Particulate Organic Carbon Research Articles

Soil organic carbon, carbon fractions, and microbial community under various organic amendments

The impact of various organic amendments on soil organic carbon (SOC) have rarely been reported. To address this, a laboratory experiment was designed to scrutinize the effects of different amendments on soil carbon fractions, microbial communities, and the underlying interactive mechanisms. The experiment encompassed a no-amendment control (CK), as well as treatments with corn straw (CS), tobacco stalks (TS), and peanut shell biochar (PB). Over a 70-day incubation, the SOC in plots amended with CS, TS, and PB displayed significant boosts of 13.9%, 17.5%, and 44.8%, respectively, compared to the CK. For soil carbon fractions, amendments with PB, TS, and CS led to a dramatic rise in particulate organic carbon (POC) of 27.4%, 20.2%, and 105.7%, respectively, in contrast to the CK plots. Mantel analysis and structural Equation Modeling uncovered strong interrelationships among the cbbL, cbbM, Bacteroidota, TOC, and POC. Organic amendments enhance soil carbon fractions, modulating the microbial community by increasing Bacteroidetes abundance and suppressing Acidobacteria richness, thereby influencing the abundance of key carbon cycle genes such as cbbL and cbbM. These results suggest that the addition of peanut shell biochar significantly boosted TOC and key carbon fractions, enhancing carbon content and soil fertility.

Distribution characteristics of soil active organic carbon at different elevations and its effects on microbial communities in southeast Tibet

Global mountain ecosystems have garnered significant attention due to their rich biodiversity and crucial ecological functions; however, there is a dearth of research on the variations in soil active organic carbon across altitudinal gradients and their impacts on microbial communities. In this study, soil samples at an altitude of 3,800 m to 4,400 m were collected from Sejira Mountain in the southeast Tibet, and soil active organic carbon components, soil microbial community diversity, composition and structure distribution and their relationships were systematically analyzed. The results revealed a non-linear relationship between the elevation and the contents of soil organic carbon (SOC) and easily oxidized organic carbon (ROC), with an initial increase followed by a subsequent decrease, reaching their peak at an altitude of 4,200 m. The Shannon diversity of bacteria exhibited a significant decrease with increasing altitude, whereas no significant change was observed in the diversity of fungi. The bacterial community primarily comprised Acidobacteria, Proteobacteria, Chloroflexi, and Actinobacteriota. Among them, the relative abundance of Proteobacteria exhibited a negative correlation with increasing altitude, whereas Actinobacteriota demonstrated a positive correlation with elevation. The fungal communities primarily consisted of Basidiomycota, Ascomycota, and Mortierellomycota, with Ascomycota prevailing at lower altitudes and Basidiomycota dominating at higher altitudes. The diversity and composition of bacterial communities were primarily influenced by altitude, SOC, ROC, and POC (particulate organic carbon). Soil carbon-to-nitrogen ratio (C/N), dissolved organic carbon (DOC), and available phosphorus (AP) emerged as key factors influencing fungal community diversity, while POC played a pivotal role in shaping the composition and structure of the fungal community. In conclusion, we believe that soil active organic carbon components had a greater impact on the bacterial community in the primary forest ecosystem in southeast Tibet with the elevation gradient increasing, which provided a theoretical basis for further understanding of the relationship between the microbial community and soil carbon cycle in the plateau mountain ecosystem under the background of climate change.

Bio-optical variability of particulate matter in the Southern Ocean

The composition and size distribution of particles in the ocean control their optical (scattering and absorption) properties, as well as a range of biogeochemical and ecological processes. Therefore, they provide important information about the pelagic ocean ecosystem’s structure and functioning, which can be used to assess primary production, particle sinking, and carbon sequestration. Due to its harsh environment and remoteness, the particulate bio-optical properties of the Southern Ocean (SO) remain poorly observed and understood. Here, we combined field measurements from hydrographic casts from two research voyages and from autonomous profiling floats (BGC-Argo) to examine particulate bio-optical properties and relationships among several ecologically and optically important variables, namely the phytoplankton chlorophyll a concentration (Chl), the particulate absorption coefficient (ap), the particulate backscattering coefficient (bbp), and the particulate organic carbon (POC) concentration. In the clearest waters of the SO (Chl < 0.2 mg m−3), we found a significant contribution to absorption by non-algal particles (NAP) at 442 nm, which was up to 10 times greater than the absorption by phytoplankton. This makes the particulate bio-optical properties there remarkably different from typical oceanic case 1 water. A matchup analysis confirms the impact of this larger NAP absorption on the retrieval of Chl from satellite ocean colour observations. For waters with Chl > 0.2 mg m−3, no significant differences are observed between the SO and temperate waters. Our findings also demonstrate consistency in predicting phytoplankton carbon from either Chl or bbp, suggesting that both methods are applicable in the SO.

Long-term recovery of compacted reclaimed farmland soil in coal mining subsidence area

During the reclamation process of coal mining subsidence areas, mechanical compaction hinders the rapid recovery of reclaimed mine soil (RMS) functions to their pre-mining levels. To optimize reclaimed farmland management, which can promote the recovery of compacted soils, this study aims to explore the key factors and underlying mechanisms affecting RMS recovery from a systemic perspective using complex network theory (CNT). Soil samples from reclaimed farmland at different recovery stages (0, 2, 6, 12, 16, and 22 years) and adjacent non-subsided cultivated soils (NCS) were collected at a depth of 0 ∼ 20 cm in the eastern plains mining region of China. Twenty-four soil indicators were measured, and their evolution over RMS recovery was analyzed. CNT was employed to systematically analyze the complex network relationships among these indicators, identifying key factors and underlying mechanisms affecting RMS recovery. The results indicated that compaction led to soil macroaggregate (MA) destruction, mineralization losses of organic carbon and nitrogen, reduced microbial activity, degraded soil fertility, and increased complexity and disorder in the relationships among soil indicators. Re-cultivation had a positive effect on the recovery of RMS. After 22 years of cultivation, significant improvements in soil structure were observed, with MA increasing by 30.95 % (P < 0.05). Based on this, organic carbon gradually accumulated, with soil organic carbon (SOC) increasing by 250.94 % (P < 0.05). Microbial abundance similarly improved, with total microbial biomass (TB) increasing by 123.07 % (P < 0.05). Soil fertility was also enhanced, with alkali-hydrolyzed nitrogen (AN), available phosphorus (AP), and available potassium (AK) increasing by 125.96 %, 304.84 %, and 61.90 %, respectively. However, RMS indicators and system structure remained distinct from those of the NCS. The first approximately 12 years after re-cultivation represented a critical period during which soil structure improvements drove RMS functional recovery, with a focus on MA recovery. Increasing particulate organic carbon (POC) accumulation is an effective strategy to promote aggregation in compacted RMS. Microbially mediated carbon–nitrogen cycling emerged as a crucial driver of the gradual recovery of the RMS system after re-cultivation.

Arbuscular mycorrhizal symbiosis enhances the accumulation of plant-derived carbon in soil organic carbon by regulating the biosynthesis of plant biopolymers and soil metabolism

Plant-derived carbon (C) is a critical constituent of particulate organic carbon (POC) and plays an essential role in soil organic carbon (SOC) sequestration. Yet, how arbuscular mycorrhizal fungi (AMF) control the contribution of plant-derived C to SOC storage through two processes (biosynthesis of plant biopolymers and soil metabolism) remains poorly understood. Here, we utilized transcriptome analysis to examine the effects of AMF on P. communis roots. Under the AM symbiosis, root morphological growth and tolerance to stress were strengthened, and the biosynthetic pathways of key plant biopolymers (long-chain fatty acids, cutin, suberin, and lignin) contributing to the plant-derived C were enhanced. In the subsequent metabolic processes, AMF increased soil metabolites contributing to plant-derived C (such as syringic acid) and altered soil metabolic pathways, including carbohydrate metabolism. Additionally, C-acquiring soil extracellular enzyme activities were enhanced by AMF, which could affect the stabilization of plant-derived C in soil. The contents of POC (21.71 g kg−1 soil), MAOC (10.75 g kg−1 soil), and TOC (32.47 g kg−1 soil) in soil were significantly increased by AMF. The concentrations of plant-derived C and microbial-derived C were quantified based on biomarker analysis. AMF enhanced the content of plant-derived C in both POC and MAOC fractions. What's more, plant-derived C presented the highest level in the POC fraction under the AMF treatment. This research broadens our understanding of the mechanism through which plant-derived C contributes to the accumulation of POC and SOC induced by AM symbiosis, and evidences the benefits of AMF application in SOC sequestration.

Soil carbon sequestration potential of different land use systems: evidence from sub-humid southern plains and Aravalli hills of Rajasthan, India.

This study has assessed total soil organic carbon (TOC) fractions, carbon management index (CMI), and carbon sequestration economic value under diverse land use systems (LUSs) of sub-humid Southern plains of Rajasthan, India. The study dealt with six LUSs: barren land (BL), agricultural land (AL), agri-horticulture (AH), horticultural land (HL), grassland (GL), and natural forest land (FL) were selected for the study. FL contained the highest TOC (13.04 ± 0.74gkg-1), particulate organic carbon (POC) (2.2 ± 0.19gkg-1), mineral-associated organic carbon (MAC) (10.84 ± 0.54gkg-1), and CMI (230.36 ± 4.13), whereas BL had the lowest amount of TOC (3.53 ± 0.4gkg-1), POC (0.47 ± 0.06gkg-1), MAC (3.06 ± 0.32gkg-1), and CMI (54.60 ± 4.2). The impact of LUSs on soil carbon lability index (LI) was minimal in all LUSs except FL exhibited statistically insignificant variations in LI. TOC stock showed the highest decline in BL (71.04%), AL (55.29%), AH (44.38%), and HL (25.92%) uses compared with the FL system. Different LUSs result in varying amounts of carbon stocks, representing the relative carbon credit gain. The maximum carbon credit was achieved by FL, which was roughly US $49,303 and 245% higher than BL. These results indicate the reinstatement of BL and AL towards HL and AH systems, and effective recycling of residues could improve the TOC storage in the study region.

Synergistic Promotion of Particulate and Mineral-Associated Organic Carbon Within Soil Aggregates After 10 Years of Organic Fertilization in Wheat-Maize Systems

How different fertilization practices modify soil organic carbon (SOC) sequestration is still unclear. Our study aimed to evaluate the changes in SOC stocks and their physical fractions after 10 years of organic and inorganic fertilization. Five treatments were established under a wheat-maize system in Northern China: control (CK), chemical fertilizer (F), straw plus chemical fertilizer (SF), manure plus chemical fertilizer (MF), and straw and manure plus chemical fertilizer (SMF). The results showed that the SOC sequestration rate at 0–20 cm depth decreased in the following order: SMF (1.36 Mg C/ha/yr) &gt; MF (1.13 Mg C/ha/yr) &gt; SF (0.72 C/ha/yr) &gt; F (0.15 Mg C/ha/yr) &gt; CK (−0.25 Mg C/ha/yr). The values indicated that straw returning and manure application were important measures to achieve the “4 per 1000” target, and the application of manure was a more effective strategy. The high input of chemical fertilizer only maintained the initial SOC level and was not a powerful C-farming practice. A minimum input of 4.93 Mg C/ha/yr was required to keep the initial SOC storage. The SOC associated with small macroaggregate (0.25–2 mm) was the most sensitive indicator for the changes of bulk SOC. In addition, the accumulation of SOC under SMF, MF, and SF treatments mainly occurred in the occluded particulate organic C (oPOC) in small macroaggregates, indicating that the physical protection of macroaggregates played a predominant role in SOC sequestration. The SMF, MF, and SF treatments also displayed higher mineral organic C (mSOC) in soil aggregates than the CK and F treatments. A transformation of oPOC towards the mSOC fraction indicated that exogenous C further shifted into stable C pools under the physical protection of soil aggregates. In conclusion, these findings confirmed the important role of straw returning and manure application in SOC accumulation and stabilization, highlighting that a combination strategy of straw + manure + chemical fertilizer had the best effect.

Characterization of Phytoplankton-Derived Amino Acids and Tracing the Source of Organic Carbon Using Stable Isotopes in the Amundsen Sea.

We utilized amino acid (AA) and carbon stable isotope analyses to characterize phytoplankton-derived organic matter (OM) and trace the sources of organic carbon in the Amundsen Sea. Carbon isotope ratios of particulate organic carbon (δ13C-POC) range from -28.7‱ to -23.1‱, indicating that particulate organic matter originated primarily from phytoplankton. The dissolved organic carbon isotope (δ13C-DOC) signature (-27.1 to -21.0‱) observed in the sea-ice melting system suggests that meltwater contributes to the DOC supply of the Amundsen Sea together with OM produced by phytoplankton. A negative correlation between the degradation index and δ13C-POC indicates that the quality of OM significantly influences isotopic fractionation (r2 = 0.59, p < 0.001). The AA distribution in the Amundsen Sea (5.43 ± 3.19 µM) was significantly larger than previously reported in the Southern Ocean and was associated with phytoplankton biomass (r2 = 0.49, p < 0.01). Under conditions dominated by P. antarctica (DI = 2.29 ± 2.30), OM exhibited greater lability compared to conditions co-dominated by diatoms and D. speculum (DI = 0.04 ± 3.64). These results highlight the important role of P. antarctica in influencing the properties of OM, suggesting potential impacts on carbon cycling and microbial metabolic activity in the Amundsen Sea.

Time-series forecasting of particulate organic carbon on the Sunda Shelf: Comparative performance of the SARIMA and SARIMAX models

Conducting in-depth studies of carbon dynamics in the Southeast Asia’s Sunda Shelf waters is challenging. Particulate organic carbon (POC) is a key component that can be monitored in the region, but ocean assessment requires advanced research and POC monitoring, especially in relation to ocean dynamics and forecasts. Accurate forecasting of POC in marine environments is difficult due to the complexity of oceanic processes and the influence of various environmental factors. Statistical forecasting models such as Seasonal Autoregressive Integrated Moving Average (SARIMA) or SARIMA with Exogenous Factors (SARIMAX) are useful for this purpose. This study compares these two time-series forecasting models to assess the role of exogenous factors in predicting POC variability in Indonesian seas. The models were specified as SARIMA (3, 1, 3)x(2, 0, 0, 60) and SARIMAX (3, 1, 3)x(2, 0, 0, 60). We obtained Akaike Information Criterion (AIC) values of 1237.767 for the SARIMA model and 1207.341 for SARIMAX[exog=SST]. The SARIMA model's validation metrics were 6.94 % for MAPE, 10.80 for RMSE, and 0.65 for the correlation coefficient, outperforming the SARIMAX model. While SARIMAX incorporates additional environmental variables, SARIMA outperforms it based on MAPE, RMSE, and correlation coefficients. Our findings reveal that only sea surface temperature can significantly influence POC forecasts, thus providing a new perspective on oceanic carbon dynamics. From 2022–2030, POC levels are expected to range between 108.3 and 135.9 mg/m³, with a mean value of 120.9±5.4 mg/m³, lower than that observed from 2002 to 2022, which was 118.8±11.9 mg/m³. The highest peak in POC is predicted for the end of 2026 and 2027. The annual trend shows that the highest POC values correspond to the northwest monsoon season, with the lowest during the intermonsoon period.

Community composition and co-occurrence of free-living and particle-attached bacteria in the source region of the Ganges and Brahmaputra Rivers.

Bacteria have two trophic lifestyles in aquatic ecosystems, i.e., free-living (FL) and particle-attached (PA), with different but essential ecological roles. However, relevant knowledge is still dearth in the upstream source region of the Himalayan Rivers. Thus, we emphasized a comparative study on community composition, co-occurrence, and geographic distribution of the FL and PA bacteria and the effect of environmental factors in the source region of the Ganges and Brahmaputra Rivers. PA bacteria relative to FL harbored a significantly higher local diversity, richness, and evenness. A significantly higher abundance of Betaproteobacteria, Verrucomicrobiota, and Planctomycetota in PA trophic lifestyle and Gammaproteobacteria and Actinomycetota in FL tropic lifestyle and indicator OTUs belonging to related taxa were observed. The spatial variation of the FL and PA bacterial communities was most significantly impacted by dispersal limitation as a discrete factor. Among the environmental parameters, the total nitrogen (TN) was found to be a significant (P < 0.001) driver of the variation in PA communities. Meanwhile, particulate organic carbon (POC) and TN considerably explained the variation of FL communities. A significant correlation (P < 0.001) of TN with dominant bacterial taxa (Pseudomonadota, Actinomycetota, and Verrucomicrobiota) and FL and PA indicator OTUs associated with these taxa further confirmed nitrogen as the limiting nutrient in the source region of the Ganges and Brahmaputra Rivers. The co-occurrence network topological characteristics showed that the PA network was more stable than the FL network, which was more complicated and unstable. Thus, it can be speculated that FL communities relative to PA are more vulnerable to shifting upon disturbances.

Phytoplankton Spring Bloom Inhibited by Marine Heatwaves in the North‐Western Mediterranean Sea

AbstractMarine heatwaves (MHWs) represent anomalously warm temperature conditions of seawater that may affect marine life and ocean biogeochemistry. Under such conditions, phytoplankton communities may modify their structure and functions, and their resilience is not assured. This study characterizes the impact of MHWs on the phytoplankton spring bloom in the North‐Western Mediterranean Sea. Here, we synergistically combine autonomous observations from BioGeoChemical‐Argo floats, satellite‐based and marine ecosystem model data, and show that MHW events occurring during winter drastically inhibit phytoplankton carbon biomass in spring by up to 70%. Such reduction is related to the enhanced stratification of the water column under MHWs which hinders the renewal of nutrients from deep‐ocean reservoirs, thus preventing surface phytoplankton from blooming. This process negatively impacts particulate organic carbon stocks within the mixed layer, while severe events cause an earlier shift of phytoplankton phenology that provokes changes in zooplankton biomass distribution.

Increased rainfall frequency enhances dry seasonal soil mineral-associated organic carbon in a subtropical forest

Climate models predict longer and more severe droughts, and alterations in the frequency and intensity of rainfall events. However, how changing precipitation patterns affect soil organic carbon (SOC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) remains unclear. Here, we conducted a three-year rainfall manipulation experiment with ambient rainfall as the control, removal of half the total rainfall amount with unaltered frequency (DRA), and increased rainfall frequency with the total amounts unchanged (IRF) in a subtropical forest. The results showed that the rainfall treatments did not change SOC content or fractions during the wet season. In the dry season, DRA significantly increased topsoil POC but did not change SOC or MAOC. IRF significantly increased POC and MAOC levels. The increased MAOC was associated with the newly formed binary complexes of Fe3+, Al3+, and Ca2+, and the adsorption/precipitation of reduced short-range-ordered Fe oxides, likely derived from the enhanced reductive dissolution in the IRF treatment. Our results suggest that changes in rainfall frequency affect organo-mineral interactions more profoundly relative to the rainfall amount and that these effects are season-dependent. Therefore, moisture-sensitive geochemical processes play an essential role in SOC stabilization in subtropical forests.

Changes in Physical Fractions within Soil Aggregates Under Nitrogen Reduction and Film Mulching Measures in Dryland Wheat Field

We studied the changes in various physical fractions within aggregates in the arid plateau of southern Shanxi Province, which has great significance for synergistically improving soil fertility and crop productivity in this region. Bulk soil samples were collected from 0-20 cm layers during a 7-year long-term experiment in Hongtong County, Shanxi Province. Wheat grain yields, SOC concentrations, proportions, and OC contents within soil aggregates were analyzed. OC contents included： unprotected coarse particulate organic carbon within macroaggregate （M-cPOC） and fine particulate organic carbon within macroaggregate （M-fPOC）, physically protected intra-aggregate particulate organic carbon within macroaggregate （M-iPOC）, chemically/biochemically protected mineral organic carbon within macroaggregate （M-MOC）, unprotected fine particulate organic carbon within microaggregate （m-fPOC）, physically protected intra-aggregate particulate organic carbon within microaggregate （m-iPOC）, and chemically/biochemically protected mineral organic carbon within microaggregate （m-MOC）. The treatments were ① farmer fertilization （FP）, ② nitrogen reduction monitoring and control fertilization （MF）, ③ nitrogen reduction monitoring and control fertilization plus ridge film and furrow sowing （RF）, and ④ nitrogen reduction monitoring and control fertilization plus flat film hole sowing （RF）. The results showed that compared with that in the FP treatment, MF reduced SOC concentration while maintaining wheat grain yield, RF and FH synergistically improved soil fertility and crop yield, especially for the FH with SOC concentration, and wheat grain yield increased by 8.44% and 48.86%, respectively. MF significantly reduced the content of M-cPOC, RF significantly increased the content of M-iPOC, and FH significantly increased the contents of M-fPOC, M-iPOC, M-MOC, and m-iPOC by 64.00%, 98.39%, 6.16%, and 17.48%, respectively. In addition, combined with redundancy analysis, we found that the M-iPOC fraction played a major role in increasing SOC concentration and wheat grain yield, with a contribution rate of 61.5%. Therefore, the contribution of macroaggregates to soil fertility and crop productivity was higher than that of microaggregates in the arid plateau area of southern Shanxi, and flat film hole sowing could increase the content of M-iPOC, thereby synergistically increasing SOC sequestration and wheat grain yield, which could promote this cultivation technology in the region and even in the country's arid agricultural areas.

Biochar affects organic carbon composition and stability in highly acidic tea plantation soil

Biochar amendments are effective in stabilizing soil aggregates and improving soil organic carbon (SOC) content. However, the effects of biochar on highly acidic soil and their relation with soil SOC stability remain understudied. The study aimed to investigate the impact of biochar on changes of aggregate distribution and SOC stability in a highly acidic tea plantation soils over an eight-year period. Soil samples were collected from plots with varying biochar application amounts (0, 2.5 t ha−1, 5 t ha−1, 10 t ha−1, 20 t ha−1 and 40 t ha−1). The content of SOC, iron bound organic carbon (OC-Fe), particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and the functional group composition of SOC was analyzed. The results indicated that in the biochar application treatments, the value of soil pH, SOC, POC and MAOC contents were increased from 3.92 to 4.28, 6.68%–187.02%, 8.31%–66.78% and 13.07%–236.47% respectively, compared with CK, while the content of macro-aggregate (particle size>0.25 mm) and soil aggregates mean weight diameter (MWD) significantly increased with higher biochar application amounts. But dissolved organic carbon (DOC) and OC-Fe content exhibited downward trend, decreased from 2.43% to 6.97% and 4.18%–19.91%. Furthermore, aromatic-C levels increased, with increased biochar application amounts. The integration of biochar not only bolstered soil aggregate stability but also amplified the presence of aromatic-C, thereby enhancing the resilience of organic carbon in highly acidic tea garden soil (BC40 > BC20 > BC5>BC2.5 > BC10 > CK), with increases ranging from 6% to 47%. The principal component analysis and structural equation modeling identified soil pH, TN, SOC, POC, MAOC, R > 0.25 and MWD as key factors of soil organic carbon stability. These findings provide crucial insights into the mechanism underlying biochar's efficiency in fortifying organic carbon stability, particularly in the context of highly acidic soil.

Elevational patterns of soil organic carbon and its fractions in tropical seasonal rainforests in karst peak-cluster depression region.

Karst ecosystems, especially tropical karst forests, are crucial to the global carbon cycle. In mountainous and hilly areas, elevation-related changes in environment and vegetation often lead to shifts in the accumulation and decomposition of soil organic carbon (SOC). However, the elevational patterns and influencing variables of SOC and its fractions in tropical karst forest ecosystems remain largely unexplored. Here, we analyzed the elevational patterns of SOC and its fractions in the topsoil and subsoil in the tropical seasonal rainforests within typical peak-cluster depression region of Southwest China. Our results indicated that the SOC content was highest at 400 m asl, which was significantly higher than that at 200 m asl (p < 0.05). Overall, SOC content demonstrated an increasing trend with rising elevation. Additionally, SOC content was significantly higher in the topsoil compared to the subsoil (p < 0.05). The majority of SOC fractions exhibited an increase with elevation but decrease with soil depth. Notably, only water-soluble organic carbon (WSOC) displayed a decrease with elevation. Meanwhile, recalcitrant organic carbon (ROC, 54.27%), particulate organic carbon (POC, 30.19%), and easily oxidizable organic carbon (EOC, 16.95%) were the main SOC fractions. Labile organic carbon (LOC) in the karst forest soil was predominantly composed of EOC and POC. Correlation analysis unveiled significant positive correlations between SOC and certain fractions with elevation, soil total nitrogen, and exchangeable magnesium. Conversely, significant negative correlations were observed with soil bulk density (SBD), soil total phosphorus, and litter phosphorus (Litter P). Redundancy analysis indicated that elevation, SBD, and Litter P were the main environmental variables influencing shifts in SOC and its fractions. Structural equation models showed that SOC was primarily directly impacted by soil properties but indirectly impacted by elevation. ROC was mainly associated with the direct effects of soil properties and litterfall, although elevation exerted a substantial impact through indirect pathways. Moreover, LOC was predominantly influenced by the direct impact of soil properties. Therefore, this study demonstrates that SOC and its fractions are strongly influenced by elevation in karst peak-cluster depression regions and have important implications for forest management and sustainable ecosystem development in these regions.

Aridity drives the response of soil total and particulate organic carbon to drought in temperate grasslands and shrublands.

The increasing prevalence of drought events in grasslands and shrublands worldwide potentially has impacts on soil organic carbon (SOC). We leveraged the International Drought Experiment to study how SOC, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) concentrations, responds to extreme drought treatments (1-in-100-year) for 1 to 5 years at 19 sites worldwide. In more mesic areas (aridity index > 0.65), SOC and POC concentrations decreased by 7.9% (±3.9) and 15.9% (±6.2) with drought, respectively, but there were no impacts on MAOC concentrations. However, drought had no impact on SOC, POC, or MAOC concentrations in drylands (aridity index < 0.65). The response of SOC to drought varied along an aridity gradient, concomitant with interannual precipitation variability and standing SOC concentration gradients. These findings highlight the differing response magnitudes of POC and MAOC concentrations to drought and the key regulating role of aridity.

Using the check dam deposit for an individual event to document the sources and erosional loss of sediment-associated organic carbon from a small catchment on the Chinese Loess Plateau

Information on the sources and fate of organic carbon mobilized by soil erosion is crucial for understanding the impact of erosion on carbon (C) cycling. However, few studies have evaluated this in low-order stream systems. In this study, we adopted a novel approach, that used the sediment deposited behind a check dam constructed at the outlet of the Fengzigou catchment (0.91 km2) on the Chinese Loess Plateau by a single high magnitude storm event, to simplify the field sampling requirements. The total sediment-associated organic carbon (TOC) was fractionated into particulate organic carbon (POC) and mineral associated organic carbon (MOC). These results were combined with those from a sediment source tracing exercise. This made it possible to explore further the source and loss of the sediment-associated organic carbon. The results showed that the inter-gully areas contributed the most eroded TOC (52.3 ± 7.6 %) and, together with the gully walls, represented the main TOC source. The primary contributor of the POC fraction in the deposited sediment was the inter-gully areas (80.5 ± 12.6 %), whereas the gully walls were the main contributor of the MOC fraction (54.9 ± 10.2 %). The total output of sediment produced by the event was 1304 ± 48 t and the equivalent value for sediment-associated TOC was 2.94 ± 0.26 t, of which the POC fraction accounted for 27.6 ± 3.6 % and the MOC fraction 72.4 ± 9.8 %. The TOC lost (0.3 ± 0.02 t) through the mobilization and delivery processes was 10.2 ± 1.2 % of the TOC mobilized by erosion, and the lost carbon was dominated by the POC fraction (86.7 ± 8.8 %). The specific TOC output from the catchment associated with the event documented was 0.3 t km−2. This work demonstrates the potential for erosion-induced carbon redistribution to provide a source for atmospheric CO2 in the study area.

Are bacterial communities and aggregation in fragile soils influenced by the management system?

Light-textured soils are widely distributed globally and, despite their limitations, have been integrated into agricultural production systems. This study aimed to assess how management systems—conventional tillage (CT) and no-till (NT)—affect aggregate formation pathways (physicogenic and biogenic) and bacterial communities. Two management systems (NT and CT) and three cover crops were evaluated: CJ: Crotalária (Crotalaria juncea (40 ​kg ​ha−1); M: Millet (Pennisetum glaucum - 60 ​kg ​ha−1); and C: Cocktail (Crotalária - Crotalaria juncea - 10 ​kg ​ha−1, Jack bean - Canavalia ensiformis - 75 ​kg ​ha−1, and Millet - Pennisetum glaucum - 30 ​kg ​ha−1). Undisturbed soil samples were collected from the crop row at a depth of 0.00–0.10 ​m. Aggregates with diameters between 9.7 and 8.0 ​mm were classified as biogenic or physicogenic. In addition to the chemical attributes of the aggregates, total organic carbon (TOC) and its fractions (mineral-associated organic carbon, MAOC; particulate organic carbon, POC; and free light fraction carbon, FLFC) were quantified. The structure and bacterial composition of the aggregates were also characterized. A higher proportion of biogenic aggregates (53–64%) was observed compared to physicogenic aggregates (36–47%). Cover crops exhibited significant differences in pH, calcium (Ca2+), base saturation, phosphorous (P), and percentage of base saturation. The management systems differed significantly for Ca2+ and P, with CT showing higher values than NT. The management system influenced organic matter accumulation and stabilization in the aggregates, with MAOC content being significantly lower in CT. POC and TOC were also significantly lower in physicogenic aggregates under CT. Bacterial community richness, diversity, and structure were significantly influenced by the management system, with greater richness and diversity in NT compared to CT. Network analysis revealed NT had more nodes and edges (65 and 406, respectively) than CT (52 and 357, respectively. Phyla abundance differed between the systems, with Firmicutes and Entotheonellaeota more abundant in CT, while WPS_2, GAL15, Bdellovibrionota, and Myxococcota were more abundant in NT. Despite the relatively short period of NT implementation (5 years), it had a positive effect on the bacterial community, which may subsequently influence nutrient and carbon content and their fractions in the aggregates.

Backscattering Linear Depolarization Ratio of Smoke Aerosols From Biomass Burning

AbstractThe linear depolarization ratio (LDR) of backscattering is a key parameter for distinguishing particle types. The measured LDRs of smoke in previous studies are highly variable and different from each other. Existing research on smoke aerosols considers only the internal mixing state of the black carbon (BC) particle population either by ignoring the externally mixed organic carbon (OC) particle population or by evaluating the LDR of smoke aerosols using the results of individual particles rather than the particle population. The field‐measured LDR of smoke then becomes difficult to interpret properly. The recent prescribed forest burning experiment in China showed that the LDR of freshly emitted smoke varies between 0.0% and 20.1% in wavelength (λ) of 532 nm. Electron microscopy images also showed that coated BC and pure OC exist simultaneously in biomass burning aerosols. Therefore, this study evaluates the influence of various parameters on the LDR of smoke by considering the internal and external mixing states. The calculated LDRs of the smoke aerosols were found to vary between 0.0% and 28.2% when λ is equal to 532 nm. The calculation results indicate that the LDR of smoke is slightly influenced by BC, considerably affected by the externally mixed OC, and is essentially dominated by the latter's morphology and particle size distribution. High levels and rapid changes of LDR of smoke can be well explained by nonsphericity and particle size distribution of externally mixed OC. This study advances the research on the measurement and evaluation of the LDR of smoke aerosols.

Satellite retrieval of oceanic particulate organic carbon: Towards an accurate and seamless dataset for the global ocean

Particulate organic carbon (POC) plays crucial roles in the global ocean carbon cycle and the oceanic biological pump. Satellite remote sensing has been demonstrated to be an effective technique for the retrieval of surface oceanic POC concentration. However, the complex spatiotemporal variations of the relationships between POC and oceanic optical properties across different waters posed challenges for accurate retrieval of POC concentration from satellite observations. Additionally, interference factors, such as cloud cover and sun glint, resulted in severe data missing problems and impeding daily coverage of the global ocean. With an attempt to generate accurate, seamless and readily available POC products for the global ocean, this study aimed to develop accurate satellite POC retrieval models for the Moderate Resolution Imaging Spectroradiometer (MODIS) data from both Terra and Aqua satellites, and to explore the possibility of using the empirical orthogonal function interpolation technique (DINEOF) to reconstruct satellite-retrieved POC data to generate gap-free global oceanic POC products. Results showed that the eXtreme Gradient Boosting (XGBoost) method could accurately retrieve POC with R2 approximately 0.80 and RMSE about 0.20 in log10 scale, obviously outperforming the operational blue-to-green band ratio algorithm and the hybrid polynomial algorithm based on two multi-band indices; and the DINEOF method, which could reconstruct approximately 88 % missing pixels for the global ocean, contributed to better revealing the global oceanic POC variations at a daily scale than the satellite-retrieved POC products. Based on the developed models, a suit of long time-series accurate and seamless POC products of the global surface ocean were generated, which is readily available for other applications and should be helpful to investigate the spatiotemporal variations of POC concentrations over global ocean and its roles in the global carbon cycle. The generated seamless products are openly accessible via the DOIs listed in the data availability section.