## 4 Enrichment Ratio for Rill versus Inter-Rill Erosional Processes
### Stages III to V: Deposition, Deep Burial, and Re-Aggregation of Dispersed Soil  
The discussions on the fate of C in stages III to V are combined because these involve inter-dependent processes of deposition, deep burial, and reaggregation of dispersed soil through the formation of organo-mineral complexes. The SOC-laden sediments are deposited on depressional sites, concave slopes, and foot slopes. The fate of SOC deposited on these sites depends on a range of factors such as soil temperature and moisture regimes, burial in the subsoil, and the degree of re-aggregation and the attendant formation of organo-mineral complexes or soil aggregates. The burial of transported C at the depositional sites is of critical importance. In the Yellow River Basin of China between 1950 and 2010, 134 \(\pm\) 24.7 Pg of soil and 1.07 \(\pm\) 0.15 Pg of SOC have been eroded. Approximately 63% of eroded soils were deposited in the river system and 37% were discharged into the ocean [28]. For SOC, approximately 0.53 \(\pm\) 0.21 Pg (49.5%) was buried in the river systems, 0.25 \(\pm\) 0.14 Pg (23.5%) was delivered into the ocean, and the remaining 0.289 \(\pm\) 0.294 Pg (27%) was decomposed during the erosion/transport processes. Thus, [PERSON] et al. [28] concluded that 20-40% of eroded SOC is oxidized after erosion, that the erosion-induced redistribution of soil is a significant source of SOC, and that human activities have and are altering its redistribution pattern.  
Understanding the fate of the buried SOC is critical to compiling a credible and accurate global C budget. [PERSON] et al. [29] used a numerical model to simulate long-term C dynamics in colluvial soils and observed that the C-burial efficiency (the ratio of current C content of the buried sediments to the original C content at the time of sedimentation) of deposited sediments may decrease over time, reaching a level of equilibrium. Half of the buried C at depositional sites may be lost over \(\sim\)300 y, and the rate of depletion of SOC at the depositional site may be accelerated by agricultural management [29]. [PERSON] et al. [30] studied the fate of SOC across two top-sequences (convex and concave) over a long gentle slope in the northeast China. [PERSON] et al. [30] observed that the net loss of surface soil along the eroding convex segment resulted in a greater depletion of SOC than that along the depositional concave segment. Soil re-aggregation at the deposition site can transform highly mobile-C rich fine primary particles into rather immobile soil aggregates [23]. [PERSON] et al. [29] observed that buried SOC in colluvial profiles was more stable with a turnover time at the centennial scale. However, [PERSON] et al. [31] observed that SOC at the depositional sites may be vulnerable to severe losses due to change in environmental factors. For example, a study in the karst graben basin of China by [PERSON] et al. [32] showed that there was a higher concentration of SOC at the depositional sites relative to the upland soils. However, there was also a higher decomposition rate of SOC at the depositional sites than at the eroding sites, particularly under the farmland conditions. Similarly, [PERSON] et al. [33] also observed that deposition increased C emission under both drying and wetting conditions for Ultisol and Entisol. In a Mollisol, however, C emissions increased under drying, but decreased under wetting conditions. Therefore, the effects of deposition on C emissions are regulated by soil texture and moisture regimes, and the development of macropores due to shrinkage in depositional soils increased both soil water evaporation and the attendant C emissions [33].