Changes In Isotopic Signatures Research Articles

Foraging habits of Northwest Atlantic hooded seals over the past 30 years: Future habitat suitability under global warming.

The Arctic is a global warming 'hot-spot' that is experiencing rapid increases in air and ocean temperatures and concomitant decreases in sea ice cover. These environmental changes are having major consequences on Arctic ecosystems. All Arctic endemic marine mammals are highly dependent on ice-associated ecosystems for at least part of their life cycle and thus are sensitive to the changes occurring in their habitats. Understanding the biological consequences of changes in these environments is essential for ecosystem management and conservation. However, our ability to study climate change impacts on Arctic marine mammals is generally limited by the lack of sufficiently long data time series. In this study, we took advantage of a unique dataset on hooded seal (Cystophora cristata) movements (and serum samples) that spans more than 30 years in the Northwest Atlantic to (i) investigate foraging (distribution and habitat use) and dietary (trophic level of prey and location) habits over the last three decades and (ii) predict future locations of suitable habitat given a projected global warming scenario. We found that, despite a change in isotopic signatures that might suggest prey changes over the 30-year period, hooded seals from the Northwest Atlantic appeared to target similar oceanographic characteristics throughout the study period. However, over decades, they have moved northward to find food. Somewhat surprisingly, foraging habits differed between seals breeding in the Gulf of St Lawrence vs those breeding at the "Front" (off Newfoundland). Seals from the Gulf favoured colder waters while Front seals favoured warmer waters. We predict that foraging habitats for hooded seals will continue to shift northwards and that Front seals are likely to have the greatest resilience. This study shows how hooded seals are responding to rapid environmental change and provides an indication of future trends for the species-information essential for effective ecosystem management and conservation.

Detrital zircon record of Phanerozoic magmatism in the southern Central Andes

Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America.

Distribution in natural abundances of stable isotopes of nitrate and retained sludge in a nitrifying bioreactor: Drastic changes in isotopic signatures

This study determined the spatial and temporal changes in natural abundance of stable isotopes (δ13C, δ15N, and δ18O) with regard to nitrate (NO3−) and retained sludge in a nitrifying bioreactor. The bioreactor was continuously fed with synthetic wastewater including ammonium for 61 days at 20 °C. After the start-up period of the bioreactor, the NO3− concentration in the effluent gradually increased. The stable isotopes (δ15N and δ18O) of NO3− in the effluent also increased in a phase of incomplete nitrification. The profile experiments showed that the concentration and stable isotopes of NO3− changed simultaneously along the wastewater flow in the bioreactor. The stable isotope analysis revealed that nitrification efficiency seems to be strongly related to the δ15N of NO3−. Moreover, the δ13C and δ15N of the retained sludge drastically changed along the reactor length, from −26‰ to −18‰ and from 5‰ to 30‰, respectively, after 61 days of operation. The isotopic composition of the retained sludge might be affected by the isotope ratios (δ15N and δ18O) of NO3− in the bioreactor. Therefore, the isotope signatures of the retained sludge seem to closely reflect process performance such as nitrification efficiency throughout the operational period. Our findings suggest that the spatial distribution of the isotopic composition of the retained sludge can be used to detect process occurrence within the bioreactor over time.

Rapid change in East Antarctic terrestrial vegetation in response to regional drying

East Antarctica has shown little evidence of warming to date1–3 with no coherent picture of how climate change is affecting vegetation4–6. In stark contrast, the Antarctic Peninsula experienced some of the most rapid warming on the planet at the end of the last century2,3,7,8 causing changes to the growth and distribution of plants9–11. Here, we show that vegetation in the Windmill Islands, East Antarctica is changing rapidly in response to a drying climate. This drying trend is evident across the region, as demonstrated by changes in isotopic signatures measured along moss shoots12,13, moss community composition and declining health, as well as long-term observations of lake salinity14 and weather. The regional drying is possibly due to the more positive Southern Annular Mode in recent decades. The more positive Southern Annular Mode is a consequence of Antarctic ozone depletion and increased greenhouse gases, and causes strong westerly winds to circulate closer to the continent, maintaining colder temperatures in East Antarctica despite the increasing global average15–18. Colder summers in this region probably result in reduced snow melt and increased aridity. We demonstrate that rapid vegetation change is occurring in East Antarctica and that its mosses provide potentially important proxies for monitoring coastal climate change.

The food web positioning and trophic niche of the non-indigenous round goby: a comparison between two Baltic Sea populations

The food web positioning of the non-native round goby (Neogobius melanostomus Pallas, 1814) was studied in a new invasive population in Mariehamn, Aland Islands (northern Baltic Sea). The trophic position and isotopic niche space was compared to other benthic-feeding fish species in the same habitat. The trophic position (TP) was estimated based on stable isotope analysis of carbon (13C:12C) and nitrogen (15N:14N) ratios and compared to that of an established invasive population in Hel, Gulf of Gdansk (southern Baltic Sea). Ontogenetic changes in isotope signatures were evaluated with a regression analysis and compared between the two populations. The round goby positioned as a second-order consumer on the third trophic level among other benthic-feeding fish. It showed a similar trophic position and significant isotopic niche overlap with large perch. The trophic position of round gobies in Mariehamn is significantly higher than in Hel, likely due to different prey items. Furthermore, the ontogenetic patterns differ between the two invasive populations, also suggesting differing resource and habitat availability and levels of intraspecific competition between areas.

Temporal variation in trophic relationships among three congeneric penguin species breeding in sympatry.

Penguins are a monophyletic group in which many species are found breeding sympatrically, raising questions regarding how these species coexist successfully. Here, the isotopic niche of three sympatric pygoscelid penguin species was investigated at Powell Island, South Orkney Islands, during two breeding seasons (austral summers 2013–2014 and 2015–2016). Measurements of carbon (δ13C) and nitrogen (δ15N) stable isotope ratios were obtained from blood (adults) or feather (chicks) samples collected from Adélie Pygoscelis adeliae, chinstrap P. antarctica, and gentoo P. papua penguins. Isotopic niche regions (a proxy for the realized trophic niches) were computed to provide estimates of the trophic niche width of the studied species during the breeding season. The isotopic niche regions of adults of all three species were similar, but gentoo chicks had noticeably wider isotopic niches than the chicks of the other two species. Moderate to strong overlap in isotopic niche among species was found during each breeding season and for both age groups, suggesting that the potential for competition for shared food sources was similar during the two study years, although the actual level of competition could not be determined owing to the lack of data on resource abundance. Clear interannual shifts in isotopic niche were seen in all three species, though of lower amplitude for adult chinstrap penguins. These shifts were due to variation in carbon, but not nitrogen, isotopic ratios, which could indicate either a change in isotopic signature of their prey or a switch to an alternative food web. The main conclusions of this study are that (1) there is a partial overlap in the isotopic niches of these three congeneric species and that (2) they responded similarly to changes that likely occurred at the base of their food chain between the 2 years of the study.

Erratum to: Ontogenetic changes in isotopic signatures of an omnivorous fish Cultrichthys erythropterus in East Lake Taihu, China

The relationship between body size and stable isotopic signatures of the omnivorous Redfin Culter (Cultrichthys erythropterus), commonly found in East Lake Taihu, was investigated. Previous analyses of C. erythropterus stomach contents have shown that this species undergoes a diet switch from being predominantly zooplanktivorous to piscivorous during its life history. This was confirmed by stable carbon isotopic signature (δ13C) in this study, in which δ13C was positively correlated with both standard length and weight. The importance of littoral-benthic resources in supporting C. erythropterus during its lifespan was also demonstrated using a two-source mixing model, the results of which showed a significant increasing trend in the contribution of littoral-benthic energy. However, the stable nitrogen isotopic signature (δ15N) exhibited an unusual pattern compared with previous studies. The δ15N of C. erythropterus showed no relationship with body size, even though dietary changes were observed. This indicated that δ15N alone cannot fully reflect a diet shift in a species and possible variability in isotopic signatures over its life history. This should be considered when using stable isotopic signatures to investigate intra-specific variations and the timing of life-history events, such as estimating the trophic positions of fish species.

δ18O and δ13C Variability In Brachiopods From Modern Shelf Sediments and Its Utility For Understanding Complex Oceanography, Southern Australian Shelf

Abstract The δ18O and δ13C values of brachiopod shell calcite are commonly used as proxies for ancient environmental marine conditions and secular changes in ancient ocean chemistry. The variability of δ18O and δ13C across modern shelf settings is, however, not well documented. This study presents δ18O and δ13C data from 407 Holocene (< 5000 yr BP) and living brachiopods collected from 220 sites across ∼ 3000 km along Australia’s southern shelf, the largest cool-water carbonate shelf in the modern world. Significant isotopic variability is present in separate specimens, at individual sites, and across the entire region. Individual specimens analyzed at multiple shell positions have an isotopic range of values up to 1.3‰ and 1.9‰ for δ18O and δ13C respectively. Multiple brachiopods at a single site have values that vary as much as 2.1‰ for δ18O and 1.8‰ for δ13C. Regional variability ranges on the order of 3.6‰ and 3.4‰, for δ18O and δ13C respectively. The distribution of intrasite variability can be divided into areas of high and low variability. Areas characterized by low variability occur in zones of relatively consistent shelf water conditions. High-variability zones usually correspond to areas of seasonal upwelling onto the shelf, and this aspect is interpreted to be the main cause of isotopic variability. Upwelling waters are cold, leading to higher δ18O values, and are commonly rich in respired dissolved inorganic carbon (DIC) and nutrients, promoting active phytoplankton growth and leading to variable δ13C values. The extensive spatial coverage across this vast latitude-parallel region provides a valuable baseline illustrating the isotopic range that could occur in a single layer in the rock record. Furthermore, changes in isotopic signatures measured across stratigraphy might not necessarily reflect secular changes in ocean chemistry but, instead, could be recording local changes in shelf circulation and upwelling intensity.

Changes in isotopic signatures of soil carbon and CO2 respiration immediately and one year after Miscanthus removal

AbstractThe removal of perennial bioenergy crops, such as Miscanthus, has rarely been studied although it is an important form of land use change. Miscanthus is a C4 plant, and the carbon (C) it deposits during its growth has a different isotopic signature (12/13C) compared to a C3 plant. Identifying the proportion of C stored and released to the atmosphere is important information for ecosystem models and life cycle analyses. During a removal experiment in June 2011 of a 20‐year old Miscanthus field (Grignon, France), vegetation was removed mechanically and chemically. Two replicate plots were converted into a rotation of annual crops, two plots had Miscanthus removed with no soil disturbance, followed by bare soil (set‐aside), one control plot was left with continued Miscanthus cultivation, and an adjacent field was used as annual arable crops control. There was a significant difference in the isotopic composition of the total soil C under Miscanthus compared with adjacent annual arable crops in all three measured soil layers (0–5, 5–10 and 10–20 cm). Before Miscanthus removal, total C in the soil under Miscanthus ranged from 4.9% in the top layer to 3.9% in the lower layers with δ13C values of −16.3 to −17.8 while soil C under the adjacent arable crop was significantly lower and ranged from 1.6 to 2% with δ13C values of −23.2. This did not change much in 2012, suggesting the accumulation of soil C under Miscanthus persists for at least the first year. In contrast, the isotopic signals of soil respiration 1 year after Miscanthus removal from recultivated and set‐aside plots were similar to that of the annual arable control, while just after removal the signals were similar to that of the Miscanthus control. This suggests a rapid change in the form of soil C pools that are respired.

Arabian Shield magmatic cycles and their relationship with Gondwana assembly: Insights from zircon U–Pb and Hf isotopes

The Arabian Shield preserves a protracted magmatic record of amalgamated juvenile terranes that host a diverse range of early Neoproterozoic to Cambrian granitoids intruding volcanosedimentary basin assemblages that have corollaries in other parts of the East African Orogen. New zircon U–Pb geochronology of 19 granitoids intruding eight Arabian Shield terranes, define four discrete magmatic events: island arc (∼845 Ma), syncollisional (∼710 Ma), post-tectonic (∼620 Ma) and anorogenic (∼525 Ma). Zircon Lu–Hf isotopic analyses indicate that all studied granitoids are juvenile with typical εHf values of >+5 to +10 and Stenian-Tonian (∼1100–900 Ma) model ages, regardless of their precise intrusive ages or spatial relationship. Subtle changes in isotopic signatures between ∼850 and 600 Ma, suggest the result from changes in granite source materials brought about by; basaltic underplating, limited crustal interaction with Palaeoproterozoic basement and a change to lithospheric delamination/subduction roll-back processes driving juvenile ANS crustal growth.The cycle of granite intrusion reflects accretionary cycles initiated during Mozambique Ocean closure and during Gondwana amalgamation and final assembly. Post-tectonic magmatism is divided into a ∼636–600 Ma phase and post 600 Ma event that reflects first subduction and then within-plate related processes. The identification of magmatism at ∼525 Ma is now the youngest granitoid identified so far in the Saudi Arabian Shield and may change the identified age of the regional, basal Palaeozoic unconformity. This late magmatism may be generated by the Najd Fault reactivation correlating with the Malagasy/Kunnga Orogeny that marked the final stages of Gondwana assembly.

Use of Stable Isotopes to Identify Sources of Mercury in Sediments: A Review and Uncertainty Analysis

Mass-dependent and mass-independent mercury isotope fractionation potentially generates unique source signatures that can be used to apportion contributions to sediment contamination. This article reviews findings from previous investigations that have used mercury isotopes to identify sources. It also discusses a mass balance mercury isotope fractionation model that simulates changes in isotopic source signatures in aquatic systems caused by natural biogeochemical cycling. According to the model, the extent of source signature alteration depends on chemical speciation, with more labile forms exhibiting greater isotopic fractionation. Apportionment is tractable when differences between δ202Hg of sources are larger than potential changes in isotopic signatures following the release of mercury into the environment.

GRAVITATIONAL FRACTIONATION OF ISOTOPES AND DISSOLVED COMPONENTS AS A FIRST-ORDER PROCESS IN CRUSTAL FLUIDS

Abstract The origin of ore-forming hydrothermal fluids and mechanisms of ore formation are commonly inferred from chemical and isotopic signatures. Fluid mixing and fluid-rock interaction are well-known processes that modify and produce these signatures, respectively. We propose that gravitational fractionation is an additional, hitherto not considered process that can significantly affect fluid signatures. Gravitational fractionation is the tendency for heavier components in a fluid to concentrate at the base of a reservoir. In stagnant fluids, gravitational fractionation can lead to significant changes in isotopic signatures, but also halogen ratios, within tens of millions of years. Rising fractionated fluids can give the same signatures that are classically and potentially erroneously interpreted as indicating mixing of surface-derived and deeper fluids. Recognition of gravitational fractionation as an additional first-order process affecting fluid composition necessitates a thorough reevaluation of models for crustal fluid flow, especially for the formation of unconformity-related hydrothermal ore deposits.

81Br, 37Cl, and 87Sr Studies to Assess Groundwater Flow and Solute Sources in the Southwestern Great Artesian Basin, Australia

The Great Artesian Basin (GAB) is a water source for more than 200,000 residents in central Australia. This study investigates the relationship of bromine and chlorine stable isotopes to groundwater chemistry in a confined aquifer in the southwestern GAB to better understand its flow regime and solute sources. δ81Br values range from +0.66‰ near the recharge area to +1.04‰, 150km down gradient, while δ37Cl ranges from 0‰ to -2.5‰. While δ37Cl decreases with distance from the recharge area, δ81Br increases slightly. Bromide in the recharge area is possibly enriched from selective atmospheric processes causing fractionation in marine aerosols during transport. When confined and isolated from the atmosphere, increases in bromide and to a lesser extent strontium concentrations may contribute through water-rock interaction to changes in isotopic signatures along the flow system. 87Sr/86Sr values range from ∼0.717 near the recharge zone to a depleted 0.708 160km down gradient.

Effect of ingestion on the stable isotope signatures of marine herbivorous fish diets

Stable isotope analysis is widely used to investigate the diet–consumer relationships of free-ranging animals, including fishes. However, measuring the isotope signature of relevant diet items can be problematic when little is known about consumer diet choice, or when individual diet items are difficult to collect in the field. Here we tested the viability of using ingested material as a proxy for fresh diet items in a field study by examining the effect of ingestion on the δ15N and δ13C values of diet items commonly consumed by the marine herbivorous fish Odax pullus (Labridae). Ingestion had a significant effect on the δ15N or δ13C value of three of the five diet items considered. Two diet items showed a clear and consistent change in isotopic signature following ingestion, indicating that discrimination between light and heavy isotopes occurs during the early stages of processing in this stomachless fish. The potential for isotope signatures to change following ingestion suggests that caution should be taken when using ingested material as a proxy for diet items in studies of marine herbivory. Additionally, the predictable effect of ingestion demonstrates the potential for digestion to make a large contribution to the fractionation observed in trophic studies of marine herbivory.

Estimating changes of isotopic fractionation based on chemical kinetics and microbial dynamics during anaerobic methane oxidation: apparent zero- and first-order kinetics at high and low initial methane concentrations

Changes in natural isotopic composition may be used to reveal metabolic pathways of substrate transformation by microbial communities (Vavilin in Ecol Model 240:84-92, 2012b). Anaerobic oxidation of methane (AOM) by sulfate has been described using a mathematical model based on chemical kinetics, microbial dynamics and equations for (13)C isotope accumulation in products as well as their redistribution between substrate and products. Experimental data for two batch cultures that originated from microbial mats covering methane seep chimneys in the Black Sea, previously obtained by Seifert et al. (Org Geochem 37:1411-1419, 2006) and Holler et al. (Env Microbiol Reports 1(5):370-376, 2009), were used to model AOM. During long-time incubation, changes of isotope signatures in CH(4) showed that in the Seifert et al. batch tests (low methane concentration), in contrast to the Holler et al. batch tests (high methane concentration), methane production occurred along with methane oxidation. In accordance with the model, apparent zero and first-order kinetics of methane oxidation were valid for the Holler et al. and Seifert et al. batch tests, respectively. The observed change of [Formula: see text] was explained by microbial kinetics reflecting that the rate is lower for heavy substrate microbial utilization when compared to light substrate microbial utilization. The model showed that small amounts of methanogenesis will change the carbon isotopic composition of methane because biogenic methane has a distinct isotopic composition and due to the large difference between the maximum specific rates of methane oxidation and production. The estimated biomass doubling time of methane-oxidizers for high and low methane concentration was 408/126 days and 4640/1160 days, respectively, depending on the value of the half-saturation constant K ( S ) (5 and 20 mM).

Foraging ecology of Mediterranean fin whales in a changing environment elucidated by satellite ­tracking and baleen plate stable isotopes

We investigated seasonal shifts in diet and distribution of fin whales Bala en optera physalus occurring in the western Mediterranean Sea. For this purpose, we combined carbon and nitrogen stable isotope ratios (δ13C, δ15N) along 10 baleen plates collected from stranded fin whales between 1975 and 2002 with satellite tag deployments on 11 fin whales during summer 2003. Baleen plate stable isotopes were compared with those of the krill Meganyctiphanes norvegica, the main prey of fin whales in the northwestern Mediterranean Sea. Two plates collected near Malaga, Spain, exhibited larger δ13C variations, while only smaller variations could be detected in the other 8. While all mean baleen plate results were consistent with the δ13C signature of Mediterranean M. norvegica, the most depleted δ13C values were intermediate between those of Atlantic and Mediterranean M. norvegica, suggesting westward migrations perhaps extending to the Strait of Gibraltar but not extensive, prolonged feeding in the Northeast Atlantic. This pattern was confirmed by satellite tracking; 1 out of 8 fin whales we successfully tracked left the Mediterranean for the Atlantic. Longer-term changes in isotopic signatures of baleen plates exhibited significant depletion trends, indicating that changes due to increasing input of nutrients and anthropogenic carbon are occurring in the western Mediterranean Sea ecosystem.

Effects of formalin preservation on stable carbon and nitrogen isotope signatures in Calanoid copepods: implications for the use of Continuous Plankton Recorder Survey samples in stable isotope analyses

Preserved and archived organic material offers huge potential for the conduct of retrospective and long-term historical ecosystem reconstructions using stable isotope analyses, but because of isotopic exchange with preservatives the obtained values require validation. The Continuous Plankton Recorder (CPR) Survey is the most extensive long-term monitoring program for plankton communities worldwide and has utilised ships of opportunity to collect samples since 1931. To keep the samples intact for subsequent analysis, they are collected and preserved in formalin; however, previous studies have found that this may alter stable carbon and nitrogen isotope ratios in zooplankton. A maximum ~0.9‰ increase of δ(15) N and a time dependent maximum ~1.0‰ decrease of δ(13) C were observed when the copepod, Calanus helgolandicus, was experimentally exposed to two formalin preservatives for 12 months. Applying specific correction factors to δ(15) N and δ(13) C values for similarly preserved Calanoid species collected by the CPR Survey within 12 months of analysis may be appropriate to enable their use in stable isotope studies. The isotope values of samples stored frozen did not differ significantly from those of controls. Although the impact of formalin preservation was relatively small in this and other studies of marine zooplankton, changes in isotope signatures are not consistent across taxa, especially for δ(15) N, indicating that species-specific studies may be required.

Assessment of gestation, lactation and fasting on stable isotope ratios in northern elephant seals (Mirounga angustirostris)

Effects of physiological processes such as gestation, lactation and nutritional stress on stable isotope ratios remain poorly understood. To determine their impact, we investigated these processes in simultaneously fasting and lactating northern elephant seals (Mirounga angustirostris). Stable carbon and nitrogen isotope values were measured in blood and milk of 10 mother-pup pairs on days 5 and 22 of lactation. As long- and short-term integrators of diet, blood cells and serum may reflect foraging data or energy reserves from late gestation and lactation, respectively. Limited changes in isotopic signatures of maternal blood over the lactating period were highlighted. Nitrogen isotope fractionation associated with mother-to-offspring

Introduction to a Virtual Special Issue: probing the carbon cycle with 13C

Introduction to a <i>Virtual Special Issue</i>: probing the carbon cycle with ¹³C

Fate of microbial biomass-derived amino acids in soil and their contribution to soil organic matter

Soil organic matter (SOM) is important for soil fertility and for the global C cycle. Previous studies have shown that during SOM formation no new compound classes are formed and that it consists basically of plant- and microorganism-derived materials. However, little data on the contribution from microbial sources are available. Therefore, we investigated previously in a model study the fate of C from 13C-labelled Gram-negative bacteria in soil (Kindler, R., Miltner, A. Richnow, H.H., Kästner, M., 2006. Fate of gram negative bacterial biomass in soil – mineralization and contribution to SOM. Soil Biology and Biochemistry 38, 2860–2870) and showed that 44% of the bulk 13C remained in the soil. Here we present the corresponding data on the fate of amino acids hydrolysed from proteins, which are the most abundant components of microbial biomass. After 224days incubation, the label in the total amino acids in the soil amended with 13C-labelled cells decreased only to >95%. The total amino acids therefore clearly showed a lower turnover than the bulk 13C and a surprisingly stable concentration. Proteins therefore have to be considered as being stabilised in soil in dead, non-extractable biomass or cell fragments by known general stabilisation mechanisms. The label in the amino acids in a fraction highly enriched in living microbial biomass decreased to a greater extent, i.e. to 25% of the initially added amount. The amino acids removed from this fraction were redistributed via the microbial food web to non-living SOM. All amino acids in the microbial biomass were degraded at similar rates without a change in isotopic signature. The nuclear magnetic resonance (NMR) spectra of the soils were very similar and indicate that the residues of the degraded microbial biomass were very similar to those of the SOM and are a significant source for the formation of the SOM.