Soluble carbon loss through multiple runoff components in the shallow subsurface of a karst hillslope: Impact of critical zone structure and land use

In karst hillslopes, the soil-epikarst system is not only the main reservoir of soluble carbon, but also the hot spots of hydrological and biogeochemical processes. However, the soluble carbon loss mechanism through various runoff components occurring in the soil-epikarst system is not clear. We hypothesize that the karst critical zone structure determines the runoff and soluble carbon loss, and that these response processes can be regulated by land use. 12 experimental plots on a typical karst hillslope (each with 5 × 20 m projected area) of four land uses (naturally recovered shrubland, forage grassland, economic forest land, and cropland) were intensively instrumented for separately monitoring surface, subsurface, and epikarst seepage runoffs and their dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentration. Results show that economic forest land had the highest total runoff and soluble carbon loss, followed by cropland, naturally recovered shrubland, and forage grassland. Generally, karst critical zone structure was the most important factors for controlling DOC and DIC loss through surface, subsurface, and epikarst seepage runoffs. Critical zones with thicker soil and less undulating soil-epikarst contact were more susceptible to runoff and soluble carbon loss. Although not as important as critical zone structures, land use can effectively regulate runoff and prevent soluble carbon loss by affecting rainfall partitioning in karst hillslopes. Planting forage grassland is an effective method of reducing carbon loss, due to its remarkable ability of reducing runoff in the soil-epikarst systems. Our results provide useful insights for determining appropriate land uses and optimizing soil and water conservation strategies in karst hillslopes.