Hydrodynamic and geochemical controls on soil carbon mineralization upon entry into aquatic systems.

Erosion is the most widespread form of soil degradation and an important pathway of carbon transfer from land into aquatic systems, with significant impact on water quality and carbon cycle. However, it remains debatable whether erosion induces a carbon source or sink, and the fate of eroded soil carbon in aquatic systems remains poorly constrained. Here, we collect 41 representative soils from seven erosion-influenced basins and conduct microcosm simulation experiments to examine the fate of soil carbon under three different scenarios. We showed that soil carbon mineralization was generally promoted (by up to 10 times) in water under turbulence relative to in soils, but suppressed under static conditions upon entering into aquatic systems. Moreover, the enhancement of mineralization in turbulent systems is primarily related to soil aggregate content, while suppression in static systems positively relates to macromolecule abundance, indicating that soil geochemistry affects the magnitude of hydrodynamic effects on carbon mineralization. Random forest model further predicts that erosion may induce significant carbon sources in basins dominated by turbulent waters and aggregate-rich soils. Our findings demonstrate hydrodynamic and geochemical controls on soil carbon mineralization upon delivery into aquatic systems, which is a non-negligible part of the boundless carbon cycle and must be considered when making region-specific conservation strategies to reduce CO emissions from inland waters.