Soil Organic Carbon Fractions Research Articles

Soil Microbial Community in Relation to Soil Organic Carbon and Labile Soil Organic Carbon Fractions under Detritus Treatments in a Subtropical Karst Region during the Rainy and Dry Seasons

Climate and detritus influence soil organic carbon (SOC) and labile SOC fractions by affecting soil microbial communities. However, it is not clear how, or to what extent, different detritus treatments affect soil microbial communities and SOC content in karst landscapes during different seasons. Plots in a karst landscape were treated with different detritus input regimes (control, no litter, no roots, no litter or roots, and double litter), and samples were collected during the dry and rainy seasons. We used Illumina sequencing of 16S rRNA to examine shifts in the diversity and composition of the associated soil microbial communities. Additionally, labile SOC fractions, including dissolved organic carbon (DOC) and microbial biomass carbon (MBC), along with soil physicochemical properties and C-degrading enzyme activities, were analyzed. The results revealed that the responses of soil properties and labile SOC fractions to detritus treatments were more pronounced during the rainy season than during the dry season, which mainly reflected that the levels of available potassium (AK), DOC, and MBC were significantly increased during the rainy season. Moreover, SOC and total nitrogen (TN) demonstrated significant changes with the double litter (DL) treatment during the rainy season. The responses of soil microbial communities to detritus treatments varied with the season, as reflected primarily in changes in the relative abundance of Ascomycota, unclassified_K_fungi, Proteobacteria, and Actinobacteriota. Climate, detritus treatments, and their interactions had significant effects on the species richness of soil bacterial communities, but did not influence fungal community diversity. Furthermore, structural equation modeling (SEM) revealed that the soil bacterial composition had the largest total effects on SOC, DOC, and MBC. In addition to directly influencing SOC, DOC, and MBC, soil properties (TN, AK, and pH) indirectly affected SOC, DOC, and MBC by altering C-degrading enzyme activity and the microbial community. We conclude that detritus treatments affect the soil microbial community and labile carbon fractions during both the rainy and dry seasons. Relationships among SOC, labile SOC fractions, enzyme activities, microbial communities, and function differed between seasons and among treatment types. This research advances our knowledge of how variation in detritus treatments affects biogeochemical cycling in karst soils during the rainy and dry seasons.

Effect of Short-Term Organic Matter Returns on Soil Organic Carbon Fractions, Phosphorus Fractions and Microbial Community in Cold Region of China

To investigate the effect of different organic matter returns on soil organic carbon (SOC) fractions, phosphorus (P) fractions and microbial communities, a pot experiment was conducted in a cold region of China for three years. There were six treatments in this study, including no rice straw return (S0), rice straw return (SR), decomposed rice straw return (DS), rice-straw-burned return (BS), rice root return (RR) and decomposed cattle manure return (DM). The results indicated that the organic matter returns had no significant effect on the rice yield after three years. The SR, DS and DM treatments significantly increased the content of the soil’s total organic carbon (TOC), light fraction organic carbon (LFOC), particulate organic carbon (POC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC). The BS treatment decreased the soil MBC content. The SR, DS, BS and DM treatments significantly increased the content of the soil’s total P, NaHCO3-P, NaOH-P and residual-P. The proportion of nonlabile P (HCl-P and residual-P) was reduced by the organic matter returns. The SOC fractions were positively correlated to the soil P fractions (except HCl-P). The organic matter returns did not affect the microbial diversity but did change the microbial community composition. The dominant phyla included Proteobacteria, Firmicutes, Chloroflexi, and Bacteroidetes. Compared with the S0 treatment, the organic matter returns increased the relative abundance of Actinobacteria, Anaerolineae and Alphaproteobacteria and decreased the relative abundance of Bacteroidetes, Clostridia and Bacteroidia. The contents of MBC, DOC and NaOH-P were the main factors affecting the microbial community composition, and the soil’s P fractions had a larger influence on the microbial community than the SOC fractions. These results indicated that the incorporation of rice straw, decomposed rice straw and decomposed cattle manure might be an effective practice for maintaining soil fertility in the cold region of China.

Effects of land reclamation on soil organic carbon and its components in reclaimed coal mining subsidence areas

The accumulation of soil organic carbon (SOC) is crucial for the development and ecosystem function restoration of reclaimed mine soils (RMSs). To optimize reclamation management practices, this study aims to explore the factors and underlying mechanisms influencing the recovery of SOC and its components in RMSs from a systemic perspective using complex network theory (CNT). This study focused on coal mining subsidence areas in the eastern mining regions of China, comparing reclaimed cultivated land with surrounding non-subsided cultivated land. Soil samples were collected at depths of 0–20 cm, 20–40 cm, and 40–60 cm, and 25 soil indicators were measured. CNT was applied to explore the intricate relationships between soil indicators and to identify the key factors and underlying mechanisms influencing SOC and its components in RMSs. The results revealed that the compaction-induced soil structural damage during the reclamation process led to a chain reaction, resulting in increased soil bulk density (11.92 % to 15.03 %), finer soil particles (5.00 % to 9.88 % more clay and silt), and enhanced SOC mineralization (SOC decreased by 10.70 % to 15.62 % with a lower C/N ratio by 2.30 % to 28.55 %). Microbial activity also decreased, with a 6.25 % to 13.16 % drop in MBC and a 0.91 % to 27.68 % decrease in enzyme activity. The utilization of active SOC fractions by more adaptable bacterial communities was crucial within this chain reaction process. The intermediate role of soil structure in the RMS ecosystem, particularly in carbon cycling, becomes more prominent. RMSs exhibited heightened sensitivity to soil structure changes, with the response of microorganisms and enzymes to soil structure changes being pivotal. In the carbon cycling process of RMSs, microbial-driven enzyme activity in response to soil structure was more critical during SOC transformation, while the role of physical-chemical protection and microbial inhibition mediated by iron‑aluminum oxides became more pronounced in stabilizing SOC.

The Different Roles of Mineralogy in Soil Organic Carbon Accumulation in Northern and Southern China

The sequestration of soil organic carbon (SOC) through mineral protection is an important approach to mitigating climate change. However, the effect of mineral composition on SOC stability is unclear at regional scales. In this study, we investigated the relationship between mineralogy and SOC in Alfisol and Mollisol from southern and northern regions of China. We analyzed soil at two layers for its SOC fractions, mineralogical characteristics and functional groups. It was found that the majority of SOC was stored as mineral-associated organic C (MAOC), which had higher δ13C values and narrower C/N ratios compared to particulate organic C. In Mollisol, the proportion of MAOC and the abundance of aromatic C were higher than that in Alfisol, while polysaccharide C was lower. Compared to Alfisol, Mollisol was dominated by illite, and had significantly (p < 0.05) lower iron (Fe) and aluminum (Al) sesquioxides contents. The SOC content was positively correlated with illite in Mollisol, and with Fe and Al sesquioxides in Alfisol. The random forest model identified sesquioxides as the most important determinant of SOC accumulation (36%), followed by SOC fractions (18%) and functional groups (18%). In summary, our study suggests that SOC protection through mineralogy depends more on the composition of the host minerals, and not just on the clay content, and aromatic C is also important in the stabilization of SOC.

Reproducibility of Rock-Eval® thermal analysis for soil organic matter characterization

Rock-Eval® (RE) is a thermal analysis technique increasingly used to characterise soil organic matter. To interpret the results, particularly when investigating differences between samples, it is necessary to know the expected ranges of analytical error associated with the RE measurements. Moreover, the RE analyzer is now at its seventh version (RE7) while most literature results were produced using the previous version (RE6). Thus, a characterization of the reproducibility of RE measurements is necessary. We measured the reproducibility of RE measurements using fifteen samples from French croplands and forests that were analysed on five different RE instruments, located in different laboratories and belonging to both generations RE6 and RE7. From each RE analysis, we extracted RE parameters commonly used for soil organic matter characterization and we performed the prediction of the active and stable soil organic carbon fractions using a machine learning model (PartySOC) that uses RE parameters. We obtained a measure of the expected relative errors in RE parameters and PartySOC predictions per instrument, across instruments of the same generation and across generations. We found that the parameters total organic carbon (TOC), mineral carbon (MinC) and R-index are well reproducible, even across the RE6 and RE7 generations. Instead, the hydrogen index (HI) and oxygen index (OI) are more sensitive to signal variations, even within the same generation, especially when TOC is low. The PartySOC predictions were well reproducible across RE6 instruments but not across RE generations. In the future, the results of this study will help discriminate relevant differences between soil samples characterised using RE thermal analysis.

Natural and regenerated saltmarshes exhibit different bulk soil and aggregate-associated organic and inorganic carbon contents but similar total carbon contents

Saltmarshes are considered to be one of the planet's most efficient carbon sinks. The continued loss of saltmarshes and induced ecological consequences promoted their restoration worldwide. Previous efforts aimed to evaluate the success of restoration in terms of organic carbon accumulation, but inorganic carbon and carbon contents within soil aggregates, which are essential for making a comprehensive assessment of the carbon sink function, were rarely studied. To fill this gap, a range of metrics including bulk and aggregate-associated soil organic and inorganic carbon contents together with the soil's physical, chemical and microbiological parameters were measured to compare natural and a 15-year restoration effort in saltmarsh habitats within the Yellow River Delta region in eastern China. The results showed that regenerated saltmarsh exhibited significantly higher soil organic carbon (SOC) contents but significantly lower soil inorganic carbon contents, resulting in no notable change in total carbon contents between the regenerated and natural saltmarshes. SOC contents within the silt and clay fractions and their contribution to the bulk SOC contents were significantly lower in the regenerated saltmarsh than those in the natural ones (P < 0.05). In regenerated saltmarsh, significantly lower soil microbial biomass and distinct microbial community composition with reduced Gram-negative to Gram-positive bacteria ratios were observed compared to natural saltmarsh. These findings indicate the stability of SOC fraction and soil microbe-mediated carbon biogeochemical processes differed between naturally occurring and artificially regenerated saltmarshes. As interest in blue carbon programs gains global attention, further research on the generation and transformation processes of different carbon fractions during restoration are needed, which can be conducive to elucidating more details in coastal carbon cycling processes.

Adaptive evaluation for agricultural sustainability of different fertilizer management options for a green manure-maize rotation system: Impacts on crop yield, soil biochemical properties and organic carbon fractions

Green manure planting can reduce the intensity of soil use, while improving farmland productivity in double-cropping systems. However, only few studies have focused on the impacts of green manure application under different fertilization management options on succeeding crop yield and soil organic carbon (SOC) process. A three-year field experiment was conducted with a winter smooth vetch-summer maize cropping system to evaluate the effects of green manure with different chemical fertilizers on soil physiochemical properties, SOC fraction, enzyme activities and maize yield. Total eight treatments were compared including different combinations of green manure and chemical fertilizers (i.e., nitrogen and phosphorus fertilizers) in the smooth vetch phase and maize phase. The results showed that compared to the control, green manure incorporation increased the soil moisture, total nitrogen, total phosphorus, basal respiration, SOC and its labile fractions, and enzyme activities, especially for the treatments of green manure with fertilization. However, the soil pH and bulk density decreased due to green manure application. Maize yield increased 34 %–53 % after green manure application, and was found to be significantly and positively correlated with soil carbon process (P < 0.05). Moreover, SOC and its labile fractions, and total nitrogen were observed as the main drivers of the maize yield. Variation partition analysis demonstrated that soil biochemical properties and their interaction with green manure by fertilization caused variations in SOC fractions. Further, structural equation models indicated that both balanced fertilization practices had positive effects on maize yield and soil carbon process via changes in SOC fractions and C cycling-related enzyme activities, respectively. In addition, the amount balance of chemical fertilizer positively impacted the soil carbon process by regulating SOC fractions through enzyme activities. These findings provide important guidance for applying optimal fertilization management in the green manure phase to improve succeeding crop yield and soil quality as well as to mitigate the adverse impacts of chemical fertilizers. The study will be equally illuminating for other green manure-crop rotation systems.

Precipitation change affects forest soil carbon inputs and pools: A global meta-analysis

The impacts of precipitation change on forest carbon (C) storage will have global consequences, as forests play a major role in sequestering anthropogenic CO2. Although forest soils are one of the largest terrestrial C pools, there is great uncertainty around the response of forest soil organic carbon (SOC) to precipitation change, which limits our ability to predict future forest C storage. To address this, we conducted a meta-analysis to determine the effect of drought and irrigation experiments on SOC pools, plant C inputs and the soil environment based on 161 studies across 139 forest sites worldwide. Overall, forest SOC content was not affected by precipitation change, but both drought and irrigation altered plant C inputs and soil properties associated with SOC formation and storage. Drought may enhance SOC stability by altering soil aggregate fractions, but the effect of irrigation on SOC fractions remains unexplored. The apparent insensitivity of SOC to precipitation change can be explained by the short duration of most experiments and by biome-specific responses of C inputs and pools to drought or irrigation. Importantly, we demonstrate that SOC content is more likely to decline under irrigation at drier temperate sites, but that dry forests are currently underrepresented across experimental studies. Thus, our meta-analysis advances research into the impacts of precipitation change in forests by revealing important differences among forest biomes, which are likely linked to plant adaptation to extant conditions. We further demonstrate important knowledge gaps around how precipitation change will affect SOC stability, as too few studies currently consider distinct soil C pools. To accurately predict future SOC storage in forests, there is an urgent need for coordinated studies of different soil C pools and fractions across existing sites, as well as new experiments in underrepresented forest types.

Soil conditioners promote the formation of Fe-bound organic carbon and its stability

The close association of soil organic carbon (SOC) with Fe oxides is an important stabilization mechanism for soil organic matter (SOM) against biodegradation. Soil conditioners are of great importance in improving soil quality and soil health. Yet it remains unclear how different conditioners would affect the fractionation of SOC, particularly the Fe-bound organic carbon (Fe-OC). Field-based experiments were conducted in farmland to explore the fractionation of organic carbon (OC) and Fe oxides under the effects of three different soil conditioners (mineral, organic, and microbial conditioners). The results showed that all soil conditioners increased the total OC and Fe-OC contents, with the contribution of Fe-OC to total OC increasing from 1.57% to 2.99%. The low OC/Fe molar ratio indicated that surface adsorption played a crucial role in soil Fe-OC accumulation. Nuclear magnetic resonance (NMR) results suggested that soil conditioner altered the composition of SOM, accelerating O-alkyl C degradation and increasing recalcitrant alkyl C and aromatic C sequestration. Scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) analysis indicated that all conditioners promoted the association of OC and Fe oxides. Furthermore, comprehensive analysis of 13C isotope and synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy showed that the mineral conditioner enhanced the association of microbial-derived OC and Fe oxides, whereas the organic conditioner increased the association of plant-derived OC with Fe oxides. These findings provide important insights into the potential mechanisms through which soil conditioners regulate the stability of OC and guide agricultural management.

Implications of surfactant application on soil hydrology, macronutrients, and organic carbon fractions: An integrative field study

Implications of surfactant application on soil hydrology, macronutrients, and organic carbon fractions: An integrative field study

Straw and straw biochar differently affect fractions of soil organic carbon and microorganisms in farmland soil under different water regimes

Incorporating amendments can change soil organic carbon (SOC) components and biological properties, but the comprehensive utilization of straw and straw biochar in paddy soil remains poorly understood. We studied the changes in the pH, SOC content, labile organic C fractions, and soil microbial communities under alternating wetting and drying (AWD) and continuous flooding (CF). The results showed that the addition of straw and straw biochar increased SOC content, and the relative changes in microbial biomass carbon (MBC), dissolved organic carbon (DOC), and easily oxidizable carbon (EOC) were more sensitive than those in SOC, but soil pH was higher under AWD management than under CF management. Proteobacteria, Bacteroidetes, and Acidobacteria were the three most abundant bacterial phyla, and the three major abundant fungal phyla were Phragmoplastophyta, norank_Eukaryta, and Apicomplexa. A redundancy discriminant analysis (RDA) found that both straw and straw biochar were conducive to bacterial growth, and their combination was more beneficial for fungal growth and development. Aggregated boosted tree (ABT) and Structural equation modeling (SEM) showed that the soil MBC and EOC were the most important factors determining bacterial abundance and diversity, while pH, EOC, and MBC were the main factors influencing the abundance and diversity of fungi. This study explored the changes in SOC fractions and bacterial and fungal communities and provided a theoretical basis for an in-depth understanding of the influence of labile organic C pools on bacterial and fungal communities.

Distribution characteristics of soil organic carbon fractions in paddy profiles with 40 years of fertilization under two groundwater levels

Distribution characteristics of soil organic carbon fractions in paddy profiles with 40 years of fertilization under two groundwater levels

Comparison of organic carbon properties in extracted soil solutions obtained underneath Cryptomeria japonica and Quercus acutissima and its implication on stream dissolved organic carbon

Dissolved organic carbon (DOC) in soils is released into streams, working as a main component of the carbon cycle. DOC in terrestrial and aquatic ecosystems participates in many biogeochemical reactions and processes such as heterotrophic respiration, sorption of metals, and transport of pollutants. In order to understand the connectivity of organic carbon among soil, soil water, and forest streams, we investigated the concentrations and dual carbon isotope ratios (δ13C and Δ14C) of soil organic carbon (SOC) and water-extractable organic carbon (WEOC) obtained from soils beneath two tree species stands, and compared these with stream Δ14C-DOC of the forest watershed. Soil samples were collected at different depths (0–10, 10–30, and 30–50 cm) beneath Japanese cedar (Cryptomeria japonica) and sawtooth oak (Quercus acutissima). Although the SOC concentration was not significantly different between the two tree species, the WEOC concentration ([WEOC]) of soil at 0–10 cm depth under Cryptomeria japonica was higher than that of Quercus acutissima, in general. The δ13C-SOC and δ13C-WEOC increased, while the Δ14C-SOC and Δ14C-WEOC decreased with increasing soil depth. The Δ14C-WEOC was higher than the Δ14C-SOC, indicating that WEOC could be primarily derived from the young, hydrophilic, and exchangeable fraction of SOC, rather than from SOC strongly bonded to mineral soils. However, the stream Δ14C-DOC was lower than Δ14C-WEOC in general, except during summer storms. The 14C-depleted DOC released from deep soils or groundwater might lower Δ14C-DOC of a stream, suggesting that relatively old DOC could be released into streams during baseflow. This is contrary to the results of previous studies that have reported positive stream Δ14C-DOC from temperate forests. The discrepancy warrants future research on forest stream Δ14C-DOC across entire seasons, particularly under Asian monsoon climates.

Shallow-incorporated straw returning further improves rainfed maize productivity, profitability and soil carbon turnover on the basis of plastic film mulching

Ridge and furrow plastic film mulching (RFM) has improved net primary productivity (NPP) to a high level in semi-arid rainfed agricultural regions. Yet, it is unclear whether there is still a room for further improvement on NPP while stabilizing soil organic carbon through returning previous maize stover pieces. To address this issue, maize stubbles were smashed into pieces and in situ ploughed into 30 cm topsoil at a semi-arid site in northwest China from 2016 to 2017. This was done using the Pioneer 335 maize variety with three treatments (CK, ridge and furrow without mulching; RFM, ridge and furrow with plastic mulching; RFML, 3.49 t ha−1 of maize stover returning in RFM; RFMH, 5.24 t ha−1 of returning in RFM). The results indicated that maize stover returning exhibited similar trend as RFM at early cool seedling stage, raising the topsoil temperature. However, in warm and dry silking stage (2017), both stover returning rates reduced soil temperatures by 1.33 and 0.8 ◦C relative to RFM and CK (P < 0.05), respectively. The soil water storage increased significantly in RFMH by 17.3% and RFML by 28.5% relative to that of RFM, in a warm and dry growing season (2017). Critically, stover returning increased (P < 0.05) soil organic carbon and light fraction organic carbon turnover across the two growing seasons. By improving hydrothermal conditions, stover returning in RFM increased biomass accumulation and grain yield (P < 0.05), leading to higher (P < 0.05) net economic benefit, with greater above-ground biomass NPP (74.5–93.2%) and below-ground biomass NPP (88.5–89.4%), compared with CK. In conclusion, maize stover returning of stover pieces into topsoil might be a promising solution to enhance carbon turnover for higher net primary productivity on the basis of plastic film mulching in semiarid rainfed region.

Mulch-derived microplastic aging promotes phthalate esters and alters organic carbon fraction content in grassland and farmland soils

Agricultural plastic mulch is a major microplastics (MPs) source in terrestrial ecosystems. However, knowledge about the aging characteristics of mulch-derived MPs entering natural and agricultural soils and their effects on phthalate esters (PAEs) and organic carbon fractions is still limited. Black (contains black masterbatches) and white polyethylene (PE) and biodegradable (Bio, Poly propylene carbonate and Polybutylene adipate terephthalate synthetic material (PPC+PBAT)) mulch-derived MPs, at 0.3% (w/w) dose, were added to grassland and farmland soils for eight-week incubation. Microplastic (MP) aging degree was explored by quantifying the carbonyl index (CI). The soil PAEs and organic carbon fractions were also analyzed. After incubation, black and white PE-MP aged greater in farmland than in grassland. PAEs accumulated highest in PE-MP treatment (5.27–6.41 mg kg−1) followed by Bio-MP (1.88–2.38 mg kg−1). Soil organic carbon (SOC), particulate organic carbon (POC), and microbial biomass carbon (MBC) were reduced by 5.3%–8.2%, 31.8%–41.6%, and 39.7%–63.0%, dissolved organic carbon (DOC) was increased by 10.1%–27.6% in grassland containing MP compared to control. MPs’ aging degree promoted PAEs content or altered nutrients, then regulated soil microbial biomass and extracellular enzyme activity directly or indirectly, ultimately affecting SOC. Environmental implicationMicroplastics are persistent environmental pollutants that gradually undergo surface aging in response to extracellular enzymes secreted by microorganisms. As microplastics age, their surface roughness and functional groups change; thus, organochemical contaminants gradually leach out. Therefore, this study analyzed the aging of mulch film-derived microplastics under the action of diverse microorganisms in farmland and grassland soils and the effect on plasticizer and organic carbon fractions. The results proved that polyethylene microplastic aging degree was highest in farmland soil. Besides, biodegradable microplastic caused lower contamination of phthalate esters than polyethylene, but they affected soil carbon balance in grassland and farmland soils. Statement of environmental implicationThis study highlights that MPs affect organic carbon fractions by influencing the PAEs, available nutrients, and extracellular enzyme activity.

Trade-offs between ligninase and cellulase and their effects on soil organic carbon in abandoned Moso bamboo forests in southeast China

A vast expanse of Moso bamboo (Phyllostachys pubescens J.Houz.) forests in subtropical areas was once intensively managed but has been abandoned in recent years. However, the response of soil organic carbon (SOC) to abandonment management remains unclear, partly because how carbon-degrading enzymes vary with abandonment management and the role of this change in the soil carbon cycle are still poorly understood, which restricts the scientific evaluation of carbon sink benefits of these abandoned Moso bamboo forests. The results of the survey, based on 40 Moso bamboo forests, showed that compared with intensive management, abandonment management for 7–10 and 11–14 years exhibited a significant decrease in ligninase activities (a reduction of 12.14 % and 44.41 %, respectively) and a significant increase in SOC content (an increase of 49.39 % and 52.64 %, respectively). However, abandonment management did not affect cellulase activities or easily oxidizable organic carbon content (p > 0.05), but significantl increased non-easily oxidizable organic carbon (p < 0.05). Furthermore, the total nitrogen (TN) content and pH value increased with prolonged abandonment, and these trade-offs between ligninase and cellulase were primarily driven by pH and TN. The ligninase-to-cellulase activities ratio is the most key factor affecting NEOC and SOC changes in abandoned Moso bamboo forests. Together, these findings demonstrate the response of carbon-degrading enzyme trade-offs to abandonment management and highlight the role of these trade-offs in controlling SOC accumulation. In addition, the different responses of different SOC fractions to abandonment management deserve attention in future studies.

Differences of SOC storage and stability between soil layers influenced by long-term fertilization in a typical paddy soil of Southern China

BackgroundThe contribution of long-term fertilization to soil organic carbon (SOC) storage has been of great concern. To assess the effects of long-term fertilization on SOC storage and stability in top and sub-soil layers, soil samples were collected from a 29-year field experimental station in a typical paddy soil in southern China. The SOC storage of whole soil and SOC fractions was quantified at three soil depths (0–20, 20–40, 40–60 cm) under four treatments: no fertilization (Control), a combination of nitrogen, phosphorus and potassium (NPK), double the rates of NPK (2NPK), NPK plus manure (NPKM).ResultsThe increase of Cinput-total was significantly higher than that of SOC storage among different treatments (p < 0.05), indicating that soil fixation of exogenous carbon is limited. Besides, the SOC accumulation and sequestration rates patterned as NPKM > 2NPK > NPK, and these rates were higher at 0–20 cm depth as compared to other depth intervals. Furthermore, for the whole profile, the SOC storage of active pool was higher in the Control (39.6 t C ha−1) than in other treatment (36.2 t C ha−1, p < 0.05). Whereas, fertilization increased the SOC storage of passive pool, ranked as NPKM > 2NPK≈NPK > Control (p < 0.05), indicating that fertilization, especially organic combined with inorganic fertilization, improved SOC stability. From the perspective of soil layers, the difference of SOC storage among treatments for passive pool was mainly resulted from the difference at surface soil, and for active pool were the deeper layers. Additionally, manure application increased the difference among soil layers.ConclusionThis study concluded that non-fertilized treatment could improve the SOC storage of active pool especially in deep soil layers, while fertilization especially manure application could improve the SOC storage and stability in surface soil and increased the difference among soil layers.Graphical

Recalcitrant organic carbon plays a key role in soil carbon sequestration along a long-term vegetation succession on the Loess Plateau

Vegetation restoration effectively promotes soil quality and enhances soil organic carbon (SOC) sequestration. However, the dynamics and driving factors of SOC fractions during long-term vegetation succession remain unclear. In this study, a complete ∼ 160 years successional chronosequence from farmland to climax forest was used to study the dynamics and driving factors of SOC fractions in topsoil (0–20 cm) and subsoil (20–40 cm). The results showed that vegetation succession age significantly affected the SOC and its fractions (p < 0.05). The content of SOC fractions increased with succession age, especially recalcitrant organic carbon (ROC), which accounted for 62%–85% of the total SOC. Long-term vegetation succession enhanced the stability of SOC pools, reduced the proportion of active C, and facilitated the fixation of C. ROC was the best indicator of SOC accumulation within the entire profile. When vegetation succession reached the pioneer forest stage (∼110 years), the SOC content and fractions increased significantly (p < 0.05) owing to continuous plant biomass inputs. Moreover, soil C sequestration was controlled by total nitrogen content in the topsoil and by belowground biomass in the subsoil. This study indicates that long-term vegetation succession can effectively improve SOC accumulation and SOC pool quality, emphasizing the need to focus on SOC pool stabilization mechanisms under future climate change.

The Different Factors Driving SOC Stability under Different N Addition Durations in a Phyllostachys edulis Forest

As one of the most widespread driving forces in the world, atmospheric nitrogen (N) deposition can significantly alter the carbon cycling of ecosystems. In order to understand how N deposition regulates soil organic carbon (SOC) dynamics and its underlying mechanisms, a 7-year N addition experiment was set in a Phyllostachys edulis forest with three N addition levels (+0, +20, and +80 kg N hm−2 year−1) to evaluate the effects of N addition on the concentration and stability of SOC fractions in the third, fifth, and seventh years. The results are as follows: (1) short-term (third year) N addition markedly increased SOC stability by decreasing the concentration of particulate organic carbon (POC) and increasing the mineral-associated organic carbon (MAOC); longer duration of N addition (5 and 7 years) had an insignificant effect on SOC stability and fractions, suggesting that the effects of N deposition on the SOC stability varied under different duration regimes; (2) N addition did not significantly affect microbial community composition while increasing the ratio of fungi to bacteria (F:B) in the seventh year, and microbial biomass carbon (MBC) and carbon use efficiency (CUE) were significantly increased in the short-term (third year) high N addition regime and enzyme activity was significantly increased in the seventh years’ high N addition regime; (3) variation partitioning analysis and multiple regression analysis showed that SOC fractions are mainly regulated by CUE and MBC under short-term N addition, while enzyme activity was mainly regulated under the longer duration of N addition. Our results show that SOC stability was more sensitive in the short term, and the role of microbial characteristics varied under different N addition durations in the P. edulis forests. Overall, our findings provide a new perspective for the responses of the SOC pool to N deposition and contribute to predicting SOC dynamics in terrestrial ecosystems under future climate change.

Soil Organic Carbon Sequestration after 20-Year Afforestation of Mangrove Plantations on Qi’ao Island, Southern China

Mangrove afforestation is considered an important measure in the “natural-based solution” for mitigating climate warming through sequestering massive carbon (C) into vegetation biomass, yet how the planted mangrove species facilitate soil C sequestration remains unclear. Here, we investigated the stock, source, and fraction of soil organic carbon (SOC) over 1 m depth after 20-year afforestation of five mangrove pure plantations (Acrostichum aureum, Acanthus ilicifolius, Aegiceras corniculatum, Kandelia obovate, and Excoecaria agallocha) on Qi’ao Island, South China. The results showed that SOC stocks did not significantly differ among the five plantations, with an average value of 16.7 kg C m−2. Based on the two-end-member mixing model with plant–soil C stable isotope signatures, the autochthonous (mangrove-derived) C source accounted for 20.2–34.1% of SOC but varied significantly among the plantations. The SOC stock in particulate fraction (1.2–2.0 g C kg−1) and mineral-associated fraction (14.3–16.0 g C kg−1) also significantly differed among the plantations. The similar SOC stock but different source contributions and C fractions among the plantations observed here may have important implications for mangrove afforestation to optimize stand structure and maximize C sequestration.