplemented with 1% curing agent. After vacuum infiltration for 24 h, they were polished to a particle size of 0.25 μm using a diamond suspension, after polishing, the surface roughness meets the requirement of Ra ≤ 50 nm. Prior to microscopic observation, the samples are cleaned by argon ion sputtering coating treatment. Under a reflection microscope, a statistical count of 500 points was conducted on the polished coal using a 50 × objective lens to determine the maceral composition and mineralogical properties of the coal, with measurement accuracy controlled within ± 5%. To ensure data stability and reliability, all operations for each sample were repeated three times, and the final statistical analysis was conducted based on the average values obtained from these three experiments.

XPS analysis of nitrogen

XPS analysis was performed based on Chinese National Standard GB/T 19,500-2004 by using a Thermo-VG Scientific ESCALAB 250 X-ray photoelectron spectrometer at the University of Science and Technology of China’s (USTC) Instruments’ Center for Physical Science. In a constant temperature and humidity laboratory (25 °C, 45% RH), 10.0 ± 0.1 mg of powder sample was pressed onto a conductive indium foil. An achromatic Al (Kα) radiation source with an operating voltage of 15 kV was employed, and the analysis was performed in constant analyzer energy mode with a pass energy of 30 eV. Operators wore lead glass protective glasses during the operation, and the sample loading process was completed in a nitrogen-filled glovebox (O2 < 0.1 ppm) to prevent oxidation of air-sensitive materials. The reproducibility of peak position measurements was within ± 0.1 eV. Data processing was carried out using XPSPEAK 4.1 software (developed by the Department of Physics, Tsinghua University), with Shirley-type background subtraction applied. Using a typical 30% Lorentzian and 70% Gaussian function, fixing the full width at half-maximum at 1.7 eV and defining but not fixing N peaks30,36. Four peaks were identified in the N 1 s spectrum, including (1) the N-5 peak at 400.6 ± 0.3 eV, the N structure refers to Pyrrolic Nitrogen (Pyrrolic N)28; (2) the N-6 peak at 398.8 ± 0.3 eV, the N structure refers to pyridinic nitrogen (Pyridinic N)30; (3) the N-Q peak at 401.3 ± 0.3 eV, the N structure refers to “protonated” quaternary N16,30; and (4) the N-X peak ranging from 402 to 405 eV, the N structure refers to Oxidized N33. The peak area error for each component was less than 5%. Three independent experiments demonstrated a binding energy measurement reproducibility of ± 0.08 eV (2σ confidence interval). A total of 615 valid data points (n = 16) were obtained, with a standard deviation of less than 1.2% for the relative content of each nitrogen species.

Nitrogen isotope analysis

Nitrogen isotopic analysis was conducted at State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum using elemental analyzer/isotope ratio mass spectrometry (EA/IRMS, Finnigan Delta V Advantage interfaced with Flash EA 1112 HT, Thermo Fisher Scientific, Bremen, Germany). Approximately 10.2 ± 0.3 mg of sample was weighed and placed into a 50 mL polytetrafluoroethylene digestion vessel. Five milliliters of pre-cooled 4 M hydrochloric acid solution was added, and the sample was acidified at 25 ± 1 °C for 24 h to remove carbonate components. The hydrochloric acid treatment should be carried out inside a fume hood, with acid and alkali-resistant gloves and goggles worn for protection. After acidification, the sample was centrifuged and washed three times with Milli-Q ultrapure water (18.2 MΩ cm) (10 mL each time, at 4000 rpm for 5 min) until the filtrate reached a neutral pH, verified using pH strips. The sample was then dehydrated in a vacuum oven at 60 °C until constant weight was achieved. The processed sample (8.5–9.8 mg) was encapsulated in a tin combustion capsule (8 × 5 mm, Elemental Microanalysis Ltd.), and combusted at 960 °C to produce N2 gas. Regular checks were conducted to ensure airtightness and prevent oxygen leakage, which could pose a combustion and explosion risk. The N2 was separated on a Porapak QS gas chromatography column at a temperature of 50 °C with a He flow rate of 90 mL/min. The results were expressed using the delta (δ) notation relative to air standards34:

$$\delta^{15} N\left( \permille \right) = \left( {V_{c} /V_{s} - 1} \right) \times 1000$$
(2)

where Vc is the 15N/14N value of the measured coal and Vs is the 15N/14N value of the air standard. The measured δ15N values were normalized with the certified reference substance acetanilide #1 (δ15N =  + 1.18‰). To ensure reproducibility and precision, measurements were repeated three times for all samples. The standard deviation of the analysis was within ± 0.25‰.

Outlook

The current study focuses on the analysis of δ15N values in samples from the eastern part of the north China Craton Basin, and the results obtained do not represent the general status of δ15N values in coal across China. The research indicates that the occurrence of negative δ15N values does not negate existing studies but rather emphasizes the regulatory role of factors such as regional geology and sedimentary environments on δ15N values. This finding aligns with the observed negative δ15N values in coal seams from the southern margin of the north China Craton, collectively showcasing the unique δ15N characteristics of coal seams in the north China Craton Basin. Meanwhile, comparative analysis in the study reveals that factors influencing nitrogen isotope composition vary across different regions within the basin. These differences are reasonably explained across multiple dimensions, ranging from technical methods and nitrogen isotope composition to sedimentary environments, diagenetic processes, and microbial nitrogen cycling. However, understanding of these differences still requires further research for validation and deepening. Therefore, future studies are planned to expand the sampling scope to include more diverse geological structures and sedimentary environments, and to delve deeper into the relationships between sulfates, nitrates, and other components in sedimentary environments and nitrogen isotope characteristics, with the aim of more comprehensively revealing the composition and distribution characteristics of nitrogen isotopes in Chinese coal.

Conclusion

  1. 1.

    This study delves into 16 coal samples from basins in the eastern north China Craton, revealing their prominent characteristics in terms of coal quality, nitrogen forms, and macerals. All coal samples belong to the category of ultra-low moisture coal (≤ 1.591%), with high ash content being predominant (30.00% < Ad ≤ 40.00%). High volatile coal dominates in terms of volatiles (Vdaf > 37.00%). FCdaf in this study (ranging from 43.652 to 65.259%), the main body consists of medium-fixed carbon coal (FCdaf 50–70%). Analysis of nitrogen forms indicates that N-5 accounts for the highest proportion in all coal samples (48 –62%), followed by N-6 (13 –45%). This conclusion is consistent with previous research findings. XPS results further confirm that nitrogen in coal primarily exists in organic forms, with no inorganic nitrogen detected. The absence of N-Q may be related to the transformation of protonated structures during thermal evolution. Additionally, vitrinite is the dominant maceral in all coal samples.

  2. 2.

    The study found that the δ15N values of the coal samples ranged from − 1.9 to + 3.15‰. Some coal samples exhibited negative δ15N values, which is consistent with previous research results from the same basin but differs from the typical δ15N value range of Chinese coal. Further analysis revealed that the δ15N values of Coal Seam No. 8 were generally higher than those of Coal Seam No. 9, and both were positive. Through comprehensive analysis of multiple aspects, including the sedimentary environment, diagenesis, microbial alteration, and δ15N values of modern peat, the study explained the reasons for the occurrence of negative δ15N values in coal from the eastern basin of the north China Craton and justified why these values are lower than the typical δ15N values of Chinese coal. In addition, this study further confirmed that the coal in the north China Craton basin has unique δ15N characteristics.

  3. 3.

    This study integrates elemental factors, macerals, and geographical variations, and explores the multivariate factors influencing the nitrogen isotope δ15N values in coal from multiple dimensions, including depositional environments, coal-forming environments, and coal ranks. Correlation analysis reveals significant correlations between δ15N values and the contents of carbon, nitrogen, hydrogen, and oxygen, as well as macerals in coal. Specifically, δ15N values are higher when nitrogen content is lower; hydrogen content positively correlates with δ15N values, while oxygen content negatively correlates with δ15N values. In addition, vitrinite content negatively correlates with δ15N values, while inertinite content positively correlates with δ15N values. These findings further confirm the complexity of nitrogen isotopes in coal, deepen the understanding of nitrogen isotope behavior in coal, and provide new perspectives for regional geological evolution and paleoenvironment reconstruction. Simultaneously, they indicate that factors such as different coal-forming environments and coal ranks have significant impacts on nitrogen isotope composition.

  4. 4.

    Coal samples from the eastern and southern margins of the north China Craton basins exhibit significant differences in ash content, V/I ratio, and δ15N values, reflecting the long-term control of paleogeographical conditions (such as water body coverage and redox conditions) on coal quality. However, the samples in this study are concentrated in a single region, and future research needs to be expanded to include more diverse geological structures and sedimentary environments to comprehensively reveal the nitrogen isotope characteristics of coal in China. Furthermore, combining molecular isotope techniques (such as compound-specific isotope analysis, CSIA) to analyze the fractionation mechanisms of specific nitrogen species, and exploring the contributions of inputs such as sulfates and nitrates to δ15N values, will help deepen our understanding of the nitrogen cycle in coal. This, in turn, will provide more precise geochemical evidence for paleoenvironmental reconstruction and studies on the origin of oil and gas.