Geochemical distribution and genesis of selenium-rich soils in Maojie Town on the Yunnan low-selenium belt

Abstract

Understanding the distribution patterns and genesis of selenium-rich soils, a valuable natural resource, constitutes a fundamental prerequisite for their effective development and utilization. To investigate the geochemical distribution characteristics and influencing factors of selenium-rich soil in Maojie Town, this study conducted a land quality geochemical survey and collected rock and soil samples. Selenium concentrations were determined using atomic fluorescence spectrometry (AFS), and multivariate statistical regression analysis was employed to analyze the data. The results showed that the selenium content in the soils of Maojie Town ranged from 0.13 to 0.85 mg·kg⁻¹, with an average concentration of 0.39 mg·kg⁻¹. The selenium distribution was uneven, with selenium-rich soils mainly concentrated in the central part of the town. The formation of selenium enrichment in Maojie Town is attributed to the geological background of Mesoproterozoic slightly metamorphosed rocks, high-altitude forestlands with abundant organic matter and slow decomposition rates, and the fixation and adsorption effects of clay minerals and acidic soils. These findings provide a scientific basis for the development and utilization of selenium-rich soils and the socioeconomic development of the region.

Introduction

Selenium (Se) is a rare and dispersed element that is also an essential trace element for humans. It possesses anticancer, antioxidant, and antagonistic effects against harmful heavy metals. Selenium deficiency can lead to diseases such as Keshan disease, Kashin-Beck disease, and certain types of cancer. Additionally, selenium is a beneficial element for plant growth and development, as it promotes plant growth, enhances crop yield, and improves product quality¹. Despite its biological significance, selenium is present in extremely low concentrations in the Earth’s crust. It exists in both organic and inorganic forms and is widely distributed in rocks, soils, groundwater, the atmosphere, and living organisms. Through various geochemical processes, selenium undergoes migration and transformation across different environmental media, completing its geochemical cycle².

A selenium-deficient belt extends from northeastern to southwestern China², with the Chuxiong region located within this belt. This region is characterized by selenium deficiency and a high prevalence of selenium-deficiency-related diseases. It has been well established that selenium deficiency is a major environmental risk factor for endemic diseases such as Keshan disease³. The majority of Chinese soils are deficient in selenium, with an average selenium concentration (0.40 mg·kg⁻¹) that is below the global average for soils. In the study area located in Chuxiong, the selenium content in cultivated soils of certain regions ranges from below 0.125 mg·kg⁻¹ to 0.125–0.175 mg·kg⁻¹, representing some of the lowest levels observed among cultivated soils in Yunnan Province⁴. Previous research on selenium has primarily focused on its role in human pathology. More recent studies have shifted towards investigating its distribution in geological carriers such as rocks, soils, and vegetation, as well as its application in the production of selenium-enriched agricultural products^5–7. The geochemical evaluation of selenium-rich soils and their potential utilization has become a key research focus of the China Geological Survey in recent years. Future research directions include the practical application of scientific findings on selenium-rich soils, the industrialization of selenium-enriched agricultural products, and the conversion of research outcomes into tangible benefits. Extensive research conducted by domestic and international scholars on the geochemical characteristics, cycling patterns, genesis and sources of selenium-rich soils in specific regions of China has revealed that geological settings serve as the primary source of soil selenium^8–13. Topography and geomorphology, soil types, soil pH, and other heavy metal elements exert secondary influences on the spatial distribution of selenium levels through complex geochemical interactions.At present, the mechanisms underlying selenium enrichment in Maojie Town, located within the selenium-deficient belt, remain unclear. Selenium-rich soils serve as the primary substrate for selenium-enriched wild fungi, which are essential for the development of selenium-enriched agricultural products. However, there are still unresolved scientific questions regarding the distribution patterns, enrichment mechanisms, and controlling factors of selenium-rich soils in Maojie Town.

This study, supported by the land quality geochemical survey project in the central Yunnan region (Chuxiong Prefecture), identified Maojie Town in Wuding County as an area with significant selenium-rich soil resources. A systematic investigation was conducted to analyze the geochemical characteristics of selenium-rich soils and the factors influencing their distribution. The findings provide crucial insights for the scientific evaluation of selenium-rich land and the sustainable development of selenium-enriched agricultural products. Additionally, this study serves as a valuable reference for future research on selenium enrichment mechanisms in wild fungi, soil selenium thresholds, and the rock-soil–plant selenium migration and transformation processes.Ultimately, these insights contribute to the advancement of selenium-enriched agricultural industries and support rural revitalization efforts by providing a solid theoretical and scientific foundation.

Materials and methods

Study area overview

The study area is located in Maojie Town, Wuding County, Chuxiong Yi Autonomous Prefecture, Yunnan Province. It belongs to the low-latitude plateau monsoon climate zone. The town borders Renxing Town in Lufeng County to the east, Bicheng Town in Lufeng Argiudic ferrosols and Hapli-udicargosols County to the south, Yangjie Town in Yuanmou County to the west, and Bailu Township and Gaoqiao Town to the north, covering an administrative area of 461 km². Geologically, the study area is part of the South China stratigraphic region, specifically the Yangtze stratigraphic subregion and the Kangdian stratigraphic subdivision. It is divided into the Chuxiong and Kunming stratigraphic subunits by the Yuanmou-Lüzhijiang fault. The region has undergone intense tectonic activity, with well-developed folds and faults and extensive stratigraphic exposure, ranging from the Mesoproterozoic Kunyang Group to the Quaternary system. The study area is situated within the Yangtze Block, specifically in the Upper Yangtze Paleo-Landmass. In terms of third-order tectonic units, it lies within the Kangdian Basement Fault-Uplift Belt. The bedrock in this area consists primarily of Mesoproterozoic low-green-schist facies low-grade metamorphic rocks. The exposed stratigraphic units include the Mesoproterozoic Lüzhijiang Formation, Etouchang Formation, Luoxue Formation, Yimin Formation, and Meidian Formation.The Lüzhijiang Formation is composed of light-gray and dark-gray dolomite containing siliceous streaks and masses, interbedded with sericite slate, argillaceous limestone, and limestone.The Etouchang Formation consists of dark-gray and gray-black slate, siliceous and silty streaked and banded slate, intercalated with limestone, argillaceous dolomite, siliceous rock, and metamorphosed sandstone.The Luoxue Formation is characterized by bluish-gray, gray-white, and flesh-red algal dolomite interbedded with siliceous dolomite, argillaceous and sandy dolomite, and slate. It exhibits horse-tail veining structures and is copper-bearing.The Yimin Formation comprises purplish-red ferruginous slate and sandy dolomite interbedded with slate, dolomitic siltstone, hematite beds, and is copper-bearing. Its base consists of polymictic conglomerate.The Meidian Formation is composed of gray-black and gray sericite slate, slate containing lenticles of argillaceous limestone, interbedded with metamorphosed siltstone, conglomeratic limestone, and algal limestone. Proven mineral resources within Maojie Town include iron, copper, and lead–zinc deposits. According to the land quality survey results, the predominant soil type is Argi-Udic ferrosols, with localized areas of Hapli-Udic argosols. The main land use types include dryland, shrubland, and arbor forestland. Moreover, Wuding County boasts abundant wild mushroom resources, encompassing over 30 high-value species including Thelephora ganbajun, Tricholoma matsutake, and Boletus. With an annual yield stabilized at approximately 8,000 metric tons, the sector achieved a comprehensive output value of 550 million yuan in 2024, establishing itself as a pivotal industry for rural revitalization in the region.

Sample collection

Soil samples

Surface soil samples (0–20 cm depth) were collected from the study area using specialized sampling tools, including a soil auger and bamboo strips. The collection process involved vertically excavating a 20 cm deep pit with a soil auger, followed by the uniform and continuous sampling of soil from the 0–20 cm segment using bamboo strips. Root residues, gravel, fertilizer clumps, and portions that came into contact with the sampling tools were removed before placing the samples into pre-labeled new cloth bags. A total of 214 surface soil samples were collected.

For soil samples associated with wild mushroom rhizospheres, approximately 1000 g of soil was collected per sample using a bamboo knife. Samples were taken from the natural soil within a 15 cm radius and 0–20 cm depth surrounding the wild mushrooms. Any grass roots, gravel, and other debris were discarded before placing the samples into labeled cloth bags for further analysis.

To enhance sample representativeness, vertical soil profiles were selected at sites with stable soil development conditions and minimal human disturbance. The classification of the soil vertical profile was conducted with reference to the principles outlined in Principles of Soil Horizon Definition and Classification¹⁴. At each selected site, a rectangular excavation area (1.5 m in length and 1 m in width) was marked. Soil was excavated layer by layer, with a step left every ~ 30 cm to prevent collapse. Excavated soil from each layer was carefully placed in sample bags, labeled accordingly, and transported to the laboratory.

Crop samples

Wild mushroom samples were collected from Damadi Village, Maojie Town, Wuding County. Due to the random nature of wild mushroom occurrence, sampling sites were evenly distributed as much as possible. Each sample weighed approximately 300 g and was placed in labeled cloth bags for further analysis. The identification of wild mushroom species was based on the reference book Mushrooms of Yunnan¹⁵.

All sample collection and processing followed the guidelines outlined in the Specifications for Multi-objective Regional Geochemical Survey (DZ/T 0258–2014) and Specifications for Land Quality Geochemical Assessment (DZ/T 0295–2016), ensuring the representativeness and reliability of the samples. Stringent quality control protocols were implemented during sample collection to ensure representativeness and accuracy. Sampling sites were systematically located using a predetermined grid, and at each site, a composite sample was formed from multiple subsamples around the central GPS-recorded point. To monitor the entire process precision, field duplicate samples were collected independently at a rate of 3–5% of the total samples. All sampling tools were meticulously cleaned between each sample to prevent cross-contamination, and comprehensive field documentation, including photographs and detailed descriptions, was maintained for every location.Sample processing was conducted in a dedicated, dust-controlled environment to avoid contamination. The samples were air-dried, disaggregated using a rubber mallet, and sieved through a 200-mesh nylon screen to achieve homogeneity. From this homogenized powder, laboratory duplicate samples were submitted at a rate of 5% to assess the combined precision of the processing and analytical stages. Furthermore, Certified Reference Materials (CRMs) and procedural blanks were inserted into each analytical batch to verify analytical accuracy and monitor for potential contamination throughout the laboratory workflow. Recovery rates for the CRMs consistently fell within acceptable ranges, and blanks demonstrated negligible contamination levels, confirming the reliability and accuracy of the analytical data.The processed samples were sent to the Sichuan Bureau of Geology & Mineral Resources and Development and the Hubei Geological Research Laboratory for analysis.

Sample analysis

The determination of selenium concentrations in rock and soil samples was performed by Atomic Fluorescence Spectrometry (AFS) pursuant to the Specifications for Quality Management of Laboratory Testing in Geological and Mineral Resources (DZ/T 0130–2006) and Implementation Plan for Analysis Methods of 54 Elements/Oxides in Geochemical Survey Samples (2010). This method entailed a complete digestion process, subsequent conversion of selenium to its volatile hydride, and final quantification by AFS, wherein methodological accuracy and precision were ensured through the systematic analysis of certified reference materials and analytical replicates.

After natural air-drying, the soil samples were freed of plant residues and gravel, and sieved through a 200-mesh screen. Soil mineral morphology and particle size characteristics were examined using a scanning electron microscope (SEM, ZEISS SIGMA 300) following the “General Guidelines for Scanning Electron Microscopy (JY/T 0584–2020)”, with an operating voltage of 10–15 kV. Clay mineral composition was determined by X-ray diffraction (XRD, Bruker D8 FOCUS) in accordance with the "Analytical Method for Clay Minerals and Common Non-clay Minerals in Sedimentary Rocks by X-ray Diffraction (SY/T 5163–2018)", with a scanning range of 5°-65°(2θ) and a scanning speed of 0.02°/s. The resulting diffraction patterns were subjected to peak identification and qualitative phase analysis using Jade/HighScore software, and the primary mineral contents were calculated using a semi-quantitative approach. The correlation between clay mineral content and soil selenium concentration was assessed by Pearson correlation analysis, and scatter plots were generated using Origin 2024. Elevation was measured using the Mobile GIS 4.0.5 application on a UniStrong A8 handheld device during the field survey, while land use types were recorded through direct field observation with the same equipment.

Soil chemical properties were analyzed according to the methods described by Bao¹⁶. The contents of Al₂O₃ and TFe₂O₃ were determined using X-ray fluorescence spectrometry (XRF; Axios MAX, Malvern Panalytical, the Netherlands). Soil pH was measured in a soil–water suspension (1:2.5 w/v ratio) using the glass electrode method. Potassium (K) was quantified by flame photometry following digestion with a mixture of hydrofluoric and perchloric acids. Total sulfur (S) content was determined by the high-temperature combustion iodometric method. Soil organic matter (SOM) was estimated from organic carbon content, which was measured by the potassium dichromate oxidation method with external heating.

Data processing and mapping

The datasets were statistically analyzed using Excel 2024 and SPSS 25, with one-way ANOVA and correlation analysis (Pearson/Spearman) applied for hypothesis testing and variable relationship quantification. Spatial visualization and trend analysis were subsequently performed through ArcGIS 10.2 (for geospatial mapping) and Origin 2024 (for generating scatterplots, trendlines, and regression models), ensuring comprehensive data interpretation.

Results

Geochemical characteristics of soil selenium in Maojie Town, Wuding County

Based on the geochemical classification criteria for soil nutrients (Table 1) in the “Specifications of Land Quality Geochemical Evaluation” (DZ/T 0295–2016)(Table 1), the surface soils of Wuding County (2,930 km²) were evaluated for selenium (Se) content. The results indicate that marginal-grade Se soils are the most widespread, accounting for the largest share (37.30%) of the total area. This is followed by adequate-grade (930 km², 31.74%) and deficient-grade (875 km², 29.86%) areas. High-grade Se soils are very limited, covering only 32 km² (1.09%), with no areas classified as excessive (Fig. 1).

Table 1.

Classification standard of selenium in soil.

index	Level 1: lack	Level 2: Edge	Level 3: appropriate amount	Level 4: high	Level 5: surplus
Se (mg·kg−1)	≤ 125	> 0.125 ~ 0.175	> 0.0.175 ~ 0.40	> 0.40 ~ 3.0	> 3.0

Fig. 1.

Evaluation map of selenium level in surface soil of Wuding County. The map was created using ArcGIS 10.8 (https://desktop.arcgis.com).

Spatially, soil Se distribution is irregular and sporadic. Selenium-deficient soils dominate the central parts of the county. In contrast, the southern region exhibits higher Se levels than the north. Notably, Maojie Town contains more extensive areas of adequate-grade Se soils and the county’s primary occurrences of high-Se soils, setting it apart from other townships.

The selenium (Se) content in the soils of Maojie Town ranges from 0.13 to 0.85 mg·kg⁻¹, with an average concentration of 0.397 mg·kg⁻¹, which is 1.8 times higher than the national average Se content in surface soils (0.29 mg·kg⁻¹)¹⁷. The standard deviation is 0.1545, and the coefficient of variation (CV) is 38.93% (Table 2). Overall, the soil in Maojie Town exhibits a relatively high Se content, but its spatial distribution is uneven. The Se-enriched soils are primarily concentrated in the central region of the town.

Table 2.

Distribution characteristics of selenium content in Maojie Town.

Study area	Sample size	Minimum	Maximum	Mean	Standard deviation	CV(%)
Maojie town	214	0.13	0.85	0.397	0.1545	38.93%

Factors influencing selenium-enriched soils in Maojie Town

Geological background

Through the collection of representative rock and corresponding surface soil samples from five vertical profiles within the study area, a comparative analysis revealed the vertical variation of selenium (Se) content in the soil profiles of Mesoproterozoic strata, as shown in Fig. 2. The Se content in surface soils generally ranges from 0.22 to 0.60 mg·kg⁻¹, indicating that most of the surface soils in the region are selenium-enriched soils. In the Etouchang Factory Formation (Pt₂e), the Se content in the soil profile shows an increase from 0 to 90 cm, followed by a decrease beyond 90 cm. In the Lüzhijiang Formation (Pt₂lz), the Se content shows an increase from 0 to 130 cm, followed by a decrease beyond 130 cm. In the Yinmin Formation (Pt₂y), the Se content increases from 0 to 50 cm, then decreases These three soil profiles show a systematic decrease in Se content with increasing soil depth. Conversely, the Meidang Formation (Pt₂m) and Luoxue Formation (Pt₂l) exhibit fluctuating trends in Se content, but overall, there is a selenium-enrichment phenomenon in these profiles. This suggests that soil Se content is influenced by both stratigraphic composition and depth, with specific trends in Se accumulation within certain formations.

Fig. 2.

Geochemical characteristics of Se elements in soil profiles. A, Mineral Surface Horizon; E: Eluvial Horizon; B: Subsurface Horizon; C: Parent Material Horizon ; R: Bedrock Layer.

By excavating vertical soil profiles and collecting typical rock samples and corresponding surface soil samples from all strata within the study area, the influence of lithological variations on the geochemistry of selenium (Se) in residual soils was investigated (Fig. 3). A comparison of selenium content in the main strata rocks and soils revealed that soil selenium inherits from the rocks, which is the primary reason for the selenium enrichment in the soils. Furthermore, the study area’s Mesoproterozoic metamorphic rocks, such as carbonaceous slate and dolomite, are generally selenium-enriched and serve as the parent material for the soils. The region also has copper ore deposits, where sulfide minerals coexist with selenium minerals, leading to a higher selenium content in the rocks compared to other regions. After weathering, these rocks form soils that are relatively selenium-enriched. Thus, the selenium geochemistry in the soils is primarily influenced by the parent rocks, with lithological composition playing a significant role in the selenium enrichment process.

Fig. 3.

Influence of lithology change on geochemistry of Se element in residual soil.

Adsorption effect of clay minerals

Scanning electron microscopy (SEM) was employed to investigate the wild fungal rhizosphere soil. The soil was found to contain an assemblage of fine-grained minerals, comprising quartz, chlorite, illite, biotite, hematite, and sericite. The clay minerals in the soil exhibit significant adsorption, expansion, ion exchange, and viscosity properties, which play a crucial role in the migration and transformation of soil nutrients and heavy metals^18–20. X-ray Diffraction (XRD) analysis (Table 3) indicated that the clay minerals in the selenium-enriched wild mushroom rhizosphere soil were predominantly gibbsite, goethite, hematite, and illite, with trace amounts of chlorite and sodium feldspar. A correlation analysis between the clay mineral content and selenium concentration in the soil (Fig. 4) revealed a positive correlation, where an increase in the proportion of clay minerals led to a corresponding increase in the selenium content. This positive relationship is attributed to the ability of clay minerals to adsorb and retain selenium, which influences the mobility and bioavailability of selenium in the soil. The presence of clay minerals in the soil not only affects the effective selenium concentration but also facilitates selenium transformation into more bioavailable forms, thus promoting its accumulation in the soil. Therefore, the adsorptive properties of clay minerals play an essential role in enhancing selenium retention, mobility, and transformation, significantly contributing to the formation of selenium-enriched soils.

Table 3.

Statistics of clay mineral content of wild bacterial root soil in the study area.

Mineral Types	Gibbsite	Illite	Albite	Limonite	Hematite	Chlorite	Microcline	Sericite	Chlorite-Serpentine interlayer minerals	Illite-Serpentine interlayer minerals
Minimum	4	3	1	2	1	3	4	10	11	2
Maximum	15	9	7	5	3	7	5	18	21	4
Mean	8.79	6.13	4.13	3.00	1.67	3.73	0.60	3.93	4.87	1.00
Standard Deviation	3.36	1.64	1.70	0.85	0.62	1.45	0.71	3.59	4.62	1.00
CV/%	38.19	26.77	41.08	28.17	37.03	38.77	117.85	91.45	94.77	100.00

Fig. 4.

Correlation between clay mineral content and selenium content in soil.

Soil types

The primary soil types in the study area are red soil and yellow–brown soil. An independent sample t-test was performed to compare the selenium content in different soil types (Table 4). The result showed that the P-value was greater than 0.05, indicating that there was no significant difference in the mean selenium content between the two soil types. The study found that selenium is more enriched in yellow–brown soil. However, it was concluded that soil type has little influence on the selenium content in the soils of the study area. This suggests that other factors, such as geological background and clay mineral content, may play a more significant role in determining the selenium concentration in the soil.

Table 4.

Comparison of the mean selenium content of different soil types in the study area.

Soil type	Sample size	Selenium content range (mg·kg⁻1)	Mean (mg·kg⁻1)	Standard deviation	CV (%)
Argiudic ferrosols	59	0.15 ~ 0.76	0.38	0.14432	37.98
Hapliudicargosols	155	0.13 ~ 0.85	0.40	0.15801	39.50

Land use types

An ANOVA (Analysis of Variance) was conducted to analyze the selenium content in soils under different land use types (Table 5). The study revealed that the main land use types in the study area include shrubland, forested land, orchards, dryland, and paddy fields. The results showed significant differences in the selenium content across the different land use types (P < 0.05), with the mean selenium content following the order: shrubland > forested land > orchard > dryland > paddy fields. This indicates that the land use type in the study area has a significant impact on the enrichment and migration of selenium in the soil. The shrubland areas exhibit the highest selenium content, while paddy fields show the lowest, suggesting that land management practices and vegetation cover play a key role in the distribution and accumulation of selenium in the soil.

Table 5.

Comparison of soil selenium content in different land use areas in the study area.

Land use type	Sample size	Selenium content mean ((mg·kg⁻1)	Standard deviation	CV (%)
Paddy field	3	0.31	0.04359	14.06
Dryland	120	0.36	0.14227	39.18
Orchard	4	0.37	0.13326	35.77
Forested land	24	0.43	0.1463	34.05
Shrubland	63	0.45	0.16705	36.77

Soil physicochemical properties

A correlation analysis was conducted between soil organic matter, pH, TF₂O₃, Al₂O₃, K, S, and selenium content in the study area (Fig. 5). The pH values of the soils in the study area ranged from 3.98 to 7.42. Among the total samples, 93.46% of the soils were acidic, indicating that the overall soil pH is acidic. The analysis found a significant negative correlation (P < 0.01) between soil pH and selenium content. In acidic soils (pH < 6.5), the adsorption and fixation process results in higher selenium content in the soil. The organic matter content in the soils of the study area ranged from 3.793 to 217.224 mg·kg⁻¹, with an average value of 43.276 mg·kg⁻¹ and a coefficient of variation of 61.40%. This indicates that the organic matter content in the soils is rich but unevenly distributed. As shown in Fig. 5, a significant positive correlation (P < 0.01) exists between soil organic matter and selenium content. The correlation between soil TF₂O₃, Al₂O₃, K, S, and selenium content was also significant (P < 0.05), but the correlation coefficients were lower than those of soil organic matter and pH. These findings suggest that soil organic matter and pH play a more dominant role in influencing selenium enrichment in the soil, while other factors, such as soil mineral content, have a lesser but still significant impact.

Fig. 5.

Scatter diagram of correlation analysis matrix between soil physical and chemical properties and selenium content in the study area.SOM, soil organic matter.

Elevation

The results of one-way ANOVA showed that soil selenium content at different altitude elevations was significantly different (P < 0.01) (Fig. 6). The altitude of the study area ranged from 1900 to 2700 m. The effects of different altitude on soil selenium content were shown in Fig. 6, which indicated that soil selenium content increased with increasing altitude. At the elevation of 1900 ~ 2100 m, the surface soil selenium content was 0.275 mg·kg⁻¹; after the elevation increased to 2100 ~ 2300 m, the surface soil selenium content was 0.351 mg·kg⁻¹; at the elevation of 2300 ~ 2500 m the surface soil selenium content increased to 0.439 mg·kg⁻¹; at the elevation of 2500 ~ 2700 m the selenium content was higher, amounting to 0.536 mg·kg⁻¹.

Fig. 6.

Comparison of soil selenium content at different elevations in the study area.

Discussion

Geological background is one of the main controlling factors for soil selenium (Se) content, and variations in geological background can trigger spatial variations in soil selenium content^21,22.Some studies have demonstrated that soil selenium enrichment is primarily controlled by selenium-rich parent rocks, such as black shales and coal-bearing strata²³. In this study, the geological background of the Mesoproterozoic metamorphic rocks was found to be the primary reason for selenium enrichment in the soil of Maojie Town. The parent material in the study area contains relatively high selenium content, and during the soil formation process, the selenium is inherited from the parent rock. This finding is consistent with the results of other studies, which have demonstrated a close relationship between the selenium content in bedrock and the selenium content and distribution in the soil^10,24,25. Clay minerals play an important role in the ecosystem. Most clay minerals have high cation exchange capacity, large surface area, and large pore volume. The absorption of heavy metals by clay minerals is characterized by direct bonding between metal cations and the clay mineral surface, surface complexation, and ion exchange²⁶.The research findings of numerous scholars demonstrate that the total content of clay minerals in soil exhibits a significant positive correlation with soil nutrients and heavy metal elements. Similarly, our study results further confirm that clay minerals play a positive role in enhancing selenium enrichment within soil systems^12,27. Soil organic matter content in the study area is relatively high and shows a significant positive correlation with soil selenium content. This is consistent with other research findings. Li X²⁸ and colleagues explored the sources and controlling factors of selenium in selenium-rich soils in Yuanzhou in China’s selenium-deficient areas. They found that in the eastern and southern regions, selenium accumulation in surface soils was mainly due to the adsorption and accumulation of selenium by soil organic matter and the enrichment effect of clay minerals.

Soil pH is one of the factors affecting selenium content. pH indirectly changes the migration and transformation of selenium in soil by affecting the form and valence state of selenium. This, in turn, is influenced by biological and precipitation processes, which affect soil selenium content^27,29. The soils in the study area are generally acidic. Under such moist, acidic conditions, selenium exhibits high mobility and exists primarily as selenite. This selenite is subsequently stabilized through associations with metal oxides and organic matter, which represents a major mechanism for selenium enrichment in the local soils. Land use types significantly impact soil physicochemical properties, which may in turn have a profound effect on the biogeochemical cycling of elements^30,31. The land use type in the study area has a significant influence on the enrichment and migration of selenium in the soil. Most of the selenium-rich soils in the study area are found in shrubland and forest land, and soil selenium content tends to increase with altitude. This is because the temperature decreases with altitude, slowing down the decomposition rate of organic matter. As a result, most selenium in the soil is adsorbed and fixed by organic matter, reducing selenium loss and leading to enrichment in the surface soil. The study area’s geological characteristics, with its Mesoproterozoic low-grade schist facies metamorphic rocks, combined with the forest land as the main land use type, high vegetation cover, and Pinus yunnanensis as the main vegetation species, provide favorable conditions for the accumulation of selenium in surface soils in Maojie Town. These factors, together with high soil organic matter content, contribute to the significant selenium enrichment observed in the study area.

Maojie Town, located in China’s selenium-deficient area, exhibits a unique phenomenon of selenium-rich soils. To explore the selenium (Se) content in local wild mushrooms, samples were collected from selenium-rich soil regions. The wild mushroom species sampled include Boletus edulis, Russula virescens, Boletus subvelutipes, and the rare Tricholoma matsutake, among others. The Se content in these mushrooms was determined, and the results are shown in Fig. 7. According to the standard of GH/T 1135–2017 for selenium-rich agricultural products, edible fungi are considered selenium-rich when their total selenium content (dry weight basis) falls within the range of 0.1 to 5 mg·kg⁻¹. Based on the formula for dry weight (dry weight = wet weight/(1 + moisture content)), a total of 63 wild mushroom samples were collected, of which 49 samples met the selenium-rich criteria. This yields a selenium-rich rate of 77.8% in the wild mushroom samples (Fig. 7). The selenium content in these wild mushrooms ranged from 0.02 to 36.9 mg·kg⁻¹, with an average of 3.51 mg·kg⁻¹. The highest selenium content was found in the Sarcodon aspratus at 36.9 mg·kg⁻¹, which is notably high. In comparison, commonly consumed mushrooms such as Lentinula edodes ( 0.13 mg·kg⁻¹)³², Agaricus bisporus (0.07 mg·kg⁻¹), Pleurotus cornucopiae (0.06 mg·kg⁻¹), and Flammulina velutipes (0.01 mg·kg⁻¹) ³³have much lower selenium contents. The high selenium levels found in some of the wild mushrooms in Maojie Town suggest significant potential for the development and utilization of selenium-rich mushrooms. These findings highlight the considerable value of these wild mushrooms for the emerging selenium-rich wild mushroom industry. Maojie Town is known for its abundant wild mushrooms, which are delivered in large quantities to the Chuxiong Wild Mushroom Market each rainy season. The discovery of selenium-rich soil and wild mushrooms further provides a valuable opportunity for the growth of this industry.

Fig. 7.

Selenium content statistics of various wild mushrooms.

Overall, Maojie Town is located within the low selenium belt, and the existence of selenium-rich soil is a selenium-enrichment phenomenon formed through the fixation and adsorption of clay minerals and acidic soil, in the geological background of shallow metamorphic rocks of the Middle Metamorphic Realm, where the organic matter of the high-elevation woodland is rich and the decomposition rate is low. Maojie Town is Wuding County grain, tobacco, vegetables, livestock, flowers, fruits and other major agricultural production base and processing base, with the development of selenium-rich agricultural products with good conditions. It has been identified that other crops in the study area are not selenium-enriched agricultural products, and further in-depth research is needed in the development of selenium-enriched wild mushrooms, which needs to consider selenium bio efficacy, selenium polymerization capacity of different wild mushroom varieties, etc., and also needs to consider increasing N-rich, K-rich and other elemental fertilizers when enhancing soil fertility, using the positive interaction effect of different elements to increase the selenium uptake efficiency by the wild mushrooms^34–37. Therefore, it is recommended that local governments and relevant departments rationally utilize the existing natural resources, scientifically and rationally plan the development, utilization and protection of selenium-rich land, develop the cultivation and development of selenium-rich agricultural products on this basis, adjust the local agricultural planting structure, efficiently cultivate natural selenium-rich wild mushrooms, and convert resource advantages into economic advantages.

Conclusions

This study clarifies the genesis and spatial distribution of selenium (Se)-rich soils in Maojie Town, Wuding County. Geochemical analysis reveals a markedly heterogeneous Se distribution, with the mean soil Se content (0.397 mg·kg⁻¹) measuring 1.8 times the national average. The enrichment is predominantly concentrated in the central region.

Se enrichment stems from both geological and pedogenic origins. The primary source is the Mesoproterozoic low-grade metamorphic parent rock. Key contributing factors include the fixation of selenium—mainly present as mobile selenite under acidic soil conditions—by clay minerals and organic matter. Land use also plays an essential role, as the widespread forest and shrubland cover in higher-altitude areas aids in Se retention by slowing organic matter decomposition and enhancing adsorption capacity.

In summary, Se-rich soils in Maojie Town originate from inheritance of Se-bearing bedrock and are further enriched under the influence of acidic conditions, clay minerals, organic matter, and favorable land-use patterns. We recommend strategic zoning to protect the central Se-rich core area, with particular attention to forest ecosystems, to maintain organic matter accumulation and Se sequestration. Identified adequate- and high-Se zones offer promising potential for developing high-quality Se-enriched agricultural products. These findings provide a scientific basis for the sustainable utilization and economic development of local Se-rich land resources.

Author contributions

CD: Investigation, Methodology, Formal analysis, Writing-Original draft, Writing—Reviewing and Editing; JH: conceptualization and data interpretation; ZL: Conceptualization, Visualization, Methodology, Data curation, Writing-Original draft, ZH: Investigation, Methodology, Data curation, WC: Investigation, Validation, software,Resources, Funding acquisition, Supervision, Project administration, YZ: Funding acquisition, Supervision, YB: Project administration, Supervision, Writing-review & editing. All authors have read and agreed to the published version of the manuscript.

Data availability

The author confirms that all data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.