Hillslope hydrologic influences on soil dissolved organic carbon fate informs extreme precipitation impacts on boreal forest mineral soil stocks

• Laboratory experiments evaluating mineral soil C responses to extreme precipitation. • Soil C accrual depends on the degree to which organo-metal complexes are C saturated. • Deep mineral soils offer increased organic C storage potential in wet boreal forests. • Greater infiltration in low relief soils likely supports deep C accrual. • Hillslope flow paths govern boreal soil C distribution and fate with climate change. In boreal forests dissolved organic carbon (DOC) from organic soil layers supports chelation of metals forming organometal complexes (OMCs), a major form of stabilized soil organic carbon (SOC) in mineral soil. However, the effectiveness of OMCs likely depends on the degree to which they are saturated by carbon. We studied DOC retention in mineral soil from contrasting hillslope positions in a humid boreal forest to help address two key questions: (1) How do soil and pre-event conditions impact SOC response to extreme precipitation events? (2) Does this response vary by hillslope position? Soil collected from both low-relief upslope and steep footslope sites, were subjected to a sequence of additions of organic soil solutions over a range of soil moisture conditions. Results of linear-mixed models assessing the effect of soil position, moisture, and OMC carbon saturation indicate variation in DOC retention is largely (75%) explained by the carbon saturation of aluminum OMCs. Soil moisture conditions prior to DOC addition and soil solution DOC concentration explained much of the remaining variation. These results help explain the greater SOC content and its steeper decline with depth in the upslope low-relief soils relative to the steeper footslope soils. The relatively C-saturated surface soils exhibited DOC loss with rewetting suggesting enhanced SOC loss with extreme precipitation on dry shallow mineral soils. Conversely, C-undersaturated deeper soils consistently retained DOC, particularly in low-relief upslope soils where greater infiltration transports DOC to depth supporting adsorption by undersaturated OMCs, indicating deep SOC storage potential. In steeper footslope soils, however, greater lateral flow may limit deeper DOC supply and thus SOC accrual. By combining hillslope soil observations with laboratory experimentation, we find evidence suggesting the fate of DOC infiltrating through and across soil profiles is impacted by hydrologic processes and the degree to which OMCs are saturated with C.