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Warming and eutrophication influence carbon (C) processing in sediments, with implications for the global greenhouse‐gas budget. Temperature effects on sedimentary C loss are well understood, but the mechanism of change in turnover through priming with labile organic matter (OM) is not. Evaluating changes in the magnitude of priming as a function of warming, eutrophication, and OM stoichiometry, we incubated sediments with 13C‐labeled fresh organic matter (FOM, algal/cyanobacterial) and simulated future climate scenarios (+4°C and +8°C). We investigated FOM‐induced production of CH4 and microbial community changes. C loss was primed by up to 17% in dominantly allochthonous sediments (ranging from 5% to 17%), compared to up to 6% in autochthonous sediments (−9% to 6%), suggesting that refractory OM is more susceptible to priming. The magnitude of priming was dependent on sediment OM stoichiometry (C/N ratio), the ratio of fresh labile OM to microbial biomass (FOM/MB), and temperature. Priming was strongest at 4°C when FOM/MB was below 50%. Addition of FOM was associated with activation and growth of bacterial decomposers, including for example, Firmicutes, Bacteroidetes, or Fibrobacteres, known for their potential to degrade insoluble and complex structural biopolymers. Using sedimentary C/N > 15 as a threshold, we show that in up to 35% of global lakes, sedimentation is dominated by allochthonous rather than autochthonous material. We then provide first‐order estimates showing that, upon increase in phytoplankton biomass in these lakes, priming‐enabled degradation of recalcitrant OM will release up to 2.1 Tg C annually, which would otherwise be buried for geological times. [ABSTRACT FROM AUTHOR]

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]

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 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 achieve this, we conducted two studies: (1) in Central Switzerland and (2) in South Korea, to investigate the seasonal and spatial dynamics in catchment erosion processes. We applied a compound-specific stable isotope (CSSI) approach using the isotopic (d13C) signature of plant-wax derived long-chain fatty acids (C24:0-C30:0) in source soils, suspended sediments and a lake sediment core to apportion the sources of the suspended sediments in the catchments of Lake Baldegg (Switzerland) and Lake Soyang (South Korea). With the R software package MixSiar (version 3.1.10) we calculated the contribution of the different land-uses to the sediments.Event-based suspended sediment sampling from the five main contributing rivers to Lake Baldegg (up to 10 high flow events over the course of one year) and subsequent compound-specific stable isotope analysis of long-chain fatty acids indicated a strong seasonal variation of the sediment origin in the rivers. In spring, forests are the main contributor to the sediments, rapidly changing to grassland as the main source in early summer. Then, during July and August, the CSSIs are indicating arable lands as the main contributor. Averaged over all events in all rivers, forests are dominating the sediment origin over agricultural land and grasslands.In Korea, we focused on the suspended sediment apportionment in a mountainous catchment, Haean Basin, which experienced rapid agricultural expansion and intensification during the last decades. Large areas of hill slope forests were transformed to paddies and vegetable fields. The intensive agriculture and easily erodible soils are a major reason for increased soil erosion resulting in high suspended sediment loads in the river systems. Our second aim in Korea was to investigate the spatial variation in sediment contribution along Soyang river from the Haean catchment to the hydro-electric reservoir Lake Soyang. In the Haean catchment and as well at four suspended sediment sampling sites along Soyang river we applied the same CSSI approach as in Switzerland, mentioned above. Source soils were identified and differentiated in the Haean catchment and used to apportion the sediment sources within the catchment and along the river. Our results show a clear shift from almost exclusive contribution of vegetable fields and forest soils to the suspended sediments in the headwater catchment to an increased contribution of maize fields downstream towards Lake Soyang. Since the areal proportion of maize in the Lake Soyang catchment is not increasing significantly downstream compared to vegetables, rice fields or forests, our findings are giving strong indication for a higher vulnerability of the agricultural soils where maize is grown. [ABSTRACT FROM AUTHOR]

Varved lake sediments are very often used as environmental archives, since the yearly temporal resolution of these archives is giving us some unique insights in past environmental and climate settings and changes. We use sediment records as an archive for erosion dynamics and thus to potentially trace back land use changes at the catchment scale.Land use changes and agricultural practices become nowadays a key factor of sediment dynamics by modifying the soils erosive risk and the catchment sediment connectivity. And while soil erosion is one of the biggest threats to soil fertility as well as to ecological health of freshwater systems, restoration and management plans of water bodies can only be efficient if the sediment sources and their respective contributions, i.e. the proportion attributable to different land uses and agricultural practices, are identified. To tackle this problem, we applied a compound-specific stable isotope approach (13C of long-chain fatty acids (FA)) combined with connectivity modelling and biomarker analysis to a 130-years old varved lake sediment core from a eutrophic Swiss lake. Todays "source" soils with different land-uses (permanent and non-permanent grasslands, arable lands, orchards, and forests) were discriminated by using the compound-specific 13C signature fingerprint of C26:0 and C28:0 FA. FA analysis of the lake sediments samples, covering the last 130 years, enabled us to apportion the sediment origin to the sources and follow the erosion dynamics of the catchment back in time. Mixing models like MixSiar give us the possibility to calculate the contribution of the different land-uses to the sediments. For the time window 1940 until 1960 we identified forests as the main source of the terrestrial sediment origin. Actually 80 to almost 100% of the fatty acids origin could be attributed to the forests. For the younger sediments a clear change in sediment origin happened. The influence of arable land and grasslands was increasing compared to forests. But quantitative attribution and differentiation between grassland and arable land contribution were difficult due to the linear distribution of the CSSI signal of different source soils in the isoplot.However, limitations in using this method became obvious for sediments older than 1940. The isotopic signal of these sediments could no longer be explained by today's terrestrial sources. Additional sources of the assumed terrestrial long-chain fatty acids were hypothesized. We discuss a potential bias of the terrestrial fatty acid signal by (1) historical peatlands, (2) in-situ FA production by algae and (3) in-situ production of FAs by methanotroph bacteria.The dynamic of sediment origin over time revealed through this integrated source-to-sink study could serve as a basis for future management options to reduce sediment inputs to rivers and lakes. [ABSTRACT FROM AUTHOR]