Investigating sediment sources using compound-specific stable isotopes and conventional fingerprinting methods in an agricultural loess catchment

Abstract

Identifying crop-specific sediment sources is important, and conventional fingerprinting methods do not do so sufficiently, which limits their usefulness. The application of compound-specific stable isotopes (CSSIs) enables crop-specific sediment sources to be identified. To this end, this study applied the CSSI method to the intensively farmed loess soil Geesgraben catchment (75 km²), in Central Germany. We used this catchment because the importance of different surface and subsurface sediment sources is unknown in temperate loess soil areas. The study also compared radionuclide and spectral fingerprinting methods, as well as spatiotemporal contribution of sediment sources. Specifically, the CSSI method, based on measuring δ¹³C signatures of fatty acids, was applied to distinguish C3 and C4 plants, and riverbank sediment sources, which were identified using a multivariate mixing model. At the midstream site, the riverbanks contributed a mean of 12 % of the sediment, while the C3 and C4 plants each contributed 44 %. At the downstream site, according to the CSSI method, the riverbanks contributed 28 %, while the C3 and C4 plants contributed 9 % and 63 %, respectively. In comparison, according to the radionuclide and spectral methods, the downstream riverbanks contributed 41 % and 10 %, respectively; generally, this shows relatively lower contribution to the surface sediment contribution. The riverbanks contribution increased with catchment size, due to downstream changes caused by the deposition of surface sediments. Thus, results showed that CSSIs of δ¹³C of fatty acids can distinguish C3 vs. C4 plants and surface vs. riverbank sources at the catchment scale. However, radionuclides remain useful in heterogeneous catchments because they are not influenced by soil type or lithology. This information is crucial for implementing agricultural practices that can decrease sediment loads to river ecosystems.