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Application of compound-specific isotope analysis (CSIA) in sediment fingerprinting source apportionment studies is becoming more frequent, as it can potentially provide robust land-use based source attribution of suspended sediments in a freshwater system. Isotopic tracers such as δ13C values of vegetation-derived organic compounds are considered to be suitable for CSIA based fingerprinting method. However, a rigorous evaluation of tracer conservativeness in terms of the stability of isotopic signature during detachment and transport of soil during erosion process is essential for the suitability of the method. With the aim to identify potential fractionation and shifts in tracer signature during early degradation of organic matter in surface soils, we measured concentrations and δ13C values of long-chain fatty acids and n-alkanes from fresh plant biomass (as vegetation is a direct source of these compounds to the soils), degraded organic horizon (O horizon) as well as mineral soil (A horizon) from various forest types with different humus forms (five sites). The bulk δ13C values showed continuous 13C enrichment through the degradation stages from fresh plant material to the O and A horizon, ranging between 3.5 and 5.6 ‰. Compound-specific δ13C values showed a general 13C enrichment for both, long-chain fatty acids (up to 5 ‰) as well as n-alkanes (up to 3.9 ‰) from fresh plant biomass to the O horizon overlying the A horizon. However, only slight or no further changes occurred from the O to the A horizon. We also compared compound-specific δ13C values between two soil particle-size classes (< 2 mm and < 63 µm) from four sites and found no significant differences of tracer values between them, with even less fractionation for the long-chain n-alkanes within the soil particle fractions, which points to the conclusion that sampling and analysing bulk soil material might be valid for the isotopic tracer applications. We further conclude, that our results support the suitability of studied isotopic tracers as representative source soil signature in CSIA based sediment source attribution, as they demonstrated necessary stability in plant-soil system during organic matter degradation. [ABSTRACT FROM AUTHOR]

Soil erosion and associated sediment transfer are among the major causes of aquatic ecosystem and surface water quality impairment. Through land-use and agricultural practices, human activities modify the soil erosive risk and the catchment connectivity, becoming a key factor of sediment dynamics. Hence, restoration and management plans of water bodies can only be efficient if the sediment sources and the proportion attributable to different land-uses are identified. To this aim, we applied two approaches, namely compound-specific isotope analysis (CSIA) of long-chain fatty acids (FA) and triterpenoid biomarker analysis, to the eutrophic Lake Baldegg and its agriculturally used catchment (Switzerland). Soils reflecting the five main land-uses of the catchment (arable lands, temporary and permanent grasslands, mixed forests, orchards) were subjected to CSIA. The compound-specific stable isotope δ13C signatures clearly discriminate between grasslands (permanent and temporary) and forests. Signatures of agricultural land and orchards fall in-between. The soil signal was then compared to the isotopic signature of a lake sediment sequence covering ca. 130 years (before 1885 to 2009). Most of the lake sediment samples lie out of the source soils polygon, most likely as a result of carbon exchanges with highly depleted material related to methanotrophic bacterial activity. The recent lake samples falling into the soil polygon indicate an important contribution of the forests, which can be explained by (1) the location of the forests on steep slopes, resulting in a higher connectivity of the forests to the lake, and (2) potential direct inputs of trees and shrubs growing along the rivers feeding the lake and around the lake. Despite the strong bacterial overprint on the isotopic signal, land-use and catchment history are clearly reflected in the CSIA results, with isotopic shifts being consistent with catchment, land-use and eutrophication history. While present in the soils, the investigated highly specific biomarkers were not detected in the lake sediment. Two trimethyltetrahydrochrysenes (TTHCs), natural diagenetic products of pentacyclic triterpenoids, were found in the lake sediments. Their origin is attributed to the in-situ microbial degradation of some of the triterpenoids. While the need to apportion sediment sources is especially crucial in eutrophic systems, our study stresses the importance of using caution with CSIA and triterpenoid biomarkers in such environments, where the presence of methanotrophic bacterial biomass might overprint original isotopic signals. [ABSTRACT FROM AUTHOR]