Climate-driven mercury dynamics in a tropical savanna: Low uptake, pulsed emissions, and weak sequestration

Savanna ecosystems, spanning approximately 20 million km² globally, play a critical yet understudied role in the global mercury (Hg) cycle. We hypothesized that the unique structural and climatic features of savannas, including open canopies, pronounced seasonal aridity, and elevated temperatures, would drive a unique Hg cycling pattern. This study presented the first comprehensive assessment of Hg dynamics in China's savanna, and showed that the annual average of atmospheric Hg⁰ concentration was 2.29 ± 0.46 ng m^-3. Throughfall Hg deposition was dominated by particulate Hg (50-80 % of total Hg). This is a sharp contrast to forest systems where dissolved Hg prevails, reflecting the savanna's dust-enriched and low dissolved organic matters (1 -2 times lower) in throughfall. Additionally, intense solar radiation and temperature during dry seasons triggered pulsed soil Hg emissions, creating a "pump-leak" dynamic in which newly deposited Hg was rapidly re-emitted. This process resulted in a markedly weak net ecosystem Hg sink (6.9 ± 17.2 μg m⁻² yr⁻¹), challenging traditional assumptions about terrestrial Hg sequestration. Vegetation Hg uptake was severely constrained by drought stress, with accumulation rates 3-4 times lower than those observed in tropical forests. Finally, the small atmospheric Hg deposition and weak sequestration in soil leaded to low surface soil Hg concentration (22.7 ± 6.1 ng^-1). These findings demonstrates that savannas act as dynamic Hg exchange zones rather than long-term storage reservoirs and underscores the imperative to integrate these biogeochemical processes into global Hg models and policy frameworks.