Particulate Organic Carbon Concentrations Research Articles

Hydrology drives export and composition of carbon in a pristine tropical river

ABSTRACTThe Ruki is a pristine blackwater tributary in the Congo Basin draining tropical lowland forest. Daily discharge and fortnightly concentrations, isotopic ratios, and molecular composition of carbon and organic matter were measured for 1 yr (2019–2020). Like the Congo River, discharge peaked from November–January, with a smaller secondary peak in June. Dissolved organic carbon (DOC), inorganic carbon (DIC), carbon dioxide (pCO2), and methane (pCH4) concentrations were high (21.3 ± 4.8 mg C L−1, 5.8 ± 0.9 mg C L−1, 6370 ± 1740 ppm, and 250 ± 100 ppm, respectively) and positively correlated with discharge, indicating transport limitation. Total suspended solids and particulate organic carbon (POC) concentrations were generally low (3.68 ± 1.61 mg L−1, 0.88 ± 0.33 mg C L−1, respectively) and varied inversely with discharge, indicating source limitation. The Ruki exported a total of 3.25 Tg C yr−1, of which DOC, DIC, and POC comprised 76%, 20%, and 3%, respectively. This DOC flux represents ~ 20% of the annual Congo Basin flux from about 5% of its area, highlighting the high yield. Isotopic ratios of DOC and POC indicate modern C3 forest vegetation as a source, except for a few older samples potentially indicating peat inputs. The bulk molecular composition of dissolved organic matter was seasonally consistent; however, a more oxidized and aromatic assemblage occurred at high discharge, corresponding with forest vegetation, while a more aliphatic, nitrogen‐, and sulfur‐enriched assemblage was found during low discharge, corresponding with soil‐derived organic matter. Overall, these results underscore how hydrology controls C concentrations in the Ruki River and how this blackwater river contributes disproportionately to C export per unit area within the Congo Basin hydrosystem.

Characteristics and factors influencing soil organic carbon composition by vegetation type in spoil heaps.

The variation of organic carbon content in spoil heaps is closely related to improving soil structure, maintaining soil fertility, and regulating soil carbon cycling balance. Analyzing the soil organic carbon content and related driving factors during the natural vegetation restoration process of spoil heaps is of great significance for promoting the accumulation of soil organic carbon in the spoil heaps. we selected spoil heaps with the same number of years of restoration to research the variations in soil organic carbon components under different vegetation types (grassland: GL, shrubland: SL, secondary forest: SF) and compared the results with those on bare land (BL). Our results showed that vegetation type and soil depth significantly affect the content of soil organic carbon components. There was no difference in soil organic carbon components between SF and SL, but both were considerably superior to GL and BL (p<0.05), and the particulate organic carbon (POC) and light fraction organic carbon (LFOC) contents of SL were the highest. A significant positive linear correlation existed between SOC and active organic carbon components. Pearson's correlation and redundancy analysis showed that the available potassium (AK) and total nitrogen (TN) contents and gravel content (GC) in the BL soil significantly impacted soil organic carbon. When vegetation is present, TN, total phosphorus (TP), and Fine root biomass (FRB) significantly affect soil organic carbon. Structural equation modelling (SEM) shows that AK and soil moisture content (SMC) directly affect the organic carbon composition content of BL, When there is vegetation cover, fine root biomass (FRB) had the largest total effect in the SEM. Soil bulk density (BD) has a negative impact on soil organic carbon, especially in the presence of vegetation. These findings suggest that vegetation restoration can significantly increase soil organic carbon content, FRB, AK, and TN play important roles in enhancing soil organic carbon. Supplementation with nitrogen and potassium should be considered in the bare land stage, and shrubs nitrogen-fixing functions and well-developed roots are more beneficial for the accumulation of soil organic carbon.

Seasonal dynamics of sea-ice protist and meiofauna in the northwestern Barents Sea

The rapid decline of Arctic sea ice makes understanding sympagic (ice-associated) biology a particularly urgent task. Here we studied the poorly known seasonality of sea-ice protist and meiofauna community composition, abundance and biomass in the bottom 30 cm of sea ice in relation to ice properties and ice drift trajectories in the northwestern Barents Sea. We expected low abundances during the polar night and highest values during spring prior to ice melt. Sea ice conditions and Chlorophyll a concentrations varied strongly seasonally, while particulate organic carbon concentrations were fairly stable throughout the seasons. In December to May we sampled growing first-year ice, while in July and August melting older sea ice dominated. Low sea-ice biota abundances in March could be related to the late onset of ice formation and short time period for ice algae and uni- and multicellular grazers to establish themselves. Pennate diatoms, such as Navicula spp. and Nitzschia spp., dominated the bottom ice algal communities and were present during all seasons. Except for May, ciliates, dinoflagellates, particularly of the order Gymnodiales, and small-sized flagellates were co-dominant. Ice meiofauna (here including large ciliates and foraminifers) was comprised mainly of harpacticoid copepods, copepod nauplii, rotifers, large ciliates and occasionally acoels and foraminifers, with dominance of omnivore species throughout the seasons. Large ciliates comprised the most abundant meiofauna taxon at all ice stations and seasons (50–90 %) but did not necessarily dominate the biomass. While ice melt might have released and reduced ice algal biomass in July, meiofauna abundance remained high, indicating different annual cycles of protist versus meiofauna taxa. In May highest Chlorophyll a concentrations (29.4 mg m−2) and protist biomass (107 mg C m−2) occurred, while highest meiofauna abundance was found in August (23.9 × 103 Ind. m−2) and biomass in December (0.6 mg C m−2). The abundant December ice biota community further strengthens the emerging notion of an active biota during the dark Arctic winter. The data demonstrated a strong and partially unexpected seasonality in the Barents Sea ice biota, indicating that changes in ice formation, drift and decay will significantly impact the functioning of the ice-associated ecosystem.

Spatiotemporal variability of organic carbon in streams and rivers of the Northern Hemisphere cryosphere

The earth's cryosphere is the outpost of climatic warming, which leads to rapid changes of organic carbon (OC) transport from terrestrial to aquatic ecosystems. OC in the cryosphere rivers plays vital roles in the carbon cycle and river ecosystem health. Yet, we still lack a comprehensive assessment of the spatiotemporal patterns of riverine OC across the Northern Hemisphere cryosphere. Here, we compiled OC concentration, radiocarbon (14C), and the specific ultraviolet absorbance (SUVA) of dissolved OC (DOC) at 254 nm data from 1007 unique sites, extracted from 138 published literature between 1972 and 2022. Overall, the average DOC and particulate OC (POC) concentrations are 6.34 and 2.61 mg C L−1, respectively, with the average age of DOC and POC being ~1100 and ~4300 years BP, respectively, indicating the release of aged carbon pools. Seasonal variations in DOC and POC concentrations, Δ14C-DOC and SUVA254 were observed, with distinct spatial variations closely linked to specific watershed characteristics. We found permafrost-impacted watersheds displayed significantly higher DOC concentrations, younger OC ages but lower POC concentrations compared to glacier-impacted watersheds. Meanwhile, in boreal forest watersheds, DOC is the most concentrated and youngest in varied ecoregions. Additionally, in permafrost regions characterized by higher permafrost extent, ground ice content, or lowlands with thick overburden cover, riverine DOC is more concentrated and aromatic. We estimated that specific OC fluxes in glacier rivers are higher than that in permafrost rivers (4.77 and 1.86 g C m−2 yr−1, respectively). Our results highlight the complex and variable spatiotemporal patterns of riverine OC in the northern cryosphere, which are essential for assessing the impact of OC on the global carbon cycle and climate warming.

Effect of ocean warming on pigment and photosynthetic carbon fixation of plankton assemblage in Pingtan Island of Southeast China

Temperature plays an important role in affecting the physiological traits of marine plankton. In this study, we conducted an outdoor incubation experiment to investigate the effects of elevated temperature on Chl a, photosynthetic carbon fixation and the composition of plankton communities in the surface seawater around Pingtan Island, the northwest Taiwan Strait in Autumn 2022. After 3–4 days of incubation, elevated temperature (1–4 °C higher than ambient temperature) led to a decrease in Chl a concentration across all three stations, did not result in significant increases in the particulate organic carbon (POC) and nitrogen (PON) concentrations in seawater with high nitrate concentrations, whereas increased POC and PON concentrations in nitrate-limited seawater. These findings suggest that the effect of temperature on the POC and PON contents of plankton is affected by the availability of nitrate. Diatoms were the dominant phytoplankton group in all three stations. Our results indicate that ocean warming has a potential to increase the POC contents of marine plankton per volume of seawater, which may increase the ability of phytoplankton to absorb atmospheric CO2 and to alleviate global warming.

The importance of transparent exopolymer particles over ballast in determining both sinking and suspension of small particles during late summer in the Northeast Pacific Ocean

Gravitational sinking of particles is a key pathway for the transport of particulate organic carbon (POC) to the deep ocean. Particle size and composition influence particle sinking velocity and thus play a critical role in controlling particle flux. Canonically, sinking particles that reach the mesopelagic are expected to be either large or ballasted by minerals. However, the presence of transparent exopolymer particles (TEP), which are positively buoyant, may also influence particle sinking velocity. We investigated the relationship between particle composition and sinking velocity during the Export Processes in the Ocean from RemoTe Sensing (EXPORTS) campaign in the Northeast Pacific Ocean using Marine Snow Catchers. Suspended and sinking particles were sized using FlowCam for particle imaging, and their biogeochemical composition was assessed by measuring the concentration of particulate organic carbon (POC) and nitrogen, particulate inorganic carbon, biogenic and lithogenic silica, and TEP. Sinking fluxes were also calculated. Overall, both suspended and sinking particles were small (&amp;lt;51 μm, diameter) in this late summer, oligotrophic system. Contrary to expectation, the ratio of ballast minerals to POC was higher for suspended particles than sinking particles. Further, suspended particles showed TEP-to-POC ratios three times higher than sinking particles. These ratios suggest that TEP content and not ballast dictated whether particles in this system would sink (low TEP) or remain suspended (high TEP). Fluxes of POC averaged 4.3 ± 2.5 mmol C m−2 d−1 at 50 m (n = 9) and decreased to 3.1 ± 1.1 mmol C m−2 d−1 at 300–500 m (n = 6). These flux estimates were slightly higher than fluxes measured during EXPORTS with drifting sediment traps and Thorium-234. A comparison between these approaches illustrates that small sinking particles were an important component of the POC flux in the mesopelagic of this late summer oligotrophic system.

A CNN–LSTM Machine-Learning Method for Estimating Particulate Organic Carbon from Remote Sensing in Lakes

As particulate organic carbon (POC) from lakes plays an important role in lake ecosystem sustainability and carbon cycle, the estimation of its concentration using satellite remote sensing is of great interest. However, the high complexity and variability of lake water composition pose major challenges to the estimation algorithm of POC concentration in Class II water. This study aimed to formulate a machine-learning algorithm to predict POC concentration and compare their modeling performance. A Convolutional Neural Network–Long Short-Term Memory (CNN–LSTM) algorithm based on spectral and time sequences was proposed to construct an estimation model using the Sentinel 2 satellite images and water surface sample data of Chaohu Lake in China. As a comparison, the performances of the Backpropagation Neural Network (BP), Generalized Regression Neural Network (GRNN), and Convolutional Neural Network (CNN) models were evaluated for remote sensing inversion of POC concentration. The results show that the CNN–LSTM model obtained higher prediction precision than the BP, GRNN, and CNN models, with a coefficient of determination (R2) of 0.88, a root mean square error (RMSE) of 3.66, and residual prediction deviation (RPD) of 3.03, which are 6.02%, 22.13%, and 28.4% better than the CNN model, respectively. This indicates that CNN–LSTM effectively combines spatial and temporal information, quickly captures time-series features, strengthens the learning ability of multi-scale features, is conducive to improving estimation precision of remote sensing models, and offers good support for carbon source monitoring and assessment in lakes.

Revegetation promotes soil mineral-associated organic carbon sequestration and soil carbon stability in the Tengger Desert, northern China

Ecological restoration is considered an effective strategy for increasing soil carbon storage and mitigating climate change. However, the impact of revegetation in desert areas on the stability of soil organic carbon (SOC) is still unclear. We investigated the content and stability of SOC in restoration sites along a chronosequence in the Tengger Desert, focusing on the mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) fractions. The content of SOC significantly increased with site age from 0.37 g kg−1 at year 0–5.32 g kg−1 at year 66. Revegetation significantly changed the SOC fraction and improved SOC stability as a factor of site age. In the 66-year-old site, the levels of MAOC and POC were increased by 255.67 and 9.24 times, respectively. The percentage of MAOC was increased from 1.50% to 28.92%, whereas that of POC was decreased from 98.50% to 71.08%. Based on our findings, the content and proportion of MAOC and POC are closely related to plant input, soil variables, and the soil microbial community. We estimate that the maximum content and proportion of MAOC are 2.65 g kg−1 and 36.71% with continuous succession, based on the soil clay and silt contents. Overall, revegetation improved the stability of SOC. Our study highlights the importance of the revegetation of temperate desert areas to further mitigate climate change.

Composition and sources of suspended particles in the Pacific Arctic region

During the 6th (2014) and 7th (2016) Chinese Arctic Expedition (CHINARE), samples of suspended particulate matter (SPM) were collected from both surface (depth: <1.0 m) and subsurface (depth: approximately between 10 and 150 m) waters over the northern shelf of the Bering Sea and in the western Arctic Ocean. To investigate the distribution and sources of organic matter in both the surface water and the vertical profile, the concentration and stable carbon isotopic composition of SPM, particulate organic carbon (POC), and particulate nitrogen (only in surface water samples) were determined, and some particle samples were selected for examination using scanning electron microscopy. Results showed apparent geographical partitioning and temporal variation in both the concentration and the composition of SPM. Higher SPM concentrations were observed in nearshore, shelf break, and sea ice edge areas; the distribution of POC concentration displayed a similar pattern, with higher values found from the northern part of the Bering Shelf to southern parts of the Chukchi Shelf. In surface water, SPM mainly comprised clay and detrital minerals with higher POC contents, lighter δ13C values, and higher POC/PN ratios, indicating organic matter predominantly derived from terrestrial sources in areas south of St. Lawrence Island and north of 73°N. The downward trend of heavier δ13C values, together with reduction in clay and detrital minerals, suggests that vertical transport of SPM is hindered by stratification, resulting in transport of terrestrial materials toward northern basin areas. In the Chukchi Slope and Canada Basin areas, extremely light δ13C values (as low as −33.41‰ PDB) were mainly observed at depths of 20–60 m, where the Polar Mixed Layer (PML) intersects with the Upper Halocline Layer (UHL). Under the condition of low sea ice extent in 2016, the POC-δ13C values were heavier in the PML than in the UHL in the Chukchi Slope and Canada Basin areas. These findings provide insights into the sources, transport, and fate of organic matter in the Pacific Arctic region, which have important implications for understanding the biogeochemical cycles and ecosystem dynamics in this remote and rapidly changing environment.

Particulate Organic Matter in the Atacama Trench: Tracing Sources and Possible Transport Mechanisms to the Hadal Seafloor

AbstractOceanic trenches are an important sink for organic matter (OM). However, little is known about how much of the OM reaching the hadal region derives from the sunlit surface ocean and other sources. We provide new insight into the OM sources in the Atacama Trench by examining the elemental and stable isotope composition of carbon and nitrogen in bulk OM throughout the entire water column and down to bathyal and hadal sediments. Moreover, we estimated the particulate organic carbon (POC) concentration and downward carbon flux. Our results, based on two‐way variance analysis, showed statistical differences in δ15NPON between the epipelagic zone and the deep zones. However, no statistical differences in δ13CPOC and C:N ratio between hadalpelagic and shallower pelagic zones were found, except for δ13CPOC in the oxygen‐deficient zone. On the contrary, whereas the isotopic signatures of hadal sediments were distinct from those over the entire water column, they were similar to the values in bathyal sediments. Thus, our results suggest that bathyal sediments could contribute more OM to hadal sediments than the different zones of the water column. Indeed, whereas POC flux estimates derived from remote sensing data indicate that ∼16%–27% of POC could evade surface remineralization within the top 200 m and potentially be exported to depths beyond the mesopelagic region, model estimates suggest that ∼3.3% of it could reach hadal depths. Our results provide a quantitative baseline of pelagic‐benthic coupling which can aid in assessment of carbon cycling changes in future climate scenarios.

Remote Sensing Estimates of Particulate Organic Carbon Sources in the Zhanjiang Bay Using Sentinel-2 Data and Carbon Isotopes

The source information of coastal particulate organic carbon (POC) with high spatial and temporal resolution is of great significance for the study of marine carbon cycles and marine biogeochemical processes. Over the past decade, satellite ocean color remote sensing has greatly improved our understanding of the spatiotemporal dynamics of ocean particulate organic carbon concentrations. However, due to the complexity of coastal POC sources, remote sensing methods for coastal POC sources have not yet been established. With an attempt to fill the gap, this study developed an algorithm for retrieving coastal POC sources using remote sensing and geochemical isotope technology. The isotope end-member mixing model was used to calculate the proportion of POC sources, and the response relationship between POC source information and in situ remote sensing reflectance (Rrs) was established to develop a retrieval algorithm for POC sources with the following four bands: (Rrs(443)/Rrs(492)) × (Rrs(704)/Rrs(665)). The results showed that the four-band algorithm performed well with R2, mean absolute percentage error (MAPE) and root mean square error (RMSE) values of 0.78, 33.57% and 13.74%, respectively. Validation against in situ data showed that the four-band algorithm derived calculated the proportion of marine POC accurately, with an MAPE and RMSE of 27.49% and 13.58%, respectively. The accuracy of the algorithm was verified based on the Sentinel-2 data, with an MAPE and RMSE of 28.02% and 15.72%, respectively. Additionally, we found that the proportion of marine POC sources was higher outside the Zhanjiang Bay than inside it using in situ survey data, which was consistent with the retrieved results. Influencing factors of POC sources may be due to the occurrence of phytoplankton blooms outside the bay and the impact of terrestrial inputs inside the bay. Remote sensing in combination with carbon isotopes provides important technical assistance in comprehending the biogeochemical process of POC and uncovering spatiotemporal variations in POC sources and their underlying causes.

The role of blue carbon stocks becomes more labile with mangrove development

Soil labile organic carbon (LOC) is a crucial component in carbon cycling in coastal wetlands and serves as an important indicator of SOC. Despite this, little is known about the stabilization of SOC and its LOC during mangrove development. The objective of our study was to quantify SOC and LOC in the soil at depths ranging from 0 to 100 cm across four sites, including a mudflat and three mangrove sites of varying ages (15-, 45-, and 80-yr old) were selected in Yingluo Bay, China. The concentration of SOC, POC (particulate organic carbon), DOC (dissolved organic carbon), MBC (microbial biomass carbon), and KMnO4-C (potassium permanganate-oxidizable carbon) were measured. The CMI (soil carbon management index) was also calculated to precisely and directly reflect the dynamic changes of soil carbon pools. Mangrove natural expansion showed a significant positive effect on both SOC and LOC. The concentration of KMnO4-C in the top 50 cm of soil layer showed a significant increase in three different mangrove forest sites. The POC, DOC and MBC were also significantly increased in response to mangrove development. The 80-yr old mangrove site had the highest SOC concentrations, LOC concentrations, soil carbon lability Index (LI) and CMI among all sites. These findings suggested that the development of mangroves increases soil organic carbon fractions through vegetation production and enhances long-term carbon sequestration rates by expanding soil labile carbon pools. As a management option, promoting the natural expansion of mangroves can maximize the sequestration potential of soil organic carbon.

Pore structure characteristics and organic carbon distribution of soil aggregates in alpine ecosystems in the Qinghai Lake basin on the Qinghai-Tibet Plateau

Soils in alpine ecosystems serve as important carbon sinks. Soil aggregates provide physical protection to soil organic carbon (SOC) and pore networks of them can greatly influence SOC sequestration. However, previous studies mainly focused on SOC dynamics at ecosystem or regional scale in cold alpine regions. The lack of quantification of alpine soil aggregate pore structure limits our understanding of SOC sequestration mechanisms of alpine soil. In this study, three typical alpine ecosystems in the Qinghai Lake basin of Qinghai-Tibet Plateau (QTP) were selected: Kobresia pygmaea meadow (KPM), Potentilla fruticosa shrubland (PFS), and Achnatherum splendens steppe (ASS). Soil pore structure, SOC and their relationships in aggregates were examined through sieving procedure and X-ray computed tomography (CT) scanning. The results showed that the mass proportion of small macroaggregates (SMA, 0.25–2 mm) was highest and they accounted for 36–51% of all aggregate fractions in the above three alpine ecosystems. The pores of soil aggregate were found to be dominated by micropores (<30 μm), which accounted for over 90% of the pore numbers. Aggregates of the ASS ecosystem had highest mean pore volume (7.761 × 10³ μm3 in average) while soil aggregates of the KPM ecosystem had the highest pore number density (4.137 × 10-5 no. μm−3 in average). The highest total organic carbon (TOC) content was mostly observed in the mA (microaggregates, 0.053–0.25 mm) fractions in the three ecosystems. Particulate organic carbon (POC) content was higher than mineral-associated organic carbon (MAOC) content of aggregates. Pores of < 15 μm served as indirect indicators for SOC protection in soil aggregates. Pores of 15–30 μm in aggregates might provide voids for SOC decomposition. TOC and MAOC content were positively correlated with pore surface area density while POC content was positively correlated with pore equivalent diameter and mean volume of aggregates. SOC was mainly affected by soil water content (SWC) in KPM and PFS ecosystems while it was mainly affected by soil particle compositions in the ASS ecosystems. Our results revealed the structural characteristics of aggregates of alpine soil and their relationships with the carbon storage function, which are helpful to understand the micro-scale mechanism of carbon sink of alpine soil.

Effect of Particulate Organic Carbon Deposition on Nitrate Reduction in the Hyporheic Zone

AbstractThe hyporheic zone (HZ), where surface water and groundwater interact in sediments beneath streams, presents unique conditions for nutrient dynamics, such as carbon and nitrogen (N) cycling. Organic carbon (OC) in aquatic systems has two distinct forms: dissolved organic carbon and particulate organic carbon (POC). OC affects N reduction and controls the occurrence of denitrification, a primary process by which nitrate (NO3−) is removed as gas (N2 or N2O) to the atmosphere. When POC is the predominant form of OC in streams, how POC influences NO3− reduction within a HZ remains unclear. We established a new reactive transport model incorporating POC transport and filtration processes to assess how POC deposition affects NO3− removal. Sediment permeability decreases as POC is filtrated in the streambed. Nitrate influx is reduced due to POC‐induced sediment clogging. The deposited POC in the shallow streambed serves as an OC source and supports higher biogeochemical reaction kinetics because OC is an important substrate promoting microbial activities. The filtrated POC pool enhances denitrification, thus causing a higher NO3− removal efficiency. In contrast, the POC pool is typically assumed to be distributed homogeneously in sediments, potentially causing a significant underestimation of stream‐borne NO3− removal. A lower hydraulic conductivity, a smaller POC grain size, and a larger POC filtration efficiency and in‐stream POC concentration allow for extensive POC deposition within the shallow streambed that favors nitrate removal. This study provides a better understanding of N processing and an accurate estimation of NO3− removal potential in HZs.

Calcium carbonate regulates soil organic carbon accumulation by mediating microbial communities in northern China

Mineral organic complexes are essential for the storage of soil organic carbon (SOC) in agricultural soils. They are also affecting the CO2 emission reduction goals of known as the “doublecarbon target” (carbon peaking and carbon neutrality), as well as a landing point for achieving the 4 per 1,000 carbon enhancement initiative. As a typical soil mineral, calcium (Ca) plays a very critical role in regulating the accumulation of global organic carbon. Although calcium carbonate (CaCO3) has been shown to mediate the SOC accumulation process, especially in northern China, the relationship between Ca and SOC, and the underlying microbial mechanisms remain unclear. In this study, five soils (black soil, light chernozem soil, fluvo-aquic soil, sierozem soil, and loess soil) with different CaCO3 contents (4.29, 17.45, 98.66, 131.85, 143.82 g/kg, respectively) were selected to reveal the relationship among CaCO3, the SOC fraction and the chemical structure, and the bacterial community. The content of CaCO3 was determined by the gas volume method. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used to analyze the chemical structure of SOC, and 16S rRNA gene amplicon sequencing was used to analyze the bacterial community. The results showed that the contents of particulate organic carbon (POC) and mineral-associated organic carbon (MOC) in macro-aggregates were higher than that in micro-aggregates, and that POC was significantly higher than MOC in soil with high CaCO3. With an increase in the CaCO3 content, there was a decrease in the proportion of aromatic C and alkyl C, but an increase in the proportion of O-alkyl C in SOC, the relative abundance of Proteobacteria and Chloroflexi grew, and that of Acidobacteria decreased. Moreover, the overall diversity of the soil bacteria likewise showed a decreasing trend. High CaCO3 soils showed a positive correlation between Acidobacteria and macro-aggregates, and a negative correlation with clay and silt, when compared to low CaCO3 soils. Also, Chloroflexi and Gemmatimonadota showed the opposite relationship with SOC aggregates. CaCO3 influenced the relationship between SOC (fraction and chemical structure) and the bacterial community, resulting in changes in the SOC content within the aggregates. Our results indicate that CaCO3 plays an essential role in the accumulation of SOC and reveal the mechanisms of carbon sequestration in calcareous soils in northern China.

Organic fertilization promotes the accumulation of soil particulate organic carbon in a 9‐year plantation experiment

AbstractManagement practices are expected to influence the capacity of forests to mitigate climate change. However, the long‐term effects of afforestation on soil carbon accumulation in response to contrasting management regimes remain poorly understood. Here, we combined organic matter fractionation with a nine‐year‐long organic fertilization experiment to investigate the influences of largely accepted practices such as biochar (BC) and biogas‐slurry (BS) inputs on the accumulation of soil particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC) in three soil horizon depths (0–25, 25–50, and 50–75 cm, respectively). Our results suggested that both BS and BC significantly enhanced the POC and total soil organic carbon (SOC) content but overall did not significantly influence MAOC. Moreover, the POC and MAOC was more responsive to BS than BC. Further, our analyses revealed that the effects of BC on POC and MAOC were indirectly regulated by changes in the SOC: total nitrogen (TN) ratio, while BS influenced POC and MAOC by regulating TN. However, the responses of MAOC to the infiltration of organic fertilizers into the mineral soil should not be ignored, especially under high BC levels. Our work revealed that management practices are critical for supporting the long‐term capacity of new forests to accumulate soil carbon, thereby facilitating the provision of nature‐based solutions in response to climate change.

Seasonality and drivers of water column optical properties on the northwestern Barents Sea shelf

Hydrographic and bio-optical measurements were conducted along a south–north transect on the northwestern Barents Sea shelf from early spring to late summer in 2021. Strong climate change manifestations observed in this region are rapidly changing the marine environment. These rare observations cover the seasonal evolution from well-mixed and sea ice-covered winter conditions, through sea ice retreat in spring, to late summer where the sea ice had largely retreated and the water column was stratified due to sea ice melt.Phytoplankton drives the spatial and temporal variability in optical properties in most of the water column, but increased scattering and absorption could also be seen in the bottom boundary layer due to resuspended particles. The relationship between chlorophyll-a and particulate absorption deviates from the globally observed relationship, likely due to light adaptations in the ice-covered water masses. We recommend developing specific models for spring phytoplankton growth in ice-covered waters to accurately account for self-shading effects. The absorption of colored dissolved organic matter (CDOM) was relatively low, due to waters of Atlantic origin in the studied region, and varied considerably less than particulate absorption. CDOM was nevertheless the optically dominant ocean constituent in the very clear waters in late winter.Regional relationships for estimating particulate organic carbon (POC) and chlorophyll-a concentrations from in situ attenuation and fluorescence measurements were developed. POC may act as an alternative indicator to chlorophyll-a for optical properties in ice-covered open ocean, which is relevant for light availability parametrizations in biogeochemical ocean models.

Duration of continuous cropping with straw return affects soil organic carbon

AbstractUnderstanding the impact of continuous cropping with straw return on soil organic matter functional groups is of great importance for maintaining chemical stabilization of SOC in arid regions. Infrared spectral characteristics of cotton field soil with different continuous cropping durations (0, 5, 10, 15 and 20 years) were determined with Fourier‐transform infrared spectroscopy for fields in Manasi River Basin, Xinjiang province, China. The effects of continuous cropping duration on the stability and chemical composition of soil organic carbon (SOC) were analysed. The results indicate an initial increase in the SOC, soil particulate organic carbon (POC), and mineral‐associated organic carbon (MOC) content of soil under continuous cropping with straw return. However, as continuous cropping duration increased, the levels of SOC, POC, and MOC in soil began to decrease. The soil POC content and POC/MOC were highest after 5 years of straw return, and started to decrease as the continuous cropping duration increased. The SOC and MOC contents were highest after 10 years of continuous cropping, and were 3.30 and 1.84 times higher than the control, respectively. As continuous cropping duration increased, the relative peak intensities for polysaccharides and aromatic groups in soil organic matter decreased, while the relative peak intensities for aliphatic compounds and hydroxyketone increased. In conclusion, continuous cropping with straw return in cotton fields promotes SOC but only for about 10 years. An increase in straw return duration boosts the degree of esterification in soil organic matter. Moreover, enhanced protection by mineral binding was observed for soil organic matter, increasing organic matter stabilization. This study aimed to provide an empirical foundation for the management of the SOC pool and the establishment of rational straw return practices for cotton fields in arid areas.

Influence of land use types on soil carbon fractions in the Qaidam Basin of the Qinghai-Tibet Plateau

Soil carbon is an essential component that influences soil health and agronomic productivity, and land-use practices significantly impact soil carbon stocks. In this study, the researchers investigated the impact of various land-use types (farmland, orchards, grasslands, and facility lands) on soil organic carbon (SOC) fractions in the Qaidam Basin of China's Qinghai-Tibet Plateau. The soil samples were taken from the 0–10 and 10–20 cm depths. The particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions of various land uses have been measured. The results revealed that grasslands had the greatest POC/SOC ratio (0.59), with POC accounting for the majority of the SOC fraction. In contrast, all other land-use types (facility land, orchards, and farmland) had greater MAOC/SOC ratios than POC/SOC. The facility land had the highest SOC content (17.75 g kg−1), and orchard had the lowest (8.51 g kg−1). The grassland had the highest POC content (9.82 g kg−1) at a depth of 0–10 cm. The trend for MAOC content among the various land-use types was facility lands > farmlands > grasslands > orchards. The soil pH was also observed to be moderately alkaline, with an average of 8. The orchard has the highest bulk density (BD), measuring 1.38 g cm−3. The grasslands contained the highest amount of total nitrogen (TN) (2.2 g kg−1) while the agricultural fields contained the highest amount of total phosphorus (TP) (1.16 g kg−1). In addition, facility lands showed the highest level of microbial activity compared to other land-use types. According to the findings, SOC, POC, and MAOC fractions had considerably positive associations with TN and microbial biomass carbon (MBC), but were significantly negatively associated with metabolic CO2. The findings imply that different land-use types can have a considerable impact on the quality and amount of soil organic carbon, which has consequences for soil health and ecosystem processes. The findings also highlight the significance of long-term land management practices that promote the preservation and enhancement of soil organic carbon in various land-use systems.

Factors affecting broadscale variation in nearshore water-column organic carbon concentrations along the Great Barrier Reef

Organic carbon is a key energy source in marine food-webs, but elevated concentrations can have negative ecological impacts. We used 12 years of marine physiochemical, climate, and near-surface sea and meteorological condition data to investigate broadscale correlates and potential drivers of variability in dissolved and particulate organic carbon (DOC and POC, respectively) concentrations in coastal waters of the Great Barrier Reef (GBR), Australia. DOC concentrations were higher when salinity was lower, when the Southern Oscillation Index was more positive, and at times of greater northward wind speeds. POC concentrations were higher in shallower waters and were positively correlated with total suspended solids and chlorophyll a concentrations. Latitude was important for structuring variation in organic carbon concentrations, likely reflecting terrestrial material from rivers and broader marine processes. We conclude that variability in broadscale patterns of DOC concentrations in GBR coastal waters is likely most sensitive to material discharge from rivers as well as oceanographic and climatic processes and forcing. Variability in broadscale patterns of POC concentrations in coastal waters of the GBR is likely most affected by sediment resuspension and phytoplankton biomass accumulation.