One of the first things we needed to briefly calculate was the theoretical input of HMs caused by the conditioner. In the above field tests, the general tillage depth was about 20 cm, and based on 667 m
2 per mu, the actual cultivated soil volume was 133.4 m
3. The general soil bulk density was 1.1~1.4 g/cm
3 (choosing 1.3 g/cm
3). Therefore, the actual cultivated soil was 173,420 kg/mu, and the maximum application amount of soil conditioner was 100 kg/mu, only accounting for 0.06%. Below,
Table 6 shows the test results of five HMs in fly ash-based soil conditioner, and
Table 7 shows the theoretical increase in soil heavy metal concentrations due to the application of soil conditioner within three years and China’s risk control standard for soil contamination of agriculture land of
GB 15618-2018.
As can be seen from the table above, when the tillage depth is 20 cm and the application amount of soil conditioner is 100 kg/mu, it was equivalent that 2.90 g Cr, 4.14 g As, 0.025 g Cd, 0.023 g Hg and 1.51 g Pb would subsequently enter the 173,420 kg soil. Based on cumulative input over the past three years, it could be calculated that the theoretical increases of soil heavy metal contents were only 0.050 mg/kg, 0.072 mg/kg, 0.00044 mg/kg, 0.00039 mg/kg and 0.026 mg/kg, respectively, which were far below the soil environmental quality assessment indicator limits of
GB 15618-2018. It can be preliminarily inferred that the potential contamination risk of HMs in the fly ash-based soil conditioner being transferred into the soil is negligible and can be ignored.
Table 8 and
Table 9 indicate that the application of soil conditioner for three consecutive years did not significantly increase the heavy metal contents in the castano-cinnamon soil and dark brown soil. Of course, more specific calculation and analysis of HMs in corn plants have been done as well.
3.2.1. Contents of Heavy Metals in Corn Plants
Table 10 below shows the test results of heavy metal contents in corn on the test field of castano-cinnamon soil.
As is shown in the
Table 10, compared with conventional fertilization, there was no significant increase of Cr, As, Cd, Hg, and Pb in the root, stem, and lamina of corn after the application of the soil conditioner in the test field of castano-cinnamon soil. At the root, the contents of Cr, As and Cd increased slightly by less than 29.5% and the content of Hg remained unchanged, while the content of Pb increased significantly by 98.1%. In general, the application of soil conditioner on castano-cinnamon soil would not significantly enhance the absorption and enrichment of Cr, As, Cd and Hg by the root of corn. At the stem, the contents of these five HMs all decreased to varying degrees. As for the HMs in lamina, the content of Cr decreased significantly by 39.7%, and the contents of the remaining HMs were basically flat. Overall, these five HMs were not significantly enriched in any part of the corn after the soil conditioner had been applied in the test field of castano-cinnamon soil. There is no environmental risk for returning straw to the field or for raising livestock.
Below,
Table 11 shows the test results of heavy metal contents in corn on the test field of dark brown soil.
From the data in the table above, compared with conventional fertilization, it can be seen that in the dark brown soil test field, there was no significant increase of Cr, As, Cd, Hg and Pb in the root, stem, and lamina of corn after the application of the soil conditioner. At the root, the contents of these five HMs were basically flat. The content of Cr increased by only 1.2%, while the contents of As, Cd, Hg, and Pb showed a downward trend. Generally speaking, the application of soil conditioner on dark brown soil had no obvious effect on the corn root for absorption and enrichment of these five HMs. At the stem, the contents of As, Cd, Hg, and Pb seemed to be basically flat, while the content of Cr increased slightly. Similarly, the contents of these five HMs in the lamina were almost unchanged. Overall, these five HMs were not significantly enriched in any parts of the corn after the soil conditioner had been applied in the test field of dark brown soil. There is no environmental risk for returning straw to the field or for raising livestock.
In view of the overall results, the application of fly ash-based soil conditioner in the two test fields had no significant effect on the absorption and enrichment of Cr, As, Cd and Hg by corn. The application of soil conditioner in the castano-cinnamon soil test field might increase the absorption and enrichment of Pb by the root of corn. In fact, Pb is most likely to migrate under acidic conditions, while the soil conditioner is alkaline. Theoretically, the application of soil conditioner could inhibit the migration of Pb. As a result, the root of corn grown on dark brown soil reduced the absorption of Pb. Furthermore, the behavior of Pb in the shoot part of the corn was consistent with that of Cr, As, Cd and Hg, whose contents basically remained unchanged without any significant increase, or even decreased. Therefore, it was speculated that the substantial increase of Pb in corn root is caused by experimental errors. Subsequent detection of heavy metal contents in corn grains has also been conducted.
3.2.2. Soil Conditioner Affects Transfer of Heavy Metals in Corn
BCF is defined as the ratio of the metal contents in plant parts (including root, stem and lamina) to the exchangeable metal contents in the soil [
20]. Larger BCF usually indicates a greater ability of one metal to migrate from soil to plant parts [
21]. TF is the ratio of the metal contents in the shoot part (including stem, lamina and fruit) of the plant to the metal contents in the root [
20]. The uptake of HMs by plants may be related to the bioavailability of these metals and the physic-chemical properties of the soil [
22]. Based on the research by Mujtaba et al. [
13], lower TF value always represents a lower degree of food chain enrichment and lower risk of heavy metal contamination. By calculating BCF and TF values and comparing their changes with different treatments, the effects of soil conditioner on HMs transfer can be explored to assess the risk of HMs contamination.
Figure 2 and
Figure 3 show the BCF and TF values of corn grown in castano-cinnamon soil.
As is shown in the
Figure 2, after applying the soil conditioner to the castano-cinnamon soil test field, the BCF
root (BCF between root and soil), BCF
stem (BCF between stem and soil) and BCF
lam (BCF between lamina and soil) of Cr and As showed a downward trend, with a decrease of 83% to 4%. The BCF
root, BCF
stem and BCF
lam (BCFs) of Hg reminded almost unchanged. As for Pb, it was previously speculated that the experimental error led to an artificial increase of Pb in corn root, which further led to the increase of the BCF
root of Pb. The BCF
root and BCF
lam of Cd increased significantly, while the BCF
stem of Cd dropped sharply. In fact, the differences of Cd contents in corn root and lamina are not obvious. It was speculated that the difference in soil background was the main reason for the increase of the BCF
root and BCF
lam of Cd.
From the data shown in
Figure 3, it can be seen that after the application of soil conditioner in the castano-cinnamon test field, the TF values of these five HMs in corn all changed significantly. On the whole, the TF
stem (TF between stem and root) of Pb, Cd, As and Cr decreased sharply, with a decrease range of 93% to 47%, while the TF
stem of Hg remained almost unchanged. The TF
lam (TF between lamina and root) of Cr, As, Cd and Pb showed a downward trend, with a decrease of 45% to 19%, and the descending order was Pb > Cr > As > Cd. The TF
lam of Hg rose slightly by 16%, which might be related to diverse cultivation methods, environmental differences, the special transportation mode of Hg in corn and experimental errors.
Figure 4 and
Figure 5 show the BCF and TF values of corn grown in dark brown soil.
As is shown in the
Figure 4, after applying the soil conditioner to the dark brown soil test field, the BCF
root, BCF
stem and BCF
lam (BCFs) of Pb, Cr, As and Cd basically showed a downward trend, with a decrease range of 26% to −12%. The BCFs of Hg dropped significantly, with a decrease of more than 38%. From the data shown in
Figure 5, it can be seen that after applying the conditioner in the dark brown soil test field, a large proportion of the TF values of these five HMs in corn did not change significantly. Specifically, TF
stem of Cr, Cd and Hg increased slightly with a range of 25% to 17%, while the TF
stem of As and Pb rose by 60% and 40%, respectively, probably due to lower background concentration and experimental errors. TF
lam of As, Pb, Cr and Hg in corn was almost unchanged, with a variation range of 7.8% to −1.7%. Although the TF
lam of Cd changed the most, its growth was only 11%.
In view of the overall results, the application of the soil conditioner had a significant effect on the BCF and TF values of As, Pb, Cr, Cd and Hg in the castano-cinnamon soil test field, in which the TF
stem and TF
lam (TFs) values showed a downward trend. Excluding the effects of sampling and testing errors, the BCFs of these five HMs should theoretically remain unchanged or even decrease. The reason for this is that the soil conditioner agent is alkaline and has a strong adsorption capacity, which can firmly immobilize the HMs in the soil [
13]. The decrease in BCFs and TFs means that the application of soil conditioner limits the transfer of HMs in soil, as well as corn, and reduces the risk of heavy metal contamination. The soil conditioner contains a large amount of active silicon, which is an important element for building plants. Also, silicon can promote plant synthesis and release chelating agents [
23], thereby forming hydroxyaluminosilicate precipitation to limit the transfer of HMs [
24]. The increased content of silicon in corn stalks, which were specifically tested, and the changes in the TFs of HMs also corroborate this view. In the dark brown soil test field, the BCFs of these five HMs all showed a downward trend, while the TFs of these five HMs remained basically flat or slightly increased with the application of the soil conditioner (deviations were within 10%). After excluding some sampling and testing errors, the landform of the test field and the change of corn mass were analyzed. As a result, it is possible that the inclined terrain and natural rainfall caused the conditioner not to be completely absorbed in the plot, which would reduce the ability of the soil conditioner to limit HMs transfer.