Influence of Different Soil Types on Dissolved Organic Matter Spectral Characteristics of Soil Leachate After Green Manure Tilling in Saline Soils

Abstract

To investigate the changes in the composition and structure of the dissolved organic matter (DOM) of the lysate solutions of different types of soil after green manure tilling treatment, we set up two types of soil materials (fluvo-aquic soil; coastal saline soil) and three green manure tilling treatments (T1: CK—without green manure, T2: tilling Dongmu70 rye, and T3: tilling rapeseed green manure); then, the soil leachate was obtained with a soil column simulation test and its DOM spectral properties were determined. The rapeseed green manure leachate demonstrated a significantly higher humic macromolecule content and aromaticity compared to Dongmu70 rye leachate. Fluorescence Index (FI) values (1.5–2.2) suggest a mixed origin of dissolved organic matter (DOM) from both terrestrial and microbial-derived sources. All Humification Index (HIX) values remained below 1, indicating low humification levels and limited stabilization of DOM within the leachate system, and Biological Index (BIX) values exceeding 1 across all soil layers highlight the predominance of a recent biological metabolism in shaping DOM autochthonous origins. The SUVA₂₆₀ values in Dongmu70 rye–moist soils and rapeseed green manure–coastal saline soil exhibited reductions of 0.020–2.573 L·(mg·m)⁻¹ relative to pre-drenching levels. After tilling rapeseed green manure, the SUVA₂₅₄ value of coastal saline soil at the 60–90 cm layer decreased by 1.941 L·(mg·m)⁻¹. This study shows that differences in green manure and soil type affect DOM sources and composition, reducing DOM leaching, with coastal saline soil + rapeseed green manure and fluvo-aquic soil + Dongmu70 rye being the advantageous combinations. The study results provide theoretical guidance for applying green manure coupled with freshwater leaching technology in the context of saline and alkaline land with multiple soil types.

1. Introduction

Soil-dissolved organic matter (DOM), comprising dissolved organic carbon, nitrogen, and other constituents [1], constitutes a crucial nutrient pool in terrestrial ecosystems [2]. Its solubility characteristics predispose DOM to mineralization, decomposition, and leaching losses during soil percolation processes [3]. Contemporary analytical approaches utilize UV–visible absorption spectroscopy and three-dimensional fluorescence spectroscopy to characterize DOM dynamics [4,5]. Specific absorbance indices (UV₂₅₄, UV₂₅₃/UV₂₀₃, SUVA₂₅₄, SUVA₂₆₀) effectively quantify aromaticity and structural complexity, while fluorescence parameters (FI, BIX, HIX) reveal DOM provenance, stability, and the humification state [6].

In the process of saline soil improvement in freshwater, salt drenching and green manure tilling have shown good benefits [7,8], and it has been shown that green manure tilling after returning to the field can slow down soil erosion to a certain extent [9]. At the same time, green manure residue decomposition releases a certain amount of organic matter and humus material, which is conducive to the enhancement of the soil’s organic matter content [10]. Although desalination irrigation measures achieve significant reductions in surface soil salinity [11], they inevitably modify soil structural integrity and promote leaching losses of soluble soil nutrients, including nitrogen and phosphorus [12]. Notably, this occurs in the Yellow River Delta region, which is a component of coastal saline soil [13] where green manure tillage and freshwater leaching are extensively implemented in agriculture [14]. However, there are several types of soils in the Yellow River Delta region, such as fluvo-aquic soil and coastal saline soil [15], with marked variations in dissolved organic matter (DOM) composition and concentration across soil types [16], which may cause changes in soil adsorption capacity [17]. Taken together, due to the change of soil structure and physicochemical properties from the application of green manure, it may affect the spectral characteristics of soil leaching solution DOM and change the composition and distribution of soil DOM. Additionally, there have been more studies analyzing the characteristics of soil type, green manure tilling, and freshwater salinity as independent factors on soil DOM but fewer studies on the results of multi-factor coupling.

Based on the above research basis, this present investigation selected fluvo-aquic soil and coastal saline soil from the Yellow River Delta to establish simulated soil column leaching experiments incorporating various green manure types. These experiments were designed to examine alterations in dissolved organic matter (DOM) spectral characteristics within saline soil leachates under combined freshwater leaching and green manure incorporation regimes. It aims to provide theoretical support for the coupled application of green manure and freshwater leaching measures in different types of saline soils.

2. Materials and Methods

2.1. Experimental Site and Soil

The test soils were two typical loamy soils in the Yellow River Delta region of Dongying City, Shandong Province: fluvo-aquic soil and coastal saline soil, which were distributed in the south and north of the region, respectively. According to the Word Reference Base for Soil Resources 2022 (WRB 2022), fluvo-aquic soil belongs to Cambisols and coastal saline soil belongs to Solonchaks. The parent material of the two soils is the alluvial material of the Yellow River, and according to the texture classification of the USDA particle size classification, the two soils belong to loamy soil and sandy soil in texture. The soil was sampled at depths of 30 cm, 60 cm, and 90 cm, and the fluvo-aquic soil was sampled from the Yellow River Delta Agricultural High-tech Industrial Demonstration Zone, Guangrao County, Dongying City, Shandong Province (37.11183° N, 118.654574° E). The composition of the soil particles from 0 to 30 cm was determined to be powdery, sandy, and clayey at 78.91%, 3.3% and 17.79%; the composition from 30 to 60 cm was 77.21%, 11.6% and 11.5%, and the composition from 60 to 90 cm was 77.82%, 13.34% and 8.82%. The coastal saline soil was extracted from a farm near Boxin Road, Kenli District, Dongying City, Shandong Province (37.566104° N, 118.725157° E). The composition of the soil particles from 0 to 30 cm was determined in the laboratory to be powdery, sandy, and clayey at 45.5%, 52.29%, and 2.19%; the composition from 30 to 60 cm was 41.11%, 54.48%, and 4.41%; the composition from 60 to 90 cm was 35.28%, 60.56%, and 4.17%;The basic soil physicochemical properties are shown in

Table 1. Basic properties of soil for testing.

Physical and Chemical Properties	Fluvo-Aquic Soil 0–30 cm	Fluvo-Aquic Soil 30–60 cm	Fluvo-Aquic Soil 60–90 cm	Coastal Saline Soil 0–30 cm	Coastal Saline Soil 30–60 cm	Coastal Saline Soil 60–90 cm
pH	8.77	9.12	9.21	8.86	9.51	9.11
EC (ms·cm−1)	0.22	0.19	0.22	0.06	0.11	0.27
Organic matter (g·kg−1)	13.48	5.07	3.96	5.24	2.61	1.46
Total nitrogen (g·kg−1)	0.77	0.33	0.27	0.21	0.13	0.09
Available K (mg·kg−1)	275.44	142.44	163.23	27.59	24.29	32.87
Available P (mg·kg−1)	6.37	4.60	2.24	3.01	1.42	0.47

. As can be seen in the Table 1, the pH and EC of the coastal saline soil were higher in each soil layer compared to the fluvo-aquic soil, while the fluvo-aquic soils had a higher content of nutrients such as organic matter, total nitrogen, quick-acting phosphorus, and quick-acting potassium.

2.2. Spectral Characteristics of Green Manure Leachate for Testing

To simulate the decay process of green manure, before the test started, the green manure residue and water were mixed in a ratio of 1:10 and fully soaked to obtain green manure leachate, and the same batch of green manure was continuously soaked three times; this leachate was used as the original solution for drenching treatment of the soil column in triplicate. The absorbance value of the obtained leachate was determined using UV–visible absorption spectrometry at a wavelength of 200–300 nm. The absorbance values of rapeseed green manure were higher than those of Dongmu70 rye at 254 nm, 253 nm, and 230 nm, which indicated that rapeseed green manure leachate possessed more humus macromolecules and a higher degree of aromatization under the same conditions (Figure 1).

2.3. Experimental Design

The experiment was conducted from May 2023 to August 2023. Soil columns were constructed using PVC pipes (100 cm height × 20 cm diameter) with predetermined three-layer soil capacities that guided subsequent stratified soil packing. Ryegrass and rapeseed green manure, which are suitable for the local environment and widely used in agricultural production in the Yellow River Delta region, were selected as green manure materials. The experimental design incorporated three green manure incorporation treatments (T1—CK, without green manure; T2—tilling Dongmu70 rye; T3—tilling rapeseed green manure) and two soil types (NC—fluvo-aquic soil, NS—coastal saline soil), resulting in five treatments with triplicate columns per treatment. Five leaching events were implemented during the experimental period, including three controlled green manure leachate applications and two natural rainfall simulations.

Following the saturation and stabilization of soil columns, initial leaching was performed on 28 May 2023 using 1142.5 mL of green manure leachate (equivalent to three months of rainfall). Leachate was collected in glass bottles and allowed to settle, and the supernatant was transferred to sample bottles for 4 °C refrigeration as the pre-leaching controls. Subsequent rinse tests were conducted on 3 June 2023 and 3 July 2023 under the same conditions. After the first three drenching cycles, the above soaked green manure residues were tilled at a depth of 10–15 cm in the soil columns of T2 and T3 at a rate of 7027.65 kg∙hm⁻² (Dongmu70 rye) and 6000 kg∙hm⁻² (oilseed and rapeseed green manure), respectively.

Natural rainfall simulations were conducted on 20 July 2023 and 3 August 2023, using natural rainwater as the eluent and final leachate collected on 3 August 2023 as the post-leachate sample. All collected samples were stored refrigerated at 4 °C prior to analysis.

2.4. Data Measurement and Analysis

2.4.1. Data Measurement

The aromaticity and conjugate structure of DOM were determined with an Agilent 8453 UV–visible Absorption Spectrometer manufactured by Agilent, Santa Clara, CA, USA, and the three-dimensional Fluorescence Index of DOM was determined with a Hitachi F-7000 Fluorescence Spectrophotometer manufactured by Hitachi, Tokyo, Japan.

2.4.2. Spectral Data Selection

UV₂₅₄, UV₂₅₃/UV₂₀₃, SUVA₂₅₄, and SUVA₂₆₀ were selected for UV spectral parameters, and the Fluorescence Index (FI), Humification Index (HIX), and Biological Index (BIX) were selected for 3D fluorescence spectroscopy. FI, HIX, and BIX were obtained by calculating the fluorescence intensity ratios at excitation wavelengths of Ex = 370 nm; emission wavelengths of 450 nm and 500 nm, Ex = 255 nm; emission wavelengths of 435–480 nm and 300–345 nm, Ex = 310 nm; and emission wavelengths of 380 nm and 430 nm, respectively.

2.4.3. Other Data Analysis

The experimental data were preprocessed using Excel 2019, and SPSS 22.0 software was used for statistical analysis, significance analysis, and correlation analysis and was plotted using Origin 2018.

3. Results

3.1. Characteristics of the UV Parameters of the DOM

3.1.1. UV₂₅₄

The UV₂₅₄ values of DOM of the leaching solution of each soil before and after freshwater salting by different treatments are shown in Figure 2 and Figure 3. In the 0–30 cm layer of the fluvo-aquic soil, the UV₂₅₄ values of the leaching solution DOM of the T1, T2, and T3 treatments decreased by 0.002 cm⁻¹, 0.003 cm⁻¹, and 0.003 cm⁻¹, respectively, after freshwater salting compared with that before the treatment, whereas in the 30–60 cm layer of the soil, the UV₂₅₄ values of the leaching solution DOM of the T1 and T2 treatments increased by 0.011 cm⁻¹ and 0.018 cm⁻¹ after freshwater salting compared with that before the salting. However, the T3 treatment decreased by 0.080 cm⁻¹ and 0.018 cm⁻¹ and 0.011 cm⁻¹ and 0.018 cm⁻¹, respectively, before and after salting but decreased by 0.032 cm⁻¹ for T3 treatment. In the 60–90 cm soil layer, the UV₂₅₄ values of DOM of drench solution in T1 and T2 treatments increased by 0.014 cm⁻¹ and 0.015 cm⁻¹ after freshwater drenching compared with that before freshwater drenching, respectively, while they decreased by 0.011 cm⁻¹ in the T3 treatment (Figure 2). Unlike fluvo-aquic soils, in coastal saline soil, in the 0–30 cm soil layer, the UV₂₅₄ values of DOM in the T1 and T3 soil drench solutions decreased by 0.002 cm⁻¹ and 0.009 cm⁻¹ after freshwater drenching, whereas the T2 treatment increased by 0.006 cm⁻¹. In the 30–60 cm soil layer, the UV₂₅₄ values of DOM in the drench solution of the T1 treatment increased by 0.007 cm⁻¹ after freshwater drenching. The UV₂₅₄ value of soil drench solution DOM of T1, T2, and T3 treatments decreased by 0.002 cm⁻¹, 0.004 cm⁻¹, and 0.013 cm⁻¹, respectively, after freshwater drenching in the soil layer from 60 to 90 cm (Figure 3). Overall, with the freshwater drenching process, the irrigation water entered the deeper layers of the soil, the UV₂₅₄ mean value of DOM in the leaching solution of each treatment of fluvo-aquic soil T1 and T2 showed a gradually increasing trend, and the T3 treatment showed a gradually decreasing trend, which indicated that the rapeseed green manure could be better in avoiding the content of DOM in the leaching solution gradually leaching out with irrigation water in order to increase the content of DOM in the fluvo-aquic soil. The coastal saline soil in the 60–90 cm layer of the T2 and T3 treatments decreased more than the T1 treatment, indicating that ryegrass and rapeseed green manure can better avoid the content of DOM in the leaching solution in the deep soil from gradually leaching away with irrigation water in order to increase the content of DOM in the deep coastal saline soil.

3.1.2. UV₂₅₃/UV₂₀₃

Figure 4 and Figure 5 show the variation of the UV₂₅₃/UV₂₀₃ values of soil drench solution DOM. For fluvo-aquic soil (Figure 4), in the 0–30 cm soil layer, the UV₂₅₃/UV₂₀₃ values of the lysate DOM of each treatment of T1, T2, and T3 decreased by 0.005 cm⁻¹, 0.012 cm⁻¹, and 0.005 cm⁻¹, respectively, after freshwater drenching and salting. In the 30–60 cm layer, the UV₂₅₃/UV₂₀₃ values of the lysate DOM of each treatment of T1 and T2 increased by 0.003 cm⁻¹ and 0.027 cm⁻¹ and by 0.003 cm⁻¹ and 0.027 cm⁻¹, respectively, after freshwater drenching and salting. The UV₂₅₃/UV₂₀₃ values of the drench solution DOM in the soil layer from 30 to 60 cm in the T1 and T2 treatments increased by 0.003 cm⁻¹ and 0.027 cm⁻¹ after freshwater drenching, while T3 decreased by 0.027 cm⁻¹; in the soil layer from 60 to 90 cm in the T1 and T2 treatments, the UV₂₅₃/UV₂₀₃ values of the drench solution DOM in the soil layer from 30 to 60 cm in the T1 and T2 treatments increased by 0.180 cm⁻¹ and 0.004 cm⁻¹ after freshwater drenching compared with that before freshwater drenching, while T3 decreased by 0.008. The overall trend was that T1 and T2 treatments increased with depth in the 30–90 cm soil layer, while T3 showed a decreasing trend.

The UV₂₅₃/UV₂₀₃ values of the T1 and T2 treatments in the 0–30 cm soil layer of the coastal saline soil (Figure 5) increased by 0.190 and 0.014 cm⁻¹, respectively, and decreased by 0.031 cm⁻¹ in T3; all treatments increased in the 30–60 cm soil layer (T1: 0.087, T2: 0.006, T3: 0.037 cm⁻¹); and in the 60–90 cm layer, respectively, they increased by 0.004, 0.033, and 0.064 cm⁻¹. Especially, in the 60–90 cm soil layer, the T3 treatment decreased significantly compared with T1, which was consistent with the changing pattern of fluvo-aquic soil. The data in the two figures jointly indicated that the rapeseed green manure (T3) could effectively inhibit the phenomenon of DOM leaching with irrigation water, and showed the enhancement effect of DOM content in the deeper layers (30–90 cm) of both fluvo-aquic soil and coastal saline soils, with the difference between the T3 treatments and T1 in the 60–90 cm layer of coastal saline soils being particularly significant.

3.1.3. SUVA₂₅₄

The SUVA₂₅₄ values of soil leaching solution DOM before and after different treatments of freshwater leaching are shown in Figure 6 and Figure 7. In the 0–30 cm layer of fluvo-aquic soil, the SUVA₂₅₄ values of T1, T2, and T3 treatments decreased by 0.680, 0.059, and 0.793 L·(mg·m)⁻¹ after salting compared with those before salting; in the 30–60 cm layer of soil, the T1, T2, and T3 treatments decreased by 0.869, 0.179, and 0.007 L·(mg·m)⁻¹, respectively; in the 60–90 cm layer of soil, the SUVA₂₅₄ values increased significantly, by 3.795, 2.527, and 3.733 L·(mg·m)⁻¹, respectively. The overall presentation showed a significant increase in SUVA₂₅₄ values in the deeper soils (60–90 cm) after leaching, with the ryegrass treatment (T3) showing the lowest leaching in the fluvo-aquic soil (Figure 6).

The coastal saline soil (Figure 7) showed a different pattern: the SUVA₂₅₄ values of the T1 and T2 treatments in the 0–30 cm soil layer increased by 1.437 and 0.721 L·(mg·m)⁻¹, respectively, and decreased by 1.485 cm⁻¹ in T3; in the 30–60 cm soil layer, T1 and T2 increased by 0.974 and 0.130 L·(mg·m)⁻¹ and decreased by 2.545 L·(mg·m)⁻¹ in T3; and in the 60–90 cm soil layer, only T1 kept increasing (+0.565 cm⁻¹), while T2 and T3 decreased by 0.859 and 1.941 L·(mg·m)⁻¹, respectively. It is noteworthy that the rapeseed green manure treatment (T3) decreased the SUVA₂₅₄ values in the whole 0–90 cm layer of the saline coastal soil.

A comparison revealed that the SUVA₂₅₄ values of treatments decreased with depth before salting of the fluvo-aquic soil, indicating that the content of aromatic compounds in DOM decreased with a deepening of the soil layer; after salting, the SUVA₂₅₄ values of deeper soils rebounded, and the ryegrass treatment (T3) showed a higher ability to retain aromatic compounds in the fluvo-aquic soil. In contrast, the whole soil layer SUVA₂₅₄ value of rapeseed green manure treatment (T3) decreased in coastal saline soil, indicating its selective retention of aromatic compounds in DOM, which can effectively reduce organic matter leaching.

3.1.4. SUVA₂₆₀

Figure 8 and Figure 9 reflect the characteristics of the effect of freshwater salting on the SUVA260 values of the DOM of the leaching solution of different textured soil materials. The differential changes in SUVA₂₆₀ values before and after freshwater salting can be clearly seen in both figures. In Figure 8, the SUVA₂₆₀ values of T1, T2, and T3 in the 0–30 cm soil layer decreased by 0.650, 0.020, and 0.793 L·(mg·m)⁻¹, respectively, and the decreases in the 30–60 cm layer continued but the differences between treatments were expanded (T1: 0.772, T2: 0.356, T3: 1.290 L·(mg·m)⁻¹): the 60–90 cm layer showed an increase (T1: 0.772, T2: 0.356, and T3: 1.290 L·(mg·m)⁻¹) and the differences in the 60–90 cm layer showed a decrease (1.290 L·(mg·m)⁻¹), while the 60-90 cm soil layer showed an increasing trend of, respectively, T1: 3.549, T2: 2.208, and T3: 3.239 L·(mg·m)⁻¹ compared to the pre-lachrymation period, which showed the significant enhancement of deep DOM hydrophobicity under this treatment. In the coastal saline soil (Figure 9), the SUVA₂₆₀ values of T1 and T2 in the 0–30 cm soil layer increased by 1.404 and 0.592 L·(mg·m)⁻¹, while T3 decreased by 1.564 L·(mg·m)⁻¹; T1 and T2 in the 30–60 cm soil layer increased by 0.970 and 0.862 L·(mg·m)⁻¹¹, while T3 decreased significantly by 2.573 cm⁻¹; and in the deeper layer (60–90 cm), T1 remained increased by 0.332 L·(mg·m)⁻¹ while T2 and T3 decreased by 0.432 and 2.069 L·(mg·m)⁻¹, respectively.

The changes of SUVA₂₆₀ and SUVA254 were highly synchronized, and the characteristic of decreasing SUVA₂₆₀ values with depth before fluvo-aquic soil salting further verified the vertical differentiation law of DOM hydrophobic compounds. Ryegrass treatment (T3) affected soil DOM hydrophobicity in fluvo-aquic soil by inhibiting shallow loss (the largest decrease from 0–30 cm) and driving deep accumulation (an increase of 3.239 L·(mg·m)⁻¹ from 60–90 cm), while oilseed and rapeseed green manure (T3) in coastal saline soils had a significantly lower decrease in deep decrease (only 2.069 L·(mg·m)⁻¹) than that of T2 despite a decrease in the whole-soil SUVA₂₆₀ value, which suggests that there is no specific regulation of green manure in soil.

3.2. DOM Fluorescence Spectral Characteristics of Soil Leaching Solution

3.2.1. Fluorescence Index FI

The FI value of soil leaching solution DOM is an important indicator to characterize the source of DOM. In the moist soil, in the 0–30 cm soil layer, the FI of the leaching solution DOM was reduced by 23.64%, 1.20%, and 12.20% for the T1, T2, and T3 treatments, respectively, with the T1 treatment having a significantly higher decrease than that for the T2 and T3 treatments; in the 30–60 cm soil layer, T1 and T2 also had a decrease of 15.99% and 0.85%, respectively, but the T3 was elevated by 10.20%; in the 60~90 cm soil layer, the T1, T2, and T3 treatments were reduced by 8.14%, 4.60%, and 8.43%, respectively (Figure 10). In Figure 11, which represents the coastal saline soil, we can see that in its 0–30 cm soil layer, the FI of the T1 and T2 treatments also decreased by 8.17% and 1.90%, respectively, but the decrease was lower than that of the fluvo-aquic soil; in the 30–60 cm soil layer, the FI of the T1, T2, and T3 treatments increased by 4.56%, 7.80%, and 5.11%, respectively; and in the 60–90 cm soil layer, the FI of the T1 treatment was elevated by 0.73%, while the T2 and T3 treatments decreased by 11.66% and 5.77%, respectively. On the whole, the FI values of DOM from the leaching solution in different soil layers of each treatment ranged from 1.5 to 2.2, indicating that the source of DOM from the soil leaching solution was mainly a mixture of terrestrial and biological sources. Meanwhile, the average FI values of the leachate DOM in each treatment gradually increased with the deepening of the soil depth, indicating that the proportion of biogenic sources of leachate DOM gradually increased. In fluvo-aquic soil, compared with T1 treatment, the decrease of the FI value of the leaching solution in the T2 treatment was smaller, indicating that less DOM was leached, while in coastal saline soil, the decrease of the T3 treatment was smaller in the 30–90 cm soil layer. So, fluvo-aquic soil ryegrass and coastal saline soil oilseed and rapeseed green manure could increase the proportion of biogenic sources of leaching solution DOM in order to reduce the input of terrestrial sources (terrestrial runoff and soil leaching).

3.2.2. Humification Index (HIX)

The variation of the HIX values of soil drench solution DOM is shown in Figure 12 and Figure 13. In this case, the fluvo-aquic soil exhibited elevated HXI values throughout the soil layers, including 17.86%, 30.59%, and 96.21% for T1, T2, and T3, respectively, in the 0–30 cm soil layer; 17.71%, 52.55%, and 108.84%, respectively, in the 30–60 cm soil layer; and 25.90%, 42.95%, and 140.95%, respectively, in the 60–90 cm soil layer, in which the T3 treatment showed a significantly higher elevation than other treatments in the 60–90 cm soil layer (Figure 12). As can be seen in the performance of coastal salt soil in Figure 13, the HIX of the T1 and T3 treatments in the 0–30 cm soil layer were elevated by 104.61% and 36.06%, respectively, but there was no significant change in T2. The trend of the HIX values of coastal saline soil in the 30–60 cm soil layer and the 60–90 cm soil layer was consistent with the fluvo-aquic soil but the magnitude of the change was lower relative to that of the fluvo-aquic soil. The HIX values of each treatment were elevated in two soil layers, 31.35%, 18.43%, and 40.75% and 29.30%, 87.59%, and 68.36%, respectively. Overall, the HIX values of DOM in the leaching solution of different soil layers of each treatment were less than 1, indicating that the degree of DOM humification in the leaching solution was relatively low and poorly stabilized. It can also be seen from the figure that the trends of the HIX values of the three treatments of the lysate DOM in the different sampling periods of each soil layer were basically the same, and the average HIX values of the lysate DOM of the T2 and T3 treatments were higher than those of the T1 in the same soil layer compared to the T1 treatment, which indicated that the green manure could increase the degree of humification and the stability of the lysate DOM and thus improve the degree of humification of the soil.

3.2.3. Biogenic Index BIX

The variation of the Biogenic Index BIX values is shown in Figure 14 and Figure 15. In fluvo-aquic soil, the T1 treatment had the same performance in the 0–30 cm soil layer as it did in the other two successive soil layers, which were elevated by 1.61%, 5.30%, and 11.86%, respectively, with the 60–90 cm being elevated the most, higher than the first two soil layers. The performance of the T2 and T3 treatments in different depths of soil layers was different: in the 0–30 cm layer, the BIX of the T2 and T3 treatments was elevated by 5.12% and 9.11%, respectively, whereas they decreased in the 30–60 cm and 60–90 cm soil layers by 2.21% and 13.52% and 4.35% 21.24%, respectively. Among them, the T3 treatment showed the largest decrease in the 60–90 cm soil layer (Figure 14). The BIX values of the T1 treatment were similar to those of the fluvo-aquic soil in the 0–30 cm soil layer of the coastal saline soil, and all of them increased, with increases of 3.69%, 4.38%, and 5.32%, respectively. The changes in the HIX values of the various layers were not significantly different. Also belonging to the 0–30 cm soil layer, the HIX value of the T3 treatment showed a decrease of 5.03%, which was different from that of T1; in the 30–60 cm and 60–90 cm soil layers, the T2 and T3 treatments were similar to the fluvo-aquic soil with decreases of 15.71%, 2.36%, and 0.76% and 6.27%, respectively, but with smaller decreases than that of the fluvo-aquic soil (Figure 15). The BIX values of the leachate DOM in different soil layers of each treatment were overwhelmingly > 1, which indicated that the leachate DOM was strongly characterized by autochthonous sources, mainly produced by the metabolism of nektonic organisms. Meanwhile, with the increase of soil depth, the difference between the mean BIX values of the T1 and T2 treatments was not significant, while the difference between the mean BIX values of the T3 treatment gradually increased with the deepening of the soil layer, and its mean value was higher than that of the T1 and T2 treatments in the same soil layer, which indicated that the green manure of oilseed and rapeseed could enhance the autochthonous source of the leachate DOM, which reflected stronger microbial activity in the soil.

4. Discussion

This experiment implemented three green manure incorporation treatments (T1: CK—without green manure, T2: tilling Dongmu70 rye, T3: tilling rapeseed green manure) across two types of soil material treatments (NC: fluvo-aquic soil, NS: coastal saline soil). A simulated soil column leaching apparatus generated leachate samples post-treatment, which were subsequently analyzed using UV–Vis and 3D-EEMs. Characteristic changes in UV₂₅₄, UV₂₅₃/UV₂₀₃, SUVA₂₅₄, SUVA₂₆₀, FI, HIX, and BIX values were measured to investigate the effects of two green manure treatments on the DOM spectral characteristics of soil leachate in different types of soil materials.

An analysis of UV spectral parameter variations in soil leachate DOM revealed that during freshwater leaching processes, as irrigation water infiltrated deeper soil horizons, the mean UV₂₅₄ and UV₂₅₃/UV₂₀₃ values in fluvo-aquic soil under T1 and T2 treatments exhibited a gradual increasing trend, whereas the rapeseed green manure treatment (T3) displayed a progressive decrease. This result suggests that tilling oilseed and rapeseed green manure can better maintain DOM levels in fluvo-aquic soil, which is consistent with the findings of ElBishlawi H et al [18]. on soil organic carbon. It has been shown that the application of oilseed and rapeseed green manure can increase the organic matter content of soil and soil DOM [19,20], and this study also found that oilseed and rapeseed green manure leachate contains highly soluble organic matter, so the supplementation of this leachate with exotic organic matter may also be one of the important reasons for its ability to maintain soil DOM levels. In coastal saline soil, the average UV₂₅₄ and UV₂₅₃/UV₂₀₃ values in the 60–90 cm layer under T2 (ryegrass amendment) and T3 (rapeseed green manure) treatments were lower than those of T1 (control), demonstrating enhanced stabilization of DOM in deep soil leachates. This stabilization effect mitigated DOM loss with irrigation water movement while promoting DOM preservation in subsoil horizons. The reasons for this analysis may be similar to their findings in fluvo-aquic soil, both because green manure application affects the content and distribution of organic matter in the soil [21].

The SUVA₂₆₀ and SUVA₂₅₄ trends in soil leachate DOM UV spectral parameters exhibited closely aligned with the variation patterns. The mean SUVA₂₆₀ values of all green manure treatments before freshwater leaching in fluvo-aquic soil gradually decreased with soil depth, indicating a gradual decrease in the relative content of hydrophobic compounds in the soil leaching solution DOM. Parallel observations regarding DOM characteristics have been documented in comparable soil systems, though mechanistic interpretations differ. Similar findings but without elucidating the reasons were found in a further study on the organic matter properties of soil solution from a German marsh [22]. This phenomenon was also found in the study of straw-treated paddy soil DOM at various depths by Huang R et al. [23], who attributed it to the stronger physical and chemical perturbation of the surface soil compared to the deep soil. However, this study prefers the idea that the application of green manure may have influenced the transformation process of hydrophobic compounds [24].

This study also found that fluvo-aquic soil exhibited smaller variations in SUVA₂₅₄ values of leachate DOM before and after leaching compared to coastal saline soil. This may be attributed to fluvo-aquic soil’s stronger adsorption capacity for hydrophobic compounds, which retains more hydrophobic compounds in the upper soil layers [25]. On the other hand, it is possible that the presence of clay minerals and other more complex mineral compositions in coastal saline soil affects the adsorption process of hydrophobic compounds [26]. Taking the results into account, it was found that the content of hydrophobic compounds in soil DOM could be effectively increased by tilling rapeseed green manure in fluvo-aquic soil and by tilling Dongmu70 rye in saline coastal soil.

An analysis of the fluorescence spectral characteristics of DOM in soil leachates revealed that FI values of DOM in leachates from different soil layers under both soil treatments and green manure treatments ranged between 1.5 and 2.2. This indicates that the sources of DOM in soil leachates are predominantly a mixture of terrestrial and biological origins. Both green manure residues and their leachates can act as effective biological sources to replenish DOM. Previous studies have also confirmed the complexity of soil DOM sources, which may include contributions from plant apoplastic material [27]. The proportion of biogenic sources in the DOM of the leachate of each treatment gradually increased with the increase of soil depth, which may be due to the change of soil moisture as well as microorganisms in the deeper soil [28], or it may be that the deeper soil contains more apoplastic material itself, which provides a more stable biogenic source [29].

Fluorescence spectral analysis revealed that HIX values of DOM in leachates from different soil layers across all treatments were less than 1, indicating a low humification degree and poor stability of DOM in leachates. However, HIX values in T2 and T3 treatments were higher than those in T1, suggesting that incorporating green manure crops enhances soil humification and improves DOM stability. This may be attributed to the fact that green manure leachates inherently contain humic-like macromolecular organic matter, which supplements soil DOM and influences humification processes [30], or possibly because green manure increases the proportion of aromatic compounds in the soil [31]. When BIX > 0.8, DOM exhibits distinct autogenic properties [32]. In this study, BIX values of DOM in leachates from different soil layers across all treatments were predominantly > 1, indicating strong autogenic source characteristics of leachate DOM. This may be attributed to either the influence of microbial activity on DOM autogenic-related properties or changes in DOM composition leading to enhanced autogenic characteristics [33,34]. Additionally, this study observed that rapeseed green manure increased BIX values in both soil textures, thereby elevating the autogenicity of DOM in soil leachates. The incorporation of rapeseed green manure, which likely alters soil elemental composition, may be a critical factor contributing to this phenomenon [35].

In summary, this study found that the application of green manure residues affects soil DOM, influenced by soil texture and green manure types. Specifically, incorporating rapeseed green manure in coastal saline soil and ryegrass green manure in fluvo-aquic soil are effective measures to reduce soil DOM loss. Although this study has provided insights into the effects of different green manures on soil DOM, it has not yet fully explored the detailed characteristics of how various green manures interact with soil materials of different textures, nor has it elucidated the underlying mechanisms of these interactions. However, clarifying the action mechanisms of green manures on soil DOM is of great significance for optimizing the composition of soil organic matter, rationally selecting green manure types, and improving soil fertility. Therefore, further in-depth investigation is still needed to fill these knowledge gaps.

5. Conclusions

(1)

The mean values of UV₂₅₄ and UV₂₅₃/UV₂₀₃ for the leaching solution DOM of fluvo-aquic soil under winter leisure and Dongmu70 rye green manure treatments exhibited a gradual increase, while the rapeseed green manure treatment showed a gradual decrease. The trend of SUVA₂₆₀ values for the leaching solution DOM across each treatment mirrored that of the SUVA₂₅₄ values. When comparing the two textured soils, the average SUVA₂₆₀ value for each treatment of drench solution DOM before freshwater leaching in fluvo-aquic soil steadily declined with increasing soil depth, indicating that the relative content of hydrophobic compounds in the soil drench solution DOM diminished over time. This suggests that applying Dongmu70 rye in fluvo-aquic soil and rapeseed green manure in coastal saline soil effectively enhances the content of hydrophobic compounds. Furthermore, utilizing Dongmu70 rye in moist soil and rapeseed green manure in coastal saline soil can significantly raise hydrophobic compounds in DOM.

(2)

The FI values of DOM from different soil layers in each treatment ranged from 1.5 to 2.2, and the sources of DOM from the soil leaching solution were mainly a mixture of terrestrial and biological sources. The HIX values of DOM in the leaching solution of different soil layers in each treatment were all less than 1; the degree of DOM humification in the leaching solution was relatively low, and the stability was poor. The BIX values of DOM in the leaching solution of different soil layers in each treatment were mostly >1. The autochthonous source of DOM in the leaching solution was strongly characterized, and the metabolism of recent organisms mainly produced it.