The ratio of stable isotopes of carbon (δ13C) is commonly used to track the flow of energy among individuals and ecosystems, including in mangrove forests. Effective use of this technique requires ...understanding of the spatial variability in δ13C among primary producer(s) as well as quantification of the isotopic fractionations that occur as C moves within and among ecosystem components. In this experiment, we assessed δ13C variation in the cosmopolitan mangrove Avicennia marina across four sites of varying physico-chemical conditions across two estuaries. We also compared the isotopic values of five distinct tissue types (leaves, woody stems, cable roots, pneumatophores and fine roots) in individual plants.
We found a significant site effect (F3, 36 = 15.78; P < 0.001) with mean leaf δ13C values 2.0‰ more depleted at the lowest salinity site compared to the other locations. There was a larger within-plant fractionation effect, however, with leaf samples (mean ± SE = −29.1 ± 0.2) more depleted in 13C than stem samples (−27.1 ± 0.1), while cable root (−25. 8 ± 0.1), pneumatophores (−25.7 ± 0.1) and fine roots (−26.0 ± 0.2) were more enriched in 13C relative to both aboveground tissue types (F4, 36 = 223.45; P < 0.001).
The within-plant δ13C fractionation we report for A. marina is greater than that reported in most other ecosystems. This has implications for studies of estuarine carbon cycling. The consistent and large size of the fractionation from leaf to woody stem (∼2.0‰) and mostly consistent fractionation from leaf to root tissues (>3.0‰) means that it may now be possible to partition the individual contributions of various mangrove tissues to estuarine food webs. Similarly, the contributions of mangrove leaves, woody debris and belowground sources to blue carbon stocks might also be quantified. Above all, however, our results emphasize the importance of considering appropriate mangrove tissue types when using δ13C to trace carbon cycling in estuarine systems.
Identifying drivers of variation in soil organic carbon (OC) at a regional scale is often hampered by a lack of historical management information. Focusing on red-brown-earth soils (Chromosol) under ...dryland agriculture in the Mid-North and Eyre Peninsula of South Australia, our aims were 2-fold: (i) to provide a baseline of soil OC stocks (0.3m) and OC fractions (mid-infrared predictions of particulate, humus, and resistant OC in 0.1m samples) in cropping and crop-pasture systems; and (ii) to evaluate whether the inclusion of management-based indices could assist in explaining regional-level variation in OC stocks and fractions. Soil OC stocks in both regions varied ~20Mgha-1, with higher OC stocks in the Mid-North (38Mgha-1) than the Eyre Peninsula (29.1Mgha-1). The humus OC fraction was the dominant fraction, while the particulate OC was the most variable. Environmental variables only partially explained soil OC variability, with vapour pressure deficit (VPD) offering the greatest potential and likely acting as an integrator of temperature and moisture on plant growth and decomposition processes. Differences between broad-scale cropping and crop–pasture systems were limited. In the Mid-North, variability in soil OC stocks and fractions was high, and could not be explained by environmental or management variables. Higher soil OC concentrations (0.1m) in the Eyre Peninsula cropping than crop–pasture soils were largely accounted for in the particulate OC fraction and are therefore unlikely to represent a long-term stable OC pool. Use of the management data in index format added some explanatory power to the variability in OC stocks over the main environmental variables (VPD, slope) within the Eyre Peninsula cropping soils only. In the wider context, the management data were useful in interpreting differences between regional findings and highlighted difficulties in using uninformed, broad-scale management categories.
Concerns about energy security and climate change have increased biofuel demand, particularly ethanol produced from cellulosic feedstocks (e.g., food crop residues). A central challenge to cropping ...for cellulosic ethanol is the potential environmental damage from increased fertilizer use. Previous analyses have assumed that cropping for carbohydrate in residue will require the same amount of fertilizer as cropping for grain. Using 13C nuclear magnetic resonance, we show that increases in biomass in response to fertilization are not uniform across biochemical classes (carbohydrate, protein, lipid, lignin) or tissues (leaf and stem, grain, reproductive support). Although corn grain responds vigorously and nonlinearly, corn residue shows only modest increases in carbohydrate yields in response to high levels of fertilization (25% increase with 202 kg N ha−1). Lignin yields in the residue increased almost twice as much as carbohydrate yields in response to nitrogen, implying that residue feedstock quality declines as more fertilizer is applied. Fertilization also increases the decomposability of corn residue, implying that soil carbon sequestration becomes less efficient with increased fertilizer. Our results suggest that even when corn is grown for grain, benefits of fertilization decline rapidly after the ecosystem’s N demands are met. Heavy application of fertilizer yields minimal grain benefits and almost no benefits in residue carbohydrates, while degrading the cellulosic ethanol feedstock quality and soil carbon sequestration capacity.
Anaerobic sedimentary conditions have traditionally been linked to the generation of the source rocks for petroleum formation. However, the influence of sedimentary redox conditions on the ...composition of freshly deposited organic matter (OM) is not clear. We assessed the effect of in situ exposure time to oxic conditions on the composition of OM accumulating in different coastal and deep-sea sediments using solid-state13C nuclear magnetic resonance (NMR).13C NMR spectra were resolved into mixtures of model components to distinguish between alkyl carbon present in protein and nonprotein structures. There is an inverse relation between the length of exposure to oxic conditions and the relative abundance of nonprotein alkyl (alkylNP) carbon, whose concentration is two orders of magnitude higher in coastal sediments with short exposure times than in deep-sea sediments with long exposure times. All alkylNP-rich samples contain a physically separate polymethylene component similar in composition to algaenans and kerogens in type I oil shales. The duration of exposure to oxic conditions appears to directly influence the quality and oil generation potential of OM in marine shales.
Better understanding the spatial distribution of soil organic carbon (SOC) stocks is important for the management and enhancement of soils for production and environmental outcomes. We have applied ...digital soil mapping (DSM) techniques to combine soil-site datasets from legacy and recent sources, environmental covariates and expert pedological knowledge to predict and map SOC stocks in the top 0.3m, and their uncertainty, across South Australia’s agricultural zone. In achieving this, we aimed to maximise the use of locally sourced datasets not previously considered in national soil C assessments. Practical considerations for operationalising DSM are also discussed in the context of working with problematic legacy datasets, handling large numbers of potentially correlated covariates, and meeting end-user needs for readily interpretable results and accurate maps. Spatial modelling was undertaken using open-source R statistical software over a study area of ~160000km2. Legacy-site SOC stock estimates were derived with inputs from an expert-derived bulk-density pedotransfer function to overcome critical gaps in the data. Site estimates of SOC were evaluated over a consistent depth range and then used in spatial predictions through an environmental-correlation regression-kriging DSM approach. This used the contemporary Least Absolute Shrinkage and Selection Operator penalised-regression method, which catered for a large number (63 numeric, four categorical, four legacy-soil mapping themes) of potentially correlated covariates. For efficient use of the available data, this was performed within a k-fold cross-validation (k=10) modelling framework. Through this, we generated multiple predictions and variance information at every node of our prediction grid, which was used to evaluate and map the expected value (mean) of SOC stocks and their uncertainty. For the South Australian agricultural zone, expected value SOC stocks in the top 0.3m summed to 0.589 Gt with a 90% prediction interval of 0.266–1.086 Gt.
Cupric oxide (CuO) oxidation is a powerful tool for tracing different forms of organic carbon (OC), particularly vascular plant-derived OC, through the environment. The method produces a suite of ...benzene carboxylic acids (BCAs) whose sources have been unclear, but some of which have been used as tracers of soil OC in the ocean. In particular, some evidence suggested they might derive from charcoal, making them a potentially powerful tracer of combustion-derived OC within the environment. In order to constrain their sources and distributions in the environment, we analyzed the concentrations of an extended suite of 18 BCAs from over 75 samples. The entire suite of compounds was produced from most pure organic samples, as well as from charcoal, indicating that a variety of processes lead to their production. Both terrestrial and marine geochemical samples yielded the suite, and OC content, not charcoal, appears to control the total BCA yield. The compounds cannot, therefore, be used as specific tracers of charcoal in soils and sediments. Types of OC produced by marine biota (i.e. glucose, protein and degraded phytoplankton) produce many of the BCAs, but not 3,5-dihydroxybenzoic acid, suggesting that previous applications of this compound for tracing soil OC in the ocean may be robust. Additionally, we present a new ratio based on the entire BCA suite, which may provide a further constraint on the amount of soil OC present in ocean sediments.
The oxidative ratio (OR) of the biosphere is the stoichiometric ratio (O2/CO2) of gas exchange by photosynthesis and respiration—a key parameter in budgeting calculations of the land and ocean carbon ...sinks. Carbon cycle‐climate feedbacks could alter the OR of the biosphere by affecting the quantity and quality of organic matter in plant biomass and soil carbon pools. This study considers the effect of elevated atmospheric carbon dioxide concentrations (CO2) on the OR of a hardwood forest after nine growing seasons of Free‐Air CO2 Enrichment. We measured changes in the carbon oxidation state (Cox) of biomass and soil carbon pools as a proxy for the ecosystem OR. The OR of net primary production, 1.039, was not affected by elevated CO2. However, the Cox of the soil carbon pool was 40% higher at elevated CO2, and the estimated OR values for soil respiration increased from 1.006 at ambient CO2 to 1.054 at elevated CO2. A biochemical inventory of the soil organic matter ascribed the increases in Cox and OR to faster turnover of reduced substrates, lignin and lipids, at elevated CO2. This implicates the heterotrophic soil community response to elevated CO2 as a driver of disequilibrium in the ecosystem OR. The oxidation of soil carbon pool constitutes an unexpected terrestrial O2 sink. Carbon budgets constructed under the assumption of OR equilibrium would equate such a terrestrial O2 sink to CO2 uptake by the ocean. The potential for climate‐driven disequilibriua in the cycling of O2 and CO2 warrants further investigation.
Key Points
Soil carbon oxidation state increased with atmospheric CO2 concentration
The carbon oxidation state is sensitive to disequilibria in O2 and CO2 fluxes
Hardwood forests may be an atmospheric O2 sink at high CO2 levels
Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for characterising the complex chemistry of soil organic carbon (SOC), but is prohibitively expensive, time-consuming and ...technically-demanding. Diffuse reflectance mid-infrared (MIR) spectroscopy is an attractive alternative because it is a high-throughput, cost-effective and easy-to-use technique that provides information on the amount and nature of soil mineral and organic components. However, interpretation of complex MIR spectra can be challenging due to difficulties with distinguishing SOC peaks from overlapping mineral-related peaks. We present a novel approach to predict the entire NMR spectra of SOC from corresponding MIR spectra using partial least-squares regression (PLSR) in an R environment. We developed a multi-response MIR–PLSR prediction model by regressing corresponding NMR and MIR spectra of 99 HF-treated <50μm fractions of soils using the pls package. The model was validated using (set-aside) test sets in four model iterations. The model provided accurate predictions of the entire average NMR spectra. Average Euclidean distance values between spectra in the training set were at least 3.5 fold greater than those between average reference and predicted NMR spectra, indicating that prediction errors were small relative to between-soil variation. Our approach accurately predicted intricate NMR spectra, demonstrating new potential for routine analysis of complex SOC chemistry.
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•We present a novel application of the pls package in R to predict NMR spectra of SOC.•We develop multi-response calibrations using MIR spectra and partial least-squares.•Method validation reveals accurate predictions of intricate NMR spectra of SOC.•This approach offers enhanced capability for routine analysis of complex SOC chemistry.
The use of subtropical perennial grasses in temperate grazing systems is increasingly being promoted for production and environmental benefits. This study employed a combination of elemental and ...stable isotope analyses to explore whether pastures sown to either kikuyu (Pennisetum clandestinum) or a combination of panic (Panicum maximum) and Rhodes grass (Chloris gayana) could increase soil organic carbon (SOC) levels in five regions across southern Australia. Carbon was sequestered under kikuyu at a rate of 0.90±0.25MgCha-1year-1 along the south coast of Western Australia. Lower but still significant sequestration rates were found for kikuyu in South Australia (0.26±0.13MgCha-1year-1). No changes in SOC were found for panic–Rhodes grass pasture systems in the northern district of Western Australia. Additionally, we found no changes in SOC when kikuyu-based pastures were established on formerly cropped paddocks in the Namoi Catchment of New South Wales. Stable isotope results corroborated these findings and suggested that, where SOC has accumulated, the gains have been dominated by SOC derived from the perennial vegetation and have been concentrated in the upper 10cm of soil.
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