Can Fluorescence Spectroscopy Evaluate Soil Dissolved Organic Matter Dynamics?

A new study published in Chemical Engineering Journal investigated the influence of biochar aging on dissolved organic matter and in remediating cadmium-contaminated soil (1). This study, which was led by Ying Zhang and Yifan Wang from Northeast Agricultural University in Harbin, China, provided new insights regarding the ability of biochar in controlling pollution (1).

Biochar is an alternate form of charcoal: it is black carbon that is comprised of several biomass sources (2). These sources include manure, wood chips, and more (2). Biochar is designed to serve several purposes, including soil aeration, carbon sequestration, and to improve plant health (3). However, the aging process of biochar in soil, particularly its impact on dissolved organic matter (DOM) release and the bioavailability of contaminants, remains underexplored.

Large heavy paper bag filled with biochar on a grassy field, representing sustainable soil improvement. Generated with AI. | Image Credit: © Yan - stock.adobe.com

As part of their experimental procedure, the researchers subjected biochar produced from corn straw at 500 °C to three artificial aging treatments: freeze-thaw cycles (FT), dry-wet cycles (DW), and ultraviolet irradiation (UI) (1). This process was meant to simulate long-term environmental exposure, so that researchers could evaluate how aging affects DOM release, microbial activity, and heavy metal bioavailability (1).

Out of the three artificial aging treatments tested, the UI treatment resulted in the highest release of biochar-derived DOM, which was 209.57 mg/L after 25 days (1). The other two treatments, DW and FT, achieved 135.75 mg/L and 129.47 mg/L, respectively (1). The findings suggest that UV exposure accelerates biochar DOM solubility.

Soil incubation experiments revealed that aging biochar alters soil DOM composition, which in turn impacts cadmium bioavailability. Notably, the S-FT and S-BC treatments, which had higher humic acid content, were more effective in reducing cadmium bioavailability compared to S-DW and S-UI (1). The researchers produced the following cadmium passivation rates across treatments: S-BC was 27.84%; S-FT was 23.76%; S-UI was 24.81%; and S-DW was 18.98% (1).

These results underscore that while aging biochar influences the bioavailability of DOM in the soil, the presence of humic acid-like substances plays a key role in immobilizing cadmium.

Another aspect to this study was the use of the excitation-emission Matrix (EEM) fluorescence spectroscopy and parallel factor analysis (PARAFAC) to learn more about the structural changes in BC-derived DOM. Using EEM fluorescence spectroscopy and PARAFAC, the researchers discovered that aging increased humic-like substances in DOM, which in turn had a direct impact on the soil DOM composition and the ability of biochar to immobilize cadmium (1).

The researchers also noticed a gradual decline in the percentage of humic acid-like substances in soil-derived DOM over time, with S-BC and S-FT treatments maintaining higher humic acid content than S-DW and S-UI after 45 days of incubation (1). This suggests that while aging biochar can enhance microbial activity, it may also reduce its long-term capacity for heavy metal stabilization.

The results highlight the need to consider how different environmental factors, such as exposure to UV radiation and wet-dry cycles, influence the release of DOM and the bioavailability of heavy metals (1). By demonstrating that aging treatments affect the microstructure of biochar and the chemical composition of soil DOM, this study lays the groundwork for optimizing biochar applications in pollution control (1).

Biochar is being used in the agriculture industry for several reasons. Because of the utility of biochar in improving plant growth or crop yield, it is expected that its usage will only continue to grow (3). As environmental concerns around soil contamination and heavy metal pollution continue to grow, studies like this one from Northeast Agricultural University provide valuable insights into sustainable remediation strategies (1).

The researchers suggest that future research endeavors should further refine biochar modifications to enhance its long-term stability and effectiveness in diverse environmental conditions (1).

References

1. Li, J.; Li, Q.; Liang, S.; et al. Aging Mechanism of Biochar based on Fluorescence Spectroscopy: Assessing Soil Dissolved Organic Matter (DOM) Dynamics and Cd Bioavailability. Chem. Eng. J. 2025, 505, 159538. DOI: 10.1016/j.cej.2025.159538
2. U.S. Department of Agriculture, Biochar. USDA.gov. Available at: https://www.ars.usda.gov/midwest-area/stpaul/swmr/people/kurt-spokas/biochar/ (accessed 2025-02-19).
3. Murray, M. What is Biochar and How Is It Used? USU.edu. Available at: https://extension.usu.edu/planthealth/research/biochar (accessed 2025-02-19).