Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially ...for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant‐derived SOC following microbial decomposition. Here, we used in situ isotope pulse‐chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long‐term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4‐dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.
Long‐term grazing exclusion dramatically shifts plant carbon allocation priorities in subtropical pasture, reducing root biomass, fine root exudates, microbial biomass, and soil carbon. Additionally, analysis of lignin phenols extracted from soil suggests that variations in soil carbon are very closely coupled to plant tissue deposition. Overall, our results support that grazing can have profound impact on soil carbon, independent of shifts in plant species composition, through effects on fine root biomass, proliferation, and exudation.
The demanding precision of triple oxygen isotope (Δ17O) analyses in water has restricted their measurement to dual-inlet
mass spectrometry until the recent development of commercially available
...infrared laser analyzers. Laser-based measurements of triple oxygen isotope
ratios are now increasingly performed by laboratories seeking to better
constrain the source and history of meteoric waters. However, in practice,
these measurements are subject to large analytical errors that remain poorly documented in scientific literature and by instrument manufacturers, which can effectively restrict the confident application of Δ17O to settings where variations are relatively large (∼ 25–60 per
meg). We present our operating method of a Picarro L2140-i cavity ring-down
spectrometer (CRDS) during the analysis of low-latitude rainwater where
confidently resolving daily variations in Δ17O (differences of
∼ 10–20 per meg) was desired. Our approach was optimized over
∼ 3 years and uses a combination of published best practices
plus additional steps to combat spectral contamination of trace amounts of
dissolved organics, which, for Δ17O, emerges as a much more
substantial problem than previously documented, even in pure rainwater. We
resolve the extreme sensitivity of the Δ17O measurement to
organics through their removal via Picarro's micro-combustion module, whose
performance is evaluated in each sequence using alcohol-spiked standards. While correction for sample-to-sample memory and instrumental drift significantly improves traditional isotope metrics, these corrections have only a marginal impact (0–1 per meg error reduction) on Δ17O. Our
post-processing scheme uses the analyzer's high-resolution data, which
improves δ2H measurement (0.25 ‰ error
reduction) and allows for much more rich troubleshooting and data processing
compared to the default user-facing data output. In addition to competitive
performance for traditional isotope metrics, we report a long-term, control
standard root mean square error for Δ17O of 12 per meg. Overall
performance (Δ17O error of 6 per meg, calculated by averaging three
replicates spread across distinct, independently calibrated sequences) is
comparable to mass spectrometry and requires only ∼ 6.3 h per
sample. We demonstrate the impact of our approach using a rainfall dataset
from Uganda and offer recommendations for other efforts that aim to measure
meteoric Δ17O via CRDS.
Tidal wetlands contain large reservoirs of carbon in their soils and can sequester carbon dioxide (CO2) at a greater rate per unit area than nearly any other ecosystem. The spatial distribution of ...this carbon influences climate and wetland policy. To assist with international accords such as the Paris Climate Agreement, national‐level assessments such as the United States (U.S.) National Greenhouse Gas Inventory, and regional, state, local, and project‐level evaluation of CO2 sequestration credits, we developed a geodatabase (CoBluCarb) and high‐resolution maps of soil organic carbon (SOC) distribution by linking National Wetlands Inventory data with the U.S. Soil Survey Geographic Database. For over 600,000 wetlands, the total carbon stock and organic carbon density was calculated at 5‐cm vertical resolution from 0 to 300 cm of depth. Across the continental United States, there are 1,153–1,359 Tg of SOC in the upper 0–100 cm of soils across a total of 24 945.9 km2 of tidal wetland area, twice as much carbon as the most recent national estimate. Approximately 75% of this carbon was found in estuarine emergent wetlands with freshwater tidal wetlands holding about 19%. The greatest pool of SOC was found within the Atchafalaya/Vermilion Bay complex in Louisiana, containing about 10% of the U.S. total. The average density across all tidal wetlands was 0.071 g cm−3 across 0–15 cm, 0.055 g cm−3 across 0–100 cm, and 0.040 g cm−3 at the 100 cm depth. There is inherent variability between and within individual wetlands; however, we conclude that it is possible to use standardized values at a range of 0–100 cm of the soil profile, to provide first‐order quantification and to evaluate future changes in carbon stocks in response to environmental perturbations. This Tier 2‐oriented carbon stock assessment provides a scientific method that can be copied by other nations in support of international requirements.
In the tidal wetlands across the United States, there is 1,152–1,359 Tg of soil organic carbon (SOC), which is twice as much carbon as the most recent national estimate. The average area‐weighted carbon density is 0.040 g cm‐3 at the 100 cm depth. Approximately 75% of the carbon is found in herbaceous estuarine emergent wetlands with freshwater tidal wetlands holding about 19%. Standardized values can be used to provide a first‐order valuation of sequestration potential.
Seagrass meadows represent globally important stores of carbon. However, environmental heterogeneity in shallow, estuarine environments may shape the quantity, composition, and postdepositional ...processing of organic carbon stocks (Corg) in such meadows. Along a persistent gradient in total phosphorus concentrations in the water column and a parallel gradient in seagrass morphology, we measured bulk carbon parameters (Corg, dry bulk density, %Corg, Corg : N, δ13C) and lignin biomarkers in Thalassia testudinum tissues and in the sediments beneath these meadows in three coastal systems. We found Corg stocks and sources differed among coastal systems, but the aforementioned parameters were not consistently related to either standing stocks of seagrass or historical nutrient concentrations. We estimated that seagrasses contributed 30–53% of the total sedimentary Corg in these three coastal systems, with the remainder derived from allochthonous sources. The coastal system with intermediate phosphorus concentrations and aboveground seagrass stock had more Corg overall, more Corg from seagrass, and sediments with lower bulk density. A consistent negative relationship between dry bulk density and %Corg suggested hydrodynamics exerted a strong influence on stocks and sources of sedimentary Corg. Lignin biomarkers refined our understanding of sources of Corg and postdepositional processing of seagrass tissues. Phenolic acid-to-aldehyde ratios were high in fresh T. testudinum tissues. Lower values in sediments indicated seagrass tissues undergo extensive loss of acidic lignin phenols after deposition resulting in a need for lignin biomarker indices designed for estuarine sediments. Future studies of seagrass Corg should account for hydrodynamic setting, especially when investigating influences of environmental heterogeneity.
Compositional changes in dissolved organic matter (DOM) matrix via solid phase extraction (SPE) remain under‐constrained due to disproportional extraction of specific groups of compounds. This study ...elucidates changes in the composition of DOM via SPE by presenting an intercomparison of recovery efficiencies of dissolved lignin phenol standards using three SPE cartridges (C‐18, PPL and HLB) and an intercomparison of free‐lignin phenols and lignin‐oxidized products (LOPs) (via SPE) for two end‐members (river and coastal) in the Suwannee River, Cedar Key, Florida. A comparison was also made between DOC and dissolved lignin recovery rates, fluorescence intensity, EEMs indices, spectral slope (SR) and specific UV absorbance at 254 nm (SUVA254) for the river end‐members. Isolates harbored lower fluorescence intensities, lower EEMs index values, lower aromatic indices and higher spectral slopes indicating changes in both concentration and composition after SPE. Our results highlight that fractionation is occurring during SPE for lignin‐phenol standards, LOPs, and DOM. Riverine end‐member DOC extraction efficiencies and dissolved lignin (Σ8) recoveries were 46–60% and 41–48%, respectively, while coastal end‐member DOC efficiencies were lower and ranged from to 41% to 47%. In terms of DOC recovery, PPL was the best for river (60%) and coastal (47%), while in terms of LOP recovery, the three cartridges gave comparable results for dissolved lignin and one cartridge type is not necessarily better than the others for coastal samples.
Climate‐driven thawing of Arctic permafrost renders its vast carbon reserves susceptible to microbial degradation, serving as a potentially potent positive feedback hidden within the climate system. ...While seemingly intuitive, the relationship between thermally driven permafrost losses and organic carbon (OC) export remains largely unexplored in natural settings. Filling this knowledge gap, we present down‐core bulk and compound‐specific radiocarbon records of permafrost change from a sediment core taken within the Alaskan Colville River delta spanning the last c. 2,700 years. Fingerprinted by significantly older radiocarbon ages of bulk OC and long‐chain fatty acids, these data expose a thermally driven increase in permafrost OC export and/or deepening of mobilizable permafrost layers over the last c. 160 years after the Little Ice Age. Comparison of OC content and radiocarbon data between recent and Roman warming episodes likely implies that the rate of warming, alongside the prevailing boundary conditions, may dictate the ultimate fate of the Arctic's permafrost inventory. Our findings highlight the importance of leveraging geological records as archives of Arctic permafrost mobilization dynamics with temperature change.
Plain Language Summary
Temperature rise in the Arctic is likely causing enhanced thawing of perennially frozen soil (permafrost), leading to potential decomposition of organic matter and release of greenhouse gases. Models forecasting the potential release of permafrost organic carbon (OC) largely rely on historical records or experimental results over the past decades, leaving large uncertainties for long‐term predictions. In this study, a sediment core from the Alaskan Colville River delta was analyzed to provide a sub‐centennial long‐term record of changes of permafrost OC export. The radiocarbon results of bulk OC demonstrated a close association with temperature change, highlighting the increase of permafrost OC export and/or deepening of mobilizable permafrost layers for the past 160 years as a result of Arctic warming. The 2,700‐year record also implies that some factors like the rate of warming and the temperature before warming may need to be considered in climate models for better predictions.
Key Points
Arctic warming has likely caused an increase of permafrost organic carbon export and/or deepening of mobilizable permafrost layers over the last 160 years
Bank erosion is likely a key mechanism of mobilizing permafrost to the coast under warming conditions
The rate of warming and the prevailing boundary conditions may be important modulators of permafrost thawing
Incomplete drill holes in shelled invertebrate prey produced by predatory gastropods are often interpreted as failed attacks, and the frequency of incomplete drill holes in a sample is thought to ...provide insight into the effectiveness of prey defensive adaptations. Here, we conduct an experimental test of the hypothesis that differences in the frequencies of incomplete drill holes do not necessarily indicate anything about the prey and can instead be triggered by disturbance competition among conspecific drilling gastropods. Our test employed the western Atlantic naticid gastropod Neverita delessertiana and its venerid bivalve prey Chione elevata. The experiment consisted of an isolation treatment, where predators fed in the absence of conspecifics, and a competition treatment, where predators fed in groups of three. Competitive interactions, which included grappling, prey theft, and cannibalism, were found to be important in determining the presence of incomplete drill holes, as this trace was absent in the isolation treatment but present in the competition treatment, with the frequency of incomplete drill holes increasing in interactions among larger gastropods. These results suggest that interpretations of both field and fossil data must consider the role of competitive disruption as an additional mechanism underlying long-term patterns in the frequency of incomplete drill holes in the fossil record.
•We test the effects of conspecific interactions on naticid predators.•Snails were fed bivalve prey in two treatments: isolated and among conspecifics.•Conspecific competition led to an increase in incomplete drill hole frequencies.•Fossil drill hole studies should consider the effects of conspecific interactions.
The Late Cretaceous sedimentary record of the North American Western Interior Seaway is characterized by cyclic deposition of organic carbon-rich sediments. One notable interval during the late ...Coniacian-Santonian is recorded by the Niobrara Formation. The organic carbon-rich interval within the Niobrara Formation has been identified as Oceanic Anoxic Event (OAE) 3. Understanding the reason for this distribution of organic carbon within the Niobrara Formation requires a refined understanding of the source and maturity of the organic matter. In this study, we present lipid biomarker records from the USGS Portland #1 core (Cañon City, CO) to constrain the thermal maturity of the organic matter and the differing contributions of organic matter sources. Sterane and hopane thermal maturity indices indicate that the samples are somewhat immature with respect to oil formation and that there is strong agreement between different proxies for thermal maturity. Based on the distribution of n-alkanes, steranes, and hopanes, there is a significant increase in the contribution of algal organic matter during and after OAE 3, coeval with increased organic carbon accumulation. Although a consistent terrestrial contribution is observed, it is only a minor source of organic matter at the Portland core location and does not drive increased organic matter accumulation during OAE 3. Of particular note is the consistent influence of even-over-odd predominantly mid-chain length (C21 to C25) organic matter. This observation within the brackish to marine, not methanogenic WIS represents an expansion of the depositional settings in which even-over-odd predominance has been observed in mid-chain length n-alkanes. Pristane (Pr) and phytane (Ph) abundances are inconsistent with a redox control on Pr/Ph ratios and suggest an increase in the delivery and/or preservation of phototrophic organic matter as the source for pristane and phytane in the Portland core.
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