Fingerprinting using compound-specific δ13C of n-alkanes reveals the temporary role of paddy fields as a secondary source for watershed sediment loss

Sediment fingerprinting generates reliable sediment provenance information which supports policy or practical strategies for catchment sediment management. But the approach remains challenging in areas with complex landscape configuration. This investigation evaluated the seasonality of biomarker signatures and their variability among particle size fractions, and accordingly apportioned target time-integrated suspended sediment to land-use based sources in an intensive farming watershed with mosaic land use patch configurations and crop-specific farming practices. Source materials (i.e., topsoil) from dry croplands, paddy fields and citrus orchards were sampled, and time-integrated suspended sediment was collected at the watershed outlet. The absolute concentrations and compound-specific δ13C of long-chain saturated n-alkanes (C23-C33) were determined for two absolute particle size fractions (i.e., <25 μm and 25-63 μm). The δ13C of monomeric n-alkanes displayed no significant variabilities between the particle size fractions nor across the whole sampling period. The MixSIAR Bayesian model was employed to quantify sediment source contributions. Due to human activities, paddy fields have become an important sediment source, but dry farmland remains the largest contributor. Based on sediment source information for the study watershed, a range of measures such as soil virginization, returning straw to fields, and pasture cultures in orchards are recommended.