Characterization of Dissolved Organic Matter Released from Aged Biochar: A Comparative Study of Two Feedstocks and Multiple Aging Approaches

Dissolved organic matter (DOM) plays important roles in environmental ecosystems. While many studies have explored the characteristics of aged biochar, limited information is available about the properties of DOM derived from aged biochar. In this study, biochar obtained from maize stalk and soybean straw were aged using farmland or vegetable-soil solution, as well as soil solution containing hydrogen peroxide (H2O2). Chemical composition of the extracted DOM from the aged biochar was analyzed via excitation–emission matrix coupled with fluorescence regional integration (FRI) and parallel factor analysis (PARAFAC). Obtained results showed that biochar aged with H2O2-enriched soil solution had higher water-soluble organic carbon, ranging from 147.26–734.13% higher than the controls. FRI analysis revealed fulvic and humic-like organics as the key components, with a considerable increase of 57.48–235.96% in the humic-like component, especially in soybean-straw-aged biochar. PARAFAC identified four humic-like substance components. Concurrently, the aromaticity and humification of the aged-biochar-derived DOM increased, while the molecular weight decreased. These findings suggest that DOM derived from aged biochar, with a high content of humic-like organics, might impact the mobility and toxicity of pollutants in soil.


Introduction
Biochar, with large specific surface area (SSA), a well-developed pore structure, and diverse oxygen-containing functional groups, is often used to remediate soils contaminated with heavy metals and organic pollutants [1,2]. Notably, physicochemical properties of biochar, such as surface characteristics, functional groups, and hydrophilicity, can change when incorporated into soil, due to exposure to biotic and abiotic stresses that contribute to biochar aging [3,4]. Research has shown that aged biochar typically exhibits greater SSA, rougher surface morphology, lower acidity, higher ion exchange capacity, and increased aromaticity [4,5]. However, the properties of dissolved organic matter (DOM) released from aged biochar are still insufficiently understood. DOM, as determined by the measurement of water-soluble organic carbon, is a crucial component of biochar [6,7]. It not only contributes to biochar stability but also supplies nutrients to plants and microorganisms in agroecosystems [8,9]. Furthermore, it plays a critical role in shaping the fate of heavy metals and organic contaminants in the environment [9,10]. The characteristics of DOM released from biochar are greatly affected by The aging approach significantly impacted the water-soluble organic carbon (WSOC) content of biochar derived from maize stalk and soybean straw (Figure 1b). For UM, the WSOC content was 0.47 mg g −1 , which increased by 1.47 and 2.07 times in FM and VM, respectively. This increase suggests that biochar aged by soil solution exhibited enhanced WSOC content, possibly due to the release of soil organic matter adsorbed onto the biochar surface from soil solution [29]. Notably, WSOC content in maize stalk biochar aged by soil solution with H2O2 further increased by 0. 40-1.66 times. This is because chemical oxidation by H2O2 facilitated the decomposition of inert organic carbon in biochar [30,31]. Comparable trends were observed for WSOC content in soybean-straw-derived biochar. High-resolution SEM images revealed the development of small pores on the surface of biochar aged by soil solution with H2O2 (Figure S1k-n). This observed phenomenon can be attributed to H2O2's potent oxidative properties, which react with organic functional groups on the biochar surface to initiate an 'etching' process that creates pores, and to the generation of hydroxyl radicals from H2O2 decomposition that further interact with and oxidize the biochar surface, promoting the formation of additional pores [32].

Fluorescent Components of Aged-Biochar-Derived DOM Identified through FRI Analysis
Fluorescence regional integration (FRI) analysis of dissolved organic matter (DOM) extracted from aged biochar delineates five distinct regions [19]: Regions I and II (excitation wavelengths (Ex) < 250 nm, emission wavelengths (Em) < 350 nm) correspond to simple aromatic proteins, Region III (Ex < 250 nm, Em > 350 nm) represents fulvic-acid-like materials, while Region IV (Ex 250-280 nm, Em < 380 nm) is associated with soluble-microbial-byproduct-like material. Finally, Region V (Ex > 280 nm, Em > 380 nm) is linked to humic-acid-like organics (Figure 2a).  The aging approach significantly impacted the water-soluble organic carbon (WSOC) content of biochar derived from maize stalk and soybean straw ( Figure 1b). For UM, the WSOC content was 0.47 mg g −1 , which increased by 1.47 and 2.07 times in FM and VM, respectively. This increase suggests that biochar aged by soil solution exhibited enhanced WSOC content, possibly due to the release of soil organic matter adsorbed onto the biochar surface from soil solution [29]. Notably, WSOC content in maize stalk biochar aged by soil solution with H 2 O 2 further increased by 0.40-1.66 times. This is because chemical oxidation by H 2 O 2 facilitated the decomposition of inert organic carbon in biochar [30,31]. Comparable trends were observed for WSOC content in soybean-straw-derived biochar. High-resolution SEM images revealed the development of small pores on the surface of biochar aged by soil solution with H 2 O 2 (Figure S1k-n). This observed phenomenon can be attributed to H 2 O 2 's potent oxidative properties, which react with organic functional groups on the biochar surface to initiate an 'etching' process that creates pores, and to the generation of hydroxyl radicals from H 2 O 2 decomposition that further interact with and oxidize the biochar surface, promoting the formation of additional pores [32].

Fluorescent Components of Aged-Biochar-Derived DOM Identified through FRI Analysis
Fluorescence regional integration (FRI) analysis of dissolved organic matter (DOM) extracted from aged biochar delineates five distinct regions [19]: Regions I and II (excitation wavelengths (Ex) < 250 nm, emission wavelengths (Em) < 350 nm) correspond to simple aromatic proteins, Region III (Ex < 250 nm, Em > 350 nm) represents fulvic-acid-like materials, while Region IV (Ex 250-280 nm, Em < 380 nm) is associated with solublemicrobial-byproduct-like material. Finally, Region V (Ex > 280 nm, Em > 380 nm) is linked to humic-acid-like organics (Figure 2a). The aging approach significantly impacted the water-soluble organic carbon (WSOC) content of biochar derived from maize stalk and soybean straw ( Figure 1b). For UM, the WSOC content was 0.47 mg g −1 , which increased by 1.47 and 2.07 times in FM and VM, respectively. This increase suggests that biochar aged by soil solution exhibited enhanced WSOC content, possibly due to the release of soil organic matter adsorbed onto the biochar surface from soil solution [29]. Notably, WSOC content in maize stalk biochar aged by soil solution with H2O2 further increased by 0.40-1.66 times. This is because chemical oxidation by H2O2 facilitated the decomposition of inert organic carbon in biochar [30,31]. Comparable trends were observed for WSOC content in soybean-straw-derived biochar. High-resolution SEM images revealed the development of small pores on the surface of biochar aged by soil solution with H2O2 ( Figure S1k-n). This observed phenomenon can be attributed to H2O2's potent oxidative properties, which react with organic functional groups on the biochar surface to initiate an 'etching' process that creates pores, and to the generation of hydroxyl radicals from H2O2 decomposition that further interact with and oxidize the biochar surface, promoting the formation of additional pores [32].

Fluorescent Components of Aged-Biochar-Derived DOM Identified through FRI Analysis
Fluorescence regional integration (FRI) analysis of dissolved organic matter (DOM) extracted from aged biochar delineates five distinct regions [19]: Regions I and II (excitation wavelengths (Ex) < 250 nm, emission wavelengths (Em) < 350 nm) correspond to simple aromatic proteins, Region III (Ex < 250 nm, Em > 350 nm) represents fulvic-acid-like materials, while Region IV (Ex 250-280 nm, Em < 380 nm) is associated with soluble-microbial-byproduct-like material. Finally, Region V (Ex > 280 nm, Em > 380 nm) is linked to humic-acid-like organics (Figure 2a).   The distribution of volumetric fluorescence among these five regions (i.e., P i,n ) is presented in Figure 2b. For aged biochar, the highest percentages (34% < P III,n < 47% and 35% < P V,n < 55%) are observed in Regions III and V, respectively. This finding is consistent with the visual analysis of the EEM peak locations within Regions III and V of the delineated regions (Figure 2a). Cao et al. [12] also reported that fulvic-acid-and humicacid-like fractions predominantly constituted the DOM derived from wood biochar. The aging process resulted in reduced volumetric fluorescence of Region III but increased that of Region V, suggesting an increase in humic-like organics following the aging process ( Figure 2b). Xing et al. [33] found that the complexation affinity between Cu(II) and humiclike substances in sludge-based biochar-derived DOM was the highest. This implies that the elevated humic-like organics may contribute to the reduction of mobility and toxicity of heavy metals [9].
Pearson's correlation analysis was conducted to investigate the relationship between WSOC and the calculated volumetric values in the five defined regions for the DOM (Figure 3). In aged biochar derived from maize stalk, a negative correlation was observed between WSOC and the fluorescence intensity at the excitation-emission wavelengths that fall within Regions I and IV ( Figure 3a). A similar trend was observed for DOM extracted from aged biochar derived from soybean straw ( Figure 3b). Furthermore, WSOC from soybean-straw-derived aged biochar exhibited a positive correlation with the content of aromatic protein II (Region II), fulvic-like (Region III), and humic-like organics (Region V) ( Figure 3b). Distinct differences in functional groups may influence the observed linear correlation between aromatic carbon and fluorescence [19]. This observation was corroborated by FTIR of aged biochar derived from both maize stalk and soybean straw ( Figure S2).
Molecules 2023, 28, x FOR PEER REVIEW The distribution of volumetric fluorescence among these five regions (i.e., Pi sented in Figure 2b. For aged biochar, the highest percentages (34% < PIII,n < 47% < PV,n < 55%) are observed in Regions III and V, respectively. This finding is consist the visual analysis of the EEM peak locations within Regions III and V of the de regions ( Figure 2a). Cao et al. [12] also reported that fulvic-acid-and humic-acid-l tions predominantly constituted the DOM derived from wood biochar. The aging resulted in reduced volumetric fluorescence of Region III but increased that of R suggesting an increase in humic-like organics following the aging process ( Fig  Xing et al. [33] found that the complexation affinity between Cu(II) and humic-l stances in sludge-based biochar-derived DOM was the highest. This implies that vated humic-like organics may contribute to the reduction of mobility and to heavy metals [9].
Pearson's correlation analysis was conducted to investigate the relationship WSOC and the calculated volumetric values in the five defined regions for the DO ure 3). In aged biochar derived from maize stalk, a negative correlation was obse tween WSOC and the fluorescence intensity at the excitation-emission waveleng fall within Regions I and IV ( Figure 3a). A similar trend was observed for DOM e from aged biochar derived from soybean straw ( Figure 3b). Furthermore, WSO soybean-straw-derived aged biochar exhibited a positive correlation with the co aromatic protein II (Region II), fulvic-like (Region III), and humic-like organics (R (Figure 3b). Distinct differences in functional groups may influence the observe correlation between aromatic carbon and fluorescence [19]. This observation was rated by FTIR of aged biochar derived from both maize stalk and soybean straw (Fi

Fluorescent Components of Aged-Biochar-Derived DOM Identified by EEM-PARAFAC Analysis
Four fluorescent components were discerned in the aged biochar derived from maize stalk and soybean straw (Figure 4a, Table 1). A quantitative comparison with the existing components listed in the OpenFluor database [34] indicated that components C1, C2, C3, and C4 are humic-like substances with respective excitation/emission (Ex/Em) maxima of 270/500, 265/415, 225/470, and 270/415 nm [35][36][37][38]. These four components, typically found in terrestrially derived DOM, have been previously reported in biochar samples [39,40].   Table 1. The aging process influences the fluorescence intensity and composition of DOM in aged biochar obtained from maize stalk and soybean straw (Figure 4b). We observed increases in both C1 and C2, along with a decrease in C4, in biochar aged by soil solution as compared to biochar aged by ultrapure water (Figure 4c). Specifically, C1, which comprised 39.57% of UM, increased by 5.47-12.69% in maize stalk biochar aged with soil solution, while C4, which constituted 34.91% of UM, decreased by 22.75-34.45% in the same aged biochar. Comparable trends were observed in aged biochar derived from soybean straw. Notably, humic-like substances exhibit distinct responses to ultraviolet (UV) light: C1 absorbs light in the UVC and UVA regions, C2 and C4 in the UVC, UVB, and UVA regions, and C3 mainly in the UVC region [26]. These components have previously been reported to facilitate the degradation of organic pollutants under solar or UV light exposure via the generation of reactive oxygen species [41].
The correlations between the maximum fluorescence (Fmax) values of individual component and WSOC for the aged biochar were evaluated to discern the fluorescent signal component of the aged biochar, based on p-values (n = 38; Figure 5). For aged biochar derived from maize stalk, C2 and C3 exhibited significant positive correlations with WSOC, while a negative correlation was observed for C4. Similarly, significant positive correlations were observed between WSOC for aged biochar derived from soybean straw and C1, C2, and C3. Evidently, the fluorescent signal components of the aged biochar were  Table 1. The aging process influences the fluorescence intensity and composition of DOM in aged biochar obtained from maize stalk and soybean straw (Figure 4b). We observed increases in both C1 and C2, along with a decrease in C4, in biochar aged by soil solution as compared to biochar aged by ultrapure water (Figure 4c). Specifically, C1, which comprised 39.57% of UM, increased by 5.47-12.69% in maize stalk biochar aged with soil solution, while C4, which constituted 34.91% of UM, decreased by 22.75-34.45% in the same aged biochar. Comparable trends were observed in aged biochar derived from soybean straw. Notably, humic-like substances exhibit distinct responses to ultraviolet (UV) light: C1 absorbs light in the UVC and UVA regions, C2 and C4 in the UVC, UVB, and UVA regions, and C3 mainly in the UVC region [26]. These components have previously been reported to facilitate the degradation of organic pollutants under solar or UV light exposure via the generation of reactive oxygen species [41].
The correlations between the maximum fluorescence (Fmax) values of individual component and WSOC for the aged biochar were evaluated to discern the fluorescent signal component of the aged biochar, based on p-values (n = 38; Figure 5). For aged biochar derived from maize stalk, C2 and C3 exhibited significant positive correlations with WSOC, while a negative correlation was observed for C4. Similarly, significant positive correlations were observed between WSOC for aged biochar derived from soybean straw and C1, C2, and C3. Evidently, the fluorescent signal components of the aged biochar were primarily impacted by the feedstock type. Notably, C2 and C3 displayed similar changes for aged biochar regardless of feedstock type, suggesting that both C2 and C3 could serve as the signal components for aged biochar.
olecules 2023, 28, x FOR PEER REVIEW for aged biochar regardless of feedstock type, suggesting that both C2 and C as the signal components for aged biochar.

Spectroscopic Characteristics of DOM Extracted from Aged Biochar
Spectroscopic parameters including absorbance coefficients at 254 nm nm (A300), molecular weight (E2/E3), spectral slope (S295), spectral slope solved organic carbon specific ultraviolet absorbance (SUVA), fluorescence ological index (BIX), and humification index (HIX) offer valuable insights i of DOM and their variations due to aging processes and feedstock type ( and A300 exhibited increases by 7.64-11.08 and 5.17-11.62 times, respectiv aged by soil solution containing H2O2, hinting at enhanced aromatic fractio bleaching of DOM in these samples [22,42]. E2/E3 values for DOM in al samples ranged from 4.44 to 9.40, higher values found in biochar aged with suggesting a decrease in DOM molecular weight subsequent to the aging p This observation was consistent with results from S295 and SR [11]. SUVA25 DOM aromaticity [14,23,43], showed significant variations. For biochar age lution from farmland or vegetable land, SUVA254 declined by 43.89-68.15% for biochar aged with soil solution containing H2O2, SUVA254 increased to 7. C m −1 , showing a rise by 8.57-80.83% in comparison to UM and US. The m in SUVA254 for DOM derived from biochar aged by H2O2 can be attributed to ization and aromatization of monomers [16]. FI values in the range of 0.87the origin of the aged-biochar-derived DOM as terrestrial [11]. Low BIX valu in aged biochar allude to the presence of older, decomposed DOM within th For aged biochar derived from soybean straw, HIX values ascended by 9.4 nifying enhanced humification and maturation of DOM [21,44].

Spectroscopic Characteristics of DOM Extracted from Aged Biochar
Spectroscopic parameters including absorbance coefficients at 254 nm (A 254 ) and 300 nm (A 300 ), molecular weight (E2/E3), spectral slope (S 295 ), spectral slope ratio (S R ), dissolved organic carbon specific ultraviolet absorbance (SUVA), fluorescence index (FI), biological index (BIX), and humification index (HIX) offer valuable insights into the origins of DOM and their variations due to aging processes and feedstock type ( Figure 6). A 254 and A 300 exhibited increases by 7.64-11.08 and 5.17-11.62 times, respectively, in biochar aged by soil solution containing H 2 O 2 , hinting at enhanced aromatic fractions and photobleaching of DOM in these samples [22,42]. E2/E3 values for DOM in all aged biochar samples ranged from 4.44 to 9.40, higher values found in biochar aged with soil solutions, suggesting a decrease in DOM molecular weight subsequent to the aging process [14,20]. This observation was consistent with results from S 295 and S R [11]. SUVA 254 , indicative of DOM aromaticity [14,23,43], showed significant variations. For biochar aged with soil solution from farmland or vegetable land, SUVA 254 declined by 43.89-68.15%. Conversely, for biochar aged with soil solution containing H 2 O 2 , SUVA 254 increased to 7.92-9.92 L mg −1 C m −1 , showing a rise by 8.57-80.83% in comparison to UM and US. The marked increase in SUVA 254 for DOM derived from biochar aged by H 2 O 2 can be attributed to the polymerization and aromatization of monomers [16]. FI values in the range of 0.87-1.12 designate the origin of the aged-biochar-derived DOM as terrestrial [11]. Low BIX values (below 0.6) in aged biochar allude to the presence of older, decomposed DOM within the biochar [11]. For aged biochar derived from soybean straw, HIX values ascended by 9.43-10.95%, signifying enhanced humification and maturation of DOM [21,44].

PCA Analysis for the Spectroscopic Properties of DOM Extracted from the Aged Biochar
Multivariate statistical analysis, specifically principal component analysis (PCA), was performed to decipher the distinct optical signatures of DOM derived from aged biochar and extract critical information from DOM characteristics (Figure 7). The first two principal components (PC1 and PC2) accounted for 54.7% and 15.5% of the variance, respectively, amounting to a cumulative total of 70.2% of the overall variance (Figure 7a). PC1 is predominantly representative of changes in fluorescence composition, featuring positive loadings associated with A254, A300, E2/E3, WSOC, C1, C2, C3, and HIX, while negative loadings corresponded to FI, BIX, and C4. In contrast, PC2 had a positive loading for SUVA254, implying its representation of the aromaticity of the biochar-derived DOM. The PCA results also suggested intimate relationships between ultraviolet indices and C3, fluorescence indices (FI and BIX) and C4, and HIX with C1 and C2.

PCA Analysis for the Spectroscopic Properties of DOM Extracted from the Aged Biochar
Multivariate statistical analysis, specifically principal component analysis (PCA), was performed to decipher the distinct optical signatures of DOM derived from aged biochar and extract critical information from DOM characteristics (Figure 7). The first two principal components (PC1 and PC2) accounted for 54.7% and 15.5% of the variance, respectively, amounting to a cumulative total of 70.2% of the overall variance (Figure 7a). PC1 is predominantly representative of changes in fluorescence composition, featuring positive loadings associated with A 254 , A 300 , E2/E3, WSOC, C1, C2, C3, and HIX, while negative loadings corresponded to FI, BIX, and C4. In contrast, PC2 had a positive loading for SUVA 254 , implying its representation of the aromaticity of the biochar-derived DOM. The PCA results also suggested intimate relationships between ultraviolet indices and C3, fluorescence indices (FI and BIX) and C4, and HIX with C1 and C2.
The aging processes greatly modified the DOM characteristics of the biochar, resulting in the emergence of two distinct clusters based on these alterations (Figure 7b). The first and second principal components depicted a formation of one cluster consisting of UM, US, FM, and FS, while HFM, HVM, HFS, and HVS comprised the other cluster. Notably, there was a significant shift in PC1 scores from negative to positive for the biochar aged by ultrapure water, soil solution, and soil solution with H 2 O 2 , mirroring the increase in WSOC as indicated in Figure 1b. Further, the samples displayed tighter clustering along the PC2 axis in comparison to the PC1 axis, indicating that the aromaticity of DOM varied greatly with the aging process and feedstock.
tively, amounting to a cumulative total of 70.2% of the overall variance (Figure 7a). PC1 is predominantly representative of changes in fluorescence composition, featuring positive loadings associated with A254, A300, E2/E3, WSOC, C1, C2, C3, and HIX, while negative loadings corresponded to FI, BIX, and C4. In contrast, PC2 had a positive loading for SUVA254, implying its representation of the aromaticity of the biochar-derived DOM. The PCA results also suggested intimate relationships between ultraviolet indices and C3, fluorescence indices (FI and BIX) and C4, and HIX with C1 and C2.

Biochar
Maize stalks and soybean straw were washed, air-dried, cut into segments less than 5 cm in length, and then pyrolyzed at 500 • C for 2 h with a heating rate of 10 • C min −1 to obtain maize stalk biochar (MB) and soybean straw biochar (SB). This process was based on our earlier research, which indicated that these specific conditions allowed for optimal biochar properties [45,46]. The biochar obtained was sieved using a 2 mm mesh. The corresponding water-soluble organic carbon (WSOC) content for MB and SB was 1.67 and 2.48 mg g −1 , respectively.

Preparation of Aged Biochar
Soil solution was prepared by mixing soil and water in a 1:2.5 weight-to-volume ratio (w/v), followed by shaking at 25 • C for 30 min, and then filtered through a 0.45 µm filter. Biochar, either derived from maize stalk or soybean straw, was then added to the obtained soil solutions from both farmland and vegetable soil, at a ratio of 1:10 (w/v). The mixture was vortexed, shaken at 25 • C for 2 h, and filtered using a 0.45 µm filter, with the filtrate collected for WSOC measurement and the remaining biochar recollected. This entire process was repeated until WSOC levels in the aged biochar remained constant, meaning no significant difference was observed across three consecutive measurements. Biochar derived from maize stalk or soybean straw and aged by farmland-soil solution were denoted as FM and FS, respectively. Similarly, biochar aged by vegetable-soil solution was marked as VM and VS, respectively. In another setup, 30% mass concentration hydrogen peroxide (H 2 O 2 ) was diluted with farmland-or vegetable-soil solution to obtain a 3% H 2 O 2 soil solution. Biochar was then aged using this soil solution, following the previously mentioned steps. Biochar derived from maize stalk and soybean straw and aged by farmland-soil solution with H 2 O 2 were named as HFM and HFS, respectively, while biochar aged by vegetable-soil solution with H 2 O 2 were tagged as HVM and HVS, respectively. Fundamentally, biochar subjected to aging via soil solution and soil solution with H 2 O 2 simulate the normal aging process and the natural oxidation that biochar experiences Molecules 2023, 28, 4558 9 of 12 in environmental conditions [4]. For comparison, biochar, derived from maize stalk or soybean straw, aged by ultrapure water were set as controls and labelled as UM and US, respectively. Each of these treatments was repeated four times.

Analytical Methods
WSOC concentration was measured using a total organic carbon analyzer (TOC-L, CPH, Shimadzu, Kyoto City, Japan). All samples were diluted to less than 10 mg C L −1 prior to UV-visible and EEM spectra measurements [47]. UV-visible absorbance spectra were collected between 200 and 800 nm at 1 nm interval using a UV-visible spectrophotometer (TU1901, Persee, Beijing, China). The fluorescence spectra of DOM were obtained using a three-dimensional fluorescence spectrometer (Hitachi F-4600, Hitachi, Tokyo, Japan). This process involved scanning at excitation wavelengths between 200 and 400 nm and emission wavelengths between 200 and 600 nm, with an increment of 5 nm. The speed was 12,000 nm min −1 and the peak voltage was 500 V. A scanning spectrum of ultrapure water was used as the control. Surface functional groups of the aged biochar were detected by Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific, Waltham, MA, USA).

Statistical Analysis
Contents of WSOC, the relative fluorescence intensity, as well as Fmax value of the individual components identified by PARAFAC modelling for the aged biochar obtained from various aging approaches, were compared using one way analysis of variance via SPSS software (version 26, IBM Corporation, Armonk, NY, USA). The percent fluorescence response in a specific region (P i,n ) was calculated according to the method of Chen et al. [19]. Pearson correlations, corrected using the Benjamini-Hochberg method, between WSOC for the aged biochar and the volume beneath region "i" of the EEM after area normalization were calculated in R software (version 4.2.0) [48].
PARAFAC modelling based on the EEM data was used to analyze the DOM composition. The model was constrained to non-negative values, and the results were validated using split-half analysis and residual analysis [27]. All data and analysis described above were performed in MATLAB 2017a using the DOMFluor toolbox. Detailed comparison and validation of the PARAFAC modelling for the excitation and emission minimum similarity score based on Tucker's congruence coefficient was, respectively, set 0.94 and 0.95. Pearson correlations, corrected using the Benjamini-Hochberg method, between WSOC for the aged biochar and the identified components from PARAFAC modelling were calculated in R software [48]. DOM spectroscopic parameters including absorbance coefficients at 254 (A 254 ) and 300 (A 300 ), molecular weight (E2/E3), spectral slope (S 295 ), spectral slope ratio (S R ), water-soluble organic carbon specific ultraviolet absorbance at 254 (SUVA 254 ), fluorescence index (FI), biological index (BIX), and humification index (HIX) were calculated, and the above-mentioned parameters are presented in Table S1.

Conclusions
Feedstock used in biochar production affected the biochar aging process. Specifically, maize stalk biochar was found to be more susceptible to aging by soil solution. DOM released from the biochar significantly increased, particularly when aged by soil solution with H 2 O 2 . Humic-acid-like substances were the main components for the aged-biocharderived DOM based on FRI and PARAFAC. Additionally, the aromaticity, humification, and maturation of these terrestrially derived DOM in the aged biochar samples increased, while the molecular weight decreased.
However, our study acknowledges certain limitations in the methods employed for aging biochar and the subsequent implications on the findings: (1) environment replication: the aging processes carried out under controlled laboratory conditions may not precisely replicate the wide range of conditions encountered in natural environments; (2) timescale constraints: the accelerated aging processes used in the lab may not accurately reflect the slower, complex processes occurring in nature due to practical time restrictions; (3) biochar type variability: the results derived from maize stalks and soybean straw may not generalize to biochars sourced from other materials; and (4) measurement limitations: utilizing WSOC as an indicator for DOM may not capture all components or nuances of the DOM, nor does it provide insights into the bioavailability or reactivity of the DOM. In light of these findings and limitations, further research should assess potential environmental implications, specifically regarding soil remediation, and weigh the potential benefits and drawbacks associated with DOM release from aged biochar.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28114558/s1, Figure S1 Table S1: Spectroscopic parameters, indices, and matrices used in this study for DOM characterization.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.