# Fate of Soil Carbon Transported by Erosional Processes
## 3 Soil Erosion as a Source or Sink of Carbon
### Hydro-Thermal Regime and Aggregation  
Emission of GHGs depends on soil moisture and temperature regimes, as moderated by aggregation, in relation to stability of aggregates along with size and continuity of pores, through their strong impacts on activity and species diversity of soil biota. For example, based on a 19-day simulated depositional experiment using three soils of contrasting texture (Ultisol, Mollisol, and Entisol) and two moisture regimes, [PERSON] et al. [22] observed that deposition increased C emissions under both drying and wetting conditions for the Ultisol and Entisol, but the trend in Mollisol varied with the soil moisture regime. Gaseous emissions increased under drying but decreased under wetting conditions. Thus, the effect of deposition on GHG emissions depend on the soil moisture regime and its control such as texture, clay minerals, SOC content and aggregation.  
Soil aggregation (structure) is a key attribute in stabilization of SOC [23]. In addition, degree and stability of soil aggregates is an important factor affecting emission of GHGs. An incubation study by [PERSON] et al. [17] involving soil aggregates obtained from diverse landscape positions indicated that CO\({}_{2}\) emissions from coarse-size aggregate fractions (0.024 \(\pm\) 0.009 g C/kg\(\cdot\)d) was six times higher than that from small size aggregate fractions (0.0038 \(\pm\) 0.0011 g C/kg\(\cdot\)d) at the depositional toe slope position. [PERSON] and colleagues concluded that the breakdown of aggregates (stage 1 of the erosional process) by accelerated erosion impacts both aggregate size distribution and CO\({}_{2}\) emissions from aggregates at different slope positions.  
Soil aggregates, being transported by fluvial processes, may increase settling velocities of mineral particles contained within the aggregate and skew the distribution of SOC along the slope. [PERSON] et al. [14] observed that redeposition of eroded SOC within terrestrial ecosystems increased by 64% considering the actual aggregate size, and the CO\({}_{2}\) emission rates also differed across settling fractions depending on soil type. [PERSON] and colleagues also observed that over a 50-day incubation, CO\({}_{2}\) emissions from sediment was 114% greater than that from the non-eroded soil, probably because of enrichment with the labile fractions.  
[PERSON] and [PERSON] [24], based on their study comprising 24 locations of a typical hillslope of the South African Highveld with a wide range of soil texture, concluded that the increase in aggregate stability resulted in a significant increase in particulate and DOC concentrations in the eroded sediments and in losses of gaseous organic C. In contrast, however, high aggregate stability induced low total loss of particulate and DOC because of increase in water infiltration rate and reduced transport in runoff. In the context of wind erosion risks of Histosols in the U.S., [PERSON] et al. [25] observed that variations in sediments and dust emissions is linked to soil properties. A study based on simulated rainfall in subtropical China by [PERSON] et al. [26] indicated that soil bulk density, moisture content, and DOC were the major factors controlling erosion-induced SOC mineralization.