Elevation-Dependent Soil Organic Matter Persistence and Molecular Traits Influence Mercury Storage in Timberline Ecotones

Abstract

Alpine timberline ecotones are climatically sensitive transition zones, where small but persistent variations in temperature can markedly affect soil organic matter (SOM) composition, persistence, and the retention of pollutants such as mercury (Hg). While SOM-Hg interactions have been extensively studied in different ecosystems, the molecular-level mechanisms governing these relationships in high-elevation timberline ecotones remain unclear. Here, we investigated SOM molecular composition, persistence, and soil Hg content in the lower (LT) and upper (UT) timberline of Mt. Gongga, China. We applied pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) to characterize SOM molecular components, calculated a persistence index (PI) using the multifunctionality method from 18 SOM persistence-related properties to quantify resistance to decomposition, and measured total Hg concentrations using cold-vapor atomic fluorescence spectrometry. The significantly higher carbohydrates and lignin at LT indicate higher incorporation of fresh plant inputs than at UT. In contrast, the predominance of aliphatic SOM at UT suggested enhanced microbial processing. The significantly higher PI at LT indicated greater SOM persistence in this zone. Interestingly, SOM-Hg coupling was stronger at UT, even though total Hg levels were higher at LT, suggesting less effective Hg retention at LT. These findings indicate that LT serves as a key zone for carbon and Hg storage, whereas UT exhibits stronger SOM-Hg associations. With projected upward timberline migration under global warming, alpine soils may sequester more carbon and Hg, but weakened SOM-Hg coupling could increase Hg mobilization risks. These findings advance the understanding of SOM-Hg interactions by integrating molecular composition and persistence metrics into natural climate gradient studies, providing a novel framework for predicting Hg mobility risks under timberline migration driven by global warming.