Linking recent changes in sediment yields and aggregate-associated organic matter sources from a typical catchment of the Loess Plateau, China

The historically environmental information of soil erosion and human activity is thoroughly “recorded” in deposited organic matter (OM) in aggregates within the catchment. Although the changes in sediment-associated organic carbon (OC) concentration and source have great impact on the stability and fate of eroded OM on Earth, the response of OM sources of sediment aggregate fluxes in eroding landscapes to land uses still remain unclear. In this study, we selected three sediment deposit profiles in a typically dam-controlled catchment of the Chinese Loess Plateau to trace the 60-year changes in sediment yields and then explored the historical distributions of aggregate-associated OM sources through using the fallout radioisotope dating (¹³⁷Cs and ²¹⁰Pbₑₓ) and the isotope mixing model based on aggregate size-specific ¹³C and ¹⁵N isotope signatures. The results showed that the deposition rates of sediment trapped the check dam displayed a decreasing trend with year except for an abrupt increase in 2013 over the past 60 year, ranging from 4.47 × 10³ to 89.16 × 10³ t km⁻² yr⁻¹. The average SOC and TN concentrations in deposit profile showed the following order of macroaggregates (> 0.25 mm, 9.55 g kg⁻¹ for SOC and 0.81 g kg⁻¹ for TN) > microaggregates (0.053–0.25 mm, 4.16 g kg⁻¹ for SOC and 0.42 g kg⁻¹ for TN) > silt-clay (< 0.053 mm, 2.41 g kg⁻¹ for SOC and 0.33 g kg⁻¹ for TN), despite there was a opposite trend of mass fraction in aggregate sizes. The data of ¹³C and C/N among aggregate sizes indicated that the OM in silt-clay fraction had higher decomposition rate than that of other aggregates. The modeling outputs further showed that cropland was the primary contributing sources to transported OM of aggregate flux, accounting for > 20% of the total. Our results demonstrated the influence of erosion processes and changes in land use on the sediment yield and aggregate-associated OM dynamics in this Loess region.