Abstract
Valorization of invasive plant biomass through pyrolysis into biochar offers opportunities for waste management and resource recovery. However, the release of biochar-derived dissolved organic matter (BDOM), which influences carbon dynamics and pollutant mobility, remains poorly understood. This study investigated BDOM from Japanese knotweed biochar (500 °C) using seven extraction methods, covering mild aqueous conditions (water and CaCl2), salt effects (NaCl), standardized acidic leaching tests (the synthetic precipitation leaching procedure, SPLP, and the toxicity characteristic leaching procedure, TCLP), and strong chemical extraction conditions (HCl and NaOH). NaOH and HCl maximized dissolved organic carbon (DOC) release (75–183 mg L-¹) while the resulting BDOM exhibited low aromaticity (SUVA254: 1.8–5.3) and relatively lower molecular weight (E2/E3 > 0.5). In contrast, mild extraction (water and CaCl2) released less DOC (5–15 mg L-¹) but preserved high-aromaticity components with larger molecular weights. Fluorescence analysis identified four distinct BDOM components: (1) a terrestrial humic-like substance (C4) preferentially extracted by water, (2) a fulvic-like component (C2) dominant in NaCl and SPLP extracts, (3) a protein-like component (C1) most abundant in NaCl extract, and (4) a transitional component (C3) that decreased under acidic conditions. The results demonstrate that extraction method influences BDOM quantity, optical characteristics, and compositional features, providing a useful framework for understanding BDOM behavior and informing the environmental management of invasive plant.
1 Introduction
Japanese knotweed (Fallopia japonica), native to East Asia, has emerged as one of the most aggressive invasive species in Europe and North America since the 19th century (1). Current management strategies for Japanese knotweed include chemical control (e.g., glyphosate and imazapyr application), mechanical removal, and biological control (2). However, these methods present limitations: herbicide treatments often fail to provide long-term suppression (3), while mechanical excavation entails high costs. Biological control using fungal pathogens (e.g., Mycosphaerella polygoni-cuspidati) has shown promise but remains constrained by the complex life cycle of the biocontrol agents (4).
Pyrolysis of invasive biomass into biochar presents a sustainable alternative, offering dual benefits of waste management and resource recovery (5). Through oxygen-limited thermochemical conversion, this process yields a carbon-rich material with unique physicochemical properties, including high surface area, porous structure, and abundant oxygen-containing functional groups (6–8). Because of these characteristics, biochar has been used for improving soil fertility, carbon (C) sequestration, and contaminant immobilization (9, 10). When applied to soils, however, biochar can release a fraction of dissolved organic matter that interacts with surrounding biogeochemical processes. Therefore, integrating waste-derived material into environmental management practices requires careful evaluation of its potential behavior and risks. Of particular importance is biochar-derived dissolved organic matter (BDOM), a highly mobile and bioactive fraction that plays a crucial role in biogeochemical processes.
On a global scale, approximately 10% of dissolved organic carbon (DOC) in major rivers originates from fire-derived charcoal, essentially a natural form of biochar (11). When biochar is added to soil, BDOM may interact with native soil organic matter (SOM), potentially enhancing decomposition of native SOM, known as positive priming effects (12). This may reduce biochar’s potential to sequester C in soils. From a remediation perspective, BDOM also influences pollutant dynamics by participating in redox reactions (13), modifying SOM interactions (14)and potentially influencing its ability to stabilize metal(loid)s. Compared to bulk biochar, BDOM exhibits higher mobility and bioavailability, making it more susceptible to photodegradation and microbial mineralization (15). Its rapid transport via leaching and runoff further underscores its environmental relevance. Therefore, predicting the release and composition of BDOM is critical for biochar applications.
Conventional protocols using neutral salt (NaCl), acidic (HCl), or alkaline (NaOH) solutions (16, 17) are used to probe ionic-strength effects, pH-dependent mobilization, and relative release potential (18). Since these procedures impose simplified and, in some cases, chemically aggressive conditions, they may yield BDOM with molecular weight distributions and aromaticity that differ from those obtained under more context-specific test conditions (19, 20). Accordingly, a more informative approach is to combine such chemical extractions with tests tailored to specific exposure contexts relevant to waste management and resource use. The toxicity characteristic leaching procedure (TCLP), for example, is designed to represent landfill leachate conditions using acetic acid (21). The synthetic precipitation leaching procedure (SPLP) employs a sulfuric/nitric acid mixture to simulate acid rain conditions (22). In addition, extractions with deionized water can be used to assess release during interaction with surface water and groundwater (23), whereas 0.01 M CaCl2 solution approximates the ionic strength of typical soil solutions (24).
This study investigates BDOM from Japanese knotweed-derived biochar using seven extraction methods that span context-specific leaching tests and controlled chemical extractions. The specific objectives are to: (1) quantify the release of DOC, metals, and nutrient elements under different extraction conditions; (2) characterize the optical and compositional properties of the released BDOM using ultraviolet-visible and fluorescence spectroscopy; and (3) compare extraction-dependent release patterns to distinguish environmentally contextual leaching responses from operationally defined chemical mobilization behavior. The outcomes will support the integration of this sustainable waste management strategy into practice, while also providing a broader basis for evaluating the release behavior of biochar produced from invasive plant biomass.
2 Materials and methods
2.1 Invasive plant-derived biochar
Japanese knotweed (Fallopia japonica) biomass was collected from Vlienderpad, Ghent, Belgium during the summer growing season. Following established protocols for invasive species handling, the plants were cut 15 cm above ground level to minimize regeneration potential from remaining rhizomes. The harvested material was immediately sealed in airtight polyethylene bags to prevent fragment dispersal during transport. Prior to processing, the biomass was thoroughly washed with tap water to remove surface contaminants and air-dried for seven days under ambient conditions (20–25 °C). Subsequent oven-drying at 75 °C ensured complete moisture removal (constant weight achieved with < 0.5% variation over 2 hours). The dried material was ground using a stainless-steel mill and sieved to obtain uniform particle sizes (0.5–2 mm).
Biochar was produced through slow pyrolysis in a lab-scale reactor under a continuous nitrogen atmosphere (detailed production parameters are provided in the Supplementary Materials). The pyrolysis protocol consisted of: (1) heating at 15 °C min-¹ to 500 °C, which is a typical temperature for biochar production (25), (2) maintaining the target temperature for 60 minutes, and (3) natural cooling under continuous N2 flow. The resulting biochar was stored in glass containers with desiccant until further analysis.
2.2 Extraction of biochar dissolved organic matter
The dissolved organic matter (BDOM) was extracted from biochar under seven extraction conditions. These methods included the toxicity characteristic leaching procedure (TCLP, US EPA Method 1311) with glacial acetic acid (pH 2.88), the synthetic precipitation leaching procedure (SPLP, US EPA Method 1312) with a sulfuric/nitric acid mixture (60:40 w/w, pH 4.20), distilled water (pH 6.50), 0.01 M CaCl2 solution (pH 6.50), 0.1 M NaCl (pH 7.00), 0.1 M HCl (pH 1.00), and 0.1 M NaOH (pH 13.00). The NaCl, HCl, and NaOH treatments were included as conventional chemical extractions to compare BDOM release under different ionic and pH conditions. No washing or other pretreatment was applied to the biochar prior to extraction to avoid removing loosely bound or readily soluble surface-associated organic fractions that could influence the measured release of BDOM and compromise comparability among extraction treatments. The biochar was therefore extracted in its original post-production state.
In the extraction procedure, 1 g of biochar was mixed with 20 mL of each extraction solution (1:20 solid-to-liquid ratio) in polypropylene centrifuge tubes (26). The tubes were tightly sealed with parafilm and aluminum foil to prevent evaporation and photochemical degradation, then shaken at 23 ± 1 °C for 24 hours. Following extraction, the mixtures were centrifuged at 10,000 × g for 10 minutes and filtered through 0.45 µm membrane filters. All extractions were performed in triplicate. The collected BDOM samples were stored in amber vials at 4 °C in the dark and analyzed within 48 hours to minimize storage artifacts. In addition, biochar-free blank experiments were conducted following the same protocol. The resulting blank solutions were used for background correction in subsequent analyses, including total organic carbon (TOC), UV-Vis spectroscopy, and excitation-emission matrix (EEM) coupled with parallel factor analysis. The extraction time was fixed at 24 h for all extractants to provide a unified basis for comparing their relative extraction performance under controlled laboratory conditions.
2.3 BDOM analyses
Prior to analytical measurements, all BDOM samples were adjusted to pH 7.0 ± 0.1 using 0.1 M HCl or NaOH to ensure consistent measurement conditions (27). Dissolved organic carbon (DOC) concentrations were quantified using a TOC analyzer (TOC-VCPN, Shimadzu Corporation, Japan). For elemental analysis, samples were acidified with one drop of concentrated HNO3 (65%, v/v) and subsequently analyzed for metal and nutrient content using inductively coupled plasma optical emission spectrometry/mass spectrometry (ICP-MS, Thermo Fisher iCAP Q Scientiffc, USA). The characteristics of the Japanese knotweed-derived biochar used in this study are summarized in Table 1.
Table 1
| Material | pHwater | C(%) | H(%) | N(%) | O(%) | Zn | Cu | Ca | K | Mg | P |
|---|---|---|---|---|---|---|---|---|---|---|---|
| (mg kg-1) | (mg kg-1) | (g kg-1) | (g kg-1) | (g kg-1) | (g kg-1) | ||||||
| Biochar | 10.15 ± 0.04 | 68.7 ± 0.8 | 0.6 ± 0.02 | 1.9 ± 0.20 | 12.3 ± 1.9 | 85.4 ± 5.2 | 27.4 ± 0.1 | 15.1 ± 0.3 | 21.7 ± 2.0 | 20.3 ± 1.2 | 5.9 ± 0.7 |
Biochar properties used for this study.
2.4 Ultraviolet-Visible analysis
The Ultraviolet-Visible (UV-Vis) absorption characteristics of the extracts were quantitatively analyzed using a Thermo Scientific Evolution 220 UV-Vis spectrophotometer. Measurements were performed across the 200–600 nm wavelength range at 1 nm resolution, employing a 1 cm pathlength quartz cuvette. To ensure accuracy, blank measurements were conducted using identical extraction solutions without biochar, and these background values were systematically subtracted from all sample measurements. For characterization of the BDOM, two key optical indices were calculated: (1) The E2/E3 ratio, which was used for relative comparison among extracts from the same biochar source, with higher values generally associated with relatively lower-molecular-weight DOM characteristics; and (2) SUVA254 (specific UV-Vis absorbance at 254 nm), a well-established indicator of aromaticity, using the following equations:
The E2/E3 ratio was calculated using the equation (28, 29).
where and are the absorbance coefficient measured at 254 and 365 nm, respectively.
where A254 is the decadic absorbance at 254 nm normalized to a pathlength of 1 cm, DOC is the dissolved organic carbon concentration (mg L–1), and SUVA254 is expressed in L mg C-1 m-1.
2.5 Fluorescence measurements coupled with parallel factor modeling
To characterize the fluorescent components of extracted BDOM, fluorescence excitation-emission matrix (EEM) spectroscopy was performed using a Shimadzu RF-5301 PC spectrofluorometer. The EEM measurements were conducted with excitation wavelengths ranging from 200 to 600 nm (5 nm intervals) and emission wavelengths from 290 to 550 nm (1 nm resolution). Prior to analysis, all samples underwent standardized pretreatment to ensure data quality: (1) samples were diluted with distilled water to achieve DOC concentrations <10 mg L-1 to minimize inner filter effects (30); (2) sample pH was adjusted to 7.0 ± 0.1 using 0.1 M HCl or NaOH to eliminate pH-induced fluorescence variations; and (3) Milli-Q water blanks were measured under identical conditions for background subtraction. Residual inner filter effects were further corrected using an absorbance-based correction approach based on the corresponding sample absorbance spectra. All obtained EEM spectra were subsequently normalized to Raman units (normalized to the daily Raman scan of Milli-Q water) to enable quantitative comparisons between samples (31). This protocol ensured the comparability of fluorescence signatures across different extraction methods while maintaining optimal signal-to-noise ratios.
The fluorescence datasets were analyzed using parallel factor analysis in MATLAB 2024a (MathWorks, Natick, MA, USA) following established protocols from our previous work (32). Parallel factor analysis model development incorporated non-negativity constraints and rigorous validation through multiple approaches: (1) split-half analysis to verify component stability, (2) residual analysis to assess model fit, and (3) visual inspection of spectral loadings. The validation process confirmed model robustness with no outliers detected. Component quantification was performed using maximum fluorescence intensities.
2.6 Statistical analysis
The IBM SPSS Statistics 23 (NY, USA) software was used for statistical analysis. Prior to analysis, normality and homogeneity of variance were examined. One-way ANOVA followed by Duncan test at a significance level of 0.05 was conducted to determine significant differences between the means in each group.
3 Results and discussion
3.1 Effects of extraction methods on the release of DOC, metals, and nutrients
The release behavior of DOC and inorganic constituents from Japanese knotweed-derived biochar varied significantly across extraction methods (p < 0.05; Figure 1). Extreme pH conditions induced the highest DOC release, with NaOH and HCl yielding 183 mg L-1 and 75 mg L-1, respectively (Figure 1a). These responses are consistent with alkaline hydrolysis of aromatic structures and the solubilization of organic functional groups under acidic conditions (33). In contrast, mild extractants (deionized water and CaCl2) released significantly lower amounts of DOC (5–15 mg L-1), suggesting limited mobilization under relatively natural scenarios.
Figure 1
The extractable micronutrient metals (Cu, Zn) were also strongly pH-dependent (Figure 1b). For instance, Cu showed marked acid sensitivity, with HCl extraction (167 μg L-1) releasing 3.7-fold more than SPLP extraction (45 μg L-1). This behavior can be explained by the dual speciation of Cu in biochar systems, where acidic conditions simultaneously dissolve oxide/hydroxide phases (CuO + 2H+ → Cu²+ + H2O) and displace organically complexed Cu (R-COO-Cu+ + H+ → R-COOH + Cu²+) (34). Zn demonstrated even more distinct pH-responsive patterns, with 354 μg L-1 released in HCl through dissolution of exchangeable, weakly complexed, and precipitated fractions, while under alkaline conditions (0.1 M NaOH), 91 μg L-1 was released, potentially as zincate ions (Zn(OH)2 + 2OH- → Zn(OH)4²-) (35). From a risk management perspective, the elevated Cu and Zn concentrations indicate a potential for trace metal release that warrants attention. For contextual comparison, the measured values were lower than the Flemish groundwater reference values (100 μg L-¹ for Cu and 500 μg L-¹ for Zn in Belgium) (36).
Macronutrient release showed element-specific patterns (Figures 1c, d). P solubility was markedly enhanced in HCl (1.1 mg L-¹ versus 0.1–0.3 mg L-¹ in neutral extractants), implying a predominant association with acid-labile compounds, including readily extractable phosphate species (e.g., inorganic phosphate salts, low-molecular-weight organic phosphates) (37). This indicates that P release may increase substantially under acidic conditions; however, such strongly acidic conditions are uncommon in most natural environments. K and Mg also showed substantial release in HCl (300 mg L-¹ and 101 mg L-¹, respectively), primarily originating from soluble salts and exchange sites. CaCl2 extraction triggered cation exchange, releasing 49 mg L-¹ Mg and 9 mg L-¹ Ca through stoichiometric displacement reactions (X-Mg²+ + Ca²+ ↔ X-Ca²+ + Mg²+) (38). The preferential release of Mg suggests weaker binding on biochar exchange complexes. Moreover, the release of K and Mg even under relatively mild extraction conditions indicates the presence of exchangeable nutrient pools that may remain mobile under environmentally relevant leaching scenarios.
Overall, these findings show that constituent release from this biochar was strongly controlled by extraction conditions. Extreme pH markedly enhanced the mobilization of DOC, Cu, Zn, P, K, Mg, and Ca, whereas water and CaCl2 extractions yielded much lower DOC and metal concentrations while still releasing exchangeable nutrient elements under relatively mild conditions.
3.2 Influence of extraction methods on the UV-Vis properties of BDOM
The UV-Vis spectral analysis revealed clear differences in the optical properties of BDOM among different extraction methods (Figure 2). NaOH extracts showed the strongest absorbance across the 200–600 nm spectrum (Figure 2a), consistent with their elevated DOC concentrations. However, these extracts exhibited much lower SUVA254 values (Figure 2b, 5.3 L mg C-1 m-1), suggesting that strong alkaline conditions may have altered aromatic structures through hydrolysis, thereby reducing the apparent aromaticity of the dissolved fraction. Although NaOH solubilized substantial amounts of organic matter, the extracted chromophores were likely enriched in less aromatic compounds, such as phenolic monomers and enolates (39).
Figure 2
Conversely, water-extracted BDOM showed the highest SUVA254 value (81 L mg C-1 m-¹). This suggests preferential dissolution of relatively aromatic components under mild extraction conditions (40). The mild nature of water extraction likely preserves the complex aromatic structures inherent in biochar. Neutral salt extracts (CaCl2: 64; NaCl: 32 L mg C-1 m-1) exhibited intermediate aromaticity, reflecting their ion-exchange-mediated release mechanism that selectively mobilizes specific aromatic constituents (41). Through interactions with charged sites on the biochar surface, these salts may facilitate the partial release of certain aromatic moieties while leaving more strongly bound structures intact. In contrast, acidic conditions can protonate functional groups and promote the release of smaller or less aromatic fractions (13). The acidic extractants (SPLP: 10; TCLP: 3.5 L mg C-1 m-1), and especially HCl (1.8 L mg C-1 m-1), generated BDOM with progressively reduced aromaticity. This indicates that more aggressive chemical treatments tend to favor the release of less aromatic or more fragmented organic matter (42). Overall, these contrasting patterns reflect fundamental differences in dissolution mechanisms among extractants. Mild extractants, such as water and dilute salts, preferentially mobilized persistent, aromatic-rich DOM, whereas extreme pH conditions disrupted biochar structure and released larger amounts of less aromatic material. These results corroborate the findings of Li et al. (17) that natural leaching processes favor the mobilization of persistent DOM constituents, while aggressive chemical treatments induce higher carbon release.
The E2/E3 ratios provided complementary information on the molecular characteristics of the released BDOM (45, 46). HCl (1.5) and TCLP (1.4) extracts showed the highest E2/E3 values, suggesting a greater contribution from relatively low-molecular-weight fractions, whereas the water extract (0.2) showed the lowest value, indicating relatively higher-molecular-weight components. Neutral salt solutions (CaCl2 and NaCl) showed intermediate values (around 0.4), which may reflect the release of DOM with intermediate molecular characteristics through ion-exchange processes (43). The NaOH extract (0.5) also fell within this intermediate range (44). Together with the SUVA254 results, these findings demonstrate that extraction method strongly influences both the amount and composition of released BDOM. NaOH extraction maximized DOC release but was associated with lower aromaticity, whereas water extraction retained stronger aromatic characteristics. Neutral salt extractants showed intermediate behavior, while acidic treatments favored the release of less condensed organic matter. However, E2/E3 should be interpreted with caution, as it may also be influenced by aromaticity and overall DOM composition, rather than reflecting molecular-weight differences alone (29). In this study, it was therefore used only for relative comparison among extracts derived from the same biochar source.
3.3 The impact of extraction methods on BDOM fluorescent components
Excitation-emission matrix fluorescence spectroscopy coupled with PARAFAC analysis identified four distinct fluorescent components (C1–C4, Figure 3) in BDOM (Table 2). The differential solubilization patterns reflect the interactions between extraction conditions and DOM characteristics, including charge properties, aromaticity, and molecular features.
Figure 3
Table 2
| Component | Spectral characteristics (Ex/Em) | Related intensity orders in different extracts |
|---|---|---|
| C1 (Tryptophan-like) | 245, 335/391 (47, 48) | NaCl > SPLP > Water > CaCl2 > NaOH > TCLP > HCl |
| C2 (Fulvic-like) | 225/416 (49, 50) | NaCl > SPLP > Water > NaOH > CaCl2 > TCLP > HCl |
| C3 (Tyrosine-like + Humic-like) | 235, 290/360, 408 (52, 53) | SPLP > NaCl > CaCl2 > TCLP > Water > NaOH > HCl |
| C4 (Humic-like) | 250, 375/452 (48, 54, 55) | NaCl > Water > NaOH > CaCl2 > SPLP > HCl > TCLP |
The spectral characteristics and related intensity orders of different components under different extraction methods.
Protein-like component C1, associated with tryptophan-containing compounds (47, 48), showed the highest fluorescence intensity under high ionic strength (NaCl extraction: 808 [R.U.]) and weakly acidic conditions (SPLP: 761 [R.U.]), which may reflect ionic stabilization of charged biomolecules and mild protonation effects, respectively. By comparison, its near absence in the strong acid extract (HCl: 62 [R.U.]) was accompanied by elevated E2/E3 values (1.5), indicating that protein-like fluorophores were unstable and prone to degradation under harsh acidic conditions.
Fulvic-like component C2 (49, 50), representing humified material of intermediate molecular weight, showed high intensities in both NaCl (374 [R.U.]) and NaOH (275 [R.U.]) extracts. This can be explained by salt-enhanced extraction of loosely bound fulvic-like fractions (51) and alkaline solubilization of partially oxidized aromatic structures. Under strongly acidic conditions, C2 was absent in the HCl extract and detected only at trace levels in the TCLP extract (3.8 [R.U.]). These results coincide with a pronounced reduction in SUVA254 (to 3.5), consistent with the sensitivity of fulvic-like fractions to protonation and possible precipitation under acidic conditions.
The transitional component C3 (52, 53), comprising tyrosine-like microbial byproducts and early-stage humic substances, exhibited peak intensities in SPLP (287 [R.U.]) and NaCl (268 [R.U.]) extracts. This pattern suggests that moderate acidity (pH 4.20) and ionic strength may favor the release or stabilization of these transitional fluorophores without causing extensive alteration. However, C3 intensity was lower in TCLP and HCl extracts (107 and 160 [R.U.], respectively). Moreover, the elevated E2/E3 ratios (> 1.4) under these acidic conditions suggest DOM characteristics distinct from those of the other extracts and potentially associated with relatively lower molecular weight.
Humic-like component C4, representing aromatic-rich humic substances with relatively high molecular weight (48, 54, 55), was most effectively extracted by water (452 [R.U.]) and NaCl solutions (524 [R.U.]). The water extract showed the maximum SUVA254 (81) and minimal E2/E3 (0.2), reflecting stronger preservation of humic-like components in the absence of ionic or pH perturbations. Although NaCl extraction produced a higher fluorescence intensity for C4 than water (524 vs. 452 [R.U.]), its lower SUVA254 (32 vs. 81) indicates that salt-induced conformational changes enhanced fluorophore emission while reducing the apparent aromaticity of the extract. This behavior may be attributed to moderate ionic strength (0.1 M) disrupting supramolecular associations while maintaining the overall integrity of humic-like components (41). By contrast, the negligible intensity of C4 in acidic extracts (TCLP: 23; HCl: 28 [R.U.]), together with extremely low SUVA254 values (< 3.6), points to protonation and destabilization of humic substances under low-pH conditions.
3.4 Environmental implications
The integrated analysis of DOC release, optical indices, and fluorescent components allows a comparative interpretation of extraction-dependent differences in the release characteristics of biochar-derived dissolved organic matter. The main implications of these extraction-dependent release patterns are summarized in Table 3.
Table 3
| Extraction method | Representative conditions | Main release characteristics | Fluorescent component performance | UV-Vis properties | Environmental implication |
|---|---|---|---|---|---|
| Water | Mild leaching, closest to natural conditions | Low DOC release; | Highest C4 (453), strong C1 (676) | SUVA254: 81 (highest), E2/E3: 0.2 (lowest) | Naturally leached BDOM is relatively aromatic and humic-like, with limited mobilization |
| CaCl2 | Mild ionic exchange system | Low DOC release; release of exchangeable Mg and Ca | Moderate C1 (445), C4 (374) | SUVA254: 64, E2/E3: ~0.4 | Reflects mobilization of exchangeable DOM and cations under mild leaching |
| NaCl | Elevated ionic strength | Intermediate optical behavior; strong fluorescence response across multiple components | Highest C1 (809), strong C2 (374), C3 (268), and C4 (525) | SUVA254: 32, E2/E3: ~0.4 | Ionic strength substantially influences BDOM release and fluorescence response |
| SPLP | Simulated acidic precipitation | Moderate DOC release; altered BDOM composition under acidic leaching | High C1 (761) and C3 (288); low C4 (48) | SUVA254: 10, E2/E3: ~0.4 | Acid rain conditions may modify both the amount and composition of released BDOM |
| TCLP | Strong acidic leaching/chemically disturbed conditions | Low aromaticity; enhanced Cu, Zn, P, K, and Mg release | Very low C4 (24); limited C1 (224), C2 (107), and C3 (232) | SUVA254: 3.5, E2/E3: 1.4 | Acidic disturbance enhances the mobility of less aromatic DOM and associated inorganic constituents |
| HCl | Extreme acidic condition | Strong compositional alteration; release of low-aromaticity DOM | lowest C2; limited C1 (62), C3 (160), and C4 (29) | SUVA254: 1.8, E2/E3: 1.5 | Strong acid causes marked alteration of BDOM composition and reduced humic-/fulvic-like character |
| NaOH | Extreme alkaline condition | Highest DOC release; substantial DOM solubilization | Moderate C2 (275) and C4 (391) | SUVA254: 5.3, E2/E3: 0.5 | Alkaline disturbance promotes strong DOM solubilization with marked compositional alteration |
Environmental implications of extraction-dependent BDOM release patterns.
Extraction with water is one of the most widely used methods for DOM extraction (56), as it is commonly used to approximate natural conditions. In the present study, water extraction yielded BDOM with the highest humic-like C4 fluorescence intensity (453), the highest aromaticity (SUVA254 > 80), and the lowest E2/E3 ratio, indicating the release of relatively aromatic DOM with more preserved humic-like characteristics under mild conditions. CaCl2 extraction also represented a comparatively mild system, with lower DOC release, intermediate optical characteristics, a more balanced composition of C1 (445) and C4 (374), and continued release of exchangeable Mg and Ca. NaCl extraction showed similarly intermediate optical behavior, but with high fluorescence intensities of C1, C2, C3, and C4, indicating that ionic strength strongly influenced the release and fluorescence response of multiple BDOM fractions. Together, water and CaCl2 indicated limited DOC mobilization under relatively mild conditions, whereas NaCl, although non-extreme in pH, produced clearer compositional shifts associated with ionic-strength effects.
By contrast, chemically aggressive extraction methods produced markedly different release patterns. NaOH extraction resulted in the highest DOC concentration and relatively strong C2 (fulvic-like) and C4 (humic-like) fluorescence intensities, but substantially lower SUVA254 than water extraction, indicating that greater DOM release was accompanied by clear compositional alteration. The strongly acidic extracts, particularly HCl and TCLP, were characterized by very low SUVA254 values, elevated E2/E3 ratios, and markedly reduced humic-like fluorescence. HCl showed complete loss of C2 and only limited residual C1, C3, and C4 signals, whereas TCLP retained low but detectable C1, C2, and C3 fluorescence together with very low C4 intensity. These shifts were accompanied by enhanced release of Cu, Zn, P, K, and Mg, showing that extreme pH can simultaneously influence the mobilization of both organic and inorganic constituents.
The SPLP method, which simulates acidic precipitation, provided additional insight into BDOM behavior under environmentally relevant acidic conditions. SPLP mobilized a mixture characterized by high C1 (761) and C3 (288) fluorescence intensities, low C4 intensity (48), and intermediate aromaticity (SUVA254 = 10), indicating that acidic leaching can modify both the amount and composition of released BDOM even when acidity is less extreme than in HCl or TCLP extraction. This suggests that acid deposition-type conditions may still shift BDOM toward less humified and compositionally altered forms.
Taken together, the results indicate that the environmental behavior of BDOM is strongly governed by leaching chemistry. Mild extraction conditions favored the release of smaller amounts of relatively aromatic and humic-like DOM, whereas acidic and alkaline conditions promoted greater solubilization and stronger compositional alteration. Extreme pH also enhanced the co-release of nutrient elements and trace metals, suggesting that chemically disturbed environments may increase the mobility of both dissolved organic matter and associated inorganic constituents. These observations provide important context for interpreting the environmental behavior of biochar under contrasting leaching conditions, although the present findings are limited to batch extraction systems.
3.5 Considerations and future research
This work provides a comparative basis for evaluating how different extraction conditions influence both the quantity and chemical characteristics of dissolved organic matter released from biochar. Japanese knotweed biochar, representing a herbaceous feedstock and produced at a commonly applied pyrolysis temperature, was used as the model material. Since herbaceous biomass is widely used as a biochar feedstock, these findings offer useful insight into the release behavior of invasive plant-derived biochar. However, the observed patterns should not be generalized to all biochars, as feedstock type and production conditions influence biochar composition and extractability. For example, wood-derived biochars often contain lower ash contents and more condensed carbon structures than herbaceous biochars, features that may reduce DOC release, whereas sewage sludge-derived biochars may show substantially different inorganic and organic release characteristics. Future research should therefore extend this comparative extraction approach to biochars from a wider range of feedstocks and pyrolysis conditions.
In addition, the present results are based on controlled batch extractions and therefore represent extraction-dependent release responses rather than direct predictions of long-term field behavior. Under field conditions, biochar is subjected to aging processes such as wet-dry cycles, freeze-thaw events, microbial decomposition, oxidation, and solar irradiation, all of which may alter surface chemistry and affect BDOM release over time. Future studies should combine laboratory extraction experiments with field-based monitoring to better assess how BDOM quantity and composition evolve under realistic exposure conditions. Particular attention should be given to the long-term dynamics of fluorescent components, shifts in aromaticity and related optical characteristics, and the coupled mobilization of dissolved organic matter with metals and nutrient elements. The potential effects of co-released metals on fluorescence behavior should also be considered.
4 Conclusion
This study elucidated the release patterns of biochar-derived dissolved organic matter (BDOM) from Japanese knotweed biochar through a multi-method extraction approach. The results showed that extraction conditions significantly influenced BDOM quantity, composition, and release characteristics. Strong chemical treatments (0.1 M HCl and 0.1 M NaOH) maximized DOC release but yielded BDOM with low aromaticity, while mild extractions (water and 0.01 M CaCl2) preserved more stable and highly aromatic fractions. The leaching of metals and nutrients was pH-dependent. The extraction methods employed in this study provided a comparative framework for evaluating BDOM release under different chemical conditions. Specifically, water and CaCl2 represented relatively mild extraction conditions relevant to aqueous and soil-solution contact, whereas aggressive chemical methods (HCl and NaOH) reflected BDOM release patterns under chemically extreme conditions.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author contributions
JQ: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing. JL: Funding acquisition, Writing – review & editing. XW: Writing – review & editing. YO: Writing – review & editing. EM: Supervision, Writing – review & editing. HL: Funding acquisition, Methodology, Supervision, Writing – review & editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. Jing Qiu acknowledges the support from High-Level Talent Introduction Program for the Guizhou Institute of Technology (No. 2025GCC032, No. 2023GCC083), and the Young Scientific Technical Talents Development Fund of Guizhou Province (No. QJJ (2024)169). Haichao Li is supported by Swedish Research Council for Sustainable Development (Formas) (2024-00502).
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsoil.2026.1809945/full#supplementary-material
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Summary
Keywords
biochar, dissolved organic matter, extraction methods, fluorescence analysis, invasive plant-Japanese knotweed, ultraviolet-visible spectroscopy
Citation
Qiu J, Lyu J, Wang X, Ok YS, Meers E and Li H (2026) Extraction method influences dissolved organic matter from invasive Japanese knotweed-derived biochar and environmental implications. Front. Soil Sci. 6:1809945. doi: 10.3389/fsoil.2026.1809945
Received
12 February 2026
Revised
24 March 2026
Accepted
30 March 2026
Published
15 April 2026
Volume
6 - 2026
Edited by
Kamel M. Eltohamy, Zhejiang University, China
Reviewed by
Huihui Du, Hunan Agricultural University, China
Fayong Li, Tarim University, China
Daniel Menezes Blackburn, Sultan Qaboos University, Oman
Updates
Copyright
© 2026 Qiu, Lyu, Wang, Ok, Meers and Li.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Haichao Li, haichao.li@slu.se
Disclaimer
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