Fraction Of Particulate Organic Carbon Research Articles

Investigating the complementarity of thermal and physical soil organic carbon fractions

Abstract. Partitioning soil organic carbon (SOC) in fractions with different biogeochemical stability is useful to better understand and predict SOC dynamics and provide information related to soil health. Multiple SOC partition schemes exist, but few of them can be implemented on large sample sets and therefore be considered relevant options for soil monitoring. The well-established particulate organic carbon (POC) vs. mineral-associated organic carbon (MAOC) physical fractionation scheme is one of them. Introduced more recently, Rock-Eval® thermal analysis coupled with the PARTYSOC machine learning model can also fractionate SOC into active (Ca) and stable SOC (Cs). A debate is emerging as to which of these methods should be recommended for soil monitoring. To investigate the complementarity or redundancy of these two fractionation schemes, we compared the quantity and environmental drivers of SOC fractions obtained on an unprecedented dataset from mainland France. About 2000 topsoil samples were recovered all over the country, presenting contrasting land cover and pedoclimatic characteristics, and analysed. We found that the environmental drivers of the fractions were clearly different, the more stable MAOC and Cs fractions being mainly driven by soil characteristics, whereas land cover and climate had a greater influence on more labile POC and Ca fractions. The stable and labile SOC fractions provided by the two methods strongly differed in quantity (MAOC/Cs=1.88 ± 0.46 and POC/Ca=0.36 ± 0.17; n=843) and drivers, suggesting that they correspond to fractions with different biogeochemical stability. We argue that, at this stage, both methods can be seen as complementary and potentially relevant for soil monitoring. As future developments, we recommend comparing how they relate to indicators of soil health such as nutrient availability or soil structural stability and how their measurements can improve the accuracy of SOC dynamics models.

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.

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.

Contribution of soil microbial necromass carbon to soil organic carbon in grassland under precipitation change and its influencing factors in loess hilly region, Northwest China.

To investigate the contribution of microbial necromass carbon (MNC) to soil organic carbon (SOC) and its influencing factors under precipitation changes in grassland, we conducted a precipitation experiment with seven different precipitation levels in the Bothriochloa ischaemum restoration area in the loess hilly region. We analyzed the contents and characteristics of fungal necromass carbon (FNC), bacterial necromass carbon (BNC), and MNC in different fractions of SOC under different treatments, including natural precipitation (CK), and increased and decreased 20%, 40%, 60% of natural precipitation (I20, I40, I60, D20, D40, D60) . The results showed that 1) MNC content in mineral organic carbon (MAOC) ranged from 1.62 g·kg-1 to 2.17 g·kg-1, which was higher than that in particulate organic carbon (POC) ranging from 0.69 g·kg-1 to 1.31 g·kg-1. The former was approximately 1.4 to 2.8 times as that of the latter. 2) FNC and MNC exhibited similar changes in both MAOC and POC fractions. BNC content in MAOC was approximately 1-3.1 times as that of FNC. FNC content in POC was generally higher than BNC except for I40 and I60 where BNC exceeded FNC. 3) Overall, both increases and decreases in precipitation resulted in elevated MNC/MAOC and BNC/MAOC ratios, but decreased MNC/POC and FNC/POC ratios. The MNC/MAOC ratios in I60 and D60 were 33.2% and 18.1% higher than CK, respectively. The BNC/MAOC ratios in D60, I40 and I60 were 28.0%, 23.0% and 19.1% higher than those in CK, respectively. Except for D60, the FNC/POC and MNC/POC ratios were significantly lower than CK under other treatments. In terms of POC fractions, the MNC/POC ratios of D40, D20, I20, I40 and I60 were 28.4%, 23.3%, 28.8%, 23.3% and 32.2% lower than that of CK, respectively. The FNC/POC ratio of D40, D20, I20, I40 and I60 was found to be lower by 23.3%, 16.1%, 21.0%, 27.0% and 31.0% compared to that of CK, respectively. 4) NH4+-N and pH were the primary factors influencing the content of MNC in different carbon fractions under varying precipitation conditions. In summary, alterations in precipitation (either increase or decrease) enhanced the contribution of BNC-dominated MNC to MAOC, but reduced the contribution of FNC-dominated MNC to POC. This study was of significance for understanding the distribution of microbial necromass across different organic carbon fractions under precipitation alterations.

Terrestrial and marine POC export fluxes estimated by 234Th–238U disequilibrium and δ13C measurements in the East China Sea shelf

The use of 234Th–238U disequilibrium has been widely employed to estimate the sinking flux of particulate organic carbon (POC) from the upper sea and ocean. Here, the deficits of 234Th relative to 238U in the water column and the carbon isotope signature (δ13C) of POC in the East China Sea (ECS) Shelf were measured, which was used to distinguish the fraction of marine and terrestrial POC export fluxes. In the ECS Shelf, very strong deficits of 234Th relative to 238U were observed throughout the water column, with 234Th/238U activity ratios ranging from 0.158 ± 0.045 to 0.904 ± 0.068 (averaging 0.426 ± 0.159). The residence times of particle reactive radionuclide 234Th (τTh–T) in the ECS shelf water varied between 9 and 44 days, which is significantly shorter than that in the continental slope area or the basin area. This phenomenon indicates that there is a more rapid particle scavenging process in the ECS shelf water compared to the continental slope and basin upper water. By applying a two-end-member mixing model based on the δ13C, the fraction of terrestrial POC was estimated to be 0 to 74% (mean: 30 ± 22%) and the fraction of marine POC was in the range of 25% to 100% (mean: 70 ± 22%). Fluxes of marine and terrestrial POC settling to the seafloor exhibited significant spatial differences among different stations, ranging from 11 to 129 mmol C/m2/day and from 2.6 to 38 mmol C/m2/day, respectively. The averaged terrestrial POC fluxes in the southern and northern ECS Shelf were similar (~ 21 to 24 mmol C/m2/day), while the marine POC fluxes in the north (86 ± 37 mmol C/m2/day) were approximately four times higher than those in the south (26 ± 20 mmol C/m2/day). Interestingly, the estimated export flux of both marine and terrestrial POC were approximately one order of magnitude higher than the previously reported burial fluxes of POC (ranging from 1.1 ± 0.1 to 11.4 ± 1.1 mmol C/m2/day) in the underlying bottom sediments, indicating that the majority (> 90%) of both terrestrial and marine POC exported from the upper water column are degraded in the sediments of the ECS Shelf. This “carbon missing” phenomenon can greatly be attributed to rapid decomposition by other processes (including microbial reworking, cross-shelf transport, and possible consumption by benthic organisms). Our findings highlight the dynamic nature of carbon cycling in the continental shelf and the need for further research to understand these processes and improve carbon budget assessments.

Straw return plus zinc fertilization increased the accumulations and changed the chemical compositions of mineral-associated soil organic carbon

Agricultural zinc (Zn) fertilization can markedly promote organo-Zn complex formation, especially when combined with straw. However, how soil organic carbon (SOC) conversion responds to Zn and/or straw plus Zn has received little attention. Here, we explored the SOC, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), as well as the OC chemical compositions’ responses in a field experiment site created in 2016. The following treatments were control (CK, no straw return and no Zn fertilization), straw return (St), Zn fertilization (Zn), and straw return combined with Zn fertilization (St+Zn). The results of our three-year study indicated that MAOC content under St+Zn increased by 4.4–6.4%, which led to a slight SOC accumulation (3.7–7.4%) compared to that of St alone. After including straw return with Zn fertilization, the Zn content of particulate organic matter and that of mineral-associated organic matter (ZnMAOM) both increased and thus helped MAOC formation. The chemical compositions of POC and MAOC were affected mainly by straw return and Zn fertilization, respectively. Both the Zn and St+Zn treatments had increased relative hydroxyl C contents and reduced O-alkyl C contents in MAOC, compared to that of CK and St, and the ZnMAOM content best explained the changes. In both the POC and MAOC fractions, the aromatic C to aliphatic C ratio decreased in Zn treatments compared to those in CK. Altogether, Zn’s effects on SOC and its fractions were closely linked to straw return, and Zn aids in MAOC and SOC accumulations when applied with straw.

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.

Quantifying the Unvoiced Carbon Pools of the Nilgiri Hill Region in the Western Ghats Global Biodiversity Hotspot—First Report

Accelerating land-use change (LUC) in the Nilgiri Hill Region (NHR) has caused its land to mortify. Although this deterioration has been documented, the destruction of buried gem soil has not been reported. Therefore, this study was conducted to assess the impact of LUC on soil-carbon dynamics in the six major ecosystems in the NHR: croplands (CLs), deciduous forests (DFs), evergreen forests (EFs), forest plantations (FPs), scrublands (SLs), and tea plantations (TPs). Sampling was conducted at selected sites of each ecosystem at three depth classes (0–15, 15–30, and 30–45 cm) to quantify the carbon pools (water-soluble carbon, water-soluble carbohydrates, microbial biomass carbon, microbial biomass nitrogen, dehydrogenase, and different fractions of particulate organic carbon). We found that the LUC significantly decreased the concentration of carbon in the altered ecosystems (49.44–78.38%), with the highest being recorded at EF (10.25%) and DF (7.15%). In addition, the effects of the LUC on the aggregate size of the organic carbon were dissimilar across all the aggregate sizes. The relatively high inputs of the aboveground plant residues and the richer fine-root biomass were accountable for the higher concentration of carbon pools in the untouched EFs and DFs compared to the SLs, FPs, TPs, and CLs. The results of the land-degradation Index (LDI) depicted the higher vulnerability of TP (−72.67) and CL (−79.00). Thus, our findings highlight the global importance of LUC to soil quality. Henceforth, the conservation of carbon pools in fragile ecosystems, such as the NHR, is crucial to keep soils alive and achieve land-degradation neutrality.

Redox oscillations destabilize and mobilize colloidal soil organic carbon

The onset of oxic-anoxic events in soil induces biogeochemical processes that may have profound influences on the fate and transport of soil organic carbon (OC). An 18-wk long laboratory soil column study was conducted to investigate the influence of static oxic (SO), static anoxic (SA), or redox oscillation (RO) events on the release of OC. Column leachate samples were collected after 9-wk and 18-wk of incubations and separated into dissolved (<2.3 nm), natural nanoparticle (2.3–100 nm), fine colloid (100–450 nm), and particulate (> 450 nm) fractions that were characterized through spectrofluorometric analyses. The concentration of released OC from different treatment columns followed the order of SA > RO > SO. After 9-wk, total OC concentrations in the leachate samples were increased by 4-fold and 54-fold in the SO and SA columns compared to the time-zero control columns, respectively. However, after 18-wk, the released amount of OC doubled in SO but decreased by 50 % in SA columns compared to the 9-wk incubation samples. The RO columns had intermediate OC concentrations between the SO and SA treatments. The RO events further led to widely varied dynamics in the release and molecular composition of the size-fractionated OC compared to static conditions, indicating the effects of redox oscillation on the organo-mineral association in soil. The wide variations in the aromaticity of OC released after the 1st and 2nd RO events further support the notion that alternating redox processes regulate OC cycling differently than the SO or SA condition. The observed increase in fine colloid and particulate OC fractions (i.e., >100 nm) from 7 % to 40 % between 1st and 2nd RO suggests the clustering of nanoaggregates and/or formation of colloidal size aggregates. The composition of the released OC as influenced by redox fluctuations provides a baseline for the size continuum of soil OC and its potential ecological and environmental roles.

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

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

Distribution of sequestered carbon in different pools in Alfisols under long-term groundnut system of hot arid region of India

Soil organic carbon (SOC) pools are indicators of soil productivity and sustainability and provide valuable information on the pathways of carbon sequestration in soils. We analysed organic C pools of different oxidizabilities, labile pools like particulate organic carbon, permanganate-oxidisable C, and microbial biomass carbon in soils under a long-term groundnut mono-cropping system with different management practices. Among the treatments, 50% NPK+farmyard manure (FYM) maintained a proportionately higher amount of soil carbon in passive pools (48.3%) followed by 50% NPK+ groundnut shell (GNS) (46.7%), FYM (44.7%), GNS (43.8%), 100% NPK (40.6%), 50% NPK (38.3%) and the control (32.4%). Particulate organic carbon fraction was the most sensitive fraction upon application of the amendments. Carbon stabilized from GNS and FYM sources had a skewed distribution along soil profile with a ratio of 1.2:1.0:1.4 and 1.8:1.0:1.3 at 0–15, 15–30, 30–45 cm depth, respectively. A critical carbon input of 0.32 Mg ha−1y−1 was needed to maintain SOC level, and the rate of conversion of crop residue C into soil organic C was about 8.1% for the present study. Combined use of chemical fertilizers and organic manure was found to be the best for enhancing SOC sequestration in groundnut mono-cropping system under hot arid eco-regions in India.

Soil organic carbon response to global environmental change depends on its distribution between mineral-associated and particulate organic matter: A meta-analysis

Soil organic carbon (SOC), as the largest terrestrial carbon pool, plays an important role in global carbon (C) cycling, which may be significantly impacted by global changes such as nitrogen (N) fertilization, elevated carbon dioxide (CO2), warming, and increased precipitation. Yet, our ability to accurately detect and predict the impact of these global changes on SOC dynamics is still limited. Investigating SOC responses to global changes separately for mineral-associated organic carbon (MAOC) and the particulate organic carbon (POC) can aid in the understanding of overall SOC responses, because these are formed, protected, and lost through different pathways. To this end, we performed a systematic meta-analysis of the response of SOC, MAOC, and POC to global changes. POC was particularly responsive, confirming that it is a better diagnostic indicator of soil C changes in the short-term, compared to bulk SOC and MAOC. The effects of elevated CO2 and warming were subtle and evident only in the POC fraction (+5.11% and − 10.05%, respectively), while increased precipitation had no effects at all. Nitrogen fertilization, which comprised the majority of the dataset, increased SOC (+5.64%), MAOC (+4.49%), and POC (+13.17%). Effect size consistently varied with soil depth and experiment length, highlighting the importance of long-term experiments that sample the full soil profile in global change SOC studies. In addition, SOC pool responses to warming were modified by degree of warming, differently for air and soil warming manipulations. Overall, we suggest that MAOC and POC respond differently to global changes and moderators because of the different formation and loss processes that control these pools. Coupled with additional plant and microbial measurements, studying the individual responses of POC and MAOC improves understanding of the underlying dynamics of SOC responses to global change. This will help inform the role of SOC in mitigating the climate crisis.

Investigation of a cold-based ice apron on a high-mountain permafrost rock wall using ice texture analysis and micro-14C dating: a case study of the Triangle du Tacul ice apron (Mont Blanc massif, France)

AbstractThe current paper studies the dynamics and age of the Triangle du Tacul (TDT) ice apron, a massive ice volume lying on a steep high-mountain rock wall in the French side of the Mont-Blanc massif at an altitude close to 3640 m a.s.l. Three 60 cm long ice cores were drilled to bedrock (i.e. the rock wall) in 2018 and 2019 at the TDT ice apron. Texture (microstructure and lattice-preferred orientation, LPO) analyses were performed on one core. The two remaining cores were used for radiocarbon dating of the particulate organic carbon fraction (three samples in total). Microstructure and LPO do not substantially vary with along the axis of the ice core. Throughout the core, irregularly shaped grains, associated with strain-induced grain boundary migration and strong single maximum LPO, were observed. Measurements indicate that at the TDT ice deforms under a low strain-rate simple shear regime, with a shear plane parallel to the surface slope of the ice apron. Dynamic recrystallization stands out as the major mechanism for grain growth. Micro-radiocarbon dating indicates that the TDT ice becomes older with depth perpendicular to the ice surface. We observed ice ages older than 600 year BP and at the base of the lowest 30 cm older than 3000 years.

Influence of edaphic and management factors on soils aggregates stability under no-tillage in Mollisols and Vertisols of the Pampa Region, Argentina

In the highly fertile and productive soils of the Pampa Region of Argentina, constraints on soil water regime as a consequence of the decline in aggregates stability and the development of platy structures, are observed. Most of agricultural plots in this region, even under no tillage, are subjected to simplification of crop sequences (low cropping intensity) with predominance of soybean. On the contrary, some farmers are intensifying the crops sequence to avoid soil degradation and equilibrate economical incomes. The aim of this work was to evaluate the effects of intrinsic soil factors and of cropping intensity on aggregates stability in the surface horizon of the main soils in this region. Three Mollisols (an Entic Haplustoll and two Typic Argiudolls) and a Vertisol (Hapludert) located across a west-east transect in the northern part of the Argentinean Pampas were selected for this study. In each site two management treatments under no-till (GAP: Good Agricultural Practices –high cropping intensity- and PAP: Poor Agricultural Practices –low cropping intensity-) and a soil without cultivation as a reference (NE: Natural Environment) were compared. In each treatment, aggregates destruction mechanisms were assessed by Le Bissonnais (1996) tests: slaking, microcraking and cohesion loss. Mollisols showed higher aggregates stability than the Vertisol. Differences on aggregates stability depended on management variables and on organic carbon contents in the Mollisols and on both clay content-clay type in the Vertisol, revealing a strong relationship of aggregation mechanisms with soils taxonomic order. In the Mollisols, the labile coarse particulate organic carbon fraction (POCc) determined the shifts on slaking and overall aggregates stability rather than other carbon fractions. In the soils studied, aggregates stability was linked mainly to management variables, best reflected by the cropping intensity index (CI). More intensive agricultural managements (GAP treatments) in Mollisols and Vertisol, resulted in an enhancement of aggregates stability; however, this relationship was stronger in the Mollisols. Surface horizons from both soil orders evidenced a high soil fragility related to slaking process (FW10s and FW tests). Thus, FW test was the best test to discriminate between management treatments. The results obtained in this work allow, on the one hand, to understand the stabilization mechanisms of the structure in the surface horizon of the main Pampas soils and, on the other hand, to highlight the effect of different NT management practice on soil aggregate stability, which highly affects soil health and the sustainability of agricultural systems.

Organic carbon fractions in temperate mangrove and saltmarsh soils

Coastal wetlands, such as mangrove and saltmarsh environments, can store significant amounts of soil organic carbon (SOC); however, most studies focus on tropical and subtropical environments. We assessed SOC stocks and fractions in temperate mangrove (two sites) and saltmarsh (sites SM1, SM2 and SM3) environments in southern Australia. The SOC fractions were separated according to particulate organic carbon (POC), humic carbon (HC) and recalcitrant carbon (RC) by size fractionation. Saltmarsh sites generally had the highest SOC content (up to 12.4% SOC). The POC fraction was the highest at the surface in the saltmarsh site and decreased relative to the HC and RC fractions with depth. Conversely, the proportion of POC at the mangrove sites did not decrease with depth, forming up to 76% of the SOC. The vertical displacement of soil of up to 5.8 mm year–1 at the saltmarsh sites, measured using root ingrowth bags, suggest significant contributions of POC via root materials. Retention of these POC inputs are likely to be related to waterlogging, which decreases decomposition rates – with much lower soil moisture content at SM1, where the lowest POC content occurred below the surface, compared with SM2 and SM3.

Responses of Soil Organic Carbon Fractions to Land Use Types in Hilly Red Soil Regions, China

Land use type exerts important influences on soil organic carbon (SOC) and its fractions, and determines the stability of the carbon pool. Taking woodland as a reference, the content of SOC and its labile fractions[dissolved organic carbon (DOC), microbial biomass carbon (MBC), and particulate organic carbon (POC)] and non-labile fractions[mineral-associated organic carbon (MAOC)] in upland and paddy surface soils in hilly red soil regions were determined to explore the responses of SOC fractions to land use type. The results showed that the contents of SOC, MBC, POC, and MAOC ranked highest in paddy compared with upland and woodland. DOC content in woodland was significantly higher than in upland and paddy (P<0.001). The proportion of each SOC fraction, i.e. DOC/SOC, MBC/SOC, POC/SOC, and MAOC/SOC, was in the range of 0.22%-0.93%, 1.62%-2.70%, 31.08%-40.00%, and 43.22%-56.82%, respectively. The contents of labile fractions (MBC and POC) and their proportions (MBC/SOC and POC/SOC) were in the order of paddy > woodland > upland. MAOC content ranked the highest in paddy but the lowest in upland, while MAOC/SOC exhibited the opposite trend. The correlation suggested that the labile fractions (MBC and POC) and inert fraction (MAOC) were significantly positively correlated with SOC (P<0.001) in the three land use types, while no significant correlations were found between DOC and SOC and its fractions (P>0.05). There was a significant positive correlation between POC and MBC in upland and woodland (P<0.001). POC was significantly positively correlated with MAOC in the three land use types (P<0.001). MAOC and MBC in paddy and upland were significantly positively correlated (P<0.001). Therefore, compared with upland and woodland, SOC in paddy had a higher proportion of labile SOC fraction, but a lower proportion of inert fraction. Moreover, MBC content in paddy was not related to the accumulation of the labile fraction of POC, but positively related to the accumulation of the inert fraction of MAOC. In summary, agricultural land uses have great influence on SOC and its fractions in hilly red soil regions. Though paddy is beneficial for SOC sequestration, the proportions of labile fractions in its SOC are relatively higher, and thus it is vulnerable to loss due to improper agricultural management.

Zooplankton response to organic carbon content in a shallow lake covered by macrophytes

ABSTRACTZooplankton are an important link between trophic levels in aquatic ecosystems, and their response to organic carbon is likely to have broad implications for lake food webs. The main objective of the research was to determine variations in zooplankton communities against the background of the structural heterogeneity of the lake and to link the observed patterns to the organic carbon content of the lake water. Spatial differences were noted in the organic carbon content of the lake water. Higher total organic carbon (TOC) and dissolved organic carbon (DOC) concentrations were recorded in the vegetated littoral area than in the lake pelagic zone. Zooplankton distribution and response to organic carbon content varied among habitats. At sites covered by plants, DOC and the bacterial sized fraction of particulate organic carbon were positively correlated with zooplankton biomass. In reeds, the grazing pressure by zooplankton on bacterial sized organic carbon was particularly strong. This implied that the microbial carbon link could be an important food web component providing carbon to higher trophic levels in areas covered by plants. This assumption corresponded well with the results of redundancy analysis (RDA).

Model study of organic carbon attenuation and oxygen mass transfer in persistent aggregate layers in the deep sea

A time series study from 1989 to 2017 indicates that, increasingly, carbon export to abyssal sediments in the California Current Ecosystem (CCE), 220 km west of the central California coast (Sta. M), occurs as rapidly sinking pulses of particulate organic carbon (POC). Nearly continuous sediment trap collections confirm that POC export to 3400 m increased significantly after 2011 with carbon attenuation (as fraction of POC remineralized) ranging from a low of 57% to a high of 77% during periods of pulsed flux. All of the major pulse events for the period resulted in the delivery of detrital aggregates that covered part or all of the sediment surface as an organic carbon rich layer at ~4000 m depth. However, the magnitude of the measured Sediment Community Oxygen Consumption (SCOC) did not increase proportionally to the organic carbon inventory change. Here, a model of oxygen consumption, informed by the time series data at Sta. M in the CCE, suggests that aggregate flocs constitute a major source for benthic carbon and a barrier to mass transport of oxygen, leading to reduced carbon attenuation in surface sediments. The correlation of POC delivery with aggregate coverage shows that the majority of POC delivery during pulse events is likely to consume all of the oxygen within the aggregates. Results indicate that an increasing fraction of POC reaches the abyssal sediments at Sta. M under conditions that support increased net burial of carbon.This finding is significant in the context of the fraction of the total POC flux that occurs as pulse events in this important upwelling region. During the period from 2011 to 2017, a third of the total POC flux resulted in aggregate flocs that covered a significant portion of the sediment surface (>47%). Following the 2015–2016 El Nino period the flocs became a dominant feature of the sediment surface. More than 80% of the POC flux to near bottom sediment traps in 2017 occurred coincident with aggregate floc cover of more than 90% of the sediments. The resultant physical barrier to oxygen transport and relatively low consumption of newly delivered carbon indicate conditions that are more typical of environments having low oxygen bottom waters.

Sediment and particulate organic carbon budgets of a subarctic estuarine fjard: Lake Melville, Labrador

Sediment and particulate organic carbon (POC) budgets constructed for oceanic regions provide a means to quantify the burial of POC, providing a baseline against which future change can be measured. Here we developed the first mass balance budgets of sediment and POC (terrestrial and marine) for the Lake Melville system (LMS; Goose Bay and Lake Melville) in central Labrador using previously published data (210Pb-based mass accumulation rates, river discharge, and total suspended solids) together with measurements of %OC and δ13C signatures within dated sediment cores collected from the LMS basins and water from main tributaries and Groswater Bay. A two end-member mixing model was used to establish fractions of terrigenous and marine POC. Overall, our mass balance budgets show that the Churchill River is the major supply of sediment (16.7 ± 7.8 × 108 kg y−1) and POCterr (18.4 ± 6.6 × 106 kg y−1) to the LMS, accounting for 47% and 56% of the total input to the system, respectively. Although the Churchill River drains directly into Goose Bay along with the smaller Goose River, the majority of sediment (12.9 ± 7.8 × 108 kg y−1) and POCterr (11.8 ± 7.0 × 106 kg y−1) is transported eastward through the surface water plume into Lake Melville. Inputs of POCmar from autochthonous primary production, estimated by applying an empirical relationship between sediment-surface flux of POCmar and water depth, to Goose Bay (1.8 × 106 kg y−1) and Lake Melville (98.3 ± 4.7 × 106 kg y−1) increase with increasing distance from the Churchill River. To investigate the potential changes to the sediment load of the Churchill River to the LMS following impoundment at Muskrat Falls, we calculated the sediment increase using the median reservoir shoreline erosion potential. This approach showed that the delivery of sediment and POCterr to the LMS by the Churchill River may double during the two years following impoundment of the lower Churchill River.

Carbon sequestration and selected hydraulic characteristics under conservation agriculture and traditional tillage practices in Malawi

Conservation agriculture (CA) is increasingly promoted among smallholder farmers of sub-Saharan Africa in a quest to improve food security while sustaining the natural resource base of the agro-ecosystems where agriculture is based. The aim of this study was to investigate the effects of CA and traditional tillage on soil organic carbon (SOC) and selected hydraulic properties in two contrasting agro-ecological zones of Malawi. Six farmers hosted on-farm trials in each location, with each farmer having the following treatments: CA with continuous sole maize (CA-SM), CA with maize–legume intercrops (CA-ML), and traditional tillage with continuous sole maize (CT-SM). Soil samples were randomly collected in October 2015, from farmers’ fields located in Chipeni, Chinguluwe, Lemu, and Zidyana where CA had been implemented for 10 years (2005–2015) at six depth intervals: 0–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm. Bulk density, soil water characteristics, and pore size distribution were determined using undisturbed core samples. At all sites, CA improved total SOC, carbon stocks, and the stable fraction of particulate organic carbon. Maize–legume intercropping under CA had 35%, 33%, and 73% more total SOC than CT-SM in Chipeni, Lemu, and Zidyana respectively. In Chinguluwe and Lemu, CA-ML had 0.54 and 0.50 g kg–1 respectively more stable fraction of particulate organic carbon (POMP) than CT-SM; whereas in Chipeni, CA-SM had 0.73 g kg–1 higher POMP compared with CT-SM. CA also improved soil porosity, pore size distribution, and water retention capacity by increasing the proportion of mesopores and micropores compared with CT-SM. Thus, changing management practices from CT-SM to CA has the potential to improve the soil organic matter and soil hydraulic properties across agro-ecological zones in Malawi, which is important for sustainable agriculture. Farmers should be encouraged to minimise tillage, retain residues as mulch on the soil surface, and practice crop rotation.