Effect of phytoplankton and microorganisms on the isotopic composition of organic carbon in the Russian Arctic seas

Carbon isotope composition of suspended organic matter (CIC_SOM) and of organic carbon of the bottom sediments (CIC_BS) was studied in a series of expeditions (starting in 1993) to the White, Kara, Chukchi, and Barents seas in the Russian Arctic. For each sea, CIC_SOM and CIC_BS was found to depend primarily on the ratio of OM produced in the water and OM of terrigenous origin. While in the White Sea, where the primary production (PP) is 5.3 times higher than the yearly inflow of terrigenous OM, δ¹³C of SOM carbon is ?29.1‰, in the Chukchi Sea, where PP is more than 300 times higher than the inflow of terrigenous OM, δ¹³C of SOM carbon is ?21.8‰. In the Barents and Chukchi seas, a considerable effect of suspended material arriving with the currents from the neighboring seas on formation of the CIC_SOM was demonstrated. The difference between CIC OM of the bottom sediments form CIC_SOM, the main component of organic matter in the sediments of all shelf seas, was demonstrated for the first time for all the seas studied. This results from production of additional microbial OM due to CO₂ assimilation at the water-sediment redox boundary or in near-bottom water.     

Abundance and activity of microorganisms at the water-sediment interface and their effect on the carbon isotopic composition of suspended organic matter and sediments of the Kara Sea

At ten stations of the meridian profile in the eastern Kara Sea from the Yenisei estuary through the shallow shelf and further through the St. Anna trough, total microbial numbers (TMN) determined by direct counting, total activity of the microbial community determined by dark CO₂ assimilation (DCA), and the carbon isotopic composition of organic matter in suspension and upper sediment horizons (δ¹³C, ‰) were investigated. Three horizons were studied in detail: (1) the near-bottom water layer (20–30 cm above the sediment); (2) the uppermost, strongly hydrated sediment horizon, further termed fluffy layer (5–10 mm); and (3) the upper sediment horizon (1–5 cm). Due to a decrease in the amount of isotopically light carbon of terrigenous origin with increasing distance from the Yenisei estuary, the TMN and DCA values decreased, and the δ¹³C changed gradually from ?29.7 to ?23.9‰. At most stations, a noticeable decrease in TMN and DCA values with depth was observed in the water column, while the carbon isotopic composition of suspended organic matter did not change significantly. Considerable changes of all parameters were detected in the interface zone: TMN and DCA increased in the sediments compared to their values in near-bottom water, while the 13C content increased significantly, with δ¹³C of organic matter in the sediments being at some stations 3.5–4.0‰ higher than in the near-bottom water. Due to insufficient illumination in the near-bottom zone, newly formed isotopically heavy organic matter (δ¹³C ～ ?20‰) could not be formed by photosynthesis; active growth of chemoautotrophic microorganisms in this zone is suggested, which may use reduced sulfur, nitrogen, and carbon compounds diffusing from anaerobic sediments. High DCA values for the interface zone samples confirm this hypothesis. Moreover, neutrophilic sulfur-oxidizing bacteria were retrieved from the samples of this zone.     

Microbiological and isotopic geochemical investigation of Lake Kislo-Sladkoe,a meromictic water body at the Kandalaksha Bay shore (White Sea)

Microbiological, biogeochemical, and isotopic geochemical investigation of Lake Kislo-Sladkoe (Polusolenoe in early publications) at the Kandalaksha Bay shore (White Sea) was carried out in September 2010. Lake Kislo-Sladkoe was formed in the mid-1900s out of a sea gulf due to a coastal heave. At the time of investigation, the surface layer was saturated with oxygen, while near-bottom water contained sulfide (up to 32 mg/L). Total number of microorganisms was high (12.3 × 10⁶ cells/mL on average). Light CO₂ fixation exhibited two pronounced peaks. In the oxic zone, the highest rates of photosynthesis were detected at 1.0 and 2.0 m. The second, more pronounced peak of light CO₂ fixation was associated with activity of anoxygenic phototrophic bacteria in the anoxic layer at the depth of 2.9 m (413 μg C L^?1 day^?1). Green-colored green sulfur bacteria (GSB) predominated in the upper anoxic layer (2.7–2.9 m), their numbers being as high as 1.12 × 10⁴ cells/mL, while brown-colored GSB predominated in the lower horizons. The rates of both sulfate reduction and methanogenesis peaked in the 2.9 m horizon (1690 μg S L^?1 day^?1 and 2.9 μL CH₄ L^?1 day^-1). The isotopic composition of dissolved methane from the near-bottom water layer (δ¹³C (CH₄) = ?87.76‰) was significantly lighter than in the upper horizons (δ¹³C (CH₄) = ?77.95‰). The most isotopically heavy methane (δ¹³C (CH₄) = ?72.61‰) was retrieved from the depth of 2.9 m. The rate of methane oxidation peaked in the same horizon. As a result of these reactions, organic matter (OM) carbon of the 2.9 m horizon became lighter (?36.36‰), while carbonate carbon became heavier (?7.56‰). Thus, our results demonstrated that Lake Kislo-Sladkoe is a stratified meromictic lake with active microbial cycles of carbon and sulfur. Suspended matter in the water column was mostly of autochthonous origin. Anoxygenic photo-synthesis coupled to utilization of reduced sulfur compounds contributed significantly to OM production.     

Benthic food web structure in the southeastern Chukchi Sea: an assessment using ��13C and ��15N analyses

The benthos of the southeastern Chukchi Sea shelf is typified by high faunal abundance and biomass resulting from settlement of a large proportion of seasonal phytoplankton under highly nutritious offshore Bering Shelf Anadyr Water (BSAW). In contrast, inshore Alaska Coastal Water (ACW) is much less productive. Yet the Chukchi Bight and Kotzebue Sound, located under ACW in the southeastern Chukchi Sea, contain a substantial faunal abundance and biomass of invertebrates, fishes and marine mammals. We examined food web structure to gain an understanding of how a relatively rich benthic fauna with a high biomass can be supported under ACW with a supposedly low flux of carbon to the benthos. We measured stable isotope (δ¹³C and δ¹⁵N) values of selected organisms (from zooplankton to fishes) as markers of food sources and trophic position to compare fauna on the shelf under BSAW with that in the Chukchi Bight and Kotzebue Sound under ACW. Relative isotope position of organisms in all three regions was similar, even though some pelagic species within the Sound were depleted in δ¹³C compared to the other regions. We attribute the depletion to the influence of terrestrially derived carbon. We suggest that the hydrodynamics along an oceanic front between the Chukchi Shelf and the Chukchi Bight support the advection of nutrient-rich POC into the Bight and Sound as additional food sources to local production. We conclude that local conditions and multiple POC sources in the Bight and Sound support the substantial population of benthic invertebrates and the fishes, seabirds, and marine mammals that feed on them.     

Establishment of trophic continuum in the food web of the Yellow Sea and East China Sea ecosystem: Insight from carbon and nitrogen stable isotopes

Stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) are used to study the trophic structure of food web in the Yellow Sea and East China Sea ecosystem. The trophic continuum of pelagic food web from phytoplankton to top preyer was elementarily established, and a trophic structure diagram in the Yellow Sea and East China Sea was outlined in combination with carbon isotopic data of benthic organisms, which is basically consistent with and makes some improvements on the simplified Yellow Sea food web and the trophic structure diagram drawn based on the biomass of main resource population during 1985–1986. This result indicates that the stable isotope method is a potential useful means for further studying the complete marine food web trophic continuum from viruses to top predators and food web stability.     

Microbiological and biogeochemical processes in a pockmark of the Gdansk depression,Baltic Sea

Comprehensive microbiological and biogeochemical investigation of a pockmark within one of the sites of gas-saturated sediments in the Gdansk depression, Baltic Sea was carried out during the 87th voyage of the Professor Shtokman research vessel. Methane content in the near-bottom water and in the underlying sediments indicates stable methane flow from the sediment into the water. In the 10-m water layer above the pockmark, apart from methane anomalies, elevated numbers of microorganisms and enhanced rates of dark CO₂ fixation (up to 1.15 µmol C/(l day)) and methane oxidation (up to 2.14 nmol CH₄/(l day)) were revealed. Lightened isotopic composition of suspended organic matter also indicates high activity of the near-bottom microbial community. Compared to the background stations, methane content in pockmark sediments increased sharply from the surface to 40–60 ml/dm³ in the 20–30 cm horizon. High rates of bacterial sulfate reduction (SR) were detected throughout the core (0–40 cm); the maximum of 74 µmol S/(dm³ day) was located in subsurface horizons (15–20 cm). The highest rates of anaerobic methane oxidation (AMO), up to 80 µmol/dm³ day), were detected in the same horizon. Good coincidence of the AMO and SR profiles with stoichiometry close to 1: 1 is evidence in favor of a close relation between these processes performed by a consortium of methanotrophic archaea and sulfate-reducing bacteria. Methane isotopic composition in subsurface sediments of the pockmark (from ?53.0 to ?56.5‰) does not rule out the presence of methane other than the biogenic methane from the deep horizons of the sedimentary cover.     

Isotope alteration caused by changes in biochemical composition of sedimentary organic matter

Stable carbon (C) and nitrogen (N) isotope ratios of sedimentary organic matter (OM) can reflect the biogeochemical history of aquatic ecosystems. However, diagenetic processes in sediments may alter isotope records of OM via microbial activity and preferential degradation of isotopically distinct organic components. This study investigated the isotope alteration caused by preferential degradation in surface sediments sampled from a eutrophic reservoir in Germany. Sediments were treated sequentially with hot water extraction, hydrochloric acid hydrolysis, hydrogen peroxide oxidation and di-sodium peroxodisulfate oxidation to chemically simulate preferential degradation pathways of sedimentary OM. Residue and extracts from each extraction step were analyzed using elemental analyzer-isotope ratio mass spectrometry and solid-state ¹³C nuclear magnetic resonance spectroscopy. Our results show that stable C and N isotope ratios reacted differently to changes in the biochemical composition of sedimentary OM. Preferential degradation of proteins and carbohydrates resulted in a 1.2‰ depletion of ¹³C, while the isotope composition of ¹⁵N remained nearly the same. Sedimentary δ¹⁵N values were notably altered when lignins and lipids were oxidized from residual sediments. Throughout the sequential fractionation procedure, δ¹³C was linearly correlated with the C:N of residual sediments. This finding demonstrates that changes in biochemical composition caused by preferential degradation altered δ¹³C values of sedimentary OM, while this trend was not observed for δ¹⁵N values. Our study identifies the influence of preferential degradation on stable C isotope ratios and provide additional insight into the isotope alteration caused by post-depositional processes.

     

Organic matter distribution and retention along transects from hilltop to kettle hole within an agricultural landscape

In agricultural landscapes, the spatio-temporal distribution of organic matter (OM) varies greatly across landscape structures and soil types. We investigated patterns of organic carbon (OC) content, polyvalent cations, and isotopic values for specific OM fractions along transects spanning topographic positions from erosional to depositional areas, including aquatic sediments within a single kettle hole. We hypothesized different drivers exist at different scales. At the transect scale, we hypothesized (1) landscape form and land management to explain patterns of isotopic and OC content from different OM fractions. At the aggregate scale, (2) we expected different OM-mineral associations to explain stabilized OM. We also hypothesized, (3) that shallow sediment δ¹³C and δ¹⁵N of the kettle hole reflected different terrestrial sources. We found that distinct differences in the OM turnover rates existed between the fractions suggesting that different processes are affecting the transformation rates that are recorded in the isotopic composition patterns. Erosion along with plant productivity drive mineral-associated fractions over the transect, while microbial decomposition and slurry influence freely available and aggregated OM fractions. The type and magnitude of OM-mineral associations changed along the transect while binding OM of different decomposition status. OM in mineral-associated fractions in kettle hole sediments were derived from clay- and silt-sized particles from the field, whereas OM in freely available and aggregated fractions potentially originated from macrophytes. We conclude that kettle holes constitute important sinks for terrestrial OM across the landscape.     

Variations in microbial isotopic fractionation during soil organic matter decomposition

The soil microbial biomass (SMB) is known to participate in key soil processes such as the decomposition of soil organic matter (SOM). However, its contribution to the isotopic composition of the SOM is not clear yet. Shifts in the ¹³C and ¹⁵N natural abundances of the SMB and SOM fractions (mineralised, water soluble and non-extractable) were investigated by incubating an unamended arable soil for 6 months. Microbial communities were also studied using Fatty Acid Methyl Ester specific isotope analysis. The SMB was significantly ¹³C and ¹⁵N-enriched relative to other fractions throughout the incubation. However, significant isotopic variations with time were also observed due to the rapid consumption of relatively ¹³C-enriched water soluble compounds. The increase in the difference in SMB and water soluble ¹⁵N compositions as the water soluble C/N ratio decreased, indicated a shift from N assimilation to N dissimilation during the incubation. These changes also induced modifications of the microbial community structure. Once the system reached a steady-state (after 1 month), the isotopic trends appeared to corroborate those obtained in long term experiments in the field in that there was a constant microbial isotopic fractionation leading to a ¹³C and ¹⁵N enrichment of the SOM over the long-term. This work also suggests that caution must be exercised when interpreting short term incubation studies since perturbations associated with experimental set-up can have an important effect on C and N dynamics, microbial fractionation of ¹³C and ¹⁵N and microbial community structure.     

Allochthonous organic matter supplements and sediment transport in a polymictic reservoir determined using elemental and isotopic ratios

Because allochthonous organic matter (OM) loading supplements autochthonous OM in supporting lake and reservoir food webs, C and N elemental and isotopic ratios of sedimenting particulate OM were measured during an annual cycle in a polymictic, eutrophic reservoir. Particulate organic C and N deposition rates were greatest during winter and lowest during spring. C:N ratios decreased through our study indicating that OM largely originated from allochthonous sources in winter and autochthonous sources thereafter. δ¹³C were influenced by C₄ plant signatures and became increasingly light from winter through autumn. δ¹⁵N indirectly recorded the OM source shift through nitrate utilization degree with maximum values occurring in May as nitrate concentrations decreased. Unlike relationships from stratified systems, δ¹³C decreased with increasing algal biomass. This relationship suggests that minimal inorganic C fixation relative to supplies maintained photosynthetic isotopic discrimination during productive periods. Water column mixing likely maintained adequate inorganic C concentrations in the photic zone. Alternatively, OM isotopic composition may have been influenced by changing dissolved inorganic nutrient pools in this rapidly flushed system. δ¹⁵N also recorded increased N₂ fixation as nitrate concentrations declined through autumn. Secondary sediment transport mechanisms strongly influenced OM delivery. Particulate organic C and N deposition rates were 3× greater near the sediment-water interface. Isotopic ratio mixing models suggested that river plume sedimentation, sediment resuspension, and horizontal advection influenced excess sediment deposition with individual mechanisms being more important seasonally. Our findings suggest that allochthonous OM loading and secondarily-transported OM seasonally supplement phytoplankton production in productive reservoirs.     

Environmental controls on benthic food web functions and carbon resource use in subarctic lakes

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15N fractionation between vegetation, soil, faeces and wool is not influenced by stocking rate

Understanding stable isotope fractionation in trophic networks is important for the interpretation of stable isotope composition of ecosystem components. This work explores the influence of grazing pressure on the nitrogen isotope composition (?? ¹⁵N) of vegetation (standing biomass), soil, and sheep??s faeces and wool in a three-years (2005?C2007) experiment with different stocking rates (0.375?C2.25 sheep ha^-1 year^-1) in semi-arid Inner Mongolia grassland. The ¹⁵N of wool (from a yearly shearing) reflects vegetation at the whole-year grazing grounds-scale while faeces reflect that of the area grazed within a few days. Stocking rate had no effect on ?? ¹⁵N of vegetation and soil, and sheep??s faeces and wool, although nitrogen content of bulk vegetation increased with stocking rate. Furthermore, ?? ¹⁵N of vegetation and diet did not differ between stocking rates. Hence, ¹⁵N fractionations between vegetation and faeces (??_veg-faeces), vegetation and wool (?? _veg-wool), faeces and soil (?? _faeces-soil) and soil and vegetation (?? _soil-veg) were constants, with ?? _veg-faeces?=?3.0?? (±0.1??, 95% confidence interval), ?? _veg-wool?=?5.3?? (±0.1??), ?? _faeces-soil?=?1.1?? (±0.4??) and ?? _soil-veg?=?-4.1?? (±0.3??). This finding is useful as it means that ?? ¹⁵N of wool or faeces can be used to estimate the ¹⁵N of grazed vegetation, even if grazing pressure is unknown.     

Contribution of leaf nitrogen to photosynthetic gas exchange in contrasting rice (Oryza sativa L.) cultivars during the grain-filling period

Photosynthetic parameters and leaf carbon isotope composition (??¹³C) in contrasting rice genotypes in relation to supplemental nitrogen (N) application and water management during the grain-filling period were compared. The changes in stomatal conductance (g _s) and ratio of intercellular to ambient CO₂ mole fraction (C _i/C _a) depended on the leaf nitrogen concentration (leaf N) in both ??Hinohikari?? (temperate japonica genotype) and ??IR36?? (indica genotype). In ??Hinohikari??, ??¹³C reflects photosynthetic gas exchange during the grain-filling period, which is indicated by the significant response of ??¹³C to leaf N. In contrast, in ??IR36?? ??¹³C did not depend on leaf N. This varietal difference in ??¹³C to leaf N can be attributed to a difference in the timing of leaf senescence. In ??IR36??, leaf N and photosynthetic parameters decreased more rapidly, indicating earlier senescence and a shorter grain-filling period in comparison with ??Hinohikari??. The significant increase in shoot dry mass in ??Hinohikari?? resulting from supplemental N application, compared with nonsignificant effect observed in ??IR36??, suggests that the timing of senescence in relation to the grainfilling period has a preponderant influence on productivity.     

Changes in the chemistry of sedimentary organic matter within the Coorong over space and time

Like many other coastal systems across the world, the Coorong lagoonal ecosystem (South Australia) has degraded over the last 100 years; in this case as a result of extensive regulation and diversions of water across the Murray-Darling Basin following European settlement. To evaluate whether the sources of organic matter (OM) supporting its food-web have changed since the inception of water management and barrage construction, sedimentary OM was characterised in cores spanning the Coorong’s salinity gradient at depths representative of the last 100 years over which the management alterations to river and estuarine flow were most marked. Detailed ²¹⁰Pb, ¹³⁷Cs and Pu dating in conjunction with palaeolimnological data (Pinus pollen) allowed for the reconstruction of the timing of substantial changes observed in the composition of the OM, most of which occur during the early 1950s, concurrent with management-related variations in water flow and salinity. Negative shifts in δ¹³C of up to 8.3‰ in the 2–10 and <2 μm fractions after the 1950s suggest a pronounced alteration in biogeochemical cycling or in the origin of OM. Elemental ratios and δ¹³C values of potential sources are inconclusive as to the cause of these biogeochemical changes. However, ¹³C-NMR spectra of the sediments suggest that degraded phytoplankton constitutes a large proportion of today’s OM and also reveal that an OM source rich in lignin was present prior to the 1950s. The high δ¹³C (?18.3‰) and low C/N (7.5) signatures of the lignin-bearing sediments are inconsistent with a C3 terrestrial OM source and instead suggest that the lignin-bearing seagrass Ruppia megacarpa (δ¹³C of ?13‰) contributed to a large degree to the sediment of the North Lagoon. R. megacarpa once was abundant in the North Lagoon but today has all but vanished from the system. Thus, only through a combination of isotopic and spectroscopic techniques was it possible to effectively decipher the changes in the composition of OM deposited throughout the Coorong over space and time. These results have important implications for research in estuarine OM dynamics in other geographic locations. Specifically, utilising complementary analytical techniques may sometimes be essential in reliably determining OM sources and processes in estuaries and lagoons.     

Spatial and temporal distribution of carbon isotopes in soil organic matter at the Dinghushan Biosphere Reserve, South China

The spatial and temporal distribution of carbon isotopes (¹³C, ¹⁴C) in soil organic matter (SOM) were studied based on SOM content, SOM ¹⁴C and SOM ¹³C of thinly layered soil samples for six soil profiles with different elevations at the Dinghushan Biosphere Reserve (DHSBR), South China. The results indicate that variations of SOM ¹³C with depth of the soil profiles at different elevations are controlled by soil development, and correlate well with SOM composition in terms of SOM compartments with different turnover rates, and SOM turnover processes at the DHSBR. The effect of carbon isotope fractionation was obvious during transformation of organic matter (OM) from plant debris to SOM in topsoil and SOM turnover processes after the topsoil was buried, which resulted in great increments of OM ¹³C, respectively. Increments of SOM ¹³C of topsoil from ¹³C of plant debris were controlled by SOM turnover rates. Both topsoil SOM ¹³C and plant debris ¹³C increase with elevation, indicating regular changes in vegetation species and composition with elevation, which is consistent with the vertical distribution of vegetation at the DHSBR. The six soil profiles at different elevations had similar characteristics in variations of SOM ¹³C with depth, alterations of SOM contents with depth and that SOM ¹⁴C apparent ages increasing with depth, respectively. These are presumably attributed to the regular distribution of different SOM compartments with depth because of their regular turnover during soil development. Depth with the maximal SOM ¹³C value is different in mechanism and magnitude with penetrating depth of ¹⁴C produced by nuclear explosion into atmosphere from 1952 to 1962, and both indicate controls of topography and vegetation on the distribution of SOM carbon isotopes with depth. Elevation exerts indirect controls on the spatial and temporal distribution of SOM carbon isotopes of the studied mountainous soil profiles at the DHSBR. This study shows that mountainous soil profiles at different elevations and with distinctive aboveground vegetation are presumably ideal sites for studies on soil carbon dynamics in different climatic-vegetation zones.     

The effect of microorganisms and seasonal factors on the isotopic composition of particulate organic carbon from the black sea

The isotopic composition of particulate organic carbon (POC) from the Black Sea deep-water zone was studied during a Russian-Swiss expedition in May 1998. POC from the upper part of the hydrogen sulfide zone (the C-layer) was found to be considerably enriched with the¹²C isotope, as compared to the POC of the oxycline and anaerobic zone. In the C-layer waters, the concurrent presence of dissolved oxygen and hydrogen sulfide and an increased rate of dark CO₂ fixation were recorded, suggesting that the change in the POC isotopic composition occurs at the expense of newly formed isotopically light organic matter of the biomass of autotrophic bacteria involved in the sulfur cycle. In the anaerobic waters below the C-layer, the organic matter of the biomass of autotrophs is consumed by the community of heterotrophic microorganisms; this results in weighting of the POC isotopic composition. Analysis of the data obtained and data available in the literature allows an inference to be made about the considerable seasonable variability of the POC δ¹³C value, which depends on the ratio of terrigenic and planktonogenic components in the particulate organic matter.     

Relationship between δ13C of chironomid remains and methane flux in Swedish lakes

1. Methanogenic carbon can be incorporated by methane‐oxidising bacteria, leading to a ¹³C‐depleted stable carbon isotopic composition (δ¹³C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ¹³C of other abundant chironomid groups and the relationship between chironomid δ¹³C and methane production in lakes. 2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ¹³C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini). 3. Remains of Orthocladiinae in the seven study lakes had the highest δ¹³C values (?31.3 to ?27.0‰), most likely reflecting δ¹³C of algae and other plant‐derived organic matter. Remains of Chironomini and Tanypodinae had lower δ¹³C values (?33.2 to ?27.6‰ and ?33.6 to ?28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ¹³C of Chironomini (r = ?0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ¹³C of Chironomini (r = ?0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05). 4. Selective incorporation of methane‐derived carbon can explain the observed correlations between methane fluxes and δ¹³C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ¹³C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ¹³C. More detailed studies with a higher number of analysed samples and detailed measurement of δ¹³C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ¹³C of chironomid remains.     

Annual burning of a tallgrass prairie inhibits C and N cycling in soil,increasing recalcitrant pyrogenic organic matter storage while reducing N availability

Grassland ecosystems store an estimated 30% of the world's total soil C and are frequently disturbed by wildfires or fire management. Aboveground litter decomposition is one of the main processes that form soil organic matter (SOM). However, during a fire biomass is removed or partially combusted and litter inputs to the soil are substituted with inputs of pyrogenic organic matter (py‐OM). Py‐OM accounts for a more recalcitrant plant input to SOM than fresh litter, and the historical frequency of burning may alter C and N retention of both fresh litter and py‐OM inputs to the soil. We compared the fate of these two forms of plant material by incubating ¹³C‐ and ¹⁵N‐labeled Andropogon gerardii litter and py‐OM at both an annually burned and an infrequently burned tallgrass prairie site for 11 months. We traced litter and py‐OM C and N into uncomplexed and organo‐mineral SOM fractions and CO₂ fluxes and determined how fire history affects the fate of these two forms of aboveground biomass. Evidence from CO₂ fluxes and SOM C:N ratios indicates that the litter was microbially transformed during decomposition while, besides an initial labile fraction, py‐OM added to SOM largely untransformed by soil microbes. Additionally, at the N‐limited annually burned site, litter N was tightly conserved. Together, these results demonstrate how, although py‐OM may contribute to C and N sequestration in the soil due to its resistance to microbial degradation, a long history of annual removal of fresh litter and input of py‐OM infers N limitation due to the inhibition of microbial decomposition of aboveground plant inputs to the soil. These results provide new insight into how fire may impact plant inputs to the soil, and the effects of py‐OM on SOM formation and ecosystem C and N cycling.     

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO₃ ^??CN, NH₄ ⁺?CN and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15?C0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural ¹⁵N abundance in NO₃ ^? during the passage of rain water through the ecosystem and bulk ??¹⁵N values in soil to detect N transformations. Depletion of ¹⁵N in NO₃ ^? and increased NO₃ ^??CN fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO₃ ^? concentrations progressively decreased and were enriched in ¹⁵N but did not reach the ??¹⁵N values of solid phase organic matter (??¹⁵N = 5.6?C6.7??). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the ??¹⁵N value of NO₃ ^? in litter leachate was smaller (??¹⁵N = ?1.58??) than in the other quarters (??¹⁵N = ?9.38 ± SE 0.46??) probably because of reduced mineralization and associated fractionation against ¹⁵N. Nitrogen isotope fractionation of NO₃ ^? between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO₃ ^??CN from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO₃ ^? gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.     

The importance of sedimenting organic matter, relative to oxygen and temperature, in structuring lake profundal macroinvertebrate assemblages

We quantified the role of a main food resource, sedimenting organic matter (SOM), relative to oxygen (DO) and temperature (TEMP) in structuring profundal macroinvertebrate assemblages in boreal lakes. SOM from 26 basins of 11 Finnish lakes was analysed for quantity (sedimentation rates), quality (C:N:P stoichiometry) and origin (carbon stable isotopes, δ¹³C). Hypolimnetic oxygen and temperature were measured from each site during summer stratification. Partial canonical correspondence analysis (CCA) and partial regression analyses were used to quantify contributions of SOM, DO and TEMP to community composition and three macroinvertebrate metrics. The results suggested a major contribution of SOM in regulating the community composition and total biomass. Oxygen best explained the Shannon diversity, whereas TEMP had largest contribution to the variation of Benthic Quality Index. Community composition was most strongly related to δ¹³C of SOM. Based on additional δ¹³C and stoichiometric analyses of chironomid taxa, marked differences were apparent in their utilization of SOM and body stoichiometry; taxa characteristic of oligotrophic conditions exhibited higher C:N ratios and lower C:P and N:P ratios compared to the species typical of eutrophic lakes. The results highlight the role of SOM in regulating benthic communities and the distributions of individual species, particularly in oligotrophic systems.