13C evidence for the selective loss of active and inert organic carbon in soil aggregates under rain-induced overland flow erosion

• Published in: Geoderma Regional Vol. 38; p. e00822
• Main Authors: Qu, Jiuqi, Huang, Jinquan, Liu, Lin
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.09.2024
• Subjects: Heavy fraction organic carbon; Light fraction organic carbon; Sheet erosion; Splash erosion; δ13C; δ13C; Heavy fraction organic carbon; Splash erosion; Sheet erosion; Light fraction organic carbon
• ISSN: 2352-0094; 2352-0094
• DOI: 10.1016/j.geodrs.2024.e00822

Due to the hierarchical structure of aggregates, raindrops cause changes in the composition, distribution and state of aggregate-wrapped organic carbon (OC) fractions during erosion, greatly affecting soil carbon turnover and sequestration. To explore these regulatory mechanisms, the selective transport of light (LFoc) and heavy (HFoc) fraction OC within aggregates of varying sizes was traced via the 13C/12C carbon isotope ratio (δ13C) under splash and sheet erosion conditions. A “three-zone” variable-slope soil pan was filled with loess soil with a high aggregate concentration to monitor rainfall erosion on a slope. The OC aggregate composition, aggregate stripping and δ13C values of the sediment aggregates of various particle sizes were measured. When the erosion intensity was low, the splash-eroded sediment was mainly enriched in 13C-rich HFoc, and as the rainfall intensity increased, the concentrations of 12C-rich LFoc and HFoc gradually increased. That is, under heavy rainfall, aggregate fragmentation exposed more large fragments rich in younger HFoc and LFoc, and these fragments underwent saltation. There was no obvious correlation between the trends in splash erosion and OC, so runoff transport was an important factor influencing the correlation between δ13C and OC (P < 0.05). Raindrop impact exposed HFoc-rich aggregate fragments of different sizes, e.g., stable mineral-associated OC. Runoff promoted the obvious preferential transport of LFoc and the redistribution of particulate OC (POC) among different sediment particles. Mineral-associated OC and 2–0.05 mm easily decomposable LFoc or POC were preferentially transported, causing OC with the highest and median δ13C values to be preferentially transported. Overall, the transport order of aggregate-exposed OC particles was clay + silt particle-bonded OC, POC, POC-bonded aggregate fragments and silt-bonded aggregate fragments. These results verified the selective loss of aggregate-detached OC fragments during erosion, which led to a change in the OC sequestration and reaggregation potential of OC in the eroded and deposited soil. •Under different rainfall conditions, the δ 13C values of splash and sheet erosion sediments vary significantly.•Selective loss of active and inert organic carbon cause the change of δ13C in aggregates.•Aggregates breakdown greatly determine the existing states of active and inert organic carbon.