Effects of S. indica on soybean growth and development
Cd has a extremely inhibitory effect on the growth and development of plants. S. indica can alleviate the stress of Cd on plants. In this research, the addition of exogenous Cd decreased the dry weight and plant height of soybean and the total dry weight (6.97 g) and the plant height (30.47 cm) were the lowest in the HH treatments. However, S. indica treatments increased the dry weight and plant height of soybean and promoted the growth and development of Soybean (Table. 2), which are consistent with the findings of Wu et al. (2018). The study of Yun Ping et al. (2018) found that under salt alkali stress, the S. indica could regulate the transfer rate of K to the aboveground part, which was beneficial to the growth and development of plants. Thus we suspected that the increase of soybean biomass was due to the fact that S. indica promoted the absorption of nutrients and photosynthesis to improve the adaptability of host plants to Cd stress.
Photosynthesis plays a decisive role in the growth and development of plants. Therefore, plants will have withering and yellow leaves and other growth performance in the presence of Cd pollution in the soil. The inhibition of Cd on photosynthesis was the decrease of photosynthetic pigment and stomatal number. At present, the research on the effect of S. indica on plant photosynthesis mainly focused on non stomatal conductance factors such as chlorophyll synthesis, PSII photochemistry and electron transport rate. However, Pn, Tr, Ci and Gs rate can also be used to estimate the photosynthetic physiological process of plants (Hui et al. 2015; Malkowski et al. 2020). There were many reports proved that S. indica can help plant resist saline alkali stress not only from promoting plant nutrient absorption, but also from improving stomatal conductance to grow and develop, which is good agreement with the results of ours (Yun et al. 2018; Liu et al. 2020).
Our result expounded that compared with the control, treatments with exogenous Cd significantly decreased Pn, Tr, Gs, and Ci (Table 2). This change may be caused by the decrease of stomatal conductance and the obstruction of CO2 entering leaves when soybeans were stressed by Cd, which led the decrease of Pn, which was the stomatal limitation of photosynthesis (Liu et al. 2020). The Ci value of the treatment with inoculation of S. indica decreased may be because CO2 was the raw material of photosynthesis, the less CO2 between cells, the more carbon dioxide consumed in photosynthesis, the greater Pn, which was opposite to Tsai et al. (2020). In the study of rice, S. indica can reduce the toxic effect of Cd by stomatal closure and oxidative stress reduction. The main reason for this result may be that the host selected was different (Klichowska et al. 2019). There was research also indicated that the decrease of photosynthesis under Cd stress was related to the decrease of carboxylase and ribose 1, 5-diphosphate carboxylase activities (Song et al. 2019). However, the application of S. indica enhanced Gs, Pn, Tr of soybean (Table 2). S. indica can effectively improve the stability of Cd and reduce the inhibition of Cd on photosynthesis.
Effects of S. indica on the activities of soil enzymes under Cd stress
The soil enzyme is an important biocatalyst in soil, which reveals ecosystem perturbations and plays an irreplaceable role in the detoxification process of pollutants. It can also facilitate the biogeochemical cycle of nutrients, maintain soil structure, and produce the necessary compounds for microorganisms and plants (Gelsomino et al. 2006; Topac et al. 2009; Hu et al. 2014). Many studies demonstrated that Cd had adverse effects on soil enzyme activities (Ali et al. 2020). On the one hand, pollutants inhibit enzyme activity by silencing catalytic active groups that led to protein conformational denaturation. On the other hand, pollutants may compete with the enzyme's substrate, thus hindering the enzyme from functioning (Kizilkaya and Bayrakli 2005). Among the different soil enzymes, soil urease, sucrase and phosphatase are often used to evaluate the nutrient absorption of plants and organic matter transformation, and catalase was often used to evaluate the detoxification ability of soil ecosystem.
In this study, we also observed that Cd contamination had adverse effects on soil enzyme activities. The activities of urease, sucrase and catalase in the LH, MH and HH treatments were significantly lower than those of control. The decrease of soil enzyme activities was due to the increase of heavy metal content and decreased pH value (Fig. 5). A result showed that the soil enzyme activities decreased with the increase of heavy metal concentration, and Cd inhibited the activities of urease and catalase (Wang et al. 2020). On the one hand, the reason for S. indica to played a role might be that S. indica promoted the growth of soybean (Table 2), stimulated the secretion of plant root metabolites, and directly enhanced soil enzyme activity. Similar to endophytic plant growth promoting bacteria and arbuscular mycorrhizal fungi, S. indica might improve the activity of urease, sucrase and phosphatase in soil to absorb sufficient C / N / P from soil, and significantly increased soybean biomass (Table 2), which can make the root system absorb and accumulate Cd; on the other hand, S. indica stimulated soil microorganisms, which increased the biomass and activity of microorganisms, and indirectly increased the activity of soil enzymes. The effect of microorganism on soil enzyme activity is more complex. We observed that the soil enzyme activity decreased with the increase of Cd concentration, and plants inoculation with S. indica in the root, which increased soil enzyme activities, in turn. The phosphatase and catalase activities in soybean soil, which inoculated S. indica were significantly increased under MH. It is similar to the conclusion of Xiao et al. (2021), which in highly Cd-polluted soils, Trifolium repens with mycorrhizal inoculation and straw treatment, phosphatase activity and catalase activity were promoted, and reduced Cd toxicity via a dilution effect.
Heavy metal ecotoxicity and soil enzyme activity are closely related to soil physical and chemical properties (Heidari et al. 2020), especially can be significantly affected by soil pH value. RDA results demonstrated a positive correlation between soil enzyme activity and soil pH value. That is, the decrease of soil pH value could represent the adverse effect of Cd on soil enzyme activity. The change of soil pH value in S. indica treatment was the result of multiple factors. The increase of Cd content led to more organic acids secreted by plant roots, decreased soil pH and increased metal availability, making plants absorb more heavy metals (Zeng et al. 2020). Notably, in HH treatments, the soil pH was enhanced from 7.60 to 7.68 with S. indica, indicating that S. indica may reduce the content of soil organic acid to effectively inhibit of soil acidification process. (Fig. 5d). The research of Yang et al. (2020) found that the citric and malic acids in rhizosphere soil of inoculating AMF were significantly higher than the control under Cd stress. Besides, the decrease of exchangeable Cd form may be due to the significant reduction of organic acid release from soybean roots by S. indica (Fig. 4).
Effects of S. indica on soil Cd fraction
The toxicity of heavy metals was mainly related to the exchangeable form, which was highly mobile and easy to enter into plants. On the contrary, carbonate form and reducible iron and manganese form were relatively stable components, which were not easy to enter the plants (Liu et al. 2014). Figure 3 illustrated that the Cd component has been gradually transformed into a more stable component from an exchangeable form and effectively immobilized in the soil. Also, the changes of chemical forms of Cd indicated that the application of S. indica could significantly reduce the bioavailability of Cd.
Moreover, the form of Cd in soil and the proportion of various forms are the key factors to determine its impact on the environment and the surrounding ecosystem (Lee et al. 2015). Previous studies also reported that the exchangeable fraction of Cd accounted for the highest proportion of Cd concentration in soil (Nemati et al. 2011). Generally, soil pH, organic matter and redox conditions all affected the forms of heavy metals in soil, but pH is the most important factor. It was found that the decrease of pH value by only 0.2 unit will lead to the increase of exchangeable Cd by 3–5 times (Zhu et al. 2016; Meng et al. 2020). The results showed that the application of S. indica to soils could contribute to higher pH, which increased the contents of carbonate bound and residual form Cd to immobilize Cd in the soil. According to the single pollution index method, under the treatment of 10 mg / kg Cd, inoculation of S. indica could effectively reduce the level of Cd pollution, indicating that the use of S. indica could reduce the risk of Cd pollution in the soil.
These results were confrmed by RDA analysis. The chemical forms of Cd in soil were negatively correlated with the content of DW, SH and photosynthetic parameters of soybean, indicating that the accumulation of Cd in soil greatly inhibited the physiological indexes of soybean. Indeed, the photosynthetic parameters of soybean was positively correlated with urease, sucrase, phosphatase and catalase in soil. In this study, S. indica caused a positive effect on the growth and development of soybean to attenuate the toxic effects of Cd on soybean, and ultimately enhanced soil enzyme activities of soybean soil to reduce the accumulation of Cd.