Light Fraction Organic Carbon Research Articles

川西亚高山不同林龄粗枝云杉人工林土壤微生物生物量及酶活性

林分结构随林龄增加发生变化，致使土壤微环境变化，进而对土壤微生物生物量和酶活性产生直接或间接影响。以川西亚高山米亚罗林区25、40、50、60年生粗枝云杉（Picea asperata）人工林表层（0-20 cm）土壤为研究对象，分析不同林龄表层土壤微生物生物量碳（MBC）、氮（MBN）和酶活性的变化以及驱动土壤酶活性变化的主要环境因子，为人工林生态系统恢复效果评价及经营管理提供科学依据。结果表明：随着林龄的增加，MBC和MBN先上升后下降，在40年生达到最大；β-葡萄糖苷酶（βG）、β-N-乙酰氨基葡萄糖苷酶（NAG）和多酚氧化酶（PHO）活性与微生物生物量碳氮变化趋势相似，其中βG、NAG活性在50年生人工林达到最大，PHO活性于40年生人工林达到最大；50年生人工林过氧化物酶（PEO）活性显著低于其他林龄人工林；纤维素水解酶（CBH）活性随林龄增加而显著增加。冗余分析显示，碱解氮（AN）、pH和可溶性有机碳（DOC）是影响土壤酶活性变化的主导因子，分别解释了土壤酶活性变异的65.4%、9.7%和7.6%。综上可知，林龄对粗枝云杉人工林表层土壤微生物生物量和酶活性变化产生了显著影响，60年粗枝云杉林地力呈现衰退态势。为此，可对近60年的粗枝云杉人工林增加碳氮元素（尤其是氮素）的输入，以提升土壤质量和酶活性。;Stand structure changes with forest age caused the variation of soil microenvironment, which can directly or indirectly affect soil microbial biomass carbon and nitrogen, and soil enzyme activities. To better understand the change characteristics of soil microbial biomass contents and enzyme activities along an age sequence of spruce plantations, this study was conducted to evaluate the theoretical basis for restoration and effective management of subalpine plantations. In this study, Picea asperata plantations of various ages (25, 40, 50, and 60 years old) were selected in Miyaluo forest area of western Sichuan, China. The soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN) and pH, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and light fraction organic carbon (LFOC), soil enzyme activity index(β-glucosidase, βG; Cellulose hydrolysis, CBH; β-N-acetylglucosaminidase, NAG; Phenol oxidase, PHO and Peroxidase, PEO) in 0-20 cm soil was measured,respectively. The results showed that the MBC and MBN increased up to 40 years, and then decreased with the increase of age. There was a similar trend in changes of the activities of βG, NAG and PHO, while βG and NAG were highest at 50 years, and PHO was highest at 40 years. PEO activity of 50 years was significantly lower than other plantations. The activity of CBH increased with the increasing age of Picea asperata plantation stands. Redundancy analysis (RDA) indicated that AN, pH and DOC were main influence factors of soil enzyme activities in the top-layer soil, accounted for 65.4%, 9.7% and 7.6% of total variation of soil enzyme activities, respectively. Overall, our results suggested that the growth of Picea asperata had a significant effect on soil microbial biomass and enzyme activities. The soil fertility in 60 years of Picea asperata plantation was the lowest in all Picea asperata plantations. Therefore, we can increase carbon and nitrogen elements (especially nitrogen) devotion to improve soil quality and enzyme activities in Picea asperata plantations nearly 60 years.

Effects of long-term straw return on soil organic carbon fractions and enzyme activities in a double-cropped rice paddy in South China

Long-term straw return is an important carbon source for improving soil organic carbon (SOC) stocks in croplands, and straw removal through burning is also a common practice in open fields in South China. However, the specific effects of long-term rice straw management on SOC fractions, the related enzyme activities and their relationships, and whether these effects differ between crop growing seasons remain unknown. Three treatments with equal nitrogen, phosphorus, and potassium nutrient inputs, including straw/ash and chemical nutrients, were established to compare the effects of straw removal (CK), straw return (SR), and straw burned return (SBR). Compared to CK, long-term SR tended to improve the yield of early season rice (P=0.057), and significantly increased total organic carbon (TOC) and microbial biomass carbon (MBC) in double-cropped rice paddies. While SBR had no effect on TOC, it decreased light fraction organic carbon (LFOC) in early rice and easily oxidizable organic carbon (EOC) in late rice, significantly increased dissolved organic carbon (DOC), and significantly decreased soil pH. These results showed that MBC was the most sensitive indicator for assessing changes of SOC in the double-cropped rice system due to long-term straw return. In addition, the different effects on SOC fraction sizes between SR and SBR were attributed to the divergent trends in most of the soil enzyme activities in the early and late rice that mainly altered DOC, while DOC was positively affected by β-xylosidase in both early and late rice. We concluded that straw return was superior to straw burned return for improving SOC fractions, but the negative effects on soil enzyme activities in late rice require further research.

Rice-straw mat mulching improves the soil integrated fertility index of apple orchards on cinnamon soil and fluvo-aquic soil

Mulching management have been proposed for protecting the soil surface in farmland. This study was a three-year field experiment to evaluate the effects of rice-straw mat mulching (RSMM) on the soil integrated fertility index (IFI). We measured the soil total organic carbon (TOC) and labile carbon fractions, and soil microbial community functional diversity in cinnamon soil and fluvo-aquic soil orchards. The RSMM significantly increased the contents of soil TOC, particulate organic carbon (POC), easily oxidized carbon (EOC) and available phosphorus (AP) in both orchards. Additionally, the soil light fraction organic carbon (LFOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) contents, and soil urease activity were significantly increased in the cinnamon soil orchard, and the soil alkaline nitrogen (AN) and available potassium (AK) contents increased in the fluvo-aquic soil orchard under RSMM. The microbial metabolic utilization of carbohydrates and amino acids, and the soil microbial functional diversity and apple fruit quality were improved by RSMM, while the microbial communities under RSMM had different metabolic functions in the two orchards. A comprehensive evaluation of soil quality based on the weighted coefficients determined by a principal component analysis of 17 fertility indicators showed that RSMM increased the IFI by 97.9 and 138.5 % in the cinnamon soil and fluvo-aquic soil orchards, respectively. The study demonstrated that RSMM can effectively improve the soil IFI by altering the soil organic carbon content and microbial community diversity, with the effects being more sensitive for the fluvo-aquic soil orchard than the cinnamon soil orchard.

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.

Comparison of soil active organic carbon fractions in different vegetation zones in purple-soil of hill slope

To explore the effect of vegetation restoration on soil carbon cycle and fractions of soil organic carbon pool on purple-soil hill slope in Hengyang City of Hunan Province, China was selected. The soil samples of 0 - 10 and 10 - 20 cm soil layers under three types of vegetation, i.e., grassland zone (GZ), grassland-forest zone (GFZ) and forest zone (FZ). The dynamics of soil active organic carbon (SAOC) fractions to provide theory basis for the influence of soil carbon cycle and different vegetation zones on the fractions of organic carbon pool and its stability. Results show: Microbial biomass carbon and easily oxidizable organic carbon exhibited a decreasing pattern: FZ, GZ, GFZ (p &lt; 0.05); Dissolved organic carbon exhibited a decreasing pattern: FZ, GFZ, GZ (p &lt; 0.05); Light fraction organic carbon was the highest in FZ (p &lt; 0.05), and the second in GZ and GFZ; The availability of active organic carbon in 0 - 10 cm soil layer was higher than that of 10 - 20 cm soil layer (p &lt; 0.05); In comparison with GFZ, the herb in GZ could increase the contents of active organic carbon.

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.

Composition Characteristics of Organic Matter and Bacterial Communities under the Alternanthera philoxeroide Invasion in Wetlands

The influence of Alternanthera philoxeroide (alligator weed) invasion on wetland organic matter (OM) accumulation and bacterial changes is rarely studied, but is possibly an important step for revealing the invasion mechanism. Thus, the distribution characteristics of light fraction organic carbon and nitrogen (LFOC and LFON), and heavy fractions organic carbon and nitrogen (HFOC and HFON) were analyzed. Sampling was done on two sediment depths (0–15 cm and 15–25 cm) of invaded and normal habitats of two natural wetlands and two constructed wetlands, and bacterial taxa and composition in surface sediments were also analyzed by high-throughput sequencing. In the surface sediments, the LFOC and LFON contents were significantly higher in the constructed wetlands (0.791 and 0.043 g·kg−1) than in the natural wetlands (0.500 and 0.022 g·kg−1), and the contents of the C and N fractions were also prominently higher in the invaded areas than in normal wetland habitats. The OM storage was relatively stable. Proteobacteria (55.94%), Bacteroidetes (5.74%), Acidobacteria (6.66%), and Chloroflexi (4.67%) were the dominant bacterial phyla in the wetlands. The abundance of Acidobacteria, Actinobacteria, and Gemmatimonadetes were significantly higher in the invaded areas than in the normal habitats. The relative high abundance-based coverage estimator (ACE) index in the constructed wetlands and invaded areas suggested the corresponding high bacterial diversity. The significant and positive relationship between Acidobacteria and organic nitrogen concentrations suggested their potential and positive interrelationships. This study demonstrated that the alligator weed invasion could significantly change the compositions of sediment organic matterand bacteria, thus further changing the nutrition cycle and wetland microhabitat.

Response of soil labile organic carbon fractions and carbon-cycle enzyme activities to vegetation degradation in a wet meadow on the Qinghai–Tibet Plateau

Vegetation degradation resulting from climate change and human activities in wet meadows is an important issue worldwide. This phenomenon is known to influence soil labile organic carbon (LOC) and enzyme activities due to changes in environmental conditions. However, little is known about the response of LOC and enzyme activities to vegetation degradation in high-altitude wet meadows. In this study, we examined the response of LOC and carbon-cycle enzyme activities to different intensities of vegetation degradation (i.e., non-degraded (ND), slightly degraded (SD) moderately degraded (MD), and heavily degraded (HD)) in a Tibetan wet meadow. The contents of soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC) and light fraction organic carbon (LFOC) and the carbon-cycle enzyme activities (i.e., cellulase, amylase and β-glucosidase) were investigated in two growing seasons (2016 and 2017). We found that the content of soil SOC and LOC fractions declined with increasing soil depth in each degraded level except for HD. Vegetation degradation significantly decreased the amount of SOC at depths of 0–10 cm and 10–20 cm, and this decrease was attributed to the relative reduction of carbon source input and higher carbon decomposition. Vegetation degradation also significantly reduced the contents of SWC, DOC, MBC, LFOC, amylase and β-glucosidase in the topsoil layers (0–10 and 10–20 cm). However, the corresponding contents in deeper soil layers had no significant differences. In addition, the SWC, DOC, LFOC and the carbon-cycle enzyme activities were higher in 2016 than in 2017. Significant correlations were obtained between SWC, SOC, LOC fractions and enzyme activities. Soil moisture was found to be the main abiotic driver for variation of soil carbon and enzyme activities. Our results indicate that vegetation degradation in the Tibetan wet meadows decreased the quantity of topsoil labile carbon fractions and enzyme activities and heavily degraded vegetation may lead to a change of profile distribution in soil carbon pool.

Soil labile organic carbon fractions and soil enzyme activities after 10\xa0years of continuous fertilization and wheat residue incorporation

Labile organic carbon (LOC) fractions and related enzyme activities in soils are considered to be early and sensitive indicators of soil quality changes. We investigated the influences of fertilization and residue incorporation on LOC fractions, enzyme activities, and the carbon pool management index (CPMI) in a 10-year field experiment. The experiment was composed of three treatments: (1) no fertilization (control), (2) chemical fertilizer application alone (F), and (3) chemical fertilizer application combined with incorporation of wheat straw residues (F + R). Generally, the F + R treatment led to the highest concentrations of the LOC fractions. Compared to the control treatment, the F + R treatment markedly enhanced potential activities of cellulase (CL), β-glucosidase (BG), lignin peroxidase (LiP), and manganese peroxidase (MnP), but decreased laccase (LA) potential activity. Partial least squares regression analysis suggested that BG and MnP activities had a positive impact on the light-fraction organic carbon (LFOC), permanganate-oxidizable carbon (POXC), and dissolved organic carbon (DOC) fractions, whereas laccase activity had a negative correlation with those fractions. In addition, the F + R treatment significantly increased the CPMI compared to the F and control treatments. These results indicated that combining fertilization with crop residues stimulates production of LOC and could be a useful approach for maintaining sustainable production capacity in lime concretion black soils along the Huai River region of China.

Effects of different organic amendments on soil organic carbon and its labile fractions in the paddy soil of a double rice cropping system

Application of organic amendments is an effective approach for improving soil organic carbon and soil fertility. To investigate the effects of different organic amendments on soil organic carbon and its labile fraction content, a batch of incubation experiments was conducted on the fluvo-aquic soil in Dongting Lake region, Hunan Province. There were six treatments, including soil amended with rice straw, soil amended with Chinese milk vetch, soil amended with bio-organic fertilizer, soil amended with pig manure, and soil amended with rice straw-derived biochar, with unamended soil as control. Each treatment had the same amount of carbon input. After 180 days of incubation, application of organic amendments increased soil labile organic carbon content. Application of bio-organic fertilizer, pig manure and rice straw-derived biochar significantly increased soil organic carbon content by 26.1%, 9.7% and 30.7%, respectively. There was no significant change in soil organic carbon content in rice straw and Chinese milk vetch treatments which were more favourable to the accumulation of soil dissolved organic carbon and microbial biomass carbon. Pig manure was more favourable to the accumulation of soil dissolved organic carbon. Bio-organic fertili-zer could benefit the accumulation of soil microbial biomass carbon and readily oxidizable organic carbon. Rice straw-derived biochar could promote the accumulation of soil microbial biomass carbon and light fraction organic carbon. Compared with rice straw, soil carbon pool management index was increased by 31.8%, 111.6%, 62.2% and 50.7% in Chinese milk vetch, bio-organic fertilizer, pig manure and rice straw-derived biochar treatments, respectively. The performance of bio-organic fertilizer, pig manure, and rice straw biochar was better than rice straw and Chinese milk vetch from the perspective of soil carbon sequestration and soil carbon pool management index.

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.

Reduction of Soil Cadmium Activity and Rice Cadmium Content by 4-year-consecutive Application of Organic Fertilizer

Because commercial organic fertilizers may contain cadmium (Cd) and may cause the dual effect of "inhibition" and "activation" on Cd availability in paddy soil with organic fertilizer input, the reduction of rice Cd following organic fertilizer application is still uncertain. Herewith, typical purple mud paddy fields were selected in the eastern Hunan Province. The effect of commercial organic fertilizer input on Cd reduction of double-rice paddy ecosystem was monitored for four consecutive years. The relationships between brown rice Cd content, soil available Cd, and soil factors (pH, soil labile organic carbon fractions, and iron oxide) at different growth stages in double-rice paddy fields were investigated. Results showed that the input of organic fertilizer reduced the Cd content in brown rice by 28%-56%. Meanwhile, the decrease of Cd content in brown rice of late rice (43%-56%) was higher than that of early rice (28%-45%), and the inter-annual fluctuation of the decrease was relatively small. On the one hand, soil available Cd content decreased by 6%-7% during several growth stages of double-rice (from tillering peak stage to full heading stage) with organic fertilizer input. Additionally, the content of soil exchangeable Cd was decreased by 11%, whereas the content of organic bound Cd was increased by 14%. This directly reflects the decrease of soil Cd availability. On the other hand, the soil pH value was steadily increased by 0.1-0.3 units following organic fertilizer input, which promoted the development of soil from acidity to slight acidity. Besides, the content of soil active organic carbon (light fraction organic carbon, coarse particulate organic carbon, and fine particulate organic carbon) was increased significantly (53%, 77%, and 107%, respectively). This indirectly reflects the decrease in soil Cd availability. This study implies that the decrease of soil Cd availability may be the primary driving force for the reduction of rice Cd content with consecutive organic fertilizer input in purple mud paddy fields.

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.

Vegetation degradation along water gradient leads to soil active organic carbon loss in Gahai wetland

Soil active organic carbon responds quickly to soil disturbances and is a sensitive indicator of early changes in soil organic carbon (SOC). In order to identify the differences in the distribution of dissolved organic carbon (DOC), light fraction organic carbon (LFOC), SOC and their changes as affected by vegetation degradation degree along water gradient in wetland, we analyzed DOC, LFOC, and SOC in the 0–100 cm soil layer under four vegetation degradation degrees: non-degradation (ND), lightly degradation (LD), moderately degradation (MD) and heavily degradation (HD). The results showed that soil DOC, LFOC and SOC in the 0–100 cm layer of ND wetland was significantly higher than the other three degradation levels. DOC, LFOC and SOC contents decreased with increasing soil depth under the four degradation degrees and the contents of soil DOC, LFOC and SOC were mainly concentrated in the soil surface (0–20 cm). The DOC, LFOC, and SOC contents in the 0–20 cm layer under all four degradation levels showed obvious seasonal changes, while the DOC, LFOC, and SOC contents in the 20–100 cm layer showed little fluctuation over the plant growing season. There was a significant positive correlation between soil DOC and SOC, and between LFOC and SOC, with correlation coefficients of 0.948 and 0.911, respectively. There was also a very significant correlation between DOC and LFOC(R2 = 0.904). Soil DOC and LFOC in the 0–100 cm layer under the four degree of degradation were linearly correlated with SOC. While there was a linear correlation between DOC and LFOC in the non-degradation wetland soils, DOC and LFOC in the three degradation soils correlated exponentially correlated with SOC.

Soil organic carbon pool and chemical composition under different types of land use in wetland: Implication for carbon sequestration in wetlands

This study was conducted to understand how different wetland vegetation-land use types influenced the storage and stability of soil organic carbon (SOC) in surface soils. We determined the concentration and chemical composition of SOC in both density (including light fraction organic carbon (LFOC) and heavy fraction organic carbon (HFOC)) and particle size fractions (including <2μm, 2-63μm, 63-200μm and 200-2000μm) in four wetland land use types covered with different vegetation: lake-sedge, reed, willow and poplar wetlands. Results showed that the concentrations and stock of SOC and LFOC in willow and poplar wetlands were significantly higher than those in lake-sedge and reed. However, a higher proportion of alkyl-C and a lower proportion of O-alkyl-C were observed in lake-sedge and reed wetlands than in willow and poplar, suggesting that accumulated C in willow and poplar wetlands was less stable than that in lake-sedge and reed. For all particle-size fractions except the silt (2-63μm), the SOC concentrations were highest in willow and lowest in reed wetland surface soils, while their alkyl-C/O-alkyl-C (A/O-A) and hydrophobic-C/hydrophilic-C ratios progressively decreased from lake-sedge and reed wetland surface soils to poplar and willow surface soils. Moreover, the ratios of A/O-A and hydrophobic-C/hydrophilic-C in surface soils generally decreased with increasing concentrations of SOC in particle-size fractions, with these stability indexes being lowest in the largest particle-size fraction. These results indicate that the wetland vegetation-land use types that could incorporate more C into finer particle-size fractions had a greater potential for sequestering more stable C in such wetland ecosystems. Different wetland vegetation-land use types resulted in significant changes in the concentration and chemical structure of SOC, which could affect soil C sequestration and dynamics, C cycling in wetland ecosystems. Although both willow and poplar forests could increase SOC stock, the stability of SOC in willow wetland was higher. Therefore, on balance (stock and stability) the land use of wetland for willow forest could be a more promising way for enhancing soil C sequestration in wetlands.

Limited carbon contents of centuries old soils forming in legacy sediment

Accelerated erosion from European agriculture overwhelmed transport capacities of relatively low-gradient Piedmont watersheds and triggered widespread and massive sediment deposition on floodplains, referred to as legacy sediments. While erosion-driven soil loss on the Southern Piedmont landscape has been widely studied, the resultant legacy sediments deposited on hillslopes, floodplains, and in channels appear remarkably understudied. In the interim between deposition and future remobilization, legacy sediments are undergoing soil formation in ways that impact carbon stocks, nutrient retention, and a number of other soil-sediment functions. Here, we use a pedological framework to understand the dynamics of these legacy sediments by constraining the time since deposition and redevelopment of vegetation and the soil profile. In this study, we characterized the properties of legacy sediment and buried, pre-legacy sediment alluvium, established the timing of legacy sediment deposition and its volume in a 600-ha watershed, and analyzed the impact of accelerated sedimentation on floodplain soil carbon (SOC) storage.We found alluvial legacy sediment in the 6 km2 Holcombe's Branch watershed to range in thickness from 0.35 to 1.25 m, averaging 1.01 m in depth (n = 20, CV = 19%). Radiocarbon (14C) dates from buried wood indicate that accelerated deposition began by the mid-1800s. During the next century, deposition rates increased and peaked at levels up to 40× higher than contemporary sedimentation rates. By the mid-1900s, accelerated deposition rates had slowed significantly, as indicated by surficial distributions of 1960s fall-out 137Cs in sediments. Upstream floodplains are today less frequently flooded and function as abandoned floodplains, or terraces. In contrast, downstream sites remain active floodplains and zones of deposition. A century of pedogenesis has formed acidic, well-drained Entisols, characterized by incipient, redeveloping A horizons over coarse-textured C horizons. Buried, pre-legacy sediment alluvium have well-developed redox features, are lower in hue and chroma than legacy sediments, and occasionally have moderate to strong aggregate structure. A century-old mixed pine-hardwood forest has diminished subsoil carbon stocks and created a shallow vertical distribution of SOC stocks, that total 95 Mg C ha−1 (CV = 25%) in the upper 2 m. Legacy sediments have nearly doubled alluvial sediment in the catchment and account for 55% of soil carbon stock. Coarse texture and porous legacy sediment-soils limit accumulation of mineral-associated soil carbon. A-horizon morphology was preserved in buried A horizons underlying legacy sediment, yet these horizons did not account for significant SOC stocks at depth. Depth profiles of active, light-fraction organic carbon revealed differences in SOC preservation between upstream and downstream sites. The hydrogeomorphic change responsible for this difference - from abandoned legacy sediment terraces upstream to active sedimentation on floodplains downstream – will continue to impact soil-sediment dynamics and functioning. Time since last deposition and revegetation are dominant forces in SOC dynamics, while gradients in soil moisture may be creating subtle longitudinal patterns in SOC preservation at depth.

武夷山不同海拔森林表层土壤轻组有机质特征

土壤轻组有机质是土壤有机质的重要组分，研究轻组有机质在不同森林生态系统土壤中的变化规律对理解土壤有机质形成与转换具有重要意义。以福建省武夷山国家级自然保护区不同海拔的常绿阔叶林（海拔600 m）、针阔混交林（海拔1000 m）和针叶林（海拔1400 m）为研究对象，利用密度分组方法分离了表层（0-5 cm和5-10 cm）土壤轻组有机质，研究了不同海拔森林土壤轻组有机质特征及其影响因素。结果表明：针阔混交林表层土壤的轻组有机质含量大于针叶林和常绿阔叶林（P < 0.05），并且轻组有机碳的含量变化亦是如此（P < 0.05），而轻组有机氮的含量无显著差异（P > 0.05）。表层土壤对应土层的轻组C：N大于土壤C：N，针阔混交林轻组C：N和土壤C：N均大于其他林分类型。0-5 cm与5-10 cm土层针阔混交林的轻组有机碳、氮储量均大于针叶林和常绿阔叶林（P < 0.05），并且针阔混交林的轻组有机碳、氮储量所占土壤有机碳与总氮的比重均大于其余两种林分。0-10 cm土层针叶林土壤有机碳与总氮含量与储量最高，并随海拔降低而减小，但差异不显著（P > 0.05）。相关分析结果表明，轻组有机碳、氮储量与SOC、DOC、MBC和细根生物量具有显著相关关系（P < 0.05），而与年凋落物量无关（P > 0.05），说明地下细根可能是土壤轻组有机质的重要来源。因此，在未来气候和植被变化共同作用下，地下细根对土壤轻组有机质的形成可能具有不可忽视的作用。;Soil light fraction organic matter is an important component of soil organic matter. It is crucial to understand the relationship between soil organic matter and climate change by studying the variation of light fraction organic matter along an elevation gradient in subtropical China. Taking evergreen broad-leaved forest (EBF), mixed coniferous and broad-leaved forest (BCF), and coniferous forest (CF) at different altitudes (from low to high elevation) in Wuyishan National Park in Fujian Province as research objects, the light fraction organic matter in surface soil (0-5 cm and 5-10 cm) was separated by density fractionation analyses, as well as the characteristics of light fraction organic matter and its influencing factors in different forest ecosystems were studied. The results showed that the content of light fraction organic matter in BCF was higher than that in CF and EBF in 0-5 cm and 5-10 cm soil layers (P < 0.05). The pattern of the content of light fraction organic carbon was similar to the light fraction organic matter, but there was no significant difference in light organic nitrogen content (P > 0.05). The light fraction C/N was greater than soil C/N, and the light fraction C/N and soil C/N in BCF were greater than those of other forest types in the same soil layer. In 0-5 cm and 5-10 cm soil layer, the storage of light fraction organic carbon and nitrogen in BCF were higher than those in CF and EBF (P < 0.05), and the contribution of light fraction organic carbon and nitrogen to soil organic matter in BCF was higher than that of the other two forests. The contents and storage of soil organic carbon and total nitrogen in CF was the highest in 0-10 cm, and decreased with the decrease of altitude, but the difference was not significant (P > 0.05). The correlation analysis showed that the storage of light fraction organic carbon and nitrogen was significantly correlated with soil organic carbon, dissolved organic carbon, microbial biomass carbon and fine root biomass (P < 0.05), but not annual litterfall (P > 0.05), suggesting that root derived organic matter might be an important source of light fraction organic matter in soil. Therefore, we concluded that belowground sources may play an important role in regulation of the formation of light fraction organic matter in soil under future climate change.

Converting evergreen broad-leaved forests into tea and Moso bamboo plantations affects labile carbon pools and the chemical composition of soil organic carbon

This study aimed to explore the effects of conversion from evergreen broad-leaved forests (EBFs) to tea plantations (TPs) and Moso bamboo (Phyllostachys heterocycla var. pubescens) plantations (MBPs) and the subsequent long-term intensive management on the soil carbon pool and the chemical composition of soil organic carbon (SOC). Soil samples from three layers (0–10, 10–30 and 30–60 cm, respectively) were collected from adjacent EBFs, TPs and MBPs in An’ji County, Zhejiang Province, China. The physico-chemical properties of soils, including bulk density, SOC and its different fractions were determined. The chemical composition of SOC was also measured using 13C-nuclear magnetic resonance spectroscopy (NMR). The results showed that conversion from EBFs to TPs and MBPs decreased the concentrations of water soluble organic carbon (WSOC), light and heavy fraction organic carbon (LFOC, HFOC) and humus carbon (HC) (P < 0.05), reduced the O-alkyl C and carbonl C content, but increased the alkyl C, Aromatic C, aromaticity and the ratio of alkyl C/O-alkyl C (A/O-A) (P < 0.05). These results suggested that intensive management markedly altered the chemical structure of SOC and labile carbon pools. Our results demonstrated that converting EBFs to TPs and MBPs had a negative effect on SOC content and a positive effect on SOC stability. Therefore, management practices such as rational fertilization and sod cultivation are recommended after land-use conversion.

Effects of Fe oxides on organic carbon variation in the evolution of clayey aquitard and environmental significance

As an important part of groundwater systems, aquitards may have a considerable impact on the quality of groundwater in an aquifer. Organic carbon (OC) serves as an important component in biogeochemical processes which affects the hydrochemical composition of pore water; the association of OC with (Iron) Fe-containing minerals is recognized as an important stabilization mechanism for OC. However, the characteristics of the process forming complexes between OC and Fe oxides in subsurface aquitards and the contribution of OC to aquifers has been poorly understood. In this study, the content, speciation and reaction types of OC and Fe oxides were investigated to reveal the characteristics and mechanisms of OC-Fe interaction in subsurface aquitards at different depths in two typical sedimentary facies of the Jianghan Plain in central China. The total organic carbon (TOC) contents of aquitard sediments in alluvial-lacustrine and residual slope facies were 7.25 ± 5.10 and 2.79 ± 0.86 mg g−1, respectively. In general, the proportion of the heavy fraction of OC (HFOC) to TOC gradually increased with increasing depth because consumption of the light fraction of OC (LFOC) that caused more HFOC to remain in sediment. On average, Fe bound-OC contributed 30.3% and 31.6% of TOC by adsorption and coprecipitation, respectively. The adsorption-stabilized OC has not changed obviously but the coprecipitation-stabilized OC increased gradually with depth. Coprecipitation stabilized more OC in a stable environment (residual slope facies) when compared with an unstable environment (alluvial-lacustrine facies), which can be supported by the greater average ratio of FePP bound-OC:TOC ratio and increased enrichment of carboxylates and aromatics in a stable environment. The transformation of OC-Fe complexes could affect the transport of As, Cr and ammonium which chemically bind to organic matter (OM) and Fe minerals from sediment to pore water by reductive dissolution. The findings of this study are helpful in understanding the interaction between OC and Fe oxides in subsurface aquitard environments, in which reaction products may affect adjacent aquifers.