Particulate Organic Carbon Concentrations Research Articles

Atmospheric Particles Are Major Sources of Aged Anthropogenic Organic Carbon in Marginal Seas.

Deposition of atmospheric particulates is a major pathway for transporting materials from land to the ocean, with important implications for climate and nutrient cycling in the ocean. Here, we report the results of year-round measurements of particulate organic carbon (POC) and black carbon (BC) in atmospheric aerosols collected on Tuoji Island in the coastal Bohai-Yellow Sea of China (2019-2020) and during a cruise in the western North Pacific. Aerosol POC contents ranged from 1.9 to 11.9%; isotope values ranged from -18.8 to -29.0‰ for δ13C and -150 to -892‰ for Δ14C, corresponding to 14C ages of 1,235 to 17,780 years before present (BP). Mass balance calculations indicated that fossil carbon contributed 19-66% of the POC, with highest values in winter. BC produced from fossil fuel combustion accounted for 18-54% of the POC. "Old" BC (mean 6,238 ± 740 yr BP) was the major contributor to POC, and the old ages of aerosol POC were consistent with the 14C ages of total OC preserved in surface sediments of the Bohai-Yellow Sea and East China Sea. We conclude that atmospheric deposition is an important source of aged OC sequestered in marginal sea sediments and thus represents an important sink for carbon dioxide from the atmosphere.

Role of fertilization regime on soil carbon sequestration and crop yield in a maize-cowpea intercropping system on low fertility soils

Achieving food security through intensive agricultural practices on low fertility soils is challenging as crop productivity is increasingly curtailed by the loss of soil structural stability and rapid depletion of soil organic carbon (SOC). As such, the conversion from traditional mono-cropping to legume-cereal intercropping, especially with integrated fertilization, may increase crop yields with the least ecological footprint. We set up a 2-year field experiment in a split-plot design with cowpea-maize monoculture and intercropping under different organic–inorganic fertilization regimes, including no fertilization (control), organic input only (compost), chemical input only (NPK), and multi-nutrient enriched compost (NPKEC). We observed that intercropped maize had a significantly higher biomass yield compared to the corresponding monoculture when fertilized with NPKEC fertilizer. However, cowpea biomass yield differences between monoculture and intercropped plots were comparable under all fertilization regimes. In contrast, the grain yield advantage of both maize and cowpea was significantly enhanced under the intercropping system compared to monoculture, with NPKEC showing the most significant effect among all fertilization regimes. When comparing the relative contribution of the fertilization regime to SOC, the NPKEC fertilizer provided the highest SOC-sequestration (0.30 Mg C/ha yr−1). At the same time, the effect of the cropping system on C-sequestration showed that intercropping provided the highest C-sequestration (0.17 Mg C/ha yr−1) compared to monocultures of both crops. Although compost application significantly increased mineral associated (MAOC) and particulate associated organic carbon (PAOC) concentrations compared to unfertilized control plots, NPKEC fertilization with intercropping system was the most effective combination causing the greatest increase of both soil C pools over time. Based on redundancy analysis (RDA), the positive association of MAOC and PAOC with C-sequestration suggests the importance of both organic fractions as primary C reservoirs conducting SOC storage. Importantly, although compost alone in association with intercropping had a lower C-sequestration, it was associated to a better soil structure as confirmed by its positive relationship with macro-and micro-aggregation, water stable aggregates (WSA), and mean weight diameter (MDA). Overall, our results indicate the importance of restoring soil structure in degraded soils through appropriate land management solutions, such as stoichiometrically balanced fertilization practices (NPKEC) and crop diversification (intercropping), in order to achieve significant gains in SOC storage and, ultimately, improve crop productivity.

Response of soil carbon fractions and enzyme activities to mowing management on in a coastal wetland of the yellow river delta

Coastal wetlands are considered as important “blue carbon” sink, and mowing management induced by anthropogenic activities is anticipated to profoundly affect soil carbon stocks in coastal wetlands. However, the impacts of mowing management on soil organic carbon (SOC) and enzyme activities and the mechanisms responsible for associated changes in Phragmites australis wetland remain uncertain. We conducted a field mowing manipulation experiment [control (CK), mowing and returning straw in December (12MS), mowing and removing straw in December (12MR), mowing and returning straw in March (3MS), and mowing and removing straw in March (3MR)] in P. australis wetland of the Yellow River Delta and quantified their impacts on soil quality, SOC, SOC fractions, and enzyme activities. Results showed that mowing treatments led to overall increases in soil nutrients [total carbon (TC), total nitrogen (TN), total phosphorus (TP), NH4+, and NO3−] and decreases in soil C/N ratio. The effects of mowing treatments on soil nutrient content were pronounced on topsoil than deep soil, and the maximum value of TC, TN, and TP reached in the 12MR treatment. Compared with CK, the 12MS, 12MR, 3MS, and 3MR treatments at 0–10 cm depth significantly enhanced SOC content by 8.78%, 32.9%, 16.5%, and 30.1%, respectively, but only the 3MS treatment enhanced SOC by 16.5% at 10–20 cm depth. Mowing treatments increased dissolved organic carbon (DOC), microbial biomass carbon (MBC), particulate organic carbon (POC), and labile organic carbon (LOC) and the contents of DOC, MBC, POC, and LOC decreased with soil depth. Mowing treatments stimulated the activities of sucrase and urease in topsoil, but only the 3MR treatment improved alkaline phosphatase activity in topsoil. Path analysis indicated that mowing management dominantly modulates SOC by changing sucrase activity, alkaline phosphatase activity, TN, TP, NH4+, NO3−, DOC, and LOC in 0–10 cm depth. However, SOC was significantly controlled by sucrase activity, urease activity, TC, TN, TP, LOC, and POC in 10–20 cm depth. Collectively, our results indicate that the continuous mowing management is beneficial to enhance soil quality and carbon storage capacity of P. australis wetlands, which will contribute to enhance carbon sequestration and sink capacity of coastal wetlands.

Particle cycling rates at Station P as estimated from the inversion of POC concentration data

Particle cycling rates in marine systems are difficult to measure directly, but of great interest in understanding how carbon and other elements are distributed throughout the ocean. Here, rates of particle production, aggregation, disaggregation, sinking, remineralization, and transport mediated by zooplankton diel vertical migration were estimated from size-fractionated measurements of particulate organic carbon (POC) concentration collected during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise at Station P in summer 2018. POC data were combined with a particle cycling model using an inverse method. Our estimates of the total POC settling flux throughout the water column are consistent with those derived from thorium-234 disequilibrium and sediment traps. A budget for POC in two size fractions, small (1–51 µm) and large (> 51 µm), was produced for both the euphotic zone (0–100 m) and the upper mesopelagic zone (100–500 m). We estimated that POC export at the base of the euphotic zone was 2.2 ± 0.8 mmol m−2 d−1, and that both small and large particles contributed considerably to the total export flux along the water column. The model results indicated that throughout the upper 500 m, remineralization leads to a larger loss of small POC than does aggregation, whereas disaggregation results in a larger loss of large POC than does remineralization. Of the processes explicitly represented in the model, zooplankton diel vertical migration is a larger source of large POC to the upper mesopelagic zone than the convergence of large POC due to particle sinking. Positive model residuals reveal an even larger unidentified source of large POC in the upper mesopelagic zone. Overall, our posterior estimates of particle cycling rate constants do not deviate much from values reported in the literature, i.e., size-fractionated POC concentration data collected at Station P are largely consistent with prior estimates given their uncertainties. Our budget estimates should provide a useful framework for the interpretation of process-specific observations obtained by various research groups in EXPORTS. Applying our inverse method to other systems could provide insight into how different biogeochemical processes affect the cycling of POC in the upper water column.

Environmentally induced reconstruction of microbial communities alters particulate carbon flux of deep chlorophyll maxima in the South China sea

Abstract The deep chlorophyll maximum (DCM) harbours diverse microbial communities and regulates carbon fixation and sequestration in marine ecosystems. Although variations in the DCM microbial community diversity and composition across oceans are reported, we still have a limited knowledge about the ecological mechanism driving the biogeography of DCM microbial communities and their contributions to ecosystem functioning. Here, based on DNA‐sequencing data (16S rRNA and 18S rRNA genes) with two size fractions collected from coastal to offshore regions of the South China sea (SCS), we found that both size‐fractionated bacterial and protistan communities in the DCM exhibited a distance‐decay pattern, which was strongly governed by deterministic processes (62.7%–72.2%) and mainly explained by environmental factors (34.1%–37.7%). Under warmer temperature and fresher salinity levels, the decreased abundance of protistan parasites induced the weakening parasitic interactions with other protists, whereas the increased abundance of protistan consumers enhanced the phagocytic interactions with bacteria. Such reconstruction of microbial communities induced by environmental variations was thought to constrain particulate organic carbon (POC) flux, as revealed by the partial least square regression model illustrating a strong correlation between the composition of functional protistan groups and POC content of DCM (R2 = 0.48; r = 0.75; p < 10−6). This work highlights the importance of environmental filtering in shaping DCM microbial communities and extends our understanding of the declined carbon flux of the DCM in the SCS under future scenarios. Read the free Plain Language Summary for this article on the Journal blog.

A Multivariable Empirical Algorithm for Estimating Particulate Organic Carbon Concentration in Marine Environments From Optical Backscattering and Chlorophyll-a Measurements

Accurate estimates of the oceanic particulate organic carbon concentration (POC) from optical measurements have remained challenging because interactions between light and natural assemblages of marine particles are complex, depending on particle concentration, composition, and size distribution. In particular, the applicability of a single relationship between POC and the spectral particulate backscattering coefficient bbp(λ) across diverse oceanic environments is subject to high uncertainties because of the variable nature of particulate assemblages. These relationships have nevertheless been widely used to estimate oceanic POC using, for example, in situ measurements of bbp from Biogeochemical (BGC)-Argo floats. Despite these challenges, such an in situbased approach to estimate POC remains scientifically attractive in view of the expanding global-scale observations with the BGC-Argo array of profiling floats equipped with optical sensors. In the current study, we describe an improved empirical approach to estimate POC which takes advantage of simultaneous measurements of bbp and chlorophyll-a fluorescence to better account for the effects of variable particle composition on the relationship between POC and bbp. We formulated multivariable regression models using a dataset of field measurements of POC, bbp, and chlorophyll-a concentration (Chla), including surface and subsurface water samples from the Atlantic, Pacific, Arctic, and Southern Oceans. The analysis of this dataset of diverse seawater samples demonstrates that the use of bbp and an additional independent variable related to particle composition involving both bbp and Chla leads to notable improvements in POC estimations compared with a typical univariate regression model based on bbp alone. These multivariable algorithms are expected to be particularly useful for estimating POC with measurements from autonomous BGC-Argo floats operating in diverse oceanic environments. We demonstrate example results from the multivariable algorithm applied to depth-resolved vertical measurements from BGC-Argo floats surveying the Labrador Sea.

A novel semianalytical remote sensing retrieval strategy and algorithm for particulate organic carbon in inland waters based on biogeochemical-optical mechanisms

The estimation of particulate organic carbon (POC) concentrations from satellite images can provide crucial spatiotemporal continuous observation data for the carbon cycle and ecological environmental governance. Here, we developed a novel inversion algorithm for deriving POC in inland water based on remote sensing and geochemical isotopes, which is summarized as follows. First, we developed empirical relationships between the phytoplankton absorption coefficient and endogenous POC concentration (CendPOC) and between the nonalgal particulate absorption coefficient and terrestrial POC concentration (CterPOC). Second, based on the valid relationships, semianalytical retrieval models were established to estimate CendPOC and CterPOC. Third, the proportions of endogenous POC (RendPOC) and terrestrial POC (RterPOC) to the total POC concentration (CPOC) were derived using a three-band empirical model. Finally, CPOC was obtained by dividing CendPOC by RendPOC (RendPOC ≥ 0.5) or dividing CterPOC by RterPOC (RendPOC < 0.5). Validation with field data shows that our proposed algorithm can accurately derive CPOC (0–20 mg/L), with a root mean square deviation (RMSD), median bias (MB), median absolute percent difference (MAPD), and median ratio (MR) of 1.15 mg/L, −0.05 mg/L, 24%, and 0.98, respectively. Synchronous validation based on Sentinel-3/OLCI images confirmed the accuracy, with RMSD, MB, MAPD, and MR values of 0.41 mg/L, −0.16 mg/L, 28%, and 0.91, respectively. The algorithm was applied to ocean and land color sensor (OLCI) images to reveal the temporal and spatial variations in POC in Lake Taihu.

Uncertainties of particulate organic carbon concentrations in the mesopelagic zone of the Atlantic ocean.

Measurements of particulate organic carbon (POC) in the open ocean provide grounds for estimating oceanic carbon budgets and for modelling carbon cycling. The majority of the published POC measurements have been collected at the sea surface. Thus, POC stocks in the upper layer of the water column are relatively well constrained. However, our understanding of the POC distribution and its dynamics in deeper areas is still modest due to insufficient POC measurements. Moreover, the uncertainty of published POC estimates is not always quantified, and neither is it fully understood. In this study, we determined the POC concentrations of samples collected in the upper 500 m during an Atlantic Meridional Transect and described a method for quantifying its experimental uncertainties using duplicate measurements. The analysis revealed that the medians of the total experimental uncertainties associated with our POC concentrations in the productive and mesopelagic zones were 2(±2) mg/m 3 and 3(±1) mg/m 3, respectively. In relative terms, these uncertainties corresponded to ∼12% and ∼35% of POC concentrations, respectively. We modelled the POC uncertainty in order to identify its main causes. This model however could explain only ∼19% of the experimental POC uncertainty. Potential sources of the unexplained uncertainty are discussed.

Human activities changed organic carbon transport in Chinese rivers during 2004-2018

Rivers serve as regulators of global climate by releasing greenhouse gases, burying particulate carbon, and connecting different ecosystem carbon pools. However, long-term organic carbon (OC) transport features across different Asian rivers are not well known due to unavailable data. Based on routinely monitored environmental and hydrological data during 2004-2018, this study investigated the spatiotemporal variations in dissolved (DOC) and particulate OC (POC) transport across 41 rivers in China. Across different rivers, both DOC (1.35 – 16.8 mg/L) and POC (0.27 – 4.48 mg/L) concentrations covered wide ranges. The DOC content was high in the north and low in the south, with significantly higher (t test, p < 0.01) values for rivers north of 30°N (5.39 ± 3.66 mg/L vs. 2.39 ± 1.14 mg/L). Human activities greatly influenced the riverine DOC and POC distributions. The riverine ammonia nitrogen (NH+ 4-N) content was positively correlated with DOC (r = 0.81 and p < 0.01) and explained 85.59% of its spatial variation. High vegetation coverage had significant effects on decreasing the riverine POC content, with r = -0.55 and p < 0.05. During 2004-2018, water pollution prevention and control strategies decreased DOC concentrations in 60.98% of rivers; meanwhile, anthropogenic vegetation restoration and dam construction led to POC content decreases in 90.48% of rivers. Importantly, along with DOC and POC changes, increasing DOC/POC ratios were found in 90.48% of the rivers, with 42.86% being significant, which indicated that Chinese rivers are losing their Asian features of low DOC/POC ratios due to artificial disturbance. This study is significant for accurately quantifying greenhouse gas emissions, carbon burial, and OC export to estuaries by Chinese rivers.

Effects of Exogenous Glucose on Soil Organic Carbon Sequestration and Carbon Metabolism in Root of Malus baccata (L.) Bork

ABSTRACT Labile carbon (C) inputs to soil affect organic C dynamics in soil and the pathway of energy metabolism in tissue of plants. However, studies focusing on orchard soil are still scarce. In this study, the fate of 13C-glucose in organic C components, and the C metabolism in root of Malus baccata (L.) Borkh. in response to exogenous glucose at two levels, 1.50 g kg−1 (GLC1) and 7.50 g kg−1 (GLC2), were evaluated. Glucose addition increased the δ13C values and contents of soil organic carbon (SOC) and active fractions including microbial biomass carbon (MBC), dissolved organic carbon (DOC) pools, and particulate organic carbon (POC). At the end of the experiment, the MBC, DOC, and POC contents in GLC1 were 3.20%, 14.26%, and 5.16% higher than those in GLC2, respectively. In addition, no marked difference was found for SOC content between these two glucose rates. At the end of incubation, glucose-C was mostly allocated to SOC (56.79–85.05%), followed by DOC (4.72–20.88%) and MBC (4.46–18.1%); the lowest percentage was in POC (3.86–5.77%). Exogenous glucose increased the contents of organic acids and activities of key enzymes related with C metabolism in roots of M. baccata. In addition, adenosine triphosphate content, the energy matter, in root under GLC1 was 22.35% higher than that under GLC2. Correlation analyses suggested that the soil organic C fractions were positively correlated with C metabolism parameters in the roots of M. baccata. Overall, glucose addition improved the C metabolism in roots and the allocation of glucose-derived C in soil.

The Differential Responses of Coastal Diatoms to Ocean Acidification and Warming: A Comparison Between Thalassiosira sp. and Nitzschia closterium f.minutissima.

Marine diatoms are one of the marine phytoplankton functional groups, with high species diversity, playing important roles in the marine food web and carbon sequestration. In order to evaluate the species-specific responses of coastal diatoms to the combined effects of future ocean acidification (OA) and warming on the coastal diatoms, we conducted a semi-continuous incubation on the large centric diatom Thalassiosira sp. (~30 μm) and small pennate diatom Nitzschia closterium f.minutissima (~15 μm). A full factorial combination of two temperature levels (15 and 20°C) and pCO2 (400 and 1,000 ppm) was examined. The results suggest that changes in temperature played a more important role in regulating the physiology of Thalassiosira sp. and N. closterium f.minutissima than CO2. For Thalassiosira sp., elevated temperature significantly reduced the cellular particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphate (POP), biogenic silica (BSi), chlorophyll a (Chl a), and protein contents, and the C:N ratio. CO2 only had significant effects on the growth rate and the protein content. However, for the smaller pennate diatom N. closterium f.minutissima, the growth rate, POC production rate, and the C:P ratio significantly increased with an elevated temperature, whereas the cellular POP and BSi contents significantly decreased. CO2 had significant effects on the POC production rate, cellular BSi, POC, and PON contents, the C:P, Si:C, N:P, and Si:P ratios, and sinking rate. The interaction between OA and warming showed mostly antagonistic effects on the physiology of both species. Overall, by comparison between the two species, CO2 played a more significant role in regulating the growth rate and sinking rate of the large centric diatom Thalassiosira sp., whereas had more significant effects on the elemental compositions of the smaller pennate diatom N. closterium f.minutissima. These results suggest differential sensitivities of different diatom species with different sizes and morphology to the changes in CO2/temperature regimes and their interactions.

Organic matter distribution in the icy environments of Taylor Valley, Antarctica

Glaciers can accumulate and release organic matter affecting the structure and function of associated terrestrial and aquatic ecosystems. We analyzed 18 ice cores collected from six locations in Taylor Valley (McMurdo Dry Valleys), Antarctica to determine the spatial abundance and quality of organic matter, and the spatial distribution of bacterial density and community structure from the terminus of the Taylor Glacier to the coast (McMurdo Sound). Our results showed that dissolved and particulate organic carbon (DOC and POC) concentrations in the ice core samples increased from the Taylor Glacier to McMurdo Sound, a pattern also shown by bacterial cell density. Fluorescence Excitation Emission Matrices Spectroscopy (EEMs) and multivariate parallel factor (PARAFAC) modeling identified one humic-like (C1) and one protein-like (C2) component in ice cores whose fluorescent intensities all increased from the Polar Plateau to the coast. The fluorescence index showed that the bioavailability of dissolved organic matter (DOM) also decreased from the Polar Plateau to the coast. Partial least squares path modeling analysis revealed that bacterial abundance was the main positive biotic factor influencing both the quantity and quality of organic matter. Marine aerosol influenced the spatial distribution of DOC more than katabatic winds in the ice cores. Certain bacterial taxa showed significant correlations with DOC and POC concentrations. Collectively, our results show the tight connectivity among organic matter spatial distribution, bacterial abundance and meteorology in the McMurdo Dry Valley ecosystem.

Effects of Close-to-Nature Transformation on Soil Enzyme Activities and Organic Carbon Fractions in Cuninghamia lanceolata and Pinus massoniana Plantations

Soil enzymes are involved in the process of mineralization of soil organic matters. The close-to-nature transformation (CNT) of plantations changes the soil enzyme activities by changing the composition of stand vegetation, which in turn affects the change process of soil organic carbon. We therefore selected two typical coniferous plantations in southwest China, Pinus massoniana and Cunninghamia lanceolate, to explore the effects of CNT on soil enzyme activities and soil organic carbon, and the relationship between them is explored through comparative study. Compared with control stands (CCK and PCK), CNT enhanced soil organic carbon; the content of water-soluble organic carbon in the 0–10 cm soil layer of the transformed C. lanceolata plantations (CCN) is 81.29% higher than those in the control stands (p &lt; 0.05); the contents of particulate organic carbon and water-soluble organic carbon in the 10–30 cm soil layer are 95.42% and 48.68% higher than those in the control stands (p &lt; 0.05), respectively; after the CNT, the protease, urease, and acid phosphatase in C. lanceolata plantations were higher than control stands, while protease and catalase in P. massoniana plantations were higher than control stands. Correlation analysis showed that catalase and protease were more closely related to organic carbon components than other organic enzymes; redundancy analysis (RDA) results show that pH and total nitrogen are key factors that cause changes in carbon fractions after the CNT. In general, CNT enhanced soil organic carbon in coniferous plantations, which was more conducive to soil organic carbon accumulation but had a negative effect on soil organic carbon stability to a certain extent. Therefore, the effect of tree species configuration on soil carbon stability components should be considered in the forest management practice.

Hydrological classification by clustering approach of time-integrated samples at the outlet of the Rhône River: Application to Δ14C-POC

Within the framework of the Rhône Sediment Observatory, monthly time-integrated samples have been collected by Particle Traps in the last decade to monitor particulate contaminants in the Rhône River and its main tributaries. In this watershed with a contrasted hydrology, a clustering approach is used to classify the samples according to the main hydrological events. This approach has been applied to riverine particulate organic radiocarbon signatures (Δ14C-POC) that are strongly affected by the origin of the material and the occurrence of nuclear power plant releases. Suspended Particulate Matter (SPM) samples were collected near the outlet of the Rhône River and analysed for 14C along with particulate organic carbon (POC), chlorophyll a and tritium contents to confirm Δ14C-POC origins. Cluster Analysis, coupled to Principal Component Analysis, was performed based on monthly average water discharges of the Upper Rhône River and the five main tributaries. The classification obtained by fuzzy C-mean logic of the Rhône River hydrology into 5 clusters is similar to that already observed in the literature with Mediterranean/Cevenol flood, oceanic pluvial flood, nival flood, low-water level and baseflow clusters. The contributions of each cluster among the Δ14C-POC values demonstrate the complexity of hydrological classification of time-integrated samples. First, the samples with a unique and significantly dominant cluster are easily explained with negative Δ14C-POC values observed in the flood clusters due to input of 14C-depleted material from soil or rock weathering, and positive values observed in the low-water level and baseflow clusters due to anthropogenic input by nuclear industry. Second, samples that present a homogeneous mixture between several clusters demonstrate the occurrence of different hydrological events during the sampling periods. This tool appears as a solution to estimate the contribution of each hydrological event in time-integrated samples.

Pelagic ecosystem dynamics between late autumn and the post spring bloom in a sub-Arctic fjord

Marine ecosystems, and particularly fjords, are experiencing an increasing level of human activity on a year-round basis, including the poorly studied winter period. To improve the knowledge base for environmentally sustainable management in all seasons, this study provides hydrographic and biological baseline data for the sub-Arctic fjord Kaldfjorden, Northern Norway (69.7° N, 18.7° E), between autumn 2017 and spring 2018. Field observations are integrated with results of a numerical ocean model simulation, illustrating how pelagic biomass, represented by chlorophyll a (Chl a), particulate organic carbon (POC), and zooplankton, is affected by stratification and circulation from October to May. We observed an unusually warm autumn that likely delayed the onset of cooling and may have supported the high abundances of holoplankton and meroplankton (5768 individuals m–3). With the onset of winter, the water column cooled and became vertically mixed, while suspended Chl a concentrations declined rapidly (&amp;lt; 0.12 mg Chl a m–3). In January and February, suspended POC concentrations and downward flux were elevated near the seafloor. The hydrodynamic model results indicate that the strongest currents at depth occurred in these months, potentially inducing resuspension events close to the seafloor. In spring (April), peak abundances of suspended biomass were observed (6.9–7.2 mg Chl a m–3 at 5–15 m; 9952 zooplankton ind. m–3 at 0–100 m), and field observations and model results suggest that zooplankton of Atlantic origin were probably advected into Kaldfjorden. During all investigated seasons, the model simulation suggests a complex circulation pattern, even in such a small fjord, which can have implications for environmental management of the fjord. We conclude that the pelagic system in Kaldfjorden changes continually from autumn to spring and that winter must be seen as a dynamic period, not a season where the fjord ecosystem is ‘at rest’.

Methane migration mechanisms for the Green Canyon Block 955 gas hydrate reservoir, northern Gulf of Mexico

In marine continental margins, high saturations of natural gas hydrate are observed within the gas hydrate stability zone, but the gas migration pathways to the hydrate reservoir are not often clear. We focus on a site in Green Canyon Block 955 (GC 955) in the Gulf of Mexico, where gas hydrate occurs in a 35 m-thick reservoir with saturations of 79-93% in numerous centimeter- to decimeter-thick sandy silt layers and little to no gas hydrate in the interbedded cm to decimeter-thick clayey silt layers. Different datasets (well logging, coring, and seismic data) suggest different types of potential methane migration mechanisms. For example, relatively high particulate organic carbon content in the reservoir and the microbial methane source indicated by gas chromatography and methane isotopes suggest gas might be made locally and migration could occur through dissolved methane sources or water advection. In contrast, the visible deeply rooted faults intersecting the gas hydrate reservoir and the gas pockets adjacent to the faults on seismic data suggest that free gas may migrate directly into the reservoir. We tested three different mechanisms to form gas hydrate at GC 955: short-range methane diffusion, short-range methane diffusion coupled with upward water advection, and free gas flow. The 1-D and 2-D numerical models show that both the short-range migration and the upward water advection are not enough to form high saturations gas hydrate at GC 955, and that free gas flow is required to form the high saturations of gas hydrate observed at GC 955.

Effects of Shellfish and Macro-Algae IMTA in North China on the Environment, Inorganic Carbon System, Organic Carbon System, and Sea–Air CO2 Fluxes

Shellfish and macro-algae integrated multi-trophic aquaculture (IMTA) contribute greatly to the sustainability of aquaculture. However, the effects of large-scale shellfish and macro-algae aquaculture on the functions of the ocean carbon sink are not clear. To clarify these effects, we studied the spatial and temporal changes of inorganic and organic carbon systems in seawater under different aquaculture modes (monoculture or polyculture of shellfish and macro-algae) in Sanggou Bay, together with the variation of other environmental factors. The results show that the summertime dissolved oxygen (DO) concentration in the shellfish culture zone was significantly lower than other zones (p &amp;lt; 0.05), with a minimum value of 7.07 ± 0.25 mg/L. The variation of pH and total alkalinity (TA) were large across different culture modes, and the seawater in the shellfish culture zone had the lowest pH and TA than the other zones. Seasonal environment and aquaculture modes significantly affected the variation of dissolved inorganic carbon (DIC), CO2 partial pressure (pCO2), dissolved organic carbon (DOC), and particulate organic carbon (POC) concentrations. The highest values of DIC, pCO2, and POC appeared in summer, and the lowest appeared in winter. For DOC concentration, the lowest value appeared in autumn. Spatially, DIC and pCO2 were highest in the shellfish culture zone and lowest in the macro-algae culture zone, DOC was highest in the macro-algae culture zone and lowest in the shellfish culture zone, and POC was lower in the shellfish culture zone and macro-algae culture zone and higher in the remaining zones. The results of sea–air CO2 fluxes showed that except for the shellfish culture zone during summertime, which released CO2 to the atmosphere, all culture zones were the sinks of atmospheric CO2 during the culture period, with the whole bay being a strong CO2 sink during autumn and winter. In summary, large-scale shellfish–macro-algae IMTA plays an important role in the local carbon cycle and contributes to mitigating ocean acidification and hypoxia.

Distribution and dynamics of particulate organic matter in Indian mangroves during dry period.

The distribution and possible sources of particulate organic carbon (POC) and particulate nitrogen (PN) in seven mangroves ecosystems along the east and west coast of India were examined, to understand their contribution to coastal biogeochemistry. Suspended particulate matter (SPM) concentration in mangrove waters were about ~ 1.6-fold higher in west coast (Gulf of Kachchh (GOK), Mandovi-Zuari (MA-ZU) and Karwar-Kumta (KR-KU)], whereas the mean POC content in SPM along east coast [Sundarbans (SUN), Bhitarkanika (BHK), Coringa (COR) and Pichavaram-Muthupet (PI-MU)] was nearly two times higher than the west coast (1.97 ± 0.91% and 1.06 ± 0.29%), respectively. The results indicated that the influence of the land-based contaminants on the water quality parameters (dissolved oxygen, pH, salinity, nutrients and chlorophyll-a, etc.), which primarily regulated the distribution and transformation of organic carbon in these mangrove waters. Among the studied systems, an extremely high DOC/POC ratio (5.72 ± 1.64) with low pH and DO in COR waters clearly indicated the labile nature of the organic matter influenced by anthropogenic stress. Strong correlation between POC and PN indicated a similar origin in particulate organic matter. The ratios of POC/PN and POC/Chl-a showed significant spatial variation ranging from 5.5 to 18.7 and 126 to 1057, respectively. The results indicated that significant fraction of in-situ primary production contributed to particulate organic matter (POM) pool in all Indian mangrove waters except the GOK and the SUN waters, where sediment resuspension and mangrove derived organic matter were the dominant POM sources.

Effects of long-term manure substitution regimes on soil organic carbon composition in a red paddy soil of southern China

Long-term fertilization regimes can substantially change soil organic carbon (SOC) dynamics within different soil fractions. However, the underlying mechanisms of these effects, particularly under manure substitution regimes, remain elusive. Soils from different fertilization regimes [i.e., control, mineral NPK fertilization, and NPK with three manure substitution rates (30%, 50%, and 70%)] in a 33-year red paddy field experiment in southern China were sampled. The mean weight diameter (MWD) of soil aggregates and soil microbial properties (PLFAs), SOC content, and chemical composition of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) under different treatments were determined to investigate the effects of manure substitution regimes on SOC quantity and quality. The results showed mineral NPK fertilization alone significantly (p < 0.05) decreased the MWD of soil aggregates by 14.4%, whereas 30%, 50%, and 70% manure replacement significantly (p < 0.05) increased the MWD by 12.0%, 19.9% and 27.2%, respectively. Compared to the mineral NPK fertilization regime, manure substitution practices continuously and significantly (p < 0.05) increased SOC along with its content in microbial biomass C and POC when the substitution rate of manure increased from 30% to 70%. Regardless of the substitution rates, manure substitution regimes decreased the O-alkyl C and aryl C proportions of POC, but increased the recalcitrant alkyl C resulting in higher alkyl C to O-alkyl C ratios compared with mineral NPK fertilization. Pearson analysis further showed that the O-alkyl C of POC were more associated with the soil microbial biomass C and N and total PLFAs. Partial least squares path modeling analysis indicated that microbial biomass C, POC, and MAOC were positive factors impacting SOC, whereas the promotion of manure substitution on SOC stabilization progressed mainly through increasing POC content. Our results suggested that substitution of 70% mineral N with manure N is the most effective fertilizer practice for improving POC-related SOC sequestration in rice paddy soil of subtropical regions.

Soil Organic Carbon Storage, Active Component Contents, and Stability Along a Flooding Gradient in the Tidal Wetland of the Jiulong River Estuary

Flooding is one of the key environmental factors affecting the carbon sequestration potential of estuarine tidal flat wetlands. In order to reveal the effect of flooding on soil carbon (C) sinks in estuarine tidal wetlands, we investigated and analyzed the soil organic carbon (SOC) storage, the contents of active SOC components, and SOC stability indicators across a tidal flat in the Jiulong River estuary in southeast China. The results showed that the SOC storage gradually decreased by 54% with the increase in flooding frequency. The change pattern of microbial biomass carbon (MBC), dissolved organic carbon (DOC), and liable organic carbon (LOC) followed the change pattern of the SOC storage. With the increase in flooding frequency, DOC/SOC and LOC/SOC increased by 80% and 26%, respectively, whereas MBC/SOC decreased by 29%. As flooding frequency increased, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) contents decreased by 81% and 35%, respectively. The decreases in POC contents were correlated with the increases in soil pH, whereas the decreases in MAOC contents were associated with the decline in clay contents. Soil carbon stability index (CSI) increased by 246% with increasing flooding frequency. These combined results indicated that SOC storage decreased, but SOC stability increased, with the increased flooding frequency. Mineral-bound organic carbon was the main protection mechanism for the SOC stability, which was of great significance to the soil C sink of the estuarine tidal wetlands.