Soil Organic Carbon Redistribution and Delivery by Soil Erosion in a Small Catchment of the Yellow River Basin.

Soil erosion modulates the atmospheric CO2 level by affecting the redistribution of young biospheric organic carbon (OCbio) and ancient petrogenic organic carbon (OCpetro) in different order streams. However, the fate of soil organic carbon (SOC) in low‐order stream systems is still uncertain due to the complex influences of terrain, land uses, and anthropogenic disturbances. Here, we used the geochemical properties fingerprinting method and a radiocarbon‐based two‐member mixing model to clarify the source, budgets, and composition of SOC in a low‐order stream catchment in the Yellow River basin. The results showed that the primary source of SOC in the catchment was agricultural activity area, which accounted for 68.5%. After vegetation restoration, the SOC delivery rate and loss rate decreased from 0.38 ± 0.06 to 0.26 ± 0.04 and 0.19 ± 0.06 to 0.14 ± 0.04 t·ha−1·yr−1, respectively. The SOC mobilized by soil erosion in the source area was 1,085.8 ± 170.5 t from 1969 to 2015, and the SOC lost during the transport process was 552.9 ± 170.1 t. The lost carbon was dominated by OCbio, whereas the loss of OCpetro was minimal. Soil erosion can mobilize the OCpetro sequestered in terrestrial ecosystems and deliver it to a river system if it is not intercepted. The oxidation of OCpetro during long‐distance fluvial transport may represent a significant carbon source for atmospheric CO2 on a geological timescale. This study contributes to our understanding of the carbon sink/source issue and quantifies the SOC budgets in stream systems. Plain Language Summary: Understanding how soil erosion affects the redistribution and delivery of soil organic carbon (SOC) is essential for clarifying the source, budgets, and composition of SOC and helps us further understand the terrestrial carbon cycle. This study calculated the source of SOC based on differences in geochemical properties in different source areas. Then, we measured the radiocarbon (14C) content to clarify the composition and fate of SOC during the erosion process. We found that the primary source of SOC in the catchment was the agricultural activity area and that agricultural activities and vegetation restoration were shown to significantly affect SOC redistribution. A large amount of modern organic carbon was lost during the transport process and may become an atmospheric CO2 source over short timescales. Almost all ancient organic carbon is intercepted by the dam at the outlet. If there is no check dam, the remaining ancient organic carbon can be exported to the Yellow River and then oxidized during fluvial transport, leading to an increase in long‐term atmospheric CO2 levels. This research clarifies the influence of soil erosion on SOC redistribution and delivery and quantifies the source, budget, and composition of SOC in the low‐order stream system. Key Points: Agricultural activities and vegetation restoration have impacted the sediment and erosion‐induced redistribution of soil organic carbonA large amount of soil organic carbon was lost during the transport process, and the lost carbon was dominated by biospheric organic carbonThe petrogenic organic carbon exported from the low‐order stream may become a carbon source on a geological timescale [ABSTRACT FROM AUTHOR]