Natural Abundance 13C Research Articles

Reduced turnover rate of topsoil organic carbon in old-growth forests: a case study in subtropical China

BackgroundOld-growth forests are irreplaceable with respect to climate change mitigation and have considerable carbon (C) sink potential in soils. However, the relationship between the soil organic carbon (SOC) turnover rate and forest development is poorly understood, which hinders our ability to assess the C sequestration capacity of soil in old-growth forests.MethodsIn this study, we evaluated the SOC turnover rate by calculating the isotopic enrichment factor β (defined as the slope of the regression between 13C natural abundance and log-transformed C concentrations) along 0–30 cm soil profiles in three successional forests in subtropical China. A lower β (steeper slope) is associated with a higher turnover rate. The three forests were a 60-year-old P. massoniana forest (PF), a 100-year-old coniferous and broadleaved mixed forest (MF), and a 400-year-old monsoon evergreen broadleaved forest (BF). We also analyzed the soil physicochemical properties in these forests to examine the dynamics of SOC turnover during forest succession and the main regulators.ResultsThe β value for the upper 30-cm soils in the BF was significantly (p < 0.05) higher than that in the PF, in addition to the SOC stock, although there were nonsignificant differences between the BF and MF. The β value was significantly (p < 0.05) positively correlated with the soil recalcitrance index, total nitrogen, and available nitrogen contents but was significantly (p < 0.01) negatively correlated with soil pH.ConclusionsOur results demonstrate that SOC has lower turnover rates in old-growth forests, accompanied by higher soil chemical recalcitrance, nitrogen status, and lower soil pH. This finding helps to elucidate the mechanism underlying C sequestration in old-growth forest soils, and emphasizes the important value of old-growth forests among global C sinks.

Efficient polynomial analysis of magic-angle spinning sidebands and application to order parameter determination in anisotropic samples.

Chemical shift tensors in C solid-state NMR provide valuable localized information on the chemical bonding environment in organic matter, and deviations from isotropic static-limit powder line shapes sensitively encode dynamic-averaging or orientation effects. Studies in C natural abundance require magic-angle spinning (MAS), where the analysis must thus focus on spinning sidebands. We propose an alternative fitting procedure for spinning sidebands based upon a polynomial expansion that is more efficient than the common numerical solution of the powder average. The approach plays out its advantages in the determination of CST (chemical-shift tensor) principal values from spinning-sideband intensities and order parameters in non-isotropic samples, which is here illustrated with the example of stretched glassy polycarbonate.

Profile distribution of soil organic carbon and its isotopic value following long term land-use changes

Soil organic carbon (SOC) content and its 13C natural abundance along soil profile are sensitive to the changes of land use since these changes alter external carbon (C) inputs and belowground C turnover processes. A study was conducted to investigate how the conversion of cropland to forest, grassland or bare land (no vegetation) affect the distribution of SOC and its 13C values in 200-cm soil profiles. After 28 years of land conversion, the cropland that was remained as cropland (treated as control) had a significantly higher δ13C value than the bare land, grassland or forest at the top 10 cm soil depth. Regardless of the land use type, the mean soil 13C values in the subsurface soil layers depleted and δ13C signature decreased by 1%−5% compared with the top 10 cm soil layer, while the corresponded SOC content decreased by 3.5% to 91.2% as soil depth went down deeper. The SOC stock in the 0–200 cm profile was 16% and 12% higher in the grassland and forest, respectively, than in the control. The conservation of cropland to grassland and forest largely increased the SOC stock in the 20–40 cm soil layer compared with the control. The newly formed SOC was higher in the grassland than in the forest, and accounted for 15.4% and 9.7% of the SOC stock in the soil depth of 0–60 cm in the grassland and forest, respectively. Soil δ13C values and SOC contents were negatively correlated with pH and clay content, while they were positively correlated with nitrogen and C/N ratio along the soil profile. We infer that the conversion of cropland to forest or grassland is a beneficial practice for SOC storage, especially in the 0–60 cm depth. Such conversion has little impact on the SOC turnovers in deep soil layers during the study period.

FLEXIBLE SOIL MICROBIAL CARBON METABOLISM ACROSS AN ASIAN ELEVATION GRADIENT

ABSTRACTThe function and change of global soil carbon (C) reserves in natural ecosystems are key regulators of future carbon-climate coupling. Microbes play a critical role in soil carbon cycling and yet there is poor understanding of their roles and C metabolism flexibility in many ecosystems. We wanted to determine whether vegetation type and climate zone influence soil microbial community composition (fungi and bacteria) and carbon resource preference. We used a biomarker (phospholipid fatty acids, PLFAs), natural abundance 13C and 14C probing approach to measure soil microbial composition and C resource use, along a 1900–4167-m elevation gradient on Mount Gongga (7556 m asl), China. Mount Gongga has a vertical mean annual temperature gradient of 1.2–10.1°C and a diversity of typical vegetation zones in the Tibetan Plateau. Soils were sampled at 10 locations along the gradient capturing distinct vegetation types and climate zones from lowland subtropical-forest to alpine-meadow. PLFA results showed that microbial communities were composed of 2.1–51.7% bacteria and 2.0–23.2% fungi across the elevation gradient. Microbial biomass was higher and the ratio of soil fungi to bacteria (F/B) was lower in forest soils compared to meadow soils. δ13C varied between −33‰ to −17‰ with C3 plant carbon sources dominant across the gradient. Soil organic carbon (SOC) turnover did not vary among three soils we measured from three forest types (i.e., evergreen broadleaved subtropical, mixed temperate, coniferous alpine) and dissolved organic carbon (DOC) turnover decreased with soil elevation. Forest soil microbial PLFA 14C and δ13C measurements showed that forest type and climate were related to different microbial C use. The 14C values of microbial PLFAs i15, a15, 16:1, br17 decreased with elevation while those of C16:0, cyC17, and cyC19 did not show much difference among three forest ecosystems. Bacteria and bacillus represented by C16:1 and brC17 showed considerable microbial C metabolism flexibility and tended to use ancient carbon at higher altitudes. Anaerobes represented by cyC17 and cyC19 showed stronger C metabolism selectivity. Our findings reveal specific C source differences between and within soil microbial groups along elevation gradients.

Formula omitted]-particle induced yield of 6.13 MeV [formula omitted]-rays in carbon

Results from a preliminary accelerator-based experiment conducted at the Harwell Tandem Van de Graaff generator to estimate the thick target integrated over angle yield of 6.129 MeV γ-rays from α-particles in the MeV range stopping in natural carbon are briefly outlined. The results should be useful in planning future detailed experiments to generate data for applications and also to benchmark thin target cross section evaluations. The estimated yields referred to a natural 13C abundance of 1.09 atom % at 5.597, 5.801, and 5.999 MeV in units of 6.129 MeV γ-rays per 108α-particles are: 1.04, 1.62 and 3.31, respectively. An indicative overall relative uncertainty at the 68 % confidence level is estimated to be about 6%, although we caution that, being undertaken only as a feasibility study, the checks and balances we would usually conduct were not performed.

Measurement of lipogenic flux by deuterium resolved mass spectrometry

De novo lipogenesis (DNL) is disrupted in a wide range of human disease. Thus, quantification of DNL may provide insight into mechanisms and guide interventions if it can be performed rapidly and noninvasively. DNL flux is commonly measured by 2H incorporation into fatty acids following deuterated water (2H2O) administration. However, the sensitivity of this approach is limited by the natural abundance of 13C, which masks detection of 2H by mass spectrometry. Here we report that high-resolution Orbitrap gas-chromatography mass-spectrometry resolves 2H and 13C fatty acid mass isotopomers, allowing DNL to be quantified using lower 2H2O doses and shorter experimental periods than previously possible. Serial measurements over 24-hrs in mice detects the nocturnal activation of DNL and matches a 3H-water method in mice with genetic activation of DNL. Most importantly, DNL is detected in overnight-fasted humans in less than an hour and is responsive to feeding during a 4-h study. Thus, 2H specific MS provides the ability to study DNL in settings that are currently impractical.

Skin hydration as a tool to control the distribution and molecular effects of intermediate polarity compounds in intact stratum corneum

The barrier function of the skin is mainly assured by its outermost layer, stratum corneum (SC), which consists of dead keratin-filled cells embedded in a lipid matrix. The skin is daily exposed to an environment with changing conditions in terms of hydration and different chemicals. Here we investigate how a molecule that has reasonable solubility in both hydrophobic and hydrophilic environments can be directed to certain regions in SC by changing the skin hydration. We use 1,2,3-trimethoxy propane (TMP) as a model substance and solid-state NMR on natural abundance 13C to obtain atomically resolved information on the molecular dynamics of TMP as well as SC lipid and protein components at varying hydration conditions. Upon dehydration, TMP redistributes from the hydrophilic corneocytes to the hydrophobic SC lipid regions. In this way, TMP can act to prevent the fluid–solid lipid transition in drying conditions and be present in the corneocytes in more humid conditions. Hydration can thereby be used as a switch to control the location and action of TMP or similar compounds in complex materials like SC. The general principles described here can also have impact on other applications including lipid-based formulations in food, drug delivery and cosmetics.

Natural 13C Abundance and Chemical Structure of Organic Matter of Haplic Chernozem under Contrasting Land Uses

Natural 13C Abundance and Chemical Structure of Organic Matter of Haplic Chernozem under Contrasting Land Uses

Dynamics of new- and old- soil organic carbon and nitrogen following afforestation of abandoned cropland along soil clay gradient

Disentangling the changes of newly input and original organic carbon (OC) and nitrogen (N) after agricultural land-use transition is crucial for understanding the mechanisms behind soil OC and N dynamics in terrestrial ecosystems. However, these dynamics have received little attention so far due to the lack of an approach separating them. In this study, we determined the natural abundance of 13C and 15N in soils from afforested forestlands and adjacent croplands, the forestlands were established at different times (10, 20 and 30 years) at four sites with different soil textures across the Loess Plateau of China. The objective was to explore the changes of new- and old- OC and N along with soil clay gradient, and to examine the proportions of these changes in total OC and N along with the clay content. We showed that the conversion of cropland to forestland increased total- and new- OC and N stocks by 0.69 kg m−2 and 0.10 kg m−2, but decreased old OC and N stocks by −0.20 kg m−2 and −0.04 kg m−2, respectively. When averaged across afforestation age, the proportions of new OC and N increased, but the proportions of old OC and N decreased with the clay content. In the soils that have a higher clay content, the accumulation of N rates were greater than that of OC in the new stock, and the loss rates of N were higher than that of OC in the old stock after afforestation. In contrast, in the soils that have a lower clay content, the accumulation rates of N were lower than that of OC in the new stock, and the loss rates of N were smaller than the loss of OC in the old stock. These findings indicated that soil texture regulates the changes of OC and N in new and old soil organic matter pools following cropland afforestation.

Conversion of the LLNL/CAMS 1 MV biomedical AMS system to a semi-automated natural abundance 14C spectrometer: system optimization and performance evaluation

The Lawrence Livermore National Laboratory – Center for Accelerator Mass Spectrometry compact 1 MV biomedical accelerator mass spectrometer was repurposed and optimized for the semi-automated radiocarbon measurement of natural abundance environmental samples. Here we present results from 15 months of routine system operation and evaluate the system performance based on 30 sample wheels measured with consistent operating conditions over 7 months from August 2019 to March 2020. The average measurement precision was 2.7 ± 0.7‰. System accuracy was assessed with measurements of 14C-free materials, standard materials with 14C-content spanning 0.5 to 1.5 F14C, and by comparison to split samples measured with the 10 MV AMS system. Overall, this system can routinely measure samples as small as 300 µgC and less than 33000 years without the need for size-specific correction protocols.

Rhizosphere Effects of Maize and Wheat Increase Soil Organic and Inorganic Carbon Release in Carbonate-Rich Soils: A Three-Source 13C Partitioning Study

In carbonate-rich soils with plants, CO2 emissions from the rhizosphere may come from as many as three sources, that is, root-derived respiration, decomposition of soil organic carbon (SOC), and dissolution of soil inorganic carbon (SIC), so partitioning of CO2 emissions by source is important to accurately quantify the rhizosphere effect (RE). Because of limited methods for three-source partitioning of soil CO2, how living roots affect SOC and SIC release (RE) has not yet been clarified, and this urgently needs to be evaluated. In this study, the RE of summer maize and winter wheat on SOC decomposition and SIC dissolution was investigated at three phenological stages in pot experiments with the aid of 13CO2 pulse labeling combined with 13C natural abundance techniques. We found that the contribution of SIC dissolution to CO2 emissions from unplanted soils ranged from 25 to 44%. As crop growth progressed, the maize rhizosphere effect on SOC- and SIC-derived CO2 emissions increased from 14 and 74% at the elongation stage to 84 and 268% at the grain filling stage compared to that in unplanted soils, respectively, while the wheat rhizosphere effect on SOC- and SIC-derived CO2 emissions increased from 51 and 34% at the elongation stage to 77 and 76% at the grain filling stage. We concluded that the rhizosphere effects increased SOC and SIC release over the entire growing season of maize (by 54% for SOC and 159% for SIC) and wheat (by 64 and 49%) compared to those in unplanted soils, indicating that ignoring SIC dissolution in carbonate-rich soils with plants will result in overestimation of SOC decomposition.

Storage of soil carbon as particulate and mineral associated organic matter in irrigated woody perennial crops

Agricultural practices such as annual crop production, land use change and grazing on marginal lands lead to a loss of soil carbon (C) stock. But soil C losses are not universal in agricultural systems and modest soil C gains can occur when constraints such as a lack of water are removed. To characterize this we used a meta-analysis of published data focused on semi-arid regions, where irrigation is required for crop production. We showed that soil C stocks declined under cereals, cotton, maize and non-woody horticultural crops when compared to native unirrigated adjacent grassland or shrubland. By contrast, cultivation of irrigated, woody perennial crops generally leads to an accumulation of soil C. Identifying the mechanisms by which C is retained in the soils beneath woody perennial crops, and any limits to C accumulation, was the main goal of this study. A mechanistic understanding of soil organic C content accumulation, upon land use change, can be gained by dividing soil into particulate organic matter (POM) and mineral associated organic matter (MAOM). Here, we analyzed the C and natural abundance 13C concentrations in POM and MAOM fractions in soils from eight apple orchards and eight vineyards irrigated using a dripline, and eight apple and eight cherry orchards irrigated with micro-spray. Samples were also taken from eight native grassland areas adjacent to the agricultural sites for comparison. Several decades of woody crop production doubled the average soil C concentration in comparison to the native sites, from to 10.1 ± 1.48 g C kg−1 to 20.1 ± 0.96 g C kg−1 over a depth of 0–15 cm. Most of the C was associated with POM, which increased in concentration from 7.9 ± 1.19 g C kg−1 to 14.2 ± 0.79 g C kg−1 in 0–15 cm soils, an increase of 80%. This was crop dependent, being highest in the cherry orchards and lowest in the vineyard soils. Although holding less C, the MAOM concentration increased by 166%, changing from 2.22 ± 0.33 to 5.91 ± 0.62 g C kg−1; no differences existed between crops but the MAOM C concentration appeared to be constrained to a maximum value of ~ 12 g C kg−1. MAOM and POM had markedly different δ13C values: MAOM was more enriched, indicative of greater microbial processing, whereas POM had a lower δ13C value consistent with the dominant standing vegetation. δ13C values were more depleted in both fractions at the agricultural sites compared to the native sites, indicating the accumulation of greater amounts of less processed C at the agricultural sites, due to higher C inputs. We conclude the soils in this region respond to irrigated perennial woody crop production by retaining C within both POM and MAOM fractions. Accumulation of C as MAOM is constrained by the minerology of the soils in this region, but is unconstrained for POM which dominates the soil C content, and is potentially vulnerable to changes in management practices and land use.

Hydrocarbon transformation pathways and soil organic carbon stability in the biostimulation of oil-contaminated soil: Implications of 13C natural abundance

Mineralization, assimilation, and humification are key processes to detoxify oil-contaminated soil by biostimulation remediation strategies, and these processes are affected by stimulants. In this study, we investigated the effects of either inorganic salts or organic stimulants (organic compost and sawdust) on hydrocarbon transformation. Total petroleum hydrocarbons (TPH) and hydrocarbon components were determined by gravimetry and gas chromatography, and the 13C of CO2, microbial biomass carbon (MBC), and humus were measured by stable isotope mass spectrometry. The results showed that organic compost was the most beneficial for the dissipation of hydrocarbons. After 60 days of remediation, the removal rates of TPH, saturates, aromatics, C7-C30 n-alkanes, and 16 PAHs were 35.7%, 39.6%, 15.9%, 80.5%, and 8.8%, respectively. A total of 84.7%–88.5% of the removed hydrocarbons were mineralized in all the treatments. The hydrocarbon degradation pathway in the control soil (without stimulant addition) was “assimilation → humification → mineralization”. The hydrocarbon transformation pathways in the biostimulation treatments were “assimilation → mineralization → humification”. The soil organic carbon (SOC) stability decreased during remediation, which was attributed to the enhanced microbial activity and the removal of recalcitrant hydrocarbons.

Aggregation, carbon, nitrogen, and natural abundance of 13C and 15N in soils under no-tillage system fertilized with injection and surface application of pig slurry for five years

Injection of pig slurry (PS) into soils under no-tillage system (NTS) is more efficient for improving soil chemical and physical attributes, and reducing C and N emissions, than surface applications. This study evaluated the effect of using injection and surface application of PS, compared to NPK and control treatments, on the soil aggregate, C and N contents, and isotopes 13C and 15N. The NTS consisted of rotations of summer (maize) and winter (black oat and wheat) grasses from 2011 to 2015. The treatments were PS injected into the soil (PSI), PS applied on the soil surface (PSS), chemical fertilization (NPK), and control (CTRL). The following soil properties were evaluated in the 0–5, 5–10, and 10–20 cm layers: aggregate stability (geometric mean diameter – GMD; aggregate mass distribution); total organic carbon (TOC) and total nitrogen (TN) in macroaggregates (8.0–0.25 mm), microaggregates (<0.25 mm), and bulk soil (<2.0 mm); and isotopes 13C and 15N in macro and microaggregates. The application of PSI improved the soil physical attributes, presenting higher GMD (0–5 cm) than the PSS, NPK, and CTRL treatments. In the 5–10 cm layer, the PSI treatments were more efficient in increasing the GMD and macroaggregate mass than the NPK. PSI also was more efficient in increasing TOC and TN when compared to PSS, and generated a higher GMD, which are protectors of these elements in the soil. The natural abundance of 15N denoted the lower soil organic matter decomposition in the PSI treatment when compared to the PSS. The natural abundance of 13C showed less-negative values in macroaggregates than in microaggregates, denoting that the soil management practices and crops used (grasses) affected positively the abundance of 13C. After seven applications of PS in maize-oat-wheat rotation in NTS, the application of PSI was more efficient in improving the soil physical and chemical attributes than the application of PSS.

The omega-loop of cobra cytotoxins tolerates multiple amino acid substitutions

Cobra cytotoxins (CTs), the three-fingered proteins, feature high amino acid sequence homology in the beta-strands and variations in the loop regions. We selected a pair of cytotoxins from Naja kaouthia crude venom to clarify the sequence-structure relationships. Using chromatography and mass spectroscopy, we separated and identified the mixture of cytotoxins 2 and 3, differentiated by the only Val 41/Ala 41 substitution. Here, using natural abundance 13C, 15N NMR-spectroscopy we performed chemical shift assignments of the signals of the both toxins in aqueous solution in the major and minor forms. Combining NOE and chemical shift data, the toxins’ spatial structure was determined. Finally, we proved that the tip of the “finger”-2, or the loop-2 of cytotoxins adopts the shape of an omega-loop with a tightly-bound water molecule in its cavity. Comparison with other NMR and X-ray structures of cytotoxins possessing different amino acid sequences reveals spatial similarity in this family of proteins, including the loop-2 region, previously considered to be flexible.

Low-temperature (

Electronic cigarette usage has spiked in popularity over recent years. The enhanced prevalence has consequently resulted in new health concerns associated with the use of these devices. Degradation of the liquids used in vaping have been identified as a concern due to the presence of toxic compounds such as aldehydes in the aerosols. Typically, such thermochemical conversions are reported to occur between 300 and 400 °C. Herein, the low-temperature thermal degradation of propylene glycol and glycerol constituents of e-cigarette vapors are explored for the first time by natural abundance 13C NMR and 1H NMR, enabling in situ detection of intact molecules from decomposition. The results demonstrate that the degradation of electronic nicotine delivery system (ENDS) liquids is strongly reliant upon the oxygen availability, both in the presence and absence of a material surface. When oxygen is available, propylene glycol and glycerol readily decompose at temperatures between 133 and 175 °C over an extended time period. Among the generated chemical species, formic and acrylic acids are observed which can negatively affect the kidneys and lungs of those who inhale the toxin during ENDS vapor inhalation. Further, the formation of hemi- and formal acetals is noted from both glycerol and propylene glycol, signifying the generation of both formaldehyde and acetaldehyde, highly toxic compounds, which, as a biocide, can lead to numerous health ailments. The results also reveal a retardation in decomposition rate when material surfaces are prevalent with no directly observed unique surface spectator or intermediate species as well as potentially slower conversions in mixtures of the two components. The generation of toxic species in ENDS liquids at low temperatures highlights the dangers of low-temperature ENDS use.

High-resolution laser spectroscopy of the [formula omitted]1[formula omitted] – [formula omitted]1[formula omitted] transition of 13C12C12C and 12C13C12C

We present a laser spectroscopic study of the Ã1Πu – X̃1Σg+ transition of 13C mono-substituted C3 in the 378–406 nm region. The 13C12C12C and 12C13C12C isotopologues are produced in a pulsed jet expansion, by DC discharging a gas mixture of ∼0.3% C2H2/Ar (with ∼1% 13C natural abundance). By using laser induced fluorescence spectroscopy, thirteen vibronic bands belonging to the Ã1Πu – X̃1Σg+ electronic transition have been recorded for 13C12C12C and 12C13C12C, eleven of which have not been reported before. The transitions cover the three fundamental vibrational modes (ν1,ν2l,ν3). For all the vibronic bands, the full width at half maximum of spectral lines is found to be narrower than 0.023 cm-1, allowing to determine the transition frequencies with an absolute accuracy of ∼0.003 cm-1. Moreover, rotational analyses of the high-resolution data have yielded accurate spectroscopic constants for the upper vibronic states studied here. The 100–000, 04+0–000 and 12−0–000 bands of 13C12C12C, and the 02+0–000 and 04−0–000 bands of 12C13C12C are found to be less blended by spectral features of 12C3, and thus may be used for astronomical searches of 13C12C12C and 12C13C12C in the space.

Mechanistic investigation of Rh(i)-catalysed asymmetric Suzuki–Miyaura coupling with racemic allyl halides

Understanding how catalytic asymmetric reactions with racemic starting materials can operate would enable new enantioselective cross-coupling reactions that give chiral products. Here we propose a catalytic cycle for the highly enantioselective Rh(i)-catalysed Suzuki–Miyaura coupling of boronic acids and racemic allyl halides. Natural abundance 13C kinetic isotope effects provide quantitative information about the transition-state structures of two key elementary steps in the catalytic cycle, transmetallation and oxidative addition. Experiments with configurationally stable, deuterium-labelled substrates revealed that oxidative addition can happen via syn- or anti-pathways, which control diastereoselectivity. Density functional theory calculations attribute the extremely high enantioselectivity to reductive elimination from a common Rh complex formed from both allyl halide enantiomers. Our conclusions are supported by analysis of the reaction kinetics. These insights into the sequence of bond-forming steps and their transition-state structures will contribute to our understanding of asymmetric Rh–allyl chemistry and enable the discovery and application of asymmetric reactions with racemic substrates. A major drive in current chemistry research is to develop asymmetric versions of widely used carbon–carbon bond-forming reactions, such as Suzuki-Miyaura cross-couplings. Now, the origins of diastereo- and enantioselectivity in a Rh-catalysed cross-coupling of boronic acid and racemic allyl halides have been established.

Shallow NV centers augmented by exploiting n-type diamond

Creation of nitrogen-vacancy (NV) centers at the nanoscale surface region in diamond, while retaining their excellent spin and optical properties, is essential for applications in quantum technology. Here, we demonstrate the extension of the spin-coherence time (T2), the stabilization of the charge state, and an improvement of the creation yield of NV centers formed by the ion-implantation technique at a depth of ∼15 nm in phosphorus-doped n-type diamond. The longest T2 of about 580 μs of a shallow NV center approaches the one in bulk diamond limited by the nuclear spins of natural abundant 13C. The averaged T2 in n-type diamond is over 1.7 times longer than that in pure non-doped diamond. Moreover, the stabilization of the charge state and the more than twofold improvement of the creation yield are confirmed. The enhancements for the shallow NV centers in an n-type diamond-semiconductor are significant for future integrated quantum devices.

Mitigation of carbon dioxide by accelerated sequestration from long-term biochar amended paddy soil

Understanding the process of carbon (C) sequestration under biochar amendment is crucial to mitigate climate change. However, the actual annual C sequestration rates after biochar application and underlying mechanisms at a scale of soil aggregates remain unclear. Thus, the C sequestration rates based on a net ecosystem C budget approach were investigated during a consecutive 6 years paddy field (2012–2018). Further, soil samples were collected in 2018 to identify the effects of long-term biochar (20 or 40 t ha−1 applied in 2012) amendment on 1) C sequestration by determining C hydrolyzing activities and mineralization in aggregate size classes and 2) C sequestration between aggregates using the 13C natural abundance. The results showed that biochar-induced annual C sequestration rate decreased during the first two years, whereas it increased during the following four years. The lower ecosystem respiration (-4%) and higher crop yield (+9%) explained the latter C sequestration, compared to N treatment. Relative to N treatment, six years aged biochar decreased soil total organic C (TOC) mineralization in the bulk soil (-12 %), macroaggregate (MacroA, 250−2000 μm, -38 %) and microaggregate (MicroA, 53−250 μm, -19 %) size classes, but increased in silt (2−53 μm, +5%) and clay (0.1−2 μm, +24 %) size classes. Partial least squares path modeling revealed that the decrease in activities of β-glucosidase (-13 %), α-glucosidase (-20 %), cellobiohydrolase (-17 %) and xylanase (-2.5 %) and the improvement in soil structure increased C accumulation. The 13C natural abundance showed that Δ13C (δ13C of aggregates – δ13C of bulk soil) decreased by increasing aggregate size. Biochar increased probability of C flow from MacroA to MicroA, relative to without biochar amended treatment. Thus, biochar protected low molecular weight C compounds in the MacroA, with subsequent fast transfer into MicroA, which having long storage and the highest stable C content among the aggregate size classes. Consequently, increase in C protection via transferring C from MacroA to MicroA and decrease in soil hydrolases activities induced by biochar in MacroA and MicroA contribute to the high C sequestration potential.