Organic Carbon Stability Research Articles

First evidence of fractionation on stable organic carbon isotopes in ancient Egyptian linen textile fibers during degradation

The study of the fractionation of the stable carbon isotopes 13C and 12C through the δ13C value finds applications in various fields of science, from quality control in agribusiness to the study of the geographical origin of materials. At the plant level, this information is useful to prove which photosynthetic cycle a particular plant belongs to: C3, C4 or Crassulacean Acid Metabolism. In this work, the Isotopic Ratio Mass Spectrometry technique was used to shed light and investigate about the degradation due to aging of an ancient linen textile of Egyptian origins, in order to see whether degradation phenomena change the content of stable carbon isotopes, over time. For this purpose, the δ13C values of the ancient sample were compared with contemporary Egyptian and French flax fibers samples. For the first time, it was observed that, contrary to what might have been expected, material degradation due to natural aging contributes to the phenomenon of the isotopic fractionation, due to the increasing in measured δ13C values from contemporary to ancient samples.

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

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

Combination of mineral protection and molecular characteristics rather than alone to govern soil organic carbon stability in Qinghai-Tibetan plateau wetlands

Wetlands in the Yarlung Tsangpo River Basin (YTR) on the Qinghai-Tibet Plateau provide immense soil organic carbon (SOC) storage, which is highly susceptible to climate warming and requires urgent deciphering SOC stabilization mechanisms of long-term protection of SOC against decomposition. Conflicting views exist regarding whether persistent SOC is controlled by molecular features or by mineral protection. As such, this study quantified SOC stability using two thermal indices (TG-T50, and DSC), described molecular features of SOC using pyrolysis-gas chromatography-mass spectrometry, and measured SOC protection by minerals using a chemical extraction method. Results indicated SOC of topsoils had higher thermal stability, with TG-T50 and DSC-T50 of 337.61 °C and 384.58 °C, than that of subsoils with TG-T50 and DSC-T50 of 337.32 and 382.67 °C, respectively. We found subsoils had significantly higher proportions of aliphatic and aromatic compounds, while existed higher SOC associated with minerals. It seemed SOC stabilization differed with soil depths, in which mineral protection dictated SOC thermal stability in topsoils while molecular features posed a more important constraint on SOC stabilization in subsoils. Overall, our findings support the hypothesis of physical and chemical protection but emphasized that SOC thermal stability largely depended on to extent of the combination between molecular features and mineral protection, which explained 55% in topsoils and 73% in subsoils, respectively.

Long-term light grazing does not change soil organic carbon stability and stock in biocrust layer in the hilly regions of drylands

Long-term light grazing does not change soil organic carbon stability and stock in biocrust layer in the hilly regions of drylands

Effects of Soil Microbiological Properties on the Fractional Distribution and Stability of Soil Organic Carbon under Different N Addition Treatments

Soil organic carbon (SOC) fractions are influenced by inputs of nitrogen (N) from globally rising N deposition; however, the mechanisms of how soil microbiological properties are influenced by N deposition and its impact on the fractional distribution and stability of SOC remain unclear. In this study, we assessed the effects on SOC fraction distribution and stability from four aspects of soil microbiological properties: soil microbial biomass (SMB), soil microbial activity, structure diversity, and functional diversity of soil microbial community in a Pinus tabuliformis plantation, which received four N addition levels (0 g N m−2 y−1 (N0), 3 g N m−2 y−1 (N3, low N addition), 6 g N m−2 y−1 (N6, mid-N addition), and 9 g N m−2 y−1 (N9, high N addition)) for 2 years. The N inputs did significantly affect some soil microbiological properties, like SMB, soil phospholipid fatty acid (PLFA), and soil microbial functional diversity. Mid- and high N addition decreased the richness (HPLFA) and evenness (EPLFA) index of the soil microbial community, from 3.24 to 2.91 and 0.93 to 0.87, respectively. In addition, the low N addition promoted the carbon management index (CMI) to 141.35, i.e., higher than the CMIs in the mid- and high-level treatments. The SOC stability also showed significant differences among N addition treatments, and SOC could be the most stable at the mid-N addition level. Regarding the effects of the four soil microbiological attributes on the CMI and stability, SMB and soil respiration positively impacted the CMI, but did not significantly affect the stability. In addition, EPLFA had positive effects, but EBIOLOG had negative effects on CMI and lability. Our findings indicate that soil microbiological properties are essential in SOC fractional distributions and stability. Further identification and study of soil microbial species used to change SOC fractions would help to clarify the detailed mechanisms involved.

Effect of Exogenous Nutrient Addition on Soil Organic Carbon Mineralization and Stabilization

Soil organic carbon (SOC) pools have the potential to attain sustainable agriculture goals under climate change. External organic and inorganic nutrient inputs considerably affect SOC mineralization and SOC pools. Therefore, a laboratory-based, three-factor soil incubation experiment was conducted to investigate the impact of different exogenous nutrient additions on the mineralization and stability of SOC. The study investigated the effects of three fertilizer types (inorganic, organic, and a combination of inorganic and organic) and three rates of fertilizer addition (low, medium, and high) while considering two temperature levels (15 °C and 25 °C). At 25 °C, the application of fertilizer at a high rate significantly increased the SOC mineralization (2.84–19.97%) compared to fertilizer applied at a low rate, while, at 15 °C, different fertilizer types had no significant impact. Overall, fertilization resulted in an increase in the total potential mineralizable carbon (7.87–84.50%), while the rate of decomposition was decreased. The priming effect of inorganic fertilizer decreased over time, with the main effect observed during the initial 14 days. The addition of organic fertilizer resulted in a lesser increment in the soil activity index while simultaneously yielding a higher Q10 compared to inorganic fertilizer. Overall, the mixed application of organic and inorganic fertilizers was suggested to improve SOC stabilization and promote sustainable agricultural development.

Which is more important in stabilizing soil organic carbon in mountain ecosystems: molecular features, mineral protection, soil nutrients, or elevation?

The mechanisms that maintain soil organic carbon (SOC) stability along the elevation gradient has not been adequately explored in mountain ecosystems, though numerous work has been performed to reveal potential impacts of multiple environmental factors on SOC stability. The objective of this study was to decipher the predominant factor and effects that affected SOC stability by linking elevation, soil nutrients, minerals, and chemical molecular composition with SOC thermal stability in Changbai Mountain (CBM), Northeast China. Results indicated that SOC ranged from 2.12% to 17.75% without obvious elevation dependency. SOC stability was evaluated using the temperature at which haft of the exothermic mass loss (TG50). The TG50 decreased with elevation at p < 0.05 level. SOC did not vary significantly across soils from predominant plant communities, which were the mixed coniferous and broad-leaved forests (HJL), mixed coniferous forests (ZYL), birch forests (YHL), and tundra vegetation zones (TYD). Total phosphorus (P) and manganese (Mn) differed significantly among these four zones. Aliphatic, alkanes, alkenes, and aromatics were predominant moieties of pyrolytic products of SOC in all zones. TP, alkanes, Mn, Th, aliphatic, Lg, and Ph were the main drivers controlling SOC in contents and stabilities. Mineral protection in conjunction with soil nutrients could explain 73% of the total variance in SOC stability, while Mn had more importance than Fe in stabilizing organic carbon in CBM. The structural equation model revealed positive effects of both mineral protection and molecular feature on SOC pool and stability.

Vegetation restoration altered the soil organic carbon composition and favoured its stability in a Robinia pseudoacacia plantation

Soil organic carbon (SOC) stabilization is vital for the mitigation of global climate change and retention of soil carbon stocks. However, there are knowledge gaps on how SOC sources and stabilization respond to vegetation restoration. Therefore, we investigated lignin phenol and amino sugar biomarkers, SOC physical fractions and chemical structure in one farmland and four stands of a Robinia pseudoacacia plantation. We observed that the content of SOC increased with afforestation, but the different biomarkers had different contributions to SOC. Compared to farmland, the contribution of lignin phenols to SOC decreased in the plantations, whereas there was no difference among the four stand ages, likely resulting from the balance between increasing lignin derivation input and increasing lignin degradation. Conversely, vegetation restoration increased the content of microbial necromass carbon (MNC) and the contribution of MNC to SOC, mainly because microbial residue decomposition was inhibited by decreasing the activity of leucine aminopeptidase, while microbial necromass preservation was promoted by adjusting soil variables (soil water content, clay, pH and total nitrogen). In addition, vegetation restoration increased the particulate organic carbon (POC), mineral-associated organic carbon (MAOC) pools and the O-alkyl C intensify. Overall, vegetation restoration affected SOC composition by regulating lignin phenols and microbial necromass and also altered SOC stabilization by increasing the physically stable MAOC pool during late afforestation. The results of this study suggest that more attention should be given to SOC sequestration and stability during late vegetation restoration.

Effects of Changing Restoration Years on Soil Nutrient Traits and Plant Community Diversity in a Phosphate Mining Area

The thinning vegetation and soil erosion problems left behind by extractive mining have caused serious environmental pollution, and vegetation restoration is one of the effective strategies to counter them. To study the effects of vegetation restoration on plant community species diversity, soil carbon, nitrogen and phosphorus, and the response of their plant community succession, four communities of different ages (1, 7, 10, 40) and one natural forest (&gt;50 years) in the Kunyang phosphate mine were selected, and the analysis was carried out using the methods and protocols for plant community inventory. The species composition was recorded, and soil was collected from 0–60 cm in each community to determine the response of soil nutrients and plant diversity to the restoration process. The results show that the species richness of the community increases with the restoration year, the species composition at 40 years of restoration is similar to that of the natural forest, and the Shannon–Wiener diversity index in the tree layer at 40 years of restoration is greater than in the natural forest. Soil pH showed a decreasing trend with restoration year, and TP and AP increased with increasing time series. And the linear stepwise regression analysis showed that soil pH, soil organic carbon (SOC), total phosphorous (TP), available phosphorous (AP), and restoration year were the main factors of plant diversity. Compared to restoration of 10 years, TP and AP at the restoration of 40 years increased to 11.9–20.0 g∙kg−1 and 33.4–75.5 mg∙kg−1. The SOC of the community reached a maximum at 40 years of restoration, 1.5, 2.8, and 2.4 times higher at 0–20 cm, 20–40, and 40–60 cm, respectively, than at 1 year. The organic carbon fraction increased with the restoration year in an ‘N’ pattern, and mineral-associated organic carbon (MOC) and unstable organic carbon fraction decreased at 10 years and 40 years of restoration. The SOC of natural forests decreased, but stable organic carbon increased. The soil pH, SOC, and organic carbon fraction of the communities decreased with increasing soil depth, while TP and AP increased with increasing soil depth at the later period of restoration. In general, with extended restoration years, 40 years plant of restoration in phosphate mines can be expected to allow for plant community succession to climax community, and the key influence on plant diversity are the phosphorus and stable carbon fractions. These results are expected to facilitate the future basis for vegetation succession and restore systems during mining area restoration.

Influence of tree species on soil microbial residue accumulation and distribution among soil aggregates in subtropical plantations of China

BackgroundMicrobial residues are significant contributors to stable soil organic carbon (SOC). Soil aggregates effectively protect microbial residues against decomposition; thus, microbial residue accumulation and distribution among soil aggregates determine long-term SOC stability. However, how tree species influence accumulation and distribution of soil microbial residues remains largely unknown, hindering the chances to develop policies for SOC management. Here, we investigated microbial residue accumulation and distribution in soil aggregates under four subtropical tree species (Cunninghamia lanceolata, Pinus massoniana, Michelia macclurei, and Schima superba) after 29 years of afforestation.ResultsAccumulation of microbial residues in the 0–10 cm soil layer was 13.8–26.7% higher under S. superba than that under the other tree species. A structural equation model revealed that tree species affected the accumulation of microbial residues directly by altering fungal biomass. Additionally, tree species significantly affected microbial residue distribution and contribution to SOC in the top 20 cm soil. In particular, microbial residue distribution was 17.2–33.4% lower in large macro-aggregates (LMA) but 60.1–140.7% higher in micro-aggregates (MA) under S. superba than that under the other species in the 0–10 cm soil layer, and 14.3–19.0% lower in LMA but 43–52.1% higher in MA under S. superba than that under C. lanceolata and M. macclurei in the 10–20 cm soil layer. Moreover, the contribution of microbial residues to SOC was 44.4–47.5% higher under S. superba than under the other tree species. These findings suggest a higher stability of microbial residues under S. superba than that under the other studied tree species.ConclusionsOur results demonstrate that tree species influence long-term microbial persistence in forest soils by affecting accumulation and stabilization of microbial residues.

Formation of soil organic carbon pool is regulated by the structure of dissolved organic matter and microbial carbon pump efficacy: A decadal study comparing different carbon management strategies.

To achieve long-term increases in soil organic carbon (SOC) storage, it is essential to understand the effects of carbon management strategies on SOC formation pathways, particularly through changes in microbial necromass carbon (MNC) and dissolved organic carbon (DOC). Using a 14-year field study, we demonstrate that both biochar and maize straw lifted the SOC ceiling, but through different pathways. Biochar, while raising SOC and DOC content, decreased substrate degradability by increasing carbon aromaticity. This resulted in suppressed microbial abundance and enzyme activity, which lowered soil respiration, weakened in vivo turnover and ex vivo modification for MNC production (i.e., low microbial carbon pump "efficacy"), and led to lower efficiency in decomposing MNC, ultimately resulting in the net accumulation of SOC and MNC. In contrast, straw incorporation increased the content and decreased the aromaticity of SOC and DOC. The enhanced SOC degradability and soil nutrient content, such as total nitrogen and total phosphorous, stimulated the microbial population and activity, thereby boosting soil respiration and enhancing microbial carbon pump "efficacy" for MNC production. The total C added to biochar and straw plots were estimated as 27.3-54.5 and 41.4 Mg C ha-1 , respectively. Our results demonstrated that biochar was more efficient in lifting the SOC stock via exogenous stable carbon input and MNC stabilization, although the latter showed low "efficacy". Meanwhile, straw incorporation significantly promoted net MNC accumulation but also stimulated SOC mineralization, resulting in a smaller increase in SOC content (by 50%) compared to biochar (by 53%-102%). The results address the decadal-scale effects of biochar and straw application on the formation of the stable organic carbon pool in soil, and understanding the causal mechanisms can allow field practices to maximize SOC content.

Crop diversification effects on soil organic carbon and nitrogen storage and stabilization is mediated by soil management practices in semiarid woody crops

Crop diversification is a promising strategy to mitigate climate change through soil carbon sequestration while ensuring soil fertility maintenance and food security. Here, we assess the impact of different inter-cropping practices on soil organic carbon (SOC) and nitrogen (N) storage and stabilization under rainfed and irrigated semiarid conditions. Under rainfed conditions, an almond (Prunus dulcis Mill.) monocrop was compared with an almond inter-cropped with caper (Capparis spinosa) or with winter thyme (Thymus hyemalis). Under irrigated conditions, a mandarin (Citrus reticulata Blanco) monocrop was compared with a mandarin inter-cropped with an annual crop rotation (including Hordeum vulgare, Vicia sativa and Vicia faba) or with a triennial crop rotation (including Vicia faba, Portulaca oleracea and Vigna unguiculata L.). The SOC and N stocks, SOC mineralization rates, water-stable aggregates and associated OC and N contents were estimated at 0–10 and 10–30 cm depth in each monocrop and inter-cropping system after three years. Contrasting effects of inter-cropping on SOC and N stabilization were found in the rainfed and irrigated systems. The combination of inter-cropping and no tillage did not affect the SOC stocks or mineralization rates at any soil depth in the rainfed system, while it significantly increased the OC and N contents within the large macro-aggregates in the subsoil. Compared to the irrigated mandarin monocrop, the mandarins inter-cropped with the annual crop rotation showed a significant increase of 10% and 18% in the topsoil and subsoil N stocks, respectively, while the SOC mineralization rates were reduced by 30% at both soil depths. On the contrary, the topsoil SOC stock in the mandarins inter-cropped with the triennial crop rotation were reduced by 38% compared to the monocrop. Significant reductions ranging between 24% and 66% were also observed in the OC and N contents associated to the macro- and micro-aggregates in the topsoil and subsoil of both crop diversifications compared to the monocrop system. Our results highlight the potential of inter-cropping rainfed woody crops with perennials for boosting SOC and N storage and stabilization. In irrigated woody monocrops, inter-cropping with annual crop rotation schemes that include two or more leguminous and reducing tillage frequency operations seems to be more appropriate in terms of SOC and N stabilization.

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.