Heavy Fraction Organic Carbon Research Articles

Effects of Straw Return Duration on Soil Carbon Fractions and Wheat Yield in Rice–Wheat Cropping System

In China’s subtropical rice–wheat cropping system, the changes in the soil organic carbon (SOC) pool due to long-term straw return and its connection with crop yield remain unclear. This study aims to provide insights into establishing a sensible straw return system by evaluating the differences in the distribution and variation rates of SOC, light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), particulate organic carbon (POC), and mineral-associated organic carbon (MOC) in the 0–20 cm soil layer under different durations of straw return. Additionally, the study analyzes the relationship between the changes in SOC and its fractions and wheat yield. The experiment was conducted in 2019 in a rice–wheat rotation field with ten years of straw return treatments: no straw return (NR) or 1, 2, 3, 4, 5, 6, 7, 8, 9 year(s) of straw return (SR1–9), and an additional treatment in 2020 (10 years of straw return, SR10). The results revealed that with an increase in the duration of straw return, the contents of SOC, LFOC, HFOC, and POC gradually increased, showing the highest increments of 45.88%, 187.22%, 41.55%, 97.89%, and 28.21%, respectively, compared to the NR treatment. However, after eight years of straw return, the compound annual increase in soil organic carbon and its components was lower than in years 1–8, indicating a trend of diminishing increments. The SOC content and its variation were significantly correlated with the content and variation of LFOC, HFOC, POC, and MOC, with the highest sensitivity observed for the variation in LFOC, indicating the strong influence of the duration of straw return. The SOC and its fraction contents showed significant positive correlations with wheat yield, with the highest contribution to wheat yield increase attributed to an increase in LFOC content. In summary, straw return enhances the 0–20 cm deep soil carbon pool, with LFOC being the most sensitive indicator, reflecting the influence of the duration of straw return on soil carbon pools.

Effect of Land Use on the Stability of Soil Organic Carbon in a Karst Region

The composition of soil organic carbon and its stability mechanism are the key to understanding the terrestrial carbon sink capacity. The stability of soil organic carbon in a karst ecosystem greatly affects the soil carbon fixation capacity. In order to understand the impact of human activities on the stability of soil organic carbon in karst areas, the karst valley area of Zhongliang Mountain in Chongqing was selected as an example, and soil samples of four typical land use modes (mixed forest, bamboo forest, grassland, and cultivated land) were collected in layers to analyze the total organic carbon (TOC) and heavy fraction organic carbon (HFOC). The distribution characteristics of light fraction organic carbon (LFOC), labile organic carbon (LOC), and recalcitrant organic carbon (ROC) were analyzed quantitatively by using a structural equation model to provide basic data for soil carbon sink assessment and soil quality protection in karst areas. The results showed that the organic carbon components under different land use patterns in karst areas had obvious surface accumulation, and the content of organic carbon components in the surface layer was 1.2 times that in the bottom layer. Except for LFOC, the content of other organic carbon components was the highest in the mixed forest, followed by that in the bamboo forest and wasteland, with the lowest in cultivated land. Mixed forest ω(TOC) content was the highest, 42.5 g·kg-1, followed by that of bamboo forest (36.6 g·kg-1) and grassland (18.7 g·kg-1), and cultivated land content was the lowest, 13.4 g·kg-1. The soil organic carbon content of cultivated land was 68.5%, 63.5%, and 28.3% lower than that of mixed forest, bamboo forest, and grassland, respectively. Mixed forest had the highest content of ω(HFOC), 21 g·kg-1, followed by those of bamboo forest (20.9 g·kg-1), grassland (18.2 g·kg-1), and cultivated land (13.5 g·kg-1). The mixed forest ω(LOC) content was the highest, 16.3 g·kg-1, followed by those of bamboo forest (14.9 g·kg-1), grassland (11.5 g·kg-1), and cultivated land (5.3 g·kg-1). Mixed forest ω (ROC) content was the highest, 25.7 g·kg-1, followed by those of bamboo forest (21.6 g·kg-1), grassland (15.9 g·kg-1), and cultivated land (10.3 g·kg-1). The bamboo forest land ω(LFOC) content was 15.9 g·kg-1, followed by those of mixed forest (13.9 g·kg-1), grassland (7.3 g·kg-1), and cultivated land (4.9 g·kg-1). The recalcitrant organic carbon index (ROCI) was used to indicate the stability of soil organic carbon. The variation range of ROCI was 33.9%-64.5%, of which the highest was mixed forest (64.5%-66.3%), and the lowest was cultivated land (33.8%-39.6%). The ROCI of mixed forest, bamboo forest, and grassland were 1.8 times, 1.6 times, and 1.4 times that of cultivated land, respectively. Karst area ω (inert organic carbon) content and ROCI showed that human agricultural activities caused the reduction in soil organic carbon content and the destruction of soil physical structure, resulting in the accelerated decomposition and turnover rate of soil organic matter. The most important factor affecting soil stability in karst areas was soil pH. Tillage activities caused soil pH to rise, reduced soil microbial activity, and were not conducive to the accumulation of the inert organic carbon and soil organic carbon pool in the soil.

Vertical distribution characteristics of soil organic carbon and vegetation types under different elevation gradients in Cangshan, Dali.

The Cangshan National Nature Reserve of Dali City was adopted as the research object to clarify the vertical distribution characteristics of soil organic carbon (SOC) and vegetation types at different elevations in western Yunnan. The contents of SOC, light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), and water-soluble organic carbon (WSOC) in the 0-30 cm soil layer at different elevations (2,400, 2,600, 2,800, 3,000, 3,200, 3,400, and 3,600 m) were determined, and the above-ground vegetation types at different elevations were investigated. Results showed that the SOC content was the highest in 0-20 cm surface soil and gradually decreased with the deepening of the soil layer. It increased then decreased with the increase in elevation, and it peaked at 3,000 m. The LFOC content was between 1.28 and 7.3515 gkg-1. It exhibited a decreasing trend and little change in profile distribution. The HFOC content ranged between 12.9727 and 23.3708gkg-1; it increased then decreased with the increase in profile depth. The WSOC content was between 235.5783 and 392.3925 mgkg-1, and the response sensitivity to elevation change was weak. With the increase in elevation, WSOC/SOC and LFOC/SOC showed a similar trend, whereas HFOC presented an opposite trend. This observation indicates that the active organic carbon content at 3,600 m was lower than that at 2,400 m, and the middle elevation was conducive to the storage of active organic carbon. Meanwhile, the physical and chemical properties of soil affected the distribution of organic carbon to a certain extent. The vegetation type survey showed that the above-ground dominant species within 2,400-2,800 m were Pinus yunnanensis and Pinus armandii. Many evergreen and mixed coniferous broadleaf forests were distributed from 3,000 m to 3,200 m. Species of Abies delavayi were mainly distributed from 3,400m to 3,600m. This research serves as a reference for the study of forest soil carbon stability in high-elevation areas and plays an important role in formulating reasonable land use management policies, protecting forest soil, reducing organic carbon loss, and investigating the carbon sequestration stability of forest ecosystems.

Response of soil organic carbon and its fractions to natural vegetation restoration in a tropical karst area, southwest China

The dynamics of soil organic carbon (SOC) and its fractions are important for evaluating the vegetation restoration effect and carbon cycle of the ecosystem. Here, SOC fractions, including light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), and labile organic carbon (LOC) fractions (including water-soluble organic carbon, WSOC, readily oxidizable organic carbon, ROC, particulate organic carbon, POC, and microbial biomass carbon, MBC), were investigated at four soil depths under five restoration stages in a tropical karst area in southwest China. This study showed that the content of SOC and its fractions significantly increased with vegetation restoration and decreased with increasing soil depth at each restoration stage (p < 0.05). Additionally, LFOC was more sensitive to vegetation restoration, whereas HFOC was the main storage form of SOC. The LOC fractions in the surface soil layer were significantly higher than those in the lower, and the percentages of some LOC fractions (POC/SOC and MBC/SOC) significantly decreased with increasing soil depth, indicating that SOC was more stable in the lower layer than in the surface layer. Correlation analysis showed that SOC was significantly and positively correlated with its fractions. Moreover, SOC and its fraction were positively correlated with soil chemical factors (TN, TP, AP, AK, ECa, EMg, NH4+-N, and NO3–-N) and negatively correlated with bulk density (BD) at a significant level (p < 0.05). Moreover, redundancy analysis showed that the 12 soil physicochemical factors explained 70.99% of the variation in SOC and its fractions, with AK, NH4+-N, and BD being the main factors, explaining 19.38, 17.24, and 10.52% of the variation, respectively. The structural equation mode analysis showed that soil properties, above-ground biomass, and litterfall explained most of the variation in SOC (59%), LFOC (79%), HFOC (81%), and LOC (61%). Soil properties and above-ground biomass significantly affected SOC, LFOC, and HFOC content mainly through indirect effects, while the total phosphorus content of the litterfall could directly and significantly affect SOC, LFOC, and HFOC content. NH4+-N and AK of soil factors had direct effects on LFOC and LOC accumulation, respectively. This study provides a valuable perspective for estimating carbon sink potential and constructing carbon sink models in karst areas.

Soil aggregate variation in two contrasting rice straw recycling systems for paddy soil amendment over two years

ABSTRACT Soil aggregate is important to soil quality. Straw return is beneficial for soil amendment, but the effects of straw and straw-derived biochar on aggregate formation and stability in paddy soil are unclear. This study carried out a field experiment for 2 years in Northeast China with four treatments: conventional fertilization (CF), straw (ST, 7.5 t ha−1 year−1), biochar (BC, 2.5 t ha−1 year−1), and biochar-based fertilizer (BCF, 0.75 t ha−1 year−1). Compared with the CF and BCF, BC and ST improved the macroaggregates and significantly increased soil total carbon and aggregate organic carbon, indicating a sustained positive role in promoting the formation and stability of aggregates. BC significantly improved aggregate stability. Correlation analysis showed that macroaggregates (2–0.25 mm) can be increased by increasing the heavy fraction organic carbon (HFOC), BC and ST significantly increased the HFOC, and BC had a greater effect. Scanning electron microscopy (SEM) showed that the combination of biochar and soil particles can be captured with strong water scouring. BCF showed an increasing trend in the formation and stability of soil aggregates. In comparison, biochar had a greater effect on promoting the formation of macroaggregates and the stability of soil aggregates with a significant sustainable effect.

Habitats generated by the restoration of coal mining subsidence land differentially alter the content and composition of soil organic carbon.

The content and composition of soil organic carbon (SOC) can characterize soil carbon storage capacity, which varies significantly between habitats. Ecological restoration in coal mining subsidence land forms a variety of habitats, which are ideal to study the effects of habitats on SOC storage capacity. Based on the analysis of the content and composition of SOC in three habitats (farmland, wetland and lakeside grassland) generated by different restoration time of the farmland which was destroyed by coal mining subsidence, we found that farmland had the highest SOC storage capacity among the three habitats. Both dissolved organic carbon (DOC) and heavy fraction organic carbon (HFOC) exhibited higher concentrations in the farmland (20.29 mg/kg, 6.96 mg/g) than in the wetland (19.62 mg/kg, 2.47 mg/g) or lakeside grassland (5.68 mg/kg, 2.31 mg/g), and the concentrations increased significantly over time, owing to the higher content of nitrogen in the farmland. The wetland and lakeside grassland needed more time than the farmland to recover the SOC storage capacity. The findings illustrate that the SOC storage capacity of farmland destroyed by coal mining subsidence could be restored through ecological restoration and indicate that the recovery rate depends on the reconstructed habitat types, among which farmland shows great advantages mainly due to the nitrogen addition.

Application of malrstone-based conditioner and plantation of Jerusalem artichoke improved properties of saline-alkaline soil in Inner Mongolia

Soil salinization is a critical environmental issue restricting agricultural production. Inner Mongolia is one of the areas with severe land salinization in China. This study aimed to investigate the effects of conditioning agent (containing marlstone and a range of enzymes) and cultivating Jerusalem artichoke on saline soils in Inner Mongolia. The effects of conditioner (0, 0.06 and 0.18 kg/m2) on soil physical, chemical and biological properties, including soil carbon fractions and microbiota in saline soils planted with Jerusalem artichoke, were characterized. The results showed that soil salinity was reduced significantly after cultivating Jerusalem artichoke and declined also after the conditioner addition. The application of conditioner increased the content of DOC (dissolved organic carbon), HFOC (heavy fraction organic carbon) and the content of aggregates >0.25 mm compared to the soil planted with Jerusalem artichoke alone. The relative abundance of halophilic bacteria such as Thioalkalivibrio and Thiohalobacter was greater in the CK (non-treated control). By contrast, the relative abundance of microorganisms with the carbon assimilation and nitrogen fixation capacities, such as Cyanobacteria and Rhodovulum, was greater in the conditioner-treated and Jerusalem artichoke-planted treatments. The planting of Jerusalem artichoke reduced soil salinity, increased soil organic carbon fractions, improved soil structure, and altered the soil microbial community, with the application of the conditioning agent enhancing these positive changes. The co-occurrence network structure of “Jerusalem artichoke-conditioner-saline soil-soil microorganism” was established, which provided scientific basis for Jerusalem artichoke-conditioner to improve saline soil.

Straw type and returning amount affects SOC fractions and Fe/Al oxides in a rice-wheat rotation system

Straw returning is a vital agronomic practice for soil organic carbon (SOC) sequestration and stabilization by changing the storage and fraction composition. Iron and aluminum (Fe/Al) oxides have a protective effect on SOC through adsorption and co-precipitation, which depends on the fraction composition of SOC itself, especially originated from externally added organic materials. However, the changes and interactions of SOC components and Fe/Al oxides in straw returning soil are not clearly understood. To evaluate the effect of straw returning on SOC composition and Fe/Al oxides, we have conducted an in situ field experiment for 10 yrs., which including five treatments: no rice and wheat straw returning (NRW, as control), total amount wheat straw returning (W), total amount rice straw returning (R), half amount rice and wheat straw returning (HRW), and total amount rice and wheat straw returning (TRW). The results showed that SOC and heavy fraction organic carbon (HFOC) increased under all five treatments, especially for R, HRW, and TRW treatments (P < 0.05). Compared with wheat straw returning, rice straw returning was more conducive to increasing the contents of SOC and its fractions, free Fe/Al oxides, complex aluminum oxides and the carbon utilization efficiency. Collectively, these properties were strongly affected by both the type and amount of straw returning. Soil organic carbon was significantly correlated with HFOC, water-soluble organic carbon, alkali-hydrolyzed nitrogen, available phosphorous, available potassium, and total nitrogen (P < 0.05). Of these, HFOC had the most significant effect on SOC and was vitally important for improving SOC sequestration and stability (P < 0.001). The increase of labile carbon fractions was more conducive to the absorption and utilization of nutrients by crops. Straw returning promoted the interaction between increasing organic matter and complex iron oxide (Fec). This served to strengthen the protective effect of SOC, which increased the content of Fec and stability of SOC, resulting in decreased soil pH. Taken together, these results highlight the importance of straw returning in promoting the storage and stability of SOC pool. Critically, rice straw returning can reduce soil pH and Fec content, therefore could be considered as an ideal model for a rice-wheat rotation system.

The main driver of soil organic carbon differs greatly between topsoil and subsoil in a grazing steppe.

Soil organic carbon (SOC) dynamics is regulated by a complex interplay of factors such as climate and potential anthropogenic activities. Livestocks play a key role in regulating the C cycle in grasslands. However, the interrelationship between SOC and these drivers remains unclear at different soil layers, and their potential relationships network have rarely been quantitatively assessed. Here, we completed a six‐year manipulation experiment of grazing exclusion (no grazing: NG) and increasing grazing intensity (light grazing: LG, medium grazing: MG, heavy grazing: HG). We tested light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC) in 12 plots along grazing intensity in three soil layers (topsoil: 0–10 cm, mid‐soil: 10–30 cm, subsoil: 30–50 cm) to assess the drivers of SOC. Grazing significantly reduced SOC of the soil profile, but with significant depth and time dependencies. (1) SOC and SOC stability of the topsoil is primarily regulated by grazing duration (years). Specifically, grazing duration and grazing intensity increased the SOC lability of topsoil due to an increase in LFOC. (2) Grazing intensity was the major factor affecting the mid‐soil SOC dynamics, among which MG had significantly lower SOC than did NG. (3) Subsoil organic carbon dynamics were mainly regulated by climatic factors. The increase in mean annual temperature (MAT) may have promoted the turnover of LFOC to HFOC in the subsoil. Synthesis and applications. When evaluating the impacts of grazing on soil organic fraction, we need to consider the differences in sampling depth and the duration of grazing years. Our results highlight that the key factors influencing SOC dynamics differ among soil layers. Climatic and grazing factors have different roles in determining SOC in each soil layer.

Comparing soil organic carbon stock and fractions under natural secondary forest and Pinus massoniana plantation in subtropical China

Comprehensive comparison of soil organic carbon (SOC) stock and its fractions in different forest types with same age is scantily considered, especially between natural secondary forest (NSF) and Pinus massoniana plantation (PMP), which are widely distributed in subtropical China. Thus, we evaluated the differences of C stocks in stand litter and 0–40 cm soil profile of an Andosols and the labile fractions [dissolved organic carbon (DOC), microbial biomass carbon (MBC), coarse particulate organic carbon (cPOC, 250–2000 μm), fine particulate organic carbon (fPOC, 53–250 μm) and easily oxidizable carbon (EOC)] and heavy fraction organic carbon (HFOC) in SOC between NSF and adjacent 70-year-old PMP. We found that NSF enhanced the C stocks in stand litter and 0–40 cm soil epipedon by 15.1% and 37.9% higher those of PMP. Compared with PMP, the stocks of MBC, cPOC, fPOC, EOC and HFOC in 0–40 cm soil epipedon under NSF were increased by 68.2%, 59.8%, 51.6%, 31.7% and 47.18% respectively, but the DOC stock was reduced by 9.9%. The impact of forest type on the SOC pool in surface soil (0–20 cm) was stronger than that in subsoil (20–40 cm). Our results suggested that NSF is more suitable for SOC sequestration and soil C sink in the study region because significantly higher SOC is incorporated as HFOC in the soil rather than stand litter under the naturally generated secondary forest.

Re-used mulching of plastic film is more profitable and environmentally friendly than new mulching

Reducing the usage and residue of plastic mulch film, but not productivity, is crucial for the sustainable development of dryland farming. The re-used mulching of plastic film (directly cropping on the already used plastic film) was tested compared to no-mulch and new mulching (mulching new film each year), in the grain maize, forage maize, and faba bean cropping systems. The new mulch significantly increased topsoil temperature (18.4% above average), upper 2 m soil moisture (20.2%), crop yield (85.8%), above-ground biomass (63.4%), net revenue (68.4%), proportion of macroaggregates (> 0.25 mm) (35.7%), mean weight diameter (MWD) (78.4%), soil organic carbon (7.0%), and heavy fraction organic carbon (10.3%) in all three crop systems compared to no-mulch. Re-used mulch significantly increased topsoil temperature (11.4% above average), moisture (13.4%), and MWD after final crop harvest (19.1%) in all three crop systems; and significantly increased crop yield (61.8%) and above-ground biomass (50.3%) in the faba bean system. However, re-used mulch had no significant effect for the grain and forage maize systems compared to no-mulch. The 2-years re-used mulch (3-years continuously mulch) had significantly lower plastic film residue (57.1% below average) after final crop harvest in all three crop systems. Re-used mulch also produced significantly higher net revenue (13.2%) in the faba bean system compared to new mulch. In summary, cropping faba bean, under re-used mulch, instead of new mulch, significantly reduce the usage and residue of plastic film and resulted in similar high productivity, and may become a willingly adopted practice for farmers due to the higher net revenue. This work can serve as a reference for sustainable use of plastic film mulch and has potential applications to effectively reduce plastic film usage and residue in cropland and develop sustainable agriculture, especially for cultivating faba bean and other temperature-insensitive crops in developing countries.

A novel organic carbon accumulation mechanism in croplands in the Yellow River Delta, China

The trends and mechanisms of organic carbon changes in coastal delta croplands are not yet clear due to the complexity physicochemical processes in soil. In this study, combing powder x-ray diffraction (XRD), microbial analysis, and density functional theory in quantum mechanics, we proposed a novel mechanism underlying OC accumulation. We investigated changes of three kinds of organic carbon (OC)-dissolved organic carbon (DOC), light fraction organic carbon (LFOC), and heavy fraction organic carbon (HFOC) in the Yellow River Delta croplands. We found that HFOC, dominant in coastal delta cropland soil, formed at different ages and its density increased with increasing reclamation time. Yet, DOC and LFOC had no significant increase or decrease tendency. Moreover, in coastal delta croplands, HFOC accumulation might be a complex progress, including the loss of indigenous OC and the accumulation of newly input OC. Based on these results, we proposed that although root exudative DOC (organic acids) could cause the indigenous OC loss by forming a specific microbial community, it still was a source of HFOC and promoted the OC accumulation. More importantly, based on density functional theory, we verified that these root exudative organic acids could adsorb on SiO2 together with crystalline Fe oxides (Fec) to form aggregates. The finding could explain the phenomenon that the XRD results showed samples were compounds of SiO2, Fec, and OC and the accumulation of HFOC in coastal delta croplands. By revealing a new OC accumulation mechanism in coastal delta croplands, this study provides novel insights into the mechanism of OC dynamics in coastal delta croplands and the global carbon budget.

Bulk Chemical and Optical Spectroscopy Characterisations of Dissolved Organic Matter Extracted from the Tropical Coastal Sediment

The study of organic matter characterisation in coastal sediment is useful for understanding how dissolved organic matter (DOM) reactivity influences the fate of pollutants in the aquatic environment. However, there is little information available on the structural properties of DOM in coastal sediment. Chemical analysis, UV-Visible (UV-Vis) absorbance, fluorescence, and Fourier-transform infrared (FTIR) spectroscopy were used in this work to characterise and compare the components of water-extractable organic matter (WEOM) fractions from sediment from an estuary, a coastal area, and a port in Peninsular Malaysia, Malaysia. The elemental analysis (H/C and C/N ratios) of the three samples differed, which coincided with the findings of optical indices estimated from UV-Visible and fluorescent spectroscopy analyses. WEOM had an average output of 7.05 to 8.47 mg/L and was very dominating with heavy fraction organic carbon (HFOC). In DOM-KS and DOM-K sediments, the allochthonous component with a high degree of condensation and the aromatic compound was the dominating composition. Meanwhile, the DOM-KT possessed a high autochthonous composition as well as carboxylic and phenolic content. Correlation analysis indicates that the aromaticity index SUVA254 and humification index (HIX) have positive correlations. The combined results of the chemical and spectroscopic analyses indicate that different coastal ecosystems, with the integration of various human and land activities, produce variations in DOM in the coastal area.

Stoichiometric regulations of soil respiration and its temperature sensitivity under erosion and deposition conditions

Soil erosion not only affects the redistribution of soil materials but also changes soil respiration (Rs) and its temperature sensitivity (Q10), which plays a significant role in the carbon cycle of terrestrial ecosystem. To explore the stoichiometric regulations of Rs and Q10 under erosion and deposition conditions, typical eroded (croplands) and deposited sites (check dams) were selected in the Luoyugou watershed of the Chinese Loess Plateau. In situ Rs, soil temperature and moisture were monitored. The results showed that soil deposition decreased soil organic carbon, total nitrogen, heavy fraction organic carbon and dissolved organic nitrogen by 20.0%, 26.7%, 20.1% and 49.1%, respectively. However, the higher ratios of carbon:nitrogen and labile carbon:organic carbon were observed in the deposited sites due to the accelerated loss of dissolved nitrogen and the selective transport of light fraction organic matter. The Rs displayed a decreasing trend from July to December, and soil temperature explained 69.5–93.6% of the temporal variation in Rs. The mean Rs was 2.18 and 2.89 μmol m−2 s−1 in the eroded and deposited sites, respectively, and the average Q10 was 2.49 and 3.38. Multiple stepwise regression analysis showed that soil organic carbon:total nitrogen and light fraction organic carbon totally explained 48.9% of the Rs variation, and the main control factors for Q10 were dissolved organic carbon: total organic carbon (34.6%), soil temperature (17.0%) and dissolved organic carbon (10.0%). Our results suggested that the ratios of carbon:nitrogen and labile carbon:organic carbon play an important role in regulating Rs and Q10, and the increase in carbon:nitrogen ratios induced by sediment deposition contributes to increasing Rs and Q10. We believe that the effects of erosion and deposition on Rs and Q10 revealed in the study will contribute to a better understanding of the terrestrial ecosystem carbon cycle.

The content, composition, and influencing factors of organic carbon in the sediments of two types of constructed wetlands.

Constructed wetland is a common measure for water purification and biodiversity conservation, but the mechanism of carbon storage is still unclear. Here, we researched the content and composition of soil organic carbon (SOC) and the influencing factors in surface sediment in surface flow constructed wetlands (SFCW) and subsurface flow constructed wetlands (SSFCW). Results showed that the content and storage of SOC in SSFCW were significantly higher than those in SFCW. However, the higher proportion of light fraction organic carbon (LFOC) and lower proportion of heavy fraction organic carbon (HFOC) in SSFCW indicated that SSFCW had less stable organic carbon storage than SFCW. The composition of SOC in the two types of constructed wetlands was mainly affected by total nitrogen, which suggesting carbon-nitrogen coupling in constructed wetlands. The abundant microbial species in SSFCW and their positive correlation with SOC could explain the higher carbon storage in SSFCW than in SFCW. In addition, plant biomass was the principle factor limiting LFOC proportion in SFCW, while it was moisture content in SSFCW. The study has important implications for understanding and management of ecological function of carbon sequestration in contrasted wetlands, and also provides a special perspective to understand the carbon storage mechanism in wetlands.

Application of earthworm cast improves soil aggregation and aggregate-associated carbon stability in typical soils from Loess Plateau

Earthworm casts exhibit remarkable fertility and have been widely used as an organic fertilizer. This study focused on the effects of earthworm cast application on soil aggregates and aggregate-associated carbon in typical soils from the Loess Plateau (China). Soil column experiments were conducted in the laboratory using cultivated loessial soil (CS), dark loessial soil (DS), and aeolian soil (AS). Application of earthworm casts significantly reduced the content of aggregates sized <0.5 mm but increased the content of water-stable aggregates. Compared to without-cast treatment, earthworm cast application increased the organic carbon content by 13.4–58.3%, 14.4–51.1%, 17.9–45.3%, 16.7–62.4%, 18.4–43.3%, and 19.8–62.9% in soil aggregate fractions of sizes <0.25, 0.25–0.5, 0.5–1, 1–2, 2–5, and >5 mm, respectively. The application of earthworm casts significantly increased heavy fraction organic carbon (HFOC), CaCO3, and exchangeable Ca contents in soil by 14.5–69.4%, 12.8–51.9%, and 33.3–63.2%, respectively. Compared with macroaggregates, microaggregates had higher CaCO3 contents but smaller light-fraction organic carbon (LFOC) to HFOC ratios, indicating that earthworm cast application improved the organic carbon stability more in microaggregates than macroaggregates. Comparison analysis of the three soils showed AS performed better in aggregation and aggregate-associated carbon stability than CS and DS after applying earthworm casts. The findings improve our understanding of the effects of earthworm cast application on soil aggregate distribution and aggregate-associated carbon stability, which will help improve the application efficiency of earthworm casts as an organic fertilizer in the Loess Plateau area.

A combined method for the source apportionment of sediment organic carbon in rivers

Organic carbon sources apportionment in river sediments is crucial to the output management of organic carbon. We conducted a source apportionment of sediment organic carbon in four rivers in Shaanxi Province, China, with a novel method that combined environmental scanning electron microscopy and energy dispersive X-ray spectrometry (ESEM-EDAX), principal component analysis (PCA), 16S rRNA sequencing, microbial community metabolic prediction, and positive matrix factorization (PMF). According to the ESEM-EDAX results, the sources of light fraction organic carbon (LFOC) were the vegetation residues and the organic carbon adsorbed on them; and the source of heavy fraction organic carbon (HFOC) was organic carbon wrapped in particles. Moreover, 16S rRNA sequencing results of LFOC and HFOC concerning microbes demonstrated that LFOC was mainly composed of carbohydrate, cellulose, and alky-aromatic compounds, and that carbohydrate with high molecular weight might be a part of HFOC. Based on the results of microbial community metabolic prediction, PCA, and PMF, we found dissolved organic carbon (DOC) was mainly from lipopolysaccharide biosynthesis, apoptosis, and decomposition of carboxylic acids. And it might be mainly composed of lipopolysaccharide, carbohydrates, and organic acid with low molecular. To reflect the appearance of a specific DOC type, three biomarkers were proposed based on the microbial relative abundance and specificity. This research proposed a new method to trace the sources of organic carbon and established microbial biomarkers for the appearance of specific DOC, which would promote the understanding of organic carbon sources into microbes. Thus, this research provides new perspectives in the source apportionment and the life cycle of organic carbon in rivers.

Factors controlling organic carbon distributions in a riverine wetland.

As a significant reservoir of organic carbon (OC), natural wetlands play an important role in mitigating greenhouse effects. To determine what factors might influence OC, we analyzed the distributions of dissolved organic carbon (DOC), light fraction organic carbon (LFOC), and heavy fraction organic carbon (HFOC) in sediments taken from the Yanghe River Wetland (YRW) and assessed the effects of several environmental variables on the distribution of the different carbon types. The microbial community abundances and compositions of the sampled sediments were analyzed by 16S rRNA amplicon sequencing. Redundancy analysis (RDA) was used to reveal the environmental factors that affect the distribution of OC. The DOC and LFOC contents varied significantly in the research area, while HFOC content showed no variation. The DOC content was significantly affected by sediment pH, vegetation height, and microbial abundances, and the LFOC content was significantly affected by water pH. We also proposed a novel indicator to study the microbial effect on the distribution of OC content in wetlands: weighted abundance of related microbes (WARM). This study identifies the environmental factors that could affect the distribution of OC in a riverine wetland and outlines the calculation of a novel indicator.

Effects of land use change on soil organic carbon and its components in karst rocky desertification of southwest China

Soil organic carbon (SOC) is the dynamic medium of carbon transfer and the main pathway of carbon transfer in the karst ecosystem. SOC and its components are the important parts in soil carbon cycling of karst ecosystem. However, few studies have focused on SOC and its components in the karst ecosystem. We analyzed the effects of land use change on the SOC content, SOC reserve (SOCS), water-soluble organic carbon (WSOC), easily oxidizable organic carbon (EOC), particu-late organic carbon (POC), and light fraction organic carbon (LFOC), and heavy fraction organic carbon (HFOC) and their distribution ratio, with six different land-use patterns [Zanthoxylum bungeanum forest (HJ), Hylocereus undulates forest (HL), mixed forest of Z. bungeanum and H. undulates (HHL), Sabina chinensis forest (YB), mixed forest of S. chinensis and Ligustrum luci-dum (YBN), and slope cropland (PD)] in Huajiang Canyon of Guanling County, Guizhou Pro-vince. Results showed that SOC and SOCS in YB, YBN and HJ were significantly higher than those in HL, HHL and PD. In the 0-20 cm soil layer, the concentrations of SOCS followed the order of HJ＞YB＞YBN＞PD＞HHL＞HL. Contents of WSOC, EOC, POC, LFOC and HFOC in YB, YBN, and HJ were all higher than those in the other three patterns. Significant positive correlations existed between SOC and each of its components (WSOC, EOC, POC, LFOC and HFOC), also between any two of those components. Z. bungeanum could be used as a priority economic species for the ecological rehabilitation of karst rocky desertification and mountain agriculture development in Southwest China. WSOC, EOC, POC, LFOC and HFOC could be used as indicators of soil organic carbon pool.

Effects of Contemporary Land Use Types and Conversions from Wetland to Paddy Field or Dry Land on Soil Organic Carbon Fractions

Soil organic carbon (SOC) concentration is closely related to soil quality and climate change. The objectives of this study were to estimate the effects of contemporary land use on SOC concentrations at 0–20 cm depths, and to investigate the dynamics of SOC in paddy-field soil and dry-land soil after their conversion from natural wetlands (20 and 30 years ago). We investigated the dissolved organic carbon (DOC), light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), and other soil properties (i.e., moisture content, bulk density, pH, clay, sand, silt, available phosphorous, light fraction nitrogen, and heavy fraction nitrogen) in natural wetlands, constructed wetlands, fishponds, paddy fields, and soybean fields. The results indicated that the content of DOC increased 17% in constructed wetland and decreased 39% in fishponds, and the content of HFOC in constructed wetland and fishponds increased 50% and 8%, respectively, compared with that in natural wetlands at 0–20 cm. After the conversion of a wetland, the content of HFOC increased 72% in the paddy fields and decreased 62% in the dry land, while the content of DOC and LFOC decreased in both types. In the paddy fields, LFOC and HFOC content in the topmost 0.2 m of the soil layer was significantly higher compared to the layer below (from 0.2 to 0.6 m), and there were no significant differences observed in the dry land. The findings suggest that the paddy fields can sequester organic carbon through the accumulation of HFOC. However, the HFOC content decreased 22% after 10 years of cultivation with the decrease of clay content, indicating that paddy fields need to favor clay accumulation for the purpose of enhancing carbon sequestration in the paddy fields.