Study on the variation of THg content in different types of soils with time
The relationship of THg content in the five types of soils with time is shown in Fig. 1. All five types of soils were contaminated with different degrees of Hg by the smelting activities of the surrounding non-ferrous metal plants, and the THg concentrations showed three different degrees. With the increase in incubation time, the overall Hg concentration of different types of soils showed a trend of decrease-increase-decrease again. Nevertheless, the periods of rising and falling THg content were completely different for various land sorts. The Hg concentration in dryland soils showed little change in the first 20 d, around 1.5 mg·kg− 1, while it suddenly increased to 3.15 mg·kg− 1 at d 25 and dropped back to 1.32 mg·kg− 1 at d 30; the Hg concentrations in paddy soils, cultivated wasteland soils and riparian wasteland soils exhibited a tendency of first falling, then rising and then falling, reaching the maximum value of 3.03 mg·kg − 1, 11.83 mg·kg − 1 and 49.79 mg·kg − 1 at d 25, respectively; the overall variation of Hg concentration in riparian soils was not significant in the first 20 d, and showed an overall increase before decrease, with the maximum value of 7.11 mg·kg − 1 within the 25th
In general, THg concentrations in soils of different land types under the same incubation conditions showed a decreasing-increasing-decreasing fluctuating variation with incubation time. But the time period of rising and falling THg content varies by land type. The dry field soil, paddy field soil and riparian soil are all distant from the smelter and are less affected by the smelter. However, the riparian soil has the highest relative THg content among the three soils due to the influence of Hg-containing river scouring by the pathway smelter.
The 30th d THg concentrations of the five soils, in descending order, were: wasteland soils (38.31 mg·kg− 1) arable wasteland soils (10.22 mg·kg− 1) riparian soils (5.51 mg·kg− 1) paddy soils (2.63 mg·kg− 1) dryland soils (1.32 mg·kg− 1), with the same distance from the soil and the colored metallurgical zone The relationship was the same.
Variation of MeHg content in soil with water content and land type
The relationship between MeHg content in the five types of soils with time is shown in Fig. 2. As shown in the figure, the five types of soils The MeHg content in paddy soil, cultivated wasteland soil, riparian soil, and riparian wasteland soil reached the maximum value (0.364 µg·kg− 1, 0.137 µg·kg− 1, 0.532 µg·kg− 1, and 4.002 µg·kg− 1 ) at the 15th day of incubation, and also the MeHg content in dryland soil reached the maximum value (0.278 µg·kg− 1 ) at the 25th day of incubation. All five different types of soil MeHg contents exhibited a tendency of increasing and then decreasing. The changes in MeHg content in soils weren't stricken by exogenous imports, that the increase in MeHg content was principally because of in place methylation of soils.
Relative to the original soil samples, Methylmercury levels in the soil were raised, indicating that pro-Hg methylation reactions were mainly carried out in the soil. Both paddy and dryland soils were agricultural soils in a local village, but the paddy soil was under flooding conditions for a long time, which made the soil Hg methylation higher than that of the dryland soil, which is consistent with the results of Zhou et al. for upland soils(Zhou Xin-Quan et al., 2022) The highest MeHg growth rate (0–30 d) for the five soils (33.3%) was found in paddy soils indicating that local agricultural irrigation activities affect soil Hg methylation rates. The MeHg increment was generally higher in flooded soils than in non-flooded environments, and the anaerobic soil environment in flooded environments suggests that anaerobic environments are more conducive to Hg methylation in soils than aerobic environments. On the one hand, the anaerobic environment provides a suitable living environment for Hg methylation microorganisms, which increases the activity of microorganisms involved in Hg methylation and the utilization of inorganic Hg in sediment by microorganisms, leading to the accumulation of MeHg in sediment( Zhu Jinshan et al., 2018); On the other hand, Hg in the soil is more easily dissolved in an anaerobic environment, resulting in more inorganic Hg available for Hg methylation(Poulin et al., 2016).
SRB content in soil and its effect on MeHg content
As shown in Fig. 3, the SRB counts of all five soils showed different degrees of increase and decrease, and the SRB content was higher in the riparian wasteland soil and the paddy field soil, which showed that SRB was suitable for survival under flooding anaerobic and high THg concentration. In the two groups of soils with low THg concentrations, the maximum SRB count in dryland soil occurred at d 15 and in paddy soil at d 10; in the two groups of soils with high THg concentrations, the maximum SRB count in arable wasteland soil occurred at d 5 and in riparian soil at d 10; in the soil with the highest THg concentration, the maximum SRB count in riparian wasteland soil The maximum number of SRB in riparian heathland soils with the highest THg concentration occurred at d 30.
The correlations between 30-d MeHg concentrations and SRB counts for the five different utilization types of soils are shown in Table 2. In the first 20 d, there was no correlation between MeHg concentrations and SRB counts for each soil, indicating that changes in soil MeHg concentrations in the first 20 d were not related to changes in SRB counts; while the two indices showed significant positive correlations in the soil at 25 d and highly significant positive correlations at 30 d, indicating that changes in soil MeHg concentration were correlated with the change of SRB number. If the increase in MeHg yield improved the correlation with the amount of SRB, it indicates an increase in the velocity of the enzymatic methylation reaction of Hg at this time and an increase in the metabolic capacity of SRB itself.
Table 2
The Pearson correlation analysis for 30-day MeHg levels and SRB counts in soil
Soil incubation time (days)
|
5
|
10
|
15
|
20
|
25
|
30
|
Correlation coefficient
|
-0.399
|
-0.489
|
-0.362
|
0.117
|
0.903*
|
0.955**
|
Influence of MeHg levels in soil on the relationship between soil physicochemical properties
As shown in Fig. 4, the MeHg content of the five different utilization types of soils showed different correlations with pH, organic matter, and THg under the same incubation conditions. All five different utilization types of soils showed positive correlations with pH. Among them, MeHg content in paddy soil and riparian soil demonstrated a significant positive relationship with pH (p < 0.05); MeHg content in arable wasteland soil demonstrated a highly significant positive relationship with pH (p < 0.01), indicating that pH played a significant effect on Hg methylation in the above three soil types; dryland soil showed a positive relationship with organic matter, while the other four Dryland soils showed positive correlations with organic matter, while the remaining four showed negative correlations, but none of them was significant (p > 0.05), indicating that organic matter was not a major effect on Hg methylation in the five soils; dryland soils showed significant positive relationship with THg, the MeHg levels of riparian soils was not strongly correlated with THg, and the remaining three soils showed negative correlations with THg. This indicates that the methylation mechanism of Hg is affected by different soil utilization methods as well as the environment they are located.
It has been proved that (Gerbig, Kim, Stegemeier, Ryan, & Aiken, 2011; Qiu, Guangle, Feng, Xinbin, Wang, Shaofeng, & Shang, Lihai, 2006), that soluble sulfide in soil has a great influence on MeHg concentration in soil, and sulfide can inhibit MeHg formation in soil because soluble sulfide can combine with Hg(II) in the soil to form insoluble mercury sulfide, which makes it difficult for the methylation microorganism of mercury to use, thus reducing MeHg formation. In this experiment, there was no significant correlation between MeHg content and sulfide in paddy soils under flooded conditions (p > 0.05), while the remaining four soils showed a negative correlation between MeHg content and soluble sulfide, among which, dryland soils and cultivated wasteland soils showed a significant negative correlation, this is consistent with the findings of Caroline E. Pierce on the effect of sulfide on mercury methylation in peatland soils(Pierce et al., 2022). The increase in sulfide may be due to the respiration of SRB, which converts soil sulfate to sulfide(Benoit, Gilmour, & Mason, 2001), which reduced the available state of Hg in the soil and inhibited the production of MeHg in the soil.