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Article

Characteristics of Soil DOM and Its Effect on the Transformation of Potentially Toxic Elements (PTE) Forms under Organic Fertilizer Return Conditions

by
Hongwei Pan
,
Lili Shi
,
Xin Liu
,
Hongjun Lei
*,
Jie Yu
and
Guang Yang
School of Water Conservancy, North China University of Water Resources and Electric Power, Zhengzhou 450046, China
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(3), 630; https://doi.org/10.3390/agronomy13030630
Submission received: 22 November 2022 / Revised: 19 February 2023 / Accepted: 20 February 2023 / Published: 22 February 2023
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Abstract

:
In order to explore the effects of the composition and structure of soil’s dissolved organic matter (DOM) and its electron transfer capacity (ETC) on the bioavailability of the potential toxic elements chromium (Cr), lead (Pb) and cadmium (Cd) after the application of decomposed pig manure organic fertilizer, three-dimensional fluorescence spectroscopy (3D-EEMs), parallel factor analysis (PARAFAC) and electrochemical methods were used to analyze the composition characteristics of DOM in soil solution and the changes in the ETC, and the dynamic relationship between the relative content of DOM, ETC and various forms of potential toxic elements was explored by means of a Pearson correlation analysis and redundancy analysis (RDA). Among them, Cr, Pb and Cd were the elements with significant biological toxicity in farmland soil. The results indicated the following: (1) The soil DOM before and after returning the organic fertilizer to the field contained four components: UV and UVA humic-like (C1), tryptophan-like and UVA humic-like (C2), Exogenous and visible humic-like (C3) and tyrosine-like (C4). Humus-like was the main component. (2) After applying organic fertilizer, the relative contents of the DOM humus and tyrosine-like components in the soil increased by 8% and 8.73%, respectively. In this process, the DOM electron-accepting capacity (EAC) and electron-donating capacity (EDC) increased by 39.98% and 27.91%, respectively. (3) The humic-like fraction showed a highly significant positive correlation with ETC (p < 0.01), and the tyrosine-like fraction showed a significant negative correlation with ETC (p < 0.05). (4) The humus-like substance and ETC were positively correlated with the total amount, reducible state and oxidizable state of the potential toxic elements and negatively correlated with the weak acid extracted state and residue state; this showed that the humus-like components and ETC were more helpful for the transformation of the weak acid extracted state to the reducible state, oxidizable state and residue state in the interaction between the DOM components and Cr, Pb and Cd. In summary, the reasonable application of organic fertilizer could improve the relative content of DOM and ETC in soil, inhibit the biological toxicity of potential toxic elements in soil and provide a theoretical basis for the safe use of organic fertilizer.

1. Introduction

With the rapid development of the economy, the population is growing rapidly and the demand for meat products is increasing, leading to the expansion of livestock and poultry farming. Thus, the recycling of organic waste from livestock and poultry farming is a current concern. Pig breeding is the largest breeding industry in China, with an output value of 1.3 trillion yuan, accounting for 56.6% of the total output value of livestock and poultry farming [1], and a huge amount of pig manure is produced every year. Pig manure contains a large amount of organic matter and high nitrogen content, which is a high-quality nutrient source. However, improper disposal may pollute soil and water bodies [2,3]. Organic fertilizer is an effective method to turn waste into treasure, and its safe development has become a hot topic of concern nowadays.
Dissolved organic matter (DOM) in agricultural soils mainly come from soil humus decomposition, microbial activity and metabolism, organic fertilizer applications and plant and animal residues [4,5]. DOM is one of the most active organic components in soil ecosystems, which has an important influence on the geochemical action of elements in the soil and plays an important role in the global carbon cycle, pollutant and nutrient transport and agricultural environment. Because it contains rich hydroxyl, carboxyl, phenolic hydroxyl and other active organic functional groups and has high aromaticity, it can significantly affect the degradation, migration, transformation, fate, bioavailability and ecotoxicity of potential toxic elements (PTE) [6] and organic pollutants [7,8,9,10].
Most studies have found that the application of organic fertilizer can reduce the biological effectiveness of PTE. Zhou et al. found that pig manure returned to the field could stabilize the biological activity of Cu, Pb and Zn in the soil [11]. Gong et al. found that the application of chicken manure and cow manure to soils with Cd concentrations below 10 mg·kg−g was effective in reducing the biological activity of PTE and improving crop yields, while earthworm manure had the best effect in reducing the toxicity of PTE and improving crop yields at Cd concentrations up to 50 mg·kg−1 [12]. Xue et al. found that the application of commercial organic fertilizer for four consecutive years increased the content of soil organic matter and reduced the concentration of the Cd exchange state in soil [13]. Hong et al. found that the application of organic fertilizer could significantly increase the content of DOM and reduce the bioavailable form of potential toxic elements, thus improving the yield and quality of crops [14]. Organic fertilizer can not only reduce the biological activity of PTE, but also improve soil fertility and increase crop yield and quality. On the contrary, some studies found that returning organic fertilizer to the field resulted in heavy metal enrichment and enhanced its biological activity [15,16]. It can be seen that the effect of organic fertilizer on the bioavailability of PTE in soil is controversial and needs further study.
The ecological function of DOM depends on its molecular composition and characteristics (such as aromatic versus aliphatic) [17]. Li et al. found that the humic-like substances of DOM were significantly negatively correlated with potential toxic elements, and protein-like components were significantly positively correlated with potential toxic elements [18]. The results of Wu et al. are consistent with those of Li et al. The higher the humic-like component of DOM, the lower the bioavailability of potential toxic elements [19]. Zhang et al. found that the application of organic fertilizer increased the humus-like components of soil DOM, increased the binding sites of potential toxic elements and reduced their toxicity [20]. In summary, it is of great significance to study the effect of organic fertilizer on the content and structural characteristics of DOM components in soil to improve soil properties and sustainable utilization.
In addition, the redox capacity of DOM plays a key role in the passivation of potential toxic elements and the degradation of organic pollutants. The redox capacity of DOM is characterized by its electron transfer capacity (ETC), including its electron-accepting capacity (EAC) and electron-donating capacity (EDC) [21]. The transformation of PTE speciation is mainly related to the quinone group and phenol group, which allows DOM to have the ability to exert ETC. As an electron shuttle, DOM transfers electrons between PTE ions and microorganisms in a cyclic manner, mediating and accelerating the transformation of effective forms of Cr, Pb, Cd and other heavy metals, and changing their valence, bioavailability and toxicity, taking advantage of the reversibility of electron transfer among quinones, semiquinones and hydroquinones [22,23]. After returning the organic fertilizer to the field, the composition and structure of DOM in the soil have changed, and the humification degree, aromatic compounds, molecular weight and fulvic-like and humus-like contents have improved, which increased the electron transfer capacity of DOM [24]. Therefore, ETC is critical to the environmental effects of DOM.
Through the current research, it can be seen that DOM in organic fertilizer has a certain effect on the biological effectiveness of PTE in soil, but these effects are exactly the components of DOM that play a key role; additionally, the mechanism of action between these components and PTE needs to be further answered. Then, this study answers the above questions through field experiments, according to the correlation analysis between the relative content of DOM and its components in soil and the biological effectiveness of PTE.

2. Materials and Methods

2.1. Experimental Material

The test soil was collected from the sprinkler irrigation test area of the agricultural high-efficiency water-saving test site of North China University of Water Resources and Electric power (Figure 1). The soil type was aquic ustochrepts, the average soil texture was silt loam with the soil bulk density of 1.59 g·cm−3 and the field capacity of 19.59%. The soil was composed of clay (0.002 mm), silt (0.002~0.02 mm) and sand (0.02~2 mm). The particle size of the silt loam was: clay 25%, silt 50% and sand 24.88%. The soil parent material was river alluvium. In the early days, the land belonged to agricultural land. Later, it was abandoned for a period of time. In recent years, the land was planned as experimental land. In the past three years, studies on the effects of organic fertilizers, such as pig manure and chicken manure, and organic fertilizers in combination with chemical fertilizers on soil fertility and crop growth have been conducted in the study area. The physical and chemical properties of the tested soil and organic materials are shown in Table 1. Wheat was planted in the experimental area, and pig manure organic fertilizer from Hebei Rundong Fertilizer Co., Ltd. (Hebei, China) was used in the experiment.

2.2. Experimental Design and Samples Collection

The field experiment was carried out from October 2021 to May 2022, and the plot area was 32 m2 (4 m × 8 m). Two treatments, the blank control (CK) and addition of organic fertilizer (CP), were set up. The amount of organic fertilizer was 30 t·hm−2, 60% base fertilizer, 25% top dressing at the green-up onset (late February) and 15% top dressing at the booting stage (late April). In this study, samples were taken once before sowing (BS), at the tillering stage (TS), overwintering stage (OS), rising stage (RS), flag leaf stage (FLS), anthesis stage (AS) and maturity stage (MS), and this was repeated 4 times to avoid excessive experimental errors. At a tillage soil sampling depth of 0–20 cm, the soil samples were collected with an earth auger according to the plum blossom point method (5 points) sampling, where equal amounts of the samples were mixed evenly after receiving quartering treatment, and then they were air-dried, ground and sieved in order to be tested.

2.3. Measurement Items and Methods

A PHS-3E precision pH meter was used to measure the pH of the solution sample (Rex Electric Chemical, Shanghai, China), the organic matter content was determined by using the potassium dichromate oxidation–oil bath heating method and the DOC was determined via colorimetry [25]. The extraction method of the soil DOM referred to reference [26]. The total amount and various forms of PTE Cr, Pb and Cd were measured with a flame atomic absorption spectrophotometer (WFX-210, Beijing Beifen-Ruili Analytical Instrument Group Co., Ltd., Beijing, China).

2.4. Determination of DOM Electron Transfer in Soil

The electron transfer capacity of DOM was determined by chronocoulometry on an electrochemical workstation (CS350H, Wuhan Corrtest Instruments Co., Ltd., Wuhan, China). The specific operation steps for the three-electrode system method referred to reference [27] (reference electrode: Ag/AgCl; working electrode: glassy carbon electrode; counter electrode: platinum mesh electrode). Before the reaction, 20 mL of phosphate buffer saline was added to keep the pH stable. We applied the oxidation potential (Eh = 0.61 V) and reduction potential (Eh = −0.49 V) under pure nitrogen (5 min) to measure its electron-accepting capacity (EAC) and electron-donating capacity (EDC). The experimental time was set to 25,000 s. After the reaction was stabled to join the soil DOM solution (DOC remained at 30 mg L−1), the experimental set up two parallel. In order to avoid experimental error as much as possible, the oxide film at the bottom of the working electrode was removed before the experiment started, and the electrode was ground with a polishing cloth for about 1 min and then dried with ethanol.

2.5. Three-Dimensional Fluorescence Spectrum Determination of Soil DOM

The three-dimensional fluorescence spectrum was measured with an F-4600 fluorescence spectrophotometer. The spectrum scanning operation steps referred to [28], and the excitation light source was a deuterium lamp. The excitation wavelength range was 220~450 nm, the step length was 5 nm, the emission wavelength was 200~500 nm, the step length was 5 nm, the slit width was 5 nm, the PMT voltage was 400 V, the scanning speed was 1200 nm·min−1 and the samples were measured in a 1 cm quartz fluorescence cuvette. In order to reduce the fluorescence quenching effect, the DOM extract was diluted before scanning. With Miller-Q ultrapure water as the blank, the measured three-dimensional fluorescence spectral data were eliminated by Rayleigh and Raman scattering using the DOMFlour toolbox of Matlab R2018b (MathWorks, Natick, MA, USA).

2.6. Determination of Total PTE

The aqua regia digestion method [29] was used and the operation was briefly described as follows: about 0.5 g (accurate to 0.0001 g) of the soil samples were accurately weighed in a 50 mL colorimetric tube, and a little water was added to wet the soil. Then, 10 mL of the aqua regia solution was added and shaken, and the glass funnel was placed above the colorimetric tube, which was fixed in a water bath pot. The aqua regia was kept in a slightly boiling state and was heated for 2 h (shaking every 30 min). After digestion, the colorimetric tube was taken out and cooled, its constant volume was measured at the line and then it was shaken and tested. This process was repeated four times for each sample to avoid experimental errors.

2.7. Fractionation of PTE

The Fractionation of Heavy Metals method was adopted by using the BCR method [30], and the specific operation steps are shown in Table 2.

2.8. Data Processing and Calculation

The data were analyzed and drawn using Microsoft Excel 2016 (Redmond, WA, USA) and Origin 2021 (OriginLab, Northampton, MA, USA). SPSS v. 22.0 (IBM, Inc., Armonk, NY, USA) was used for the Pearson correlation analysis, multicollinearity test and heteroscedasticity test. The multicollinearity test showed that the VIF was < 10 and the variance proportions were less than 0.89. An analysis using White’s test in SPSS showed no collinearity between the independent variables and residuals. The DOMFlour toolbox of Matlab R2018b (MathWorks, Natick, MA, USA) [31] was used for the parallel factor analysis of the three-dimensional fluorescence spectral data.

3. Results

3.1. Effect of Applying Organic Fertilizer on Soil DOM Components

The fluorescence spectral data of 56 samples were analyzed by PARAFAC, and the split-half test and the spectral characteristics of each component are presented in Figure 2. A total of four fluorescent components were resolved (Figure 2). These four fractions were UV and UVA humic-like (C1), tryptophan-like and UVA humic-like (C2), Exogenous and visible humic-like (C3) and tyrosine-like (C4) (Table 3). The first three components all had two peaks, a primary peak and a secondary peak. All four were matched in the OpenFlour database (similarity score > 0.95), and the maximum value of the C3 fluorescence peak was red shifted by 20 nm along the excitation wavelength and 55 nm along the emission wavelength compared with that of C1, which showed that the nature and structure of C3 were more complex.
The maximum fluorescence intensity (Fmax) is usually used to reflect the relative content of DOM while the fluorescence intensity percentage characterizes the composition and structural features of DOM [45,46]. The changes in the fluorescence intensity of the four fractions and the proportion of DOM fractions under the CK and CP treatments are shown in Figure 3 and Figure 4, respectively, from which it can be seen that the main DOM fraction in the soil was humic-like. The average increase in the DOM humus-like components (C1 and C3) and tyrosine-like components (C4) in the soil after organic fertilizer application were 8% and 8.73%, respectively, throughout the growth period. The trends of the C1 and C3 changes in the CP treatments were consistent. After the application of the base fertilizer and topdressing, C1 and C3 showed an increasing trend in other periods, except for the anthesis stage when C1 and C3 did not show an increasing trend. The main reason was that the organic fertilizer applied at the heading stage was smaller (15%) while the organic fertilizer applied at the sowing and green-up onset was larger (60% and 25%). The tryptophan-like and UVA humic-like (C2) components had different change trends with the other three components. There was an upward trend from the sowing stage to the tillering stage and from the rising stage to the anthesis stage and a downward trend in the other periods. The trend of C4 was opposite to that of C1 (or C3). Different from the CK treatment, the relative contents of the other three components under CP treatment decreased at maturity except for the C4 component. Overall, the relative content of each component of the CP treatment was always greater than the CK treatment. To sum up, the application of organic fertilizers enhanced the DOM content in the soil, among which the humic-like components and tyrosine-like components increased most significantly. Zhao et al. found similar results [47], mainly because the organic fertilizer itself contains a large amount of easily decomposed DOC.

3.2. Relationship between DOM Components of Soil and ETC

A Pearson correlation was used to analyze the possible correlation between the relative content of DOM and its ETC (Table 4), and the results showed that C1 was highly significantly and positively correlated with the EDC and EAC of DOM (p < 0.01). This indicates that the trend of C1 in the soil was similar to ETC. This was mainly because the C1 component was UV and UVA humic-like, which is closely related to microbial activity that promotes DOM electron transport [48]. The trends of the relative contents of C1 and C3 were consistent after the application of organic fertilizer, but the relative contents of C3 did not change significantly. Therefore, the correlation between the C1 + C3 and ETC of DOM was analyzed. The results showed that C1 + C3 was significantly positively correlated with EDC (p < 0.01) and EAC (p < 0.05). C4 was significantly negatively correlated with ETC (p < 0.05).
The changes in the EDC and EAC of DOM under the CK and CP treatments are shown in Figure 5. The trends of EDC and EAC were basically similar to those of the C1 and C3 fractions, and the EAC and EDC increased by 39.98% and 27.91%, respectively, after the application of organic fertilizers. The comparison of the ETC of the two treatments showed that the ETC of the CP treatment was always greater than that of the CK treatment, which further indicated that the application of organic fertilizer increased the soil DOM humus-like fraction, promoted the formation of redox-active functional groups in DOM and improved the DOM electron transfer capacity. In this study, the EAC of DOM was found to be 9-to-26 times higher than that of EDC. This was similar to the research results of Tao et al. [49]. The main reason is that the main electroactive group of the soil DOM is the quinone group, which belongs to the oxidation state, and it is easier to transfer electrons to the terminal electron acceptors of PTE and promote the conversion of the effective forms of PTE.

3.3. Effects of DOM Components and ETC on PTE

Figure 6 shows the redundancy analysis between the DOM components and their ETCs and total PTE Cr, Pb, Cd, weak acid extracted states (water-soluble state, exchangeable state, carbonate bound state), reducible state (Fe-Mn hydroxide bound state), oxidizable state (organic matter and sulfide bound state) and residue state (Figure 5). According to the relative concentration of the DOM components, the angle between ETC and the concentration of various forms of PTE and the length of the arrow, it could be judged that the total amount, reducible state and oxidizable state of PTE were positively correlated with the C1 components, C3 components and ETC and negatively correlated with the C4 components. The weak acid extraction state and residual state of the heavy metals were negatively correlated with the C1 and C3 components and ETC and positively correlated with the C4 components. This showed that C1, C3 and ETC could promote the transformation of the weak acid extracted state to reducible, oxidizable and residue states.

4. Discussion

4.1. Effects on Soil DOM Characteristics

The application of organic fertilizers to maintain soil fertility and crop yield has a history of thousands of years in China. The high content of DOM in organic fertilizers improves soil carbon sequestration [50]. During the incubation period of this study, the relative content of DOM in the soil increased after the application of organic fertilizer, and the proportion of DOM fluorescence components was changed. The humic-like and protein-like (tryptophan-like, tyrosine-like) components of DOM were significantly increased during the whole growth period of wheat. Humus-like fractions have an improved role in soil fertility, soil structure and plant nutrient enhancement [51]. Except for the maturity stage of the wheat, the proportion of humic-like substances in other growth stages was significantly higher than that of other components, which was consistent with the results of most studies that soils preferentially adsorbed macromolecular substances [52,53]. The largest proportion of tyrosine-like fractions of DOM was found in soils at wheat maturity. The application of organic fertilizer can convert other components into easily decomposed and degradable tyrosine-like components [54]. This is consistent with the results of Zhang et al., which indicates that fertilization can improve the degree of soil humification and is conducive to soil DOC fixation [55].
The structure and composition of DOM are the decisive factors for the change in its redox capacity. Tan et al. showed that the ETC of soil DOM was related to the type and abundance of redox functional groups in its structure [56]. It was also found that EDC was closely related to the phenolic content of phenol in DOM, and EAC was positively correlated with the aromaticity of DOM [57]. Hu et al. showed that the functional groups of DOM binding to other electron acceptors were mainly carboxyl and phenolic hydroxyl groups [58]. Xiao et al. found that humic acid-like and tryptophan-like substances in DOM mainly play their EDC and aromatic substances (tyrosine-like) mainly play their EAC [59]. The results of this study showed that the application of organic fertilizer changed the structure and composition of soil DOM and increased the humification degree, aromatic compounds, molecular weight and humus-like components of soil DOM, which effectively improved the ETC of soil DOM. This was in agreement with the results of Tan et al. [24]; the main reason is that the organic fertilizers changed the composition and structure of DOM after high temperature humification in the soil, and the degree of humification, aromatic compounds, molecular weight, fulvic acid and humic-like substances content were increased, and this transformation increased the ETC of DOM.

4.2. Effects on Potential Toxic Elements

Organic fertilizers have been used as an economical and effective material for the remediation of potentially toxic elements or soils contaminated by organic pollutants due to their huge specific surface area, chargeability, redox properties and diverse functional groups [60]. In addition, DOM released by organic fertilizer applications is an important component of soil organic matter, which controls the release, migration and transformation of potential toxic elements in farmland soil to a certain extent [61]. The water-soluble state of heavy metals can be directly absorbed and utilized by plants, while the ion-exchange state of heavy metals is the most easily absorbed part by plants. Therefore, the weak acid extracted state has the highest bioavailability, the reducible state has high mobility in the environment, the oxidizable state is more easily converted to other forms and the residual state is the most stable [19,62]. Cheng et al. found that the water-soluble state of Cu/Pb; the water-soluble, ion-exchange, carbonate-bound and partially organic sulfide states of Zn; the water-soluble, ion-exchange and carbonate-bound states of Mn; and the Fe-Mn oxide state were effective for aquatic root plants using a continuous extraction method combined with biological exposure tests [63]. The humic-like components of DOM rich in carboxyl and phenolic groups may exhibit a more substantial binding ability to PTE (such as Pb) [64]. This study showed that the application of organic fertilizer increased the humus-like components and ETC of soil DOM; promoted the transformation of the weak acid extractable state to the reducible state, oxidizable state and residual state; and reduced the biological effective form of PTE. This is similar to the results of Mongkolchai et al. [65]. It was mainly due to the application of organic fertilizers to increase the content of soil DOM; promote the ion exchange mechanism of DOM to absorb the ions of potential toxic elements, thus reducing the bioavailability of Cd; and promote the transformation of exchangeable Cd to the sulfide-bound organic state and manganese oxide-bound state. Liu et al. confirmed through pot experiments that the combination of organic fertilizer and selenium (Se) promoted the conversion of the weak acid extracted state of Cd to other forms in the soil and reduced the bioavailability [66]. However, some studies have found that the greater the application amount of organic fertilizer, the stronger the biological effectiveness of potential toxic elements, which is mainly related to the length of the organic fertilizer application and the amount of potential toxic elements carried by itself. Long-term application will increase the input of potential toxic elements in the soil. This study took 210 days, while the study by Duan et al. took 6 years [17], so the long-term application of organic fertilizers is prone to the enrichment of heavy metals leading to the stronger biological effectiveness of PTE. From this study, the short-term application of organic fertilizer reduced the biological effectiveness of PTE in soil. In addition, from other related studies, the long-term application of organic fertilizer may increase the biological effectiveness of PTE, probably because the content of PTE in organic fertilizer is relatively high. After long-term accumulation, the biological effectiveness of PTE increased. In this study, the effect of applying organic fertilizer on reducing the biological effectiveness of PTE in soil came from the electron transfer between DOM and PTE in the organic fertilizer. From the perspective of DOM, the application of the organic fertilizer was beneficial to reduce the biological effectiveness of PTE in the soil. Therefore, reasonable control of the PTE content and relative content of DOM in organic fertilizers is beneficial to effectively control the biological effectiveness of PTE in the soil. Yang et al. found that the application of organic fertilizer affected the bioavailability of potentially toxic elements by changing the content of DOM in the soil through 154 days of rice culture experiments. Specifically, in the early stage of rice culture, DOM complexed with Cd to reduce the biological activity of potential toxic elements; in the later stage of rice culture, DOM was decomposed into small molecular weight organic acids under the action of roots, microorganisms and enzymes. The release of chelated organic matter led to the enhancement of the biological activity of potential toxic elements because the release of root exudates in the later stage of rice affected the stability of DOM complexes [18]. In this study, a PARAFAC analysis revealed that organic matter macromolecules showed an increasing trend and therefore the application of organic fertilizer reduced the biological effectiveness of heavy metals.

5. Conclusions

In this paper, the effect of organic fertilizer application on the DOM characteristics of cultivated soils was investigated, and the relationship between DOM components and structure, ETC and potentially toxic elements was studied. This study showed that the short-term application of organic fertilizer or increasing the electron transfer capacity of organic fertilizer (increased DOM content) were two feasible ways to safely utilize organic fertilizer. These results favor the safe development of organic fertilizers in agricultural production. This study is conducive to the safe development of organic fertilizer in agricultural production.

Author Contributions

Conceptualization, H.P. and L.S.; software, L.S.; writing—review and editing, H.P. and L.S.; data curation, L.S.; supervision, H.P., X.L. and H.L.; resources G.Y. and J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 51809095, No. 52079052), the Major Science and Technology Innovation Project of Shandong Province (No. 2019JZZY010710) and the Fund of the Innovative Education Program for Graduate Students at North China University of Water Resources and Electric Power, China (No. YK-2021-35, YK-2021-53).

Data Availability Statement

Data is contained within the article.

Acknowledgments

We fully appreciate the editors and all anonymous reviewers for their constructive comments on this manuscript.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Study area diagram. (A,B) represent the location of the study area, (C) represents the spatial distribution of the study area and the boundary of cultivated land. CK represents the treatment without fertilization, and CP represents the treatment with organic fertilizer.
Figure 1. Study area diagram. (A,B) represent the location of the study area, (C) represents the spatial distribution of the study area and the boundary of cultivated land. CK represents the treatment without fertilization, and CP represents the treatment with organic fertilizer.
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Figure 2. Four component maps and split-half test maps identified by PARAFAC. (A) Split-half test diagram of C1 component, (B) C1 fluorescence component diagram, (C) Split-half test diagram of C2 component, (D) C2 fluorescence component diagram, (E) Split-half test diagram of C3 component, (F) C3 fluorescence component diagram, (G) Split-half test diagram of C4 component, (H) C4 fluorescence component diagram. F represents the position of the fluorescence peak, Ex represents the excitation wavelength, and Em represents the emission wavelength.
Figure 2. Four component maps and split-half test maps identified by PARAFAC. (A) Split-half test diagram of C1 component, (B) C1 fluorescence component diagram, (C) Split-half test diagram of C2 component, (D) C2 fluorescence component diagram, (E) Split-half test diagram of C3 component, (F) C3 fluorescence component diagram, (G) Split-half test diagram of C4 component, (H) C4 fluorescence component diagram. F represents the position of the fluorescence peak, Ex represents the excitation wavelength, and Em represents the emission wavelength.
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Figure 3. The change in Fmax for soil DOM fluorescence components obtained by PARAFACFmax is the maximum fluorescence intensity. CK, no fertilizer application. CP, applying organic fertilizer. BS, before sowing. TS, at the tillering stage. OS, overwintering stage. RS, rising stage. FLS, flag leaf stage. AS, anthesis stage. MS, maturity stage. C1, UV and UVA humic-like. C2, tryptophan-like and UVA humic-like. C3, Exogenous and visible humic-like. C4, tyrosine-like. Fmax, the maximum fluorescence intensity (arbitrary unit, a.u.).
Figure 3. The change in Fmax for soil DOM fluorescence components obtained by PARAFACFmax is the maximum fluorescence intensity. CK, no fertilizer application. CP, applying organic fertilizer. BS, before sowing. TS, at the tillering stage. OS, overwintering stage. RS, rising stage. FLS, flag leaf stage. AS, anthesis stage. MS, maturity stage. C1, UV and UVA humic-like. C2, tryptophan-like and UVA humic-like. C3, Exogenous and visible humic-like. C4, tyrosine-like. Fmax, the maximum fluorescence intensity (arbitrary unit, a.u.).
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Figure 4. The proportion of soil DOM fluorescence components obtained by PARAFAC.CK, no fertilizer application. CP, applying organic fertilizer. BS, before sowing. TS, at the tillering stage. OS, overwintering stage. RS, rising stage. FLS, flag leaf stage. AS, anthesis stage. MS, maturity stage. C1, UV and UVA humic-like. C2, tryptophan-like and UVA humic-like. C3, Exogenous and visible humic-like. C4, tyrosine-like.
Figure 4. The proportion of soil DOM fluorescence components obtained by PARAFAC.CK, no fertilizer application. CP, applying organic fertilizer. BS, before sowing. TS, at the tillering stage. OS, overwintering stage. RS, rising stage. FLS, flag leaf stage. AS, anthesis stage. MS, maturity stage. C1, UV and UVA humic-like. C2, tryptophan-like and UVA humic-like. C3, Exogenous and visible humic-like. C4, tyrosine-like.
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Figure 5. Comparison of EAC and EDC of soil DOM (the nested diagrams represent the chronoamperometric plots of EAC and EDC, respectively). (A) EAC comparison of CK and CP, (B) EDC comparison of CK and CP.
Figure 5. Comparison of EAC and EDC of soil DOM (the nested diagrams represent the chronoamperometric plots of EAC and EDC, respectively). (A) EAC comparison of CK and CP, (B) EDC comparison of CK and CP.
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Figure 6. Redundancy analysis of soil DOM components and their ETC with total amount and forms of heavy metals (R, H, Y and C represent the weak acid extractable state, reducible state, oxidizable state and residual state of heavy metals, respectively. RDA2 represents the variance proportions that the constraint axis can explain.
Figure 6. Redundancy analysis of soil DOM components and their ETC with total amount and forms of heavy metals (R, H, Y and C represent the weak acid extractable state, reducible state, oxidizable state and residual state of heavy metals, respectively. RDA2 represents the variance proportions that the constraint axis can explain.
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Table
Table 1. Basic physical and chemical properties of soil and organic materials.
Table 1. Basic physical and chemical properties of soil and organic materials.
Experimental MaterialpHOrganic Matter (g·kg−1)DOC (mg C·L−1)Cr (mg·L−1)Pb (mg·L−1)Cd (mg·L−1)
Soil7.4411.96.7121.2030.6610.360
Organic fertilizer8.0151.4425.235101.69245.192.895
Table
Table 2. Operation steps of fractionation of PTE.
Table 2. Operation steps of fractionation of PTE.
Speciation of PTEOperation Steps
Weak acid extractable state1 g soil—centrifuge tube—40 mL, 0.11 mol·L−1 CH3COOH—oscillation box (25 °C, 250 rpm, 16 h)—centrifuge (4000 rpm, 15 min)
Reducible stateResidual samples—40 mL, 0.5 mol·L−1NH2OH·HCI (pH = 2)—oscillation box (ditto)—centrifuge (ditto)
Oxidizable stateResidual samples—10 mL H2O2 (pH = 2~3)—water bath (85 °C, 1 h)—10 mL H2O2—water bath (85 °C,1 h)—Cooling −40 mL, 1 mol·L−1CH3COONH4 (pH = 2)—oscillation box (ditto)
Residual stateTotal heavy metals—Weak acid extractable state—Reducible state—Oxidizable state
Table
Table 3. Comparison table of PARAFAC model components.
Table 3. Comparison table of PARAFAC model components.
ComponentsEx/Em(nm)PARAFAC Model Ex/Em(nm)
in Other Studies		Source and DescriptionPeak
C1260(350)/410C1,255(305)/426;
C3,250(340)/424;
C1,260/440;
C3,255(355)/420.		UV and UVA humic-like, significant characteristics of microbial sources, related to agricultural activities [32,33,34,35].A, M [36]
C2275(310)/380C2,315/380;
C4,310/400;
C3,350/428;
C3,255/395.		Tryptophan-like and UVA humic-like, derived from microbial metabolites and human activities [37,38,39,40].T, M
C3280(365)/465C2,280(370)/475;
C1,275(365)/484;
C4,260(305)/404.		Exogenous and visible humic-like, widely exists in fresh water and seawater, related to high molecular weight aromatic compounds and microbial transformation [36,41,42].A, C
C4265/280C3,305/344;
C2,265/305;
C3,270/302.		Tyrosine-like, free protein-like components, sediment source [34,43,44].B
Table
Table 4. Correlation analysis between DOM relative content and ETC.
Table 4. Correlation analysis between DOM relative content and ETC.
C1C2C3C4C1 + C3EDCEACETC
C11
C20.731 **1
C30.915 **0.710 **1
C4−0.965 **−0.865 **−0.944 **1
C1 + C30.993 **0.738 **0.955 **−0.976 **1
EDC0.615 **0.2810.372−0.473 *0.557 **1
EAC0.579 **0.1850.396−0.438 *0.537 *0.898 **1
ETC0.580 **0.1850.396−0.438 *0.538 *0.899 **1.000 **1
Different letters indicate significant differences at the level of p < 0.05. * p < 0.05; ** p < 0.01; ns, not significant at p < 0.05. C1, UV and UVA humic-like. C2, tryptophan-like and UVA humic-like. C3, Exogenous and visible humic-like. C4, tyrosine-like. ETC, electron transfer capacity. EAC, electron-accepting capacity. EDC, electron-donating capacity.
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Pan, H.; Shi, L.; Liu, X.; Lei, H.; Yu, J.; Yang, G. Characteristics of Soil DOM and Its Effect on the Transformation of Potentially Toxic Elements (PTE) Forms under Organic Fertilizer Return Conditions. Agronomy 2023, 13, 630. https://doi.org/10.3390/agronomy13030630

AMA Style

Pan H, Shi L, Liu X, Lei H, Yu J, Yang G. Characteristics of Soil DOM and Its Effect on the Transformation of Potentially Toxic Elements (PTE) Forms under Organic Fertilizer Return Conditions. Agronomy. 2023; 13(3):630. https://doi.org/10.3390/agronomy13030630

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Pan, Hongwei, Lili Shi, Xin Liu, Hongjun Lei, Jie Yu, and Guang Yang. 2023. "Characteristics of Soil DOM and Its Effect on the Transformation of Potentially Toxic Elements (PTE) Forms under Organic Fertilizer Return Conditions" Agronomy 13, no. 3: 630. https://doi.org/10.3390/agronomy13030630

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