Organic δ13C Research Articles

Influence of starch deficiency on photosynthetic and post-photosynthetic carbon isotope fractionations.

Carbon isotope (13C) fractionations occurring during and after photosynthetic CO2 fixation shape the carbon isotope composition (δ13C) of plant material and respired CO2. However, responses of 13C fractionations to diel variation in starch metabolism in the leaf are not fully understood. Here we measured δ13C of organic matter (δ13COM), concentrations and δ13C of potential respiratory substrates, δ13C of dark-respired CO2 (δ13CR), and gas exchange in leaves of starch-deficient plastidial phosphoglucomutase (pgm) mutants and wild-type plants of four species (Arabidopsis thaliana, Mesembryanthemum crystallinum, Nicotiana sylvestris, and Pisum sativum). The strongest δ13C response to the pgm-induced starch deficiency was observed in N. sylvestris, with more negative δ13COM, δ13CR, and δ13C values for assimilates (i.e. sugars and starch) and organic acids (i.e. malate and citrate) in pgm mutants than in wild-type plants during a diel cycle. The genotype differences in δ13C values could be largely explained by differences in leaf gas exchange. In contrast, the PGM-knockout effect on post-photosynthetic 13C fractionations via the plastidic fructose-1,6-bisphosphate aldolase reaction or during respiration was small. Taken together, our results show that the δ13C variations in starch-deficient mutants are primarily explained by photosynthetic 13C fractionations and that the combination of knockout mutants and isotope analyses allows additional insights into plant metabolism.

High-resolution bio- and chemostratigraphy of an expanded record of Oceanic Anoxic Event 2 (Late Cenomanian–Early Turonian) at Clot Chevalier, near Barrême, SE France (Vocontian Basin)

A newly located exposure of the Niveau Thomel, an organic-­‐rich level at the Cenomanian–Turonian boundary, provides a highly expanded record of Oceanic Anoxic Event (OAE) 2, excepted for the lower relatively condensed glauconite-­rich part of the section. The new locality, close to Barreme in the Vocontian Basin, SE France, is developed in deep-­water hemi-­pelagic facies (shales, marls, marly limestones, variably enriched in organic matter) and provides an improved understanding of palaeoceanographic events associated with OAE 2. Investigation of the biostratigraphy (nannofossils and planktonic foramininfera), organic and inorganic geochemistry (bulk carbonate δ18O, total organic carbon (TOC), bulk organic, biomarker-specific and carbonate δ13C, major and trace elements, and Rock-­Eval data) has allowed characterization of the sediments in great detail. The combined study further constrains the detailed relationship between bio-­ and chemostratigraphy (particularly with respect to the details of the well-­displayed positive carbon-­‐isotope excursion) for this interval. The section also provides new evidence, in the form of a positive oxygen-­isotope excursion and an offset between carbonate and organic-­carbon carbon-­isotope records, which confirms the importance of cooling accompanied by a drop in dissolved CO2 in near-­surface waters during the Plenus Cold Event that characterized the early part of OAE 2. Evidence for increased oxygenation of bottom waters, together with elevated concentrations of redox-­sensitive and chalcophilic elements registered elsewhere through the level of the Plenus Cold Event, may be reflected in enhanced concentrations of iron (in glauconite) and nickel in coeval strata from the Clot Chevalier section.

Compositional and biogeochemical variations of sediments across the terrestrial-marine continuum of the Baker-Martínez fjord system (Chile, 48°S)

The Baker-Martínez fjord system (Chile, 48°S) is a transitional environment between the terrestrial ecosystems of Patagonia and the SE Pacific Ocean. This unique setting makes it an ideal system to evaluate land-ocean gradients in sediment composition and in a range of biogeochemical variables. Here, we studied the composition and organic geochemistry of surface sediments deposited in the Baker-Martínez fjord system to assess spatial changes in sediment properties and identify the best tracers of terrestrial input. We determined concentrations in biogenic opal, organic carbon, calcium carbonate, and lithogenic particles, and we measured diatom abundance, bulk elemental and isotopic organic geochemistry, and n-alkanes distribution, on a series of surface sediment samples distributed along two proximal-to-distal transects (one in the Martínez Channel and a second one in the Baker Channel). Results reveal clear proximal-to-distal trends in the proportion of freshwater diatoms and in the fraction of organic carbon of terrestrial origin calculated from the bulk organic δ13C measurements. Diatoms are particularly sensitive to freshwater input in proximal environments, whereas bulk organic geochemistry better reflects terrestrial input over long distances across the fjord system. Other terrestrial proxies frequently used in the literature, such as the N/C ratio and the n-alkane-based terrestrial/aquatic ratio, did not show clear variations with distance from the fjord head. Our results suggest that the isotopic composition of bulk organic matter (δ13Corg) and the relative proportion of diatoms of freshwater origin are the best-suited proxies to estimate past changes in terrestrial input from fjord sediment archives, irrespective of the glacial status of the fjord and its watershed.

Carbon and Nitrogen Burial and Response to Climate Change and Anthropogenic Disturbance in Chaohu Lake, China.

Lakes are a crucial component of the global carbon and nitrogen cycle. As a trend of enhanced human activities and climate change, the mechanisms of burial remain poorly understood. In this study, diverse biogeochemical techniques were applied to analyze the temporal variation of organic carbon and nitrogen burial rates in Chaohu Lake. The results showed that burial rates have ranged from 9.39 to 35.87 g C m−2 yr−1 for carbon and from 1.66 to 5.67 g N m−2 yr−1 for nitrogen since the 1860s. The average rates were 19.6 g C m−2 yr−1 and 3.14 g N m−2 yr−1 after the 1970s, which were significantly higher than the rate before the 1970s, showing an increasing trend. The decrease of C/N ratios as well as organic matter δ13C values indicates that the major organic matter source in sediment has been algal production since the 1970s. The increase of δ15N values indicated that the promotion in productivity was stimulated by nutrient input from sewage and agricultural runoff. The burial rates of organic carbon and nitrogen were significantly positively related to socio-economics and temperature, indicating that Chaohu Lake will become an increasing carbon and nitrogen pool under conditions of enhanced human activities and intensive precipitation.

Enhanced mercury deposition by Amazonian orographic precipitation: Evidence from high-elevation Holocene records of the Lake Titicaca region (Bolivia)

Very few studies have focused on historical mercury (Hg) deposition in the Southern Hemisphere, especially for the pre-mining period. Therefore, the respective contributions of Hg emissions from anthropogenic and natural sources are not fully constrained and the long-term influence of climate variability is still in debate. In this study, we reconstructed Hg depositions over the last 13,500 years based on two sedimentary records located at the fringe between Amazonia and the Altiplano. Multiproxy analyses carried on the same sedimentary archives (i.e. Hg accumulation rates (AR), major elements, total organic carbon, δ13C and pollen analyses) show evidence that Hg deposition has risen according to Amazonian orographic moisture and precipitation during the Holocene, except for the last 200 years, for which modern industry and goldmining overwhelmed the natural signal. Comparisons with existing Hg records indicated that metallurgy and volcanic emissions were not perceptible due to the eastern down-wind position of the sites. Altogether, it argues that the historical variability of Hg deposition in the central eastern Andes has been primarily driven by changes in Amazonian moisture. At least six main phases of higher Hg AR (2.6 ± 1.4 μg m−2 y−1) compared to baseline values (0.9 ± 0.3 μg m−2 y−1) were identified over the Holocene. During the early and late Holocene, phases of higher Hg AR coincided with wet episodes recorded in Lake Titicaca and Sajama records. During the Middle Holocene arid phase, reconstructed wetter episodes by Hg together with vegetation dynamics were attributed to the set-up of moisture niches in the eastern flanks of the Andes. Comparisons to other paleoclimatic records supports the role of North Atlantic sea-surface temperature in forcing precipitation during the late Holocene period while other mechanisms allowing moisture niches by orography are discussed for the Mid-Holocene dry phase in the eastern Andes region.

A stable and productive marine microbial community was sustained through the end-Devonian Hangenberg Crisis within the Cleveland Shale of the Appalachian Basin, United States.

The end-Devonian Hangenberg Crisis constituted one of the greatest ecological and environmental perturbations of the Paleozoic Era. To date, however, it has been difficult to precisely constrain the occurrence of the Hangenberg Crisis in the Appalachian Basin of the United States and thus to directly assess the effects of this crisis on marine microbial communities and paleoenvironmental conditions. Here, we integrate organic and inorganic chemostratigraphic records compiled from two discrete outcrop locations to characterize the onset and paleoenvironmental transitions associated with the Hangenberg Crisis within the Cleveland Shale member of the Ohio Shale. The upper Cleveland Shale records both positive carbon (δ13 Corg ) and nitrogen (δ15 Ntotal ) isotopic excursions, and replenished trace metal inventories with links to eustatic rise. These dual but apparently temporally offset isotope excursions may be useful for stratigraphic correlation with other productive end-Devonian epeiric marine locations. Deposition of the black shale succession occurred locally beneath a redox-stratified water column with euxinic zones, with signs of strengthening denitrification during the Hangenberg Crisis interval, but with an otherwise stable and algal-rich marine microbial community structure sustained in the surface mixed layer as ascertained by lipid biomarker assemblages. Discernible trace fossil signals in some horizons suggest, however, that bioturbation and seafloor oxygenation occurred episodically throughout this succession and highlight that geochemical proxies often fail to capture these rapid and sporadic redox fluctuations in ancient black shales. The paleoenvironmental conditions, source biota, and accumulations of black shale are consistent with expressions of the Hangenberg Crisis globally, suggesting this event is likely captured within the uppermost strata of the Cleveland Shale in North America.

Carbon cycling in the Sargasso Sea water column: Insights from lipid biomarkers in suspended particles

This study compares lipid biomarker and bulk constituents (organic carbon and δ13C, nitrogen and carbonate) in particles (30–4400 m depth) collected at the Oceanic Flux Program site in the northern Sargasso Sea off Bermuda during three periods of contrasting oceanographic conditions: following the spring bloom (April 2015), during the low productivity period (November 2015), and two weeks after the passage of the Category 3 Hurricane Nicole (October 2016). Lipids biomarkers were used to evaluate the contributions of phytoplankton, zooplankton and bacterial sources to the particulate organic carbon (POC), diagenetic transformations of suspended POC throughout the water column and within the nepheloid layer, and seasonal and non-seasonal temporal variability, including the impact of an extreme weather event.Depth profiles of lipid concentration and molecular composition showed seasonal and non-seasonal variability in particle composition throughout the water column. Higher lipid concentrations in April versus November 2015 reflected differences in plankton productivity, while relative abundances in diagnostic phytosterols, C18 polyunsaturated fatty acids and alkenones reflected differences in phytoplankton community structure. A rapid decrease in lipid biomarker concentrations below the euphotic zone was accompanied by a marked transition in lipid composition, e.g. increases in the relative percentages of cholesterol, C16-C18 fatty alcohols, odd- and branched- chain fatty acids. This reveals the intense remineralization of algal-derived carbon and fresh inputs of organic materials from zooplankton and microbial production within the upper mesopelagic zone. Particulate lipids within the bathypelagic zone reveal a close connectivity of deep particulate carbon composition with upper ocean properties, while compositional heterogeneity with depth reflects continuous particle turnover and de novo particle production throughout the deep water column. Additionally, depth trends in the within-class composition of fatty alcohols and bacterial fatty acids were observed, suggesting depth zonation of zooplankton and microbial community structure.Hurricane Nicole (October 2016) strongly impacted upper ocean physics and nutrient supply, resulting in a transient phytoplankton bloom/flux event that increased concentrations of fresh particulate phytodetritus (e.g., C18 polyunsaturated fatty acids) throughout the mesopelagic and upper bathypelagic zones. Concurrent increases in zooplankton and bacterial lipid biomarkers (e.g., 18:1ω9, odd- and branched-chain fatty acids, cholesterol) indicated that this transient pulse of labile carbon stimulated mesopelagic zooplankton and microbial activity, resulting in an enrichment of labile materials in the suspended particle pool throughout the deep ocean. Our study demonstrates that extreme weather events can have a major impact on carbon flux and particle cycling in the deep ocean, with pronounced implications for the ocean carbon pump and deep ocean ecosystems.

Multi‐proxy constraints on the significance of covariant δ13C values in carbonate and organic carbon during the early Mississippian

AbstractThis study investigates the covariation between carbonate and organic δ13C values in a proximal to distal transect of four outcrops in the Madison Limestone in the Western United States Rockies, combined with δ34S values of carbonate associated sulphate, the concentration of acid‐insoluble material and measurements of total organic carbon. These new geochemical datasets not only allow for an evaluation of carbon isotope covariance during one of the largest perturbations to the global carbon cycle over the past 550 Myr, but also constrain the cause of the excursion in carbonate δ13C values. The results support the hypothesis that a period of anoxia did not play a role in generating the positive carbonate δ13C values, but rather favour interpretations by previous workers that the proliferation of land plants destabilized the Carboniferous carbon cycle, setting the stage for a significant change in the carbonate δ13C values of contemporaneous marine carbonates. These results also demonstrate that one of the largest perturbations to the global carbon cycle did not produce synchronous variations in carbonate and organic δ13C values, emphasizing the importance of local depositional controls on carbon isotope covariance in the geological record in both modern and ancient environments.

Does elevated atmospheric CO2affect soil carbon burial and soil weathering in a forest ecosystem?

Most experimental studies measuring the effects of climate change on terrestrial C cycling have focused on processes that occur at relatively short time scales (up to a few years). However, climate-soil C interactions are influenced over much longer time scales by bioturbation and soil weathering affecting soil fertility, ecosystem productivity, and C storage. Elevated CO2can increase belowground C inputs and stimulate soil biota, potentially affecting bioturbation, and can decrease soil pH which could accelerate soil weathering rates. To determine whether we could resolve any changes in bioturbation or C storage, we investigated soil profiles collected from ambient and elevated-CO2plots at the Free-Air Carbon-Dioxide Enrichment (FACE) forest site at Oak Ridge National Laboratory after 11 years of 13C-depleted CO2 release. Profiles of organic carbon concentration, δ13C values, and activities of 137Cs, 210Pb, and 226Ra were measured to ∼30 cm depth in replicated soil cores to evaluate the effects of elevated CO2 on these parameters. Bioturbation models based on fitting advection-diffusion equations to 137Cs and 210Pb profiles showed that ambient and elevated-CO2 plots had indistinguishable ranges of apparent biodiffusion constants, advection rates, and soil mixing times, although apparent biodiffusion constants and advection rates were larger for 137Cs than for 210Pb as is generally observed in soils. Temporal changes in profiles of δ13C values of soil organic carbon (SOC) suggest that addition of new SOC at depth was occurring at a faster rate than that implied by the net advection term of the bioturbation model. Ratios of (210Pb/226Ra) may indicate apparent soil mixing cells that are consistent with biological mechanisms, possibly earthworms and root proliferation, driving C addition and the mixing of soil between ∼4 cm and ∼18 cm depth. Burial of SOC by soil mixing processes could substantially increase the net long-term storage of soil C and should be incorporated in soil-atmosphere interaction models.

Bioenergy crop induced changes in soil properties: A case study on Miscanthus fields in the Upper Rhine Region.

Biomass as a renewable energy source has become increasingly prevalent in Europe to comply with greenhouse gas emission targets. As one of the most efficient perennial bioenergy crops, there is great potential in the Upper Rhine Region to explore biomass utilization of Miscanthus to confront climate change and land use demand in the future. Yet, the impacts of Miscanthus cultivation on soil quality have not been adequately explored. This study investigated the soil profiles of five- and 20-year-old Miscanthus fields (1 m depth) as well as grassland for reference in eastern France and Switzerland. The soil organic carbon (SOC) concentrations and δ13C compositions of four soil layers (0–10 cm, 10–40 cm, 40–70 cm and 70–100 cm) were determined. The CO2 emission rates of the topsoil were monitored for 42 days. Our results showed that Miscanthus, in general, could increase the SOC stocks compared to grassland, but the benefits of SOC sequestration were constrained to the surface soil. Isotopically, the Miscanthus-derived SOC ranged from 69% in the top 10 cm of soil down to only 7% in the 70 cm to 100 cm layer. This result raises the risk of overestimating the total net benefits of Miscanthus cultivation, when simply using the greater SOC stocks near the surface soil to represent the SOC-depleted deep soil layers. The Miscanthus fields had greater CO2 emissions, implying that the Miscanthus fields generated greater ecosystem respiration, rather than larger net ecosystem exchanges. Compared to the grassland soils, the surface soils of the Miscanthus fields tended to have a risk of acidification while having higher concentrations of phosphorus and potassium, calling for the inclusion of soil characteristics and SOC stability when evaluating the impacts of long-term Miscanthus cultivation on both current and future land use changes.

Biogeochemical probing of microbial communities in a basalt-hosted hot spring at Kverkfjöll volcano, Iceland.

We investigated bacterial and archaeal communities along an ice-fed surficial hot spring at Kverkfjöll volcano-a partially ice-covered basaltic volcano at Vatnajökull glacier, Iceland, using biomolecular (16S rRNA, apsA, mcrA, amoA, nifH genes) and stable isotope techniques. The hot spring environment is characterized by high temperatures and low dissolved oxygen concentrations at the source (68°C and <1mg/L (±0.1%)) changing to lower temperatures and higher dissolved oxygen downstream (34.7°C and 5.9mg/L), with sulfate the dominant anion (225mg/L at the source). Sediments are comprised of detrital basalt, low-temperature alteration phases and pyrite, with <0.4 wt. % total organic carbon (TOC). 16S rRNA gene profiles reveal that organisms affiliated with Hydrogenobaculum (54%-87% bacterial population) and Thermoproteales (35%-63% archaeal population) dominate the micro-oxic hot spring source, while sulfur-oxidizing archaea (Sulfolobales, 57%-82%), and putative sulfur-oxidizing and heterotrophic bacterial groups dominate oxic downstream environments. The δ13 Corg (‰ V-PDB) values for sediment TOC and microbial biomass range from -9.4‰ at the spring's source decreasing to -12.6‰ downstream. A reverse effect isotope fractionation of ~3‰ between sediment sulfide (δ34 S ~0‰) and dissolved water sulfate (δ34 S +3.2‰), and δ18 O values of ~ -5.3‰ suggest pyrite forms abiogenically from volcanic sulfide, followed by abiogenic and microbial oxidation. These environments represent an unexplored surficial geothermal environment analogous to transient volcanogenic habitats during putative "snowball Earth" scenarios and volcano-ice geothermal environments on Mars.

Significance of carbon, nitrogen and their isotopic changes in a Permian-Triassic non-marine boundary section at Carlton Heights (Karoo Basin), South Africa

We performed analyses of Corg, Norg, δ13C org, and δ15N org from the non-marine Permian-Triassic boundary section at Carlton Heights in the Karoo Basin, South Africa. The Carlton Heights section is thus far unique in the Karoo in containing the Permian-Triassic palynological boundary between the Upper Permian Klausipollenites schaubergeri Zone and the Lower Triassic Kraeuselisporites-Lunatisporites Zone, separated by a 1-m thick “fungal event” zone, marked by abundant fungal cell remains (Reduviasporonites) and woody debris. The Corg values obtained are very low (below 0.05%), but reach a maximum of 0.1% in the middle of the fungal event zone. Norg varies little from 0.02%, except in the middle and top of the fungal event, where it rises to 0.04%. C/N ratios are extremely low throughout the section (less than 3), reaching a maximum of 2.49 during the fungal event, coincident with the peaks in Norg and Corg. The very low C/N ratios resemble those found in modern burned soils with low bacteria/fungi ratios, and the lack of variation suggests no change in the organic matter source. δ15Norg values vary between 2.46‰ and 4.25‰, showing no significant changes in the fungal event zone. The δ13Corg values are all below −25‰, and reach a low of −27.41‰ in the fungal event zone. This negative shift in δ13Corg is an example of the global negative shift in δ13C found associated with the end-Permian extinctions and fungal event.

Benthic Carbon Mineralization in Hadal Trenches: Insights From In Situ Determination of Benthic Oxygen Consumption

AbstractHadal trenches have been proposed as depocenters of organic material and hot spots for organic matter mineralization. In this study, we for the first time quantified the total benthic O2 uptake in hadal trenches using in situ chamber incubations. Three trenches in the tropical Pacific were targeted and exhibited relatively high diagenetic activity given the great water depths, that is, the Mariana Trench (2.0 × 102 μmol O2 m−2 d−1, 10,853 m), the Mussau Trench (2.7 ± 0.1 × 102 μmol O2 m−2 d−1, 7,011 m), and the New Britain Trench (6.0 ± 0.1 × 102 μmol O2 m−2 d−1, 8,225 m). Combined with the analyses of total organic carbon and δ13C of total organic carbon in the sediments and previously published in situ O2 microprofiles from hadal settings, we suggest that hadal benthic carbon mineralization partly is governed by the surface production and also is linked to the distance from land. Therefore, we highlight that terrestrial organic matter can be of importance in sustaining benthic communities in some hadal settings.

Organic matter geochemical signatures (TOC, TN, C/N ratio, δ13C and δ15N) of surface sediment from lakes distributed along a climatological gradient on the western side of the southern Andes

Paleolimnological studies in western South America, where meteorological stations are scarce, are critical to obtain more realistic and reliable regional reconstructions of past climate and environmental changes, including vegetation and water budget variability. However, climate and environmental geochemical indicators must be tested before they can be applied with confidence. Here we present a survey of lacustrine surface sediment (core top, 0 to ~1cm) biogeochemical proxies (total organic carbon [TOC], total nitrogen [TN], carbon/nitrogen ratio [C/N ratio] and bulk organic δ13C and total δ15N) from a suite of 72 lakes spanning the transition from a Mediterranean climate with a patchwork of cultivated vegetation, pastureland, and conifers in central Chile to a rainy temperate climate dominated by broadleaf deciduous and evergreen forest further south. Sedimentary data are compared to the latitudinal and orographic climatic trends of the region based on the climatology (precipitation and temperature) produced with Climate Forecast System Reanalysis (CFSR) data and the modern Southern Hemisphere Westerly Winds (SWW) location. The geochemical data show inflection points at ~42°S latitude and ~1500m elevation that are likely related to the northern limit of influence of the SWW and elevation of the snow line, respectively. Overall the organic proxies were able to mimic climatic trends (Mean Annual Precipitation [MAP] and temperature [MAT]), indicating that they are a useful tool to be included in paleoclimatological reconstruction of the region.

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups, India.

Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro-organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite-rich microbialites. Stromatolites from the 1.6Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine-grained in situ-precipitated apatite mats with average δ13 Corg indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone-like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite-carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis-respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen-producing biotas in the formation of Paleoproterozoic shallow-basin phosphorites.

Changes from pasture to a native tree plantation affect soil organic matter in a tropical soil, Panamá

We examined changes in soil organic matter arising from conversion of a 45-year old pasture to a 10 yr. old native tree plantation in Panama, to evaluate the effect of monoculture and mixtures. We intensively sampled the soil 0–10 cm depth in the pasture in 2001 and in 22 plantation plots in 2011, ranging from 5 monocultures to 3- and 6-species treatments; samples were also taken from an undisturbed forest site. Soil analyses included organic carbon (SOC) and δ13C. Conversion of the pasture to tree plantation resulted in an overall loss of SOC of 0.6 kg m−2 (18%) in the top 10 cm, but neither tree species nor diversity had a significant effect. End-member δ13C values suggested that the contribution of C3 plants to SOC was increased from 26% in the pasture to 55% after 10 years of plantation and SOC turnover times were calculated to be 21–36 yr. The magnitude of the loss in soil SOC is smaller than the increases in tree biomass (~3 kg C m−2) and litter (~0.3 kg C m−2) in the plantation, but still a significant part of the ecosystem C balance.

Soil carbon redistribution and organo-mineral associations after lateral soil movement and mixing in a first-order forest watershed

We test the hypothesis that erosion driven soil movement on hillslopes results in an increase in new organomineral associations and overall organic matter storage in colluvial deposits within a forested hillslope. We measured mineral specific surface area (SSA), organic carbon (OC), meteoric radioisotopes (210Pb, 137Cs, 10Be), soil physical properties, C/N, δ15N, δ13C, and ∆14C in bulk soil and density fractions in a hillslope transect of soil pits. The quantity of OC per unit of mineral surface area (OC/SA) and OC inventories increased by a factor of 2–3 in depositional sites as result of soil mixing due to erosional movement as confirmed by 210Pb, 137Cs, and 10Be profiles and inventories. Soil mixing systematically decreased C/N and enriched stable isotopes of δ13C and δ15N, revealing that formation of organomineral associations instead of microbial processing was responsible for depth trends in organic matter composition. Our findings indicate that the processes that associate organic matter and minerals are fundamentally linked with organic matter composition, and OC/SA, C/N, δ13C, and δ15N provide proxies for organic matter stabilization by soil minerals.

黔西北次生林优势树种叶片-凋落物-土壤连续体有机质碳稳定同位素特征

PDF HTML阅读 XML下载 导出引用 引用提醒 黔西北次生林优势树种叶片-凋落物-土壤连续体有机质碳稳定同位素特征 DOI: 10.5846/stxb201807091489 作者: 作者单位: 贵州师范大学喀斯特研究院/国家喀斯特石漠化防治工程技术研究中心,贵州师范大学喀斯特研究院/国家喀斯特石漠化防治工程技术研究中心,贵州师范大学地理与环境科学学院,贵州师范大学地理与环境科学学院 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划项目课题（2016YFC0502601）；贵州省科技计划项目（黔科合LH字[2016]7201号） Carbon stable isotopic characteristics of organic matter in the leaf-litter-soil continuum of dominant tree species in a secondary forest in northwestern Guizhou Author: Affiliation: School of Karst Science,Guizhou Normal University,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:稳定性碳同位素自然丰度（δ13C）能够揭示生态系统长时间尺度的有机碳动态变化，阐明生态系统功能的变化特征。以黔西北次生林14个优势树种为研究对象，测定叶片、凋落物以及根区土壤有机碳含量和δ13C值，分析不同层次碳含量和δ13C丰度之间的相关性。结果表明：14个优势树种叶片碳含量为404.67-487.14 g/kg，总体为针叶树种较高、常绿灌木较低；δ13C值为-31.2‰—-27.1‰，随生活型的变化规律不明显。凋落物碳含量为414.62-561.31 g/kg，与叶片碳含量的变化规律较为一致；δ13C值为-31.5‰—-27.3‰，随树种生活型的变化特征也不明显。根区土壤碳含量为10.02-91.59 g/kg，δ13C值为-26.8‰—-22.5‰，碳含量以光皮桦、银白杨等落叶乔木较高。叶片、凋落物和根区土壤3个层次的碳含量与δ13C丰度之间均呈不显著相关，不同层次的δ13C丰度之间和碳含量之间均为正相关。研究结果有助于反映森林生态系统碳循环过程的关键信息，为森林植被恢复提供理论依据。 Abstract:The natural abundance of stable carbon isotopes (δ13C) can reveal the dynamic changes of organic carbon over time and illustrate the characteristics of functional changes in ecosystems. Using 14 dominant tree species in the secondary forest in northwestern Guizhou, the organic carbon contents and δ13C values in leaves, litter, and root-zone soil were measured to analyze the correlation between carbon content and δ13C abundance at different levels. The results showed that:the carbon content of leaves of the 14 dominant tree species was 404.67-487.14 g/kg. Generally, the carbon content was higher in conifers and lower in evergreen shrubs. The δ13C value was -31.2‰ to -27.1‰, and the change with life form was not obvious. The litter carbon content was 414.62-561.31 g/kg, which was consistent with the change of leaf carbon content; the δ13C value was -31.5‰ to -27.3‰, and the changing characteristics of life form in tree species were not obvious. The carbon content in root-zone soil was 10.02-91.59 g/kg, the δ13C value was from -26.8‰ to -22.5‰. The carbon content was higher in deciduous trees, such as Rhododendron simsii and Populus alba. There was no significant correlation between the carbon content of leaves, litter, and root-zone soil at different levels and the abundance of δ13C value. A positive correlation was found between the abundance of δ13C value and carbon content at different levels. The results could help to reveal fundamental information of the carbon cycle process in forest ecosystems and provide a theoretical basis for forest vegetation restoration. 参考文献 相似文献 引证文献

Acid digestion on river influenced shelf sediment organic matter: Carbon and nitrogen contents and isotopic ratios.

Natural stable isotope ratios (δ13 Corg and δ15 N values) and associated elemental concentrations (i.e. total organic carbon and total nitrogen contents) preserved in marine sediments are frequently used for the determination of paleo-environmental processes such as the origin of organic matter. Previous studies highlighted biases in the determination of such geochemical proxies due to pre-analysis acid treatment methods. This study is the first systematic comparison of the effect of acid treatment methods on bulk organic matter using a unique sedimentary system, under two contrasting climatic contexts (i.e. glacial vs interglacial). We used the most common method for pre-treatment analysis, which consists of the acidification of bulk sediments followed by several rinses with deionised water. We investigated the effect of acid type (hydrochloric and acetic acid), and also acid strength (from 0.2 to 10mol.L-1 ) on the δ13 Corg , δ15 N, TOC, TN and C/N values of three samples from the Gulf of Lion. Two of them (S.304 and S.102) were deposited during glacial maxima (i.e. high sedimentation rate, low porosity and high terrestrial inputs) whereas S.157 characterizes interglacial conditions (high porosity, high foraminifera content, low sedimentation rates and low terrestrial inputs). For all three samples the δ13 Corg values are between -21.7 and -24.4‰ with the TOC varying from 0.56 to 0.84 %wt/wt. The δ15 N values are more stable with an average value of 3.0±0.1‰ with a TN average of 0.08±0.002 %wt/wt. We show that acid type did not significantly affect results. We also find that (i) glacial and interglacial samples do not react similarly to acid pre-treatment, (ii) high acid strength (>1.5mol.L-1 ) induces significant bias on δ13 Corg and TOC values, and therefore on C/N ratios; (iii) 25% of an isotopically distinct pool of organic carbon was lost when using acid concentrations of 0.2-1.5mol.L-1 , affecting δ13 Corg values by more than 1.5‰; and (iv) geochemical evidence indicates that the leachable organic carbon pool is preferentially composed of terrestrial organic matter. These findings call for precautions when using C/N ratios and associated δ13 Corg values for paleo-environmental and climate reconstructions.

Correspondence between δ13C and δ15N in soils suggests coordinated fractionation processes for soil C and N

Although a number of different factors influence C and N isotopic fractionation of organic matter, the δ13C and δ15N values of soil organic matter both tend to increase with soil depth, following similar trajectories. This similarity has not been investigated at the global scale. As microbial decomposition increases organic matter δ13C and δ15N values, soil isotopic values are hypothesized to generally increase with depth across local and global scales. Soil δ13C and δ15N values for 16 soil depth-profile sites were used for local-scale investigation, and 5447 global single-depth sites were used for global-scale investigation of the correspondence between δ13C and δ15N. Correlative and boosted regression tree analyses were used to determine the main drivers of the variance in soil δ15N globally and also the environmental association of variability in the correlation with depth between δ13C and δ15N at a number of sites. Strong positive correlations between δ13C and δ15N values through soil profiles were found at a number of sites and were found to be independent of vegetation type. Globally, soil δ13C and δ15N values were also found to be significantly positively correlated across a wide range of climates and biomes. The global correspondences between δ13C and δ15N values may suggest a mechanistic link between δ13C and δ15N through the process of SOM decomposition and microbial processing and highlight the importance of soil-related processes in determining isotopic signals in soils. The variability in these soil processes should be considered when interpreting soil isotopic values of δ13C and δ15N as indicators of ecosystem sources of soil C and N and inferring vegetation inputs.