Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation

Understanding the origin of the carbon (C) stabilised in soils is crucial in order to device management practices that will foster Caccumulation in soils. The relative contributions to soilC pools of roots vs. shoots is one aspect that has been mostly overlooked, although it appears a key factor that drives the fate of plant tissueC either as mineralized CO₂ or as stabilized soil organic matter (SOM). Available studies on the subject consistently indicate that rootC has a longer residence time in soil than shootC. From the few studies with complete datasets, we estimated that the mean residence time in soils of root-derived C is 2.4times that of shoot-derived C. Our analyses indicate that this value is biased neither by an underestimation of root contributions, as exudation was considered in the analysis, nor by a priming effect of shoot litter on SOM. Here, we discuss the main SOM stabilisation mechanisms with respect to their ability to specifically protect root-derived SOM. Comparing in situ and incubation experiments suggests that the higher chemical recalcitrance of root tissues as compared to that of shoots is responsible for only a small portion, i.e. about one fourth, of the difference in mean residence time in soils of root-derived vs. shoot-derivedC. This suggests that SOM protection mechanisms other than chemical recalcitrance are also enhanced by root activities: (1)physico-chemical protection, especially in deeper horizons, (2)micrometer-scale physical protection through myccorhiza and root-hair activities, and (3)chemical interactions with metal ions. The impact of environmental conditions within deeper soil layers on rootC stabilisation appear difficult to assess, but is likely, if anything, to further increase the ratio between the mean residence time of root vs. shootC in soils. Future advances are expected from isotopic studies conducted at the molecular level, which will help unravel the fate of individual shoot and root compounds, such as cutins and suberins, throughout soil profiles.     

Whales in the carbon cycle: can recovery remove carbon dioxide?

The great whales (baleen and sperm whales), through their massive size and wide distribution, influence ecosystem and carbon dynamics. Whales directly store carbon in their biomass and contribute to carbon export through sinking carcasses. Whale excreta may stimulate phytoplankton growth and capture atmospheric CO₂; such indirect pathways represent the greatest potential for whale-carbon sequestration but are poorly understood. We quantify the carbon values of whales while recognizing the numerous ecosystem, cultural, and moral motivations to protect them. We also propose a framework to quantify the economic value of whale carbon as populations change over time. Finally, we suggest research to address key unknowns (e.g., bioavailability of whale-derived nutrients to phytoplankton, species- and region-specific variability in whale carbon contributions).     

Is soil carbon disappearing? The dynamics of soil organic carbon in Java

Research in the soil of the tropics mostly has demonstrated the decline of soil organic carbon (SOC) after conversion of primary forest to plantation and cultivated lands. This paper illustrates the dynamics of SOC on the island of Java, Indonesia, from 1930 to 2010. We used 2002 soil profile observations containing organic carbon (C) analysis in the topsoil, which were collected by the Indonesian Center for Agricultural Land Resources Research & Development from 1923 to 2007. Results show the obvious decline of SOC values from around 2% in 1930–1940 to 0.8% in 1960–1970. However, there has been an increase of SOC content since 1970, with a median level of 1.1% in the year 2000. Our analysis suggests that the human influence and agricultural practices on SOC in Java have been a stronger influence than the environmental factors. SOC for the top 10 cm has shown a net accumulation rate of 0.2–0.3 Mg C ha^?1 yr^?1 during the period 1990–2000. These findings give rise to optimism for increased soil C sequestration in the tropics.     

Initial soil organic carbon stocks govern changes in soil carbon: Reality or artifact?

Changes in soil organic carbon (SOC) storage have the potential to affect global climate; hence identifying environments with a high capacity to gain or lose SOC is of broad interest. Many cross-site studies have found that SOC-poor soils tend to gain or retain carbon more readily than SOC-rich soils. While this pattern may partly reflect reality, here we argue that it can also be created by a pair of statistical artifacts. First, soils that appear SOC-poor purely due to random variation will tend to yield more moderate SOC estimates upon resampling and hence will appear to accrue or retain more SOC than SOC-rich soils. This phenomenon is an example of regression to the mean. Second, normalized metrics of SOC change—such as relative rates and response ratios—will by definition show larger changes in SOC at lower initial SOC levels, even when the absolute change in SOC does not depend on initial SOC. These two artifacts create an exaggerated impression that initial SOC stocks are a major control on SOC dynamics. To address this problem, we recommend applying statistical corrections to eliminate the effect of regression to the mean, and avoiding normalized metrics when testing relationships between SOC change and initial SOC. Careful consideration of these issues in future cross-site studies will support clearer scientific inference that can better inform environmental management.     

Current forest carbon stocks and carbon sequestration potential in Anhui Province,China北大核心CSCD

Aims This study was conducted to investigate carbon stocks in forest ecosystems of different stand ages in Anhui Province, and to identify the carbon sequestration potential of climax forests controlled by the natural environment conditions. Methods Data were collected based on field investigations and simulations were made with the BIOME4 carbon cycle model. Important findings Currently, the total forest carbon stocks in Anhui Province amounts to 714.5 Tg C: 402.1 Tg C in vegetation and 312.4 Tg C in soil. Generally, both the total and vegetation carbon density exhibit an increasing trend with the natural growth of forest stands. Soil carbon density increases from young to near mature forests, and then gradually decreases thereafter. Young and middle-aged forests account for 75% of the total forest area in Anhui Province, with potentially an additional 125.4 Tg C to be gained after the young and middle-aged forests reach near mature stage. Results of BIOME4 simulations show that potentially an additional 245.7 Tg C, including 153.7 Tg C in vegetation and 92 Tg C in soil, could be gained if the current forests are transformed into climax forest ecosystems in Anhui Province.     

Indonesia’s blue carbon: a globally significant and vulnerable sink for seagrass and mangrove carbon

The global significance of carbon storage in Indonesia’s coastal wetlands was assessed based on published and unpublished measurements of the organic carbon content of living seagrass and mangrove biomass and soil pools. For seagrasses, median above- and below-ground biomass was 0.29 and 1.13 Mg C ha^?1 respectively; the median soil pool was 118.1 Mg C ha^?1. Combining plant biomass and soil, median carbon storage in an Indonesian seagrass meadow is 119.5 Mg C ha^?1. Extrapolated to the estimated total seagrass area of 30,000 km², the national storage value is 368.5 Tg C. For mangroves, median above- and below-ground biomass was 159.1 and 16.7 Mg C ha^?1, respectively; the median soil pool was 774.7 Mg C ha^?1. The median carbon storage in an Indonesian mangrove forest is 950.5 Mg C ha^?1. Extrapolated to the total estimated mangrove area of 31,894 km², the national storage value is 3.0 Pg C, a likely underestimate if these habitats sequester carbon at soil depths >1 m and/or sequester inorganic carbon. Together, Indonesia’s seagrasses and mangroves conservatively account for 3.4 Pg C, roughly 17 % of the world’s blue carbon reservoir. Continued degradation and destruction of these wetlands has important consequences for CO₂ emissions and dissolved carbon exchange with adjacent coastal waters. We estimate that roughly 29,040 Gg CO₂ (eq.) is returned annually to the atmosphere–ocean pool. This amount is equivalent to about 3.2 % of Indonesia’s annual emissions associated with forest and peat land conversion. These results highlight the urgent need for blue carbon and REDD+ projects as a means to stem the decline in wetland area and to mitigate the release of a significant fraction of the world’s coastal carbon stores.     

Air-sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?

To limit global warming below 2°C by 2100, we must drastically reduce greenhouse gas emissions and additionally remove ~100–900 Gt CO₂ from the atmosphere (carbon dioxide removal, CDR) to compensate for unavoidable emissions. Seaweeds (marine macroalgae) naturally grow in coastal regions worldwide where they are crucial for primary production and carbon cycling. They are being considered as a biological method for CDR and for use in carbon trading schemes as offsets. To use seaweeds in carbon trading schemes requires verification that seaweed photosynthesis that fixes CO₂ into organic carbon results in CDR, along with the safe and secure storage of the carbon removed from the atmosphere for more than 100 years (sequestration). There is much ongoing research into the magnitude of seaweed carbon storage pools (e.g., as living biomass and as particulate and dissolved organic carbon in sediments and the deep ocean), but these pools do not equate to CDR unless the amount of CO₂ removed from the atmosphere as a result of seaweed primary production can be quantified and verified. The draw-down of atmospheric CO₂ into seawater is via air-sea CO₂ equilibrium, which operates on time scales of weeks to years depending upon the ecosystem considered. Here, we explain why quantifying air-sea CO₂ equilibrium and linking this process to seaweed carbon storage pools is the critical step needed to verify CDR by discrete seaweed beds and nearshore and open ocean aquaculture systems prior to their use in carbon trading.     

Resonance assignment of methionine methyl groups and χ3angular information from long-range proton—carbon and carbon—carbon J correlation in a calmodulin—peptide complex

Summary Several simple 3D experiments are used to provide J correlations between methionine C methyl carbons and either the CH₂ protons or C and C. The intensity of the J correlations provides information on the size of the three-bond J couplings and thereby on the ³ torsion angle. In addition, a simple 3D version of the HMBC experiment provides a sensitive link between the CH₃ methyl protons and C. The methods are demonstrated for a 20 kDa complex between calmodulin and a 26-residue peptide fragment of skeletal muscle myosin light chain kinase.     

Can heterotrophic uptake of dissolved organic carbon and zooplankton mitigate carbon budget deficits in annually bleached corals?

Annual coral bleaching events due to increasing sea surface temperatures are predicted to occur globally by the mid-century and as early as 2025 in the Caribbean, and severely impact coral reefs. We hypothesize that heterotrophic carbon (C) in the form of zooplankton and dissolved organic carbon (DOC) is a significant source of C to bleached corals. Thus, the ability to utilize multiple pools of fixed carbon and/or increase the amount of fixed carbon acquired from one or more pools of fixed carbon (defined here as heterotrophic plasticity) could underlie coral acclimatization and persistence under future ocean-warming scenarios. Here, three species of Caribbean coral—Porites divaricata, P. astreoides, and Orbicella faveolata—were experimentally bleached for 2.5 weeks in two successive years and allowed to recover in the field. Zooplankton feeding was assessed after single and repeat bleaching, while DOC fluxes and the contribution of DOC to the total C budget were determined after single bleaching, 11 months on the reef, and repeat bleaching. Zooplankton was a large C source for P. astreoides, but only following single bleaching. DOC was a source of C for single-bleached corals and accounted for 11–36 % of daily metabolic demand (CHAR_DOC), but represented a net loss of C in repeat-bleached corals. In repeat-bleached corals, DOC loss exacerbated the negative C budgets in all three species. Thus, the capacity for heterotrophic plasticity in corals is compromised under annual bleaching, and heterotrophic uptake of DOC and zooplankton does not mitigate C budget deficits in annually bleached corals. Overall, these findings suggest that some Caribbean corals may be more susceptible to repeat bleaching than to single bleaching due to a lack of heterotrophic plasticity, and coral persistence under increasing bleaching frequency may ultimately depend on other factors such as energy reserves and symbiont shuffling.     

How organic carbon derived from multiple sources contributes to carbon sequestration processes in a shallow coastal system?

Carbon captured by marine organisms helps sequester atmospheric CO₂, especially in shallow coastal ecosystems, where rates of primary production and burial of organic carbon (OC) from multiple sources are high. However, linkages between the dynamics of OC derived from multiple sources and carbon sequestration are poorly understood. We investigated the origin (terrestrial, phytobenthos derived, and phytoplankton derived) of particulate OC (POC) and dissolved OC (DOC) in the water column and sedimentary OC using elemental, isotopic, and optical signatures in Furen Lagoon, Japan. Based on these data analysis, we explored how OC from multiple sources contributes to sequestration via storage in sediments, water column sequestration, and air–sea CO₂ exchanges, and analyzed how the contributions vary with salinity in a shallow seagrass meadow as well. The relative contribution of terrestrial POC in the water column decreased with increasing salinity, whereas autochthonous POC increased in the salinity range 10–30. Phytoplankton‐derived POC dominated the water column POC (65–95%) within this salinity range; however, it was minor in the sediments (3–29%). In contrast, terrestrial and phytobenthos‐derived POC were relatively minor contributors in the water column but were major contributors in the sediments (49–78% and 19–36%, respectively), indicating that terrestrial and phytobenthos‐derived POC were selectively stored in the sediments. Autochthonous DOC, part of which can contribute to long‐term carbon sequestration in the water column, accounted for >25% of the total water column DOC pool in the salinity range 15–30. Autochthonous OC production decreased the concentration of dissolved inorganic carbon in the water column and thereby contributed to atmospheric CO₂ uptake, except in the low‐salinity zone. Our results indicate that shallow coastal ecosystems function not only as transition zones between land and ocean but also as carbon sequestration filters. They function at different timescales, depending on the salinity, and OC sources.     

Distribution of soil phytolith-occluded carbon in the Chinese Loess Plateau and its implications for silica–carbon cycles

Background and aims

Plants absorb and carry soluble silica from soils and then deposit SiO₂?·?nH₂O within themselves producing amorphous silica particles known as phytoliths. Trace amount of organic carbon is occluded during phytolith formation referred to as phytolith-occluded carbon (PhytOC). This carbon fraction has been recognized as an important way of carbon biosequestration. Previous studies have investigated the PhytOC contents of many crop plants and their contribution to global carbon sink. However, the PhytOC in soil is less focused. In this study, we investigated the distribution of soil PhytOC in the Chinese Loess Plateau (CLP).

Methods

Twenty-six soil profiles were collected in the Chinese Loess Plateau. A wet oxidation method was used for phytolith extraction. Occluded carbon was determined by element analyzer.

Results

Our results showed that the soil PhytOC density (SPCD) ranged from 0.757 to 23.110 g/m² among different soil profiles. The SPCD of profiles in the Southern CLP was generally higher than that in the Northern CLP. It was estimated that 5.35 Mt of PhytOC was stored in the upper soil of the CLP. We also estimated the annual phytolith flux into the Yellow River from the CLP by soil erosion and about 2.5 Mt of phytoliths eroded and transported into rivers per year.

Conclusions

Our study indicated that PhytOC was one of the potential biosequestration way and phytoliths had an important influence on biogeochemical cycle of silica. Our results suggested that the soil PhytOC was mainly influenced by different plant communities.     

Can carbon credits fund riparian forest restoration?

Ecological restoration is increasingly called on to provide ecosystem services (ES) valuable to humans, as well as to benefit biodiversity and improve wildlife habitat. Where mechanisms to pay for ES exist, they may serve as incentives to embark on habitat restoration projects. We evaluated the potential of newly established carbon markets in the United States to incentivize afforestation along riparian corridors, by comparing the income earnable by carbon offset credits with the costs of planting, maintaining, and registering such a restoration project in California. We used a 20‐year chronosequence of riparian forest sites along the Sacramento River as our model project. We found that carbon credits can repay more than 100% of costs after two decades of regrowth, if sufficient effort is put into sampling intensity in the first post‐restoration decade. However, carbon credits alone are unlikely to entice landowners currently engaged in agricultural activities to switch from farming crops to farming habitat.     

δ13C signature of organic carbon in estuarine bottom sediment as an indicator of carbon export from adjacent marshes

The ¹³C signature of organic carbon in estuarine bottom sediment in Louisiana Barataria Basin was used for estimating carbon flux from adjacent marsh. The stable carbon isotope composition of plants, soils and sediments from the basin were determined. The ¹³C content of marsh vegetation ranged from -26.3 to -27.8% for C₃ freshwater vegetation in the upper basin to -13.0 to -13.3% for C₄ vegetation in the lower basin. The ¹³C content of the highly organic marsh soils were similar to ¹³C content of vegetation present. The ¹³C content of organic carbon from bottom sediment of open water bodies ranged from 27.3 in the upper basin (freshwater) to 16.4 in bottom sediment of salt marsh ponds. The¹³C signature of organic carbon in bottom sediment from saline regions corresponded to the size of the body of water. The smaller salt marsh ponds contain sediment with ¹³C values close to that of the C₄ plantSpartina alterniflora. Results suggest that phytoplankton rather thanSpartina alterniffora is the likely organic source in bottom sediment of the larger bay near the coast (e.g. Caminada Bay).     

An inducible hydrolase from Aspergillus niger, acting on carbon–carbon bonds, for phlorrhizin and other C-acylated phenols

1. An inducible enzyme catalysing the hydrolysis of phloretin to form phloroglucinol and phloretic acid has been extracted from the acetone-dried powders of the mycelial felts of an Aspergillus niger strain grown in the presence of phlorrhizin. The enzyme was partially purified by treatment with protamine sulphate, ammonium sulphate fractionation, negative adsorption on tricalcium phosphate gel, and DEAE-cellulose column chromatography. 2. The hydrolytic activity on phloretin appeared to be maximal at about pH9.6. However, the characteristics of the enzyme were studied at pH7.2, because of the lability of the product, phloroglucinol, under alkaline conditions. 3. The apparent K(m) value at pH7.2 was about 0.3-0.4mm for phloretin and 0.15mm for 3'-methylphloracetophenone. 4. Maximum activity of the enzyme was obtained without the addition of any cofactor or metal ion. The involvement of thiol groups in the reaction was demonstrated by the potent inhibitory action of both heavy-metal ions and p-chloromercuribenzoate. 5. The enzyme showed a rather broad substrate specificity, and some other C-acylated phenols related to phloretin were hydrolysed. It was found that 3'-methylphloracetophenone, phloracetophenone and 2',4,4'-trihydroxydihydrochalcone were attacked more efficiently than phloretin. We propose the systematic name C-acylphenol acylhydrolase for the enzyme. This enzyme belongs to EC group 3.7.1.     

Photoproduction of dissolved inorganic carbon in temperate and tropical lakes – dependence on wavelength band and dissolved organic carbon concentration

We have evaluated photoeffects of UV-B, UV-A and PAR radiation on dissolved organic matter (DOM). Photochemical production of dissolved inorganic carbon (DIC) was measured in sterile lake water from Sweden and Brazil after 6 hours of sun exposure. Tubes were exposed to four solar radiation regimes: Full-radiation, Full-radiation minus UV-B, Full-radiation minus UV-B and UV-A (PAR) and darkness.In both areas, lakes with most DOC (varying between 3 and 40 mg C l^-1) were highly humic, resulting in high UV-B attenuation coefficients (K_d = 5–466 m^-1). Under Full-radiation, photooxidative DIC-production varied from 0.09 to 1.7 mg C l^-1per 6 h, without UV-B from 0.07 to 1.4 mg C l^-1 and with PAR only from 0.02 to 0.7 mg C l^-1. UV-B radiation explains a minor part (17%) of the photoooxidative DIC-production, while UV-A and PAR have larger effects (39% and 44%, respectively). Photooxidation was proportional to DOC-content and DIC-production was positively related to decrease in DOC and to loss of absorbance at 250 nm. There was no significant difference in DOC and radiation normalized DIC-production between Swedish and Brazilian lakes. The UV-B dose during incubations was approximately 3 times higher in Brazil compared to Sweden, while UV-A and PAR doses were similar. We conclude that DOC from tropical and temperate freshwaters do not seem to differ with respect to sensitivity to photooxidation.     

Historical and projected carbon balance of mature black spruce ecosystems across North America: the role of carbon–nitrogen interactions

The role of carbon (C) and nitrogen (N) interactions on sequestration of atmospheric CO₂ in black spruce ecosystems across North America was evaluated with the Terrestrial Ecosystem Model (TEM) by applying parameterizations of the model in which C–N dynamics were either coupled or uncoupled. First, the performance of the parameterizations, which were developed for the dynamics of black spruce ecosystems at the Bonanza Creek Long-Term Ecological Research site in Alaska, were evaluated by simulating C dynamics at eddy correlation tower sites in the Boreal Ecosystem Atmosphere Study (BOREAS) for black spruce ecosystems in the northern study area (northern site) and the southern study area (southern site) with local climate data. We compared simulated monthly growing season (May to September) estimates of gross primary production (GPP), total ecosystem respiration (RESP), and net ecosystem production (NEP) from 1994 to 1997 to available field-based estimates at both sites. At the northern site, monthly growing season estimates of GPP and RESP for the coupled and uncoupled simulations were highly correlated with the field-based estimates (coupled: R ²= 0.77, 0.88 for GPP and RESP; uncoupled: R ² = 0.67, 0.92 for GPP and RESP). Although the simulated seasonal pattern of NEP generally matched the field-based data, the correlations between field-based and simulated monthly growing season NEP were lower (R ² = 0.40, 0.00 for coupled and uncoupled simulations, respectively) in comparison to the correlations between field-based and simulated GPP and RESP. The annual NEP simulated by the coupled parameterization fell within the uncertainty of field-based estimates in two of three years. On the other hand, annual NEP simulated by the uncoupled parameterization only fell within the field-based uncertainty in one of three years. At the southern site, simulated NEP generally matched field-based NEP estimates, and the correlation between monthly growing season field-based and simulated NEP (R ² = 0.36, 0.20 for coupled and uncoupled simulations, respectively) was similar to the correlations at the northern site. To evaluate the role of N dynamics in C balance of black spruce ecosystems across North America, we simulated historical and projected C dynamics from 1900 to 2100 with a global-based climatology at 0.5° resolution (latitude × longitude) with both the coupled and uncoupled parameterizations of TEM. From analyses at the northern site, several consistent patterns emerge. There was greater inter-annual variability in net primary production (NPP) simulated by the uncoupled parameterization as compared to the coupled parameterization, which led to substantial differences in inter-annual variability in NEP between the parameterizations. The divergence between NPP and heterotrophic respiration was greater in the uncoupled simulation, resulting in more C sequestration during the projected period. These responses were the result of fundamentally different responses of the coupled and uncoupled parameterizations to changes in CO₂ and climate.     

Are old forests underestimated as global carbon sinks?

Old forests are important carbon pools, but are thought to be insignificant as current atmospheric carbon sinks. This perception is based on the assumption that changes in productivity with age in complex, multiaged, multispecies natural forests can be modelled simply as scaled‐up versions of individual trees or even‐aged stands. This assumption was tested by measuring the net primary productivity (NPP) of natural subalpine forests in the Northern Rocky Mountains, where NPP is from 50% to 100% higher than predicted by a model of an even‐age forest composed of a single species. If process‐based terrestrial carbon models underestimate NPP by 50% in just one quarter of the temperate coniferous forests throughout the world, then global NPP is being underestimated by 145 Tg of carbon annually. This is equivalent to 4.3–7.6% of the missing atmospheric carbon sink. These results emphasize the need to account for multiple‐aged, species‐diverse, mature forests in models of terrestrial carbon dynamics to approximate the global carbon budget.     

The carbon cycle on early Earth--and on Mars?

One of the goals of the present Martian exploration is to search for evidence of extinct (or even extant) life. This could be redefined as a search for carbon. The carbon cycle (or, more properly, cycles) on Earth is a complex interaction among three reservoirs: the atmosphere; the hydrosphere; and the lithosphere. Superimposed on this is the biosphere, and its presence influences the fixing and release of carbon in these reservoirs over different time-scales. The overall carbon balance is kept at equilibrium on the surface by a combination of tectonic processes (which bury carbon), volcanism (which releases it) and biology (which mediates it). In contrast to Earth, Mars presently has no active tectonic system; neither does it possess a significant biosphere. However, these observations might not necessarily have held in the past. By looking at how Earth's carbon cycles have changed with time, as both the Earth's tectonic structure and a more sophisticated biology have evolved, and also by constructing a carbon cycle for Mars based on the carbon chemistry of Martian meteorites, we investigate whether or not there is evidence for a Martian biosphere.