δ13C Abundance Research Articles

Functional characterization of the water-soluble organic carbon of size-fractionated aerosol in the southern Mississippi Valley

The chemical content of water-soluble organic carbon (WSOC) as a function of particle size was characterized in Little Rock, Arkansas in winter and spring 2013. The objectives of this study were to (i) compare the functional characteristics of coarse, fine and ultrafine WSOC and (ii) reconcile the sources of WSOC for periods when carbonaceous aerosol was the most abundant particulate component. The WSOC accounted for 5 % of particle mass for particles with δp > 0.96 μm and 10 % of particle mass for particles with δp < 0.96 μm. Non-exchangeable aliphatic (H–C), unsaturated aliphatic (H–C–C=), oxygenated saturated aliphatic (H–C–O), acetalic (O–CH–O) and aromatic (Ar–H) protons were determined by proton nuclear magnetic resonance (1H-NMR). The total non-exchangeable organic hydrogen concentrations varied from 4.1 ± 0.1 nmol m−3 for particles with 1.5 < δp < 3.0 μm to 73.9 ± 12.3 nmol m−3 for particles with δp < 0.49 μm. The molar H/C ratios varied from 0.48 ± 0.05 to 0.92 ± 0.09, which were comparable to those observed for combustion-related organic aerosol. The R–H was the most abundant group, representing about 45 % of measured total non-exchangeable organic hydrogen concentrations, followed by H–C–O (27 %) and H–C–C= (26 %). Levoglucosan, amines, ammonium and methanesulfonate were identified in NMR fingerprints of fine particles. Sucrose, fructose, glucose, formate and acetate were associated with coarse particles. These qualitative differences of 1H-NMR profiles for different particle sizes indicated the possible contribution of biological aerosols and a mixture of aliphatic and oxygenated compounds from biomass burning and traffic exhausts. The concurrent presence of ammonium and amines also suggested the presence of ammonium/aminium nitrate and sulfate secondary aerosol. The size-dependent origin of WSOC was further corroborated by the increasing δ13C abundance from −26.81 ± 0.18 ‰ for the smallest particles to −25.93 ± 0.31 ‰ for the largest particles and the relative distribution of the functional groups as compared to those previously observed for marine, biomass burning and secondary organic aerosol. The latter also allowed for the differentiation of urban combustion-related aerosol and biological particles. The five types of organic hydrogen accounted for the majority of WSOC for particles with δp > 3.0 μm and δp < 0.96 μm.

Soil organic carbon storage and dynamics after C3-C4 and C4-C3 vegetation changes in sub-Andean landscapes of Colombia

The soil C capture capacity and organic matter turnover rate vary according to photosynthetic pathways; therefore the evaluation of C at sites suffering changes from C3 to C4 vegetation and vice versa, is important to identify impacts of land use change on C cycle. This study aims to evaluate C storage under different land uses, and soil C dynamics using the 13C technique to identify the origin of soil C. In the Municipality of Alcala, Department of Valle del Cauca, Colombia, the natural abundance of δ13C was estimated, and data on land use history were gathered to calculate the organic matter turnover rate. The contribution of each type of vegetation to total percentage organic C and to storage at 0.30 m was estimated at sites suffering changes from C3 to C4 vegetation and vice versa. Average δ13C ranged between -25.79 and -20.72‰ at the three depths evaluated. Over a period of 13 yr, mature fallow lands replaced more than 70% of the C fixed by pastures over a period of 60 yr, whereas paddocks, over a period of 17 yr, only managed to replace 37.9% of the C fixed by associated coffee plantations during a period of 50-100 years. We conclude that the use of 13C avoided that C storage would have been attributed to current land uses when they are actually fixed by previous vegetation; and that C deposit from C3 vegetation is recalcitrant, while that corresponding to C4 vegetation has a relatively fast turnover rate.

Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ13C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ13C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of 13CO2 to maize (Zea mays L.). Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested. The δ13C abundance before and after the continuous labeling was −24.20 ± 0.05 ‰ and −23.80 ± 0.05 ‰ in the rooted bulk soil. The labeling caused increases in δ13C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ13C abundance was enriched in the >2 mm and 1–2 mm aggregates (−23.17 ± 0.12 ‰ and −23.26 ± 0.05 ‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ13C abundance in pore water (−24.0 ± 0.01 ‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ13C abundance was in general greater in the horizontal cell than in the vertical cell. The δ13C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (−23.66 ± 0.11 ‰ and −23.5 ± 0.10 ‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ13C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell. Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.

Organic vs. Conventional Grassland Management: Do 15N and 13C Isotopic Signatures of Hay and Soil Samples Differ?

Distinguishing organic and conventional products is a major issue of food security and authenticity. Previous studies successfully used stable isotopes to separate organic and conventional products, but up to now, this approach was not tested for organic grassland hay and soil. Moreover, isotopic abundances could be a powerful tool to elucidate differences in ecosystem functioning and driving mechanisms of element cycling in organic and conventional management systems. Here, we studied the δ15N and δ13C isotopic composition of soil and hay samples of 21 organic and 34 conventional grasslands in two German regions. We also used Δδ15N (δ15N plant - δ15N soil) to characterize nitrogen dynamics. In order to detect temporal trends, isotopic abundances in organic grasslands were related to the time since certification. Furthermore, discriminant analysis was used to test whether the respective management type can be deduced from observed isotopic abundances.Isotopic analyses revealed no significant differences in δ13C in hay and δ15N in both soil and hay between management types, but showed that δ13C abundances were significantly lower in soil of organic compared to conventional grasslands. Δδ15N values implied that management types did not substantially differ in nitrogen cycling. Only δ13C in soil and hay showed significant negative relationships with the time since certification. Thus, our result suggest that organic grasslands suffered less from drought stress compared to conventional grasslands most likely due to a benefit of higher plant species richness, as previously shown by manipulative biodiversity experiments. Finally, it was possible to correctly classify about two third of the samples according to their management using isotopic abundances in soil and hay. However, as more than half of the organic samples were incorrectly classified, we infer that more research is needed to improve this approach before it can be efficiently used in practice.

Phosphorus enrichment and carbon depletion contribute to high Microcystis biomass and microcystin concentrations in Ugandan lakes

We investigated the factors influencing cyanobacterial biomass and microcystin (MC) concentrations in several Ugandan lakes from September 2008 to February 2009. We characterized thermal structure, light availability, nutrient concentrations, chlorophyll a, phytoplankton δ13C (as an indicator of CO2 limitation), and phytoplankton community composition and abundance as well as MC concentrations. We used these data to test several hypotheses based on previous research in temperate lakes regarding the factors that encourage high cyanobacterial biomass and MC concentrations. Site characteristics that appeared to favor high cyanobacterial biomass (especially Microcystis) included: high total phosphorus concentrations, low total nitrogen to total phosphorus (TN : TP) ratios, and possibly low CO2 availability. Light availability, total nitrogen concentrations, and thermal structure of the water column were not related to cyanobacterial biomass. MC concentrations were strongly related to Microcystis biomass (and were not related to the biomass of any other cyanobacterial taxa), which was positively correlated with total phosphorus and chlorophyll a concentrations. MC cell content may be moderated by CO2 availability, with MC cell quotas tending to be lower where the potential for C‐limitation of photosynthesis was higher. In these phosphorus‐rich tropical lakes, the shallowest study sites were most conducive to the development of large standing crops of Microcystis and high MC concentrations. The environmental conditions that appear to favor high cyanobacterial biomass and MC concentrations in our Ugandan study lakes are similar to what has been observed for temperate lakes.

Soil carbon stocks and carbon sequestration rates in seminatural grassland in Aso region, Kumamoto, Southern Japan

Global soil carbon (C) stocks account for approximately three times that found in the atmosphere. In the Aso mountain region of Southern Japan, seminatural grasslands have been maintained by annual harvests and/or burning for more than 1000years. Quantification of soil C stocks and C sequestration rates in Aso mountain ecosystem is needed to make well-informed, land-use decisions to maximize C sinks while minimizing C emissions. Soil cores were collected from six sites within 200km(2) (767-937m asl.) from the surface down to the k-Ah layer established 7300years ago by a volcanic eruption. The biological sources of the C stored in the Aso mountain ecosystem were investigated by combining C content at a number of sampling depths with age (using (14) C dating) and δ(13) C isotopic fractionation. Quantification of plant phytoliths at several depths was used to make basic reconstructions of past vegetation and was linked with C-sequestration rates. The mean total C stock of all six sites was 232Mg Cha(-1) (28-417Mg Cha(-1) ), which equates to a soil C sequestration rate of 32kg Cha(-1) yr(-1) over 7300years. Mean soil C sequestration rates over 34, 50 and 100years were estimated by an equation regressing soil C sequestration rate against soil C accumulation interval, which was modeled to be 618, 483 and 332kg C ha(-1) yr(-1) , respectively. Such data allows for a deeper understanding in how much C could be sequestered in Miscanthus grasslands at different time scales. In Aso, tribe Andropogoneae (especially Miscanthus and Schizoachyrium genera) and tribe Paniceae contributed between 64% and 100% of soil C based on δ(13) C abundance. We conclude that the seminatural, C4 -dominated grassland system serves as an important C sink, and worthy of future conservation.

Changes in soil C and N distribution assessed by natural δ13C and δ15N abundance in a chronosequence of sugarcane crops managed with pre-harvest burning in a Cerrado area of Goiás, Brazil

Pre-harvest sugarcane straw burning affects the dynamics of soil chemistry and δ13C and δ15N abundance, but further studies are needed to understand the long-term effects of this practice on the degradation of chemical attributes of soil and sugarcane production. The present study evaluates chemical modifications of soil and natural δ13C and δ15N abundance in sugarcane crops managed with pre-harvest straw burning for 1, 5, 10 and 20 years. For comparison purposes, soil covered with natural Cerrado vegetation and forage grass (pasture) were also evaluated. After chemical characterization of the areas studied, according to Embrapa (1997) total organic carbon (TOC), bulk density (Bd) and natural δ15N and δ13C abundance were determined in soil samples, and carbon stocks calculated from TOC values. In general, the pasture area had the highest δ13C and δ15N content, the Cerrado vegetation area had the lowest values for these elements and sugarcane crops exhibited intermediary levels. Areas subjected to sugarcane field burning for a shorter period had the highest Ca and Mg and lowest P and K content. Sugarcane crops managed with straw burning for 1 and 5 years produced lower carbon stocks in the top soil layer (0–10cm) than sugarcane crops with 10 and 20 years, pasture and Cerrado areas. The replacement of natural Cerrado vegetation by sugarcane crops changed δ13C profile, likely because sugarcane is a C4 plant, which has a specific carbon accumulation pattern. Areas cropped with sugarcane for 1, 10 and 20 years and subjected to different types of soil use after replacement of the original Cerrado vegetation had the highest δ15N values, indicating accelerated mineralization of soil organic matter.

Flow discharge influences on input and transport of particulate and sedimentary organic carbon along a small temperate river

The relative importance of hydrological conditions and upstream processes on the input and transport of organic carbon from land to sea was investigated in the Mullica River, a small system located in the southeastern New Jersey (USA), using molecular and δ13C compositions. Sediment and particulate organic carbon (POC) samples were collected along the river, from upstream (draining predominantly pine-oak forests) in the Pinelands to the Great Bay Estuary (bordered by salt marshes) under normal and high flow conditions. During base river flow, terrestrial biomarkers (e.g., diterpenoids – conifer biomarkers) were detected predominantly upstream and sharply decreased downstream. From mid to down river sites, a mixture of higher plants (primarily from the salt marsh vegetation) and microbial biomarkers (e.g., mycose, cholesterol) predominated. δ13C abundances of individual fatty acids presented a consistent pattern, with depleted values (e.g., δ13Csed=−30.9‰ on average) upstream, becoming heavier toward the Great Bay (e.g., δ13Csed=−23.1‰; δ13CPOC=−22.8‰). During the high discharge event, significant amounts of upland biomarkers (e.g., pimaric acid, ferruginol, ursolic acid) and depleted δ13C values were observed from the Pinelands (δ13Csed=−34.6‰) to mid river (δ13Csed=−32.0‰; δ13CPOC=−30.8‰), clearly demonstrating the transport of terrestrial material farther down river. However, downstream of this point, the occurrence of the terrestrial material strongly decreased, demonstrating that terrestrial plant biomarkers were not transported in significant amounts to the estuarine region, even at high river discharge. Since the mid river site is situated in the upper limit of salt water intrusions, it is likely that flocculation and sedimentation provide a depositional sink for terrestrial OC in the mid Mullica River, thereby depleting the terrigenous signature in downstream OC.

Effects of lignin‐modified Populus tremuloides on soil organic carbon

AbstractSeveral genes in the aspen genome have been modified to generate stem wood with lower lignin content and an altered lignin composition. Lower lignin in wood reduces the time and energy required for pulping. Further, this modification can also increase the allocation of photosynthate to cellulose and total biomass production, potentially increasing CO2‐sequestration capacity. However, widespread planting of trees with altered lignin content and composition could alter soil organic‐C dynamics in complex ways. To further examine the effects of altered lignin biosynthesis on plant growth and accrual of soil organic C (SOC), we conducted a repeated greenhouse study with four lines of transgenic aspen (Populus tremuloides Michx.) and one wild‐type (control) aspen. Accrual of aspen‐derived SOC was quantified by growing aspen trees (C3 plants) in C4 soil and measuring changes in the natural abundance of δ13C. We measured plant growth, biomass, and C content and combined these data with SOC measurements to create C budgets for the plant mesocosms. Lignin modifications resulted in differences in the accrual of aspen‐derived SOC and total mesocosm C, primarily due to differences in biomass between genetically modified lines of aspen. One genetic alteration (low lignin, line 23) was able to perform similarly or better than the wild‐type aspen (control, line 271) without altering SOC. Alterations in lignin structure (S : G ratios) had negative effects on biomass production and SOC formation. The addition of new (aspen‐derived) SOC was proportional to the loss of existing SOC, evidence for a priming effect. The pool of new SOC was related to total plant biomass, suggesting that the effects of lignin modification on SOC are driven by changes in plant growth.

Carbono total e δ13C em agregados do solo sob vegetação nativa e pastagem no bioma cerrado

Diferentes sistemas de uso e manejo alteram o teor de carbono no solo (C), porém diminuições no C do solo têm sido reportadas quando áreas de Cerrado nativo são convertidas em pastagem. Disso resulta a necessidade da realização de pesquisas em diferentes sistemas e condições edafoclimáticas, a fim de elucidar as tendências encontradas. O objetivo deste trabalho foi avaliar a agregação, os teores de C e a abundância natural de δ13C do solo em uma área cultivada há nove anos sob lavoura seguida de 20 anos sob pastagem, a qual atualmente apresenta sinais de degradação (PA), comparando-a a uma vegetação nativa no bioma Cerrado (CE) que ocorre em área adjacente. As áreas de estudo estão localizadas no município de Rio Verde (GO), em solo classificado como Latossolo Vermelho distrófico, de textura argilosa. Em amostras de solo coletadas nas camadas de 0-5, 5-10 e 10-20 cm de profundidade, foram determinados a distribuição de agregados estáveis em água (micro, meso e macroagregados), o diâmetro médio aritmético (DMA) e geométrico (DMG), os teores de C total e de δ13C, o tempo de residência média do C (TRM) e a fração da matéria orgânica leve livre (FLL). Os resultados indicaram que o uso sob lavoura seguido de pastagem reduziu em 21 % o estoque de C original, possivelmente devido ao menor aporte de material na FLL. O solo apresentou elevada agregação e não houve diferenças significativas entre as áreas avaliadas. No entanto, os teores de C nos macroagregados foram menores na PA em relação ao CE, enquanto nos microagregados não houve diferença entre os sistemas de uso, sugerindo, respectivamente, maior e menor sensibilidade ao manejo e menor e maior proteção do C por essas classes de agregados. Os valores de TRM e as análises isotópicas reforçaram esses resultados, mostrando, ainda, substituição de 83 % do C original durante os 29 anos de cultivo. O constante aporte de resíduos vegetais, assim como a manutenção do C no interior dos macro e mesoagregados, são importantes fatores para que ocorra o acúmulo de C nesse tipo de solo.

Whiting-related sediment export along the Middle Miocene carbonate ramp of Great Bahama Bank

Modern aragonite needles are present all along the modern leeward margin of Great Bahama Bank (ODP Leg 166), while Middle Miocene sediments contain needles only in more distal areas (Sites 1006 and 1007). In contrast to the rimmed, flat-topped platform topography during the Plio-Pleistocene, the Miocene Great Bahama Bank morphology is a carbonate ramp profile. This might imply a different location and precipitation type for Miocene aragonite needles. In this study, aragonite needles in Miocene sediments were isolated using a granulometric separation method. Furthermore, the isolation of the various carbonate components enables the identification of primary versus diagenetic components. The Miocene aragonite needles are concentrated in the finest granulometric sediment fractions (<12 μm). The fraction-specific geochemical analyses (δ13C, δ18O and Sr elemental abundance) represent useful tools to assess the possible sources of the aragonite mud. The geochemical variation of the fractions, rich in pristine aragonite needles, and the characteristics of the needle morphology point to whiting phenomena as the main sediment source and algal fragmentation as a minor component. Both components indicate shallow-water environments as the main sediment source area. Ramp-top-related fine-grained particles now present at distal sites were likely exported as suspended material similar to present-day transport mechanisms. The scarcity of needles at proximal sites is probably linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded. The granulometric separation approach applied here enables a better characterization of the finest carbonate particles representing an important step towards the discrimination between primary and diagenetic fine-grained components.

The Natural 13C Abundance of Plasma Glucose Is a Useful Biomarker of Recent Dietary Caloric Sweetener Intake

There is a need for objective biomarkers of dietary intake, because self-reporting is often subject to bias. We tested the validity of a biomarker for the fraction of dietary carbohydrate (CHO) from cane sugar and high fructose corn syrup (C4 sugars) using natural 13C abundance of plasma glucose. In a randomized, single-blinded, crossover design, 5 participants consumed 3 weight-maintaining diets for 7 d, with a 2-wk washout between diet periods. Diets differed in the fraction of total CHO energy from C4 sugars (5, 16, or 32%). During each diet period, blood samples were drawn at hours 0800 and 1600 on d 1, 3, and 5 and at 0800, 1000, 1200, 1400, and 1600 on d 7. The δ13C abundance of plasma glucose was analyzed via GC- isotope ratio MS. Within each diet period, δ13C abundance of the 0800 fasting glucose did not change from baseline with increasing time during a diet period; however, there was a strong positive correlation (R2 = 0.89) between δ13C abundance of the glucose concentration at 1000 on d 7 and the percent of breakfast CHO from C4 sugars. Also, δ13C abundance of the combined plasma glucose samples on d 7 demonstrated a strong positive correlation (R2 = 0.90) with the percent of total daily CHO from C4 sugars. The natural δ13C abundance of postprandial plasma glucose relative to dietary C4 CHO content was a valid biomarker for contributions of C4 caloric sweeteners from the previous meal.

Soil respiration in four different land use systems in north central Alberta, Canada

This study compares soil respiration and its heterotrophic and autotrophic components in four land use types: agriculture, 2 and 9 year old hybrid poplar plantations, grassland, and a native aspen stand in north central Alberta, Canada, over a period of two growing seasons (2006 and 2007). The differences were examined with respect to substrate quality and quantity, fine root biomass, and nutrient availability, in addition to soil temperature and soil water content. Cumulative soil C loss via soil respiration averaged over the two growing seasons was (in decreasing order) 781, 551, 523, 502, and 428 g C m−2 for native aspen stand, 9 year old hybrid poplar plantation, grassland, agriculture and 2 year old hybrid poplar plantation, respectively. We found that ∼75% of soil respiration in the native aspen stand originated from the top 7.5–10 cm litter‐fibric‐humus layer. Seasonal heterotrophic and autotrophic respiration among the land uses ranged from 97 to 272 and 333 to 560 g C m−2, respectively, contributing up to 35% and 83% of total soil respiration, respectively. The variability in soil respiration across different land uses was explained mainly by site differences in soil temperature (88–94%). Soil respiration followed a pronounced seasonal trend: increasing during the growing season and converging to a minimum in the fall. Soil respiration under different land uses was influenced by (1) ecosystem C stock, (2) temperature sensitivity (Q10) of organic matter present, and (3) organic matter decomposability as indicated by the natural abundance of δ13C. Heterotrophic respiration was influenced by soil temperature, while autotrophic respiration was influenced by fine root biomass and nutrient (NO3− and P) availability. These results are useful in estimating potential responses of soil respiration and its components to future land management and climate change.

PREDICTION OF ATMOSPHERIC 13CO2 USING PLANT CUTICLE ISOLATED FROM FLUVIAL SEDIMENT: TESTS ACROSS A GRADIENT IN SALT CONTENT

Abstract In order to test the assertion that the carbon-isotopic composition of the ancient atmosphere (δ13CO2) can be reconstructed from the carbon-isotopic composition of fossil terrestrial plant tissues across a variety of environments, the δ13C value of land-plant tissues isolated from modern fluvial sediments was compared to that of today's atmosphere. Plant stem and leaf fragments were isolated from organic carbon-rich sediments of the Black River in Jamaica, which drains a basin containing only C3 ecosystems. Sediment was sampled at 12 sites along a dissolved salt-content gradient, from the coastal plain to near its mouth, which allowed evaluation of the effect of salt influence on the organic carbon-isotope signature. Many properties of the sediment varied systematically with salt content (e.g., mass-percent carbonate, abundance of palynomorphs, δ13C and δ18O values of carbonate), confirming a significant and increasing marine influence closer to the mouth of the river. The δ13C value of total org...

Natural 13C abundance of plasma glucose as a potential biomarker for dietary “simple” sugar intake.

INTRODUCTIONA need exists for objective biomarkers of dietary intake, as self‐report is often subject to inaccuracies.PURPOSEWe tested the validity of measuring the fraction of dietary carbohydrate (CHO) from cane sugar and high fructose corn syrup (C4 sugars) using natural 13C abundance of plasma glucose (Glc).METHODSIn a randomized cross‐over design, 5 subjects were fed 3 weight‐maintaining diets for 7 days, with a two‐week washout between diets. Diets differed in the fraction of total CHO energy from C4 sugars (5%, 16%, or 32%). During each diet, blood samples were drawn at 8am and 4pm on days 1, 3, and 5 and at 8am, 10am, 12pm, 2pm, and 4pm on day 7. δ13C abundance of plasma Glc was analyzed using isotope ratio mass spectrometry.RESULTSWithin each diet, δ13C of 8am Glc did not change from baseline with increasing time on diet. There was a positive correlation (R2 = 0.88) between Day 7 ‐ 10am δ13C Glc and % of breakfast CHO from C4 sugars. Also, δ13C Glc of combined 10am ‐ 4pm plasma from Day 7 within each diet demonstrated a positive correlation (R2 = 0.90) with % of total daily CHO from C4 sugars.CONCLUSIONGluconeogenesis attenuates δ13C abundance of fasting Glc, as the δ13C abundance of Glc from C4 sugar consumption does not persist overnight. Postprandial plasma Glc could be used as a biomarker for contributions of C4 sugars to a previously ingested meal. (Funding provided by The National Dairy Council).

Soil Organic Carbon and Nitrogen and Distribution of Carbon‐13 and Nitrogen‐15 in Aggregates of Everglades Histosols

Oxidation of Histosols in the Everglades Agricultural Area (EAA) of south Florida leads to decreases in soil depth, changes in biogeochemical properties, and may limit land use options in the future. The objectives of this study were to determine how long‐term cultivation influences organic matter dynamics and C and N distribution throughout the profile of a drained Histosol. We measured organic C and N stocks, aggregation, and the natural abundance of δ13C and δ15N in aggregates from Histosols 100 yr after drainage for two land uses: sugarcane (Saccharum officinarum L.) and prairie. Macroaggregates comprised the bulk of total soil for both land uses, averaging 81% of the total soil in fractions &gt;0.25 mm. Macroaggregation increased with depth and the proportion to whole soil was 65% higher at 30 to 45 cm than 0 to 15 cm. Cultivated soil averaged 13% higher organic C, but 11% lower organic N than prairie throughout the profile (0–45 cm). The majority of the organic C (76%) and N stocks (77%) was in macroaggregate fractions &gt;0.25 mm. The distribution of organic matter among aggregate‐size fractions generally did not differ between land uses, except that organic C and N were 39 and 44%, respectively, greater for macroaggregates in prairie than cultivated soil at 0 to 15 cm. The δ15N decreased with depth for both land uses, indicating that organic matter was more decomposed and humified in surface soil (0–15 cm). The decrease with depth likely resulted from inundation of subsurface soils and low O2 levels, which subsequently lowered rates of decomposition. The δ13C decreased with depth for cultivated soil but increased for prairie, and was significantly higher for soil cropped to sugarcane (‐25.37‰) than prairie (‐26.20‰). Soil organic matter under cultivation was less humified than prairie soil due to recent C inputs from sugarcane. The 2‐mm fraction had 12% lower δ15N than other fractions, indicating that recent organic matter inputs accumulated in macroaggregate fractions. Smaller aggregates contained higher δ15N and older organic matter. In contrast to most studies of mineral soils, cultivation of Histosols increased C storage relative to prairie, with the major difference between land use being higher soil organic matter levels in the subsurface (15–45 cm). Thus, cropping may reduce the rate of oxidation of Histosols in southern Florida relative to the prairie ecosystem.

Characterization of a Mesorhizobium loti α-Type Carbonic Anhydrase and Its Role in Symbiotic Nitrogen Fixation

Carbonic anhydrase (CA) (EC 4.2.1.1) is a widespread enzyme catalyzing the reversible hydration of CO(2) to bicarbonate, a reaction that participates in many biochemical and physiological processes. Mesorhizobium loti, the microsymbiont of the model legume Lotus japonicus, possesses on the symbiosis island a gene (msi040) encoding an alpha-type CA homologue, annotated as CAA1. In the present work, the CAA1 open reading frame from M. loti strain R7A was cloned, expressed, and biochemically characterized, and it was proven to be an active alpha-CA. The biochemical and physiological roles of the CAA1 gene in free-living and symbiotic rhizobia were examined by using an M. loti R7A disruption mutant strain. Our analysis revealed that CAA1 is expressed in both nitrogen-fixing bacteroids and free-living bacteria during growth in batch cultures, where gene expression was induced by increased medium pH. L. japonicus plants inoculated with the CAA1 mutant strain showed no differences in top-plant traits and nutritional status but consistently formed a higher number of nodules exhibiting higher fresh weight, N content, nitrogenase activity, and delta(13)C abundance. Based on these results, we propose that although CAA1 is not essential for nodule development and symbiotic nitrogen fixation, it may participate in an auxiliary mechanism that buffers the bacteroid periplasm, creating an environment favorable for NH(3) protonation, thus facilitating its diffusion and transport to the plant. In addition, changes in the nodule delta(13)C abundance suggest the recycling of at least part of the HCO(3)(-) produced by CAA1.

Linking water sources with foliar nutrient status in upland plant communities in the Everglades National Park, USA

AbstractPlant communities within an ecosystem can vary in water source usage and nutrient availabilities, thus permitting community coexistence and diversity. We investigated the differences in water source utilization in two ecologically important upland plant communities: tropical hardwood hammocks and pine rocklands in the predominantly flooded Everglades ecosystem. We then linked these differences with their foliar nutrient levels and photosynthetic performance as measured by δ13C abundance. Based on a comparison of δ18O of plant stem waters with those of potential water sources (nutrient‐poor groundwater and nutrient‐rich water in organic litter referred to as soilwater), we observed that during the wet season hammock plants relied on soilwater while in the dry season they relied on groundwater. A similar seasonal shift was observed in pineland plants, however, groundwater constituted the major part of water uptake throughout the year except for late wet season. Consistent with the nutrient concentration of different water sources used in the two communities, hammocks had a greater annual mean foliar nitrogen and phosphorus concentration at the community level over pinelands as well as a higher leaf area index. High foliar N concentration in hammock plants was associated with eventual stomatal limitation of photosynthesis. Hammock species being intolerant of flooded soils are restricted to water uptake in the shallow unsaturated soil layer in the wet season, yet access the lowered groundwater table in the dry season. This dependence on a relatively narrow annual range of water table levels should be considered in South Florida water management and Everglades restoration. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Nutrient cycling relative to δ15N and δ13C natural abundance in a coastal wetland with long-term nutrient additions

Because nitrogen and phosphorus are primary resources for plant, algal, and microbial production, increases in nutrient inputs can markedly alter aquatic ecosystems. Coastal wetland plots at Belle W. Baruch Marine Field Laboratory (South Carolina, USA) have been amended with nitrogen and phosphorus for ~20 years to determine the effects of nutrient loading on coastal wetlands. We conducted a survey of δ15N and δ13C natural abundance in coastal wetland organic pools (sediment, vegetation) with long-term nutrient amendments (control, no addition; nitrogen; phosphorus; and nitrogen + phosphorus additions). Additionally, we conducted laboratory assays to quantify pore water nutrient availability and nitrification rates. Marsh vegetation (Spartina alterniflora) had enriched δ13C values (mean −14‰) relative to bulk sediment samples (mean −18‰). Nitrogen-amended plots (alone and in combination with phosphorus) had enriched δ13C values in the surface sediment (0–5 cm; mean −16.1‰) relative to control (mean −16.5‰) and phosphorus-amended plots (mean −16.8‰). Nitrogen-amended plots also had depleted δ15N in S. alterniflora leaf tissues (−3.3‰) and surface sediment samples (mean 2.1‰) relative to leaf tissues (mean 2.1‰) or sediment samples (mean 5.8‰) from control or phosphorus-only amended plots. Nitrate availability (as increased pore water concentration) was higher in N-amended plots although ammonium availability did not differ. Phosphorus availability was higher only in phosphorus-only amended plots. Overall, we found that long-term nutrient amendments to coastal wetlands significantly altered nutrient availability and uptake rates as well as natural abundance of δ13C and δ15N in multiple organic matter sources.

Sludge‐derived organic carbon in an agricultural soil estimated by 13C abundance measurements

SummaryThe objective of this study is to develop a method to follow the dynamics of sludge‐derived organic carbon, which will allow us to understand the behaviour of trace metals in the sludge‐treated soils. We studied, in a sandy agricultural soil of southwest France, cultivated with maize and amended with sewage‐sludge over 20 years, the dynamics of different sources of organic matter and compared this with a control, which had never received any treatment. For the first time, a method is proposed that will distinguish and quantify sludge‐derived organic carbon, maize‐derived organic carbon, and native organic carbon. This method is based on the mean differences in δ13C abundances between native (−26.5‰), maize (−12.5‰) and sludge (−25.4‰) organic carbon. Three hypotheses on the dynamics of soil organic matter sources are proposed: (i) isotopic differences observed between control and sludge‐treated soils are due only to the incorporation of sludge C, whereas in the others, the control was used to model the incorporation of (ii) maize C or (iii) native C in the sludge‐treated soils. The comparison of the stocks of each source (native C, maize C and sludge C) found in the bulk soil with the sum of corresponding stocks found in particle‐size fractions allowed us to reject the two first hypotheses and to validate the last one. Repeated applications of sewage‐sludge induced accumulation of sludge‐derived organic carbon in the topsoil, and simultaneously contributed to the preservation of maize‐derived organic carbon. When sludge applications ceased, the rapid decrease in soil organic matter stocks was mostly caused by the degradation of the sludge‐derived organic carbon sources. At the same time, the maize‐derived organic carbon shifted from the coarsest fraction (200–2000 μm) to the finest fraction (0–50 μm). Therefore, this study has shown that repeated applications of sewage‐sludge induced changes in soil organic matter dynamics over time.