Control mechanism of short-term fertilization with cattle manure on the release characteristics of soil colloids in farmland: grain size and physicochemical properties

Background Understanding the release characteristics of soil colloids is a prerequisite for studying the co-transport of colloids and pollutants in subsurface environment. As a crucial agricultural management measure, fertilization not only alters the material composition of farmland soil, but also significantly regulates the properties and release patterns of soil colloids. This study systematically investigated the regulatory mechanism of short-term cattle manure fertilization on the macroscopic release and microscopic properties of soil colloids with different particle sizes, providing a theoretical foundation for subsequent research on the fate and transport of agricultural non-point source pollutants. Results The colloids in natural agricultural soil primarily consist of inorganic components. Graded extraction of the colloids has revealed that the combined proportion of colloids with particle sizes of 1–2 μm and 0.45–1 μm accounts for approximately 80.5%. Applying cattle manure inhibits the release of soil colloids, and the content of large particle size (1–2 μm) components increases. The content of organic colloids is increased due to the high total organic carbon (TOC) in cattle manure, particularly those with a particle size less than 1 μm. The characterization of organic colloid components revealed a significant increase in aromatic carbon and oxygen-containing functional groups, while the aliphatic content decreased. The response sequence regarding changes in functional groups within organic colloids induced by fertilization was as follows: –CH 3 , –CH 2 > C–O > –OH > C=C. Fertilization promotes the release of 1:1-type inorganic mineral colloids, increasing the content of poorly crystalline minerals. The retention of aromatic carbon and oxygen-containing functional groups by poorly crystalline mineral colloids served as the primary mechanism leading to their increased content levels. Changes in environmental factors significantly impacted the release and properties of soil colloids. Conditions such as low cationic valence, high ionic strength, and high pH promoted the release of soil colloids. Conclusions The short-term fertilization resulted in a reduction in the release of soil colloids and brought about significant alterations in their particle size composition and properties. The findings of this study provide valuable insights into understanding the impact of fertilization-induced colloid release on the environmental behavior of agricultural non-point source pollutants.


Introduction
Soil colloids, acknowledged as one of the most active components of soil, have been extensively demonstrated to play a pivotal role in regulating the migration and transformation of pollutants [15,[21][22][23].In contrast to larger soil particles, colloids possess the capacity to bind with pollutants through adsorption and complexation reactions.They can facilitate the transport of certain less mobile pollutants to deeper soil layers, thereby posing a consequential threat to groundwater and drinking water safety [3,33,34,42].Alterations in environmental conditions can induce variations in the released components and transport characteristics of soil colloids, which leads to significant distinctions in their interaction mechanisms with soil pollutants [7,43].Frequent agricultural fertilization activities alter the chemical composition of the soil and the structure of soil aggregates, inevitably influencing the fate of soil colloids and pollutions.Therefore, the investigation of the influence of fertilization on the release of soil colloids in agricultural fields is of immense significance.
Soil colloids typically comprise clay minerals (montmorillonite, kaolinite, and illite), metal oxides (iron and aluminum oxides), and soil organic matter (SOM, such as humus, polysaccharides, and proteins) [10].Applying cattle manure could directly introduce organic colloids into the agricultural soil and induce changes in the aliphatic carbon to aromatic carbon ratio within these colloids [38].The application of organic fertilizer could also cause alterations in the composition and form of mineral elements such as aluminum (Al) and iron (Fe) in inorganic colloids [41].Compared with chemical fertilization, long-term fertilization (22 years) with cattle manure significantly increased amorphous Al and decreased exchangeable Al [36].Chen et al. [8] found that the long-term application of organic fertilizers (NPKC2) increased poorly crystalline mineral components within colloids, such as allophane, imogolite, and ferrihydrite.These mineral colloids could also augment the aromatic carbon content of soil organic colloids by selectively adsorbing aromatic organic compounds.All aforementioned findings were obtained from an extensive study on fertilization conducted over a lengthy period.While it is acknowledged that short-term fertilization may not induce chemical transformations in mineral composition, further investigation is required to ascertain its potential impact on the release of distinct mineral colloids.
The colloidal characteristics released from the soil are not only influenced by the inherent properties of the soil itself but also by variations in environmental conditions [33,34].The increase in ionic strength (IS) reduces the thickness of the double electric layer on the surface of soil colloids, thereby diminishing electrostatic repulsion between colloid particles and subsequently decreasing their release capacity [18].When the pH value deviates from the point of zero charge (PZC) of soil colloids, either lower or higher, their release capacity is enhanced; however, when the pH approaches its PZC, soil colloids tend to flocculate and precipitate [26].Additionally, dissolved organic matter (DOC) in the soil is an essential environmental factor that controls colloid release [9].Studies have indicated that humic acid can adsorb onto the surface of soil colloids, augmenting their release capacity by intensifying electrostatic repulsion among them [25].Therefore, it is imperative to conduct comprehensive research on the impact of agricultural fertilization-induced environmental changes on the release characteristics of soil colloids.
In summary, current studies primarily focus on the effects of long-term fertilization on soil colloid release, which consider the soil colloids as a whole for research purposes.However, further research is required to explore the release characteristics of colloids under shortterm fertilization, particularly in terms of the release patterns and properties pertaining to different-sized colloidal particles.The objectives of this study are as follows: (1) to analyze the types and compositions of organic and inorganic colloids in typical agricultural soils; (2) to determine the particle size composition of soil colloids and characterize the properties specific to each size fraction; and (3) to elucidate the regulatory mechanisms governing release patterns of soil colloids at different particle sizes induced by environment changes resulting from the application of cattle manure.

Soil and cattle manure preparation
An agricultural silt loam (9.71% clay, 51.91% silt, and 38.38% sand) was collected from the top layer (0-20 cm) of Shenbei Qianjin Farm in Shenyang City, where maize, vegetables, and fruits are predominantly grown.The farm primarily focuses on the development of organic agriculture, with a particular emphasis on utilizing cattle manure for fertilization.To conduct relevant experimental research, cattle manure samples from a farm within the same agricultural facility were collected.Soil and cattle manure were subjected to rigorous processing in the laboratory following transportation to remove plant roots and stems.The samples were subsequently air-dried in a shaded area, followed by grinding and sieving through a 1 mm mesh for further utilization.The soil and cattle manure were then divided into two segments separately: one segment was used to determine the basic physicochemical properties (Table S1), while the other was utilized for subsequent colloid extraction experiments.

Water-dispersible colloid extraction
The natural agricultural soil collected from the farm served as the control group without fertilization.In contrast, the fertilized soil (experimental group) was a laboratory-prepared mixture of natural agricultural soil and cattle manure.The procedures for water-dispersible colloid extraction were referenced to the methods of Liu et al. [21,22] and Zhang et al. [44].Briefly, 2.5 g pretreated agricultural soil samples were placed in a 500-mL beaker and supplemented with cattle manure at specific mass ratios (0%, 2%, 5%, 8%).Subsequently, 500 mL of background solution (10 mmol/L NaCl) was added, and the solution's pH was adjusted to 7 using 1 mol/L HCl or NaOH.The mixture was then transferred to a magnetic stirrer and vigorously stirred for 24 h at room temperature before being left to stand.To obtain the soil colloid suspension with particle sizes smaller than 2 μm, the settling time was determined as 2 h based on Stokes' law.The soil colloid suspension was separated using a siphoning method, followed by vacuum drying.Finally, the resulting colloids powders were stored in amber glass bottles at room temperature.The colloids were divided into two parts: one for determining the total release of soil colloids and the other for subsequent experiments of particle size classification.The concentration of soil colloids was quantified by measuring absorbance values at 400 nm using ultraviolet-visible spectroscopy [21,22].Moreover, to examine the impact of various environmental factors on the release properties of soil colloids after fertilization, the experiment was carried out under conditions with varying background solutions (10 mmol NaCl, 5.0 mmol CaCl 2 , and 3.3 mmol AlCl 3 ), pH (3,5,7,9,11), and IS (1, 10, 100 mmol/L NaCl).

Particle size classification of colloids
60 mg of soil colloids extracted with a specific mass ratio of cattle manure (0 and 5%) were weighed accurately and transferred into 50-mL centrifuge tubes.Then, 30 mL of background solution (10 mmol/L NaCl) was added, and the system pH was adjusted to 7 using HCl or NaOH.The particle size classification of colloids was accomplished via the centrifugation method [12].In brief, the soil colloid suspension was initially centrifuged at 50g for 4 min.The resulting suspension was collected in another centrifuge tube, and the residue was the colloid fraction with particle size ranging between 1 and 2 μm.Subsequently, the suspension was centrifuged at 420g for 5 min to isolate the precipitate as the colloid fraction containing particles ranging from 0.45 to 1 μm.The remaining suspension consisted of the colloid fraction with a particle size smaller than 0.45 μm.The above steps were repeated three times to ensure complete separation.
The concentrations of soil colloids with different particle sizes were quantified by measuring absorbance values at a wavelength of 400 nm using ultraviolet-visible spectroscopy.

Characterization of colloidal properties
The zeta potential and particle size of the soil colloids were determined using the Laser Nanometer Particle Size Analyzer (Zetasizer Nano ZS, Malvern Instruments, Ltd., UK).The oxide compositions were analyzed via X-ray fluorescence (XRF) (ZSX primus, Rigaku, Japan).The functional groups of C and chemical morphology of Al, Si, and Fe were ascertained via X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific, USA).The C 1s, Al 2p, and Si 2p spectra were obtained by using a monochromatic Al Kα X-ray source XPS (1486.6 eV).The binding energy scale was adjusted by C 1s spectrum (C 1s = 284.8eV) of the adventitious hydrocarbon.Advantage software (Version 5.9921, Thermo Fisher Scientific, USA) was used to analyze and deconvolve high-resolution XPS spectra with Shirley background correction.Spectra were optimized by Gaussian-Lorentzian value.The major mineral phase identification were obtained by X-ray diffraction (XRD, D8 Advance, Bruker, Germany) analysis with Co Kαradiation (40 kV, 40 mA, λ = 1.79026A) in the range 5° < 2θ < 90°.The mineral composition was analyzed semi-quantitatively using the Jade 6.5 software (Materials Data, USA).Total carbon (TC) and TOC were measured using a total organic carbon analyzer (TOC-L, Shimadzu, Japan).The composition of organic functional groups was qualitatively identified by Fouriertransform infrared spectroscopy (FTIR) (PerkinElmer 1725X) with a resolution of 4 cm −1 and a measurement range of 400-4000 cm −1 .In order to identify the soil colloid responses to fertilization, a two-dimensional infrared synchronous and asynchronous spectrum analysis was conducted in the wavenumber ranges of 1800-1000 cm −1 and 3600-2800 cm −1 with data analysis performed using the 2D-shige software (Version 1.3, Kwansei-Gakuin University, Japan) [37].The addition of cattle manure at varying concentrations (0%, 2%, 5%, and 8%) was employed as a perturbation factor, while keeping all other conditions constant.The methods mentioned above were utilized to evaluate the physicochemical characteristics of total water-dispersible colloids and specific fractions of their particle size that were extracted pre-and post-fertilization.

Data processing
One-way analysis of variance (ANOVA) with Tukey's multiple comparisons test (p < 0.05) was used to evaluate the considerable differences in colloid release, colloid stability, and particle size characteristics among different treatments (varying in pH, IS, and fertilization levels).Pearson correlation analysis was employed to assess the correlation coefficients between the relative percentages of iron and calcium oxides, organic functional groups (C=C, C-C/C-H, C-O), clay minerals, and TOC in soil colloids extracted under varying fertilization levels (0% and 5%).All mathematical calculations, statistical analysis, and figure generation were executed using Microsoft Excel (Edition 2021, Microsoft Corp., USA), IBM SPSS Statistics (Version 27, SPSS Inc., USA), and Origin (Version 2018, OriginLab, USA).

Release of water-dispersed colloid with different particle sizes
The release amount of the water-dispersible soil colloids with particle size less than 2 μm in 2.5 g of agricultural soil was 770.67 mg/L, accounting for 30.82% of the total soil mass (less than 1 mm) (Fig. 1a).Among the colloids, those with particle sizes range of 1-2 μm and 0.45-1 μm were the predominant components, contributing 40.7% and 39.8%, respectively, to the total colloid.In contrast, the colloids with particle sizes smaller than 0.45 μm constituted only 19.5% of the total (Fig. 2).Soil colloids primarily consist of organic and inorganic constituents.As revealed by XPS and XRF data (Table S2 and Table S3), the principal elemental constituents of soil colloids include O, C, Si, Al, and Fe.Therefore, the trends in TOC and the contents of Si, Al, and Fe elements were used to represent the changes in organic and inorganic colloids.The TOC in soil colloids was determined to be merely 60.85 g/kg, while the contents of Si, Al, Fe, and Ca exceeded those of C and N (Table S2), and the oxide contents of Si and Al were higher than 73% (Table S3).It was indicated that inorganic colloids (clay minerals or metal oxides) predominantly constitute the colloidal fraction of agricultural soil.A decrease in particle size was accompanied by an increase in the TOC of soil colloids, suggesting an elevation in the content of organic colloids.Simultaneously, the concentrations of Si, Al, and Fe elements decreased with the reduction in colloid particle size, indicating a corresponding decrease in the content of inorganic colloids.

Characteristics of water-dispersed colloids in agricultural soils
The organic components in soil colloids included various functional groups (Figs. 3 and 4a; Table S4).FTIR spectra of soil colloids and their different particle size components are shown in Fig. 3a.Observed functionality included [8] the broad band between 3300-3600 cm −1 ascribed to O-H stretching vibration; the peaks at 2950 and 2860.cm −1 corresponding to aliphatic C-H stretching; the absorption peak at 1710 cm −1 attributed to C=O stretching of COOH and ketones, and the peaks at 1510 and 1645 cm −1 attributed to structural vibrations of C=C within the aromatic ring skeleton.In addition, the strong peak at 1030 cm −1 was associated with the stretching vibration of C-O in carbohydrates, or Si-O in silicate.The peaks at 790 cm −1 , 527 cm −1 , and 469 cm −1 were attributed to Fe-O, Al-O-Si, and Si-O-Si, respectively.The peak fitting results of XPS C 1s also demonstrated that aromatic carbon and aliphatic carbon were the main components of organic functional groups, accounting for 61.54% in total (Fig. 4a; Table S4).In terms of the characteristics of soil colloidal functional groups at different particle sizes, a decrease in particle size leads to a sequential increase in the content of aromatic carbon and oxygen-containing polar functional groups in organic colloids, while the content of aliphatic carbon experiences a corresponding decrease (Fig. S1a, b, and c; Table S4).
The inorganic component of soil colloids was mainly composed of SiO 2 , Al 2 O 3 , and Fe 2 O 3 , which accounted for 89.72%.Moreover, some trace amounts of sulfate and phosphate components were also present (Table S3).The molar ratio of SiO 2 /Al 2 O 3 reached a high value of 1.57 (Table S3), suggesting that the inorganic colloid component may be dominated by 2:1-type minerals (e.g., montmorillonite and illite) [45].The conclusion was in accordance with the XRD analysis results.The predominant mineral constituents present in the soil colloid include Quartz, Albite, Kaolinite, Illite, Chlorite, and Muscovite.Notably, 2:1-type minerals (illite, chlorite, and muscovite) collectively constitute 69.9% of the total composition (Fig. 3c, Table S5).Further analysis of the chemical forms of Si and Al by XPS revealed that the content of allophane (poorly crystalline mineral) and C-Si-O (a short-range ordered structure) in inorganic colloids was 20.69% and 17.61%, respectively (Fig. 4b and 4c; Table S4) [6].The oxide composition of inorganic colloids in different particle-sized soil colloids was similar (Table S3).The content of Al 2 O 3 augmented as the colloid particle size decreased, whereas the SiO 2 content and the molar ratio of SiO 2 /Al 2 O 3 progressively diminished with decreasing particle size, which implied that the types of inorganic mineral colloids might simplify with decreasing particle size and exhibiting an increased proportion of 1:1-type (such as kaolinite) mineral colloids [30].Additionally, the content of allophane and C-Si-O structures in inorganic colloids escalated incrementally with decreasing colloid particle size, especially in small-sized colloids smaller than 0.45 μm, where the content of allophane and C-Si-O structures significantly increased (Fig. S2, Fig. S3; Table S4).

Regulation mechanism of soil colloid properties by fertilization
The content of organic colloids in the extracted soil colloids significantly increased after fertilization (P < 0.05) (Table S2), which was mainly attributed to the high TOC content of the cattle manure (Table S1).Cattle manure could release many organic matters, particularly under irrigation conditions.At the same time, the inorganic components in soil colloids, such as metal oxides and clay minerals, could promote the retention of organic components through adsorption and chelation processes [4,31].
Although the total TOC content of soil colloids increased after applying cattle manure, the TOC of large-sized colloids (1-2 μm did not escalate; only the TOC of smallsized colloids (less than 1 μm) increased.These findings indicate that the organic colloids found in cattle manure consisted primarily of small-sized particles.(Table S2).
Furthermore, organic colloids could improve the dispersibility of colloids and reduce their particle size, which has been investigated by previous studies [35].A comparison of the SiO 2 /Al 2 O 3 molar ratio and TOC content of different particle-sized colloid components revealed inverse trends (Table S2 and Table S3).Furthermore, the content of 1:1 mineral colloids increased from 3.8% to 6.8% after fertilization, indicating that organic colloids were more likely to bind with 1:1-type mineral colloids compared to 2:1-type minerals.Chen et al. [5] also found that kaolinite exhibited a stronger adsorption capacity for humic acid compared to montmorillonite, which was consistent with the result of this study.As the particle size of soil colloids decreased, the SiO 2 /Al 2 O 3 molar ratio concurrently reduced, suggesting a higher proportion of 1:1-type mineral colloids, which could combine with more organic components from cattle manure and increase the TOC content within small-sized colloids.A noticeable increase in the quantity of aromatic carbon and polar oxygen-containing functional groups was observed in soil colloids of various particle sizes after fertilization, accompanied by a corresponding decrease in aliphatic carbon content (P < 0.05) (Table S4, Table S6).Notably, although aliphatic carbon constituted the majority of the chemical bonds in cattle manure (Fig S1h; Table S6), the content of aliphatic carbon in soil colloids of various particle sizes decreased (Table S4, Table S6).Furthermore, a positive correlation between the aliphatic  S7).The fertilization process may promote the release of 1:1-type mineral colloids (decreasing SiO 2 /Al 2 O 3 molar ratio), which possess a less potent affinity for binding with aliphatic carbon than 2:1-type minerals.The content of aliphatic carbon in colloid components exhibited an opposite trend to that of organic carbon as particle size decreased, and both the aliphatic carbon and the SiO 2 /Al 2 O 3 molar ratio displayed similar trends (Table S4, Table S6).The response order of functional group changes in organic colloids under fertilization was analyzed using 2D infrared correlation spectroscopy (Fig. 5).-OH, C=C, and C-O showed consistent changes, but -CH 3 and -CH 2 did not demonstrate consistent alterations.According to Noda's rule [24,28], the main functional groups in the order of response for organic colloids are -CH 3 , -CH 2 > C-O > -OH > C=C.It could be concluded that the addition of cattle manure promoted the decomposition of the long aliphatic carbon chain structure in organic colloids, generating numerous small molecular organic components with oxygen-containing functional groups (hydroxyl, ester, or ether bonds).Large molecular organic components with multi-phenyl ring structures (such as humic substances) were formed through the possible combination with exogenously input soluble organic matter.This further explains the reduction in aliphatic carbon in soil colloids following fertilization [11,37,47].
After fertilization, the proportion of SiO 2 and Fe 2 O 3 in soil colloids decreased, while MgO, CaO, and K 2 O increased (Table S3).This difference in the elemental composition may be due to higher MgO, CaO, and K 2 O content compared to the soil and the decreased concentrations of SiO 2 and Fe 2 O 3 in the added manure.The contents of Fe 2 O 3 and aliphatic carbon decreased as the size of colloid particles reduced (Table S3, Table S4).A positive correlation (R = 0.745, P < 0.05) was observed between the variables (Table S7), suggesting that Fe 2 O 3 exhibited a preference for binding with aliphatic carbon.It further elucidated the factors that contributed to the decrease in aliphatic carbon in the colloidal organic components after fertilization, which was consistent with the findings of Wan et al. [32].Following the application of cattle manure, the concentrations of allophane and C-Si-O structures in total colloids and colloids of different particle sizes incrementally increased, especially in small-sized colloids (Table S4).The results of 27 Al nuclear magnetic resonance, as reported by Wen et al. [36], indicated that aluminum in soils treated with organic fertilizer primarily exists in poorly crystalline octahedral coordination.FTIR spectra confirmed the presence of poorly crystalline imogolite structures in situ.Huang et al. [17] discovered that applying organic fertilizer fosters the development of poorly crystalline minerals via high-resolution transmission electron microscopy, silicon nuclear magnetic resonance spectra, and XPS techniques.It has been demonstrated that highly reactive, poorly crystalline minerals were more likely to form organic-inorganic composite colloids through anion and internal ligand exchange reactions, thereby promoting the retention of exogenously added organic colloids [17].
In this study, the content of allophane and C-Si-O structures increased as the particle size of soil colloids decreased, accompanied by an increase in aromatic carbon and oxygen-containing functional groups (Table S4 and Table S6).This result provided additional insight into the cause of the increase in aromatic carbon and oxygen-containing functional groups after fertilization.It confirmed that poorly crystalline minerals favored the retention of such functional groups.This finding is consistent with the results reported by Ye et al. [40] and He et al. [16].

Effect of fertilization on soil colloidal release in farmland
Applying organic fertilizer can alter soil aggregate structures and the quantity of soil clay particles, which is closely related to the release of soil colloids.Consequently, the changes in soil clay properties induced by the addition of cattle manure served as the main regulators of soil colloid release [13].Water-soluble colloid extraction experiments showed that the release of soil colloids decreases with increasing fertilizer application (P < 0.05) (Fig. 1a).The inclusion of cattle manure is believed to have facilitated the development of soil aggregates, decreased the amount clay particles, and ultimately limited the release of soil colloids.The extraction of soil colloids by particle size after fertilization revealed an increase in the proportion of colloids with particle sizes of 1-2 μm.Concurrently, the proportions of colloids with particle sizes of 0.45-1 μm and below 0.45 μm decreased (Fig. 2a).This further illustrated that the application of cattle manure promoted the aggregation of soil colloids, resulting in an increased proportion of large-sized aggregates.Cattle manure contained approximately 30% metal oxides, such as iron, aluminum, calcium, and magnesium, which had a cementing effect on soil particles (Table S3).Thus, the contribution of iron-aluminum oxides introduced by cattle manure in the aggregation of soil clay particles should not be overlooked [39,46].Although short-term fertilization decreased the release of water-dispersible colloids in small particles and promoted the formation of colloids in micrometer size, it also contributed to the stability of soil aggregates and pore structure, thereby preventing the clogging of soil pores by small particle colloids [19].Particularly under long-term fertilization, ensuring a relatively loose and well-structured soil is imperative for maintaining sustainable agricultural development.
The addition of cattle manure introduced a considerable amount of TOC into the soil, which contained microbial metabolites such as polysaccharides.These microbial metabolites could also interact with clay minerals to promote soil particle aggregation and precipitation [2,20].Analysis of cattle manure using XPS and FTIR (Fig. 2b and Fig. S1h; Table S2) revealed the presence of high levels of soluble microbial metabolites and aliphatic carbon (Fig. S1h; Table S3 and Table S6).The aliphatic carbon introduced by fertilization, in conjunction with the microbial metabolites, was likely an innegligible factor in promoting the aggregation of soil clay minerals [13].

The impact of environmental factors
Before fertilization, the release of soil colloids increased slightly as the ionic strength (IS) increased (Fig. 1d).This finding contradicted previous results by Singh et al. [27] and Sun et al. [29].This discrepancy arose from using NaCl to control the IS of the background solution in this study.The critical coagulation concentration (Na + ) of soil colloids significantly exceeds 500 mmol/L (Table S8).Furthermore, the zeta potential of the colloids became more negative as the IS increased, which reduced the process of the colloidal precipitation (Fig. 6a).Graded extraction of soil colloids at different IS reveals a decline in the proportion of colloids with particle sizes between 1 and 2 μm and an increase in the proportions of colloids with particle sizes of 0.45-1 μm and less than 0.45 μm (Fig. 2b).These findings suggest that the elevated IS induced by agricultural activities does not lead to soil colloid coagulation.On the contrary, it augments the negative charge on the colloid surface, thereby improving colloid dispersion and increasing soil colloids release.The distinct valence states of ions significantly influence soil colloid release (P < 0.05) (Fig. 1b) [18].Compared to Na + , high-valence cations such as Ca 2+ and Al 3+ exert a Fig. 6 Zeta potential of soil colloids (0, 5%) before and after fertilization under different environmental factors.a Ionic strengths (1, 10, 100 mmol/L NaCl); b pH more substantial impact on the thickness of the colloid diffusion layer and possess a stronger ability to compress the double electric layer, resulting in a reduction in the zeta potential value on the soil colloid surface and leading to a decrease in colloid release [35].A significant decrease (P < 0.05) in the release of soil colloids occurs when the environmental pH < 7, whereas an increase in soil colloid release is relatively modest when the environmental pH > 7 (Fig. 1c).Graded extraction of soil colloids at varying pH illustrates that the proportion of colloids with particle sizes of 1-2 μm increases as the environmental pH decreases.Conversely, the proportions of colloids with particle sizes of 0.45-1 μm and < 0.45 μm decrease (Fig. 2a).The primary reason for this phenomenon is the reduction in soil colloid surface potential as environmental pH decreases (Fig. 6b), leading to weakened electrostatic repulsion and subsequent colloid particle coagulation [26].
After fertilization, a noticeable increase in soil colloid release was observed only under conditions of pH = 3, IS = 10 mmol/L (Na + ), and pH = 7, IS = 100 mmol/L (Na + ); For other environmental conditions, fertilization inhibited soil colloid release (Fig. 1b and c).Cattle manure contains a high content of Na + , which can be released under flooded conditions (Table S1).The increase in exchangeable Na + in the soil suspension promotes the disruption of soil aggregates, facilitating their dispersion and thus increasing the quantity of soil clay particles [13,14].The differential release of soil colloids after fertilization under various environmental conditions may be attributed to the following reasons: when cohesive substances, such as organic and inorganic colloids, play a more significant role in aggregation than the dispersing effect of Na + , it induces soil aggregate coagulation and reduces the release of soil colloids.Conversely, it inhibits the aggregation of soil aggregates, thereby increasing the release of soil colloids.The soil's cation exchange capacity has been widely proven to decrease as pH decreases [1].Therefore, at a background solution pH = 3, Is = 10 mmol/L, the increase in Na + after fertilization is higher than in other pH conditions.Thus, the dispersing effect of Na + is stronger than the aggregating effect of binders.When the background solution pH = 7 and IS = 100 mmol/L, the concentration of Na + is significantly greater than the soil's cation exchange capacity.As a result, the extent to which Na + released from fertilization are exchanged by soil cations is minimal, thereby enhancing the release of soil colloids.

Conclusion
This study comprehensively explored the impact of short-term cattle manure fertilization on the release characteristics of soil colloids and its underlying control mechanism.The application of cattle manure substantially impacted the distribution of particle size and the ratio of organic to inorganic composition in colloids.The organic colloids released from cattle manure can be retained by inorganic components in soil colloids through adsorption and chelation.Different organic functional groups exhibited varied responses to fertilization, which is a crucial factor in driving alterations in the properties of organic colloids.While short-term fertilization might not induce significant transformation of inorganic mineral colloids, it exerts a substantial influence on their release capacity.Environmental factors such as pH, IS, and coexisting ions primarily affected the release characteristics of soil colloids by altering the electronegativity and stability of soil colloids.The findings of this study provide a scientific foundation for subsequent research on co-transport and pollution control targeting agricultural pollutants and colloidal particles.

Fig. 4
Fig. 4 XPS peak fitting spectra of C, Al, and Si chemical forms in soil colloids before and after fertilization (0, 5%).a-c Before fertilization; d-f after fertilization

Fig. 5
Fig. 5 Two-dimensional infrared spectra.a Two-dimensional infrared synchronous spectra; b two-dimensional infrared asynchronous spectra