From bedrock to life activity and atmospheric deposition: Drivers of soil element coupling across horizons

Unraveling the intricate coupling of multiple elements and their underlying drivers in natural soils is crucial for comprehending ecosystem functions, yet this knowledge has remained elusive. Using a comprehensive dataset of 900 soil samples collected from 116 sites across 26 mountains, this study dissected the coupling relationship of 23 elements within three soil development horizons, spanning five climate zones in China. Our findings revealed a robust continental-scale coupling of soil elements, influenced by plants and environmental factors including spatial distance, climate, soil properties, and atmospheric nitrogen deposition, accounting for 36% of the observed variance in element coupling. Notably, our study unveiled the horizon-specific nature of element coupling mechanisms. In the parent horizon, rock type exerted the primary control on the dynamics of element coupling. However, as soil developed, life activities and atmospheric deposition of anthropogenic trace metals concurrently reshaped the element coupling patterns, particularly in the organic and surficial mineral horizons. Elements were divided into two distinct elemental groups, exhibiting opposite fitting trends with atomic mass and crustal abundance, and the effect of these properties on coupling diminished with soil depth. Heavy metals enriched by human activity deviated from property-based predictions with lower coupling. This study represents the first continental-scale quantification of multi-element coupling across soil horizons, underscoring the paramount importance of life activity and atmospheric deposition in modulating the initial lithological-mediated multi-element coupling. Our insights advance understanding of terrestrial ecosystem biogeochemistry and urge further research on the impacts of anthropogenic activities and environmental changes on these delicate elemental interactions.