Effect of acid production from forest litter on available heavy metals in soil of surrounding farmland

• Forest litter leaching reduced soil pH and increased exchangeable Al 3+ . • Leaching increased soil heavy metal availability by up to 5.48-fold. • Longer leaching and stronger acidity increased heavy metal availability. • Mantel test revealed strong links between acidity, soil properties, and metal levels. • Forest litter decomposition acidifies soil and enhances heavy metal availability. Litter decomposition is a central ecological process in forests because of its role in nutrient cycling and the maintenance of soil fertility. Products of litter decomposition affect the physicochemical properties of soil and the activity of mineral elements. However, the effects of these decomposition products on soil properties and heavy metal availability in surrounding farmland remain poorly understood. In this study, we examined three types of typical forest litter— Myrica rubra leaves, Bambusa cerosissima leaves, and Pinus sylvestris leaves—collected from Daluo Mountain, Wenzhou, China, to investigate their effects on soil chemical properties and heavy metal availability in adjacent farmland. Litter decomposition and leaching experiments showed that leachate exposure caused soil acidification, with pH decreasing by up to 0.15 units, while the maximum level of exchangeable Al 3+ (Ex-Al 3+ ) increased by 133 %. At the same time, the availability of heavy metals such as Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb rose by up to 5.48-fold. Higher levels of low-molecular-weight organic acids and longer leaching durations intensified heavy metal availability, especially in the 0–20 cm and 20–40 cm soil layers. The Mantel test revealed significant correlations ( p < 0.001) between soil acidity, chemical properties, and available heavy metals, particularly Mn, Ni, Cu, Zn, As, and Cd. Partial least squares path modeling (PLS-PM) showed that forest litter decomposition led to soil acidification, which further affected available heavy metals ( p < 0.001). With an estimated 65 % decomposition rate, annual forest litter in Daluo Mountain was calculated to contribute approximately 3.6 × 10 3 mol of H + , highlighting its role in altering soil chemistry and increasing heavy metal risks in adjacent farmland. These findings provide a theoretical foundation for assessing and controlling heavy metals in farmland and contribute to the sustainable management of farmland ecosystems.