Characteristics Of Soil Organic Carbon Research Articles

黄土丘陵区退耕小流域土壤有机碳分布特征及地形植被对其的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 黄土丘陵区退耕小流域土壤有机碳分布特征及地形植被对其的影响 DOI: 10.5846/stxb202107081840 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点研发计划项目（2021YFD1900700）；陕西省杰出青年科学基金项目（2021JC-19） Distribution characteristics of soil organic carbon and the effect of topographic and vegetation on small watershed of returned farmland in the loess hilly region Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:针对黄土丘陵区退耕还林（草）工程实施20年固碳效果研究薄弱的问题，以典型退耕小流域为对象，在不同地形（峁坡、沟肩、沟谷）和植被类型（次生草地、撂荒山杏林、撂荒坡耕地）共布设147个样点采集0-100 cm土层样品并测定，以研究土壤有机碳（SOC）分布特征及地形、植被对其的影响。结果表明：小流域峁坡剖面（0-100 cm）土层SOC含量平均为2.43 g/kg。地形和植被类型对小流域SOC分布特征产生了重要影响：沟肩表层和剖面SOC含量均最高且显著（P<0.05）高于沟谷，但与峁坡无显著差异；次生草地表层（0-20 cm）和亚表层（20-40 cm） SOC含量均显著（P<0.05）高于撂荒山杏林和撂荒坡耕地。地统计学分析显示小流域0-20 cm土层SOC含量有最大块金值且块金系数为49.6%，即表层SOC具有最大块金效应且受到结构因素与随机因素共同影响；剖面SOC分布格局表现出与表层土壤相似的特征。总之，退耕还林（草）碳汇效应显著，且在地形和植被类型作用下呈现显著的空间异质性特征。 Abstract:The "Grain to Green" project has been implemented for more than 20 years in the loess hilly region. However, its carbon sequestration effect is rarely evaluated. To this end, a total of 147 sample sites were collected in different topography (hillslope, gully shoulder, and gully) and vegetation types (secondary grassland, abandoned apricot forest, and abandoned slope cropland) in a small watershed in the region to collect samples from 0-20 cm, 20-40 cm, 40-60 cm, 60-80 cm and 80-100 cm soil layers. The distribution characteristics of soil organic carbon (SOC) and the influence of topography and vegetation were studied by statistics and geostatistics. The results showed that the average SOC content in the 0-100 cm profile of Xifuzigou watershed was 2.43 g/kg. Topography and vegetation types had important effects on the distribution characteristics of the SOC in small watershed. The SOC content in the surface and profile of gully shoulder was the highest and significantly higher (P<0.05) than gully, but had no significant difference with hillslope. The SOC in the surface layer (0-20 cm) and subsurface layer (20-40 cm) of the secondary grassland was significantly higher (P<0.05) than that of abandoned apricot forest and abandoned slope cropland. Geostatistical analysis showed that the 0-20 cm soil layer had the largest nugget and the nugget coefficient was 49.6%, and the 0-100 cm profile's nugget coefficient was 15.7%. The results indicated that the surface SOC had the largest nugget effect and was influenced by both structural and random factors, and the profile SOC had strongly spatial correlation which distribution was dominated by structural factors. The distribution pattern of the SOC in 0-100 cm profile was similar to that in surface soil which high values of SOC content were distributed in the secondary grassland and abandoned apricot forest, while the low values were mainly in abandoned slope cropland and dam land. At the small watershed scale, the distribution of SOC was consistent with vegetation types, but had no significant correlation with microtopography (slope, aspect and position), which indicated that the land-use/land cover dramatic changes weakened the effect of topography on SOC distribution. However, the results cannot reflect the carbon sequestration effect in the region because there are great differences in landform in the loess hilly region, and the implementation of the project varies greatly in shape and form. In conclusion, the carbon sink effect of "Grain to Green" is significant, and presents significantly spatial heterogeneity under the effect of terrain and vegetation type. 参考文献 相似文献 引证文献

Mechanism and Evolution of Soil Organic Carbon Coupling with Rocky Desertification in South China Karst

To study the spatial distribution characteristics of soil organic carbon (SOC) coupled with rocky desertification, 1212 soil samples from 152 soil profiles were sampled from different karst landforms, including karst low hills/virgin forest (KLH) in Libo County, a karst peak-cluster depression (KPCD) in Xingyi County, a karst canyon (KC) in Guanling County, a karst plateau basin (KPB) in Puding County and a karst trough valley (KTV) in Yinjiang County. The spatial distribution characteristics of the responses of SOC, SOC density (SOCD), rocky desertification and soil bulk density (SBD) to different influencing factors were analyzed. The relationships among SOC, SOCD, rocky desertification and SBD were analyzed using Pearson correlation analysis. The SOC storage capacity was characterized by using SOCD, and then the SOC storage capacity in different evolution stages of karst landforms was assessed. The SOC contents of KLH, KPCD, KC, KPB and KTV ranged from 6.16 to 38.20 g·kg−1, 7.42 to 27.08 g·kg−1, 6.28 to 35.17 g·kg−1, 4.62 to 23.79 g·kg−1 and 5.24 to 37.85 g·kg−1, respectively, and their average SOCD values (0–100 cm) were 7.37, 10.79, 7.06, 8.51 and 7.84 kg·m−2, respectively. The karst landforms as ordered by SOC storage capacity were KPCD &gt; KPB &gt; KLH &gt; KTV &gt; KC. The SOC content was negatively correlated with the SBD; light rocky desertification may lead to SOC accumulation. The rocky desertification degree and SBD were closely associated with slope position and gradient. Rocky desertification first increased, then decreased from mountain foot to summit, and increased with increasing slope gradient. However, the SBD decreased from mountain foot to summit and with increasing slope gradient. The SOC contents on the northern aspect of the mountains were generally higher than the other aspects. In summary, rock outcrops controlled the SOC contents in the studied regions. The slope position, gradient and aspect influenced the composition and distribution of vegetation, which influenced the evolution of rocky desertification. Therefore, these factors indirectly affected the SOC content. Additionally, the SOCD decreased with increasing rocky desertification. During the different evolution stages of karst landforms, the SOC storage capacity first decreases, then increases.

Soil erosion impacts on nutrient deposition in a typical karst watershed

Soil erosion has a significant influence on nutrient redistribution and deposition. However, the effect of soil erosion on nutrient deposition remains unclear in karst areas such as southwest China, which represents an ecologically fragile zone experiencing severe soil erosion. The objective of this study was to investigate the characteristics of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) deposition in a karst watershed of southwest China over the past 60 years and evaluate the relationship between soil erosion and nutrient deposition. The peak-cluster depressions in southwest China are typical for the geomorphological type, which is an ideal place to determine the sediment chronology, and the estimation of sediment and nutrient deposition rates. The 137Cs, 210Pbex, particle size distribution, and nutrients of 194 soil samples in three sediment cores retrieved from a karst depression were investigated to evaluate the effect of soil erosion on nutrient deposition. Results showed that there was a significant negative correlation between nutrient concentrations and clay content (P < 0.001). Generally, compared with 137Cs, 210Pbex had a higher correlation with SOC and TN. In an undisturbed sediment profile, Pb/Cs can reflect nutrient dynamics better than a single nuclide. The nutrient deposition rates increased before 1953, reached its maximum in 1954–1956, and then dropped rapidly from 1957 to 2015. The sediment deposition rates were negatively correlated with nutrient concentrations (P < 0.01), but had a positive influence on nutrient deposition rates (P < 0.01). This implied that the temporal variation in nutrient deposition rates over the past 60 years was dominated by soil erosion rather than nutrient concentrations. This study provides a new insight to explore the historical nutrient deposition rates in a peak-cluster karst depression, and may help effectively control soil erosion and sustainable development of agro-ecosystems.

Distribution characteristics of surface soil organic carbon in different land types in the loess hilly region

Management of land and soil resources is important in the loess hilly region to explore the change characteristics of soil organic carbon in different land types. Four major land use types, including jujube orchard, 7-yr-old grassland, Cropland, and 30-yr-old grassland, were selected to study the distribution characteristics of soil organic carbon content from 0-100 cm in the small watershed of Yuanzegou watershed in the loess hilly region. The results showed that the soil organic carbon content of different land use types decreased with soil layer increasing. The contents of soil organic carbon at different depths were as follows: 7-yr-old grassland < Cropland < jujube orchard < 30-yr-old grassland, and there were significant differences among different land use types in 0-20 cm soil layer (P<0.05).

Characteristics of soil organic carbon under different karst landforms

Characteristics of soil organic carbon under different karst landforms

Response of the chemical structure of soil organic carbon to modes of maize straw return

Elucidating the chemical structure of soil organic matter (SOM) is important for accurately evaluating the stability and function of SOM. Aboveground vegetation directly affects the quantity and quality of exogenous organic matter input into the soil through plant residues and root exudates, which in turn affects soil microbial species, community structure, and activity, and ultimately impacts the chemical structure of SOM. In this study, a 13C nuclear magnetic resonance technique was used to analyze the chemical structure characteristics of soil organic carbon (SOC) under various rates of straw returning combined with rotary tillage and under full straw mulching. The results showed that full straw returning with rotary tillage and full straw mulching more effectively increased the SOC content than reduced rate of straw returning (1/2 and 1/3 of full straw) with rotary tillage. The contents of alkyl C and alkoxy C in the functional groups of SOC under various straw returning treatments were increased compared with those under the treatment of maize stubble remaining in soil (CK). Furthermore, the contents of aromatic C and carboxyl C were decreased, which were consistent with the chemical shift changes of SOC. Compared with CK treatment, straw returning decreased the content of aromatic C in the functional groups of SOC, but increased the content of alkoxy C, which could be associated with the change in integral areas of absorption peaks of alkyl C and alkoxy C moving toward left and right, respectively. The content of total SOC was significantly positively (P < 0.05) correlated with that of alkoxy C and significantly negatively (P < 0.01) correlated with that of aromatic C. The molecular structure of SOC tends to be simplified due to the decreasing in refractory C and the increasing in easily decomposed C after straw returning to the field.

Effects of ant disturbance on soil organic carbon cycle: A review.

The impact of ants, as consumer and decomposer in ecosystems, on soil organic carbon pool is a hotspot in soil fauna research. Recently, effects of ants on soil organic carbon pool dynamics have been mainly documented in terms of soil macro-elements storage, physical and chemical properties, and microbial community. We summarized the effects of ant disturbance on soil organic carbon cycle. Ant nesting alters microhabitat, microclimate, and physicochemical properties of ant nested soils. By re-shaping soil microbial community structure, regulating the succession process and pattern of vegetation, ant nesting directly or indirectly affects the source, allocation, stability, and microcosmic molecular characteristics of soil organic carbon, scaling from micro, local, to landscape scale. In the future, more attentions should be paid on quantifying the contribution of ant disturbance combined with the fluctuations in environmental factors to soil carbon flux dynamics, establishing quantitative model to link and unify ant-induced soil carbon cycle process, and clari-fying mechanisms underlying the impacts of ants on the stability of soil organic carbon pool, and finally revealing the "ecological engineers" role of ants in regulating soil organic carbon cycling.

Soil C, N and P stocks and stoichiometry under different vegetation on the surface of the Leshan Giant Buddha

The accumulation of soil organic matter and nutrients is an important pathway in effectively understanding the mechanisms of plant settlement and rock weathering, while the characteristics of soil organic carbon (C), nitrogen (N) and phosphorus (P) under different vegetation remain unclear. In this study, the stocks and stoichiometry of soil organic C, N and P were determined in different positions and types of vegetation on the surface of the Leshan Giant Buddha. We found that the total stocks of soil organic C, N and P were 1689.77, 134.6 and 29.48 kg, respectively, for the Buddha. The stocks of soil organic C, N and P under vascular plants were higher than those under other vegetation, with highest values observed under herb. Higher stocks per unit area (m2) of soil organic C, N and P were found on the left and right arms, shoulders, and two platforms. These results provide a full primary picture in understanding soil organic C, N and P accumulation and distribution on the surface of the Buddha, which could supply the fundamental data on weathering management of the Buddha and other similar open-air stone carvings.

The Spatial Distribution Characteristics of Soil Organic Carbon and Its Effects on Topsoil under Different Karst Landforms.

Karst landforms are widely distributed in Guizhou Province, and the karst terrain is complex. To investigate the spatial distribution characteristics of soil organic carbon (SOC) in topsoil in different karst landforms, a total of 920 samples were taken from different karst landforms. The study areas, Puding, Xingyi, Guanling, Libo and Yinjiang in Guizhou Province, represent the karst plateau (KP), karst peak-cluster depression (KPCD), karst canyon (KC), karst virgin forest (KVF) and karst trough valley (KTV) landforms, respectively. The characteristics of the SOC contents in areas with different vegetation, land use and soil types under different karst landforms were analyzed. The dimensionality of the factors was reduced via principal component analysis, the relationships among SOC content and different factors were subjected to redundancy analysis, and the effects of the main impact factors on SOC were discussed. The results showed that there was a large discrepancy in the SOC contents in the topsoil layers among different types of karst landforms, the changes in the SOC content in the topsoil layer were highly variable, and the discrepancy in the upper soil layer was higher than that in the lower soil layer. The SOC contents in the 0–50 cm topsoil layers in different karst landforms were between 7.76 and 38.29 g·kg−1, the SOC content gradually decreased with increasing soil depth, and the descending order of the SOC contents in different karst landforms was KTV > KVF > KC > KPCD > KP.

Seawall effects in a coastal wetland landscape: spatial changes in soil carbon and nitrogen pools

Seawalls are common features in coastal landscapes. They can alter ecological processes in coastal wetland ecosystems at multiple scales. Evaluation those ecological effects requires consideration of spatial characteristics of variables. We used a scaling method, quadrat variance analysis, to quantify the patterns and scale characteristics of soil organic carbon (SOC) and total nitrogen (TN) in a wetland reclaimed by a 30-year-old seawall and a natural wetland in the Yellow River Delta, China, and then analyzed their spatial relationships with different plant and soil variables. The results revealed that spatial variances in SOC and TN in the seawall-reclaimed wetland had two scales. The smaller scale (40 m) was the distance between soil salinity patches, reflecting the influences of soil salinity on SOC and TN. The larger one (130 m) was the distance between shrub communities and the grass patches beneath them, reflecting the influences of shrubs on SOC and TN. However, in the natural wetland, both SOC and TN had only one scale of variance (90 m), which reflected the influences of soil salinity. Soil salinity determined the spatial patterns of dominant grass patches, and thus SOC and TN. Seawall altered plant distributions and shrub–grass interactions, thereby affected the patterns of SOC and TN. Scaling method can help us to efficiently evaluate the landscape impacts of seawalls on coastal wetlands.

Differences in properties of long-term planting of different formulated soils

The characteristics of soil organic carbon, polysaccharides and calcium carbonate content in different proportions of compound soil were studied in the compound soil of arsenic rock zone and sand with rotation for 2 years, and the ripening of the mixed soil of arsenic rock zone and sand in different proportions was revealed. The results showed that the ratio of easily oxidized organic carbon in the 1:2 ratio of arsenic rock zone to sand is the highest in total organic carbon, the activity of organic carbon is the highest. And the content of polysaccharides and calcium carbonate in soil is higher, the water-soluble aggregates in soil were better developed.

Stability characteristics of soil organic carbon pool following development of sand-fixing forest in Mu Us sandy land, China

The study aimed to reveal the cumulative effects and stability characteristics of soil organic carbon (SOC) during forest development at the Mu Us sandy land, China. Using space for time substitution, surface soil samples were collected from semi-fixed sandy lands and restored arbor and shrub lands with the ages of 22, 32 and 53 years in the Yulin sand control region in Northern Shaanxi Province. The content of total organic carbon (TOC), oxidizable labile organic carbon, and resistant carbon and the characteristics of mineralized carbon emission and decomposition ratio were analyzed. The results showed that the increment of TOC for 22 to 53 years shrub and arbor lands from resistant carbon were 3.5-6.2 g·kg-1 and 3.2-7.7 g·kg-1, and from oxidized labile carbon were 2.8-3.4 g·kg-1 and 1.3-2.8 g·kg-1, respectively, compared with semi-fixed sand land. The ratio of soil oxidizable labile carbon in shrub land and arbor land were stable and maintained at 37.0% and 26.8%, respectively. However, the ratio decreased to 25.7% and 17.4% after incubated at a constant temperature for 60 days. The mineralization rate of shrub and arbor lands with 22-53 years was not significant at the ending of soil incubation. Carbon losses from oxidized liable carbon were 76.9%-98.7%, and only 1.3%-23.5% from resistant carbon in all sand-fixing forest plots. Compared with the maximum carbon emission rate, the soil cumulative carbon release exhibited a higher correlation with soil enzyme activities of β-glucosidase and dehydrogenase, but the enzyme activities did not change from 32-53 years. In conclusion, SOC pool showed stable characteristics of lower emission and higher sequestration with the increases of sand-fixing forest stand age. The carbon fixation effect of arbor sand-fixing forest was better than that of shrub sand-fixing forest.

Co-accumulation characters of soil organic carbon and nitrogen under broadleaved Korean pine and Betula platyphylla secondary forests in Changbai Mountain, China.

The retrogressive succession is an important driver for dynamics of soil organic carbon (SOC) and total nitrogen (TN). We studied the quantitative distribution and synergistic accumulation characteristics of soil organic carbon and nitrogen in the primary broadleaved Korean pine (KP) forest and Betula platyphylla (BP) secondary forest in Changbai Mountain through paired plot approach. Further, we analyzed the changes of carbon pool and carbon sink effect in temperate forest soil caused by secondary succession and their carbon-nitrogen coupling mechanism. The results showed that the BP forest accumulated more organic carbon and nitrogen in the surface and subsurface soil (0-20 cm) than the KP forest, with relatively low soil C/N. Compared with KP forest, soil organic carbon storage in BP forest (0-20 cm) was higher by 14.7 t·hm-2, equivalent to a soil carbon sink gain of 29.4 g·m-2·a-1. SOC and TN concentrations were positively correlated in each soil layer of all forest types, causing a co-accumulative relationship between SOC and TN. The coefficient of determination (R2) between SOC and TN in the upper soil layers of BP forest was significantly higher than that of the KP forest, indicating that SOC accumulation under the relatively N-rich BP forest was more dependent on the accumulation of organic nitrogen. In the upper soil layers (0-10 cm) where organic matter concentrated, there was no significant difference in light fraction organic carbon and nitrogen stock between the two forest types, whereas the content, stock, and allocation percentage of heavy fraction organic carbon and nitrogen of BP forest were all significantly higher than that of the KP forest, with an average increment of 8.5 t·hm-2 in heavy fraction organic carbon stock. Those results indicated that the increase of soil organic carbon and nitrogen during secondary succession was mainly due to the increases of soil organic carbon and nitrogen pools in mineral-bound stability. The carbon-nitrogen coupling mechanisms in litter decomposition and soil organic matter formation was an important driving mechanisms underlying the changes of soil organic carbon and nitrogen pools during secondary succession.

Characteristics of Soil Organic Carbon and Mineralization with Different Stands in Jinyun Mountain

Soil organic carbon (SOC) is the largest organic carbon stock on land, and slight changes in SOC can significantly affect the atmospheric CO2 concentration, among which forest soil carbon reservoir accounts for approximately 70% of the global soil carbon stock. Therefore, the implementation of efficient management for SOC stock in the forest ecosystem has become a popular research subject. The mineralized characteristics of SOC in different soil layers (0-20, 20-40,40-60, and 60-100 cm) were analyzed in five typical stands of Jinyun Mountain:broadleaf forest, coniferous forest, coniferous and broadleaf mixed forest, bamboo forest, and 15a abandoned grassland (control soils) in the study area. The results showed that forest type, cultivation duration, and soil depth had significant effects on the SOC mineralization rate. The mineralization rate of SOC in different forest stands decreased with the deepening soil layer, among which the mineralization rate at the 0-20 cm soil layer[11.97-25.12 mg·(kg·d)-1] was significantly higher than that of other soil layers (P<0.05), and there were no significant differences between the mineralization rates of other soil layers[4.79-6.51 mg·(kg·d)-1]. The accumulated mineralization of SOC in the five forests decreased with the deepening soil layer. The accumulated mineralization of SOC in the bamboo forest and broadleaf forest in the 0-20 cm soil layer was the highest at 177.66 mg·kg-1 and 120.38 mg·kg-1, respectively. With the deepening soil layer in the 60-100 cm soil layer, the accumulated SOC mineralization in the coniferous forest reached the highest (46.96 mg·kg-1). The SOC mineralization process in the different stands of Jinyun Mountain can be well fitted by the double reservoir first-level kinetic equation. The content of easily decomposable SOC in different forest stands decreased with the deepening soil layer. Coniferous forest soil exhibited a stronger mineralization ability and higher utilization degree of refractory organic carbon stock, while bamboo forest and broadleaf forest soils had higher microbial activity, which could effectively promote the carbon cycle and improve the soil carbon fixation ability.

Impacts of land use change on soil organic matter chemistry in the Everglades, Florida - a characterization with pyrolysis-gas chromatography–mass spectrometry

Reclamation of marshlands for agricultural land use is widespread. An example of large-scale land use shifts is evident in the Everglades Agricultural Area of Florida where freshwater wetlands were converted to sugarcane (Saccharum officinarum) production. Such land-use shifts can markedly influence soil organic matter cycling and processes. The effect of such land use change on molecular characteristics of soil organic carbon is unknown. In this study, three surface soils cores (0–5 cm, 5–10 cm, and 10–19 cm) from a representative marsh and a sugarcane field in the Florida Everglades with known land use history and management practices were characterized using pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS) to investigate molecular characteristics of soil organic matter. Soil organic carbon (SOC), total nitrogen, total sulfur, and water extractable organic carbon contents in sugarcane soil were 37.02%, 2.44%, 0.46% and 0.11%, while they were 44.57%, 2.62%, 0.73% and 0.38%, respectively. On the whole, marshland reclamation to sugarcane caused increase of aromatics (from 11.37% to 13.20%), polyaromatic hydrocarbon (from 0% to 1.63%), n-alkanes (from 8.90% to 16.17%), n-alkenes (from 25.10% to 28.77%), other aliphatics (from 7.33% to 11.77%) and fatty acids (from 1.93% to 3.87%) in pyrolysis products of soil organic matter (SOM), whereas this shift reduced lignin (from 12.87% to 2.47%), phenols (from 23.33% to 16.13%), polysaccharides (from 2.40% to 1.03%), benzofuran (from 1.67% to 0.27) and nitrogen containing compounds (from 5.07% to 4.67%), respectively. Overall, the conversion of marsh to sugarcane land use not only reduced total SOC contents but also altered molecular features of SOM with significant increases in aliphatic fractions and almost complete exhaustion of lignin fractions, signifying SOC dynamic shift due to land change in the Everglades.

Spatial distribution characteristics of soil organic carbon in subtropical forests of mountain Lushan, China.

The study on the spatial distribution of forest soil organic carbon (SOC) is of great significance for accurate assessment of carbon storage in forest ecosystems. In the present study, by taking eight kinds of forest soils of Mountain Lushan in the subtropical area as the research object, we studied the spatial distribution characteristics of SOC in this mountainous area. The results showed that the SOC content and SOC density (SOCD) of main forest types in the Mountain Lushan were lower than the national and the world average. The soil layer of Lushan forest was thinner, and the SOC and active SOC (ASOC) contents of different forest types and SOCDs are the highest in the surface soil. SOCD of the topsoil accounts for 32.64-54.03% of the total SOCD in the whole soil profile. Surface litter is an important source of SOC, and the different vegetation types are the important reason for the different spatial distribution of SOC in this area. Soil SOC contents in the high-altitude forest (bamboo forest, deciduous broadleaf forest, Pinus taiwanensis forest, evergreen-deciduous forest, and coniferous-broadleaved mixed forest) were higher than those in the low-altitude forest (evergreen broadleaf forest, shrub, and Pinus massoniana forest). However, the difference in SOC content exhibited at the altitude gradient is significantly lower than that in SOC in the soil profile. This indicates that both soil depth and elevation are the important factors that affected SOC distribution. However, the influence of soil depth on spatial distribution of SOC may be more complex than that of altitude. Vegetation types and soil properties are the main reasons for the large differences of reduction rate in the contents of SOC and ASOC.

Phenanthrene adsorption on soils from the Yangtze River Delta region under different pH and temperature conditions.

The phenanthrene (PHE) adsorption on soils from the Yangtze River Delta region under different pH and temperature conditions was studied in the laboratory. Results showed that the sorption of PHE on all soils was nonlinear and fitted to the Freundlich isotherm. The PHE adsorption on the soils is related to the content of organic carbons and the environmental conditions. There was a positive correlation (the correlation coefficient was 0.956) between the PHE adsorption and the soil organic carbon content. Adsorption on the soils at 15°C ambient temperature was higher than at 25°C, which was related to PHE solubility enthalpy. Adsorption on the soils in background solution at pH 5.0 was higher than in those at pH 6.2 and 7.5, which may be related to alteration of the hydrophobic character of humic substances. This study showed that intrinsic organic carbons influenced the adsorption of PHE, which was affected by environmental conditions, such as pH and temperature. Therefore, the characteristics of soil organic carbon should be considered first for implementing effective schemes for the remediation of contaminated soils and in the formulation of soil environmental quality standards.

Correlation Between Soil Organic and Inorganic Carbon and Environmental Factors in Cotton Fields in Different Continuous Cropping Years in the Oasis of the Northern Tarim Basin

Studying the characteristics of soil organic carbon (SOC) and soil inorganic carbon (SIC), and their relationship with environmental factors is helpful for understanding soil ecological processes, and providing theoretical support for research on the global carbon budget. Based on the oasis in the northern margin of Tarim Basin, this study analyzed the characteristics of the soil carbon pool, and, combined with redundancy and path analyses, explored the relationship between SOC and SIC, and their environmental factors. The results showed that there was a significant difference (P<0.05) in SOC and SIC contents in cotton fields with different continuous cropping years (P<0.05). With an increase in continuous cropping years, the SOC content increased firstly and then decreased, while the SIC content decreased firstly and then increased (P<0.05). The SOC content was the highest in the 0-20 cm layer, while the SIC content was the highest in the 20-50 cm layer. The importance of environmental factors on the characteristics of the soil carbon pool was deduced by redundancy analysis. The order of importance was phosphatase activity > pH > sucrase activity > catalase activity > total N > available P > available K > soil moisture content > urease activity > electrical conductivity (EC); phosphatase activity, pH value, invertase activity, catalase activity, total nitrogen, available phosphorus, and available potassium showed significant correlation with SOC and SIC (P<0.01). Soil moisture and urease activity showed significant correlation with SOC and SIC (P<0.05). The path analysis showed that available P had a direct effect on the SOC content; it was the main factor affecting it. The urease activity had a direct effect on SIC; it was the main factor affecting it. This study of soil carbon pools in arid and semiarid regions is the basis for evaluating the "source/sink" effect of soil carbon in farmland ecosystems, and is of great significance for studying the global carbon budget and land carbon cycle mechanism.

Characteristics of soil organic carbon and its fractions in subtropical evergreen broad-leaved forests along an urbanization gradient.

Subtropical evergreen broad-leaved forests were selected along an urban (Guangzhou) - suburban (Dinghushan) - rural (Huaiji) gradient in the Pearl River Delta, from which soil samples in different layers were collected. The changes in total organic carbon (TOC), recalcitrant organic carbon (ROC), and active organic carbon (AOC) including readily oxidizable organic carbon (ROOC), microbial biomass carbon (MBC), and water-soluble organic carbon (WSOC) of samples were examined along this urbanization gradient to reveal the influence of urbanization on forest soil organic carbon. Results showed that no significant differences in both TOC and ROC contents were observed in 0-5 cm soil layer along the gradient. In 5-60 cm soil layer, the TOC content was significantly higher in the rural forest than that in the suburban and urban forests, the ROC content was the highest in the suburban forest and no significant difference was observed between the urban and rural forests. The ROOC content was significantly lower in the suburban forest than in the rural (0-60 cm soil layer) and urban (0-10 cm soil layer) forests. The MBC content was significantly lower in the urban forest than that in the suburban and rural forests. The suburban forest had significantly lower WSOC than the urban forest (0-10 cm soil layer). In 0-20 cm layer, the percentage of AOC to TOC of the urban and rural forests was significantly higher than those of the suburban forest, while the percentage of ROC to TOC was the lowest in the rural forest. The significant difference in the percentage of ROC to TOC was only observed in 5-10 cm depth layer between the suburban and urban forests. The results indicated that urbanization increased the active components of soil organic carbon and reduced the stable ones, which could be detrimental to organic carbon accumulation in soils. The rural forest soils were more sensitive to the urbanization.

Ants alter molecular characteristics of soil organic carbon determined by pyrolysis-chromatography/mass spectrometry

Colonization by ecosystem engineers, such as ants, have important but occasionally unnoticed roles in regulating soil carbon cycles. However, little information is available pertaining to ant effects on changing molecular features of soil organic matter. In this study, we characterized total soil organic carbon (SOC), total nitrogen, water extractable organic carbon (WEOC) as well as molecular characteristics of humic acids (HAs) of ant mound soils (SoilAnt) and the surrounding soils (SoilCon) using pyrolysis-chromatography/mass (py-GC/MS). SOC, total nitrogen and WEOC contents of SoilAnt were significantly lower than those of SoilCon. SoilAnt samples had significant higher nitrogen containing compounds proportions in pyrolysis products of HAs as opposed to SoilCon samples. Another remarkablecharacteristic was that no polysaccharides, lignin, and phenols were identified in HAs of SoilAnt samples. It indicated that ant colonization significantly changed the molecular composition of soil organic matter.