Average Soil Organic Carbon Density Research Articles

Global distribution of surface soil organic carbon in urban greenspaces

Urban greenspaces continue to grow with global urbanization. The global distribution and stock of soil organic carbon (SOC) in urban greenspaces remain largely undescribed and missing in global carbon (C) budgets. Here, we synthesize data of 420 observations from 257 cities in 52 countries to evaluate the global pattern of surface SOC density (0–20 cm depth) in urban greenspaces. Surface SOC density in urban greenspaces increases significantly at higher latitudes and decreases significantly with higher mean annual temperature, stronger temperature and precipitation seasonality, as well as lower urban greenness index. By mapping surface SOC density using a random forest model, we estimate an average SOC density of 55.2 (51.9–58.6) Mg C ha−1 and a SOC stock of 1.46 (1.37–1.54) Pg C in global urban greenspaces. Our findings present a comprehensive assessment of SOC in global urban greenspaces and provide a baseline for future urban soil C assessment under continuing urbanization.

Research on soil organic carbon pool and distribution characteristics in farmland

In this paper, the researches on the estimation of cultivgated soil organic carbon storage and its variation trend were conducted in Shayang County, a typical agricultural county in Jianghan Plain in China. The results showed that the order of soil organic carbon density (SOCD) was paddy soil > moisture soil > yellow brown soil, and the mean values were 4.46, 3.12 and 2.55 kg/m2, respectively. No matter which soil type, the organic carbon content of soil decreased gradually from the surface to the deep soil in the form of exponential function. By analyzing and processing the organic carbon data from 2001 to 2018, the average SOCD in the topsoil increased from 3.69 to 4.25 kg/m2, with an increase of 15.18%.

Soil organic carbon changes in China's croplands: A newly estimation based on DNDC model

Soil Organic Carbon (SOC) in cropland represents a significant facet of the terrestrial ecosystem's carbon reservoirs, playing a pivotal role in global climate change mitigation efforts. Within the specific context of China, cropland SOC not only extends its implications beyond environmental impact but also serves as a critical factor in ensuring the stability and security of the nation's food supply. However, there is an ongoing argument about the changes in SOC and their spatial and temporal distribution patterns within China's croplands. In this study, we constructed a new county-level DNDC database for 2020, building upon 2003 research that quantified SOC stock in China's cropland using the DNDC model. Our aim was to assess the SOC storage and temporal changes of China's cropland in 2020 using same methodology to enhance estimation accuracy. The simulation results of the validated DNDC model revealed that the average SOC storage of China's croplands (0–30 cm) in 2020 was 6.02 Pg C, with the Northeast region contributing 23 % (1.37 Pg C). The SOC density in China varied from 18.55 to 152.57 t C ha−1, averaging at 49.65 t C ha−1. In 2020, China's cropland transitioned from a net loss of SOC in 2003 to a carbon sink, with cropland SOC density and SOC storage increased by 18.2 % and 21.6 % respectively. Notably, despite experiencing a loss of SOC compared to 2003, the Northeast region had the highest average SOC density in China. This study highlights that despite the increase in SOC density and storage in China's croplands over the last 17 years, there remains substantial potential for carbon sequestration given the current spatial distribution of SOC density's significant heterogeneity within China. The findings of this study offer data support for China's strategy to achieve food security and carbon neutrality.

China's wetland soil organic carbon pool: New estimation on pool size, change, and trajectory.

Robust estimates of wetland soil organic carbon (SOC) pools are critical to understanding wetland carbon dynamics in the global carbon cycle. However, previous estimates were highly variable and uncertain, due likely to the data sources and method used. Here we used machine learning method to estimate SOC storage and their changes over time in China's wetlands based on wetland SOC density database, associated geospatial environmental data, and recently published wetland maps. We built a database of wetland SOC density in China that contains 809 samples from 181 published studies collected over the last 20 years as presented in the published literature. All samples were extended and standardized to a 1-m depth, on the basis of the relationship between SOC density data from soil profiles of different depths. We used three different machine learning methods to evaluate their robustness in estimating wetland SOC storage and changes in China. The results indicated that random forest model achieved accurate wetland SOC estimation with R2 being .65. The results showed that average SOC density of top 1 m in China's wetlands was 25.03 ± 3.11 kg C m-2 in 2000 and 26.57 ± 3.73 kg C m-2 in 2020, an increase of 6.15%. SOC storage change from 4.73 ± 0.58 Pg in 2000 to 4.35 ± 0.61 Pg in 2020, a decrease of 8.03%, due to 13.6% decreased in wetland area from 189.12 × 103 to 162.8 × 103 km2 in 2020, despite the increase in SOC density during the same time period. The carbon accumulation rate was 107.5 ± 12.4 g C m-2 year-1 since 2000 in wetlands with no area changes. Climate change caused variations in wetland SOC density, and a future warming and drying climate would lead to decreases in wetland SOC storage. Estimates under Shared Socioeconomic Pathway 1-2.6 (low-carbon emissions) suggested that wetland SOC storage in China would not change significantly by 2100, but under Shared Socioeconomic Pathway 5-8.5 (high-carbon emissions), it would decrease significantly by approximately 5.77%. In this study, estimates of wetland SOC storage were optimized from three aspects, including sample database, wetland extent, and estimation method. Our study indicates the importance of using consistent SOC density and extent data in estimating and projecting wetland SOC storage.

Soil organic carbon stock in China’s tea plantations and their great potential of carbon sequestration

China is the biggest tea producer in the world, and soils in the tea plantations play important roles in the carbon cycle, and regional and national carbon neutrality. The knowledge regarding the variation and the key drivers of soil organic carbon (SOC) stock in tea plantations could provide a scientific basis for assessing the potential of regional and national carbon sequestration and neutrality. This is also helpful for the sustainable management of tea plantations. However, little information is available on soil organic carbon (SOC) stock and influential factors in tea plantations. Thereby, SOC density (SOCD) and stock were estimated based on 1723 datasets collected from 322 published studies. Subsequent estimations reveal that the total SOC stock in China’s tea plantations is 207.13 Tg with 124.58 Tg SOC stored in top soil layer (0–20 cm), and the average SOCD is 59.17 Mg ha−1 ranging from 0.9 Mg ha−1 to 589.54 Mg ha−1. The SOCD showed a significant three-dimension zonal pattern and was significantly influenced by managerial practices. Tea plantations managed in ecological ways and/or with long stand age even showed higher SOCD than that in neighbored forests. The lower average but wide range of SOCD in tea plantations indicates their great potential in carbon sequestration. This makes tea plantations critically important in regional and national carbon accounting and carbon neutrality, and cannot be ignored in future research and policymaking initiatives.

Zonal patterns and driving factors of soil organic carbon density in Zhejiang Province, China

The complex topography and intensive land use and land cover change (LUCC) might shift the spatial pattern of soil organic carbon (SOC) density. However, the effects of LUCC caused by development on the distribution and driving factors of SOC density remain unclear. As one of the most developed regions in China, Zhejiang Province has experienced intensive land use change and has various vegetation types, making it an ideal location to analyse the zonal pattern and driving factors of SOC density in terrestrial ecosystems. To do that, a dataset was built with data collected from existing literature. The average SOC density in Zhejiang Province was 69.75 Mg ha−1, ranging widely from 6.44 to 301.90 Mg ha−1 depending on land and vegetation types. We found that forests had the highest mean SOC density (78.30 Mg ha−1), while cropland had the lowest (45.69 Mg ha−1). Meanwhile, the SOC density of unmanaged land was significantly higher than that of managed land (P < 0.001). Additionally, SOC density varied greatly with longitude and altitude, showing a decreasing zonal pattern from west to east and from high to low altitude. The RF model indicated that vegetation type, stand age, mean annual temperature (MAT), soil pH, and mean annual precipitation (MAP) were the five most important factors affecting SOC density in this area, but the effects differed between managed and unmanaged lands. These results aid with understanding the dynamics of SOC pools in terrestrial ecosystems and inform policy-making to mitigate global climate change. These findings provide valuable insights into the dynamics of SOC pools in terrestrial ecosystems, enabling more effective policymaking to mitigate the impact of global climate change.

Northeast China holds huge wetland soil organic carbon storage: an estimation from 819 soil profiles and random forest algorithm

As a huge natural carbon storage, wetlands play an important role in the global carbon cycle. However, the spatial pattern and storage of soil organic carbon (SOC) in wetland ecosystems remain largely uncertain due to large spatial heterogeneity and insufficient field observations. In this study, we predict the spatial pattern of SOC density and estimated SOC storage in wetlands of Northeast China based on 819 field samples and multiple geospatial data using random forest algorithm. The SOC density of wetlands at different depths was affected differently by environmental factors and the SOC density in the surface layer (0–30 cm) was more susceptible to climatic change. The correlation coefficients (r) between the SOC density predicted by the random forest model and the measured SOC density were 0.86, 0.77 and 0.73 in 0–30, 30–60 and 60–100 cm soil depths, respectively. Our estimation showed that Northeast China holds huge wetland SOC storage in the amount of 3.40 ± 0.13 Pg C. The average wetland SOC density was 44.30 ± 1.72 kg C m−2, which decreased gradually from north to south in the study area. The wetland SOC density in Northeast China decreased with soil depth, and the influence of environmental factors on wetland SOC gradually decreased. Surface wetland SOC may be more sensitive to global climate change. Our results examined the relationship between wetland SOC and environmental factors, which benefits the understanding of the responses of wetland SOC to climate change.

Spatial-Temporal Variability of Soil Organic Carbon Density and Its Related Factors in Fengqiu County of Yellow River Basin, China: A Model and GIS Technique Approach

The accurate estimation of the soil organic carbon (SOC) sequestration rate is very important for studying farmland soil fertility and environmental effects. In this research, a typical fluvor-aquic soil area, Fengqiu county, located in the Yellow River basin of the Huang-Huai-Hai Plain of China, was chosen as a study area. The physicochemical properties of 70 soil samples collected from the surface layer (at a depth of 0–20 cm) in 2011 were analyzed, and related data about the sampling sites were also collected from the Second State Soil Survey of China (SSSSC), conducted in 1981. The results revealed that the SOC density (SOCD) in Fengqiu county increased greatly on a spatio-temporal scale. The average SOCD increased from 15.66 to 26.09 Mg ha−1, and the SOCD sequestration rate was more than 0.20 Mg C ha−1 year−1 in most regions. Few areas showed lost carbon in the past 30 years (1981–2011). In addition, the study suggested that all the areas present strong carbon sequestration potential in the coming decades from 2011, and the carbon sequestration potential was mainly between 32–40 Mg ha−1. Finally, the SOCD sequestration rate was not only affected by natural factors, such as soil type and pH, but also positively correlated with artificial soil management measures, such as fertilization and straw returning. Therefore, we concluded that the farmland in Fengqiu county showed significant carbon sequestration characteristics in the past 30 years (1981–2011). Considering that soil has a great potential for carbon sequestration in the future, the trend of carbon sequestration in farmland soil might continue for a period of time. Furthermore, the results of this study emphasized that strengthening soil scientific management may play a positive role in improving soil carbon sequestration.

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.

Role of Mangrove Rehabilitation and Protection Plans on Carbon Storage in Yanbu Industrial City, Saudi Arabia: A Case Study

Mangroves are one of the main considerations that might be used to mitigate the effects of climate change in coastal areas. Mangrove populations can be affected by industrial and civil activities on coasts. According to the Kyoto Protocol, protection and rehabilitation programs may play a pivotal role in conserving mangroves in industrial areas. Therefore, this study was designed to examine and evaluate the possible impact of conservation plans, implemented by the Royal Commission of Yanbu, in preserving mangrove trees’ ability to store carbon in the soil. Soil and plant samples were collected from three distinct locations, including a mangrove conservation site in Yanbu and natural unprotected sites in Umluj and Ar-Rayis. Organic-carbon (OC) stock, in both soil and plants, was calculated. Our results showed that at different depths, soil bulk density (SBD) in Yanbu ranged between 0.32–0.94 g cm−3. In Ar-Rayis and Umluj, SBD ranged between 1.43 to 1.99 and 0.90 to 1.57g cm−3, respectively. The average SBD values in Yanbu, Umluj, and Ar-Rayis were 0.68, 1.71, and 1.20 g cm−3, respectively. Similarly, the average soil OC density in Yanbu, Umluj, and Ar-Rayis was 165.19, 30.82, and 18.90 g C cm−3, respectively. Generally, the conserved mangrove tress grown in Yanbu industrial city showed higher (P ≤ 0.001) soil OC stock (0.39 t C ha−1) compared to the unprotected trees grown in Umluj (0.12 t C ha−1) and Ar-Rayis (0.11 t C ha−1) cities. Similarly, the highest (P ≤ 0.001) plant OC stocks (13.93 t C ha−1) were observed in protected mangroves of Yanbu, compared to the plant OC stocks observed in Umluj (8.06 t C ha−1) and Ar-Rayis (8.80 t C ha−1) cities. The results of the current study showed that the protected mangrove trees grown in Yanbu industrial city store more carbon in their sediments than those grown in the Umluj and Ar-Rayis sites without conservation or rehabilitation. These findings may provide evidence for the beneficial role of protecting mangrove forests in mitigating the effects of climate change.

Simulated response of soil organic carbon density to climate change in the Northern Tibet permafrost region

Climate warming can enhance the decomposition of soil organic matter (SOM), thereby increasing the rate of carbon release from soils. In permafrost regions, climate warming alters the nature of freeze–thaw cycles by affecting sub-surface hydrology and soil temperature, thereby impacting the decomposition of SOM. However, this process is rarely considered in projections of the long-term dynamics of soil organic carbon (SOC) on the Tibetan Plateau (TP) in response to climate change. Here, we employ the CENTURY-FTC model, which implements a freeze–thaw module in the CENTURY model, to simulate the response of top soil organic carbon density (SOCD) at 0–20 cm depth to climate change in the permafrost region of Northern Tibet. Our findings suggest that (i) the average SOCD was 2.123 kg C m−2 in 2015; (ii) the SOCD decreased at an average rate of 0.7 × 10−3 kg C m−2 year−1 for the period 1961–2015; and (iii) SOCD decreases spatially from south to north across Northern Tibet. Under various climate scenarios, the SOCD is projected to decrease significantly throughout Northern Tibet from 2016 to 2050, with the decline being most pronounced under the representative concentration-pathway (RCP) 8.5 scenario, which projects an increase in global mean surface temperature of 4.5 °C by 2100. Superimposed on this pattern, regional differences might be driven by vegetation type, with the largest decrements occurring in southern, alpine-meadow-dominated areas and the smallest in the northern alpine desert. We propose that this declining trend will be enhanced as climate warming continues and might be amplified by the increase in freeze–thaw processes. The addition of a freeze–thaw module to the CENTURY model changes projections of SOCD change due to climate. Whereas the freeze–thaw cycle has had little impact during the baseline period, its influence is likely to increase as climate warming continues. By 2050, freeze–thaw processes are projected to contribute 3% to SOCD decline under the RCP2.6 scenario, which projects a rise in global mean surface temperature of 1.5 °C by 2100, and as much as 10% under the RCP8.5 scenario. In general, future warming is likely to result in declining SOCD throughout Northern Tibet and a reduction in the capacity of alpine soils to sequester carbon.

Simulating Grassland Carbon Dynamics in Gansu for the Past Fifty (50) Years (1968–2018) Using the Century Model

China is one of the countries most impacted by desertification, with Gansu Province in the northwest being one of the most affected areas. Efforts have been made in recent decades to restore the natural vegetation, while also producing food. This has implications for the soil carbon sequestration and, as a result, the country’s carbon budget. Studies of carbon (C) dynamics in this region would help to understand the effect of management practices on soil organic carbon (SOC) as well as aboveground biomass (ABVG), and to aid informed decision-making and policy implementation to alleviate the rate of global warming. It would also help to understand the region’s contribution to the national C inventory of China. The CENTURY model, a process-based model that is capable of simulating C dynamics over a long period, has not been calibrated to suit Gansu Province, despite being an effective model for soil C estimation. Using the soil and grassland maps of Gansu, together with weather, soil, and reliable historical data on management practices in the province, we calibrated the CENTURY model for the province’s grasslands. The calibrated model was then used to simulate the C dynamics between 1968 and 2018. The results show that the model is capable of simulating C with significant accuracy. Our measured and observed SOC density (SOCD) and ABVG had correlation coefficients of 0.76 and 0.50, respectively, at p &lt; 0.01. Precipitation correlated with SOCD and ABVG with correlation coefficients of 0.57 and 0.89, respectively, at p &lt; 0.01. The total SOC storage (SOCS) was 436.098 × 106 t C (approximately 0.4356% of the national average) and the average SOCD was 15.75 t C/ha. There was a high ABVG in the southeast and it decreased towards the northwest. The same phenomenon was observed in the spatial distribution of SOCD. Among the soils studied, Hostosols had the highest SOC sequestration rate (25.6 t C/ha) with Gypsisols having the least (7.8 t C/ha). Between 1968 and 2018, the soil carbon stock gradually increased, with the southeast experiencing the greatest increase.

Soil Organic Carbon Changes for Croplands across China from 1991 to 2012

Accurate estimates of soil organic carbon (SOC) are critical for evaluating the impacts of crop and nutrient management practices on soil sustainability and global climate change. Temporal and spatial variations in topsoil (0–0.20 m) SOC were analyzed using 43,743 soil samples in China’s croplands. The soil database in our study was collected from the International Plant Nutrition Institute (IPNI) China Program. The results showed an increasing trend in SOC density (SOCD) for both grain and cash crops from 1991 to 2012. The average SOCD increased by 16.8, 17.4, 11.8 and 8.7% in the north central, northwest, southeast and southwest regions, respectively, whereas average SOCD decreased by 1.3% for the northeast region between the 1991–2001 and 2002–2012 periods. For both grain and cash crops, the SOCD frequency distribution (%) increased in the ranges of 10–20, 20–30 and 30–40 Mg C ha−1 from the 1991–2001 to the 2002–2012, but decreased in the ranges of 0–10 and 50–60 Mg C ha−1. Additionally, SOCD increased in most major soil types across China’s cropland regions, except in phaeozems, chernozems and umbrisols, where it decreased by 8.6–18.7% mainly due to water runoff, soil erosion, and low C input. The overall SOC stock (SOCS) in China’s cropland increased by 260 Tg C (23.7 Tg C yr−1) from 1991–2001 to 2002–2012, which was partially due to the increased crop residue return, improved fertilization and adopted conservation tillage over the period. This SOC increase represents a potential offset in carbon dioxide (CO2) emissions that could help reduce the overall net CO2 emissions in China.

Spatial-temporal change of soil organic carbon in Anhui Province of East China

This paper was oriented to characterise spatio-temporal variation of soil organic carbon (SOC) over a 30-year period and to explore its driving forces in East China. A comparative study was done of data obtained during 1980 and 2010 in Anhui Province. Spatio-temporal changes of SOC density and storage in the surface (0–30 cm) and 0–100 cm layers were analysed using geographic information system (GIS) spatial analysis techniques. Relationships between soil erosion, land use change, agricultural management, and changes in SOC are discussed. Results show: (1) over 30 years, the average SOC density decreased by 0.59 kg C m−2 in the surface layer and 1.63 kg C m−2 in 0–100 cm layer. The average SOC density increased in Calcaric Cambisols and Haplic Fluvisols, and decreased in other soil types.The average SOC density in upland increased but decreased as a result of other land use practices. (2) SOC density in the surface layer increased to the north but decreased to the south in Anhui. SOC density in the 0–100 cm layer increased in the central area and decreased to the south. About 59% of all soil increased in SOC density. (3) SOC storage decreased by 117.91 Tg C in the surface layer and decreased by 237.65 Tg C in the 0–100 cm layer. The SOC storage increased mainly in the northern and central areas. (4) Severe soil erosion was a main reason for the large reduction in SOC storage in western and southern Anhui. Cultivated land use that changed to paddy field from upland improved SOC more than changes to other land use types. SOC storage change in farmland was significantly and positively related to organic matter content in crop roots.

Estimation of soil organic carbon storage based on digital soil mapping technique.

Accurate spatial distribution information of soil properties would be helpful for improving the accuracy of soil organic carbon storage estimation. In this study, terrain factors were used as predictors, and the fuzzy C-means (FCM) clustering method was used to make digital soil prediction mapping for soil organic carbon content, soil bulk density, soil depth, and soil gravel content in Nanshan forest farm in Jiyuan City of Henan Province. Based on the digital mapping results, the prediction mapping of soil organic carbon density and the estimation of soil organic carbon storage were realized. The results showed that the average soil organic carbon density in the study area based on the digital soil mapping method was 4.24 kg·m-2, the mean error (ME), mean absolute error (MAE) and root mean square error (RMSE) of the prediction map were 0.08, 2.80 and 5.03 kg·m-2, respectively. The accuracy, stability and reliability of the prediction results were higher than the tradiation methods. The soil organic carbon storage in the study area was estimated to be 3.08×108 kg. Based on the digital soil mapping technology, only a small number of soil samples could be used to map and estimate the soil organic carbon density with high accuracy, which could characterize the spatial distribution characteristics of soil organic carbon density. This study provided a new way to estimate soil organic carbon storage, which would help to improve the accuracy and efficiency of soil organic carbon storage estimation.

The three-dimension zonal pattern of soil organic carbon density in China’s forests

The three-dimensional zonality of soil organic carbon (SOC) density (C storage per unit area) is a scientific basis for estimating the potential of regional forest carbon sequestration on a large scale. To understand the three-dimensional zonality of forest SOC density in China and the effect of plantations on these zonal patterns, data on China’s forest SOC density were collected from 231 published studies. The average forest SOC density was 122.72 Mg ha−1, and SOC density in natural forests was significantly higher than that in plantations in China. In addition, forest SOC density significantly increased with increasing latitude (r2 = 0.04, P < 0.001) and elevation (r2 = 0.108, P < 0.001) and showed a quadratic relationship with longitude (r2 = 0.204, P < 0.001). However, the three-dimensional zonality of SOC density in natural forests was significantly higher than that in plantations. Specifically, plantations weakened the linkages between forest SOC density and geographical factors. The results imply that increasing plantations may weaken the three-dimensional zonality of China’s forest SOC density. Thus, these results are particularly important for improving carbon cycle models and evaluating the carbon sequestration potential of China’s forest.

Discrepancies in Karst Soil Organic Carbon in Southwest China for Different Land Use Patterns: A Case Study of Guizhou Province.

The assessment of soil organic carbon (SOC) in mountainous karst areas is very challenging, due to the high spatial heterogeneity in SOC content and soil type. To study and assess the SOC storage in mountainous karst areas, a total of 22,786 soil samples were collected from 2,854 soil profiles in Guizhou Province in Southwest China. The SOC content in the soil samples was determined by the oxidation of potassium dichromate (K2Cr2O7), followed by titration with iron (II) sulfate (FeSO4). The SOC storage was assessed based on different land uses. The results suggested that the average SOC density in the top 1.00 m of soil associated with different land uses decreased in the following order: Croplands (9.58 kg m−2) > garden lands (9.07 kg m−2) > grasslands (8.07 kg m−2) > forestlands (7.35 kg m−2) > uncultivated lands (6.94 kg m−2). The SOC storage values in the 0.00–0.10 m, 0.00–0.20 m, 0.00–0.30 m and 0.00–1.00 m soil layers of Guizhou Province were 0.50, 0.87, 1.11 and 1.58 Pg, respectively. The SOC in the top 0.30 m of soil accounted for 70.25% of the total within the 0.00–1.00 m layer in Guizhou Province. It was concluded that assessing SOC storage in mountainous karst areas was more accurate when using land use rather than soil type. This result can supply a scientific reference for the accurate assessment of the SOC storage in the karst areas of southwestern China, the islands of Java, northern and central Vietnam, Indonesia, Kampot Province in Cambodia and in the general area of what used to be Yugoslavia, along with other karst areas with similar ecological backgrounds.

Hummocks in alpine tundra, northern British Columbia, Canada: distribution, morphology and organic carbon composition

Hummocks develop by cryoturbation in fine-grained frost-susceptible soils and their stage of maturity may affect the translocation of organics in Cryosols. This study examines the distribution and morphology of hummocks in the Chuck Creek Trail Valley (northern British Columbia) and determines the quantity, distribution, and composition of organic matter in their soils. Hummocks occupy about 5%–20% of the valley and their morphology is largely affected by their silt content. Cryoturbated intrusions, radiocarbon dated to 2814 and 1648 cal year B.P., suggest that hummock development was initiated during the cooler late Holocene. Hummocks have an average soil organic carbon density of 16.3 kg m−2 in the uppermost 1 m, with 62% stored in the top 25 cm. Organics are mainly present as particulate organic matter in the O-horizon (25%–80%), characterized by degradable alkyl C and O/N-alkyl groups, but occur as mineral-associated organic matter (96%–98%) composed of recalcitrant aromatic and aliphatic C groups in the underlying B and C horizons. Minor differences in organic content and composition occur between hummock tops and troughs, and between hummocks showing different stages of maturity. In the absence of an observed frost table, contemporary hummock activity is attributed to seasonal freezing and thawing.

Soil organic carbon density and its driving factors in forest ecosystems across a northwestern province in China

Forest soils are an important component of the global carbon cycle, as they sequester and store much more organic carbon than other terrestrial ecosystems. Thus, reliable information on the forest soil organic carbon (SOC), along with the factors that influence it, is crucial from the perspective of regional carbon budgets and climate modeling. Gansu Province in northwestern China is characterized by complex climatic gradients and diverse forest types, suggesting a large variability in the spatial distribution of SOC, yet the SOC stock in this region has not been comprehensively investigated because of insufficient soil profile data. Our study covered the entire forest area in the province via a regional-scale sampling campaign, and provided soil organic carbon density (SOCD) data obtained from 112 natural forests and 100 plantations. Influencing factors including forest type, forest stand age, soil type, litter carbon, climate, and topography were evaluated in both natural forests and plantations. The results indicated a SOC stock of 433.4 ± 13.2 Tg C and a SOCD value of 175.3 ± 33.0 Mg C ha−1 for the 0–100 cm layer at the regional scale. Specifically, the SOC stock and the forest area-weighted average SOCD in natural forests were estimated at 349.0 ± 16.5 Tg C and 207.3 ± 23.2 Mg C ha−1, which were about quadruple and double, respectively, of the values for the plantations. Regardless of natural forests and plantations, SOCD increased with both stand age class and elevation and decreased with mean annual temperature (MAT). SOCD also increased with mean annual precipitation (MAP) for natural forests with elevation <2500 m and for all plantations. The overall importance to SOCD of the explanatory variables increased with increasing soil depth in natural forests while decreased with increasing soil depth in plantations. Forest type, stand age, soil type, MAT, MAP, elevation, litter carbon, slope, and aspect explained 58 and 65% of the total variation in the SOCD at 100 cm depth in natural forests and plantations, respectively. In natural forests, the most important explanatory variable for the uppermost 30 cm was soil type, while it was MAT for the whole 100 cm; in plantations, the most prominent factors were forest type and elevation, regardless of the soil depth. These suggest that environmental changes will likely have different effects on SOCD in natural forests and plantations.

Effects of past land use on soil organic carbon changes after dam construction

A large-scale and high-dropout water-level-fluctuation zone (WLFZ) was formed by the construction of cascaded large dam in the southwest hydropower base of China. The original land use patterns changed significantly by the formation of the WLFZ, which possibly affected the soil organic carbon (SOC) stocks and local carbon cycle. However, little is known about the effects of land uses before dam construction on the SOC stocks in the WLFZ. To address this, we chose the Nuozhadu mega reservoir built in 2014 in the upper Mekong River basin as our study area. We established five sampling transects around the reservoir and sampled them in 2015. Each transect contained three vertical sampling lines representing three land-use types: natural forest, farmland, and rubber plantations. SOC stocks were determined in the WLFZ and in the infralittoral reference zone (IRZ), and the interactions among SOC, land uses, soil erosion, inundation, and other soil physicochemical properties were analyzed to establish the factors controlling SOC in the WLFZ. The average soil organic carbon density (SOCD) was not significantly different between the WLFZ and IRZ, but it differed significantly among the three types of land uses. The SOC stocks in natural forests of the WLFZ and IRZ were significantly greater than those in the other land use types. The SOCD decreased with increasing elevation in the natural forest and rubber plantation in the WLFZ, while no significant trend was found in farmland. These results indicated that the construction of a large dam in southwestern China may not lead to significant changes in regional SOC stocks, but it can result in the redistribution of SOC in the large-scale and high-dropout WLFZ. Land use before dam construction can affect the background concentrations of SOC, soil physicochemical properties, and soil erosion rate in the WLFZ, which controlled the redistribution of SOC.