Abundance 13C Research Articles

Labile C dynamics reflect soil organic carbon sequestration capacity: Understory plants drive topsoil C process in subtropical forests

AbstractThe huge background pool of soil organic carbon (SOC) is likely to impede the ready detection of SOC changes. We propose to explore SOC changes by monitoring the dynamics of soil labile organic carbon (LOC); namely if LOC could be largely retained in soils rather than respired rapidly, the SOC would be ready to be sequestered. The effects of the two major functional groups of plants, that is, canopy trees and understory plants, on SOC accumulation were then illustrated with this LOC‐based approach. The characteristics of LOC and SOC of topsoils (0–20 cm) in a field manipulation experiment with 5‐yr treatments of understory removal and tree girdling in both a young and a mature Eucalyptus plantations were examined. The concentration and potential turnover time of soil LOC were used to indicate the state of vegetation‐induced C accumulation in soils, which were estimated by a sequential fumigation‐incubation procedure. Soil natural abundances of 13C and 15N were measured to reflect the proportion of newly retained LOC in soils. We found that, in the young plantation, understory removal did not significantly affect both soil LOC and SOC concentrations, but significantly increased the potential turnover time of soil LOC. In contrast, in the mature plantation, understory removal significantly decreased soil LOC and SOC concentrations, but did not significantly alter the potential turnover time of soil LOC. However, tree girdling did not significantly affect SOC concentration, soil LOC concentration, or potential turnover time in either the young plantation or the mature plantation. These results demonstrated that understory plant‐derived C was one of the major components of LOC pool in topsoils, and it may be readily mineralized in the young plantation but accumulated as an important fraction of SOC in the mature plantation. This study suggests that the LOC‐based approach is potentially useful in monitoring SOC changes and improves our understanding of how plant functional groups and soil fertility status could jointly affect LOC and SOC dynamics. In considering the great contribution of understory plants to SOC processes, we propose that understory plants should be maintained in subtropical plantation ecosystems.

High-Resolution Nuclear Magnetic Resonance Spectroscopy with Picomole Sensitivity by Hyperpolarization on a Chip.

We show that high-resolution NMR can reach picomole sensitivity for micromolar concentrations of analyte by combining parahydrogen-induced hyperpolarization (PHIP) with a high-sensitivity transmission line microdetector. The para-enriched hydrogen gas is introduced into solution by diffusion through a membrane integrated into a microfluidic chip. NMR microdetectors, operating with sample volumes of a few μL or less, benefit from a favorable scaling of mass sensitivity. However, the small volumes make it very difficult to detect species present at less than millimolar concentrations in microfluidic NMR systems. In view of overcoming this limitation, we implement PHIP on a microfluidic device with a 2.5 μL detection volume. Integrating the hydrogenation reaction into the chip minimizes polarization losses to spin-lattice relaxation, allowing the detection of picomoles of substance. This corresponds to a concentration limit of detection of better than , unprecedented at this sample volume. The stability and sensitivity of the system allow quantitative characterization of the signal dependence on flow rates and other reaction parameters and permit homo- (1H-1H) and heteronuclear (1H-13C) 2D NMR experiments at natural 13C abundance.

Carbon-use strategies in stem radial growth of two oak species, one Temperate deciduous and one Mediterranean evergreen: what can be inferred from seasonal variations in the δ13C of the current year ring?

Tree ring synthesis is a key process in wood production; however, little is known of the origin and fate of the carbon involved. We used natural 13C abundance to investigate the carbon-use process for the ring development in a temperate deciduous (Quercus petraea (Matt.) Liebl.) and a Mediterranean evergreen (Quercus ilex L.) oak. The sapwood carbon reserves, phloem sucrose contents, stem respired CO2 efflux and their respective carbon isotope compositions (δ13C) were recorded over 1 year, in the native area of each species. The seasonal δ13C variation of the current year ring was determined in the total ring throughout the seasons, as well as in slices from the fully mature ring after the growth season (intra-ring pattern). Although the budburst dates of the two oaks were similar, the growth of Quercus ilex began 50days later. Both species exhibited growth cessation during the hot and dry summer but only Q. ilex resumed in the autumn. In the deciduous oak, xylem starch storage showed clear variations during the radial growth. The intra-ring δ13C variations of the two species exhibited similar ranges, but contrasting patterns, with an early increase for Q. petraea. Comparison between δ13C of starch and total ring suggested that Q. petraea (but not Q. ilex) builds its rings using reserves during the first month of growth. Shifts in ring and soluble sugars δ13C suggested an interspecific difference in either the phloem unloading or the use of fresh assimilate inside the ring. A decrease in ring δ13C for both oaks between the end of the radial growth and the winter is attributed to a lignification of ring cell walls after stem increment. This study highlighted the differences in carbon-use during ring growth for evergreen and deciduous oaks, as well as the benefits of exploring the process using natural 13C abundance.

Methodological clarification for estimating the input of plant-derived carbon in soils under elevated CO2 based on a 13C-enriched CO2 labeling experiment

BACKGROUND AND AIMS: Under the scenario of global change, continuous ¹³C-enriched CO₂ labeling is a powerful tool for evaluating the interaction between plants and soil, especially the influence of elevated CO₂ on the input of plant-derived C (new C) into soil in the short term. However, the methodological validity concerning the acquisition of isotopic signals and their implications in plants and soil remains ambiguous. METHODS: We conducted an experiment simulating elevated CO₂ with wheat planted in growth chambers. ¹³C-enriched CO₂ with identical ¹³C abundance was supplied at two CO₂ concentration levels (350 versus 600 ppm) until wheat harvest. The δ¹³C values of plant tissues and soil were measured periodically during the growing season. RESULTS: The δ¹³C values of wheat tissues under elevated ¹³CO₂ were 15–17% higher than those under ambient ¹³CO₂ after the heading stage, implying that the ¹³C signals themselves in plants and soil were not directly representative of the influence of elevated CO₂ on the C fixation in the plant and thereby the flow of plant-derived C into soil. The proportion of plant-derived C in the soil (fₙₑw) was calculated separately at each CO₂ concentration by taking the initial soil as a reference and the corresponding δ¹³C values as parameters in the mixing model, and we found that elevated CO₂ significantly enhanced the new soil C input by approximately 35.5% during our 62-day labeling. In our experiment, the fₙₑw value under elevated CO₂ could be overestimated by up to 1.7 times if the alteration of ¹³C signatures in photosynthates caused by the change in CO₂ concentration is ignored. CONCLUSIONS: For methodological clarification, we suggest that 1) ¹³C labeling is essential in all CO₂ treatments to achieve CO₂ concentration-dependent ¹³C signals in plants and soil, and that 2) the influence of elevated CO₂ on soil C turnover can be estimated by the difference in the fₙₑw under the two CO₂ concentrations.

Natural 13C and 15N abundance of moss-substrate systems on limestones and sandstones in a karst area of subtropical China

Epilithic mosses (EM) and organic substrates (OS) constitute a special ecosystem developing on rocks and a critical interface regulating rock-atmosphere interactions and biological weathering. Carbon (C) and nitrogen (N) are critical elements influencing biotic-abiotic interactions in EM-OS systems. However, the dynamics of C and N in EM-OS systems and their differences among rock types remain unclear. This study investigated contents ([C], [N]) of C and N and their isotope compositions (δ13C, δ15N) in EM and OS layers on limestones and sandstones in Huanjiang karst observatory of Guangxi, China. Significant higher [C]EM (by ca. 5.0%) and lower δ13CEM values (by ca. 3.0‰) on sandstones than on limestones might reflect a lower water use efficiency in mosses on sandstones. Differences in δ13C values between EM and OS correlated positively with those in C contents, elucidating increasing 13C discriminations with the C decomposition in substrates. The δ15NEM and δ15NOS values were correlated positively, indicating mosses as a major contributor of organic N in OS. The [N]EM did not differ between two rock types, but significant lower [N]OS and higher δ15NOS values (by ca. 0.2% and 1.5‰, respectively) occurred on limestones than on sandstones, suggesting larger N losses and 15N enrichments in OS on limestones. These results underscore that elemental and isotopic signatures can elucidate different C and N dynamics in the EM-OS ecosystems as a function of rock types.

Applications of stable carbon isotopes in soil science with special attention to natural 13C abundance approach

Since the invention of the isotope ratio mass spectrometer, isotope analysis has shed light on many key processes in the Earth’s ecosystems. Stable isotope analysis was first applied in the field of chemistry and geochemistry, while the use of isotopic fractionation for various biochemical reactions was elaborated later. The knowledge gained from isotope research led to a better understanding of the dynamics of the biosphere and to the more efficient study of interactions between the geosphere and biosphere. In soil research, stable isotopes are ideally suited to provide a wider insight into the element cycles in soil ecosystems. Stable carbon isotopes, in particular, have been in the focus of soil research, since soil organic matter (SOM) plays an important role not only in soil fertility, soil water management and many other physical, chemical and biological soil functions, but also in the global carbon cycle. If processes connected with these soil functions are isotopically labelled with stable carbon isotopes, the key reactions of C input, exchange and output in the soil and other soil organic matter functions can be studied accurately. Moreover, analysing the isotopic composition of CO2 exchange between the soil and the atmosphere helps to predict ecosystem responses to global changes.

Ten-year application of cattle manure contributes to the build-up of soil organic matter in eroded Mollisols

Amendment of animal manures into eroded soils is an important approach to improving nutrient status and increasing the concentration of soil organic carbon (SOC). However, the contribution of the manure carbon to SOC and its variation along soil profile has not been quantified. We simulated soil erosion in a mollisol by removing the top soils of 0-, 5-, 10-, 20-, and 30-cm depth and compared SOC in soil profiles 10 years after either chemical fertilization alone or combined with cattle manure application. Increasing erosion depth decreased SOC concentration and weakened soil aggregation. Compared to the chemical fertilization only, the addition of cattle manure significantly increased SOC accumulation and soil aggregation, which mainly occurred in 0–40-cm depths. The greatest effect of manure application was observed in the 10-cm erosion treatment. The application of cattle manure increased the 13C abundance in aggregates and bulk soil in the top 40 cm of soil profile. Using the natural 13C abundance method, we quantified the contribution of the cattle manure to SOC at 0–40-cm depths ranging from 1.1 to 8.4% across erosion treatments. The greatest contribution of the manure-C to SOC occurred in surface layer with 10 cm of soil removal. The application of animal manures was recommended for restoring severely eroded soils.

Mechanism of 6-Hydroxynicotinate 3-Monooxygenase, a Flavin-Dependent Decarboxylative Hydroxylase Involved in Bacterial Nicotinic Acid Degradation.

6-Hydroxynicotinate 3-monooxygenase (NicC) is a Group A FAD-dependent monooxygenase that catalyzes the decarboxylative hydroxylation of 6-hydroxynicotinic acid (6-HNA) to 2,5-dihydroxypyridine (2,5-DHP) with concomitant oxidation of NADH in nicotinic acid degradation by aerobic bacteria. Two mechanisms for the decarboxylative hydroxylation half-reaction have been proposed [Hicks, K., et al. (2016) Biochemistry 55, 3432-3446]. Results with Bordetella bronchiseptica RB50 NicC here show that a homocyclic analogue of 6-HNA, 4-hydroxybenzoic acid (4-HBA), is decarboxylated and hydroxylated by NicC with a 420-fold lower catalytic efficiency than is 6-HNA. The 13( V/ K), measured with wild-type NicC by isotope ratio mass spectrometry following the natural abundance of 13C in the CO2 product, is inverse for both 6-HNA (0.9989 ± 0.0002) and 4-HBA (0.9942 ± 0.0004) and becomes negligible (0.9999 ± 0.0004) for 5-chloro-6-HNA, an analogue that is 10-fold more catalytically efficient than 6-HNA. Covalently bound 6-HNA complexes of NicC are not observed by mass spectrometry. Comparative steady-state kinetic and Kd6HNA analyses of active site NicC variants (C202A, H211A, H302A, H47E, Y215F, and Y225F) identify Tyr215 and His47 as critical determinants both of 6-HNA binding ( KdY215F/ KdWT > 240; KdH47E/ KdWT > 350) and in coupling rates of 2,5-DHP and NAD+ product formation ([2,5-DHP]/[NAD+] = 1.00 (WT), 0.005 (Y215F), and 0.07 (H47E)]. Results of these functional analyses are in accord with an electrophilic aromatic substitution reaction mechanism in which His47-Tyr215 may serve as the general base to catalyze substrate hydroxylation and refine the structural model for substrate binding by NicC.

Identification of dominant sulfamethoxazole-degraders in pig farm-impacted soil by DNA and protein stable isotope probing

Increasing introduction of antibiotic residues from humans and animal farming into the environment impacts the functioning of natural ecosystems and significantly contributes to the propagation of antibiotic resistance. Microbial degradation is the major sink of antibiotics in soil but the identification of in situ degrading populations is challenging. Here, we investigated sulfamethoxazole-degrading bacteria in soil microcosms by culture-independent DNA and protein stable isotope probing. 0.5% of the carbon from 13C6-labeled sulfamethoxazole amended to soil microcosms was transformed to 13CO2 demonstrating partial mineralization of the antibiotic. DNA stable isotope probing revealed incorporation of 13C from 13C6-labeled sulfamethoxazole into Actinobacteria and among them into the families Intrasporangiaceae, Nocardioidaceae, and Gaiellaceae and the order Solirubrobacterales. Protein stable isotope probing demonstrated the incorporation of 13C from 13C6-labeled sulfamethoxazole into proteins of bacteria of the families Intrasporangiaceae, Nocardioidaceae and the order Solirubrobacterales, which is consistent with the results of DNA stable isotope probing. The 13C abundance of 60 to 80% in several taxonomically relevant proteins indicated that Intrasporangiaceae directly acquired carbon from 13C6-labeled sulfamethoxazole. The results highlight the crucial role of yet-uncultivated indigenous bacteria for antibiotics degradation, and the potential of cultivation-independent stable isotope based molecular approaches to elucidate the structure of antibiotic-degrading populations in complex microbial communities under natural conditions.

Targeted expression and purification of fluorine labelled cold shock protein B by using an auxotrophic strategy

High resolution NMR spectroscopy is a seminal method in modern structural biology to obtain insights into proteins' structure, dynamics and function at dilute condition as well as in a cell-like environment or even intracellularly. Usually, 1H, 15N or 13C nuclei are predominantly used for the characterization of the protein of interest. These measurements are limited due to the wealth of chemical shifts and background signals arising from all molecules present in the NMR test tube. On top of that, the protein under study has to be isotopically enriched in nitrogen and/or carbon nuclei enabling to overcome the inherently low natural abundance of 13C and 15N NMR active isotopes. In this way switching to 19F NMR spectroscopy strongly reduces the total amount of signals seen in an NMR spectrum as it turns off background signals and is for this reason extremely attractive for highly-resolved investigations of proteins performance measured directly in cells or in a cell-like environment.Here we show the effective expression and purification of cold shock protein B from Bacillus subtilis (BsCspB) using fluorine labelled phenylalanine or fluorine labelled tryptophan residues. We reveal that fluorine labelled BsCspB represents the same fold on a secondary as tertiary level as seen for the wild type protein independent of the labelling position illuminating the soft character of fluorine insertion. This experimental setup of targeted fluorine labelling sets a profound ground for a broad range of highly-resolved 19F NMR applications to be performed in a complex cellular environment.

Functional composition has stronger impact than species richness on carbon gain and allocation in experimental grasslands.

Numerous experiments have shown positive diversity effects on plant productivity, but little is known about related processes of carbon gain and allocation. We investigated these processes in a controlled environment (Montpellier European Ecotron) applying a continuous 13CO2 label for three weeks to 12 soil-vegetation monoliths originating from a grassland biodiversity experiment (Jena Experiment) and representing two diversity levels (4 and 16 sown species). Plant species richness did not affect community- and species-level 13C abundances neither in total biomass nor in non-structural carbohydrates (NSC). Community-level 13C excess tended to be higher in the 16-species than in the 4-species mixtures. Community-level 13C excess was positively related to canopy leaf nitrogen (N), i.e. leaf N per unit soil surface. At the species level, shoot 13C abundances varied among plant functional groups and were larger in legumes and tall herbs than in grasses and small herbs, and correlated positively with traits as leaf N concentrations, stomatal conductance and shoot height. The 13C abundances in NSC were larger in transport sugars (sucrose, raffinose-family oligosaccharides) than in free glucose, fructose and compounds of the storage pool (starch) suggesting that newly assimilated carbon is to a small portion allocated to storage. Our results emphasize that the functional composition of communities is key in explaining carbon assimilation in grasslands.

Aromatic SOFAST-HMBC for proteins at natural 13C abundance.

We propose here SOFAST-HMBC as a new complementary NMR tool for aromatic side chain assignment of protein samples at natural 13C abundance. The characteristic peak patterns detected in SOFAST-HMBC for each aromatic side chain allow straightforward assignment of all protons and carbons (including quaternary ones) of the aromatic ring, and for tyrosine and phenylalanine, connection to the CB of the aliphatic chain. The performance of SOFAST-HMBC is demonstrated for three small proteins (7-14 kDa) at millimolar sample concentration using modern high-field NMR instruments equipped with cryogenically cooled probes. Despite the low amount of NMR-active 13C nuclei in these samples, 1H-13C multiple-bond correlation spectra of good quality were obtained in reasonable experimental times of typically less than 24 h.

Isotopic ratios of Saturn's rings and satellites: Implications for the origin of water and Phoebe

Isotopic ratios have long been used to learn about physical processes acting over a wide range of geological environments, and in constraining the origin and/or evolution of planetary bodies. We report the spectroscopic detection of deuterium in Saturn's rings and satellites, and use these measurements to determine the (D/H) ratios in their near-surface regions. Saturn's moons, Phoebe and Iapetus, show a strong signature of CO2 and the 13C component of this molecule is detected and quantified. Large averages of spectra obtained by the Cassini Visual and Infrared Mapping Spectrometer, VIMS, were computed for the rings and icy satellites. The observed intensities of the infrared absorptions in H2O and CO2 and their isotopes were calibrated using laboratory data and radiative transfer models to derive the D/H and 13C/12C ratios. We find that the D/H in Saturn's rings and satellites is close to the Vienna Standard Mean Ocean Water (VSMOW) and bulk Earth (4% lower than VSMOW) value except for Phoebe, which is 8.3 times the VSMOW value. This is the highest value for any Solar-System surface yet measured, and suggests that Phoebe formed from material with a different D/H ratio than the other satellites in the Saturn system. Phoebe's 13C/12C ratio is also unusual: 4.7 times greater than terrestrial, and greater than values measured for the interstellar medium and the galactic center. The high 13C abundance in the CO2 suggests that Phoebe was never warm enough for the large D/H ratio in its surface to have originated by evaporative fractionation of its water ice (e.g., from heating in the inner Solar System before its eventual capture by Saturn). We also report the detection of a probable O-D stretch absorption due to OD in minerals on Phoebe at 3.62 μm. This absorption is not detected on other Saturnian satellites. Stronger signatures of bound water absorptions are found in the dark material of Iapetus and we report a new detection of bound water at 1.9 μm. The position of this absorption matches that seen in spectra of hydrated iron oxides but does not match absorptions seen in spectra of tholins. Despite the strong bound water signature in the Iapetus dark material, no 3.62 μm OD absorption is seen in the spectra, further indicating the high deuterium level on Phoebe is unusual. As such, it is likely that Phoebe originated in a colder part of the outer Solar System, relative to the prevailing temperatures at Saturn's distance from the Sun.

Natural 13C abundance reveals age of dietary carbon sources in nematode trophic groups

We determined the 13C/12C ratio (expressed as δ13C ‰) of microbial biomass and nematode trophic groups in a small-plot field experiment with soil converted from C3- to C4-crop (silage maize) 20 years ago. During this period, the plots were subjected to three different organic input treatments: 1) maize stubbles and roots left after harvest (MS), 2) MS plus annual addition of aboveground maize biomass (MS + B), and 3) MS plus annual addition of faeces from sheep fed exclusively with maize (MS + F). The different δ13C value of C3- and C4-crops allowed us to distinguish between old (>20 years old) C3-derived C and recent (<20 years old) C4-derived C incorporated into microbial biomass and nematodes.The δ13C value of phytophagous nematodes closely matched that of the maize. Bacterivorous nematodes had higher δ13C values than fungivorous nematodes and microbial biomass indicating that the C sources of bacterivorous nematodes are more recent than those of fungivorous nematodes and microbial biomass. At low abundance of fungivorous nematodes (MS and MS + F), the microbial biomass had higher δ13C values than the fungivorous nematodes, whereas their δ13C values were comparable at higher densities of fungivorous nematodes (MS + B). The higher C4-derived input in MS + F and MS + B treatments increased the δ13C values of bacterivorous nematodes and microbial biomass.In MS and MS + B treatments, recent C4-derived C accounted for 50 and 70% of microbial biomass-C, respectively. Corresponding values for fungivorous and bacterivorous nematodes were 30 and 75%, and 65 and 85%, respectively.We conclude that fungal-based decomposition pathways contribute more to the turnover of old soil C than bacterial-based decomposition. A substantial fraction of the microbial biomass and fungivorous nematode C in the MS treatment (50 and 70%, respectively) was C deposited in the soil more than 20 years ago, confirming that decade-old SOC remains biologically active.

Mineralization of plant residues and native soil carbon as affected by soil fertility and residue type

Crop residue return is an effective and low-cost agricultural approach for soil organic carbon (SOC) sequestration. Yet, it is largely unknown to what extent the soil fertility and residue type affect the mineralization of maize (Zea mays L.) residue carbon (C) and the decomposition of native SOC. Therefore, a better understanding of the mineralization of C derived from residues and its priming on native SOC is crucial to accurate assessment of the benefits of crop residue returning in agricultural systems. A 360-day laboratory incubation experiment was carried out with a Cambisol of low and high fertilities amended with three types of 13C-labeled maize residues (root, stem, leaf). The abundance of 13C (δ13C) in the soil samples was measured during different incubation stages. The results showed that the total mineralization of residue C was significantly higher in the low fertility soil than in the high fertility soil, but there were no significant differences among residue types. For the high fertility soil, all the residue types induced a negative priming on native SOC mineralization during the early incubation stage, but a significant total positive priming by the end of incubation, whereas for the low fertility soil, there was no significant effect of residue return on SOC mineralization. The accumulated priming by the end of incubation did not vary across residue types. Moreover, the sum of mineralization of residue C and native SOC in the high fertility soil was 1.4 times as large as that in the low fertility soil. We conclude that mineralization of crop residue C and native SOC is affected by soil fertility rather than residue type.

Determining age of high-explosive to support nuclear warhead dismantlement verification

The future international nuclear disarmament may involve the dismantlement of nuclear warheads. After the dismantlement of a nuclear warhead, the separated explosive needs authentication that it comes from the dismantled nuclear warhead. In this paper, the Monte Carlo numerical simulation method was used to study the feasibility and result of determining the source of the explosive by analyzing the nuclide abundances of the explosive and determining the age of the explosive (calculated since the explosive was placed in the nuclear warhead). First, the JMCT software was used to analyze the nuclei produced by the neutron reactions in the explosive of the nuclear warhead. As a result, it was found that 14C is the most promising to be used to determine the source of the explosive. Second, the relationships between the average abundance of 14C and the age of the explosive, the spatial distribution of the 14C abundance in the explosive were calculated by using the JMCT software. Finally, it is found that, compared to the WgPu warheads (nuclear warheads with weapons-grade plutonium cores), the 14C abundances of the explosives of the WgU warheads (nuclear warheads with weapons-grade uranium cores) are much lower (the ages of the explosives are the same), and it is more difficult to measure the latter; for the WgPu warheads with the structures based on Model 3 and Model 4 (the warhead models employed were proposed by Steve Fetter), it can be determined that whether the separated explosives come from the dismantled nuclear warheads or are common ones (or the fake ones prepared by the verified party) after the dismantlement of the nuclear warheads by measuring the 14C abundances in the explosives (including the average abundances and the spatial distributions of the abundances) and determining the ages of the explosives, which realizes the source authentication of the explosives.

Changes in soil carbon stocks after land-use change from native vegetation to pastures in the Atlantic forest region of Brazil

Millions of hectares (ha) of the Atlantic forest of Brazil have been deforested and replaced by pastures, quite a large proportion of this in the last 60 years. There have been few studies on the impact of this land-use change on stocks of soil organic matter (SOM) only one study reported the state of vigour of the pastures. The aim of this study was to estimate the overall change in SOM stocks 16 years after the removal of forest vegetation in this biome in southern Bahia and the installation of pastures of Brachiaria brizantha fertilized with N and maintained under controlled grazing. Soil samples were taken for evaluation of density and texture and for analyses of C and N total and 13C abundance to a depth of 100 cm at 100 m intervals along four transects of 400 m from the pastures into the forest. Grazing was found not to have any significant effect on soil density (compaction). The live weight gain of the Nellore cattle on both cultivars of B. brizantha, fertilized with 120 kg N ha−1 yr−1 as urea during 12 years, was close to 500 kg ha−1 yr−1. The gain in soil C was similar under the two grass cultivars, being approximately 15 Mg C ha−1 to a depth of 30 cm and 20 Mg C ha−1 to 100 cm. The 13C abundance data showed that the large gain in soil C was due to the slow decomposition of the forest-derived C (total loss 12.6 Mg C ha−1 over 16 years) and the large accumulation of C derived from the Brachiaria (total gain 43.2 Mg C ha−1). These results confirm the potential of productive Brachiaria pastures to accumulate soil C in a tropical climate with year-round rainfall.

Biomass and fossil fuel combustion contributions to elemental carbon across the San Francisco Bay Area

Ambient black carbon (BC) has detrimental effects on both human health and near-term global warming. To mitigate these negative effects, there have been significant efforts to reduce emissions of BC from anthropogenic and biomass burning sources in California's Bay Area since the 1960s. Recent reductions in BC have mainly been from fossil fuel combustion sources such as diesel but additional reductions may be needed for contemporary carbon sources like biomass burning and meat cooking. In this study, PM10 (particulate matter with aerodynamic diameter of less than or equal to 10 μm) samples were collected at seven sites across the San Francisco Bay Area from November 2011 to October 2012 to create winter and non-winter composites for each site. Radiocarbon (14C) abundance and chemical mass balance (CMB) modeling were used for source apportionment of ambient elemental carbon (EC, a proxy for BC). The 14C abundance in the EC fraction was used to quantify the relative contributions of fossil carbon and contemporary carbon sources. The average biomass burning contributions are 48 ± 8% and 41 ± 5% for winter and non-winter seasons, respectively, across the Bay Area. Ambient concentrations of EC are approximately two to three times higher during the winter compared to the non-winter season, except for Cupertino. A CMB model, using bulk aerosol composition and select inorganic compounds, was used to understand the contributions of natural gas combustion, gasoline exhaust, and diesel exhaust to fossil carbon as well as the contribution of biomass burning and meat cooking to contemporary carbon. The different apportionment methods for EC (14C and CMB analysis) agree within 16 ± 12% for fraction fossil and biomass burning for both winter and non-winter seasons. The contemporary contribution to EC is much higher than estimated by current emission inventories, suggesting that the inventories may overestimate the contribution from fossil fuels, particularly diesel exhaust. The results from this study are compared to literature values from other 14C-EC or BC studies from across the world.

Variation of soil anaerobic microorganisms connected with anammox processes by 13C-phospholipid fatty acid analysis among long-term fertilization regimes in a crop rotation system

An anaerobic incubation experiment was conducted to evaluate the impacts of different fertilizer treatments on the activity and diversity of anaerobic bacteria combined anammox process. The treatments included no fertilization (CK), inorganic fertilization (IF), pig manure combined with inorganic fertilization (PF), and straw combined inorganic fertilization (SF) during wheat and rice seasons. Various 13C-PLFAs, especially 14:0, 16:1ω9c, 16:1ω5c, 16:0, 18:1ω9c, and 18:0, played significant roles in the anammox reaction and could be biomarkers of anammox. Additionally, fertilizer practices and seasonal changes could significantly affect soil properties, microbial biomass, and bacterial community structure. The abundance of total 13C and anaerobic microbes was higher in the PF and SF treatments than in the IF and CK treatments in the same season, indicating anaerobic activity and abundance can be enhanced by applying organic–inorganic compound fertilizers. The anammox microbial abundance, marked by the 13C-PLFAs 14:0, 16:1ω9c, 16:1ω5c, 16:0, 18:1ω9c, and 18:0, increased significantly during the crop rotation from the wheat season to the rice season. The proportions of bacteria were higher than those of fungi however the proportions of fungi were much higher than those of actinomycetes in all soils, suggest that fungi also play an important role in metabolizing the 13C-matrix with strong anaerobic activity.

Light intensity-regulated stomatal development in three generations of Lepidium sativum

Stomata are of crucial importance for CO2 uptake and the water economy of terrestrial plants. They evolve during the early phases of leaf development according to genetically conserved information modulated in its expression by environmental conditions. Mature leaves experience the environment, e.g. light intensity, and signal to younger developing leaves, modifying stomatal density (SD) and probably other leaf traits as well. In addition, the parental environment can affect the offspring’s phenotypes in an adaptive manner. However, the transgenerational effect of light on SD and interactions with the offspring’s light environment have not been studied so far. Here we compare transgenerational (parental) and growth-light (environmental) effects on SD. We grew three generations of garden cress (Lepidium sativum) in two contrasting light environments in a full factorial design. Stomatal and pavement cell densities (SD, PCD), 13C abundance as a proxy of leaf internal CO2 concentration (Ci), and leaf area were analysed in cotyledons and first true leaves of parental plants and two generations of offspring. Our results indicate that SD of offspring reflects both current and maternal light intensities. The transgenerational effect of light on SD was much smaller than the influence of current growth light and the former’s manifestation was dependent on light experienced by offspring. The cotyledons’ SD was unresponsive to variations in Ci but retained its light sensitivity, though it was smaller than in true leaves. High light reduced amphistomy in favour of the lower (abaxial) leaf side. SD and PCD were linearly proportional irrespective of light level, indicating invariant portions of spacing and amplifying cell divisions, and causing lower sensitivity of stomatal index at high SD. We show that light-stimulated stomatal development in garden cress differs in cotyledons and true leaves, varies between adaxial and abaxial leaf sides, and retains transgenerational information about environmental clues.