Biomarkers as a soil organic carbon tracer of sediment: Recent advances and challenges

Soil organic carbon (SOC) tracing can provide important information for effective and accurate management of soil erosion. However, tracing erosion-induced SOC is still challenging, due to a spatiotemporal variability of SOC and diversity of its sources. Selection of effective tracers is a vital way in coping with these challenges. Biomarkers, which present tight correlationships with soil and vegetation, and characterized with source-specific, have been considered to be a promising tracer in tracing SOC, despite specialized analytical instruments are also required for biomarker analysis. Even so, there are still uncertain factors in tracing SOC, that is, instability factors that affect the strength of biomarker signal. A fully understanding of the generation, transport, and conservative behavior of biomarkers is beneficial to better application. In this review, several representative lipid biomarkers were compared to identify different sediment/soil organic matter sources. Additionally, the general sediment/soil tracing mechanism and the transport mechanism of the biomarker are described clearly. Furthermore, the potential of biomarkers in assessing the contributions of organic carbon sources was demonstrated through correlation application analysis. Nevertheless, challenges remain because of the potentially high degrees of environmental and analytical uncertainty. The eroded soil source contributions to sediment cannot be obtained accurately by using biomarkers alone, according to uncertainty analysis. Hence, a combination with other fingerprint indices, such as spectroscopic indices or stable isotopes, is highly recommended for accurate tracing of soil organic carbon sources. Future research directions include (1) understanding the conservative behavior of tracer biomarkers in the processes of source separation, transportation, and deposition and (2) verifying the source assignment accuracy of biomarkers by field/laboratory testing or model fitting.