4.1 Effect on soil properties and Cd fraction
Prior research had indicated that the application of biochar-based amendments had an impact on soil properties (Awasthi et al., 2019; Bagheri Novair et al., 2023; Zhang et al., 2023; Zhong et al., 2023), and the findings were in accordance with our research. Our results indicated that soil properties including pH, TN, TP, CEC, SOC, and DOC were improved by applying biochar-based amendments.
Soil pH was recognized as a key determinant of metal solubility, which could directly influence the chemistry of soil Cd such as accumulation by plants and leachability (Lv et al., 2021). Leaching of heavy metals from soil is soil pH dependent. Increasing soil pH might lead to increased hydroxyl groups of metal cations and enhanced metal adsorption. Increased soil pH by treatments was due to the alkalinity of biochar-based amendments (Zhang et al., 2017). Our study found that soil pH showed an increasing trend with amounts of applied biochar (Fig. 2a). The surface of biochar contained negatively charged function groups, for instance, phenols, hydroxyl, amine, carboxyl, and mercaptan compounds, which were capable of binding hydrogen ions from soil solution and increase soil pH (Bagheri Novair et al., 2023; Nguyen et al., 2017). In addition, there was the possibility that biochar-based amendments could stimulate or enrich the activity of soil microorganisms, increasing soil pH (Lin et al., 2022; Xie et al., 2023; Ibrahim et al., 2021). Increased soil pH may also provide active sites enhancing Cd adsorption or immobilization. (Nguyen et al., 2023; Wei et al., 2023).
The value of Eh in the soil depended on the relative concentrations of oxidized and reduced substances in the soil solution (Chacón et al., 2020). The main factors affecting soil Eh were soil aeration, soil moisture, the metabolism of plant root, and the proportion of decomposable soil organic matter (Yang et al., 2021). According to this research, soil Eh decreased with increasing biochar contents (Fig. 2b). Xing et al. (2022) found that adding biochar to rice soil resulted in a broader range of soil Eh within 6 hrs (219 ~ + 334 mV) than control (-271 ~ + 391 mV), which had been confirmed by Yang et al., (2020). Klüpfel et al. (2014) and Rashid et al. (2022) indicated that biochar enhanced redox reaction by providing electrons through phenol group or accepting electrons through quinone aromatic structure. These results showed the critical role of biochar in mediating the redox environment soil. Much higher or lower soil Eh was not conducive to rice growth. Higher soil Eh presents a state of oxidation, a faster organic matter decomposition or nutrient leaching. Lower soil Eh could cause poor ventilation and restrict rice growth and development.
CEC is one of the critical soil properties that is strongly influenced by soil organic matter, which varies from < 1 cmol/kg in sandy soil to > 25 cmol/kg in clay soil (Arabi et al., 2018; Guibert et al., 1999). Our experiments revealed that applying of biochar-based amendments, especially those with high biochar content, significantly increased soil CEC (Fig. 2f). These results were consistent with findings found by Awasthi et al. (2019), Hussain Lahori et al. (2017), and Mohamed et al. (2015). Increasing soil CEC by biochar was a result of very porous character and high specific area, which subsequently increased the adsorption capacity (Liu et al., 2012).
Our data indicated that amendments significantly increased TN, TP, SOC, and DOC contents in soil (Fig. 2), which were consistent with published outcomes (Y. Li et al., 2022; Jin et al., 2019; Blanco-Canqui, 2017). This may be because biochars, ROF, and CMPF were enriched in carbon, nitrogen, and phosphorus, which provided a source of organic material and increased inputs of these compounds in soil (Arabi et al., 2018; Farhangi-Abriz et al., 2021). Previous research had also demonstrated that biochar could have a positive on crop growth and efficiency of nutrient availability (Azeem et al., 2023; Backer et al., 2016; Hagemann et al., 2017). Other surveys had demonstrated that biochar application could positively affect soil microorganisms and enzyme activity (Luis Moreno et al., 2022; Song et al., 2022; Zhu et al., 2022). Moreover, TN, TP, SOC, and DOC contents increased with biochar content, and as a result, rice yield increased (Fig. 5e). Therefore, it was necessary to conduct long-term experiments in multiple geographical sites to validate the impacts of application of biochar or derived materials on TN, TP, SOC, and DOC contents, in effort to provide reliable evidence for a large-scale application of biochar-based materials.
DTPA-Cd was the bioavailable form that could be absorbed and utilized by plants. In comparison with total soil Cd, DTPA-Cd had a greater impact on metal toxicity and uptake by plants. DTPA-Cd had an essential importance in managing Cd absorption by crops and forecasting the Cd toxicological effect in agroecosystems (Lahori et al., 2017; Wu et al., 2020; Qiao et al., 2015). In our study, the DTPA-Cd content of each treatment decreased rapidly in the early stage, increased slightly in the later stage, and finally stabilized. After harvest, the ultimate results showed that treatments decreased soil DTPA-Cd, in which only the effect of T1, T4, and T6 was significant. A comparable findings were presented by Hong et al. (2022) who found that co-application of biochar, apatite, and seaweed organic fertilizer at a ratio of 1.5:0.5:1 significantly reduced soil bioavailability of Cd, Cr, and Pb, enhanced soil conditions, improved nutrient availability and yields. Liang et al., (2022) revealed that simultaneous application of rape straw biochar, rice straw, and Chinese milk vetch was effectively reduced soil DTPA-Cd by 27.80% ~ 46.62%. Furthermore, Xiong et al. (2019) investigated that rice straw, biochar, and engineered bacteria P. putida X4/pIME compost applied as a soil amendment dramatically reduced Cd concentrations. This study also showed that biochar-based amendments decreased Ex-Cd in soil while increasing other Cd fractions, including Ca-Cd, Ox-Cd, and Re-Cd (Fig. 4). Similarly, Liu et al. (2022) as well as Xiong et al. (2019) revealed that biochar and biochar-compost lowered the Ex-Cd while increased other Cd fractions.
The potential explanation for reduction of DTPA-Cd in soil by biochar-based amendments could be explained as that biochar-based amendments contained alkaline particles and produced OH− groups that could bind Cd ions from soil solution to form insoluble complexes, thus reducing Cd bioavailability. Also, the biochar-based amendments contained high amounts of Ca2+ and Mg2+ ions and large surface area that could interact with soluble Cd to enhance Cd adsorption or immobilization (Singh et al., 2023; Zanin Lima et al., 2023; Fan et al., 2020). pH was a prominent factor regulating the effectiveness of Cd, which showed a negative correlation to DTPA-Cd. It was reported that elevated soil pH caused an increased negative surface charge of soil, leading to facilitated surface precipitation, such as CdCO3 or Cd(OH)2, and decreased DTPA-Cd (Liang et al., 2022; Suzuki et al., 2020; Xiao et al., 2014). According to our research, soil pH was higher in biochar treatments than CK, which was beneficial in reducing Cd mobilization and effectiveness. Furthermore, the addition of biochar-based amendments could result in strengthening Cd complexation due to improved soil CEC and SOM, thereby contributing to the reduction of soil DTPA-Cd (Alaboudi et al., 2019). Additional studies had indicated that some soil microorganisms had the ability to absorb Cd. It might be possible that the addition of biochar-based amendments could enrich this group of soil microorganisms and therefore reduced the Cd content in soil (Qi et al., 2023; Haider et al., 2021; Mei et al., 2022; Wang et al., 2024; Zuo et al., 2022; Chen et al., 2023).
4.2 Effect on rice yield and tissue Cd content
According to data of rice growth in Table 2, yield components in Table 3, and biomass in Fig. 5, both maize straw and bamboo biochar-based amendments significantly increased rice yield and yield components as well as dry biomass. This result was in agreement with those reported by Mohamed et al. (2015), Arabi et al. (2018), and Li et al. (2019). Whereas coconut shell activated carbon-based amendments severely reduced rice yield and caused the whitening of leaves during the seedling stage, which was opposite to Břendová et al. (2016) who utilized biochar and coconut shell activated carbon to reduce the phytoavailability and phytotoxicity of Cd, Pb, and Zn and reported that both carbon-based amendments increased spinach dry biomass by 114% and 55% at non-contaminated soil, 359% and 308% at contaminated soil. Therefore, the effectiveness of coconut shell activated carbon needs to be further validated. It is hypothesized that high adsorption capacity of coconut shell activated carbon might reduce nutrient availability for rice growth.
Biochar exhibited strong adsorption of nutrients, such as ammonium, nitrate, phosphate, potassium, and trace elements, which could retain nutrients in soil and reduce the loss from soil, enhancing more nutrient uptake by plants and improving growth and yield (Lv et al., 2021). Furthermore, biochar-based amendments improved soil properties by increasing soil pH and CEC, and lowering soil Eh, which would enhance nutrient efficiency, especially phosphorus and potassium (Alvarez et al., 2017), reduce nutrient leaching or loss from soil (Agegnehu et al., 2016), and slow down soil organic matter decomposing (Xing et al., 2022). Furthermore, biochar-based amendments would enrich soil bacterial communities, stimulate the activities of enzymes, and improve soil health and rice growth (Hong et al., 2022; Ibrahim et al., 2021; Lin et al., 2022; Luis Moreno et al., 2022).
This study demonstrated that Cd uptake and accumulation were dominated by roots, and subsequently by shoots, leaves and finally the brown rice grain (Fig. 6), and maize straw biochar-based amendments were more effective than bamboo and coconut shell activated carbon in reduction of the Cd concentration in brown rice. T2 treatment achieved the best reduction of brown rice Cd by 71.77%. Liang et al. (2022) revealed that simultaneous application of Chinese vetch, rice straw, rape straw biochar, and iron-modified biochar remarkably reduced 65.38% or 62.65% of Cd concentration in rice grain, respectively. A comparable study was reported by Hong et al. (2022) who applied a combination of seaweed organic fertilizer, apatite, and biochar that reduced Cd, Pb, and Cr by 68.9%, 68.9%, and 65.7% in maize grain, respectively. The analysis showed that the incorporation of biochar, ROF, CMPF, and MFA dramatically restrained Cd uptake by plants from soil. From the information in Table 4 indicated that BCFsoil−root was reduced by all treatments, illustrating that biochar addition was more effective in decreasing Cd enrichment in brown rice as TFleaf−rice > TFroot−shoot > TFshoot−rice.
Cd transport from roots to brown rice grain was predominantly through shoots and leaves (Zhou et al., 2018). Biochar-based amendments inhibited Cd enrichment in various tissues of rice, which was most likely because of the reduction of DTPA-Cd in soil. DTPA-Cd was found positively correlated with Cd concentration in rice grain, and biochar-based amendments significantly lowered soil DTPA-Cd and raised Re-Cd. Thus, the biochar amendments reduced Cd uptake in rice by decreasing the bioavailability and transport of Cd in soil (Sun et al., 2016). Furthermore, the development of Fe/Mn plaque was an effective barrier to prevent metal absorption by roots (Huang et al., 2022). Additionally, increased Fe2+ or Mn2+ might compete with Cd for the adsorption sites, responsible for decreased Cd movement and storage in rice (Cai et al., 2020).
4.3 Effect on root Fe/Mn plaque
Fe/Mn plaque formed on the root surface could act as an effective barrier to prevent metal absorption by roots and decrease Cd accumulation in tissues (Huang et al., 2022; Liang et al., 2022). The formation of Fe/Mn plaques was primarily influenced by Fe and Mn concentration, soil oxidation-reduction potential, oxygen concentration, and temperature (Lin et al., 2017; Wu et al., 2016). Previous studies had shown that the incorporation of organic materials, such as organic fertilizers, biochar, and livestock manure, could stimulate the generation of Fe/Mn plaques (Li et al., 2020; Liu et al., 2021). According to our research, applying biochar-based amendments enhanced the Fe/Mn plaques content, which simultaneously reduced the Cd concentration in the edible portion of rice (Fig. 7; Fig. 6d). This was because enriched Fe2+ and Mn2+ in biochar-based amendments were released into soil solution, and the Fe/Mn plaque formation was enhanced under improved soil redox conditions as induced by the biochar amendments. Application of biochar-based amendments had modified soil microenvironment, thereby promoting Fe release and contributing to Fe and Mn plaque formation (Zhou et al., 2020). Additionally, soil pH and Eh also affect the formation of Fe/Mn plaque. Soil pH showed a highly positive correlation with Fe2+ and Mn2+ concentrations in soil, while a negative correlated with Eh. Under soil condition of near neutral or higher pH and lower Eh, the predominance of iron-reducing bacteria was enhanced, which resulted in higher Fe2+ concentration and promoted the Fe/Mn plaque formation (Tong et al., 2021; T. Liang et al., 2022; Kong et al., 2023; Zheng et al., 2024).
4.4 Potential mechanism of biochar-based amendments on soil Cd immobilization, Cd accumulation in rice grain, and rice yield increased
The distribution of DTPA-Cd in soil directly affects the crop uptake and utilization of Cd. In our research, biochar-based amendments reduced the DTPA-Cd content in soil. Therefore, it is important to explore its potential passivation mechanism. Figure 10 illustrated the potential mechanism of biochar-based amendments on soil Cd immobilization, Cd accumulation in rice grain, and rice yield increased. This study showed that biochar-based amendments decreased Ex-Cd in soil and shifted it to a more stable morphology (Fig. 4), as a result, DTPA-Cd was reduced. Alternatively, the formation of Fe/Mn plaque on the root surface reduced the accumulation of Cd in rice grains (Fig. 7; Fig. 6d). Biochar-based amendment material contained rich nutrients that contributed to the crop yield.