The static environmental capacity and existing environmental capacity of soil reflected the maximum carrying capacity of heavy metal pollutants in a static state (Lv et al., 2020). The calculation formula (Equations 1, 2) was as follows: W b = M × C ic − C ib × 10 − 6 (1) W i = M × C ic − C ip × 10 − 6 (2) where W _b and W _i are the static environmental capacity and existing environmental capacity of heavy metal element i in cultivated soil, respectively, in kg·hm⁻². M is the mass of the 0–20 cm tillage layer soil per hectare of land, with an empirical value of 2.25 × 10⁶ kg·hm⁻² (Fan et al., 2022). C _ic , C _ib and C _ip are the critical value, regional background value, and measured value of heavy metal element i in soil at monitoring points, respectively, with units of mg·kg⁻¹. Most of the dry land in the county comes from the transformation of paddy fields, so the critical values in the study are based on the screening values in GB 15618-201823 (Ministry of Ecology and Environment of the People’s Repulic of ChinaState Administration for Maket Regulation, 2018), as shown in Table 1.

The dynamic environmental capacity of soil refers to its annual carrying capacity for heavy metals during the processes of heavy metal migration, transformation, and self purification (Eli et al., 2020). The calculation formula (Equation 3) is as follows: W n = M × C i n − C i p × K n × 1 − K K × 1 − K n × 10 − 6 (3) where W _n is the annual dynamic environmental capacity of heavy metal element i in the soil, measured in kg·hm⁻². M is the mass of the 0–20 cm tillage layer soil per hectare of land, with an empirical value of 2.25 × 10⁶ kg·hm⁻². C _in is the critical value of heavy metal element i in the soil after n years, measured in mg·kg^-1. C _ip is the measured content of heavy metal element i at the monitoring point, measured in mg·kg⁻¹. K is the residual coefficient, representing the ratio of the amount of a given element remaining in the soil after 1 year of cultivation to the sum of the previous year’s soil content and the current year’s input. Based on previous research and parameter settings, the value of K in this study is set to 0.9 (Liu et al., 2016; Yu et al., 2009). n is the control period.

The comprehensive index method calculates pollution sub indices based on measured values and evaluation criteria, and then uses these sub indices to calculate the comprehensive index (Cheng et al., 2011). The calculation formula (Equations 4, 5) is as follows: P i = W i W b (4) P I = 1 n ∑ i = 1 n P i (5) where P _i is the single environmental capacity index of heavy metal element i in soil; W _i and W _b are the existing and static environmental capacities of heavy metal element i in soil, respectively, measured in kg·hm^-2. n is the number of types of elements, PI is the comprehensive index of environmental capacity. The evaluation criteria for soil heavy metal environmental capacity are based on the environmental capacity index, as shown in Table 2.

The enrichment factor method evaluates the impact of human activities on heavy metal enrichment by calculating the ratio of heavy metal element concentration to reference element concentration in soil (Hernandez et al., 2003). The calculation formula (Equation 6) is: E F = C i / C S i o 2 B i / B S i o 2 (6) where C _i and B _i are the measured concentration and background concentration of heavy metal element i in the soil, respectively, measured in mg·kg⁻¹. C S i o 2 and B S i o 2 are the measured concentration and background concentration of SiO₂ in the soil, measured in mg·kg⁻¹. The grading criteria for enrichment factors (Sutherland, 2000) are shown in Table 3.

Based on the background and critical values of heavy metals in the soil of the research area, combined with the soil environmental capacity calculation model, the static and existing environmental capacity of heavy metals in cultivated soil could be calculated. The static environmental capacity of heavy metals in cultivated land within the research area, from high to low, was as follows: Cr > Zn > Pb > Ni > Cu > As > Hg > Cd. The static environmental capacities of each element were 428.92, 271.20, 146.70, 98.94, 63.60, 44.00, 0.96, and 0.32 kg·hm⁻², respectively. According to Table 5, the ranking of the existing capacity and static capacity of heavy metals in cultivated land was the same, and the average existing capacity was as follows: Cr 428.77, Zn 239.99, Pb 140.56, Ni 93.19, Cu 55.12, As 31.56, Hg 0.90, Cd −0.35 kg·hm⁻². The environmental capacity values of different elements varied greatly, with Cr, Pb, Ni, and Hg having the highest proportion of existing environmental capacity and static environmental capacity. Their average existing capacity accounts for 99.97%, 95.82%, 94.19%, and 93.97% of the total static environmental capacity, respectively; The next best ones are Zn, Cu, and As, whose average existing capacity to total static environmental capacity ratios are 88.49%, 86.68%, and 71.72%, respectively; the worst was Cd, with a negative average existing capacity, indicating that the Cd in cultivated land has exceeded the soil’s carrying capacity as a whole. The existing capacity reduction of each heavy metal was as follows: Cr 0.15, Zn 31.21, Pb 6.14, Ni 5.75, Cu 8.47, As 12.44, Hg 0.06, Cd 0.67 kg·hm⁻². In terms of the reduction amplitude, Cd had the most significant reduction, reaching 208.6%.

The statistical analysis of the static environmental capacity and the existing environmental capacity of the two different land use types, dry land and paddy field in the cultivated land was also conducted in this study. The average static environmental capacity of heavy metals in dry land was in the order of: Cr 425.77, Zn 268.05, Pb 138.96, Ni 94.07, Cu 6.08, As 44.32, Hg 0.95, Cd 0.29 kg·hm⁻². According to Table 6, the reduction amount of the existing environmental capacity of heavy metals in dry land was: Cr 0.16, Zn 27.5, Pb 5.42, Ni 4.37, Cu 9.84, As 22.17, Hg 003, Cd 0.39 kg·hm⁻², among which the reduction of Cd was the most significant, reaching 134.48%; The average static environmental capacity of heavy metals in paddy fields was: Cr 430.03, Zn 272.31 Pb 149.43, Ni 100.67, Cu 64.49, As 43.89,g 0.96, Cd 0.33 kg·hm⁻². According to Table 6, the reduction amount of the existing environmental capacity heavy metals in paddy fields was: Cr 0.15, Zn 31.51, Pb 6.39, Ni 0.24, Cu 7.99, As 9.00, Hg 0.07, Cd 0.77 kg·hm⁻², among which the reduction of Cd was the most significant, reaching 233.33%.

According to the results of the independent sample t-test shown in Table 6, the means of Cd, Pb, Ni and Cu in the static capacity of the paddy field were significantly higher than those in the dry land (p < 0.05). However, in terms of the existing capacity, the mean of Cd in the paddy field was significantly lower than that in the dry land (p < 0.05), while the contents of As and Pb in the paddy field were significantly higher than those in the dry land (p < 0.05). No significant differences were found for the capacity of other elements. In addition, the reduction range of the main pollution element Cd in the paddy field was significantly greater than that in the dry land (p < 0.05). The results also revealed that there were points in the study area where the existing environmental capacity of heavy metals in some soils is negative, indicating that the concentration of heavy metals in some areas’ soils had exceeded their environmental carrying capacity, posing potential pollution risks.

A systematic analysis of the environmental capacity of heavy metals in cultivated soil in the study area was conducted using the comprehensive index method. The environmental capacity indices and comprehensive environmental capacity indices of various heavy metals were obtained, and the capacity indices were classified into levels. Subsequently, ArcGIS software was used to assign neighboring point values to the environmental capacity indices of various heavy metals, generating spatial distribution maps of the environmental capacity indices of various heavy metals in all soil patches of cultivated land in the study area. According to the statistical results of the point environmental capacity index, significant differences were observed in the environmental capacity indices of Cr, Pb, Hg, Ni, Zn, Cu, As, and Cd within the study area. The average environmental capacity indices of these 8 heavy metals are 1.00, 0.96, 0.94, 0.96, 0.89, 0.86, 0.70, and −1.05, respectively. The average value of the comprehensive environmental capacity index PI is 0.66, indicating that the overall heavy metal environmental capacity of the soil in the study area is relatively low and the pollution is severe. Specifically, the capacity indices of Cr, Pb, Hg, Ni, Zn, and Cu are between 0.7 and 1, belonging to the medium capacity region, indicating that these elements are generally mildly polluted. The capacity index of As is 0.70, approaching the upper limit of the low capacity area, indicating severe pollution. The capacity index of Cd is −1.05, located in the overload zone, and the pollution exceeds the risk benchmark. The study also showed (Figure 5) that, except for Cd, the environmental capacity levels of other heavy metals are mainly concentrated in high-capacity and medium capacity areas, accounting for 75% of the total area. Among them, Cr has the largest proportion of high-capacity and medium capacity areas, reaching 99.26%. The order of spot area from large to small is Cr > Pb > Hg > Ni > Zn > Cu > As. In contrast, the environmental capacity level of Cd is mainly distributed in high-capacity and overloaded areas, accounting for 70.84% of the total area.

The soil Cr environmental capacity in the research area was mainly composed of high capacity and medium capacity zones (Figure 6Cr), accounting for 52.36% and 46.90% of the total area, respectively. The areas of low capacity zone, alert zone, and overload zone were relatively small, accounting for 0.64%, 0.04% and 0.06% respectively. The excessive area was located in the southern part of Fangcun Town, and the warning area was located in Xindu Industrial Park, Jinchuan Street, with a local distribution and extremely small area. The low capacity areas were locally distributed in the northern part of Fangcun Town, Yawu Village in Huibu Town, the eastern part of Tonggong Township, the periphery of Xindu Industrial Park in Jinchuan Street, the southern part of Qingshi Town, and the southeastern part of Qiuchuan Town. High capacity areas were widely distributed in the eastern part of Qiuchuan Town, Tonggong Township, Jinchuan Street, Qingshi Town, Zhaoxian Town, and most areas of Dong’an Township.

The areas of high and medium capacity Pb in the soil of the research area accounted for 49.22% and 45.79%, respectively. Their distribution was closely related to the geological background and was widely distributed in areas such as Qiuchuan Town, Hejia Township, Qingshi Town, and Zhaoxian Town. The low capacity area accounted for 9.61%, which mainly concentrated in Qiuchuan Town, Hejia Township, Fangcun Town, and Xinchang Township. The areas of overload zone and warning zone accounted for 0.78% and 0.76% respectively, which mainly distributed in the rock contact zone and surrounding areas of Xinchang Township, Fangcun Town, Tonggong Township and other places (Figure 6Pb).

The high and medium capacity areas of soil Hg accounted for 46.49% and 46.11% respectively, while the low capacity areas accounted for 6.12%, mainly distributed in Qiuchuan Town, Hejia Township, Tonggong Township, Huibu Town, Zigang Street, and Fangcun Town. The overloaded area and warning area accounted for 0.79% and 0.49% respectively, mainly located in Qiuchuan Town, Hejia Township, and Huibu Town (Figure 6Hg). These areas were mostly located around five large industrial parks or near industrial facilities such as cement plants and lime plants.

The high and medium capacity areas of Ni in the research area accounted for 52.05% and 34.57% respectively, while the low capacity area accounted for 10.17%, which had been mainly distributed in Baishi Town, Hejia Township, Huibu Town, Qingshi Town, Zhaoxian Town, Fangcun Town, and Xinchang Township. The areas of overload zone and warning zone were 1.60% and 1.61% respectively, similar to the distribution of low capacity zone, but with a smaller area. The lithology was mainly concentrated in the Ordovician Fangyan Formation, Changwu Formation, and Black Rock Series (Figure 6Ni).

The high and medium capacity areas of soil Zn in the research area respectively accounted for 50.49% and 34.16%, which were widely distributed in areas such as Qiuchuan Town, Hejia Township, Qingshi Town, and Zhaoxian Town. The low capacity area accounts for 9.61%, mainly concentrated in Baishi Town, Hejia Township, Qingshi Town, Tonggong Township, Huibu Town, Zhaoxian Town, Fangcun Town, and Xinchang Township (Figure 6Zn). The areas of overload zone and warning zone are 2.61% and 3.13% respectively, which are distributed in the same way as the low capacity zone, concentrated in the Cambrian and Ordovician strata, rock contact zones, and industrial areas. The areas of Qiuchuan Town, Hejia Township, and Huibu Town are located near five large industrial parks and are distributed in blocks.

The high capacity and medium capacity areas of soil Cu in the research area accounted for 49.47% and 30.70% respectively, with large areas distributed in various townships. The areas of low capacity zones accounted for 12.51%, with scattered distribution in various towns and villages, and the area was relatively small. The areas of the warning zone and overload zone accounted for 2.70% and 4.61% respectively, mainly concentrated in Qiuchuan Town, Qingshi Town, Tonggong Township, Jinchuan Street, Tianma Street, Huibu Town, Zhaoxian Town, Fangcun Town, and Xinchang Township (see Figure 6Cu).

The areas of high and medium capacity As in the soil of the study area accounted for 48.27% and 26.67%, respectively. Their distribution areas are similar to Pb elements and are closely related to the geological background. The low capacity area accounted for 13.09% and was widely distributed, appearing in all towns except Dong’an Township and Qingshi Town. The overloaded area and warning area accounted for 7.80% and 4.15% respectively, mainly distributed in the northern part of Qiuchuan Town and Hejia Township, the eastern part of Tonggong Township, the western and southeastern parts of Huibu Town, the northern part of Zigang Street, Fangcun Town and Xinchang Township (see Figure 6As). These areas are mainly concentrated near the contact zone between black rock formations and rock masses.

The areas of high and medium capacity Cd in the soil accounted for 42.15% and 13.24% respectively, mainly distributed in Qiuchuan Town and Tonggong Township, and also distributed in other towns. The areas of low capacity zones, warning zones, and overloaded zones accounted for 9.82%, 6.10%, and 28.69%, respectively, and were also distributed in various towns and villages. Among them, Cd overload areas were widely distributed, with a focus on Huibu Town, Hejia Township, Qingshi Town, Jinchuan Street, Zhaoxian Town, Fangcun Town, and Xinchang Township (Figure 6Cd). The high-value areas were mainly located in the Cambrian and Ordovician strata, as well as around five large industrial areas, mines, and related enterprises, presenting a blocky distribution.

Tn the research area, the average pH value of the sampling points was 5.57, and the measured average value of Cd was 0.53 mg·kg^-1, which had exceeded the critical value set by the paddy field screening value (5.5<pH ≤ 6.5) in the GB 15618–2018 standard. Therefore, based on this condition, the dynamic environmental capacity of heavy metal Cd was not calculated. According to the data in Table 7, there were differences in the dynamic environmental capacity of heavy metals in the soil of the study area under different control years. In the same year, the dynamic environmental capacity of the seven heavy metals in the study area was ranked from high to low as Cr > Zn > Pb > Ni > Cu > As > Hg, and the order of dynamic environmental capacity was consistent with that of static environmental capacity. In addition, the order of cumulative reduction rate of dynamic environmental capacity is Hg > Cr > As > Pb > Cd > Ni > Zn > Cu.

With the passage of time, the rate of decrease in the dynamic environmental capacity of the seven heavy metals also decreased gradually with the increase of control years, showing a significant downward trend in the first 20 years. After that, the changes gradually stabilized and eventually showed a relatively consistent rate of decrease. According to the prediction, when the control period reaches 100 years, the dynamic environmental capacities of Cr, Zn, Pb, Ni, Cu, As, and Hg in the soil would be 64.11, 51.61, 24.13, 17.62, 13.72, 7.34, and 0.13 kg·hm⁻², respectively.

This study also analyzed the dynamic environmental capacity of two land use types, dryland and paddy field, in cultivated land, with a focus on the cumulative reduction rates of these seven heavy metal elements (Figure 7). The results showed that the dynamic environmental capacity of these 7 heavy metal elements decreased year by year over time, and the cumulative reduction rate of Hg was the highest, exceeding 85%. There are differences in the ranking of heavy metal accumulation reduction rates between paddy fields and dry land. The cumulative reduction rate of paddy fields, in order of magnitude, is as follows: Hg > Cr > Pb > Ni > As > Zn > Cu, The order of arid land is: Hg > Cr > Pb > Ni > Zn > As > Cu. Specifically, the cumulative reduction rate of As in paddy fields was significantly higher than that in dry land (p < 0.05).

Through the analysis of the static and existing environmental capacity of heavy metals in the soil of the study area, the results show that the static environmental capacity of each heavy metal in the study area is generally greater than the existing environmental capacity, indicating that the input rate of pollutants exceeds the self-purification capacity of the soil. Among all analyzed elements, the static environmental capacity of Cr, Zn, Pb, Ni, and Cu is much greater than that of Cd, Hg, and As, with Cr having an existing environmental capacity 1340 times that of Cd. This indicates that the soil has a strong ability to accommodate elements such as Cr, Zn, Pb, Ni, and Cu, but a low sensitivity, while having a lower ability to accommodate elements such as Cd, Hg, and As, but a higher sensitivity. When the input amount of these elements increases slightly with high sensitivity, it may quickly destroy the existing state of the soil, leading to varying degrees of pollution, especially Cd, whose sensitivity is most obvious. Overall, the difference between the existing environmental capacity and the static environmental capacity is generally small, and some heavy metals have approached or exceeded the soil’s carrying capacity, especially Cd, whose existing capacity is negative, indicating that overloading pollution has occurred in the region. This phenomenon is consistent with the research results of Tong et al. (2011). On the soil of the Baili Rhododendron mining area in Guizhou, indicating that Cd has strong mobility in the soil, resulting in a much greater decrease in its existing capacity than other heavy metals. In addition, the existing capacity of heavy metals such as Pb, Ni, and As has also decreased, indicating that the accumulation trend of these heavy metals is more severe in some areas, especially in areas with intensive industrial activities. This phenomenon is consistent with the research results of Yan et al. (2024) on soil in industrial pollution areas, which indicate that industrial pollution sources are the main factor leading to a decrease in heavy metal capacity.

The research results indicate that there is a significant difference in the static environmental capacity of heavy metals between dryland and paddy fields. In terms of existing capacity, the Cd existing capacity in paddy fields is much lower than that in dry land, which may be related to the slightly acidic pH value of paddy soil. Acidic environment can increase the solubility of Cd, leading to enhanced adsorption capacity of paddy soil for this element (Chen et al., 2022). Similar to the study by Deng et al. (2024), paddy soil has a stronger ability to fix heavy metals due to its higher organic matter content and special hydrological conditions. The difference is that the existing capacity of As and Pb in paddy fields is higher than that in dry land, which may be due to leaching in paddy fields promoting the migration of these heavy metals to the lower soil layers, resulting in overall lower levels of these two heavy metals in paddy fields than in dry land.

The enrichment degree of different heavy metals significantly affects the environmental capacity of soil. The enrichment factors of elements such as Cr, Zn, Pb, Hg, and Ni are relatively low, exhibiting high environmental capacity and light pollution levels, mainly influenced by natural background factors, especially the significant impact of geological conditions on Pb and Ni. In contrast, Cd has a higher enrichment factor and exhibits a significant overload phenomenon. Zhang et al. (2021) found that the active states of Cd (water-soluble, ion exchanged, and carbonate bound) in weathered black shale soils in Zhejiang Province are relatively high, indicating their strong migration ability and environmental activity. Especially in industrial areas and specific geological backgrounds, the environmental capacity of Cd is severely limited, and the risk of pollution increases. The environmental capacity index of As and Cu is relatively low, especially in areas with frequent agricultural and industrial activities, where pollution pressure is high. The study by An et al. (2022) also pointed out that the overloading phenomenon of Cd is particularly prominent in areas with dense industrial and mining activities, indicating that the environmental risk of this element is relatively high in industrial activity areas. Therefore, heavy metals with higher enrichment levels (such as Cd, As, and Cu) are more prone to overloading and require special attention, while heavy metals with lower enrichment levels (such as Cr, Zn, Pb, Hg, and Ni) exhibit higher environmental capacity and lower pollution risk.

The impact of industrial activities on the environmental capacity of heavy metals in soil is particularly significant. The enrichment of heavy metals such as Hg, Cd, Cu, etc. is mainly concentrated near industrial parks and mines. Combined with source apportionment, these elements are closely related to atmospheric deposition, industrial emissions, and mining activities. Especially Hg is mainly from industrial sources, and its warning and overload areas are concentrated in industrial activity areas, reflecting the strong impact of industrial emissions and atmospheric deposition. Previous studies have also shown that in black rock formations affected by human activities such as mining development, the heavy metal content in the soil is significantly higher than the natural background values in undeveloped areas (Wang, 2023). Meanwhile, this study further found that Cu and Cd are greatly affected by emissions from high polluting industries such as metallurgy and chemical engineering. The discharge of mining and industrial waste further exacerbates the enrichment of these heavy metals in the soil, resulting in a significant reduction in soil environmental capacity in these areas.

Agricultural fertilization is one of the important factors affecting the environmental capacity of heavy metals in soil. The organic fertilizers commonly used in study area exceed the standard content of Cu and Cd, and long-term fertilization leads to the acceleration of the accumulation of these heavy metals in the soil, especially in with frequent agricultural activities. This study also found that agricultural activities significantly affected the capacity of Cd and Cu, especially in areas where organic fertilizers containing Cd and Cu were, the phenomenon of overloading of Cd and Cu was significantly intensified, resulting in a significant decrease in the existing capacity. Therefore, the fertilizer management in agricultural activities urgently need to be improved to avoid further accumulation of heavy metals.

Different geological backgrounds have a significant impact on the environmental capacity of soil heavy metals. Due to the variation of heavy metal elements released during rock weathering due to differences in lithology, the content of heavy metals in soil and environmental capacity are affected. The research area is widely distributed with black rock formations rich in heavy metals, mainly consisting of dark gray and gray black thin layer carbonaceous siliceous rocks, mudstone siliceous rocks, and siliceous mudstones. The statistical results show that the average current environmental capacity of Cd, Ni, Cu, and Zn in this black rock series are −1.79, 72.23, 26.81, and 178.84 kg·hm⁻², respectively, which are significantly lower than the average current environmental capacity of the study area, which are −5.12, 0.78, 0.49, and 0.75 times higher, respectively. Among them, the mudstone of the Xiyangshan Formation in the Upper Cambrian is rich in As, and the current environmental capacity of As in its formation is −54.98 kg·hm⁻², indicating an overloaded area, which is much lower than the current environmental capacity of As in the study area of 31.56 kg·hm⁻². Combining the distribution of the black rock series (Figure 1) with the spatial distribution of the soil environmental capacity index (Figure 6), it could be clearly seen that there is a significant correspondence between the environmental capacity overload areas of heavy metals such as Cd in the soil and the exposed areas of the black rock series strata, while As mainly corresponds to the limestone of the Xiyangshan Formation. This further proves that the main source of heavy metals such as Cd, Ni, Cu, and Zn in the soil of the study area is the parent material of the black rock series. These rocks release a significant amount of heavy metal elements during weathering, resulting in high soil background values and low environmental capacity, which can easily lead to overloading. On the contrary, rocks such as sandstone, mudstone, and siltstone that are poor in heavy metals generate soils with lower background values, larger environmental capacity, and can withstand more pollution inputs. In addition, special geological structures such as mining areas or rock contact zones are rich in heavy metals, and the soil in these areas has a higher natural content, making the environmental capacity more susceptible to limitations. The weathering and sedimentation processes of rocks also have a significant impact on the migration and enrichment of heavy metals. Rocks rich in metals have a faster weathering rate and accumulate more heavy metals, leading to faster consumption of environmental capacity. Overall, lithology determines the background value of heavy metals in soil, thereby affecting its environmental capacity and pollution carrying capacity.

Research has shown that the soil heavy metal environmental capacity index in the study area exhibits significant spatial heterogeneity, with some areas reaching or exceeding overload levels, especially in industrial and agricultural activity intensive areas such as Huibu Town, Jinchuan Street, and Fangcun Town. The distribution of elemental environmental capacity index is highly consistent with the source apportionment results. For example, the mixed source characteristics of elements such as Cd, Ni, Cu, Zn, etc. are consistent with the overloaded and low capacity areas in the distribution of environmental capacity index, mainly concentrated in high geological background strata and human activity areas, especially within a 2-km range of large industrial parks and mines and other industrial activity areas. The analysis of As, Pb, and Zn sources reflects the characteristics of natural sources, with overloaded and low capacity areas distributed at the northeast and southwest rock mass contact surfaces and near the Xiyangshan Formation limestone. The source analysis of Cr and Ni indicates that they are a mixture of natural and industrial sources. The environmental capacity index map shows that low capacity and overloaded areas are concentrated in hilly areas, while valley and plain areas are lower. The Hg element mainly comes from industrial sources, and its overloaded and low capacity areas are concentrated around industrial parks and large industrial and mining enterprises. Based on this, the study suggests taking targeted pollution prevention and control measures in these high-risk areas, such as controlling industrial emissions, repairing mining environments, and strengthening agricultural pollution prevention and control, to reduce the negative impact of heavy metals on soil environment and ensure the sustainability of soil quality.