Abstract. As sediment loads impact freshwater systems and infrastructure, their origin in complex landscape systems is of crucial importance for sustainable management of agricultural catchments. We differentiated the sediment source contribution to a lowland river in central Switzerland by using compound-specific isotope analysis (CSIA). We found a clear distinction of sediment sources originating from forest and agricultural land use. Our results demonstrate that it is possible to reduce the uncertainty of sediment source attribution in: (i) using compound content (in our case, long-chain fatty acids; FAs) rather than soil organic matter content to transfer δ13C signal of FAs to soil contribution and (ii) restricting the investigation to the long-chain FAs (> C22 : 0) not to introduce errors due to aquatic contributions from algae and microorganisms. Results showed unambiguously that during base flow, agricultural land contributed up to 65 % of the suspended sediments, while forest was the dominant sediment source during high flow. This indicates that connectivity of sediment source areas within the river changes between base and high flow conditions. Uncertainty, which might occur in complex, large-scale studies due to undetected source attribution and/or CSSI signature degradation, is low because of limited data complexity in our study (i.e., two–three sources and two tracers). Our findings are the first published results highlighting (i) significant differences in compound-specific stable isotope (CSSI) signature of sediment sources from land uses dominated by C3 plant cultivation and (ii) the use of these differences to quantify sediment contribution to a small river.
Abstract. For centuries European peatlands have been degrading along with drainage, land use and climate changes. Increasing pressure on peatland ecosystems calls for a more cost-efficient method to indicate the current state of peatlands and the success of restoration efforts. Metabolic pathways in peatland soils are imprinted in stable isotope compositions due to differences in microorganism communities and their metabolic pathways. Therefore, we hypothesize that depth profiles of nitrogen stable isotope values provide a promising opportunity to detect peatland decomposition or restoration. We studied five peatlands, namely Degerö Stormyr (northern Sweden), Lakkasuo (central Finland) and three mires in the Black Forest (southern Germany). At all locations, cores were taken from adjacent drained (or rewetted) and natural sites to identify δ15N trends that could indicate changes due to drainage and restoration. At all drained (and rewetted) sites we found a distinct peak (“turning point”) of the δ15N values in the center of the drained horizon. We did a fatty acids (FAs) analysis to link our results to microbial community composition. As markers, we distinguished between one fungal-derived FA (C18:2ω9c) and four bacterial-derived FAs. For bacteria, we looked for one general bacterial-derived FA (C14:0), two FAs for gram-positive bacteria (i-C15:0; a-C15:0), and one FA for gram-negative bacteria (C16:1ω9c). In accordance with other studies, our results suggest that fungi dominate the microbial metabolism in the upper aerobic peat horizon. This is reflected by depleted δ15N values. Moving downwards, the drained horizon conditions slowly switch to oxygen limitation. Consequently, fungal-derived FAs decrease whereas bacterial-derived FAs rise. The highest diversity of microbial-derived FAs is indicated by the δ15N turning point. Below the δ15N turning point, oxygen is increasingly limited and concentrations of all microbial-derived FAs are decreasing down to the onset of the permanently waterlogged anaerobic horizon. Peatland cores with restoration successes again show, above the formerly drained horizon, no depth trend of the isotopic values. Hence, we conclude that δ15N stable isotope values reflect microbial community composition, which differs between drained and natural peatlands.
<p><strong>Abstract.</strong> 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 &#948;<sup>13</sup>C 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 &#948;<sup>13</sup>C values of long-chain fatty acids and <i>n</i>-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 &#948;<sup>13</sup>C values showed continuous <sup>13</sup>C enrichment through the degradation stages from fresh plant material to the O and A horizon, ranging between 3.5 and 5.6&#8201;&#8240;. Compound-specific &#948;<sup>13</sup>C values showed a general <sup>13</sup>C enrichment for both, long-chain fatty acids (up to 5&#8201;&#8240;) as well as <i>n</i>-alkanes (up to 3.9&#8201;&#8240;) 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 &#948;<sup>13</sup>C values between two soil particle-size classes (<&#8201;2&#8201;mm and <&#8201;63&#8201;&#181;m) from four sites and found no significant differences of tracer values between them, with even less fractionation for the long-chain <i>n</i>-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.</p>
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