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.
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.
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.
Rapid Arctic warming is expected to increase global greenhouse gas concentrations as permafrost thaw exposes immense stores of frozen carbon (C) to microbial decomposition. Permafrost thaw also ...stimulates plant growth, which could offset C loss. Using data from 7 years of experimental Air and Soil warming in moist acidic tundra, we show that Soil warming had a much stronger effect on CO2 flux than Air warming. Soil warming caused rapid permafrost thaw and increased ecosystem respiration (Reco), gross primary productivity (GPP), and net summer CO2 storage (NEE). Over 7 years Reco, GPP, and NEE also increased in Control (i.e., ambient plots), but this change could be explained by slow thaw in Control areas. In the initial stages of thaw, Reco, GPP, and NEE increased linearly with thaw across all treatments, despite different rates of thaw. As thaw in Soil warming continued to increase linearly, ground surface subsidence created saturated microsites and suppressed Reco, GPP, and NEE. However Reco and GPP remained high in areas with large Eriophorum vaginatum biomass. In general NEE increased with thaw, but was more strongly correlated with plant biomass than thaw, indicating that higher Reco in deeply thawed areas during summer months was balanced by GPP. Summer CO2 flux across treatments fit a single quadratic relationship that captured the functional response of CO2 flux to thaw, water table depth, and plant biomass. These results demonstrate the importance of indirect thaw effects on CO2 flux: plant growth and water table dynamics. Nonsummer Reco models estimated that the area was an annual CO2 source during all years of observation. Nonsummer CO2 loss in warmer, more deeply thawed soils exceeded the increases in summer GPP, and thawed tundra was a net annual CO2 source.
In spring excess snow is removed from the Soil warming treatment to prevent delayed phenology and higher melt‐water input. Soil warming had a much stronger effect on CO2 flux than Air warming. Soil warming caused rapid permafrost thaw and increased ecosystem respiration (Reco), gross primary productivity (GPP), and net summer CO2 storage (NEE). Summer CO2 flux across treatments could be explained by changes in thaw, water table depth and plant biomass. In the initial stages of thaw Reco, GPP, and NEE increased linearly with thaw and plant biomass. As thaw continued to progress in Soil warming, ground surface subsidence created saturated microsites and suppressed Reco and GPP, reducing summer CO2 sink strength in the most deeply thawed areas. Adding winter Reco losses to summer NEE showed that the tundra was a net annual CO2 source as Reco in warmed and in un‐warmed winter soils exceeded the summer CO2 sink.
In the last few decades, temperatures in the Arctic have increased twice as much as the rest of the globe. As permafrost thaws in response to this warming, large amounts of soil organic matter may ...become vulnerable to decomposition. Microbial decomposition will release carbon (C) from permafrost soils, however, warmer conditions could also lead to enhanced plant growth and C uptake. Field and modeling studies show high uncertainty in soil and plant responses to climate change but there have been few studies that reconcile field and model data to understand differences and reduce uncertainty. Here, we evaluate gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem C exchange (NEE) from eight years of experimental soil warming in moist acidic tundra against equivalent fluxes from the Community Land Model during simulations parameterized to reflect the field conditions associated with this manipulative field experiment. Over the eight-year experimental period, soil temperatures and thaw depths increased with warming in field observations and model simulations. However, the field and model results do not agree on warming effects on water table depth; warming created wetter soils in the field and drier soils in the models. In the field, initial increases in growing season GPP, Reco, and NEE to experimentally-induced permafrost thaw created a higher C sink capacity in the first years followed by a stronger C source in years six through eight. In contrast, both models predicted linear increases in GPP, Reco, and NEE with warming. The divergence of model results from field experiments reveals the role subsidence, hydrology, and nutrient cycling play in influencing the C flux responses to permafrost thaw, a complexity that the models are not structurally able to predict, and highlight challenges associated with projecting C cycle dynamics across the Arctic.
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.
Transport of particles plays a major role in redistributing organic carbon (OC) along coastal regions. In particular, the global importance of fjords as sites of carbon burial has recently been shown ...to be even more important than previously thought. In this study, we used six surface sediments from Fiordland, New Zealand, to investigate the transport of particles and OC based on density fractionation. Bulk, biomarker, and principle component analysis were applied to density fractions with ranges of <1.6, 1.6–2.0, 2.0–2.5, and >2.5 g cm−3. Our results found various patterns of OC partitioning at different locations along fjords, likely due to selective transport of higher density but smaller size particles along fjord head‐to‐mouth transects. We also found preferential leaching of certain biomarkers (e.g., lignin) over others (e.g., fatty acids) during the density fractionation procedure, which altered lignin‐based degradation indices. Finally, our results indicated various patterns of OC partitioning on density fractions among different coastal systems. We further propose that a combination of particle size‐density fractionation is needed to better understand transport and distribution of particles and OC.
Key Points
There are distinct patterns of OC partitioning on density fractions in Fiordland fjords
Preferential leaching of OC during density fractionation impacts the use of biomarker proxies
Partitioning of OC on density fraction in this study was compared with other coastal systems
Temperature rise in the Arctic is causing deepening of active layers and resulting in the mobilization of deep permafrost dissolved organic matter (DOM). However, the mechanisms of DOM mobilization ...from Arctic soils, especially upper soil horizons which are drained most frequently through a year, are poorly understood. Here we conducted a short‐term leaching experiment on surface and deep organic active layer soils, from the Yukon River basin, to examine the effects of DOM transport on bulk and molecular characteristics. Our data showed a net release of DOM from surface soils equal to an average of 5% of soil carbon. Conversely, deep soils percolated with surface leachates retained up to 27% of bulk DOM while releasing fluorescent components (up to 107%), indicating selective release of aromatic components (e.g., lignin and tannin), while retaining nonchromophoric components, as supported by spectrofluorometric and ultrahigh‐resolution mass spectroscopic techniques. Our findings highlight the importance of the lateral flux of DOM on ecosystem carbon balance as well as processing of DOM transport through organic active layer soils en route to rivers and streams. This work also suggests the potential role of leachate export as an important mechanism of C losses from Arctic soils, in comparison with the more traditional pathway from soil to atmosphere in a warming Arctic.
Key Points
Significant amount of DOC export from surface soil by leaching
Net retention of surface DOM leachates in deep soils
Selective molecular exchange of soil DOM leachate during percolation
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