Effects of combined passivators and foliar inhibitors on soil properties
The application of combined passivators and foliar inhibitors into the red paddy soil generally significantly changed the soil's chemical properties (Table 1). All of the combined passivators and foliar inhibitors increased the soil pH as compared with the control (p < 0.05 for most of the treatments). Application of quicklime into acidic soil could effectively increase the soil pH because of the decrease in exchangeable acidic cations (Muhammad et al. 2020). Regarding other studied parameters of soil properties, the combined passivators and foliar inhibitors generally decreased TN, TP, and SOM in paddy soil as compared with the control (p < 0.05 for most of the treatments). Although the loss of TN and TP in soil could not be excluded, another important reason for the decrease in TN and TP contents after application of the combined passivators was attributed to rice's promotion of nitrogen and phosphorus utilisation (Watson et al. 2016; Cao et al. 2018; Mohammadi and Shariatmadari 2020). Although there was the introduction of organic components in some combined passivators, the decrease in SOM in all the treatments as compared with the control indicated that the combined passivators, especially the inorganic components, enhanced the mineralisation of soil organic carbon (Paradelo et al. 2015; Neto et al. 2021; Wu et al. 2021). The different from the aforementioned soil properties, most of the combined passivators had little influence on CEC in the paddy soil studied (p > 0.05 with few exceptions).
The significantly negative correlation between TP and soil pH (p < 0.05, Fig. 1) was most likely due to the increase in use efficiency of phosphorus by rice with increasing soil pH (Muhammad et al. 2020; Zhao et al. 2020). A significantly positive correlation was observed between the concentrations of SOM and TN in paddy soil (p < 0.001, Fig. 1). The consistent variation trend of TN and SOM was most likely due to the fact that TN in soil was largely related to the balance of accumulation and decomposition of SOM (Groffman et al. 2001).
Effects of combined passivators and foliar inhibitors on the availability of heavy metals in red paddy soil
The combined passivators and foliar inhibitors decreased the available Cd, Pb, and Cu by 14.29%-42.86% (except for QSi2), 10.18%-63.17%, and 6.95%-36.81% (except for QSi2) as compared with the control, respectively (p < 0.05, with the exception of very few cases; Table 2). Others have also reported the decrease in the availability of heavy metals in soils with the incorporation of passivators (He et al. 2021). For a given treatment, the trend of the concentration of available heavy metals was Pb > Cu > Cd, which was most likely related to their total contents in soil. From the perspective of application rates of combined passivators, the increase of Q1, QA1, and QS1 to Q2, QA2, and QS2 did not significantly change the available heavy metals in soil (p > 0.05, with the exception of Cd in Q1 and Q2; Table 2 and Table S2). Therefore, when combined with the quicklime, polyacrylamide, and sepiolite (QAS), the significant decrease of available heavy metals in soil with the increase of QAS1 to QAS2 (p < 0.05; Table 2 and Table S2) was most likely due to the interaction of polyacrylamide and sepiolite as they enhanced the immobilisation of heavy metals. Unlike the changes in available heavy metals in QAS-treated soils, the available heavy metals in the soils treated with QSi2 and QSe2 were significantly higher than those in the soils treated with QSi1 and QSe1 (p < 0.05; Table 2 and Table S2) (Li et al. 2020a; Wang et al. 2020). This observation was primary attributed to the fact that foliar Si- or Se-inhibitors prevented the uptake of heavy metals by rice, resulting in the relative high concentration of available heavy metals in soil (Wang et al. 2020; Hussain et al. 2020).
The correlations between the available heavy metals in red paddy soil and soil properties are presented in Fig. 1. The soils treated with the combination of quicklime and foliar inhibitor were excluded in this correlation analysis due to its different mechanism on heavy metals. As seen in Fig.1, there were significant negative correlations between available heavy metals and soil pH (p < 0.05 for Cd and Pb, and p < 0.01 for Cu), indicating the important role of increasing soil pH on the immobilisation of heavy metals in red paddy soil (Vrinceanu et al. 2019; Mohamed et al. 2015). The increase of soil pH after adding alkaline substances such as quicklime and sepiolite into acidic soil contributed to the release of hydroxyl ions during hydrolysis, thereby promoting the conversion of heavy metal cations to precipitates and residual fraction (Huang et al. 2019). Additionally, soil pH was also suggested as related to the adsorption capacity of soil colloid to heavy metals. It has been reported that the adsorption capacity of soil colloid to heavy metals increases with the rise of soil pH (Liu et al. 2017). The concentrations of available heavy metals were significantly positively correlated with SOM (p < 0.001 for Cd, and p < 0.01 for Pb and Cu), which indicates the important role of decreasing SOM on the immobilisation of heavy metals in soil (Liu et al. 2015). The effect of SOM on heavy metals could be due to bidentate sites where the SOM-binding sites controlled the release of heavy metals from soils (Liu et al. 2019). The significantly positive correlations between the available heavy metals (p < 0.05 for Cd, and p < 0.01 for Cu) and TP indicated the positive role of combined passivators in the stabilisation of heavy metals in red paddy soil studied, which was attributable to the formation of insoluble phosphate compounds through the reaction of water-soluble phosphate with heavy metal cations in soil (Jin et al. 2019; Li et al. 2020a; Li et al. 2021). These results indicate that the change in available heavy metals in soil was not only affected by soil pH, but also by other soil properties such as SOM and TP (Song et al. 2017).
The results from the rank of factors on available heavy metals in soil (Fig. 2) indicated that the dominant factors for available heavy metals were different depending on the types of the heavy metals. The first two dominant factors regulating the available Cd, Pb, and Cu were TN and TP, CEC and TP, and TP and SOM. Evidently, TP was one of the key factors on the three available heavy metals studied.
Effects of combined passivators and foliar inhibitors on the uptake of heavy metals by rice
For heavy metals-contaminated farmland soil, although the concentrations of total and available heavy metals in red paddy soil should not be ignored, the more critically important issue was to prevent and control rice's uptake of heavy metals in order to ensure food safety. From Table 2, the effect of the given combined passivators and foliar inhibitors on the uptake of heavy metals by rice was related to the types of heavy metals. Specifically, the combined passivators and foliar inhibitors significantly decreased rice's uptake of Cd by 22%-93% as compared with the control (p < 0.05 excluding QS2), but generally had little influence on rice's uptake of Pb (p > 0.05 excluding QA1 and QSe2). The variation of Cu in rice depended on the types and application rates of combined passivators and foliar inhibitors. For example, QA1, QSi1, QSi2, and QSe2 significantly decreased the uptake of Cu in rice (p < 0.05), while others had little influence on (p > 0.05) or significantly increased (p < 0.05) the uptake of Cu by rice.
The significantly positive correlation between available Cd in red paddy soil treated with combined passivators (excluding foliar Si- or Se-inhibitors) and the uptake of Cd by rice (p < 0.01) indicated that the decrease of available Cd in red paddy soil studied was one of the effective approaches to control the uptake of Cd (Khan et al. 2017; Jing et al. 2019; Lei et al. 2020; Yang et al. 2020; Huang et al. 2019). Unlike the observation of Cd, there was a significant negative correlation between available Pb in soil and the rice's uptake of Pb (p < 0.05), indicating that rice cannot uptake a part of chemical-extractable Pb (Zhang et al. 2016). There was no significant correlation between available Cu in soil and the uptake of Cu by rice (p > 0.05). Although the similar reason to the case of Pb could not be ruled out, the reason for finding Cu was most likely due to the fact that exogenous Cu was introduced into soil at varying degrees depending on the amount of Cu in different types and dosages of combined passivators (Table S2 and Table S4). Similar fluctuations were also observed after applying chicken or swine manure into soil (Wan et al. 2020).
With the increasing application dosage of QSi1 and QSe1 to QSi2 and QSe2, the uptake of heavy metals significantly decreased (p < 0.05 excluding Pb and Cu in soil treated with QSi). This observation was most likely due to the fact that Si and Se effectively inhibited the uptake of heavy metals because the increasing dosage of quicklime significantly increased their uptake (p < 0.05 with the exception of Pb; Table 2) (Li et al. 2020a; Wang et al. 2020). Foliar sprays of Si and Se can inhibit the transportation of heavy metals from stem to rice or reduce the bio-concentration factor of Cd in the roots of rice (Li et al. 2020a; Guo et al. 2021). The increase of Si or Se with the increasing application dosage of QSi or QSe further confirmed the antagonism of Si and Se with heavy metals in rice.
The results from the rank of factors on uptake of heavy metals by rice (Fig. 2) indicates that the amount of available Cd in soil primary controlled its uptake. This finding further confirmed that the decrease of available Cd in red paddy soil was one of the effective approaches to control the uptake of Cd, as mentioned in Section 3.3. The main factors affecting the uptake of Pb and Cu were pH and CEC, respectively. The result indicates that soil chemical properties in the acidic red paddy soil studied played a more important role in affecting the uptake of Pb and Cu by rice as compared with the concentration of available heavy metals assessed by chemical extraction.
The variation trend of the BCF of heavy metals was similar to that of heavy metal uptake (Table S3 and Table 2). The BCF of Cd in the control was as high as 87.39%, indicating that the rice under natural conditions in this study showed a superb uptake of Cd. The combined passivators and foliar inhibitors significantly decreased the BCF of Cd (p < 0.05 excluding QS2), indicating that combined passivators and foliar inhibitors effectively inhibited the transport of Cd from soil to rice. However, the combined passivators and foliar inhibitors had little influence on the BCF of Pb (p > 0.05 excluding QA1 and QSe2), indicating that the Pb translocation rate from soil to rice has not changed. This finding was different from a previous study that reported that the combination application of passivators and foliar inhibitors could suppress the formation of Fe/Mn plaques on rice roots and uptake by rice (Li et al. 2020b). The BCF of Cu increased significantly after applying Q2, QS, QAS, and QSe1 (p < 0.05), indicating the increased Cu translocation rate from soil to rice. Therefore, the combined passivators and foliar inhibitors should be used carefully for the safe utilisation of Cu-contaminated acidic red soil.
Microbial community response to the addition of combined passivators and foliar inhibitors into red paddy soil
The combined passivators and foliar inhibitors had little influence on the richness (ACE and Chao index) and diversity (Shannon and Simpson index) of bacteria in red paddy soil studied (p > 0.05, Fig. S2). However, combined passivators and foliar inhibitors had influence on the bacterial community at the phylum and genus level. The most abundant phylum of microbes found were Proteobacteria (15%–23%) and Chloroflexi (14%–25%), followed by Acidobacteria (11%–17%) and Actinobacteria (8%–12%) (Fig. 3). This result is in agreement with Wang et al. (2021) and Wen et al. (2020), who also found Proteobacteria and Acidobacteria as dominant phyla in soils in southern China and acidic red loam soil contaminated by heavy metals. As compared with the CK, the decrease in the relative abundances of Acidobacteria in most treatments was due to the increase in soil pH that was detrimental to the survival of Acidobacteria (Wang et al. 2021; Wan et al. 2020; Debnath et al. 2016). At the genus level, the top 53 genera of bacteria were analysed by bistercluster analysis (Fig. 4). The clustering results from different treatments, indicating that the genera of bacteria in the CK were similar to those in the soils treated with most of the treatments, with the exception of Q1, QA1, QSi1, and QSi2, indicating that most of the treatments did not dramatically change the genus composition of bacteria in the red paddy soil studied. The high similarity of genera of bacteria in soil treated with Q1, QA1, QSi1, and QSi2, which was evidently separated with the other treatment groups. This result indicated that bacteria at the genus level were more sensitive to the quicklime, polyacrylamide, and foliar Si-inhibitors (Liang et al. 2021; Watson et al. 2016).
The application of inorganic and/or organic passivators into soil influenced the composition of the bacterial community via the changes of soil physicochemical properties (Feng et al. 2020; Wang et al. 2021). The redundancy analysis (RDA) was used to examine the relationships between the soil physicochemical properties and the genera of bacteria (Fig. 5). The first two axes for the community composition of bacteria accounted for 23.21% and 13.43% of the total variations, respectively. At the genera level, many bacteria, such as Subgroup_18_norank, Vicinamibacterales_uncultured, and Subgroup_17_norank, each belonging to the phylum of Acidobacteria, had a negative correlation with the CEC but a positive correlation with TP. Xanthobacteraceae_uncultured, Xanthobacteraceae_unclassified, and SC–I–84_norank belonging to the phylum of Proteobacteria had a positive correlation with the CEC, but had a negative correlation with TP. In addition, pH had a positive correlation with RBG–13–54–9_norank, SBR1031_norank, and Anaerolineaceae_uncultured belonging to the phylum of Chloroflexi. These observations indicated that the changes in soil physical-chemical properties influenced the bacterial community structure at the genus level (Ren et al. 2018; Wang et al. 2021).
Fig. 5 displays the correlation between availability of heavy metals and genera of bacteria in paddy soil. The available heavy metals in red paddy soil studied were negatively correlated with the abundance of Thermodesulfovibrionia_uncultured, indicating the positive role of this genus in the stabilisation of heavy metals. The microbes in the genus of Thermodesulfovibrionia can reduce SO42 - to S2-, facilitating the formation of sulfide precipitation (Jiang et al. 2021). However, the available heavy metals are positively correlated with the abundance of Subgroup_17_norank, Vicinamibacterales_uncultured, and Subgroup_18_norank belonging to the phylum of Acidobacteria. The phylum of Acidobacteria is an acidophilic bacteria that survives better in an acidic environment and thereby has the ability to promote the release of heavy metal cations (Wang et al. 2021; Wen et al. 2020). The available Cd and Cu in red paddy soil studied is negatively correlated with the abundance of Xanthobacteraceae_uncultured, Xanthobacteraceae_unclassified, and SC–I–84_norank belonging to the phylum of Proteobacteria, which was similar to the results from Luo et al. (2019) who reported that Proteobacteria was sensitive to Cd. These results indicate that adding combined passivators might indirectly affect the heavy metals in the soil by changing the soil bacterial community at the genus levels.
Agricultural and environmental implications
The results in this study showed that applying combined passivators and foliar inhibitors (except for QS2) into the red paddy soil studied could reduce the concentrations of Cd in rice to permitted food-safe levels (< 0.2 mg kg−1), according to National Food Safety Standard of China (GB2762-2017) (MLRPRC 2014). Additionally, the application of combined passivators and foliar inhibitors did not lead to the other heavy metals being present in excess of standards. Among the combined passivators and foliar inhibitors, QA1, QSi2, and QSe2 showed the best effect for decrease in the uptake of Cd by rice in the red paddy soil studied. After taking the safety and stability of soil structure and function into consideration (Table 1 and Fig. 3), QSe2 (1875 kg·ha-1 Quicklime + 250 mL·mu-1 foliar selenium inhibitors) was the primary recommendation for remediating the heavy metals-contaminated red paddy soil, particularly for remediation of Cd-contaminated red paddy soil.
This study provided an efficient, inexpensive, and easy-to-operate strategy for remediating heavy metals-contaminated red soil in a paddy field. However, different from traditional knowledge, it found that foliar inhibitors may also affect soil's physical-chemical properties and soil bacterial community. Some combined passivators decreased the concentrations of available Cu in soil, but increased Cu uptake in rice. In addition, the change of bacterial community at the genus levels may benefit the stabilisation of heavy metals in red paddy soil. In the future, it is necessary to determine the pathways or mechanisms by which foliar inhibitors affect soil chemistry and microbial structure. Future monitoring is necessary to explore the long-term effects of combined passivators on the safe utilisation of heavy metals-contaminated red paddy soil. Additionally, further studies are needed to investigate the change mechanism of soil physical and chemical properties and to elucidate the interaction mechanism between heavy metals and microorganisms.