Assessing the Impact of Soil Erosion by Water on Soil Particle Size Fractions and Soil Organic Matter Pools in an Eroding Landscape.
N.S. Nwokporo, S. Nortcliff, and J.S. Robinson. Dept of Soil Science, Univ of Reading, Whiteknights, Reading, RG6 6DW, United Kingdom
It is widely accepted by researchers as well as farmers that during the process of soil erosion, the fertile topsoil is lost, resulting in changes to soil physical, chemical and biological properties. As topsoil is eroded, associated soil organic carbon (SOC) is significantly reduced, as it is mostly located in the upper horizon. In this experiment we investigated the soil particle size fractions that are most erodible, as well as the SOC pools that are more susceptible to loss during soil erosion by water. Additionally, we investigated and compared various SOC pools between depths along tramline soils and adjacent soils. This experiment was conducted in an eroding landscape in Moorfield, Herefordshire, England. The field was classified into Eroded, Uneroded and Depositional regions based on topographical location, particle size data, and physically observed erosional features. Soil samples were collected from all regions and analysed for total carbon (TC), readily oxidisable carbon (ROC), loss on ignition carbon (LOI), hot-water extractable carbon (HWC), and cold water extractable carbon (CWC). Samples were collected at two depths (0-1 and 1-15 cm) along tramline soils and adjacent soils. Dissolved organic carbon (DOC) was determined in the surface and subsurface water in the landscape. Results from this work showed that the coarse silt fraction is the most highly erodible soil fraction (significant p=0.001) in the study site. Increases in the levels of HWC were observed along the depositional tramlines which may indicate that the HWC is preferentially lost from the upper to the lower slopes compared to other carbon pools. SOC was also highly significantly (p=0.001) higher in the 1-15 cm depth than the 0-1 cm depth indicating that the top 0-1 cm depth are more vulnerable to losses during soil erosion by water than the underlying 1-15 cm depth. Management practices e.g. tramlines, form the major pathway through which both soil and carbon associated with the soil are lost during soil erosion by water. However, these losses vary with the position of the tramlines on the landscape and other soil and environmental variables. The mean amount of DOC (both in surface and subsurface water) can be comparable with the CWC measured in the field indicating that a large proportion of carbon could be lost in dissolved rather than particulate form during soil erosion. We conclude that erosion preferentially detaches the coarse silt fractions during erosion and that the labile C pools are more vulnerable to erosion losses from land to water during soil erosion.