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Article

Comparison of the Effects of Different Organic Amendments on the Immobilization and Phytoavailability of Lead

by
Xiaojie Wang
1,2,
Jingwen Chen
1,3,
Jiahui An
1,
Xueping Wang
1 and
Yun Shao
1,*
1
College of Life Sciences, Henan Normal University, Xinxiang 453007, China
2
School of Biological Engineering, Xinxiang University, Xinxiang 453003, China
3
School of Education, Luoyang Vocational College of Culture and Tourism, Luoyang 471000, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(7), 2981; https://doi.org/10.3390/su16072981
Submission received: 23 January 2024 / Revised: 20 March 2024 / Accepted: 28 March 2024 / Published: 3 April 2024
(This article belongs to the Section Sustainable Agriculture)
Browse Figures
Versions Notes

Abstract

:
Organic materials, such as straw, animal manure, and their processed product biochar, are known to exhibit agronomic effects and the ability to remediate heavy metal contamination. However, knowledge regarding the relative effects of different organic amendments in soils on heavy metal immobilization and phytoavailability remain limited. Consequently, the effects of maize straw (MS), chicken manure (CM), mushroom cultivation waste (MW), and sawdust biochar (SB) on the immobilization and phytoavailability of lead (Pb) in wheat plants were investigated in this study using pot experiments. The results showed that the artificial application of Pb reduced soil pH, while increasing the total organic carbon (TOC) and cation exchange capacity (CEC) to various extents. Furthermore, the Pb treatment increased the adsorption of Pb by wheat grains (0.83 mg∙kg−1), resulting in decreased above-ground dry biomass (43.16 g∙pot−1) during the maturity growth period when compared with the control check (CK) treatment. The MS + Pb and CM + Pb treatments increased the exchangeable Pb fractions in the soil, but had a limited effect on Pb accumulation in wheat grains compared with the Pb treatment. In contrast, the SB + Pb treatment effectively increased soil pH and TOC, while decreasing the fraction of exchangeable Pb forms and increasing the oxidizable and residual Pb fractions, compared with the Pb treatment. Moreover, the MW + Pb treatment also increased the soil pH and CEC, displaying the potential to increase soil TOC, in addition to substantially modifying the portioning of Pb from exchangeable forms to less bioavailable fractions. Both the MW and SB amendments significantly reduced Pb concentrations in wheat grains (0.49 and 0.70 mg∙kg−1,∙respectively), resulting in increased above-ground dry biomass (51.59 and 54.12 g∙pot−1, respectively). In summary, the application of organic amendments, especially MW, could be an effective measure for enhancing Pb immobilization in polluted soils, thereby reducing its uptake and translocation to crops.

1. Introduction

Pb is non-essential, highly toxic, and non-biodegradable element [1,2]; therefore, the uncontrolled introduction of Pb into soil via artificial and natural activities is harmful to plants and humans. Artificial activities, such as mining and industrial processes, can release large amounts of Pb into the environment, thereby contaminating soil [3]. Natural activities, such as the weathering of rocks and the deposition of atmospheric heavy metals, can also release Pb into soil [4]. In addition, Pb accumulated in soils can be consumed by humans via food chains, where it accumulates in the nervous, blood, kidney, and immune systems, although low concentrations can also increase ecosystem and organ health risks [5]. Therefore, measures are needed to reduce the toxicity and mobility of Pb in order to reduce its ecological risks and bioaccumulation in plants.
Several technologies for remediating heavy metal-contaminated soils have been developed in recent decades, including soil cleaning, capping, bioremediation, electrokinetic remediation, thermal desorption, chemical precipitation, ion exchange, adsorption, biosorption processes, and combined remediation technology [6,7]. However, the application of these methods in soils with large-scale metal contamination is not feasible due to high costs and the destruction of soil structures. Therefore, alternative, lower-cost strategies are applied for the reduction of the mobility and toxicity of heavy metals, including soil stabilization and immobilization.
Organic amendments have played important roles in improving soil properties, modifying soil architecture, increasing nutrition utilization efficiency, and controlling soil contamination [8]. Most organic amendments are derived from agricultural and forestry materials, including maize straw (MS), chicken manure (CM), mushroom cultivation waste (MW), and sawdust biochar (SB), which are wastes that can cause serious environmental pollution if improperly used. However, converting these waste materials into biosorbents for bioremediation is a win-win approach for improving waste treatment, while removing heavy metals from farmland environments.
Organic material remediation is simple, efficient, and low-cost, making it a very promising technology [9]. In particular, the return of straw improves soil physio-biochemical properties, fertility, organic carbon fixation, and N levels, in addition to microbial properties, while also reducing greenhouse gas emissions and nutrient loss from runoff [10,11]. Furthermore, Shan et al. observed that straw addition in conjunction with N fertilization influenced the accumulation, distribution, and availability of Cd [12]. Livestock manure is organic waste that harbors rich nutrients which are useful for plant growth and can supplement macronutrients and micronutrients in soils [13]. Moreover, livestock manure has the potential to promote heavy metal immobilization and reduce uptake. Wan et al. demonstrated that the application of chicken or pig manure increased soil pH and decreased available Cd and Pb in soils, thereby significantly reducing the accumulation of Cd and Pb in rice grains [14]. MW is another waste substrate produced during mushroom cultivation that has low bulk density and a high organic matter content. The addition of MW may limit the migration of Pb and Zn in Paulownia fortunei, thereby increasing the phytoremediation capacity of P. fortunei in lead–zinc slag, and leading to the reduced toxicity of the slag toward P. fortunei [15]. The effectiveness of this strategy was also validated by Castanho et al. [16], who observed that MWs from both Lentinula edodes and Agaricus bisporus were effective biosorbents for removing Pb2+ and Cu2+ from wastewater. Biochar is a carbon-rich substance generated from a variety of organic waste feedstocks, such as agricultural waste and forestry materials. Numerous studies have indicated that applying biochar to agricultural soils can lead to various benefits. For example, biochar can reduce soil bulk density [17], increase soil nitrogen use efficiency [18], improve soil organic carbon content [19,20], increase soil water-stable aggregate content [21], increase soil pH in acidic soils, increase exchangeable Ca and Mg content [22], and even boost plant root development [23]. Moreover, the porous structure of biochar leads to the presence of abundant surface functional groups and high cation exchange capacity, leading to a great affinity for heavy metals in soils [24]. Indeed, abundant evidence suggests that using biochar can greatly reduce Cd/Cu mobility and bioaccumulation [25,26]. Additionally, Cd- and Zn-polluted soils can be remediated through increasing the alkalinity and altering the potentially mobile portions of heavy metals, thereby immobilizing Cd and Zn [27]. Although the impacts of these materials in heavy-metal-contaminated soil remediation have been studied, few studies have compared the metal remediation capacity of MS, CM, MW, and SB, leaving a relative paucity of information regarding the differential effects of these organic materials for soil heavy metal immobilization and phytoavailability.
Determining the best heavy-metal-contaminated soil remediation method can help reduce the cost and improve the efficiency of soil remediation, as well as guide its practical application, thus promoting the wide application of soil remediation technology. In this study, scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectrometry were used to characterize the organic amendments MS, CM, MW, and SB. Atomic absorption spectrometry (AAS) was performed to determine the Pb concentrations. Conventional methods were employed to determine the physical and chemical soil indicators and the crop growth indicators. The specific objectives of this study were to (1) evaluate the characteristics of the above organic materials and their effects on soil properties; (2) assess the effects of the materials on heavy metal immobilization; and (3) evaluate the materials’ effects on metal bioaccumulation and wheat growth.

2. Materials and Methods

2.1. Preparation of Soils and Amendments

Loamy soils used in these experiments were collected from experimental wheat fields at the Henan Normal University in Xinxiang of China. Soil samples were taken from a depth of 0–20 cm.
Organic amendments included MS, CM, MW, and SB. MS (variety Zheng Dan 958) was collected from an experimental field in Qianli Village of Xinxiang City in the Henan Province. The collected materials were washed several times, ensuring the dirt particles were removed. CM was commercially purchased. MW was generated from corn cobs and a small amount of lime following the composting, bagging, sterilizing, cooling, inoculating, managing, and picking of the mushrooms. SB was produced from sawdust in Huojia County (Xinxiang, China) via charring at 800 °C under oxygen-limited, ambient conditions. All amendments were dried to a constant weight and sieved to <1 mm particle sizes.

2.2. Pot Experimental Design

The experimental protocol is shown in Figure 1. There were six treatments, including (1) a control group, without heavy metal and organic amendments (CK); (2) a group with 600 mg·kg−1 Pb without organic amendments (Pb); (3) a group with 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); (4) a group with 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); (5) a group with 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); and (6) a group with 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). The pot experiment was arranged in a randomized block design in triplicate. Initially, 10 kg of dry soil was mixed with a Pb(NO3)2 solution and the different amendments, along with 2.07 g·pot−1 urea, 8 g·pot−1 superphosphate, and 2.16 g·pot−1 potassium sulfate, followed by being filled into the plastic pots (diameter, 35 cm; height, 27 cm). Soil samples were allowed to sit for 7 days to allow the equilibration of Pb, soil, and amendments. Thirty seeds of winter wheat (Triticum aestivum L.) were sown in each pot, then being thinned to ten following germination.
Soil samples were collected during the wintering, flowering, and maturity periods, followed by air drying and grinding to pass through a 0.25 mm sieve for subsequent analysis. Plants across treatments were harvested at the same periods, followed by the separation of different organs, washing with deionized water, oven drying, and sieving to <1 mm for subsequent analysis after recording the dry weights.

2.3. Pb Analysis

The total Pb concentrations were determined using AAS (ICE3000; Thermo Fisher Co., Waltham, MA, USA) after digesting with HNO3 and HF via the use of a microwave digestion system (Mars 6; CEM Co., Matthews, NC, USA). Pb fractionations were measured using a modified three-step procedure which was proposed and validated by the Community Bureau of Reference (BCR) [28], as shown in Table 1.

2.4. Characterization and Analysis

(1)
Organic amendment characterization
The surface area and pore distribution were determined using an instrument (Micromeritics ASAP 2460, Norcross, GA, USA) and the BET (Brunauer–Emmett–Teller) model. Amendment surface morphologies were observed using a scanning electron microscope (Hitachi TM3030, Tokyo, Japan). In addition, functional groups were characterized using a Fourier transform infrared spectrometer (Bruker Tensor II, Ettlingen, Germany). Spectra were collected in the range of 400–4000 cm−1, and samples were prepared via milling with KBr in a 1:200 mixing ratio.
(2)
Physico-chemical property analysis
Several physico-chemical characteristics were evaluated in the initial soils and organic amendments, as well as following different treatments, including the organic matter content (OM), pH, total nitrogen (TN), available nitrogen (AN), total organic carbon content (TOC), and cation exchange capacity (CEC). OM was first measured with a TOC analyzer and then converted into organic matter content using a Bemmelen coefficient value of 1.732. pH was also measured in a deionized water suspension using a pH meter (Mettler Toledo FE20, Aargau, Switzerland). TN was measured using the micro-Kjeldhal method [29], while AN was determined following the methods described in the previous methodology [30]. The TOC content was measured with a TOC analyzer (Vario TOC; Elementar Co., Langenselbold, Germany). The CEC of soil samples was determined with the ammonium acetate method at a pH of 7.
(3)
Growth parameter analysis
Plant heights, root lengths, and shoot dry biomass from wheat plants across different treatments were measured during the wintering, flowering, and maturity periods. Plant samples were collected via breaking the plastic pot, slowly shaking off the root soils, then completely removing the plant. The samples were then rinsed with tap water, and the roots were soaked in 20 mmol·L−1 EDTA-Na2 for 15 min, followed by twice rinsing with deionized water. After measuring plant heights and root lengths, 10 plants were individually bagged and incubated at 105 °C for 30 min, followed by drying at 85 °C to a constant weight, and then weighing the dry matter mass.

2.5. Statistical Analysis

All experiments were conducted in triplicate. One-way variance analysis (ANOVA) tests were used to compare means among different treatments. The differences between individual means were evaluated with Duncan’s multiple range tests using the SPSS 20.0 program, where a p < 0.05 was considered statistically significant.

3. Results

3.1. Organic Amendment Characterization

3.1.1. Basic Properties of the Organic Amendments

The basic properties of the various organic materials used in this study’s experiments are shown in Table 2. The BET surface area of the organic amendments followed the order of SB > MW > CM > MS. Similar trends were observed for the total pore volume and Pb content. The average pore diameters of SB were much smaller than in other organic materials (i.e., only 2.61 nm). The organic matter content was highest in MS. In addition, the pH of the MW and SB was alkaline, while the CM was acidic, and that of MS was almost neutral.

3.1.2. SEM Results of Organic Amendments

SEM imaging was used to characterize the organic materials (Figure 2), with each sample exhibiting a different structure. The MS surfaces exhibited a multi-layered flaky structure, while the particle size in CM varied greatly, with surfaces exhibiting irregular sheet and layer structures. The MW and SB surfaces were porous and exhibited numerous cracks. The polylaminate or porous structures of materials might favor the adsorption and immobilization of heavy metals.

3.1.3. FTIR Spectrometry Results of Organic Amendments

The vibrations of surface functional groups were confirmed with FTIR spectroscopy analysis (Figure 3), based on the wave numbers of the dominant peaks (Table 3). Broad peaks between 3406 and 3435 cm−1 are indicative of bound hydroxyl groups in macromolecules like cellulose and pectin [31], which might remove hydroxyl radicals, thus reducing the damage of free radicals to plant cells. Peaks observed between 2921 and 2924 cm−1 can be assigned to C-H stretching vibrations. Bands between 1732 and 1796 cm−1 are due to C=O stretching within aliphatic acids. Bands between 1630 and 1637 cm−1 correspond to the stretching vibration of unsaturated groups like C=C and C=O. These functional groups could bind metal, meaning they could also reduce the metal toxicity of lead. In addition, a peak at 1515 cm−1 may derive from aromatic C-NO2 stretching [32], while a peak near 1426 cm−1 is attributable to the stretching vibration of C-O from a carboxyl group [33]. C-OH stretching vibrations were identified at 1400 cm−1 in MW and SB, which might contribute to the alkalinity of these organic amendments. Finally, an intense band between 1033 and 1042 cm−1 can be assigned to the C-O bonds of alcohols and carboxylic acids [31]. FTIR spectroscopy analysis indicated differences between the four organic materials, resulting in different adsorption capacities, as well as different repair outcomes.

3.2. Soil Physico-Chemical Property Analysis

3.2.1. Basic Soil Characteristics

The basic characteristics and Pb contents of the soil are shown in Table 4. The soil was weakly alkaline, the OM was extremely high, the TN was measured to be in the upper middle level, the AN was low, the CEC was medium, and the Pb content was within the safe range.

3.2.2. Impact of Organic Amendments on soil Chemical Properties

(1)
Soil pH
The effects of the organic material amendments on the soil pH were evaluated (Figure 4). Variation in the soil pH generally followed the trend of maturity period > wintering period > flowering period. Furthermore, throughout the wintering period, the soil pH of each treatment was MW + Pb > SB + Pb > CK > MS + Pb > CM + Pb > Pb. Notably, the pH in the MW and SB treatments were much higher than other treatments, and the differences among MS, CM, and Pb were not significant; their pH values were also all lower than that of the control. During the flowering period, the pH decreased, and similar trends were observed. During the maturity period, the pH of each treatment increased significantly, but the degree of this increase was different, instead following the ranking of MW + Pb > SB + Pb > CK > Pb > CM + Pb > MS + Pb. In summary, when compared with the CK treatment, a slight decrease in pH was observed after the addition of Pb(NO3)2 during different periods. Furthermore, when compared with the Pb treatment, the addition of MS or CM decreased the soil pH during the flowering and maturity periods, with the lowest pH being observed as 7.93 in the flowering period following the addition of MS. However, the addition of MW or SB increased the soil pH during all three periods, and MW exerted a more pronounced influence than SB.
(2)
Content of TOC
The effects of organic material amendments on the TOC in amended soils were evaluated (Figure 5). The TOC in soil was significantly affected by the treatments and over time. Relative to CK soils, the TOC in soils increased in the order of SB + Pb > MW + Pb > Pb > MS + Pb > CM + Pb > CK during the wintering period, after which, the trend changed to MW + Pb > SB + Pb > Pb > MS + Pb > CM + Pb > CK. In the maturity stage, the TOC in soils with organic material amendments exhibited continued downward trends in the order of SB + Pb > MS + Pb > MW + Pb > Pb > CM + Pb > CK. The above results indicate that the addition of Pb(NO3)2 increased soil TOC levels when compared with the CK treatment during all three periods. The addition of MW or SB increased the soil TOC levels when compared with the Pb treatment during all three periods.
(3)
Capacity of CEC
The CEC is a measure of soil’s fertility retention and buffering ability, representing the total amount of exchangeable ions in soil colloids. The effects of organic material amendments on the soil CEC were evaluated (Figure 6). Over time, the soil CEC gradually decreased, then maintaining a dynamic balance. During the wintering period, the results indicate that the soil CEC followed the order of MW + Pb > MS + Pb > Pb > CM + Pb > SB + Pb > CK. During the flowering period, the trend changed to MW + Pb > CM + Pb > MS + Pb > SB + Pb > Pb > CK. During the maturity stage, the CEC in soils with organic material amendments exhibited balanced trends in the order of MW + Pb > MS + Pb > CM + Pb > Pb > SB + Pb > CK. These results indicated that the addition of Pb resulted in increased cation exchange at different growth stages when compared with CK soils, and that the addition of MW and MS resulted in increased soil CEC compared with the Pb treatment across all three periods, with the maximum CEC (14.80 cmol·kg−1) being measured for the MW + Pb treatment during the wintering period.

3.3. Impact of Organic Amendments on Soil Pb Fractions

The migration and bioavailability of heavy metals in soils are primarily related to their combined forms. Thus, only evaluating the total amount of heavy metals in soils does not accurately reflect their biological effects. Sequential extraction was consequently conducted in order to analyze the distribution of soil Pb fractions (Figure 7). Compared to the Pb treatment, the MS + Pb and CM + Pb treatments led to increased exchangeable forms of Pb, with the MS + Pb treatment exhibiting a more pronounced effect, while the fraction of exchangeable Pb forms decreased in the MW + Pb and SB + Pb treatments. In addition, the MW + Pb and SB + Pb treatments increased the fraction of residual forms. Notably, the results indicated that the MW + Pb and SB + Pb treatments immobilized Pb in the investigated soil more effectively than the other treatments.

3.4. Impact of Organic Amendments on Plant Growth

The effects of different organic material amendments on plant growth was investigated (Figure 8). During the wintering period, the addition of Pb(NO3)2 resulted in the highest increases in plant height and shoot dry biomass when compared to the CK plants. However, the analysis of morphological characteristics and wheat biomass among different organic material treatments revealed a decreased trend compared with levels observed in the Pb treatment (with the exception of MW + Pb plant root lengths). During the flowering period, the morphological characteristics and biomass of wheat increased relative to the Pb plants with the addition of different organic materials (except for CM + Pb plant root lengths). During the maturity period, the Pb treatment exhibited decreased morphological characteristics and wheat biomass compared with the CK treatment. However, the MW + Pb treatment exhibited a significant improvement effect on wheat root elongation, while shoot dry biomass increased with the additions of CM, MW, and SB by 13.67%, 19.53%, and 25.39%, respectively, compared with the Pb treatment.

3.5. Impact of Organic Amendments on Pb Uptake

The effects of different organic materials on wheat Pb accumulation was evaluated (Figure 9). During the wintering period, the Pb concentrations in MW + Pb treatment soils were lower than in other treatments, which may be due to the larger volume of MW that was added to the soil, which would have a diluting effect on soil Pb concentrations. The Pb concentrations varied in roots among different treatments, wherein the addition of MS and MW was effective in reducing Pb concentrations in stems and leaves when compared with the Pb treatment. During the flowering period, the four treatments amended with organic materials exhibited increased wheat root, stem, and leaf Pb concentrations, but decreased concentrations in the spikes, compared with the Pb treatment. During the maturity period, four organic materials led to effectively reduced Pb concentrations in roots, stems, and leaves. In addition, the MS + Pb, CM + Pb, MW + Pb, and SB + Pb treatments significantly reduced glume Pb concentrations. Moreover, the MS + Pb and CM + Pb treatments slightly reduced grain Pb concentrations, and the MW + Pb and SB + Pb treatments significantly reduced grain Pb concentrations compared with the Pb treatment.

4. Discussion

Various organic amendments have been widely used to immobilize metals in polluted areas [34]. Indeed, these methods have been validated in soils contaminated with many heavy metals, including Pb, Zn, Cu, and Ni, in addition to compound pollution [8,35]. Nevertheless, it is well known that organic additions alter the transformation of heavy metals into diverse chemical forms in a type-dependent manner, thus leading to differential effects on soil characteristics. Furthermore, an increasing body of evidence indicates that different types of organic amendments improve plant growth in response to heavy metal stress. In the present study, we characterized four different organic material supplements, with the SB amendment (followed by the MW) leading to larger BET surface areas compared to the rest of the amendments. Such changes would favor Pb adsorption on any available sites. In addition, the SB and MW amendments led to the presence of greater soil meso-scale pores, resulting in further additional Pb adsorption sites. In contrast, the CM led to smaller BET surface areas than the SB and MW amendments, as well as the presence of lamellar structures that would allow Pb to adsorb and form complexes on accessible sites. When compared to the other organic supplements, the MS exhibited the smallest BET surface area and adsorption structures, indicating that it is unable to store Pb for longer periods of time, thereby resulting in metal leaching. Furthermore, MS is rich in cellulose and lignin, and the functional groups (such as hydroxyl groups and carboxyl groups) in these components can chemically interact with Pb ions to achieve effective adsorption. Similar functional groups exist in CM, MW, and SB, which play an important role in the adsorption of Pb. These functional groups can become fixed on the surface or inside of the Pb ions by forming chemical bonds or ion exchange, thus aiding the effective removal of Pb [36].
Amendments typically alter soil properties such as pH, TOC, and CEC. A study by Zong et al. (2021) [37] demonstrated that heavy metal immobilization could be due to improved soil properties like pH, TOC, and CEC. In this study, the CM + Pb and MS + Pb treatments decreased soil pH, likely because the original soils were alkaline, and the amendments exhibited lower pH levels than the soils. However, the MW and SB used in this study were alkaline and effectively increased soil pH. Both the addition of lime in the production of the mushroom cultivation medium and the higher pH of SB, which was due to higher pyrolyzing temperatures, might explain the relatively higher pH following their amendment into soils. Moreover, the TOC and CECs increased relative to CK after the organic material and Pb application. Both amendments likely regulated soil pH and supplemented nutrients into soils. In addition, the porous and lamellar structure of organic materials, as well as their propensity to sorb soluble organic C, could produce habitats and nutrients for soil microbiota, thus resulting in increased soil microbial activity and soil organic matter levels [38]. However, Pb applications inhibit soil microbial functional and enzymatic activities, thereby decelerating the decomposition rate of organic matter. In the present study, SB + Pb and MW + Pb treatments increased soil pH and the TOC at different growth stages compared with Pb application, with MW + Pb treatment also exhibiting a supportive effect on soil CECs, while other treatments exhibited irregular effects. These results suggest that the Pb immobilization could occur due to improved pH, TOC, and CECs, thus leading to the increased transformation of heavy metals from free to stable states. Therefore, soil Pb fraction distributions following amendment with the four organic materials requires further investigation.
As expected, altered soil properties due to organic amendments regulated the Pb forms within soils. The MS + Pb and CM + Pb treatments of the current study increased the fraction of exchangeable Pb forms, which could be due to their reducing effects on soil pH, since Pb adsorption levels in soils decrease at lower pH values. Many studies have observed a negative relationship between soil pH and Cd or Pb solubility and/or accumulation in plants [39]. The MW + Pb and SB + Pb treatments reduced the fraction of exchangeable Pb forms and increased the fraction of oxidizable and residual forms. The results suggest that exchangeable Pb might be converted into organic-bound, inorganic precipitates, and residuals due to the increased soil TOC, pH, and CECs. Wang et al. (2021) [35] observed that increased soil pH, CECs, and TOC contents were the most important factors affecting the distribution of Pb, Cu, and Zn in soils. The mobility and bioavailability of heavy metals depends on total concentrations, but also depends very much on the presence of specific chemical forms. In particular, the reducible and exchangeable forms of metals in soils are representative of their bioavailability and toxicity to plants, while the oxidizable and residual forms are less bioavailable [40]. The results of this study suggest that the MW + Pb and SB + Pb treatments can help to immobilize Pb in soils by converting the metals from available forms to stable forms.
Organic materials alter soil physicochemical characteristics, soil pH, and Pb bioavailability via numerous processes, thereby affecting what plant Pb uptake, which in turn affects wheat growth. Adding organic amendments to soil improves soil structures, thereby lessening heavy metal toxicity to plant growth [41,42]. Consistent with previous studies [37], this study showed that the addition of organic materials (relative to Pb) led to Pb being converted into extraction states with low migratory activities, thereby significantly reducing Pb toxicity to stems and leaves throughout the wintering period, Pb toxicity to spikes during the flowering period, and Pb toxicity to grains during the maturity period. Overall, the application of the four organic materials did not increase the enrichment of Pb in edible portions of the wheat; however, Pb contents in grains were significantly decreased after MW + Pb and SB + Pb treatments, which was consistent with results of other studies, wherein biochar immobilized Pb in soils [35]. Therefore, these amendments may alleviate Pb phytotoxicity via the reduction of Pb bioavailability and accumulation, thereby favoring plant growth. The CM + Pb, MW + Pb, and SB + Pb applications promoted the growth of over-ground parts of wheat during the anthesis and maturity periods; therefore, wheat biomass increased to different extents in the three treatments when compared with the Pb treatment. Among these treatments, MW exerted a remarkable effect on root elongation compared with the Pb treatment. Mushroom residues can help loosen soils, improve permeability, boost fertility, and aid root growth, similarly to the observations of Han et al. (2020) [15]. Taken together, these results suggest that organic material amendment reduced Pb accumulation in wheat grains, partly due to reduced migration abilities.

5. Conclusions

In conclusion, the influence of organic amendments on heavy metal immobilization and phytoavailability in soils varied with the amendment type and characteristics. MS and CM applications to Pb-polluted soils decreased the soil pH and increased the fraction of bioavailable Pb forms. However, this did not increase the accumulation of Pb in the edible parts of wheat plants, which may be the result of a balance between the soil and plant responses to the MS and CM materials. In contrast, MW and SB exhibited a more porous and distorted structure, which enhanced the exposure and availability of hydroxyl (-OH) functional groups. Moreover, the MW + Pb and SB + Pb treatments increased the soil pH and TOC, with the MW + Pb treatment also exhibiting a promotional effect on soil CECs. Concomitantly, these two organic materials increased the fraction of oxidizable and residual forms of Pb. Moreover, MW + Pb and SB + Pb treatments reduced heavy metal uptake by wheat, decreased the Pb content in wheat grains, and increased plant biomass more effectively than Pb amendment.

Author Contributions

Conceptualization, X.W. (Xiaojie Wang) and Y.S.; methodology, J.C.; software, X.W. (Xiaojie Wang) and J.C.; validation, J.C., J.A., X.W. (Xueping Wang) and Y.S.; formal analysis, J.C.; investigation, J.A.; resources, Y.S.; data curation, X.W. (Xiaojie Wang) and J.C.; writing—original draft preparation, J.C.; writing—review and editing, X.W. (Xiaojie Wang) and Y.S.; visualization, X.W. (Xueping Wang); supervision, Y.S.; project administration, Y.S.; funding acquisition, Y.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Project of China (No.2023YFD2300200).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Acknowledgments

We would like to thank the reviewers for their constructive comments and suggestions that helped to improve the quality of this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A schematic diagram of the experimental procedure of this study, including amendment preparation, wheat cultivation, sampling, various parameter measurements, and data analysis.
Figure 1. A schematic diagram of the experimental procedure of this study, including amendment preparation, wheat cultivation, sampling, various parameter measurements, and data analysis.
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Figure 2. Surface morphology of four organic material particles. (a) MS; (b) CM; (c) MW; (d) SB.
Figure 2. Surface morphology of four organic material particles. (a) MS; (b) CM; (c) MW; (d) SB.
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Figure 3. FTIR spectrometry characteristics of the four raw organic materials used in this study.
Figure 3. FTIR spectrometry characteristics of the four raw organic materials used in this study.
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Figure 4. Soil pH in different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
Figure 4. Soil pH in different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
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Figure 5. Total organic content (TOC) of soils in different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
Figure 5. Total organic content (TOC) of soils in different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
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Figure 6. Soil cation exchangeable capacities (CEC) in different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
Figure 6. Soil cation exchangeable capacities (CEC) in different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
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Figure 7. Pb fractions in soils of different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb).
Figure 7. Pb fractions in soils of different treatments. Note: no added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb).
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Figure 8. Wheat plant height, root length, and shoot dry biomass among different treatments and periods. Note: (a) wintering stage; (b) flowering stage; (c) maturity stage. No added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
Figure 8. Wheat plant height, root length, and shoot dry biomass among different treatments and periods. Note: (a) wintering stage; (b) flowering stage; (c) maturity stage. No added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
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Figure 9. Pb concentration in soil and different parts of wheat plants of different treatments and periods. Note: (a) wintering stage; (b)flowering stage; (c) maturity stage. No added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
Figure 9. Pb concentration in soil and different parts of wheat plants of different treatments and periods. Note: (a) wintering stage; (b)flowering stage; (c) maturity stage. No added Pb and organic material (CK); addition of 600 mg·kg−1 Pb without organic material (Pb); addition of 3 g·kg−1 maize straw + 600 mg·kg−1 Pb (MS + Pb); addition of 6 g·kg−1 chicken manure + 600 mg·kg−1 Pb (CM + Pb); addition of 150 g·kg−1 mushroom cultivation waste + 600 mg·kg−1 Pb (MW + Pb); addition of 50 g·kg−1 sawdust biochar + 600 mg·kg−1 Pb (SB + Pb). Each value represents the mean of three replicates ± standard deviation, and different lowercase letters indicate significant differences between treatments in the same period at p < 0.05.
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Table 1. Modified BCR three-step sequential extraction procedure.
Table 1. Modified BCR three-step sequential extraction procedure.
FractionPhase(s)Extraction Reagent(s)Procedures
Exchangeable formSoil solution, carbonates, exchangeable metals0.11 mol·L−1 HOAc40 mL of acetic acid is added to 1 g of the sample in a centrifuge tube and shaken for 16 h at 25 °C. The extract is separated from solid residues via centrifugation and then filtered for measurement.
Reducible formOxides Fe/Mn0.5 mol·L−1 NH2OH·HCl (pH = 2)40 mL of freshly prepared hydroxylammonium chloride is added to the residue from step 1 and re-suspended by shaking for 16 h at 25 °C. The remaining operations are the same as in step 1.
Oxidizable formOrganic matter and sulfides(1) 30% H2O2The residue from step 2 is treated with hydrogen peroxide and digested for 1 h at room temperature and 1 h at 85 °C several times, until the volume is reduced to 2–3 mL. Then, 50 mL of 1.0 M ammonium acetate is added to the cold mixture and the rest of the operations are the same as in step 1.
(2) 1 mol·L−1 NH4OAc (pH = 2)
Residual formRemaining, non-silicate-bound metalsHNO3/HClThe residue from step 3 is microwave digested using a mixture of HNO3 and HCl.
Table 2. Physical and chemical properties of organic amendments.
Table 2. Physical and chemical properties of organic amendments.
ParameterMSCMMWSB
BET surface area (m2·g−1)2.44.836.9174.85
Total pore volume (cm−3·g−1)0.00770.0130.020.1139
Average pore diameter (nm)12.9410.7511.612.61
Organic matter content (g·kg−1)789.94265.34647.13443.74
pH6.915.188.499.88
Pb content (mg·kg−1)9.6211.7614.0930.98
Table 3. Wave number (in centimeters) for the dominant observed FTIR spectra peaks.
Table 3. Wave number (in centimeters) for the dominant observed FTIR spectra peaks.
Functional GroupsMSCMMWSB
Surface O-H stretching3406341134073435
Aliphatic C-H stretching292129232924—
Aliphatic acid C=O stretching1732—17931796
Unsaturated group like alkene163716301635—
Aromatic C-NO2 stretching1515———
Carboxylic C-O stretching1426—14261431
Alcoholic and phenolic C-O stretching1042103310351040
Table 4. Basic soil properties and Pb contents.
Table 4. Basic soil properties and Pb contents.
OM (g·kg−1)TN (g·kg−1)AN (mg·kg−1)pHCEC (cmol·kg−1)Pb Content (mg·kg−1)
52.541.4330.268.0513.5617.12
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Wang, X.; Chen, J.; An, J.; Wang, X.; Shao, Y. Comparison of the Effects of Different Organic Amendments on the Immobilization and Phytoavailability of Lead. Sustainability 2024, 16, 2981. https://doi.org/10.3390/su16072981

AMA Style

Wang X, Chen J, An J, Wang X, Shao Y. Comparison of the Effects of Different Organic Amendments on the Immobilization and Phytoavailability of Lead. Sustainability. 2024; 16(7):2981. https://doi.org/10.3390/su16072981

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Wang, Xiaojie, Jingwen Chen, Jiahui An, Xueping Wang, and Yun Shao. 2024. "Comparison of the Effects of Different Organic Amendments on the Immobilization and Phytoavailability of Lead" Sustainability 16, no. 7: 2981. https://doi.org/10.3390/su16072981

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