Organic Carbon Stability Research Articles

Grazing and reclamation-induced microbiome alterations drive organic carbon stability within soil aggregates in alpine steppes

Grazing and reclamation are the major modulators of soil organic carbon (SOC) stability in alpine steppes. However, our understanding of changes in SOC caused by grazing and reclamation at the aggregate scale is limited, and the mediating role of microorganisms remains unclear. Here, based on a long-term observation transect, we selected three land-use types: a long-term enclosed natural steppe (NS), a grazed steppe (GS), and a steppe reclaimed as cropland (CL). We investigated the impacts of grazing and reclamation on soil aggregate distributions, SOC fractions and the microbial community, and microbial contributions to SOC stability. Overall, GS and CL significantly decreased the proportion of >2 mm aggregates (by 32.25–33.72%), and enhanced SOC stability within different aggregate sizes, with the highest SOC stability observed in 0.25–2 mm and <0.25 mm sizes. GS significantly decreased bacterial diversity and increased the relative abundance of Gammaproteobacteria, while CL significantly increased microbial diversity and the relative abundance of Alphaproteobacteria. Importantly, GS enhanced the complexity of the bacterial OTUs networks in >2 mm and <0.25 mm aggregates. GS and CL caused changes in the assembly processes of bacterial communities, shifting from being mainly controlled by variable selection to being mainly controlled by homogenizing dispersal. However, there were only minor differences observed in the microbial community assembly within different aggregate sizes under the same treatment. Random forest models and correlation analysis revealed that the bacterial community, especially the network patterns and community assembly, was a key determinant of SOC stability in soil aggregates. In conclusion, we emphasize the important contribution of soil bacterial network patterns and community assembly to SOC stability in the context of human activities, providing information that may be useful for developing alpine steppe management strategies to enhance soil carbon sequestration.

Temporal stability of soil organic carbon in grazing lands of Eastern Australia

ABSTRACT Sequestering soil organic carbon (SOC) has been identified as a critical tool to mitigate anthropogenic climate change. Therefore, the response of SOC to climate variability needs to be quantified to achieve carbon sequestration over decadal to centennial scales. This study assesses the temporal variability of SOC for a grazed catchment in Australia that has been exposed to periods of drought and flood over the last 100 years. The analysis utilises field-measured SOC data obtained 2006–2018, along with modelled SOC data derived from a linear regression model and BIOS2 model outputs (1908–2015). Catchment SOC did not vary significantly, with mean values of 3.40% (2006), 3.21% (2014) and 3.10% (2018), despite the catchment experiencing extreme drying and wetting events during the sampling period. This stability was further supported by the long-term modelling, which showed a mean SOC of 3.68% and coefficient of variance of 0.16%. SOC was believed to be buffered against short term fluctuations in climate that persist for less than the SOC turnover times (i.e., multi-decadal). This indicates that temporal changes in SOC reflects only long-term climate variability. Current SOC and sequestered SOC are therefore unlikely to become a carbon sink in the short-term due to climate variability.

Effects of long-term tillage practices on the stability of soil aggregates and organic carbon in black soil farmland.

We examined the effects of different tillage practices on plough layer soil structure and organic carbon stabilization in black soil farmland with a long-term positioning platform. The wet-sieving method and infrared spectroscopy method were used to investigate the impacts of conventional tillage (CT), no-tillage (NT), sub-soiling tillage (ST), and moldboard plowing tillage (MP) on soil aggregates distribution and organic carbon characteristics in 0-40 cm soil layers. Compared to CT, both NT and ST treatments significantly increased the proportion of large macroaggregates (>2 mm) in the topsoil layer (0-20 cm)and that of small macroaggregates (0.25-2 mm) in the subsoil layer (20-40 cm) for NT, ST, and MP. NT, ST, and MP treatments resulted in higher mean weight dia-meter (MWD) and mean geometric diameter (GMD) of soil aggregates in both the topsoil and subsoil layers. NT treatment improved organic carbon contents in bulk soil and large macroaggregates in the topsoil layer, while ST and MP enhanced organic carbon contents in bulk soil and large macroaggregates in the subsoil layer. The contribution rate of small macroaggregates organic carbon content to the total was between 68.9% and 83.4%. Furthermore, the organic carbon chemical stabilization of soil body and aggregates increased in the topsoil and subsoil layers under NT treatment compared to others. The MWD had a positive correlation with the organic carbon content and chemical stability of soil body and small macroaggregates. These findings offered a theoretical basis for understanding the impacts of different tillage practices on the stability of soil aggregate and organic carbon in black soil region.

Effect of Wheat Straw Addition on Organic Carbon Mineralisation and Bacterial Community in Orchard Soil

Crop straw returning can stimulate organic carbon mineralisation and C sequestration simultaneously, which affects soil fertility. However, the effects of crop straw on organic carbon mineralisation and soil bacterial community in orchards are not fully understood. A 90-day incubation experiment was performed to investigate the effects of wheat straw (0, 1, 4, 6, 8, and 10 t·ha−1) on organic carbon mineralisation and bacterial community in orchard soil. Wheat straw addition enhanced the CO2 efflux rate and cumulative organic carbon mineralisation (Cmin), especially high level. The trend of CO2 efflux rate was increased sharply, especially during the early incubation stage (the first 13 days), and then decreased in the later phase. Furthermore, soil bacterial community structure displayed distinct changes in response to straw addition. Available nitrogen, potassium, organic carbon, β-glucosidase, and pH were the key factors driving soil bacterial community changes. The bacterial taxa in networks were significantly related to Cmin. The Proteobacteria, Actinobacteria, and Chloroflexi were positively related to Cmin; while Planctomycetes, Patescibacteria, and Gemmatimonadetes showed a negative relationship with Cmin by correlation and redundancy analyses. Co-occurrence network analysis showed a discrete bacterial network in 10 t·ha−1 of straw, while cohesive networks in others. Straw addition promoted organic carbon mineralisation by improving the soil biochemical properties, including enzymes activities, and nutrient contents, and regulating bacterial community composition. On the whole, 4 t·ha−1 of straw could be considered an economical level for improving soil organic carbon and bacterial community stability in orchards.

Evaluating carbon dynamics in soil aggregates using δ13C following long-term vegetation restoration near a surface mine in a semi-arid region

The vegetation restoration of land post‑mining can promote soil structure development and increase organic carbon storage. The response of soil carbon sequestration pathways to different recovery modes is poorly understood, especially in soils of reclaimed surface-mine areas. Here, long-term carbon dynamics were characterized by examining aggregate-associated carbon contents and carbon flow pathways following vegetation restoration of reclaimed land near a surface mine. Six long-term restoration areas were represented by Populus alba trees, Amorpha fruticosa shrubs, and grassland. During the long-term restoration process the contribution of aggregates to SOC (12.5 to 46.1%), δ13C values (-25.91 to –23.65‰) and percentages of particulate organic carbon in large soil aggregates of tree plantations showed the maximum increase and shrubland showed the minimum. The direction of carbon flows was from small aggregates to silt + clay size classes, and tree plantations slowed the carbon flows of aggregates. With increasing soil depth, roots were more likely to be the starting point of carbon flow in aggregates. This analysis of the natural δ13C signature reveals the mechanism of organic carbon stabilization under long-term vegetation restoration near a surface mine in a semi-arid region.

Sources of organic matter and carbon stocks in two mangrove sediment cores and surface sediment samples from Qinglan Bay, China

The carbon stocks (Corg stocks) in mangrove sediments and the distribution and source changes of sedimented organic matter in Qinglan Bay are not understood as the mangrove forests decrease. In this paper, we collected two sediment cores in the interior mangrove and 37 surface sediment samples from mangrove-fringe, tidal flat and subtidal habitats and then analysed the total organic carbon (TOC), total nitrogen (TN), and the stable organic carbon isotope (δ13C) and nitrogen isotope (δ15N) in the sediment samples to obtain the organic matter sources and carbon stocks in two different mangrove sediment cores from Qinglan Bay. The δ13C and TOC/TN values showed that mangrove plants and algae were the main sources of organic matter. Relatively high contributions of mangrove plants (>50 %) were distributed in the mangrove areas in Wenchang estuary, the northern region of Bamen Bay and the eastern side of the Qinglan tidal inlet. The enriched δ15N values might be related to anthropogenic nutrient inputs, including increased aquaculture wastewater, human sewage and ship wastewater. The Corg stocks in cores Z02 and Z03 were 357.79 Mg C ha−1 and 265.78 Mg C ha−1, respectively. This Corg stock difference might have been related to the salinity and the benthos activities. The high Corg stock values measured in Qinglan Bay were caused by the mangrove maturity and stand age. The total Corg storage of the mangrove ecosystem in Qinglan Bay was estimated to be approximately 263.93 Gg C. This study contributes to organic carbon stocks and sources of sedimented organic matter in global mangroves.

Duration of continuous cropping with straw return affects soil organic carbon

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

Influence of long-term fertilization on soil aggregates stability and organic carbon occurrence characteristics in karst yellow soil of Southwest China.

Current research has long focused on soil organic carbon and soil aggregates stability. However, the effects of different long-term fertilization on the composition of yellow soil aggregates and the characteristics of the occurrence of organic carbon in the karst region of Southwest China are still unclear. Based on a 25-year long-term located experiment on yellow soil, soil samples from the 0-20 cm soil layer were collected and treated with different fertilizers (CK: unfertilized control; NPK: chemical fertilizer; 1/4 M + 3/4 NP: 25% chemical fertilizer replaced by 25% organic fertilizer; 1/2 M + 1/2 NP: 50% chemical fertilizer replaced by organic fertilizer; and M: organic fertilizer). In water-stable aggregates, soil aggregates stability, total organic carbon (TOC), easily oxidized organic carbon (EOC), carbon preservation capacity (CPC), and carbon pool management index (CPMI) were analyzed. The findings demonstrated that the order of the average weight diameter (MWD), geometric mean diameter (GWD), and macro-aggregate content (R0.25) of stable water aggregates was M > CK > 1/2M +1/2NP > 1/4M +3/4NP> NPK. The MWD, GWD, and R0.25 of NPK treatment significantly decreased by 32.6%, 43.2%, and 7.0 percentage points, respectively, compared to CK treatment. The order of TOC and EOC content in aggregates of different particle sizes was M > 1/2M +1/2NP > 1/4M +3/4NP> CK > NPK, and it increased as the rate of organic fertilizer increased. In macro-aggregates and bulk soil, the CPC of TOC (TOPC) and EOC (EOPC), as well as CPMI, were arranged as M > 1/2M +1/2NP > 1/4M +3/4NP> CK > NPK, but the opposite was true for micro-aggregates. In bulk soil treated with organic fertilizer, the TOPC, EOPC, and CPMI significantly increased by 27.4%-53.8%, 29.7%-78.1%, 29.7-82.2 percentage points, respectively, compared to NPK treatment. Redundancy analysis and stepwise regression analysis show that TOC was the main physical and chemical factor affecting the aggregates stability, and the TOPC in micro-aggregates has the most direct impact. In conclusion, the primary cause of the decrease in SOC caused by the long-term application of chemical fertilizer was the loss of organic carbon in macro-aggregates. An essential method to increase soil nutrient supply and improve yellow soil productivity was to apply an organic fertilizer to increase aggregates stability, storage and activity of SOC in macro-aggregates.

Environmental controls on density‐based soil organic carbon fractionations in global terrestrial ecosystems

AbstractCategorization of soil organic carbon (SOC) into different functional subpools according to their recalcitrance and protective mechanisms helps better understand ecosystems organic carbon (OC) dynamics, and various attempts have been made to explore the suitable experimental fractionation method for such purpose. However, most previous studies neglected the influences of environmental factors on the effectiveness of varying fractionation methods. Density fractionation has shown great promise in elucidating SOC immobilization mechanisms. Here, we compared three varying types of density fractionation methods (density, density + dispersion, and density + other procedures) for categorizing the SOC into three functional pools, that is, active OC (OCactive), moderately stable OC (OCm‐stable), and stable OC, (OCstable) using global data compiled for 95 sites in 31 published studies, and examined the influences of climate (mean annual temperature [MAT] and annual precipitation), vegetation type, and soil properties (soil depth, clay content, and soil type) on SOC fractions determined by the three density fractionation methods. The percentage of OCm‐stable fraction was found to be highest using the density method and lowest using the density + dispersion method, due to differential density ranges between the two methods. At a global scale, the contents of total SOC and its OC fractions decreased with temperature. Precipitation had no apparent influences on the subdivided SOC fractions using either the density + dispersion method or the method of density + other procedures, whereas soil type constrained the effect of precipitation on SOC fractions using the density method. The percentage of OCm‐stable determined by the density + dispersion method was more responsive to MAT and vegetation type than that by the other two methods. The percentage of OCstable determined by the method of density + other procedures was significantly and positively related to the clay content as the OCstable based on this method included small particles. For all the three methods of fractionation, soil type had a greater influence than the clay content on the SOC fractions, especially the OCm‐stable and the OCstable. For soil type characterized by rich metal oxides, both the density method and the method of density + other procedures could be used for SOC fractionation. For soil type rich in nutrients, the density + dispersion method would have higher sensitivity for distinguishing the OCm‐stable.

Mineral organic carbon interactions in dry versus wet tundra soils

Mineral organic carbon interactions (aggregation, organo-mineral associations and organo-metallic complexes) enhance the protection of organic carbon (OC) from microbial degradation in soils. The northern circumpolar permafrost region stores between 1,440 and 1,600 Pg OC of which a significant portion is already thawed or about to thaw in coming years. In the light of this tipping point for climate change, any mechanism that can promote OC stabilization and hence mitigate OC mineralization and greenhouse gas emissions is of crucial interest. Here, we study interactions between metals (Fe, Al, Mn and Ca) and OC in the moist acidic tundra ecosystem of Eight Mile Lake, near Healy, AK, USA. We collected thirteen cores (124 soil samples) in late summer 2019 with shallow and deep active layers (45 to 109 cm deep) and varying water table depths. We find that between 6% and 59% of total OC in Eight Mile Lake tundra soils is mineral-associated (mean 20%), in organo-mineral associations (association between poorly crystalline oxides and OC) and in organo-metallic complexes (associations between Fe, Mn, Al, Ca polyvalent cations and organic acids). We find that total Fe and Mn concentrations can be used as good proxies to assess the reactive pool of these metals able to form associations with OC, i.e., poorly crystalline oxides or metals complexed with OC. We observe that in the active layer, mineral OC interactions are mostly as organo-metallic complexes with Fe cations, with an accumulation at the water table level acting as a soil redox interface. In waterlogged soils with a water table level above surface, no such accumulation of OC-Fe complexes is found due to the absence of a redox interface below soil surface. In the permafrost layer, we find that a combination of complexed metals and poorly crystalline Fe oxides act as reactive phases towards OC. Knowing that upon permafrost thaw tundra soils will become wetter or drier, the assessment of mineral-bound OC in drier or wetter tundra soils is a needed step to better constrain the changes in the proportion of non-protected OC more likely to contribute to C emissions from tundra soils.

Long-Term Fertilization Alters the Storage and Stability of Soil Organic Carbon in Chinese Paddy Soil

The storage of soil organic carbon (SOC) in cropland soils is an essential strategy that serves the dual purpose of enhancing soil fertility and mitigating climate change. However, how the stability of stored carbon is altered under long-term fertilization has not been well understood, especially in the double rice cropping system in Chinese paddy soils. In this study, we explored the SOC storage and consequent stability of SOC under long-term fertilization. The soil samples were fractionated chemically to isolate various fractions and constituent pools of SOC (i.e., very labile C/VLC, labile C/LC, less labile C/LLC, and non-labile C/NLC). The following treatments were tested: control (CK), recommended rate of inorganic fertilizer (NPK), double the amount of recommended rate of inorganic fertilizer (2NPK), and NPK combined with manure (NPKM). The results showed that, relative to the initial level, the application of NPKM significantly improved the SOC storage as compared to the control. The long-term NPKM increased the total SOC in the paddy soil and this increased SOC was mainly stored in LLC, as revealed by the highest increase (142%) over the control. Furthermore, the highest proportion of labile pool was associated with unfertilized CK, while the reverse was true for the recalcitrant pool, which was highest under NPKM. This supports the role of combining manure with NPK to improve the stability of SOC, further verified by the high recalcitrance index under NPKM (56.75% for 0–20 cm and 57.69% for 20–40 cm) as compared to the control.

Accumulation of SOC and Carbon Fractions in Different Age Red Fescue Permanent Swards

One of the practices often mentioned to achieve climate change mitigation is the long-term cultivation of perennial plants. The objective of the study was to estimate changes in the accumulation of soil organic carbon (SOC) and its fractions in 0–10, 10–20, 20–30 cm, and within 0–30 cm soil layer of red fescue (Festuca rubra L.) swards that differ in age (5, 10 and 15 years) as well as to compare them with the arable field. Our results show that SOC accumulation at 5-year-old cultivation of red fescue is high, later this SOC increase slowed down from 71% in the 0–30 cm soil layer when land use was converted from arable field to 5-year-old sward to 1% from 10 to 15 years. The level of water extractable organic carbon (WEOC) in the 0–30 cm soil layer of swards was significantly higher compared to the arable field. The positive effect of these swards in the accumulation and stabilization of organic carbon during humification in the soil was also determined. The largest amounts of mobile humic substances (MHS) and mobile humic acids (MHA) accumulated in the 0–10 cm layer of sward soil (3.30–4.93 and 1.53–2.48 g kg−1, respectively). In conclusion, the findings suggest that a conversion from arable to soil under permanent grass cover significantly improves carbon status.

Storage and Stability of Soil Organic Carbon in Two Temperate Forests in Northeastern China

Forests worldwide store large quantities of carbon (C), particularly in soils as soil organic C (SOC). In northeastern China, two dominant forest types, secondary mixed forest (MF) and larch plantation forest (LF), cover extensive areas. However, we lack an understanding of the patterns and the mechanisms of SOC storage and stabilization in MF and LF, especially in deep soil layers. This research aims to illustrate the vertical distribution and mineral protection of SOC over soil depth; we also used δ13C values of soil fractions to evaluate SOC stability. Samples from the surface litter (Oi), organic layer (Oa+e), and 0–40 cm mineral soils were collected from both MF and LF plots. We used two different methods to separate bulk soils into distinguished fractions: (1) macro- and micro-aggregates and the non-aggregated fraction, and (2) particulate organic matter (POM) and mineral-associated organic matter (MAOM). The C concentrations, C stocks, and δ13C of all soil fractions were determined. Our findings were as follows: (1) SOC was mainly stored in mineral soils and was 13.6% lower in LF (8609 ± 1180 g C m−2) than MF (9969 ± 2084 g C m−2). (2) In both MF and LF, the SOC stock was mainly stored in aggregates (averaged 92.7%); macroaggregates dominated in the surface layers (Oa+e layer and 0–10 cm) but microaggregates dominated in the deep layers (10–20 cm and 20–40 cm). In mineral soils, MAOM was the dominant fraction of the C stock (averaged 81.6%). (3) The proportion of C distributed in microaggregates and MAOM increased from Oa+e to the 20–40 cm layer. (4) The C/N ratios and δ13C values of MAOM were smaller and heavier compared to those of POM. Our study demonstrated that in both forests, aggregate formation and mineral association predominantly contributed to SOC storage, and large stocks of SOC were distributed in the deep soil. The increasing proportion of SOC in microaggregates and MAOM along the soil depth, most likely derived from microbial turnover and microbial necromass, influenced SOC stability in both forest types.

Soil water retention modulates effects of soil nutrients and stoichiometry on stability of organic carbon in a karst region, Southwestern China

Soil water retention modulates effects of soil nutrients and stoichiometry on stability of organic carbon in a karst region, Southwestern China

Changes in land use are associated with the accumulation of soil phytolith-occluded organic carbon

Organic carbon occluded in phytoliths is extremely stable and plays important roles in long-term preservation of soil and stable carbon sequestration in terrestrial ecosystems. To clarify the effects of land-use changes on the accumulation of soil phytolith-occluded organic carbon (PhytOC), the typical six land-use types of forest, open forest, shrub, grassland, corn field and abandoned farmland in Southwest China were used to study the associations of soil PhytOC accumulation with land-use change. The present results showed that land-use changes significantly affected soil PhytOC accumulation and its proportion to TSOC (total soil organic carbon). Compared to vegetation coverage, vegetation productivity increased soil PhytOC accumulation more effectively. The content and reserves of soil PhytOC in the open forest all ranked highest among the six land uses studied. Forest thinning not only substantially increased soil stable organic carbon sequestration but also obviously increased the TSOC pool. When land use changed from forest to grassland, there were obvious releases or run off of soil organic carbon, but the stability of the soil organic C pool improved obviously. After the corn field was abandoned, the PhytOC pool did not vary obviously. Soil pH was the important factor influencing the soil PhytOC pool, and acidic soils were made against PhytOC accumulation in soils. Soil PhytOC accumulation was significantly affected by soil C, N and P nutrition contents and stoichiometric ratios. Significant negative effects of bulk density on PhytOC accumulation in soils were observed. The present results are of great significance in land use regulation for increasing the soil carbon sink of terrestrial ecosystems.

Periodic flooding enhances the function of soil Fe/Al oxides in stabilizing particulate organic carbon in a water level drawdown zone

The stabilization of soil organic carbon is closely linked to edaphic properties and environmental variables. Despite this, the mechanism of soil organic carbon stabilization, as well as the primary factor controlling soil organic carbon in riparian ecosystems, remains unclear. To address this gap, our study investigated the impact of flooding regimes on soil organic carbon fractions and other chemical properties in the riparian zone, focusing on the dominant intrinsic factors controlling soil organic carbon content within the three soil types (Anthrosols, Luvisols, and Regosols). Our results demonstrated that flooding regimes significantly affect soil chemical properties in the riparian zone, with varying effects among the three soil types. Specifically, Regosols showed an increase in the particulate organic carbon with flooding intensity. Organic carbon fractions in Anthrosols and Luvisols are found to respond similarly to flooding regimes. The effects of soil chemical properties on organic carbon fractions varied across the three soil types. The variations in POCc, POCf, and MOC for Anthrosols were explained by path models by 43 %, 95 %, and 33 %, respectively. Similarly, for Luvisols, the path models accounted for 66 %, 90 %, and 29 % of the variations, whereas for Regosols, the variations were explained by 43 %, 93 %, and 21 %. Overall, for all tested soils combined, the path models explained 36 %, 89 %, and 12 %, respectively. Therefrom, under flooding regimes, soil oxides played a more significant role in stabilizing soil organic carbon for the three soil types than clay, silt, and cations. Specifically, in Anthrosols, Luvisols, and Regosols, the path coefficient for soil oxides was 0.90, 0.95, and 0.97, respectively, for POCf; while it was 0.30, 0.21, and 0.22 for POCc. In conclusion, we revealed that flooding regimes enhance the role of soil oxides in stabilizing organic carbon. Therefore, we recommend that soil oxides shall be included in organic carbon saturation models for riparian zones subjected to flooding regimes.

Interlayer organic matter within hydroxy-interlayered clay minerals enhances soil organic carbon stability under long-term organic fertilization

The intercalation of organic matter in clay minerals is believed to be essential for soil carbon sequestration because nanoscale interlayer spaces can effectively protect organic matter from degradation. However, the lack of a direct method for the detection of interlayer organic matter has limited the identification of organic matter and the evaluation of the properties of organic matter–clay complexes. In this study, the association between the organic matter and hydroxy-interlayered clay minerals (HIMs) in red earth collected from Qiyang County (Hunan Province) in southeast China was investigated by combining X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM)-electron energy loss spectroscopy (EELS) methods. The presence of organic matter in the interlayer space of HIMs was confirmed by HRTEM-EELS for the first time, providing direct visible evidence of interlayer organic matter in the HIMs of red earth. Hydroxy-Al was identified as the interlayer hydroxy cation in the HIMs. The organic molecules were associated with Al-OH in the interlayered hydroxy-Al polymer and exhibited resistance to oxidation by H2O2. Organic fertilization increased the amount of organic matter on the external surface and in the interlayer space of the HIMs, implying the feasibility of enhanced carbon sequestration in red earth.

Multi-scale modeling of natural organic matter–heavy metal cations interactions: Aggregation and stabilization mechanisms

Interaction between natural organic matters (NOM) and heavy metal cations in aqueous environment are of great significance for maintaining stability of organic carbon and restraining transport of heavy metal contaminants in (bio)geochemical processes. We systematically explore the aggregation process and complexation between NOM and heavy metal cations (Ag+, Cd2+, Pb2+, Zn2+, Eu3+) under different pH condition by molecular dynamics (MD) simulations, umbrella sampling method, and quantum chemistry calculations. The character of molecular structures NOM-heavy metal complexes and association are quantified. In acidic pH condition, aggregation proceeds via H-bonding and π-π interactions between NOM fragments. In neutral condition, Ag+, Cd2+, Pb2+, and Eu3+ can form inner-sphere complexes with the surface carboxylic groups and therefore reduce intermolecular charge repulsion, eventually leading to NOM aggregation, and it shows that even without direct binding, the outer-sphere adsorbed Zn2+ can also result in the formation of NOM assemble through H-bonding. Consequently, these heavy metals are capable of promoting NOM aggregation regardless of the complexing ways. Complexing free energy calculations characterized the dynamic processes of cations binding to the carboxylic groups of NOM fragment and the related energy landscape. This study provides quantitative insights for understanding the environmental processes of heavy metals and cycle of C in aquatic ecosystem, and contributes to developing environment-friendly strategies for controlling heavy metal contaminants.

Root architectures differentiate the composition of organic carbon in bauxite residue during natural vegetation

Understanding plant root architectures induced changes in organic carbon accumulation and conversion is critical to predicting carbon cycling and screening appropriate plant species for ecological restoration on bauxite residue disposal areas. According to the ecological investigation of a weathered bauxite residue disposal area, three plants with different root architectures including Artemisia lavandulaefolia (A. lavandulaefolia), moss, and Zanthoxylum simulans (Z. simulans) were selected to investigate the rhizosphere effects on the composition and structure of organic carbon in bauxite residue. The physic-chemical properties, the contents and structure of different organic carbon fractions, and microbial communities of bauxite residue from rhizosphere and non-rhizosphere were analyzed. Plant growth decreased the saline-alkalinity, increased the contents of total organic carbon, particulate organic carbon and dissolved organic carbon, whilst enhancing the enzymatic activities of bauxite residue. Meanwhile, the rhizosphere effects had significant effects on the accumulation and stabilization of organic carbon in bauxite residue. A. lavandulaefolia had the strongest rhizosphere effects on the composition and structure of total organic carbon and dissolved organic carbon, whilst moss was more effective on the accumulation of particulate organic carbon in bauxite residue. Plant growth and root architecture changed the abundance of specific functional microorganisms and the complexity of microbial co-occurrence networks, thus elevating organic carbon levels in bauxite residue. During natural vegetation encroachment, rhizosphere exciting effects of the salt-tolerated plants could change the composition and structure of organic carbon fractions due to the comprehensive effectiveness of the improvement of physic-chemical properties and microbial communities. The findings improve our understanding of the responses of sequestration and stabilization of organic carbon pools to ecological restoration on bauxite residue disposal areas.

Changes in soil organic carbon stability in association with microbial carbon use efficiency following 40 years of afforestation

The effectiveness of afforestation in sequestrating carbon (C) depends on soil organic C (SOC) stability, yet the underlying mechanisms on the SOC stabilization in response to afforestation remain unresolved. Here, we conducted a random block sampling to explore the changes in SOC relative stability by comparing the proportions of stabilized C in total SOC before and after afforestation. We also calculated microbial C use efficiency (CUE), and analyzed its relation to SOC relative stability to test whether SOC relative stability would be enhanced by increasing microbial CUE following afforestation. The 40 years of afforestation significantly increased the concentration of all C fractions but it decreased SOC relative stability by lowering the proportions of stabilized C in SOC. Microbial CUE showed a significant and positive relationship with the proportions of stabilized C in total SOC. Lower microbial CUE in the afforested soils was primarily associated with lower soil N availability, and higher soil moisture and root density. Additionally, the concentration of stabilized C in the topsoil (0–10 cm) rapidly increased following afforestation and then became level off after total SOC had reached up to 20 g kg−1, while the macroaggregate-occluded C fraction kept fast-growing. Overall, our result revealed the decline in SOC relative stability following afforestation, which was regulated by microbial CUE and the inherent soil C capacity, thereby providing new insights into the assessment of “carbon neutrality” under global afforestation.