Accumulation Of Particulate Organic Carbon Research Articles

Regulation of soil organic carbon dynamics by microbial communities during reforestation of Chinese fir plantations after clear‐cutting

AbstractReforestation after forest clear‐cutting is an effective measure to increase soil organic carbon (SOC) sequestration; still, the soil C balance under reforestation and the role of microbial communities in that process remain to be determined. Samples of organic (0–2 cm) and mineral (2–10 cm) horizons were collected from the 7‐, 15‐, 20‐, 29‐, and 36‐year‐old forest stands of Chinese fir (Cunninghamia lanceolata) after plantation clear‐cutting in subtropical zone under the condition of phosphorus limitation. Particulate organic carbon (POC), mineral‐associated organic carbon (MAOC), microbial phospholipid fatty acids (PLFAs), and enzymatic activities for C, nitrogen (N), and phosphorus (P) acquisition were analyzed. The lowest contents of POC (10%) and MAOC (13%) in the organic horizon were found in 7‐year‐old stands due to the slow tree regrowth and extensive decomposition of SOC in the first years of forest regrowth. POC (2.0×) and MAOC (0.8×) increases in the organic horizon with forest age were attributed to the stand development and accumulation of above‐ and belowground litter. The organic horizon had a higher POC:MAOC ratio than the mineral (0.7–1.1 vs. 0.2–0.5), indicating lower SOC stability in the first one. The ratio of POC:MAOC increased with the Gram‐positive to Gram‐negative bacteria (G+:G‐) ratio, pointing out that microbial communities developed a specific community structure and substrate utilization strategies of organic matter under plantation restoration. The increase of total PLFAs and the G+:G‐ ratio was closely linked with the microbial C and P limitations, indicating that microorganisms shifted community structure to slow‐growing species and increased their content to cope with the C and P restrictions. In the soils of young plantations, microorganisms were limited by C and P; however, the C limitation was alleviated in the 36‐year‐old plots in the organic horizon due to increased litter input, whereas the P limitation was not. This discrepancy between C and P limitation suppressed the decomposition of litter entering the soil, which was seen in decreased specific activity of C degrading enzymes and led to the accumulation of POC in the organic horizon. Thus, soil C sequestration under reforestation of Chinese fir can be controlled by the amount of litter entering the soil and by metabolic C, N, and P limitations that force microorganisms to shift community structure and change their activity.

Nitrogen Addition Promotes the Accumulation of Soil Particulate Organic Carbon in a Subtropical Forest

Nitrogen (N) deposition rates of terrestrial ecosystems have gradually declined but are still high in some areas. Previous studies have reported that N addition elicits diverse impacts on soil organic carbon (SOC) pools. SOC can be divided into different functional fractions, namely, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). The responses of these fractions to N addition should be elucidated to better understand the changes in SOC pools. Here, we conducted a N addition experiment (0, 40, and 80 kg N ha−1 yr−1) in a subtropical Castanopsis fabri forest to simulate N deposition. The surface (0−10 cm) SOC fractions, aboveground litter product, fine root (diameter < 2 mm) biomass, soil exchangeable cation content, and soil enzyme activity under different N addition treatments were measured. The results showed the following: (1) N addition showed a positive effect on POC and SOC contents but did not significantly affect MAOC content; (2) POC content was negatively correlated with pH and soil enzyme activity and positively correlated with aboveground litter product, suggesting that POC accumulation was influenced by aboveground litter input and microbial decomposition; (3) a close negative relationship was observed between exchangeable Al3+ and Ca2+ or K+ contents, indicating that there is likely to be a trade-off between the mineral sorption and desorption, thus resulting in an insignificant reaction of MAOC to N addition. Overall, the accumulation of SOC under short-term N addition was found to be primarily driven by POC, and the response of different SOC functional fractions to N addition was inconsistent. By incorporating these nuances into ecosystem models, it is possible to predict SOC dynamics more accurately in response to global change.

Nitrogen availability regulates the effects of a simulated marine heatwave on carbon sequestration and phycosphere bacteria of a marine crop

AbstractGreat hope has been pinned on seaweed cultivation as being a potent way of removing CO2 to reduce rates of sea surface warming and acidification. Marine heatwaves and nitrogen pollution in coastal ecosystems are serious current issues that need to be better understood to inform decision making and policy. Here, we investigated the effects of a simulated heatwave and nitrogen pollution on carbon sequestration by an important seaweed crop species and its phycosphere bacteria. Gracilaria lemaneiformis was grown in ambient and high nitrogen conditions (14 and 200 μM L−1). Photosynthetic rate, seaweed biomass and particulate organic carbon accumulation were significantly increased in “high nitrogen‐no heatwave” conditions. In “ambient nitrogen heatwave” conditions, the expression of genes related to photosynthesis was down regulated and the seaweeds lost more dissolved organic carbon (DOC) to the surrounding water, resulting in more refractory dissolved organic carbon (RDOC). In “high nitrogen heatwave” conditions, photosynthetic gene expression was upregulated; bacterial abundance was also increased that can explain the reduced DOC and RDOC accumulation. The simulated heatwave reduced bacterial diversity while high nitrogen alleviated this effect. These findings suggest that the economically important alga G. lemaneiformis may lose more DOC and RDOC to nearshore waters during marine heatwave events, enhancing carbon sequestration, while nitrogen enrichment has a counteractive effect.

Effects of Straw Returning with Lime on SOC and Carbon Pool Management in Acidic Paddy Soil

Soil acidification and low SOC are the main limiting factors in acidic paddy soils. Straw returning with lime is an effective measure to alleviate soil acidification and improve soil fertility; however, its interaction effects on SOC and carbon pool management are still unclear. To investigate the impact of straw returning with lime on the organic carbon pool of acidic paddy soil, field experiments were conducted on acidic paddy soil in Baiyun District and Huiyang District of Guangdong Province. The changes in soil total organic carbon (TOC), water-soluble organic carbon (DOC), active organic carbon (LOC), particulate organic carbon (POC), microbial biomass carbon (MBC), carbon pool index (CPI), stable organic carbon (IOC), carbon pool activity (L), carbon pool activity index (CPAI), and carbon pool management index (CPMI) were analyzed under three treatments (CK, conventional fertilization; RS, straw returning+conventional fertilization; RS+L straw returning with lime+conventional fertilization). The results demonstrated that compared with that in CK, the TOC, LOC, POC, and MBC in the RS+L treatment were significantly increased by 10.24%-17.79%, 34.49%-44.37%, 19.27%-23.59%, and 33.36%-43.26%, respectively (P<0.05). Compared with that in CK, the RS+L treatment significantly increased the DOC content during the early growth stage (15-45 days after transplanting) of rice (P<0.05) but had no significant influence on the DOC content during the late growth stage of rice. Compared with that in RS, the TOC, LOC, POC, and MBC in the RS+L treatment were increased by 2.15%-6.95%, 1.17%-17.90%, 4.27%-8.65%, and 12.99%-14.53%, respectively. Compared with that in CK, the RS+L treatment significantly increased IOC and CPI by 8.32%-15.57% and 14.00%-20.00%, respectively (P<0.05). Compared with that in the CK treatment, the RS treatment significantly increased CPI by 14.00%-18.00% (P<0.05). No significant differences in L, CPAI, or CPMI were detected among the different treatments. The soil pH in the RS+L treatment was significantly higher than that in the CK treatment (P<0.05). No significant differences in rice yield were detected among the different treatments. Principal component analysis demonstrated that rice yield was primarily correlated with DOC, LOC, CPAI, and CPMI but its contribution to SOC and carbon pool management index was low. Principal component analysis also indicated that straw returning with lime could improve soil pH and nutrient contents of acidic paddy soil, driving the formation and accumulation of organic carbon fraction such as MBC and POC, thus boosting the increase in SOC. In conclusion, straw returning with lime is beneficial to the accumulation of MBC, POC, LOC, and IOC in acidic paddy soil to improve the content and stability of soil total organic carbon, which is an effective way to improve the carbon sequestration of acidic paddy soil.

Persistent soil organic carbon deficits from converting primary forests to plantations and secondary forests in subtropical China

Soil organic carbon (SOC) plays a critical role in supporting key ecosystem functions and services for humanity. Despite the widespread conversion of primary forests to plantations and secondary forests, it remains uncertain how it affects the content and distribution of SOC. We evaluated SOC content, its components in two fractions, particulate organic carbon (a mean residence time shorter than decades) and mineral-associated organic carbon (a mean residence time of multiple decades to centuries), and the fine root characteristics (biomass, necromass, and C: N of dead fine roots) across Chinese fir plantations and secondary forests during stand development (5–41 years old) in subtropical China, employing primary forests as a reference. We found that SOC content differed significantly between forest types and soil depths, but not with stand age within plantations or secondary forests. In the topsoil (0–10 cm), the SOC content of primary and secondary forests was 47 % and 25 % higher than that of Chinese fir plantations, respectively. The SOC content in the subsoil (40–60 cm) was approximately 6 ± 1.7 mg C g−1 and did not change significantly with forest types or stand ages. The higher SOC contents in primary forests and secondary forests in topsoil resulted primarily from particulate organic carbon accumulation caused by higher fine root biomass and dead root C: N. The fraction of mineral-associated organic carbon to SOC increased with soil depth and was higher in Chinese fir plantations than in secondary forests, suggesting that more SOC is composed of mineral-associated organic carbon that was associated with a higher amount of fine root necromass. Contrary to our expectations, we found SOC content or its fractions did not increase with 40 years of plantation and secondary forest development. Our results reveal persistent SOC deficits from converting primary forests to plantations and secondary forests and highlight the urgent need to conserve the remaining primary forests and protect secondary forests and plantations from disturbances, allowing them to evolve towards older-aged primary forests.

Coupling amendment of microbial and compound fertilizers increases fungal necromass carbon and soil organic carbon by regulating microbial activity in flue-cured tobacco-planted field

Microbial necromass carbon (MNC) is an essential component of soil organic carbon (SOC). The contribution of MNC to SOC has been acknowledged in previous studies; however, there is a gap in understanding the effect of fertilization treatment on MNC, especially in rhizosphere soil. In the current study, four types of treatments were selected to examine the impact of fertilization on MNC and its contribution to SOC, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). These treatments included conventional compound fertilizer (Control), conventional compound fertilizer combined with microbial fertilizer (T1), a mixture of 75% conventional compound fertilizer with microbial fertilizer (T2), and microbial fertilizer (T3). The results indicated that the type of fertilization treatments and sampling time affected the MNC accumulation, especially fungal necromass carbon (FNC), and its contribution to SOC. Combined application of microbial and compound fertilizers increased the amino sugar (TAS) and MNC contents, especially during the harvest period (3.47–27.35%). The accumulation of MNC (5.13–9.42 g kg−1) and its contribution to SOC (38.03–50.49%) increased with the growth period, and the ratios of FNC to SOC, POC, and MAOC were about three times than that of the bacterial necromass carbon (BNC). Further, the regression analysis revealed a higher POC accumulation than MAOC. In addition, POC was positively correlated with MNC and FNC (p < 0.05). RDA analysis exhibited that microbial biomass phosphorus, phosphorus acquiring enzyme activity, POC, and the ratio of fungal to bacterial biomass had significant effects on amino sugar and MNC accumulation and contribution (p < 0.05). Further, T2 was found to be beneficial for FNC accumulation, promoting POC formation and increasing SOC content in tobacco planting soil. Taken together, our findings suggest that fertilization treatments affected microbial activity by regulating the N or P demand, leading to modulations in MNC accumulation and SOC storage.

Root architectures differentiate the composition of organic carbon in bauxite residue during natural vegetation

Understanding plant root architectures induced changes in organic carbon accumulation and conversion is critical to predicting carbon cycling and screening appropriate plant species for ecological restoration on bauxite residue disposal areas. According to the ecological investigation of a weathered bauxite residue disposal area, three plants with different root architectures including Artemisia lavandulaefolia (A. lavandulaefolia), moss, and Zanthoxylum simulans (Z. simulans) were selected to investigate the rhizosphere effects on the composition and structure of organic carbon in bauxite residue. The physic-chemical properties, the contents and structure of different organic carbon fractions, and microbial communities of bauxite residue from rhizosphere and non-rhizosphere were analyzed. Plant growth decreased the saline-alkalinity, increased the contents of total organic carbon, particulate organic carbon and dissolved organic carbon, whilst enhancing the enzymatic activities of bauxite residue. Meanwhile, the rhizosphere effects had significant effects on the accumulation and stabilization of organic carbon in bauxite residue. A. lavandulaefolia had the strongest rhizosphere effects on the composition and structure of total organic carbon and dissolved organic carbon, whilst moss was more effective on the accumulation of particulate organic carbon in bauxite residue. Plant growth and root architecture changed the abundance of specific functional microorganisms and the complexity of microbial co-occurrence networks, thus elevating organic carbon levels in bauxite residue. During natural vegetation encroachment, rhizosphere exciting effects of the salt-tolerated plants could change the composition and structure of organic carbon fractions due to the comprehensive effectiveness of the improvement of physic-chemical properties and microbial communities. The findings improve our understanding of the responses of sequestration and stabilization of organic carbon pools to ecological restoration on bauxite residue disposal areas.

Dynamics of organic matter in the changing environment of a stratified marine lake over two decades

The marine lake (Rogoznica Lake), which fluctuates between stratified and holomictic conditions, is a unique environment on the eastern Adriatic coast affected by environmental changes. These changes are reflected in the warming of the water column, the apparent deoxygenation of the epilimnion, and the accumulation of organic matter (OM), toxic sulfide, and ammonium in the anoxic hypolimnion. Since the early 1990s, the volume of anoxic water has increased as the chemocline has moved to the surface water layer. A trend toward enrichment of refractory dissolved organic carbon (DOC) was observed in the anoxic hypolimnion, while a decreasing trend was observed in the oxic epilimnion in the spring DOC. At the same time, the most reactive surface-active fraction of DOC showed the opposite trend. In addition, there is evidence of accumulation of particulate organic carbon (POC) in the water column, followed by an increase in the fraction of POC in total organic carbon (TOC).On a multi-year scale (1996–2020), this work presents a unique time series of the dynamics of OM in the stratified marine system, showing a significant change in its quantity and quality due to climate and environmental variability. DOC-normalized surfactant activity is shown to be a good indicator of environmental change.

Seasonal dynamics and export of biogenic silica in the upper water column of a large marginal sea, the northern South China Sea

Biogenic silica (BSi), primarily derived from marine diatoms, is a major component of the oceanic silicon cycle. It links with the global carbon cycle through the silicate pump via photosynthesis and the so-called “ballast” effect in many areas of the world’s oceans, particularly in ocean margins. Here we examined the distribution and export of BSi based on a large data set collected from the upper water column of the northern South China Sea (NSCS), the largest marginal sea of the Pacific Ocean. Generally, BSi concentrations decreased from the shelf to the basin in all seasons. The highest value was observed in nearshore surface waters in summer, associated with a diatom bloom in the far-reaches of river plumes. Seasonally, BSi concentrations were overall higher in autumn and winter than in spring and summer, due to elevated diatom productivity supported by an increased vertical supply of nutrients in dry seasons. Patterns of BSi spatial and seasonal variability were well represented by those of the pigment fucoxanthin, demonstrating the dominant role of diatoms in controlling BSi dynamics in the upper NSCS. Mainly regulated by BSi standing stock, BSi export flux from the euphotic zone, determined using thorium-234/uranium-238 disequilibrium, was on average higher on the shelf than in the basin, and in the dry than wet season. Furthermore, the high BSi export helped to promote the export of particulate organic carbon (POC) on the shelf, while the POC standing stock was comparable between different regions of the NSCS. Thus, the silicate pump, driven by diatoms in the upper water column, functioned more efficiently under eutrophic conditions. A comparison with prior measurements of particle sinking fluxes using sediment traps showed that the correlation of BSi and POC increased and became more significant with increasing water depth. This provides direct evidence of the pronounced ballast effect of BSi in transferring POC into deep waters of the NSCS basin, even though the contribution of diatoms to POC accumulation in the euphotic zone was minor in this oligotrophic environment.

Nutrients and particulate organic matter dynamics in the outer-shelf of the South Brazil Bight: Two distinct scenarios during summer 2013

The pool of particulate organic matter driven by primary production in western boundary continental margins is known to be smaller than the highly eutrophic upwelling systems of eastern margins. Its production and export, however, can be high compared to productive ocean environments due to inherent specific physical dynamics such as mesoscale instabilities driven by the boundary current. To investigate how such physical process influence nutrients, chlorophyll-a and particulate matter on the outer shelf of the South Brazil Bight, two oceanographic cruises were conducted three months apart (November 2012 and January 2013), both under wind conditions that favor intrusions of South Atlantic Central Water (SACW). Our results showed two distinct hydrographic scenarios derived by the relative positions of oceanographic fronts in the outer shelf and shelf break region, associated with the Brazil Current (BC). In November 2012, the oligotrophic Tropical Water occupied a larger area of the continental shelf. Low nutrients and chlorophyll-a concentrations were found, typical of a regenerated production system. In contrast, maximum concentration of particulate matter and particulate inorganic carbon occurred at deeper layers in this period, together with low POC:PON ratio — probably from sediment resuspension. In January 2013, a strong shelf break upwelling was detected that enhanced SACW intrusion, notably increasing nutrient availability in the euphotic zone and the chlorophyll-a concentration at deeper layers. These deep chlorophyll maximum layers contribute to the production and accumulation of particulate organic carbon and nitrogen under these conditions. BC mesoscale instabilities appear to act as a particulate matter transport and export agent to the deep waters throughout the South Brazil Bight outer shelf.

Provenances, distribution, and accumulation of organic matter in the southern Mariana Trench rim and slope: Implication for carbon cycle and burial in hadal trenches

It is generally accepted that organic matter content and accumulation rate at the seafloor attenuate with increasing water depth. However, hadal trenches, which represent the deepest portion of the hydrosphere on Earth, do not abide by this universal rule. It has been speculated that hadal trenches would serve as organic matter depocenters, where bacteria-mediated organic matter degradation intensifies. Here we examine the contents of total organic carbon (TOC) and total nitrogen (TN), as well as the δ13C values of TOC in three gravity cores, two box cores, and three grab samples with water depth from 4900 to 7118m, in order to reveal the provenances of organic matter, spatial distribution and accumulation rates of particulate organic carbon (POC) in the southern Mariana Trench rim and slope. Although the deepest area was not sampled, trench rim and slope is also of importance in terms of transportation and accumulation processes of organic matter. A vast majority of the sediment samples have bulk TOC/TN molar ratios and δ13C values of TOC ranging from 4.2 to 11 and from −21.8 to −18.9‰, respectively, implying that the organic matter was primarily sourced from marine algae. Two exceptions have been found at 101cm and 201cm depths of core GC05 with a possible input of terrestrial material suggested by an abrupt increase in TOC contents and TOC/TN molar ratios accompanied by marked decreases in δ13C values of TOC. Moreover, TOC contents in surface sediments basically increased with water depth. Based on the published excess 210Pb data and by assuming a simplified one-dimensional sediment loading despite of frequent occurrences of mass wasting transports in hadal trenches, the sedimentation rate in the southern Challenger Deep (6037m) was estimated to be 0.02cmyr−1. This much higher sedimentation rate compared to the globally averaged values in deep ocean sediments coupled with the trend of increasing in TOC contents with water depth may serve as evidence for the lateral transport of sediment particles induced by the funnel-like topography and localized current dynamics within the trench. The average POC accumulation rate within the southern Challenger Deep roughly amounts to 1.5×10−5gcm−2yr−1, equivalent to about a seventh of the globally averaged POC accumulation rate in deep-ocean seafloor. To our knowledge, POC accumulation in hadal trenches has been quantified for the first time, and we highlight that organic matter degradation in deep hadal trenches should not be neglected and may represent a significant component of the global carbon cycle.

Intertidal zone particulate organic carbon redistribution by low‐tide rainfall

AbstractWe present field data and data from the literature to highlight the effects of low‐tide rainfall on particulate organic carbon (POC) redistribution in intertidal landscapes. The POC exchanges reported from disparate but related studies were standardized to a storm‐induced exchange rate (gPOC m−2 mmRain−1) and compared. Results show that these intertidal areas have a characteristic response to rainfall with an average flux of 0.040 ± 0.038 gPOC m−2 mmRain−1. Further, low‐tide rainfall can entrain and redistribute 7–54% of annual sedimentary POC accumulation, or 12–75% of annual POC export, based on current outwelling assessments. Finally, we provide a conceptual model describing the variability of rainfall‐driven POC exchange through the hierarchical structure of intertidal landscapes and how observations of POC flux can be expected to change across the intertidal landscape. This information should be used to guide sampling strategies for continued intertidal zone rainfall work.

Carbon Sequestration and Soil Accretion in Coastal Wetland Communities of the Yellow River Delta and Liaohe Delta, China

Rates of accumulation of particulate organic carbon (POC) and particulate inorganic carbon (PIC) as well as soil accretion rates were determined in the Yellow River Delta (YRD) and Liaohe Delta (LHD) in northeastern China. Rates of carbon accumulation were dominated by PIC accumulation in the YRD and by POC accumulation in the LHD. Rates of POC accumulation at sites above mean sea level were similar in the two deltas, 141 ± 17 g C m−2 y−1, but were much higher, 381 ± 10 g C m−2 y−1, at sites in the LHD below mean sea level. High rates of PIC accumulation in the YRD, 916 ± 133 g C m−2 y−1, reflect the very high concentration of dissolved inorganic carbon in the Yellow River, which in turn reflects the erosion of carbonate-rich glacial dust and evaporates minerals in the Loess Plateau, the excess of evaporation over precipitation in the watershed, and the fact that much of the flow in the river is diverted to irrigate crops. In both river deltas, accumulation of PIC was positively correlated with accumulation of POC, although the sources of the PIC are allochthonous and the sources of the POC are primarily autochthonous. Rates of POC accumulation were closely correlated with the species of wetland plants in the deltas. Although the rates of PIC accumulation in the YRD are exceptional, anthropogenic effects, including climate change, are creating conditions that will likely enhance PIC accumulation in many other river deltas.

Deposition, burial and sequestration of carbon in an oligotrophic, tropical lake

The amount of biogenic carbon that may be deposited, buried and eventually preserved (sequestered) in the sediments of a tropical, oligotrophic lake, was evaluated based on i) the temporal variation of the particulate organic carbon (POC) concentration in the superficial sediments in the deep zone of lake Alchichica, Puebla, Mexico; and ii) the POC accumulation and preservation in a 210Pb-dated sediment core from the lake. In lake Alchichica the POC concentration in the surficial sediments ranged between 12 and 60 mg POC g-1 (25 ± 12 mg POC g-1 dry weight). The magnitude of the sedimented POC in Alchichica was high and mostly of autochthonous origin. The POC concentrations recorded in the sediment core (16.6 to 31.6 mg g-1 dry weight) were comparable to the concentration range observed in the surface sediment samples collected during the study period, which signaled a high POC preservation capacity in the sedimentary column of lake Alchichica. The POC fluxes, estimated from the 210Pb-dated sediment core, varied between 14.9 and 35.3 g m-2 year-1 within the past century; and the maximum POC losses through diagenesis during this period were estimated to be lower than 25%. This study concludes that deep tropical lakes, exemplified by lake Alchichica, accumulate and preserve most of the POC deposited, playing an important role in regional carbon balances.

Estimation of the global inventory of methane hydrates in marine sediments using transfer functions

Abstract. The accumulation of gas hydrates in marine sediments is essentially controlled by the accumulation of particulate organic carbon (POC) which is microbially converted into methane, the thickness of the gas hydrate stability zone (GHSZ) where methane can be trapped, the sedimentation rate (SR) that controls the time that POC and the generated methane stays within the GHSZ, and the delivery of methane from deep-seated sediments by ascending pore fluids and gas into the GHSZ. Recently, Wallmann et al. (2012) presented transfer functions to predict the gas hydrate inventory in diffusion-controlled geological systems based on SR, POC and GHSZ thickness for two different scenarios: normal and full compacting sediments. We apply these functions to global data sets of bathymetry, heat flow, seafloor temperature, POC input and SR, estimating a global mass of carbon stored in marine methane hydrates from 3 to 455 Gt of carbon (GtC) depending on the sedimentation and compaction conditions. The global sediment volume of the GHSZ in continental margins is estimated to be 60–67 × 1015 m3, with a total of 7 × 1015 m3 of pore volume (available for GH accumulation). However, seepage of methane-rich fluids is known to have a pronounced effect on gas hydrate accumulation. Therefore, we carried out a set of systematic model runs with the transport-reaction code in order to derive an extended transfer function explicitly considering upward fluid advection. Using averaged fluid velocities for active margins, which were derived from mass balance considerations, this extended transfer function predicts the enhanced gas hydrate accumulation along the continental margins worldwide. Different scenarios were investigated resulting in a global mass of sub-seafloor gas hydrates of ~ 550 GtC. Overall, our systematic approach allows to clearly and quantitatively distinguish between the effect of biogenic methane generation from POC and fluid advection on the accumulation of gas hydrate, and hence, provides a simple prognostic tool for the estimation of large-scale and global gas hydrate inventories in marine sediments.

The Global Inventory of Methane Hydrate in Marine Sediments: A Theoretical Approach

The accumulation of methane hydrate in marine sediments is controlled by a number of physical and biogeochemical parameters including the thickness of the gas hydrate stability zone (GHSZ), the solubility of methane in pore fluids, the accumulation of particulate organic carbon at the seafloor, the kinetics of microbial organic matter degradation and methane generation in marine sediments, sediment compaction and the ascent of deep-seated pore fluids and methane gas into the GHSZ. Our present knowledge on these controlling factors is discussed and new estimates of global sediment and methane fluxes are provided applying a transport-reaction model at global scale. The modeling and the data evaluation yield improved and better constrained estimates of the global pore volume within the modern GHSZ ( ≥ 44 × 1015 m3), the Holocene POC accumulation rate at the seabed (~1.4 × 1014 g yr−1), the global rate of microbial methane production in the deep biosphere (4−25 × 1012 g C yr−1) and the inventory of methane hydrates in marine sediments ( ≥ 455 Gt of methane-bound carbon).

Effects of rising temperature on pelagic biogeochemistry in mesocosm systems: a comparative analysis of the AQUASHIFT Kiel experiments

A comparative analysis of data, obtained during four indoor-mesocosm experiments with natural spring plankton communities from the Baltic Sea, was conducted to investigate whether biogeochemical cycling is affected by an increase in water temperature of up to 6 °C above present-day conditions. In all experiments, warming stimulated in particular heterotrophic bacterial processes and had an accelerating effect on the temporal development of phytoplankton blooms. This was also mirrored in the build-up and partitioning of organic matter between particulate and dissolved phases. Thus, warming increased both the magnitude and rate of dissolved organic carbon (DOC) build-up, whereas the accumulation of particulate organic carbon (POC) and phosphorus (POP) decreased with rising temperature. In concert, the observed temperature-mediated changes in biogeochemical components suggest strong shifts in the functioning of marine pelagic food webs and the ocean’s biological carbon pump, hence providing potential feedback mechanisms to Earth’s climate system.

Dynamics of organic carbon under different inorganic nitrogen levels and phytoplankton composition

We examined the production and partitioning of organic carbon under different inorganic nitrogen concentrations and phytoplankton composition by incubating unaltered seawater collected from 10, 100, 400 and 700 m depths off Suruga Bay, Japan. Particulate organic carbon (POC) accounted for 60–112% of net accumulated total organic carbon (TOC) during exponential growth and blooms of phytoplankton in all incubations regardless of the concentrations of nutrients and the forms of dissolved inorganic nitrogen (DIN) as well as the composition and size of phytoplankton. After the blooms of phytoplankton, microzooplankton (mainly ciliates) (>20 μm) increased rapidly in majority of incubations. This suggests that a large fraction of new production may also be transferred to higher trophic levels rather than immediately exported. After the depletion of initial DIN, accumulation of both POC and dissolved organic carbon (DOC) increased. DOC accounted for major fraction of TOC in most of the incubations. Continued accumulation of high C/N organic matter implies that recycling through the food web is not necessarily the dominant fate of organic carbon produced under low DIN condition but rather that a major fraction of the organic carbon is exported to deep water through sinking.

Particulate organic carbon (POC) in surface sediments of the Baltic Sea

In this study, particulate organic carbon (POC) contents and their distribution pattern in surficial sediments of the Baltic Sea are presented for 1,471 sampling stations. POC contents range from approx. 0.1% in shallow sandy areas up to 16% in deep muddy basins (e.g. Gotland Basin). Some novel relationships were identified between sediment mass physical properties (dry bulk density (DBD), grain size) and POC levels. Notably, the highest POC concentrations (about 10–17 mg cm–3) occur in sandy mud to mud (60–100% mud content) with intermediate POC contents of about 3–7% and DBDs of 0.1–0.4 g cm–3. Areas with this range in values seem to represent the optimum conditions for POC accumulation in the Baltic Sea. The maximum POC contents (8–16%) are found in fluid mud of the central Baltic Sea characterized by extremely low DBDs (<0.1 g cm–3) and moderate POC concentrations (4–7 mg cm–3). Furthermore, sediment mass accumulation rates (MAR), based on 210Pb and 137Cs measurements and available for 303 sites of the Baltic Sea, were used for assessing the spatial distribution of POC burial rates. Overall, these vary between 14 and 35 g m–2 year–1 in the mud depositional areas and, in total, at least 3.5 (±2.9) Mt POC are buried annually. Distribution patterns of POC contents and burial rates are not identical for the central Baltic Sea because of the low MAR in this area. The presented data characterize Baltic Sea sediments as an important sink for organic carbon. Regional differences in organic carbon deposition can be explained by the origin and transport pathways of POC, as well as the environmental conditions prevailing at the seafloor (morphology, currents, redox conditions). These findings can serve to improve budget calculations and modelling of the carbon cycle in this large brackish-water marginal sea.

A transfer function for the prediction of gas hydrate inventories in marine sediments

Abstract. A simple prognostic tool for gas hydrate (GH) quantification in marine sediments is presented based on a diagenetic transport-reaction model approach. One of the most crucial factors for the application of diagenetic models is the accurate formulation of microbial degradation rates of particulate organic carbon (POC) and the coupled formation of biogenic methane. Wallmann et al. (2006) suggested a kinetic formulation considering the ageing effects of POC and accumulation of reaction products (CH4, CO2) in the pore water. This model is applied to data sets of several ODP sites in order to test its general validity. Based on a thorough parameter analysis considering a wide range of environmental conditions, the POC accumulation rate (POCar in g/m2/yr) and the thickness of the gas hydrate stability zone (GHSZ in m) were identified as the most important and independent controls for biogenic GH formation. Hence, depth-integrated GH inventories in marine sediments (GHI in g of CH4 per cm2 seafloor area) can be estimated as: GHI = a · POCar · GHSZb · exp (– GHSZc/POCar/d) + e with a = 0.00214, b = 1.234, c = –3.339, d = 0.3148, e = –10.265. The transfer function gives a realistic first order approximation of the minimum GH inventory in low gas flux (LGF) systems. The overall advantage of the presented function is its simplicity compared to the application of complex numerical models, because only two easily accessible parameters need to be determined.