Imbalances in dissolved elemental export fluxes disclose “hidden” Critical Zone pathways

Streams are integrators over all biogeochemical reactions taking place in the Critical Zone. The resulting export of dissolved elements from a watershed is commonly used to infer chemical weathering fluxes. Yet, this approach rests on the assumption that the mass of a given element released from primary minerals that was not incorporated into secondary solids is quantitatively transferred into the stream in the dissolved form. The comparison of element-specific solute stream fluxes with chemical weathering fluxes determined in the residual solids shows that this is often not the case. An imbalance persists even after correcting for a bias potentially introduced by changes in water flow over these entirely different timescales.

To explore causes for imbalances between short-term and long-term weathering fluxes, described by their ratio in form of a “Dissolved Export Efficiency” (DEE), we sampled six Critical Zone water compartments for one year in the Conventwald (Black Forest, Germany). We found deficits in the dissolved load, which emerged in the deep saprolite. For Si, Al, and Fe the deficit can be attributed to an export pathway that includes a “hidden” Critical Zone compartment or pool of unsampled colloids that are exported preferentially during flushing events. In contrast, deficits found for nutritive elements (Ca, K, Mg, P) can be explained by deep nutrient uptake followed by nutrient retainment in re-growing forest biomass or export in form of biogenic particulates. Given the collective evidence for these imbalances the deep Critical Zone warrants attention towards a complete budget of element cycles.