Forest leaf litter is an important source of soil nutrients, but how its decomposition products affect the availability of soil heavy metals is not totally clear. In order to understand the effects of leaf litter decomposition on soil heavy metal availability in a forest ecosystem, leaf litter samples were collected from Daluoshan in Wenzhou and the Wuyanling National Nature Reserve of Zhejiang and subjected to analysis. The leaf litter was produced by the forests’ dominant tree species, such as Myrica ruba, Bambusa cerosissima, Pinus sylvestris, Machilus thunbergia, Cunninghamia lanceolata, and Quercus acutissima. Decomposition of the leaf litter samples at room temperature and leaching decomposition of the samples were carried out to analyze the acid production characteristics of the litter samples and their impact on soil heavy metal availabilities. Oxalic acid, lactic acid, fumaric acid, succinic acid, malic acid, and citric acid were the dominant organic acids in the leaf litter decomposer, and the sum of these six organic acids accounted for more than 50% of the total organic acid. During a 30-day litter decomposition, the levels of organic acids basically increased, with significant increases appearing in the early stage. After leaching, the available contents of Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb increased by a maximum of 11.95, 2.33, 12.00, 0.80, 0.83, 0.54, 51.50, and 0.28-fold, respectively, compared with those of the original soil sample. During leaching, the higher the content of low molecular weight organic acids in the leaf litter, the longer the leaching time required and the more obvious the influence on the level of available heavy metals. A stronger leaching effect was found for the soil samples taken from a depth of 0–20 and 20–40 cm accompanied by a more obvious increase in the available heavy metals in the soil. PLSPM results showed that there were significant differences between the soil acidity index and the available heavy metals in Daluoshan (p < 0.05) and that there were significant differences between the soil acidity index and the available heavy metals in Wuyanling (p < 0.01). The decomposition of litter produced low molecular weight organic acids, which caused a decrease in soil pH and an increase in exchangeable H⁺ and Al³⁺. Both changes had an impact on soil organic matter and further led to an increase in the activity of heavy metals in the soil. This will further lead to the increase of ecological and environmental risks to forest soil.

Global forest soil acidification has become a significant environmental concern, making it essential to gain a comprehensive understanding of research hotspots in this field. Acidic substances in forest soil originate from both external and internal factors. To investigate this issue, we conducted a visual analysis of 2325 papers published between 2004 and 2024 using the Web of Science Database, along with the visualization and analysis tools CiteSpace and VOSviewer. Over the past 20 years, the number of publications on global forest soil acidification has steadily increased. China and the United States have far more publications than any other country. Key research hotspots include soil acidification, atmospheric deposition, nitrogen deposition, heavy metals, soil pH, plant growth, impacts and governance, each displaying distinct characteristics at different stages. This review offers a comprehensive overview of recent advances in global forest soil acidification research and serves as a valuable reference for both research and practical applications. It examines the current state of this global environmental problem, the long-term effects of acidification and forest succession, and the eco-environmental effects associated with soil acidification. It also proposes sustainable solutions to mitigate forest soil acidification and outlines potential future research topics. These efforts aim to support the stable development of forest ecosystems and promote ongoing research in this critical area.

Abiotic humification, dominated by catalytic oxidation, is one of the critical mechanisms for organic carbon preservation in nature. However, the effects of biochar catalysis on abiotic humification have not yet been elucidated. This study investigated the catalytic power of biochar from walnut shells at different temperatures (300 °C, 600 °C, and 900 °C) for the abiotic transformation of hydroquinone (HQ) as a representative polyphenol. All the biochar samples catalyzed HQ polymerization, resulting in the formation of humic polymers such as fulvic acids (FAs) and humic acids (HAs). Light and oxygen promoted HA formation. HO^• was detected in the BC600–HQ reaction system, and HO^• quenching resulted in a 41.22% decrease in HA production, indicating that HO^• plays a major role in the oxidative polymerization. In the proposed pathway for the abiotic humification, biochar active sites and generated reactive oxygen species accept an electron from HQ, resulting in oxidation to (semi)quinone radicals, which subsequently undergo cleavage or a coupling reaction to form the oligomerized products. Under BC600 catalysis, the weight-average molecular weight (Mw) of the reaction products of HQ, glucose, and glycine reached 14,449 Da. These findings provide new insights into the application potential of biochar for promoting soil carbon sequestration.

Forest soil acidification is a global environmental problem, and forest ecosystems play an important role in terrestrial ecosystems. To provide a theoretical basis for the sustainable development of areas, this study selected Daluoshan and Wuyanling National Nature Reserve, Wenzhou, Zhejiang Province, China and determined the characteristics of soil acidification in the two forested areas and the relationship between forest soil acidification and soil physicochemical properties. The forest soil pH value of Daluoshan was 4.25–5.40, and that of Wuyanling was 4.14–5.71. There was a significant difference between the mean soil pH values of the two woodlands (p < 0.01). The two forest soils were dominated by strongly acidic soils, and the top layer of the soils was susceptible to disturbances by environmental factors. With the increase in altitude, different layers of soil in the forests in the two woodland areas showed deepening acidification. Regarding the different plant types and slope directions, the soils under trees in the forests showed the most serious acidification, followed by the soils under shrubs and herbs. Compared with the soils on the leeward slopes, the soils of the forest sites on the windward slopes showed a more severe situation of soil acidification. Redundancy analysis between forest soil acidity indicators and soil physicochemical properties in the two forest sites showed that the main environmental factor affecting soil acidity indicators was salt base saturation, and soil pH and active state Cr, Ni, and Pb showed negative correlations in both forest sites.