## 4 Enrichment Ratio for Rill versus Inter-Rill Erosional Processes
### Stage VI: Fate of Soil Carbon in Aquatic Ecosystems  
There are several factors that affect CO2 emission from reservoirs or aquatic ecosystems. Important among these are temperature, oxygen level, properties of SOM and sediments, primary production, etc. For a study in a sub-tropical hypoxic reservoir in southeastern China, [PERSON] et al. [34] observed that an increased supply of organic matter from storm runoff and to a lesser extent, primary production in summer, enhanced subsequent oxygen consumption. [PERSON] et al. [34] also observed that the surface reservoir was usually a source of CO2, except in summer, due to high primary productivity. The anaerobic respiration of OM can lead to CH4 emission. [PERSON] et al. [35] estimated that Lake Erie emits 1.3 \(\pm\) 0.6 \(\times\) 10\({}^{5}\) kg CH4 -C/d in late summer, with 30% of CH4 derived from natural gas infrastructure. In addition to O2 availability, other factors included sub-surface currents and high winds, which may also affect CH4 efflux. The potential increase in precipitation, algal biomass, and the attendant incidence of hypoxia may aggravate the efflux of CH4, leading to positive climate change feedback [35].  
River ecosystems have also been reported to generate N2O efflux. A study in the Yellow River of China by [PERSON] et al. [31] indicated N2O concentrations in the lower reaches of 8.78 to 24.26 nmol/L. Total N2O flux from the Yellow River to the Bohai Sea was about 2.27 \(\times\) 10\({}^{5}\) mol/y. [PERSON] et al. [31] observed an 8-fold increase in N2O flux at the beginning of the water sediment regulation, and the increase may be due to stimulation of nitrification in the water column.  
[PERSON] et al. [36] measured CO2 and CH4 concentrations and emissions in a head-water stream network of interior Alaska and identified consistent areas of elevated CO2 and CH4. Likewise in Alaska, [PERSON] et al. [37] also observed that watershed slope and precipitation interact to control gaseous C fluxes from the stream, reporting the highest efflux from low-gradient watersheds following rainstroms.  
Riparian ecosystems and flood plains also impact the global C cycle. A study in a Thur River flood plain (northeastern Switzerland) by [PERSON] et al. [38] showed that soil-water content and C availability were the key determinants controlling N pools and processes. These two factors strongly depended on the sedimentation and inundation dynamics. [PERSON] and colleagues also observed locally high N2O efflux. A study in the Prairie Pothole Region of Manitoba, Canada, by [PERSON] et al. [39] showed that spring thaw increased the concentrations and contents of N2O at 15 to 35 cm in the lower and riparian area before reaching the soil surface. Furthermore, the riparian area consistently exhibited the highest CH4 emissions. [PERSON] et al. [39] observed that these spikes in CH4 occurred after the spring thaw N2O emissions, with the accumulation of CH4 and a decline in O2 concentration. Whereas lakes are considered as sinks for sediment-laden C, [PERSON] et al. [40] reported that lakes can be super-saturated with CO2 and CH4, thus emitting these GHGs. The ratio of C emission into the atmosphere to C burial into the sediment was 0.08 for Lake Donghou, China. [PERSON] et al. [40] estimated that gaseous C emission accounted for 15% of the total C output.