Total Particulate Organic Carbon Research Articles

Ecological investigations of giant Phaeocystis colonies in Viet Nam: I. Cell abundance and elemental composition.

Phaeocystis globosa is an important bloom-forming marine phytoplankton species that often accumulates to large levels in temperate and tropical waters and has significant impacts on food webs and biogeochemical cycles. It can form "giant" colonies that reach 3 cm in diameter. Microscopic observations, colony elemental composition, and pigment composition were analyzed to assess the characteristics of colonies as a function of colony size. Particulate organic carbon (POC) per unit surface area, colonial cell density, and chlorophyll a per unit surface area all increased with colony size, in contrast to results from temperate waters. Cellular chl a averaged 0.85 pg chl · cell-1. Colonies had both photosynthetic and protective pigments, with fucoxanthin being the dominant accessory pigment. Based on chl a and pigment levels, it appears colonies were acclimated to relatively low irradiances, likely due to their life cycle and the extremely turbulent environment in which they grew. Mucous carbon ranged from 16.2% to 79.2% of the total POC, and mucous carbon per unit surface area increased with colony size, suggesting that the mucous envelope did not thin as the colony grew. Based on elemental composition, nitrogen did not appear to limit growth, but phosphorus:carbon ratios were similar to those of P-limited cultures. Giant colonies represent an extreme response to the environment, but they do not appear to have greatly different characteristics than other tropical strains.

Ecological investigations of giant Phaeocystis colonies in Viet Nam: I. Cell abundance and elemental composition.

Phaeocystis globosa is an important bloom-forming marine phytoplankton species that often accumulates to large levels in temperate and tropical waters and has significant impacts on food webs and biogeochemical cycles. It can form "giant" colonies that reach 3 cm in diameter. Microscopic observations, colony elemental composition, and pigment composition were analyzed to assess the characteristics of colonies as a function of colony size. Particulate organic carbon (POC) per unit surface area, colonial cell density, and chlorophyll a per unit surface area all increased with colony size, in contrast to results from temperate waters. Cellular chl a averaged 0.85 pg chl · cell-1. Colonies had both photosynthetic and protective pigments, with fucoxanthin being the dominant accessory pigment. Based on chl a and pigment levels, it appears colonies were acclimated to relatively low irradiances, likely due to their life cycle and the extremely turbulent environment in which they grew. Mucous carbon ranged from 16.2% to 79.2% of the total POC, and mucous carbon per unit surface area increased with colony size, suggesting that the mucous envelope did not thin as the colony grew. Based on elemental composition, nitrogen did not appear to limit growth, but phosphorus:carbon ratios were similar to those of P-limited cultures. Giant colonies represent an extreme response to the environment, but they do not appear to have greatly different characteristics than other tropical strains.

Retrieval of particulate organic carbon concentration in Erhai Lake using sentinel-3 remote sensing data

ABSTRACT Particulate organic carbon (POC) plays a crucial role in the carbon cycle of inland lake ecosystems. The utilization of remote sensing satellite data provides an effective approach for monitoring the temporal and spatial variations in POC concentration within inland water. However, Erhai Lake, situated in the unique natural environment of the Yunnan-Kweichow Plateau, poses distinct challenges due to the complex and diverse origin of its water quality elements. Existing methods face difficulties in accurately detecting POC concentration in Erhai Lake. In this study, 112 water samples and 574 in-situ remote sensing reflectance curves were collected from Erhai Lake during April, May, and June 2023. The analysis of the optical characteristics of the water in Erhai Lake revealed the closest relationship between the concentration of total particulate matter and POC. Consequently, a POC concentration inversion algorithm was developed, utilizing bands 8, 12, and 16 of Sentinel-3 Ocean and Land Colour Instrument (OLCI). Various POC concentration inversion algorithms were evaluated utilizing a separate dataset. The results indicate that the three-band method offers superior accuracy in POC concentration inversion for Erhai Lake, with a root mean square error (RMSE) of 0.13 mg/L and a mean absolute percentage error (MAPE) of 28.80%. The three-band method was effectively applied to OLCI images from April, May, and June 2023, enabling the analysis of the spatiotemporal distribution of POC in Erhai Lake.

Toward the future integration of land-to-ocean observing systems to characterize organic carbon fluxes from storms

Storms can drive massive fluxes of carbon from land to ocean via rivers, contributing a large but relatively uncertain term to coastal carbon budgets. To estimate storm-driven aquatic organic carbon fluxes and develop a framework for future observations, we conducted a case study of extreme precipitation followed by river discharge to the Chesapeake Bay watershed, on the east coast of the United States over two decades from June 2001 to July 2021. Anomalous high precipitation events were defined as the top 5% of five-day accumulated precipitation over the watershed and we identified 80 distinct large storms over the Chesapeake Bay region. The top 1% of five-day accumulated river discharge from the United States Geological Survey (USGS) gage stations provided secondary storm criteria to distinguish extreme precipitation that manifest in very high flow: over the same period, we identified 16 storms that led to very high streamflow and combined streamflow with observations of POC and DOC to estimate daily mass flux from the seven major rivers. An extremely large convective storm and subsequent river discharge in July 2018 was studied to determine the association between precipitation, river discharge, and satellite ocean color in combination with in situ observations. MODIS-Aqua satellite derived dissolved organic carbon (DOC) concentration was calculated for all images in 2004–2021 and the estimated DOC values in 2018 were used to assess the feasibility of the optical satellites to monitor coastal storm fluxes of carbon. Using the USGS Load Estimator stream flow model and historical observations, the 2018 event exported approximately 12.9 Gg C particulate organic carbon (POC) and 12.5 Gg C DOC into the bay for 7.2% and 4.4% of the annual total, respectively. The largest storm over the last 20 years exported up to 16% of the annual total POC and 11% of the annual total of DOC from the major rivers over 10 days. However, monitoring the evolution of riverine carbon in estuarine environments from optical satellites alone remains challenging due to cloud cover, thus additional observing platforms assimilated through models are essential.

Variable carbon isotope fractionation of photosynthetic communities over depth in an open-ocean euphotic zone

Marine particulate organic carbon (POC) contributes to carbon export, food webs, and sediments, but uncertainties remain in its origins. Globally, variations in stable carbon isotope ratios (δ13C values) of POC between the upper and lower euphotic zones (LEZ) indicate either varying aspects of photosynthetic communities or degradative alteration of POC. During summertime in the subtropical north Atlantic Ocean, we find that δ13C values of the photosynthetic product phytol decreased by 6.3‰ and photosynthetic carbon isotope fractionation (εp) increased by 5.6‰ between the surface and the LEZ-variation as large as that found in the geologic record during major carbon cycle perturbations, but here reflecting vertical variation in δ13C values of photosynthetic communities. We find that simultaneous variations in light intensity and phytoplankton community composition over depth may be important factors not fully accounted for in common models of photosynthetic carbon isotope fractionation. Using additional isotopic and cell count data, we estimate that photosynthetic and non-photosynthetic material (heterotrophs or detritus) contribute relatively constant proportions of POC throughout the euphotic zone but are isotopically more distinct in the LEZ. As a result, the large vertical differences in εp result in significant, but smaller, differences in the δ13C values of total POC across the same depths (2.7‰). Vertical structuring of photosynthetic communities and export potential from the LEZ may vary across current and past ocean ecosystems; thus, LEZ photosynthesis may influence the exported and/or sedimentary δ13C values of both phytol and total organic carbon and affect interpretations of εp over geologic time.

Cycling Rates of Particulate Organic Carbon Along the GEOTRACES Pacific Meridional Transect GP15

AbstractUnderstanding particle cycling processes in the ocean is critical for predicting the response of the biological carbon pump to external perturbations. Here, measurements of particulate organic carbon (POC) concentration in two size fractions (1–51 and >51 μm) from GEOTRACES Pacific meridional transect GP15 are combined with a POC cycling model to estimate rates of POC production, (dis)aggregation, sinking, remineralization, and vertical transport mediated by migrating zooplankton, in the euphotic zone (EZ) and upper mesopelagic zone (UMZ) of distinct environments. We find coherent variations in POC cycling parameters and fluxes throughout the transect. Thus, the settling speed of POC in the >51 μm fraction increased with depth in the UMZ, presumably due to higher particle densities at depth. The settling flux of total POC (>1 μm) out of the EZ was positively correlated with primary production integrated over the EZ; the highest export occurred in the subarctic gyre while the lowest occurred in the subtropical gyres. The ratio of POC settling flux to integrated primary production was low (<5%) along GP15, which suggests an efficient recycling of POC in the EZ in all trophic regimes. Specific rates of POC remineralization did not show clear variations with temperature or dissolved oxygen concentration, that is, POC recycling was apparently controlled by other factors such as microbial colonization and substrate lability. Particle cohesiveness, as approximated by the second‐order rate constant for particle aggregation, was negatively correlated with trophic regime: particles appeared more cohesive in low‐productivity regions than in high‐productivity regions.

Sinking fate and carbon export of zooplankton fecal pellets: insights from time-series sediment trap observations in the northern South China Sea

Abstract. The sinking of zooplankton fecal pellets is a key process in the marine biological carbon pump, facilitating the export of particulate organic carbon (POC). Here, we analyzed zooplankton fecal pellets collected by two time-series sediment traps deployed on mooring TJ-A1B in the northern South China Sea (SCS) from May 2021 to May 2022. The results show a seasonal variability in both fecal pellet numerical (FPN) flux and fecal pellet carbon (FPC) flux, with peaks in November to April and June to August. It implies that the fecal pellet flux is largely regulated by the East Asian monsoon system. Vertical analysis further shows that FPN and FPC fluxes are higher at 1970 than at 500 m water depth, with larger pellets occurring in the deeper water, indicating a significant influence of mesopelagic and bathypelagic zooplankton community and lateral transport on deep-sea FPC export. However, the biovolume of amorphous pellets decreases significantly from 500 to 1970 m water depth, implying that these fecal pellets are broken and fragmented during the sinking process, possibly due to zooplankton grazing and disturbance by deep-sea currents. The contribution of fecal pellets to total POC export in the northern SCS is on average 3.4 % and 1.9 % at 500 and 1970 m water depth, respectively. This study highlights that the sinking fate of fecal pellets is regulated by marine primary productivity, deep-sea-dwelling zooplankton communities, and deep-sea currents in the tropical marginal sea, thus providing a new perspective for exploring the carbon cycle in the world ocean.

Influence of Land Uses on Soil Carbon Mineralization in Selected Agro-ecological Units of South Kerala, India

Soil is a major reservoir of terrestrial carbon and it plays an important role in the global carbon cycle. The soil organic carbon (SOC) is the fundamental factor for sustainable agriculture because of the ability in maintaining the soil fertility which is critical to soil productivity. Soil carbon pool has gained paramount importance in the recent decades owing to the alarming issues of climate change and global warming. Land use change was found to have a larger net effect on SOC storage than projected climate change. Depending on the land use management practices, soils can be a net sink or source for CO2. In this regard the present study was carried out to assess the impact of different land uses on carbon storage and mineralization in soils of selected agro-ecological units of south Kerala.The agro-ecological units (AEUs) of south Kerala namely, southern coastal plain (AEU 1), Onattukara sandy plain (AEU 3), southern laterites (AEU 8), south central laterites (AEU 9) and southern and central foothills (AEU 12) were selected. In each AEU, different agricultural land use categories as described by IPCC for the carbon inventory such as, garden land (coconut), wet land (rice), fallow (uncultivated) and plantation (rubber) were also selected for the study. The potential for carbon mineralization in soil was assessed by evaluating factors such as Total organic carbon, mineralizable carbon, particulate organic carbon, global warming potential, carbon distribution, and turnover. The total organic carbon and particulate organic carbon were higher in rubber land use and AEU 12. The mineralizable C content in soil varied from 1.40 to 3.45, 1.18 to 3.41, 1.04 to 3.04, 1.23 to 3.35 and 1.01 to 3.02 mg g⁻¹ in AEU 1, 3, 8, 9 and 12 respectively. The highest value was observed from AEU 1 and rice land use. Similar trend was obtained for global warming potential of soils based on CO2 evolution which varied from 31.82 to 78.41, 26.89 to 79.02, 22.89 to 69.02, 28.03 to 76.06 and 22.96 to 68.64 in AEU 1, 3, 8, 9 and 12 respectively. The C proportion and turnover rates were in the range of 0.25 to 0.77 and 0.04 to 0.17 respectively. The C proportion was the highest in AEU 12 and rubber land use whereas the C turnover was the highest in AEU 1 and rice land use.

Reconciliation of total particulate organic carbon and nitrogen measurements determined using contrasting methods in the North Pacific Ocean as part of the NASA EXPORTS field campaign

Measurements of particulate organic carbon (POC) are critical for understanding the ocean carbon cycle, including biogenic particle formation and removal processes, and for constraining models of carbon cycling at local, regional, and global scales. Despite the importance and ubiquity of POC measurements, discrepancies in methods across platforms and users, necessary to accommodate a multitude of needs and logistical constraints, commonly result in disparate results. Considerations of filter type and pore size, sample volume, collection method, and contamination sources underscore the potential for dissimilar measurements of the same variable assessed using similar and different approaches. During the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) 2018 field campaign in the North Pacific Ocean, multiple methodologies and sampling approaches for determining POC were applied, including surface inline flow-through systems and depth profiles using Niskin bottles, in situ pumps, and Marine Snow Catchers. A comparison of results from each approach and platform often resulted in significant differences. Supporting measurements, however, provided the means to normalize results across datasets. Using knowledge of contrasting protocols and synchronous or near-synchronous measurements of associated environmental variables, we were able to reconcile dataset differences to account for undersampling of some particle types and sizes, possible sample contamination and blank corrections. These efforts resulted in measurement agreement between initially contrasting datasets and insights on long-acknowledged but rarely resolved discrepancies among contrasting methods for assessing POC concentrations in the ocean.

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.

Influence of atmospheric dust deposition on sinking particle flux in the northwest Pacific

We examined the flux and composition of sinking particles collected at a water depth of 800 m in the northwest Pacific from November 2017 to August 2018 to assess the impact of dust deposition on organic carbon export. The fluxes of total particulate matter and particulate organic carbon averaged over the study period were 88 ± 63 mg m-2 d-1 and 9.0 ± 5.8 mg m−2 d−1, respectively. Biogenic particles accounted for 82% of the sinking particles, on average. There were two notable pulses in the particle fluxes of both biogenic and lithogenic material in February and May 2018. These flux peaks were decoupled from net primary production in the surface waters but coincided with intervals of high rates of atmospheric dust deposition. The biogenic component of the two peaks was dominated by two different phytoplankton communities, which may have influenced carbon export efficiency. Correlations between the sinking particle flux and the lithogenic flux are found at several locations in the northwest Pacific, implying that East Asian dust deposition has a prevalent influence on the biological pump. Attention should be paid to the effects of changes in the continental dust supply to the oceans on oceanic carbon export.

Effects of Combined Application of Biochar and Different Types of Nitrogen Fertilizers on Rapeseed Root Growth and Properties of Purple Soil in Southwest China

To demonstrate the effects of combined application of biochar and different types of nitrogen fertilizers on the growth of plant roots and on purple soil properties such as soil nutrients, soil carbon content and soil respiration, a 206-day greenhouse pot experiment with rapeseed was conducted. Three types of nitrogen fertilizer were used: urea (UR), controlled-release urea (RU), a mixture of 60% urea and 40% controlled-release urea (40% RU), and biochar was added at mass fractions of 0% (C0), 2% (C1) and 4% (C2), with a control treatment (CK) without nitrogen fertilizer and biochar. The results showed that biochar significantly improved soil nutrient status, with the best effect observed when 40%RU was co-applied with biochar. The addition of biochar significantly increased soil total organic carbon (TOC) and particulate organic carbon (POC). Soil respiration increased with increasing biochar application, and the combination of 2% biochar and 40% RU showed a reduction in carbon emissions compared to the UR and RU treatments. The number of rapeseed root tips (NT), forks (NF) and crossings (NC) increased significantly with the addition of biochar, and the combination of biochar and 40% RU was more beneficial for root growth and development than RU and UR. Considering the improvement in soil nutrition, increased soil organic carbon content, reduced carbon emissions, and enhanced rapeseed growth and development, the co-application of 2% biochar and 40% RU is recommended for large-scale application in rapeseed cultivation in the hilly purple soil of southwest China.

Greater mineral and aggregate protection for organic carbon in the soil amended by weathered coal than by biochar: Based on a 3-year field experiment

Soil carbon pool stability plays an important role in reaching carbon neutrality and mitigating global warming. Applying soil amendments is a practical strategy in agricultural production to improve soil environment. Weathered coal (WC) is an organic amendment that can be used to improve soil quality. However, the effects of WC application on soil organic carbon pool stability, and its differences from the effects of biochar (BC, a common amendment) application remains unclear. In this study, BC was selected as a comparison to further evaluate the potential of WC based on a 3-year field experiment, in which WC and BC were individually applied into a loamy clay soil at 0%, 1%, and 3% (w/w) rates. Soil organic carbon and its fractions (including particulate organic carbon and mineral-bound organic carbon), soil aggregate fractions and its stability, and the organic carbon content in aggregates were examined. The results showed that both WC and BC significantly increased soil total organic carbon, particulate and mineral-bound organic carbon (P < 0.05). The mineral-bound organic carbon content in WC treatment was significantly higher than that in BC treatment (by 32% under the 3% rate) (P < 0.05), whereas, there were no significant differences in soil total organic carbon content. Both WC and BC increased the soil organic carbon content in all aggregate fractions. While only the WC improved the soil aggregate stability, and which was 15% (under 1% rate) and 28% (under 3% rate) higher in WC treatments than in BC treatments (P < 0.05). The proportion of mineral-bound organic carbon to soil total organic carbon content and the soil aggregate stability were obviously related the soil C/N, and the WC treatments had a higher proportion of mineral-bound organic carbon and soil aggregate stability than BC treatments under the same soil C/N. The results indicated that the application of WC may be more effective than BC in increasing mineral and aggregate protection for soil organic carbon and thus improving soil carbon pool stability. Additionally, the purchase cost of WC was clearly lower than that of BC. Combined with the low cost and the benefits in soil structure and carbon pool stability, the application of WC appeared to have advantages over BC. Our findings provide robust evidence that WC is more effective than BC in improving soil carbon pool stability.

Spatial and temporal variability of the sources and sinks of carbonate system in the southwest bay of Bengal from 2014 to 2020

This study presents a synthesis of surface water partial pressure of CO2 (pCO2) and nutrient measurement in the southwest Bay of Bengal (swBoB) from 2014 to 2020 and characterizes the spatial and temporal variability. pCO2 rates found to be high (1191 μatm) during the 2015 monsoon and low (176 μatm) in the summer season during the month of May 2015. The inter-annual CO2 fluxes varied from −4.79 to 9.97 mmol Cm−2d−1. The significant a negative CO2 flux (−4.79 mmol Cm−2d−1) was recorded during the summer season in the year 2019, whereas a positive significant CO2 flux (9.97 mmol Cm−2d−1) was observed during the monsoon in 2014. Major physical parameters are at their highest during summer owing to increased high solar radiation during cloud-free circumstances, reduced or inadequate riverine flux, and a lack of vertical mixing of the water column, which results in the lowest nutrients concentration, Dissolved Oxygen (DO), Dissolved Inorganic Nitrogen (DIN), Dissolved Organic Carbon (DOC), chlorophyll-a, Particulate Organic Carbon (POC), pCO2, that leads to negative CO2 flux to the atmosphere. In contrast during monsoon season colossal discharge of freshwater high DO, DIN, DOC, chlorophyll-a, POC, pCO2 as results source CO2 flux to the atmosphere. Statistical analysis the correlation coefficient depicts Total Alkalinity (TA), DIC, POC, DIN, and DO found a positive correlation with pCO2 and fCO2 during the monsoon season. In the swBoB, pCO2 had a negative relationship with sea surface temperature (SST), sea surface salinity (SSS), and pH because CO2 solubility changes with SST and increases in cold water rather than warm water. In this study, we examine the association between all carbonate variables and the SSS and SST to better understand seasonal fluctuations.

Chlorophyll and POC in polar regions derived from spaceborne lidar

Polar regions have the most productive ecosystems in the global ocean but are vulnerable to global climate changes. Traditionally, the long-term changes occurred in an ecosystem are studied by using satellite-derived estimates of passive ocean color remote sensing measurements. However, this technology is severely limited by the inability to observe high-latitude ocean areas during lengthy polar nights. The spaceborne lidar can address the limitations and provide a decade of uninterrupted polar observations. This paper presents an innovative feed-forward neural network (FFNN) model for the inversion of subsurface particulate backscatter coefficients (bbp), chlorophyll concentration (Chl), and total particulate organic carbon (POC) from the spaceborne lidar. Non-linear relationship between lidar signal and bio-optical parameters was estimated through FFNN. The inversion results are in good agreement with biogeochemical Argo data, indicating the accuracy of the method. The annual cycles of Chl and POC were then analyzed based on the inversion results. We find that Chl, bbp, and POC have similar interannual variability but there are some subtle differences between them. Light limitation appears to be a dominant factor controlling phytoplankton growth in polar regions according to the results. Overall, the combined analysis of bbp, Chl, and POC contributes to a comprehensive understanding of interannual variability in the ecosystem in polar regions.

The Outsized Role of Salps in Carbon Export in the Subarctic Northeast Pacific Ocean.

Periodic blooms of salps (pelagic tunicates) can result in high export of organic matter, leading to an "outsized" role in the ocean's biological carbon pump (BCP). However, due to their episodic and patchy nature, salp blooms often go undetected and are rarely included in measurements or models of the BCP. We quantified salp-mediated export processes in the northeast subarctic Pacific Ocean in summer of 2018 during a bloom of Salpa aspera. Salps migrated from 300 to 750m during the day into the upper 100m at night. Salp fecal pellet production comprised up to 82% of the particulate organic carbon (POC) produced as fecal pellets by the entire epipelagic zooplankton community. Rapid sinking velocities of salp pellets (400-1,200md-1) and low microbial respiration rates on pellets (<1% of pellet C respired day-1) led to high salp pellet POC export from the euphotic zone-up to 48% of total sinking POC across the 100m depth horizon. Salp active transport of carbon by diel vertical migration and carbon export from sinking salp carcasses was usually <10% of the total sinking POC flux. Salp-mediated export markedly increased BCP efficiency, increasing by 1.5-fold the proportion of net primary production exported as POC across the base of the euphotic zone and by 2.6-fold the proportion of this POC flux persisting 100m below the euphotic zone. Salps have unique and important effects on ocean biogeochemistry and, especially in low flux settings, can dramatically increase BCP efficiency and thus carbon sequestration.

Zooplankton Fecal Pellet Characteristics and Contribution to the Deep‐Sea Carbon Export in the Southern South China Sea

AbstractZooplankton fecal pellets play a significant role in particulate organic carbon (POC) export, which has been widely reported in different ocean regions. However, zooplankton fecal pellet in the tropical marginal sea and its contribution to the total POC flux has not been sufficiently investigated yet. Here, for the first time we report zooplankton fecal pellet characteristics and fluxes in the South China Sea from June 2020 to May 2021 via one mooring with two sediment traps deployed at 500 and 2,190 m depths. Average fecal pellet numerical flux was 2.30 × 104 and 4.22 × 104 pellets m−2 d−1 at 500 and 2,190 m, respectively, corresponding to average fecal pellet carbon (FPC) flux from 0.31 to 0.71 mg C m−2 d−1. Small ellipsoidal and spherical pellets accounted for more than 90% of the total numerical flux, while large cylindrical pellets, although relatively rare (6%), accounted for almost 15% of the FPC flux. Both fecal pellet numerical and carbon fluxes were higher in winter, when the strong East Asian winter monsoon winds and heavy rainfall promoted the marine primary production. Higher fecal pellet fluxes combining with the presence of extra‐large pellets at 2,190 m compared to those at 500 m suggested the in‐situ repackaging of deep‐dwelling zooplankton communities in the mesopelagic and bathypelagic zones. Contribution of fecal pellets to the total POC flux ranged from 0.4% to 30.0% (averaging 9.0%), with higher values occurring during winter monsoon, at which time fecal pellets became a critical conveyor of carbon export to the deep sea.

Long-term adoption of reduced tillage and green manure improves soil physicochemical properties and increases the abundance of beneficial bacteria in a Mediterranean rainfed almond orchard

Soil health and function is one of the most important components for sustainable management of woody orchards. Intensive use of machinery and tillage creates heavy pressure on agroecosystems by altering the soil physicochemical and biological characteristics. In this study we aimed to assess the changes in soil physicochemical properties and the soil bacterial community structure, as well as the main drivers involved in shifts in the microbial community following 10 years implementing reduced tillage and reduced tillage plus green manure in a rainfed Mediterranean almond orchard. The treatments were: i) conventional tillage (CT); ii) reduced tillage (RT); and iii) reduced tillage and diversification with Avena sativa and Vicia sativa (RTD). After ten years, the results showed that the different managements had no significant effect on almond yields. RTD significantly increased total organic carbon (TOC), with an average content of 19.5 g kg−1 compared to 17.1 g kg−1 in CT. RTD also contributed to an increase in the fraction of soil macro-aggregates. Both RT and RTD significantly increased soil total nitrogen (NT) and particulate organic carbon (POC). However, other physical and chemical variables such as exchangeable Ca, Mg and K, bioavailable Fe, Cu, Zn and Mn, cation exchange capacity, bulk density, wilting point and field capacity were not significantly affected by the management. The bacterial community significantly changed with management: RTD showed a higher relative abundance of the family Solirubrobacteraceae and the genera Streptomyces and Solirubrobacter. The bacterial community structure was explained by changes in TOC, POC, pH, and NT values. Thus, a combination of reduced tillage and green manure could represent an appropriate sustainable management for rainfed almond orchards in very fragile environmental conditions, due to an increase in soil organic matter, total nitrogen, and the higher presence of beneficial bacteria related to soil productivity.

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 (&amp;gt; 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.

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.