Lateral CO2 emission from eroded scarps and terrace sidewalls: A non-negligible but long-ignored carbon source

The “U” or “V” shaped gullies on the Chinese Loess Plateau are highly susceptible to landslides and bank failure. The lateral carbon emissions from the bare scarps (valley banks or gully sidewalls) in theory promise a non-negligible carbon source, but had long been ignored due to their inaccessibility and underrepresentation in orthographic projection. In this study, the all-year-round CO₂ fluxes from the flatland, gully sidewalls and terrace sidewalls were monitored in-situ in a gully-dominated catchment on the Chinese Loess Plateau. The instantaneous responses of CO₂ fluxes to bank failure and drying-rewetting cycles were further examined by manual detachment and rainfall simulation. We observed that: 1) The sidewalls emitted CO₂ in all seasons, from warm rainy summer to freezing dry winter. The average CO₂ emission rates from the sidewalls reached 26% of that from the flatland, even though the soil organic carbon, dissolved organic carbon and soil microbial biomass carbon content of the barren soil on the sidewalls was disproportionally lower as 57%, 44%, and 76% of that on the flatland. 2) The simulation of bank failure and drying-rewetting cycles could rejuvenate the sidewalls and thus enhance the CO₂ emission rates, respectively by 49%, 27%, and collectively by 82%. 3) Based on the digital terrain model developed from the imagery acquired by the unmanned aerial vehicle, the actual exposed area of the study site was 46% larger than its ortho-projected area. This not only challenges the plausibility of conventional orthographic projection to estimate the actual soil surface area in similar regions predominated with eroded scarps and bench terraces, but also highlights the non-negligible contributions from lateral CO₂ emissions to local carbon source strength. The vigorous responses of lateral CO₂ emissions to soil water, temperature and layer detachment further emphasize the vulnerability of bank failure, which may potentially introduce more exposed area and thus trigger more CO₂ fluxes under future climate conditions. Therefore, the lateral CO₂ emissions from steep slopes or eroded scarps should not be ignored but deserve systematic investigations in complex terrains similar to the Chinese Loess Plateau.