Effects of Organic Wastes on Soil Organic Carbon and Surface Charge Properties in Primary Saline-alkali Soil

Abstract

High salinity and low fertility have restricted crop production in primary saline-alkali soils. Soil organic carbon (SOC) and surface charge characteristics affect the soil fertility and soil colloid characteristics of primary saline-alkali soils, respectively. In this paper, the SOC and surface charge properties of primary saline-alkaline soil under organic wastes applications were assessed. Five treatments were involved in this experiment: chemical fertilizer combined with sheep manure (SM), corn straw (CS), fodder grass (FG), and granular corn straw (GS), while chemical fertilizer only was used as control (CK). The content of SOC was significantly different under different organic wastes application (p < 0.05). Treatment GS recorded the highest content of SOC compared with the other treatments. In addition, the content of each SOC density fraction increased after the application of organic wastes. Similarly, the application of organic wastes, increased the proportion of organic carbon in free light fraction (Fr-FLOC) and organic carbon in occluded fraction (Oc-FLOC) in the soil however the proportion of organic carbon in heavy fraction (HFOC) decreased. In this study, we found that treatment GS has a greater impact on soil surface charge properties than other treatments, and through redundancy analysis (RDA) the content of SOC and Fr-LFOC (F = 24.704, p = 0.004; F = 19.594, p = 0.002) were identified as the main factors affecting the surface charge properties of soil organic carbon. In conclusion, GS is the recommended organic waste for ameliorating primary saline-alkali soil, as compared to the other organic waste treatments.

1. Introduction

The saline-alkali land in the Songnen Plain of Northeast China is 300 × 10⁴ hm² [1], which is one of the three major areas of soda saline soil distribution in the world [2]. A saline-alkali soil is usually described as salt-affected soil with many soluble salts, the content of exchangeable sodium greater than 15, and pH value usually less than 9.5. The ecological environment of the saline-alkali land in the Songnen Plain of Northeast China is fragile and it is one of the most serious desertification areas in northern China [3]. With the rapid development of China’s intensive agriculture [4], the amount of agricultural organic waste is also increasing and brings a series of problems such as environmental pollution and waste of resources [5,6]. In order to solve this series of problems, many studies have proposed using agricultural organic waste as a soil fertilizer. This is because the application of agricultural organic waste has proven to be an efficient way of improving the fertility and quality of saline-alkali soil [7,8,9].

Soil colloid refers to particles in the range of sizes less than 2 μm, which is the subtlest and active part of the soil. It has a large specific surface area and surface charge characteristics [10], which largely determine the physical and chemical properties of the soil [11,12]. Soil surface charge is a relatively active and nonnegligible component in soil [13]. It is generally affected by the charge quantity, specific surface area, charge density, electric field strength, and charge properties [14]. The difference in surface charge properties directly affects the soil in the chemical, physical and biochemical processes that occur between the particle surfaces [15,16,17]. The particles of the soil affect the ability of the soil to retain water and fertilizer and to supply water and nutrients [18]. Therefore, an accurate understanding of the electrochemical properties of soil is of great significance to the rational use of soil resources. However, there is still a lack of research on the surface charge properties of soils with the lack of reliable methods being one of the important reasons [19]. At present, the measurement methods mainly include the ion adsorption and potentiometric titration, but these two methods also have their shortcomings. The complication of experimental operation and inaccuracy in measurements are the main observed shortcomings. According to the advantages and disadvantages of ion adsorption and potentiometric titration, Liu, Li, Tian, et al. proposed a joint analysis method of surface properties, which can be used to determine the surface potential, charge density, electric field strength, specific surface area and charge quantity in one experiment [20]. This process produces a large amount of data with a small workload and simple operation.

The dynamics in the balance of soil organic carbon pool directly affects soil fertility accumulation and crop nutrient supply [21], which in turn affects soil structural traits [22] and crop yield [23]. The soil organic carbon pool also plays an important role in improving the fertility of soil [24], regulating soil physical and chemical properties [25] and also helps to reduce global environmental problems [26]. Studying the photosynthetic carbon conversion of crop straw waste to soil organic carbon in the context of global climate change is of great significance for evaluating the carbon sequestration potential of soil ecosystems. Studying the composition of soil organic carbon pool is relatively complicated, however, the physical fractionation method has less damage on the original structure of soil organic carbon, and can objectively reflect the structure and composition of soil organic carbon pool [27]. Density fractionation is one of the methods commonly used in the physical fractionation of organic matter. According to the different sedimentation characteristics of different soil organic carbon components in a certain specific gravity liquid, it can be divided into two fractions namely; light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC) [28]. LFOC is mainly divided into organic carbon in free light fraction (Fr-LFOC) and organic carbon in occluded fraction (Oc-LFOC). Generally, LFOC includes the undecomposed and semi-decomposed animal and plant residues while the HFOC mainly refers to stable carbon which is closely combined with soil minerals [29].

The purpose of this experiment was to study the changes in soil organic carbon density fractionation and soil surface charge under different agricultural waste applications and to explore the relationship between the two. We hypothesize that returning different agricultural wastes to this field can have different effects on the content of soil organic carbon components and soil surface charge properties.

2. Materials and Methods

2.1. Experimental Site

The study site was located at Haituo town in Daan City, Jilin province (124°1′48″ E, 45°19′47″ N). The site has a temperate continental monsoon climate which is dry, with annual precipitation of 413.7 mm. The study site has an annual evaporation potential of 1702.44 mm, which is four times as high as the annual precipitation in the region. The average annual temperature is 4.3 °C and the frost-free period is approximately 157 d, with average yearly sun exposure of 3012.8 h. The effective accumulated temperature of ≥10 °C is about 2921 °C. Soil type is saline-alkali soil (halosols, according to the international soil taxonomy classification). The soil type is mainly due to the presence of excess soda in the soil and the main soil nutrients and total salt contents in the study area (0–20 cm) are listed in

Table 1. Basic properties of soil.

Organic Matters g·kg−1.	Available Phosphorous mg·kg−1	Available Potassium mg·kg−1	Available Nitrogen mg·kg−1	Total Salt g·kg−1	EC ms·cm−1	CEC cmol·kg−1	Exchangeable Sodium Percent cmol·kg−1	pH	Clay %	Silt %	Sand %
2.91 ± 0.03	20.61 ± 0.04	143.33 ± 2.32	11.28 ± 0.02	3.39 ± 0.03	0.52 ± 0.02	3.82 ± 0.12	2.29 ± 0.05	9.94 ± 0.05	40.5%	18.3%	41.2%

Note: Values are mean ± standard deviation.

. The drought combined with the saline-alkali nature of the soil makes the soil in the study site unsuitable for agricultural development.

2.2. Experimental Design

A three-year field experiment was conducted in June 2016 in Haituo town. The experimental site was a bare land prior to conducting this experiment. In this experiment, rice was sowed on the study site after the application of the organic wastes. The experiment followed a randomized block design consisting of fifteen plots with five treatments and three replicates. The area of each plot was 6 × 5 m². The treatments were: chemical fertilizer plus sheep manure (SM), chemical fertilizer plus corn straw (CS), chemical fertilizer plus fodder grass (FG), and chemical fertilizer plus granular corn straw (GS), while chemical fertilizer only was used as control (CK). The same rate of chemical fertilizer was applied in the control and other treatments, where N, P, and K were applied at a rate of 175 kg/ha, 135 kg/ha, and 50 kg/ha respectively. The granular corn straw was formed by crushing the corn straw and finally pressing them into 3–5 cm length particles under high temperature and pressure. The addition of treatment was calculated so that an equal amount of carbon will be returned to the soil. The total amount of corn straw returned to the field followed the standard straw application rates whereby 7500 kg/ha was applied once during the first year of the experiment. The application rates of CS, GS, FG, and SM were 22, 22, 25, and 21 kg respectively for each treatment plot. Properties of the organic wastes used are presented in

Table 2. Basic properties of the dry weight of the different organic wastes.

Organic Wastes	Organic Matter (g kg−1)	Total Nitrogen (g kg−1)	Total Phosphorus (g kg−1)	Total Potassium (g kg−1)	C:N Ratio	pH
FG	422.4 ± 2.09	16.75 ± 0.11	1.02 ± 0.02	4.05 ± 0.05	14.63	6.14 ± 0.05
CS	493.4 ± 2.16	8.33 ± 0.09	1.12 ± 0.04	12.3 ± 0.08	34.36	6.42 ± 0.11
SM	506.2 ± 1.98	9.82 ± 0.12	3.6 ± 0.06	8.32 ± 0.04	29.9	7.27 ± 0.14
GS	493.4 ± 2.16	8.33 ± 0.07	1.12 ± 0.02	12.3 ± 0.12	34.36	6.42 ± 0.08

Note: Values are mean ± standard deviation.

. All fertilizers were applied once prior to sowing the rice.

Soil samples were collected in the 0–20 cm depth with a manual soil coring tube in June 2019. In each plot, five points were randomly selected and soil samples were collected following the “S” method. The collected soil samples were mixed to form a single composite soil to represent the specific plot. The samples were air-dried and passed through a 2-mm sieve after removal of crop residues and stones. Analysis of soil basic properties was done following the methods described by Bao [30]. Thus, the soil organic matter content, total nitrogen, total phosphorus, total potassium, C: N ratio, EC, ESP, CEC, pH, percentage of clay, silt and sand were all determined following the standard methods by Bao [30].

2.3. SOC Density Fractionation

The content of SOC in the composite soil samples was determined by the dichromate oxidation method as described by Yeomans and Bremner [31]. The method of SOC density fractionation used in this study was according to the method from Golchin, Oades, Skjemstad, et al. [32], which divides SOC as organic carbon in free light fraction (Fr-LFOC), the organic carbon in occluded fraction (Oc-LFOC) and organic carbon in heavy fraction (HFOC). The organic carbon of all fractions was quantified by dichromate oxidation. The specific details of the method are as follows: (1) Weigh 10.0 g of soil sample into a weighed 100 mL centrifuge tube, add 50 mL of NaI solution (d=1.8 g·cm⁻³), gently shake by hand, and let stand at room temperature overnight. The next day, the mixture was centrifuged at 3500 r·min⁻¹ for 15 min, and the solution was subjected to suction filtration after centrifugation, and the filtrate was recovered for reuse. Then, 50 mL of NaI solution was added to the centrifuge tube, and the mixture was shaken, centrifuged, and suction filtered. This process was repeated twice. The material remaining on the filter paper was washed with 50 mL of 0.01 mol·L⁻¹ CaCl₂ solution and 100 mL of distilled water, then transferred to a dry beaker, and allowed to stand for 24 h. The remaining material was dried at 30 °C in an oven at a constant temperature, weighed and its organic carbon quantified by dichromate oxidation. The obtained fraction was taken as free light organic carbon (Fr-LFOC). (2) Precipitation in the centrifuge tube was continuously done with the addition of 50 mL of NaI solution, followed by shaking. Ultrasonic dispersion (40 HZ, 100 W) was carried out for 15 min. The remaining steps are the same as the steps described above, and the component was taken as organic carbon in occluded fraction (Oc-LFOC); (3) At this stage, centrifuge tube precipitation plus 50 mL distilled water was carried out followed by shaking for 20 min. Centrifugation was done at 4000 r·min⁻¹ for 20 min. The tube sediment was repeatedly washed with 95% ethanol until colorless, dried in an oven at 40 °C to constant weight. This component was taken as organic carbon in heavy fraction (HFOC).

2.4. Determination and Calculation of Surface Charge Properties

2.4.1. Determination of Ca²⁺ and Na⁺

According to the surface property combined determination method proposed by Li, Hou, Liu, et al. [33] and Liu, Li, Tian, et al. [20], the electrochemical properties of the soil surface of the primary saline soil were determined: (1) Since the tested soil is primary saline-alkali soil with high relative carbonate content as described by Li, Hou, Liu, et al. and Liu, Li, Tian, et al., the soil sample needs to be decalcified. 40 g of soil (0.25 mm) was weighed into a 500 mL beaker, and 0.5 mol·L-1 HCl was added to 200 mL. The supernatant was stirred evenly with a glass rod, oscillated for 5 h, and centrifuged at 5000 rpm for 15 min. The supernatant was discarded and the same volume and concentration of HCl solution were added. The above steps were repeated 3 times until no CO₂ bubbles were produced in the soil sample. (2) The decalcified soil sample was transferred to a 1 L large beaker, 400 mL of a 0.1 mol·L⁻¹ HCl solution was added, and the mixture was shaken for 5 h, and the supernatant was discarded by centrifugation. The addition of the same volume and concentration of the HCl solution was continued, and the whole operation was repeated three times. After the last centrifugation, the same volume of deionized water was added, the shaking and centrifugation process was repeated, and the soil sample was dried at 60 °C. (3) 5 g of a hydrogen saturated soil sample (0.25 mm) was weighed into a 150 mL flask, and 55 mL of a 0.01 mol·L⁻¹ Ca(OH) 2 and NaOH 1:1 mixed solution was added. After shaking for 24 h, 1 mol·L⁻¹ HCl solution was added dropwise to adjust the pH of the soil suspension, and finally, the pH of the soil suspension was maintained at about 7. The concentration of Ca²⁺ and Na⁺ in the supernatant was determined by the atomic absorption spectrometer and flame photometer respectively [30].

2.4.2. Calculation of Surface Charge Properties of Soil

(1)

surface potential φ₀

$${\phi }_0=\frac{2\mathrm{RT}}{(2{\beta }_{\mathrm{Ca}}-{\beta }_{\mathrm{Na}})F}\ln \frac{a_{\mathrm{Ca}}^0{N}_{\mathrm{Na}}}{a_{\mathrm{Na}}^0{N}_{\mathrm{Ca}}}$$

(1)

where φ₀ (V) is the surface potential, R (J·K⁻¹·mol⁻¹) is the gas constant; T(K) is the absolute temperature; F (C·mol⁻¹) is the Faraday Constant; β_Ca and β_Na are the effective charge coefficients of Ca²⁺ and Na⁺, respectively, and β_Ca = −0.0213 ln(I^0.5) + 1.2331, β_Na = 0.0213 ln(I^0.5) + 0.766; I is the ionic strength, unit is mol·L⁻¹. a⁰_Ca, a⁰_Na (mol·L⁻¹) the equilibrium concentration of Na⁺ and Ca²⁺, in bulk solution respectively; N_Na, N_Ca (mol·L⁻¹) are the adsorbed quantities of Na⁺ and Ca²⁺ respectively in the diffuse layer at equilibrium. a⁰_Ca, a⁰_Na were measured atomic absorption spectrometer and flame photometer respectively [20,33].

(2)

Surface charge density σ0:

$$\sigma 0=\mathrm{sgn}(\varphi 0)\sqrt{\frac{\epsilon \mathrm{RT}}{2\Pi }(a_{\mathrm{Na}}^0\exp (-\frac{{\beta }_{\mathrm{Na}}F\varphi 0}{\mathrm{RT}})+a_{\mathrm{Ca}}^0\exp (-\frac{2{\beta }_{\mathrm{Ca}}F\varphi 0}{\mathrm{RT}}))}$$

(2)

where σ0 (C·dm⁻²) is the surface charge density, ε is the dielectric constant (8.9 × 10⁻¹⁰ C²·J⁻¹ dm⁻¹ for water) [20,33].

(3)

Surface electric field strength E₀

$$E_0=\frac{4\Pi }{\epsilon }\sigma 0$$

(3)

where E₀ (V·dm⁻¹) is the electrostatic field strength at the surface [20,33].

(4)

specific surface area S:

$$S=\frac{N_{\mathrm{Na}}k}{m{a}_{\mathrm{Na}}^0}\exp (\frac{{\beta }_{\mathrm{Na}}F\varphi 0}{2\mathrm{RT}})=\frac{N_{\mathrm{Ca}}k}{m{a}_{\mathrm{Ca}}^0}\exp (\frac{{\beta }_{\mathrm{Ca}}F\varphi 0}{2\mathrm{RT}})$$

(4)

where S is the specific surface area, dm²·g⁻¹; m = 0.5259 ln (c⁰_Na/c⁰_Ca) + 1.992, c⁰_Na, c⁰_Ca (mol·L⁻¹) are concentration of Na⁺ and Ca²⁺ in bulk solution. c⁰_Na and c⁰_Ca were measured by a flame photometer and an atomic absorption spectrometer, respectively. κ is the Debye parameter, and the unit is L·dm⁻¹, the reciprocal κ⁻¹ represents the thickness of the colloidal particle diffusion electric double layer, and its value can be expressed as κ = (8πF²(a⁰_Ca + 3a⁰_Na)/εRT)^0.5 [20,33].

(5)

Surface charge number SCN:

$$SCN={{\displaystyle 10}}^5\frac{S\sigma 0}{F}$$

(5)

In the formula, SCN is the surface charge number, cmol·kg⁻¹ [20,33].

2.5. Data Analysis and Processing

All data were recorded and summarized in Excel 2003. After performing a linear correlation analysis, data collected was subjected to Variance Analysis (ANOVA) using SPSS version 16.0 statistical software. Least Significant Difference (LSD) at a significant level of 5% and 1% was used to compare treatment means to test their significance in variation. Canoco 4.5 was used to perform the redundancy analysis. Results were presented in tables and graphs using Origin software version 9.0.

3. Results

3.1. SOC Density Fractionation

In

Table 3. SOC concentrations in the whole soil and different density fractions.

Treatments	SOC	Fr-LFOC	Oc-LFOC	HFOC
CK	3.051 ± 0.095 e	0.067 ± 0.001 d	0.086 ± 0.016 e	2.896 ± 0.002 e
FG	3.678 ± 0.128 d	0.409 ± 0.01 c	0.175 ± 0.002 d	3.094 ± 0.007 d
CS	3.809 ± 0.068 c	0.414 ± 0.01 c	0.118 ± 0.009 c	3.274 ± 0.003 c
SM	6.174 ± 0.089 b	0.751 ± 0.021 b	0.257 ± 0.013 b	5.168 ± 0.005 b
GS	7.679 ± 0.154 a	0.816 ± 0.006 a	0.319 ± 0.003 a	6.534 ± 0.017 a

Note: The values are mean ± standard deviation. Different lowercase letters indicate a significant difference between the different treatments. (P < 0.05).

, the content of SOC (g per kg) was significantly different under different organic wastes application (p < 0.05). The content of SOC in the different treatments was ranked as follows: GS>SM>CS>FG>CK, whereby the content of SOC of treatment GS increased by 151.69%, compared with CK (p < 0.05) (Table 3) and was also higher than the other organic waste treatments. The content of SOC in treatment SM, CS, and FG increased by 102.36%, 24.84%, and 20.55%, respectively, compared with CK treatment. The return of organic wastes to the field increased the SOC content in all cases.

After the application of organic wastes in the primary saline land, the LFOC (included Fr-LFOC and Oc-LFOC) contents increased significantly, accounting for 13.98–16.32% of SOC. The LFOC increased by 178.49–225.1% after the application of organic wastes compared with CK (Figure 1). Compared to CK, the content of Fr-LFOC of GS, SM, FG, and CS increased by 383.64%, 452.73%, 405.45%, and 394.55%, respectively. The Oc-LFOC contents after the application of GS, SM, FG, and CS also increased by 47.52%, 47.52%, 68.79%, and 9.93% respectively which was significantly higher than CK. However, the content of HFOC decreased after application of organic wastes. Compared with CK, the application of GS, SM, FG, and CS, decreased the proportion of total organic carbon in the HFOC by 10.3%, 11.9%, 11.43%, and 9.43% respectively.

3.2. Soil Surface Charge Properties

Equilibrium activity and equilibrium concentration of primary saline-alkali soil under the application of organic wastes were measured by a flame photometer and an atomic absorption spectrometer respectively (

Table 4. Equilibrium activity and equilibrium concentration of Ca²⁺ and Na^+.

Treatments	a0Ca	a0Na	c0Ca	c0Na
	mmol·L−1
CK	0.68 ± 0.02 e	16.18 ± 0.42 a	0.83 ± 0.04 e	16.53 ± 0.39 a
FG	1.25 ± 0.04 c	13.01 ± 0.17 c	1.73 ± 0.05 c	13.97 ± 0.21 d
CS	1.45 ± 0.03 a	13.83 ± 0.18 b	1.85 ± 0.05 b	14.71 ± 0.27 b
SM	1.18 ± 0.02 d	13.14 ± 0.14 c	1.67 ± 0.08 d	14.23 ± 0.22 c
GS	1.34 ± 0.06 b	12.36 ± 0.08 d	1.96 ± 0.11 a	13.47 ± 0.38 e

Note: The values are mean ± standard deviation. Different lowercase letters indicate a significant difference between the different treatments. (P < 0.05).

). After the application of organic materials, significant differences were observed in the φ₀, σ₀, E₀, S, and SCN of the primary saline-alkali soil. The φ₀ was ranked as follows: GS > SM > CS > FG > CK. The φ₀ value in treatment GS, SM, CS, and FG increased by 23.44%, 19.60%, 18.87%, and 18.58% respectively compared with CK treatment. The σ₀ value of GS treatment, SM treatment, CS treatment, and FG treatment increased by 56.80%, 51.85%, 46.91%, and 48.15% respectively compared with CK treatment. The value of E₀ was significantly different under the different organic wastes application (p < 0.05). The E₀ value of treatment GS, SM, CS, and FG increased by 55.87%, 51.57%, 46.50%, and 47.47% respectively compared with CK treatment. The value of S in treatment GS, SM, CS, and FG increased by 22.21%, 28.23%, 17.99%, and 57.74% respectively compared with CK. The SCN was ranked as follows: GS > SM > FG > CS > CK, and the SCN value of GS, SM, CS, and FG increased by 85.69%, 37.90%, 15.56%, and 38.02% respectively compared with CK (

Table 5. Soil surface electrochemical properties under different organic materials.

Treatments	φ0/mv	σ0/c·m−2	E0 108/v·m−1	S/m2·g−1	SCN/cmol·kg−1
CK	−95.99 ± 2.47 d	0.24 ± 0.03 a	3.43 ± 0.47 a	39.85 ± 2.29 e	10.04 ± 1.98 a
FG	−78.15 ± 2.07 c	0.42 ± 0.04 b	6.01 ± 0.32 c	47.02 ± 3.02 c	20.75 ± 1.34 c
CS	−73.49 ± 2.37 a	0.36 ± 0.02 c	5.05 ± 0.23 e	48.70 ± 2.35 c	18.03 ± 1.15 d
SM	−77.15 ± 2.55 b	0.39 ± 0.07 c	5.54 ± 0.11 d	51.1 ± 2.17 b	20.79 ± 1.06 c
GS	−77.88 ± 2.92 b	0.43 ± 0.01 b	6.12 ± 0. 24 b	62.86 ± 2.52 a	28.27 ± 1.85 b

Note: The values are mean ± standard deviation. Different lowercase letters indicate a significant difference between the different treatments. (P < 0.05).

).

3.3. Correlation and Redundant Analysis

All the correlations shown in

Table 6. Correlation analysis between SOC and soil organic carbon fractions.

	SOC	Fr-LFOC	Oc-LFOC	HFOC
SOC	1
Fr-LFOC	0.859 *	1
Oc-LFOC	0.932 **	0.889 *	1
HFOC	0.994 **	0.801 *	0.901 *	1

Note: * denotes significant difference at p < 0.05 while ** denotes significant difference at p < 0.01.

are positive. All the carbon fractions increased with SOC Such that, was a significant positive correlation between SOC contents and Fr-LFOC, Oc-LFOC, and HFOC contents (p < 0.05). Correlation analysis between SOC and soil surface charge properties (

Table 7. Correlation analysis between soil organic carbon and surface electrochemical properties.

	SOC	Fr-LFOC	Oc-LFOC	HFOC
φ0	0.429	0.711 **	0.486	0.365
σ0	0.655 **	0.832 **	0.793 **	0.601 *
E0	0.632 *	0.823 **	0.773 **	0.575 *
S	0.927 **	0.901 **	0.91 **	0.914 **
SCN	0.841 **	0.909 **	0.908 **	0.807 **

Note: * denotes significant difference at p < 0.05 while ** denotes significant difference at p < 0.01.

) shows that the correlation between SOC and φ₀ is not significant, however, SOC is significantly positively correlated with E₀ (p < 0.05), and other soil surface charge properties (p < 0.01). The Fr-LFOC was significantly positively correlated with all surface charge properties (p < 0.01). The Oc-LFOC was not significantly correlated with φ₀, however, there was a significant positive correlation between Oc-LFOC and other soil surface charge properties (p < 0.01). The correlation between HFOC content and φ₀ was not significant, however, there was a significant positive correlation between σ₀ and E₀ (p < 0.05). Similarly, there was a significant positive correlation between S and SCN (p < 0.01).

Figure 2 shows the redundant analysis (RDA) of soil organic carbon and soil surface charge properties under different organic materials applications. It can be seen from the figure that the first axis and the second axis respectively account for 71.2% and 17.5% of soil organic carbon. Treatment CK is distributed in the first quadrant, GS is distributed in the second quadrant, SM is distributed in the third quadrant, while CS and FG are distributed in the fourth quadrant. This showed that there were differences in the five treatments. The content of SOC and Fr-LFOC (F = 24.704, p = 0.004; F = 19.594, p = 0.002) were the main factors affecting the surface charge properties, and the interpretation rates were 71.2% and 19.594%, respectively (

Table 8. Interactions between soil organic carbon and surface electrochemical properties.

Index	Interpretation Rate %	F-value	p-value
SOC	71.2	24.704	0.004
Fr-LFOC	16.4	19.594	0.002
Oc-LFOC	11.1	1.7	0.199
HFOC	0.1	0.2	0.78

Note: Data was derived from RDA.

).

4. Discussion

Many studies have shown that the application of organic waste is conducive to the accumulation of SOC [34,35]. Hu, Wu, Qu [36] found that applying agricultural organic waste to the field under semi-arid drip irrigation increases SOC content. Cui, Meng, Wang, et al. [35] also reported that the addition of crop straw enhances the fixation and maintenance of organic carbon. The main reason for these observations is because, agricultural wastes are rich in carbon source materials [37], and the application of organic waste is equivalent to supplementing the carbon sources in its organic carbon pool [38]. Secondly, the application of fresh organic waste promotes the original undecomposed organic matter of the soil [39], such as animal and plant residues [40]. This helps to release organic carbon into the soil through the process of mineralization [41]. The application of organic waste to the soil does not only improve SOC accumulation however it also enhances soil nutrients which promotes crop growth and increases the root coefficient of crops thereby improving the adsorption and fixation of atmospheric CO₂ by crop roots [42]. In our study, treatments GS and SM were more conducive to the accumulation of SOC than the other treatments. Treatments GS and SM increased SOC content by 151.69% and 101.47% respectively (Table 3). The main reason is that the organic waste itself has different C: N ratios and studies have shown that when the C: N ratio of organic material is closer to 25:1 the greater the accumulation of organic carbon in the soil during the decomposition process as this favors microbial activity [43]. The C: N ratio of treatments SM and GS were 29.9:1 and 34.36:1 respectively (Table 2), which was closer to 25:1. This increased microbial activity and carbon production. Part of the carbon evaporates into the atmosphere in the form of carbon dioxide and the rest stays in the soil in the form of organic carbon through bacterial carbon fixation. Another reason could be due to the destruction of the structure of these two kinds of organic waste (SM and GS) which makes them more easily decomposed and transformed to form organic matter compared to the other treatments [44]. This is because sheep manure (SM) comes from sheep that feed on grass. The grass is chewed, digested and excreted by sheep to form sheep manure. This process of digestion destroys the part of the grass that is difficult to break down [45]. The granular corn straw (GS) was the normal corn straw which has been crushed and compacted under high temperature. This process breaks the phloem and xylem of corn stalks thereby making it easy to decompose [46].

LFOC includes Fr-LFOC and Oc-LFOC. They usually form the non-decomposed or incompletely decomposed animal and plant residues. HFOC also refers to the carbon which is adsorbed by soil minerals or exists between soil aggregates [47]. The application of organic waste is conducive to the accumulation of soil SOC, as such the organic carbon content of each component also increased during the study (Table 3). Sun, Huang, Yu, et al. [48] reported similar results. Correlation analysis (Table 6) showed that soil Fr-LFOC content, soil Oc-LFOC carbon content and SOC content was positively correlated (p < 0.05). Also, there was a positive correlation between HFOC, Fr-LFOC, and Oc-LFOC (p < 0.05). Rong and Ji [49] also found out that tree litter increases the carbon content and its components in the soil. Guan, Liu, Zhang, et al. [50] returned charred and uncharred maize straw to the field and reported that both straws could increase the soil organic carbon content and its components. This is consistent with our research however, our study found that after the application of organic waste, the proportion of Fr-LFOC and Oc-LFOC in the soil increased, while the proportion of HFOC decreased (Figure 1). The main reason for this observation was attributed to the fact that the amount of non-decomposed or undecomposed crop residues in the soil increased after the application of organic waste, while the addition of fresh organic waste stimulated the mineralization of the HFOC in the soil [51]. The rate of formation of HFOC is much lower than the rate of mineralization and decomposition of HFOC [52]. The test site is an original saline-alkali land, with a dry climate and the high salinity of the soil environment makes the decomposition of organic waste and the formation of stable organic carbon components not conducive [53,54].

Many studies have shown that the effect of organic waste on the surface charge properties of the soil is mainly due to a large number of organic colloids produced by the decomposition of organic waste [55,56,57]. The soil organic colloid is a small component of the soil that plays a very significant role by acting as an active component that affects soil surface electrical charge properties [58,59]. At the same time, the soil colloidal particles are small and have a large specific surface area [60]. Our study found that treatment GS had a greater impact on soil surface charge properties compared with the other treatments (Table 4 and Table 5), which is related to the different rates of decomposition of various organic wastes in the soil. The faster the decomposition rate, the more organic matter is produced [61]. On the one hand, after pulverization of the corn straws under high temperature and high-pressure treatment to form the granules, the xylem and phloem parts which are difficult to be decomposed are damaged [62]. The pulverization process also increased the specific surface area of the original materials which provides a place for microbial growth and reproduction to promote the decomposition of organic waste by microorganisms [63]. Hence SM and GS had more SOC content compared to the other treatments mainly as a result of the increase in the rate of organic matter production compared to the other treatments.

Although clay particles and soil organic matter are known to be the major factors that affect the surface charge properties of soil however in this study, the factors affecting the soil surface charge properties were analyzed with respect to the carbon fractions (organic matter). Thus the carbon fractions that significantly affect surface charge properties of soil were determined in this study (Table 7). The content of SOC and Fr-LFOC (F = 24.704, p = 0.004; F = 19.594, p = 0.002) were identified as the main factors affecting the surface charge properties through redundancy analysis. The interpretation rates were 71.2% and 19.594%, respectively. SOC and Fr-LFOC generally exist in soil as colloids, while Oc-LFOC, HFOC have a higher degree of humification compared with Fr-LFOC, and are physically protected by soil aggregates and minerals especially clay, which highly affects soil charge properties. Wu, Liu, Li [64] found that more than 75% variable surface charge originated from soil organic matter. The reason why soil SOC and Fr-LFOC affect surface charge properties is that soil organic carbon is usually negatively charged, and studies have shown that every 1% increase in organic carbon content, can increase the soil charge by 1 cmol·kg⁻¹ [65]. Soil organic carbon is generally present in organic colloids, and the organic colloid is an amphoteric colloid [66]. As the content of organic colloid increases, the amount of cation exchange increases and the surface charge density increases thereby strengthening the soil electric field [67]. The specific surface area of organic carbon such as humus carbon in the soil organic carbon pool is about 800–900 m²·g⁻¹, which is 10 times larger than the specific surface area of general inorganic minerals. Therefore, the soil organic carbon content increases with increasing soil specific surface area.

5. Conclusions

The content of SOC increased after the application of different organic wastes. Moreover, the application of organic wastes increased the content of each SOC density fraction. Similarly, the application of organic wastes, increased the proportion of organic carbon in free light fraction (Fr-FLOC), the organic carbon in occluded fraction (Oc-FLOC) and the total heavy fraction (HFOC) in the soil. However, the proportion of organic carbon in heavy fraction (HFOC) decreased. Among all the treatments, GS recorded the highest content of SOC and also had a greater impact on soil surface charge properties compared to the other treatments. The redundancy analysis (RDA) also showed that the content of SOC and Fr-LFOC (F = 24.704, p = 0.004; F = 19.594, p = 0.002) were identified as the main factors affecting the surface charge properties of the soil organic carbon. In conclusion, the application of different organic wastes can effectively improve the SOC content and soil surface charge properties and we recommend GS as the most effective organic waste for ameliorating primary saline-alkali soil, as compared with the other organic waste treatments.