Minor topography governing erosional distribution of SOC and temperature sensitivity of CO2 emissions: comparisons between concave and convex toposequence

Abstract

Erosion processes spatially redistribute soil particles and the associated carbon across landscapes. Their spatial redistribution pattern is governed by the transport distances of individual displaced soil particles, which is not only dependent on their settling velocity, but also affected by slope topography. However, the potential impacts of fine-scale variation of slope topography on the erosion-induced lateral and carbon fluxes are often over-generalized by coarse digital elevation models. In this study, two topo-sequences, convex and concave, over a long gentle slope in the northeast China were investigated. Surface soils were sampled at predetermined space intervals from upslope to downslope along the two toposequences, and then fractionated by the settling velocity of individual fractions into four classes: > 250, 63 – 250, 20 – 63 and < 20 μm. The soil organic carbon (SOC) and δ13 C of the unfractionated soils and all the settling classes were measured, and their CO2 emission rates were also determined at six temperature gradients: 5°C, 10°C, 15°C, 20°C, 25°C and 30°C. Our results show that: 1) The soil fractions along the upper lying convex segment showed a coarsening effect toward the knee point and then a fining trend at the slope toe, whilst the soil compositions along the lower lying concave segment stayed fairly comparable as the slope descended. 2) The net loss of surface soil along the eroding convex segment resulted in depleted SOC and more positive δ13 C signatures than that along the depositional concave segment. 3) The CO2 emission rates of almost all the settling fractions were enhanced compared with that of the unfractionated soil, and the settling class-specific CO2 emission rates and their temperature sensitivity (Q10 ) also differed along the two topo-sequences. This demonstrates that fine scale topographic variations had a strong control over the lateral and vertical carbon fluxes, which has been often disguised by coarse grid size in digital elevation models or average sediment delivery ratios. Topography-dependency must be properly accounted for when calculating slope-scale carbon balances.