Environmentally persistent free radicals (EPFRs) in humic substances play an essential role in soil geochemical processes. Light is known to induce EPFRs formation for dissolved organic matter in aquatic environments; however, the impacts of light irradiation on the variation of EPFRs in soil humic substances remain unclear. In this study, humic acid, fulvic acid, and humin were extracted from peat soil and then in situ irradiated using simulated sunlight. Electron paramagnetic resonance spectroscopy results showed that with the increasing irradiation time, the spin densities and g-factors of humic substances rapidly increased during the initial 20 min and then gradually reached a plateau. After irradiation for 2h, the maximum spin density levels were up to 1.63 × 10¹⁷, 2.06 × 10¹⁷, and 1.77 × 10¹⁷ spins/g for the humic acid, fulvic acid, and humin, respectively. And the superoxide radicals increased to 1.05 × 10¹⁴–1.46 × 10¹⁴ spins/g while the alkyl radicals increased to 0.47 × 10¹⁴–1.76 × 10¹⁴ spins/g. The light-induced EPFRs were relatively unstable and readily returned back to their original state under dark and oxic conditions. Significant positive correlations were observed between the concentrations of EPFRs and reactive radical species (R²= 0.65–0.98, p < 0.05), which suggested that the newly produced EPFRs contributed to the formation of reactive radical species. Our findings indicate that under the irradiation humic substances are likely to be more toxic and reactive in soil due to the formation of EPFRs.

腐殖质中的环境持久性自由基（EPFR）在土壤地球化学过程中起着至关重要的作用。已知光会诱导水生环境中溶解的有机物形成EPFR。然而，光辐照对土壤腐殖质中EPFR变化的影响尚不清楚。在这项研究中，从泥炭土中提取了腐殖酸，黄腐酸和腐殖质，然后使用模拟阳光进行原位照射。电子顺磁共振波谱结果表明，随着辐照时间的增加，腐殖质的自旋密度和g因子在最初的20min内迅速增加，然后逐渐达到平稳。照射2h后，最大自旋密度水平高达1.63×10 ¹⁷，2.06×10腐殖酸，黄腐酸和腐殖质分别为¹⁷和1.77×10 ¹⁷ spins / g。并且超氧自由基增加到1.05×10 ¹⁴ –1.46×10 ¹⁴ spins / g，而烷基自由基增加到0.47×10 ¹⁴ –1.76×10 ¹⁴ spins / g。光诱导的EPFR相对不稳定，并且在黑暗和有氧条件下容易返回到其原始状态。EPFRs的浓度与反应性自由基之间存在显着的正相关（R ² = 0.65-0.98，p<0.05），这表明新产生的EPFR有助于形成反应性自由基。我们的发现表明，在辐射下，腐殖质可能由于EPFR的形成而在土壤中更具毒性和反应性。