Contrasting carbon processing and stabilization pathways in low- and high-mercury contaminated soils

• Accumulation and stability of SOC pools were higher in low Hg stress soils. • Plant-derived C decreased and microbial-derived C increased under high Hg stress. • Carbon degradation and accumulation prevails in high and low Hg soils, respectively. The interplay between plant-derived inputs and microbial processes govern soil organic carbon (SOC) dynamics. Environmental contamination, particularly heavy metal stress, disrupts both plant input and microbial activity, thereby altering soil carbon sources and transformation processes. However, the specific contributions of microbial- and plant-derived carbon to SOC accumulation and the underlying mechanisms of carbon sequestration under contaminant stress, such as mercury (Hg), remain unclear. In this study, we collected 19 soil samples under varying Hg stress levels and quantified microbial- and plant-derived necromasses using amino sugars and lignin phenol as biomarkers. The SOC pool size, including both mineral-associated and particulate organic C decreased under high Hg stress (HgH). Additionally, SOC stability, evaluated using a persistence index based on the Hill number multifunctionality framework, was significantly lower for HgH, indicating lower carbon persistence. Furthermore, the source composition of the SOC pool differed across Hg levels, with lower plant-derived C and higher microbial-derived C observed in HgH soil, indicating a compensatory pattern. Fourier transform ion cyclotron resonance mass spectrometry analysis revealed contrasting SOC sequestration and accumulation pathways in low- and high-Hg-contaminated soils. In HgH soils, a degradation-dominated pathway (i.e., priming effect) prevailed, whereas an accumulation-driven pathway (i.e., entombing effect) dominated in low-Hg soils. These findings underscore the importance of considering pollution-induced shifts in carbon cycling processes.