Low Soil Organic Carbon Content Research Articles

Differences in pedotransfer functions of bulk density lead to high uncertainty in soil organic carbon estimation at regional scales: Evidence from Chinese terrestrial ecosystems

AbstractAccurate estimation of soil organic carbon (SOC) storage is important for evaluating carbon sequestration of terrestrial ecosystems at regional scale. How the selected pedotransfer functions (PTFs) of bulk density (BD) influence the estimates of SOC storage is still unclear at large scales, although BD is an important parameter in all equations. Here we used data from the second national soil survey in China (8210 soil profiles) to evaluate the influence of eight selected PTFs on the estimation of SOC storage. The results showed that different PTFs may result in a higher uncertainty of SOC storage estimation and the coefficient of variation (CV, %) for the eight PTFs varied from 10.61% to 70.46% (mean = 12.75%). The observed CV values were higher in the 0–20 cm layer (12.48%) than in the 20–100 cm layer (10.05%). CV values were relatively stable (10–15%) when SOC content ranged from 0.13% to 3.45%. The findings indicate that PTFs may be used cautiously in soils with higher or lower SOC content. Estimates of SOC storage in the 0–100 cm soil layer varied from 67.19 to 95.97 Pg C in the eight PTFs in China, with an average of 87.36 ± 8.93 Pg C (CV = 10.23%). Our findings provide the insight that differences in PTFs are important sources of uncertainty in SOC estimates. The development of better PFTs, or the integration of various PFTs, is essential to accurately estimate SOC storage at regional scales.

Impacts of forest types on soil C, N and DOC loss in runoff in the laterite hilly region of southern China

Forest plantations significantly impact soil properties and hydrological processes. Field investigation was carried out to provide insight into the impacts of three forest types (Cunninghamia lanceolata,Pinus elliottii, and mixed forest of Schima superba and Cyclobalanopsis jenseniana) on soil organic carbon (SOC), total nitrogen (TN) and the removal of dissolved organic carbon (DOC) by runoff. SOC and TN contents were measured in soils (0–50 cm) under three forest types on both uphill and downhill. Runoff samples were collected and DOC concentrations were analyzed after 27 erosive rainfalls during the period of April 2011 to April 2012. Results showed that the lowest contents of SOC and TN in top soil layer (0–10 cm) were observed in pure C. lanceolata stands. Vertical distributions of SOC and TN in pure C. lanceolata and pure P. elliottii followed negative power functions on downhill (P C. lanceolata > P. elliottii in 27 erosion rainfall events. The runoff and DOC loss density (DOCld) showed significantly positive correlations with throughfall (P mixed S. superba and C. jenseniana > P. elliottii. The lowest SOC content and the highest DOCld value were observed in pure C. lanceolata stands, which should be well considered when the large-scale reforestation was conducted in the laterite hilly region of southern China.

Storage, Landscape Distribution, and Burial History of Soil Organic Matter in Contrasting Areas of Continuous Permafrost

This study describes and compares soil organic matter (SOM) quantity and characteristics in two areas of continuous permafrost, a mountainous region in NE Greenland (Zackenberg study site) and a lowland region in NE Siberia (Cherskiy and Shalaurovo study sites). Our assessments are based on stratified-random landscape-level inventories of soil profiles down to 1 m depth, with physico-chemical, elemental, and radiocarbon-dating analyses. The estimated mean soil organic carbon (SOC) storage in the upper meter of soils in the NE Greenland site is 8.3 ± 1.8 kg C m-2 compared to 20.3 ± 2.2 kg C m-2 and 30.0 ± 2.0 kg C m-2 in the NE Siberian sites (95% confidence intervals). The lower SOC storage in the High Arctic site in NE Greenland can be largely explained by the fact that 59% of the study area is located at higher elevation with mostly barren ground and thus very low SOC contents. In addition, SOC-rich fens and bogs occupy a much smaller proportion of the landscape in NE Greenland (∼3%) than in NE Siberia (∼20%). The contribution of deeper buried C-enriched material in the mineral soil horizons to the total SOC storage is lower in the NE Greenland site (∼13%) compared to the NE Siberian sites (∼24%–30%). Buried SOM seems generally more decomposed in NE Greenland than in NE Siberia, which we relate to different burial mechanisms prevailing in these regions.

The Importance of Soil Fertility Constraints in Modeling Crop Suitability Under Progressive Climate Change in Tanzania

Spatial crop suitability models are important planning tools for agricultural development, especially regarding climate change adaptation. The original version of the EcoCrop model predicts crop suitability based on monthly temperature and precipitation, without taking into account soil constraints. Recently, continuous soil property maps of Tanzania were produced (Vagen et al, submitted) based on data from systematic surveys across sub-Saharan Africa. The aim of the present study was to assess the effects of incorporating information on soil fertility constraints, such as low soil organic carbon content (SOC) into the EcoCrop model. Crop suitability maps derived with and without consideration of low SOC were compared. SOC values were extracted for 1036 locations of common bean (Phaseolus vulgaris L.) presence and the 2-percentile (0.9% SOC) was taken as a lower limit for suitability. The EcoCrop model was then run with current climate data, with and without SOC as a limiting factor, to create suitability maps. An independent spatial point dataset of registered bean presence/absence (n=1113), and regional production statistics were used for validation. The agreement between bean presence and bean suitability was higher (Cohen's unweighted kappa=0.06) when low SOC was included in the model compared to when only climate was considered (kappa=0.02). The regional statistics showed that the proportion of the area where low SOC restricted the suitability was negatively correlated with the proportion of the area planted with beans (r =-0.42, p=0.07). In summary, we identified a lower limit of SOC for the soil to be suitable for common beans production and investigated the importance of taking this soil fertility constraint into account for accurate suitability modelling. It was concluded that for more accurate suitability modelling, e.g. for decision support for adaptation to climate change, low SOC should be considered as a constraining factor.

改变施肥管理后不同肥力稻田土壤CO2的排放特征

Paddy soils show significant potential of carbon sequestration. The soil organic carbon( SOC) content of red paddy soils have been reported to be steady after 30 years' cultivation and it varies with different fertilization practice. In this study,three red paddy soils,which cultivated under different organic fertilization treatments in a 30 years fertilizer experiment in Changsha, China, was adjusted to the following seven treatments without compromising the original experiment: the original high organic fertilization treatment( HOM),the high organic fertilization treatment changed from the original normal organic fertilization treatment( N-H),the original normal organic fertilization treatment( NOM),the normal organic fertilization treatment changed from chemical fertilization( C-N),the chemical fertilization treatment changed from high organic fertilization treatment( H-C),the chemical fertilization treatment changed from normal organic fertilization treatment( N-C),the original chemical fertilization treatment( CF). CO2 flux of the three original and fourchanged treatments was measured to study the effects of the following-up fertilization reforming on the CO2 flux in red paddy soils with different fertilities in 2012—2013. The results clearly showed that the following-up changing of fertilization had obvious impacts on the dynamics of CO2 flux. The CO2 flux in the soils under long-term organic fertilization treatments decreased after the adjustment from organic fertilization to chemical fertilization. However,the CO2 flux in the soils under long-term chemical fertilization treatments increased remarkably after the adjustment from chemical fertilization to organic fertilization. The results also indicated that both organic fertilizer and soil organic carbon( SOC) had important impacts on CO2 flux. The amount of organic carbon inputted by organic fertilizers had a significantly positive relationship with the total amount of annual CO2-C flux( r = 0.9015**,n = 21),and the inherent SOC content( x) also had a positive relationship with the total amount of annual CO2-C( y)( y = 10. 962x- 68. 86,R2= 0. 7507,n = 9,P 0. 01) in the paddy soils received chemical in 2012. The fertilization reforming from organic fertilization to chemical fertilization would lead to the loss of SOC in paddy soils due to its mineralization,and the SOC loss increased with the rising of inherent SOC content. The SOC content in the paddy soils under long-term organic fertilization would be consistent with that in the paddy soils under long-term chemical fertilization after the fertilization reforming from organic to chemical fertilization for a certain time. The fertilization reforming from chemical to organic fertilization,or from low organic to high organic fertilization would lead to the SOC accumulation in paddy soils,and the accumulating rate of SOC had a positive relationship with the amount of the inputted organic material. Under same organic fertilization treatment,the apparent decomposition rate of organic material applied in the soils with high SOC content was larger than that in the soils with low SOC content,which would result in lower SOC accumulation. So,the paddy soils with different SOC content would have the same SOC level when they received same organic fertilization management for a certain time. As a conclusion,the sustained organic fertilization in red paddy soils with high or low SOC content is essential to maintain or improve the soil organic carbon content in Southern China.

可溶性有机碳在米槠天然林不同土层中的迁移特征

PDF HTML阅读 XML下载 导出引用 引用提醒 可溶性有机碳在米槠天然林不同土层中的迁移特征 DOI: 10.5846/stxb201310272590 作者: 作者单位: 福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地,湿润亚热带山地生态国家重点实验室培育基地,福建师范大学地理科学学院,湿润亚热带山地生态国家重点实验室培育基地,湿润亚热带山地生态国家重点实验室培育基地 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学青年基金项目(31100467); 福建师范大学优秀青年骨干教师培养基金(fjsdky2012008); 国家自然科学基金重点项目(31130013); 福建省自然科学青年基金项目(2011J05105); 高等学校博士学科点专项科研基金(优先发展领域)(20113503130001) Transport characteristics of dissolved organic carbon in different soil horizons in natural Castanopsis carlesii forest Author: Affiliation: State Key Laboratory of Humid Subtropical Mountain Ecology,State Key Laboratory of Humid Subtropical Mountain Ecology,State Key Laboratory of Humid Subtropical Mountain Ecology,State Key Laboratory of Humid Subtropical Mountain Ecology,State Key Laboratory of Humid Subtropical Mountain Ecology,State Key Laboratory of Humid Subtropical Mountain Ecology 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:选取我国中亚热带典型的常绿阔叶林米槠天然林(Castanopsis carlesii)为研究对象,采集林内米槠凋落物并通过挖剖面法分6个土层采集土样至1 m。通过浸提米槠凋落物得到可溶性有机碳(dissolved organic carbon,DOC)溶液并在室内模拟其在不同土层的淋溶过程,不仅分析了土壤性质对DOC淋溶的影响,还研究了淋溶前后DOC化学结构的变化,以阐明DOC在不同土层中的迁移特征及影响因素,探寻米槠天然林土壤的固碳潜力和DOC在土壤有机碳循环中的作用。结果表明:(1)下层土壤比上层土壤吸附DOC的能力更强,亲水性DOC与疏水性DOC间会争夺土壤颗粒表面的吸附位点,而且芳香化合物和大分子物质等疏水性DOC组分会被优先吸附;(2)红外光谱表明,芳香类和醚类等疏水性物质会优先被吸附,烷烃类物质却不易被吸附,土壤中原有的酚、醇类亲水性物质会被初始DOC中的疏水性物质置换出来;(3)土壤DOC的截留能力与粘粒、游离氧化铁含量呈极显著正相关,而与土壤有机碳和砂粒含量呈极显著负相关,其中土壤有机碳的含量是影响米槠天然林不同土层DOC截留量的关键因素。 Abstract:Dissolved organic carbon (DOC) is a mixture of organic compounds with varying molecular sizes and weights. In forest ecosystems, litterfall is a major aboveground resource of DOC, which supplies soils with carbon as a crucial component of terrestrial biogeochemical cycles. During the migration of litterfall-derived DOC from topsoils to subsoils, soils might retain, transform, or release DOC, depending on the chemical nature of DOC and its interactions with soil surfaces. These physicochemical processes contribute considerably to soil organic carbon (SOC) accumulation. It is generally accepted that soil properties have a great influence on the interaction between soil surface and DOC, with Fe/Al oxides constraining DOC losses and high SOC content being usually negatively correlated to DOC sorption. However, an inconsistency still remains regarding the extent to which soils properties impact DOC movement. In some cases topsoils rich in SOC might demonstrate higher DOC retention capacity. Although subsoils with abundant Fe/Al oxides are considered to have a stronger DOC retention capacity, relatively lower SOC content is often found in subsoils, and direct evidence for their higher carbon sequestration potential is rare. One reason for this maybe the amount of DOC from aboveground litterfall, which is not sufficient and little DOC can reach subsoils. Another reason might be that highly sorptive DOC is preferentially absorbed by topsoils, and DOC that reaches subsoils has a weak affinity for soil surfaces. Supplying individual soil horizons with a common DOC solution is thus a good way to differentiate their carbon sequestration capacities. Natural Castanopsis carlesii forests are dominant evergreen broad-leaf forests in mid-subtropical China, and interaction between litterfall derived DOC and red soil (Ferralic Cambisols, World Reference Base) is now recognized as one of the most important mechanisms of C sequestration in this region. To solve inconsistencies about factors controlling DOC translocation in soils and to provide direct information to differentiate carbon sequestration potential between soil horizons, a 200-year-old natural Castanopsis carlesii forest without human interference was selected for undecomposed litter collection and soil sampling at the depth of 0-10 cm, 10-20 cm, 20-40 cm, 40-60 cm, 60-80 cm, 80-100 cm from each of the three profiles in the site. After extracting DOC from the collected litter with ultrapure water, we supplied individual soil cores with the common DOC in the laboratory. We not only analyzed the influences of soil properties on DOC interception, but also studied the changes in structural composition of DOC before and after leaching. The results showed that: (1) DOC interception in subsoils was greater than that in topsoils. Hydrophilic and hydrophobic DOC competed for binding sites on soil surfaces, and aromatic compounds and macromolecular substances of hydrophobic components were preferentially adsorbed by soils; (2) infrared spectrum suggested that hydrophobic materials such as aromatic substances and ethers were much more ready to be adsorbed than alkane materials, while indigenous hydrophilic substances such as phenols, alcohols could be displaced by hydrophobic components of DOC; (3) there was significant positive correlation between DOC interception and the content of clay and free iron oxide and significant negative correlation between DOC interception and the content of sand and soil organic carbon. Soil organic carbon content was the key factor affecting DOC interception in different soil horizons in natural Castanopsis carlesii forest. 参考文献 相似文献 引证文献

Occurrence of iron and aluminum sesquioxides and their implications for the P sorption in subtropical soils

The effect of the pedo-climatic variation in qualitative and quantitative soil sesquioxide contents and soil phosphorus (P) sorption capacities has been studied. In four soils (samples from A horizons) located along a southern Brazilian environmental gradient (EG) shown a decrease in crystallized iron oxides with the increase of altitude. Hematite and goethite were found at low EG altitudes in association with low soil organic carbon content, and goethite–ferrihydrite and gibbsite were found at high altitudes in association with high organic carbon content. The adsorbed soil P (measured by the remaining P) was predicted by the goethite content, and the desorbed P (measured by successive P extractions using anion exchange resin membranes) was predicted mainly by the goethite and clay content. In subtropical soils, even a smooth environmental gradient could determine the qualitative and quantitative iron and aluminum sesquioxide distributions that control soil P sorption capacities.

Soil organic carbon characteristics relating to geomorphology near Vestre Lovénbreen moraine in Svalbard

Soil organic carbon (SOC) in the Arctic is vulnerable to climate change. However, research on SOC stored in the high Arctic regions is currently very limited. Thus, this study was aimed at understanding the distribution and characteristics of SOC with respect to geomorphology and vegetation in Svalbard. In August 2011, soil samples were collected near the Vestre Lov<TEX>$\acute{e}$</TEX>nbreen moraine. Sampling sites were chosen according to altitude (High, Mid, and Low) and differences in levels of vegetation establishment. Vegetation coverage, aboveground biomass, and SOC contents were measured, and density-size fractionation of SOC was conducted. The SOC content was the highest in the Mid site (<TEX>$126.9mg\;g^{-1}$</TEX>) and the lowest in the High site (<TEX>$32.1mg\;g^{-1}$</TEX>), although aboveground biomass and vegetation coverage were not different between these two sites. The low SOC content measured at the High site could be related to a slower soil development following glacial retreat. On the other hand, the Low site contained a high amount of SOC despite having low vegetative cover and a high ratio of sand particles. These incompatible relationships between SOC and vegetation in the Low site might be associated with past site disturbances such as runoff from snow/glacier melting. This study showed that geomorphological features combined with glacier retreat or melting snow/glacier effects could have affected the SOC distribution and vegetation establishment in the high Arctic.

Soil organic carbon dynamics 75 years after land-use change in perennial grassland and annual wheat agricultural systems

The dynamics of roots and soil organic carbon (SOC) in deeper soil layers are amongst the least well understood components of the global C cycle, but essential if soil C is to be managed effectively. This study utilized a unique set of land-use pairings of harvested tallgrass prairie grasslands (C4) and annual wheat croplands (C3) that were under continuous management for 75 years to investigate and compare the storage, turnover and allocation of SOC in the two systems to 1 m depth. Cropland soils contained 25 % less SOC than grassland soils (115 and 153 Mg C ha−1, respectively) to 1 m depth, and had lower SOC contents in all particle size fractions (2000–250, 250–53, 53–2 and 40 cm) layers and mineral-associated (<53 μm) SOC. Grassland soils had significantly more visible root biomass C than cropland soils (3.2 and 0.6 Mg ha−1, respectively) and microbial biomass C (3.7 and 1.3 Mg ha−1, respectively) up to 1 m depth. The outcomes of this study demonstrated that: (i) SOC pools that are perceived to be stable, i.e. subsoil and mineral-associated SOC, are affected by land-use change; and, (ii) managed perennial grasslands contained larger SOC stocks and exhibited much larger C allocations to root and microbial pools than annual croplands throughout the soil profile.

Soil Organic Carbon, Black Carbon, and Enzyme Activity Under Long-Term Fertilization

The present study aims to understand the effects of long-term fertilization on soil organic carbon (SOC), black carbon (BC), enzyme activity, and the relationships among these parameters. Paddy field was continuously fertilized over 30 yr with nine different fertilizer treatments including N, P, K, NP, NK, NPK, 2NPK (two-fold NPK), NPK+manure (NPKM), and CK (no fertilization), N, 90 kg urea-N ha−1 yr−1; P, 45 kg triple superphosphate-P2O5 ha−1 yr−1; K, 75 kg potassium chloride-K2O ha−1 yr−1; and pig manure, 22 500 kg ha−1 yr−1. Soil samples were collected and determined for SOC, BC content, and enzyme activity. The results showed that the SOC in the NPKM treatment was significantly higher than those in the K, P, and CK treatments. The lowest SOC content was found in the CK treatment. SOC content was similar in the N, NP, NK, NPK, 2NPK, and NPKM treatments. There was no significant difference in BC content among different treatments. The BC-to-SOC ratios (BC/SOC) ranged from 0.50 to 0.63, suggesting that BC might originate from the same source. Regarding enzyme activity, NPK treatment had higher urease activity than NPKM treatment. The urease activity of NPKM treatment was significantly higher than that of 2NPK, NP, N, P, K, CK, and NPKM treatment which produced higher activities of acid phosphatase, catalase, and invertase than all other treatments. Our results indicated that long-term fertilization did not significantly affect BC content. Concurrent application of manure and mineral fertilizers increased SOC content and significantly enhanced soil enzyme activities. Correlation analysis showed that catalase activity was significantly associated with invertase activity, but SOC, BC, and enzyme activity levels were not significantly correlated with one another. No significant correlations were observed between BC and soil enzymes. It is unknown whether soil enzymes play a role in the decomposition of BC.

Possible effect of soil organic carbon on its own turnover: A negative feedback

Soil carbon (C) sequestration plays an important role in mitigating climate change; therefore, it is essential to understand the mechanisms underlying soil organic carbon (SOC) turnover and dynamics. The humification coefficient of input organic materials (h) and the SOC decomposition rate (k) together control the SOC turnover, and they have been well known to be affected by factors including climatic factors, organic material qualities, soil characteristics, and anthropogenic activities. However, the SOC content changes over time, and thus far, whether the SOC content is an inherent factor has not been understood clearly. By using a new mathematical method, we found that SOC is possibly able to regulate its own turnover through a negative feedback: h decreases and k increases simultaneously with an increase in the SOC content. This feedback is caused by a series of chemical, biological, and physical mechanisms. We suggest that the response of the SOC pool to climate change is a function of the C input level to soil, temperature, and negative feedback of the SOC turnover; in addition, organic materials should be applied in priority to soils with low SOC content to enhance the soil C sequestration.

Effect of Land Use History and Pattern on Soil Carbon Storage in Arid Region of Central Asia

The purpose of this study is to investigate variations in soil organic carbon (SOC) in arid areas due to differences in the cultivation history, land use, and soil salinization. The study area is the lower Sangong River basin on the piedmont of the northern TianShan mountains, which experiences heavy land-use activities. In 1982 and 2005,127(152) and 74 (161) samples in old (new) oasis were collected from each site at the surface soil (i.e., 0–20 cm). The data reveal that the mean value of the surface soil organic carbon content of the old oasis was higher than that of the new oasis by 4.01 g/kg in 1982 and 3.79 g/kg in 2005. Additionally, the soil organic carbon content decreased more rapidly in the newly reclaimed oasis than in the old oasis from 1982 to 2005. The spatial pattern of the SOC content was correlated with the exploitation time in the new oasis, the agricultural land use history, and the SOC content. The decreasing trend is clearer in the high SOC content area than in the low SOC content area. Farmland is the largest carbon pool in both the new and old oases. The carbon density of the old oasis was higher than that of the new oasis by 4.01 and 3.79 g/kg in 1982 and 2005 respectively. The loss of SOC in the agricultural watershed of the arid region in NW China is obvious. Improvements of land management practices, such as no tillage, straw returning to soil, and balanced fertilization techniques, should be adopted to increase the SOC content.

Soil organic matter of Iberian open woodland rangelands as influenced by vegetation cover and land management

Spanish rangelands occupy more than 90,000km 2 of land grazed by millions of domestic and wild animals. Organic matter content of soil is an essential component for fertility and productivity in both, natural and human-induced ecosystems. Despite its importance to soil quality, soil organic carbon has been little studied in these rangelands. The main goal of this study is to assess the amounts and characteristics of soil organic matter in grazed open woodlands of SW Iberia as influenced by vegetation cover and land management. Soil properties were analysed from 22 environmental units selected from 10 farms distributed throughout the region of Extremadura being representative of different rangeland types: dehesas (wooded rangelands), scrublands of Retama sphaerocarpa and treeless grasslands. Some of the soil samples were used for a chemical fractioning of soil carbon. Farmer interviews were conducted in order to quantify livestock densities and stocking rates and to assess land management. Results showed that soils are generally shallow, acidic, with low soil organic carbon contents and poor in nutrients. Areas with higher soil carbon stocks corresponded to units encroached by Retama sphaerocarpa shrubs and dehesas with 49.2T·;ha -1 and 46.0T·ha -1 , respectively, particularly those developed on schist. Soil organic carbon concentrated mainly in the top 5cm soil layer, displaying large variability (mean: 2.30%). SOC was relatively high beneath the tree canopy (mean: 3.35%) and less concentrated and reduced in variability with depth. Regardless of the variation of total amount of carbon among cover types, the proportion of each carbon fraction from the total carbon was kept relatively constant whatever the cover type. A multivariate analysis performed and other results suggest that land mismanagement and excessive stocking rates could be related with the appearance of bare soil areas, therefore negatively affecting soil organic matter by removing the sensitive topsoil materials through water erosion. © 2013 Elsevier B.V.

Parameterizing soil organic carbon’s impacts on soil porosity and thermal parameters for Eastern Tibet grasslands

This study investigates the stratification of soil thermal properties induced by soil organic carbon (SOC) and its impacts on the parameterization of the thermal properties. Soil parameters were measured for alpine grassland stations and North China flux stations, with a total of 34 stations and 77 soil profiles. Measured data indicate that the topsoils of alpine grasslands contain high SOC contents than underlying soil layers, which leads to higher soil porosity values and lower thermal conductivity and bulk density values in the topsoils. However, this stratification is not evident at the lowland stations due to low SOC contents. Evaluations against measured data show that three thermal conductivity schemes used in land surface models severely overestimate the values for soils with high SOC content (i.e. topsoils of alpine grassland), but they are better for soils with low SOC content. A new parameterization is then developed to take the impacts of SOC into account. The new one can well estimate the soil thermal conductivity values in both low and high SOC content cases, and therefore, it is a potential candidate of thermal conductivity scheme to be used in land surface models.

Key variables explaining soil organic carbon content variations in croplands and non-croplands in Chinese provinces

Soil organic carbon (SOC) plays an important role in global carbon cycles. Large spatial variations in SOC contents result in uncertain estimates of the SOC pool and its changes. In the present study, the key variables explaining the SOC contents of croplands (CPs) and non-croplands (NCPs) in Chinese provinces were investigated. Data on SOC and other soil properties (obtained from the Second National Soil Survey conducted in the late 1970s to the early 1990s), climate parameters, as well as the proportion of the CP to the total land area (Pcp) were used. SOC content variations within a province were larger than those among provinces. Soil clay and total phosphorus content, ratio of annual precipitation to mean temperature, as well as Pcp were able to explain 75% of the SOC content variations in whole soil samples. Soil pH, mean temperature during the growing season from May to October, and mean annual wind velocity were able to explain 63% of the SOC content variations in NCP soils. Compared with NCP soils, CP soils had lower SOC contents, with smaller variations within and among provinces and lower C/N ratios. Stepwise regression showed that the soil clay content was a unique factor significantly correlated with the SOC content of CP soils. However, this factor only explained 24% of the variations. This result suggested that variables related to human activities had greater effects on SOC content variations in CP soils than soil properties and climate parameters. Based on SOC contents directly averaged from soil samples and estimated by regression equations, the total SOC pool in the topsoil (0–20 cm) of China was estimated at 60.02 Pg and 57.6 Pg. Thousands of years of intensive cultivation in China resulted in CP topsoil SOC loss of 4.34–4.98 Pg.

Large-scale spatial variability and distribution of soil organic carbon across the entire Loess Plateau, China

Soil organic carbon (SOC) plays an important role in soil productivity and the global carbon cycle. However, little is known about the regional distribution of SOC across the entire Loess Plateau region of China. We investigated 382 sampling sites across the region (620 000 km2) and collected 764 soil samples from the topsoil (0–20 cm) and subsoil (20–40 cm). Standard statistics were used to identify the regional SOC content and the relationships with 11 selected environmental variables. Concentrations of SOC varied within a wide range throughout the region from 0.38 to 54.03 g kg–1, with mean values of 10.34 and 6.78 g kg–1 for the topsoil and subsoil, respectively. Coefficient of variation values showed moderate variation for SOC in both soil layers. Significant correlations were detected between SOC and these environmental variables, notably with soil total nitrogen (TN), soil pH, and clay content. Multiple linear regression analysis indicated that TN, clay content, soil pH, elevation, and temperature had greatest effects on regional SOC variability among all the selected soil and site variables. Geostatistical analysis showed that the maximum autocorrelation ranges were 384 and 393 km for SOC in the topsoil and subsoil, respectively. Nugget-to-sill ratios were 0.52 and 0.50, which also indicated moderate spatial dependence. Maps of SOC distribution produced by the geostatistical method showed that the overall spatial pattern was characterised by an area of low SOC content surrounded by bands with higher values, which generally increased towards the region’s boundaries. The distribution pattern corresponded to that of the major regional landforms, which also influenced land use, whereby the sandy Ordos Plateau is surrounded by relatively fertile plains and valleys, where the human population density is highest, and the regional boundary is mountainous. The spatial data of SOC could be useful as an important initial state in regional SOC modelling and possibly be used in calibration and prediction processes in the remote sensing method to estimate SOC content for large-scale areas.

Cattle trampling alters soil properties and changes soil microbial communities in a Swiss sub-alpine pasture

Stock farming plays an important role in the agriculture of alpine regions although deleterious effects on the soils are most pronounced here. We investigated the effects of cattle trampling on soil physical, chemical and microbial properties in a Swiss sub-alpine pasture. About 10% of the study site was bare of vegetation as a result of repeated cattle trampling and the bulk density of these bare steps was 20% higher than of the soils unaffected by trampling. In the upper 25cm, soil organic carbon (SOC) concentrations and total SOC stocks were 35% and 20% respectively lower than on the vegetated slope. As compared with the vegetated slope, topsoils of the bare steps featured narrower C:N-ratios and were more enriched in the 15N isotope, with typical values of deeper soil layers. This indicates that bare soils primarily evolved by erosion and not by a compaction, which might, together with the reduced litter input, explain the lower SOC contents. The abundances of soil microbes, estimated by the concentrations of phospholipid fatty acid (PLFA), were 30% smaller in the bare soils than in the vegetated areas. This depletion was most pronounced for fungi as expressed in the lower concentrations of the fatty acid 18:2ω6.9 (45%) and ergosterol (50%). The lower fungal abundance very likely has negative consequences for the stability of the bare soils, since fungi play an important role in the formation of soil aggregates. In summary, our results show that cattle trampling decreases soil carbon storage and alters soil microbial community structure.

Carbon accumulation and aggregate stability in an Acrisol under different fallow management in Ghana

Soil organic carbon (SOC) in relation to aggregate stability, plant biomass accumulation and other properties of a Ferric Acrisol under different fallow management practices was determined to ascertain their potential for sequestering carbon. Three minor season fallow treatments replicated four times were natural and burning (T1), natural and plough-in (T2), pigeon pea (T3), bare land (T4), cowpea (T5), mucuna (T6) and natural and fertilized cropped (T7).Inherent nutrient status of the soil was low. Generally, dry matter (DM) yield increased for all the treatments in the minor season of 2006 more than in 2005. A high DM yield for T3 resulted from the shrubby and semi-woody nature of pigeon pea.Moisture stress from low rainfall decrease DM yield in 2007. In 2005, SOC contents of the treatment plots were lower than the initial amount although, T3 produced relatively the highest SOC accumulation (20,293 ± 326 kg C ha-1). In 2006, the legume-amended treatments (T3, T5 and T6) had similar SOC contents as the control (T4); the lower SOC contents in the natural fallow plots confirmed the negative effect of burning, especially in T1. Soil OC accumulation was greater in 2007 than in 2006 (except for T4). Water dispersible silt fraction decreased with increasing SOC accumulation (r value = - 0.88**). Dispersion ratios, more related to SOC (R2 value of -0.95** in the natural fallows and - 0.76* in the legume fallows), generally decreased from an average of 0.88 in 2006 to 0.50 in 2008 emphasizing the positive role of aggregate stability in SOC accumulation. Key words: Organic carbon, crop fallow, dispersion ratio, bulk density, dry matter yield, erodibility, plant biomass, particle size distribution.

The effect of soil carbon on symbiotic nitrogen fixation and symbiotic Rhizobium populations in soil with Trifolium repens as host plant

Soil organic carbon (SOC) is the main attribute of high-quality soil. The amount of nitrogen fixed by Rhizobium symbiotically with Trifolium repens (white clover) is ultimately determined by the quality of the soil environment. The effect of SOC on the total number of symbiotic and saprophytic rhizobia was determined. Subsequently, the amount of nitrogen (N) fixed was assessed by using the N difference technique. Most Rhizobium was detected between a SOC content of 2.03% to 3.80% in both inoculated and non-inoculated soils. Inoculation increased the number of rhizobia in soil. Most N was fixed in the soil with the lowest SOC content. Although the amount of N fixed increased as the level of SOC decreased, the efficiency of N fixation decreased proportionally to SOC. Subsequently, more N was rhizodeposited. It was concluded that symbiotic rhizobia introduced by the inoculant were more efficient than free-living rhizobia in soils with higher carbon content, which highlights the importance of inoculation in improving the sustainable production of T. repens pastures.

Soil organic carbon distribution in relation to land use and its storage in a small watershed of the Loess Plateau, China

Abstract Soil organic carbon (SOC) is an important component in agricultural soil, and its stock is a major part of global carbon stocks. Estimating the SOC distribution and storage is important for improving soil quality and SOC sequestration. This study evaluated the SOC distribution different land uses and estimated the SOC storage by classifying the study area by land use in a small watershed on the Loess Plateau. The results showed that the SOC content and density were affected by land use. The SOC content for shrubland and natural grassland was significantly higher than for other land uses, and cropland had the lowest SOC content. The effect of land use on the SOC content was more significant in the 0–10 cm soil layer than in other soil layers. For every type of land use, the SOC content decreased with soil depth. The highest SOC density (0–60 cm) in the study area was found in shrublandII (Hippophae rhamnoides), and the other land uses decreased in the SOC density as follows: natural grassland > shrublandI (Caragana korshinskii) > abandoned cropland > orchard > level ground cropland > terrace cropland > artificial grassland. Shrubland and natural grassland were the most efficient types for SOC sequestration, followed by abandoned cropland. The SOC stock (0–60 cm) in this study was 23,584.77 t with a mean SOC density of 4.64 (0–60 cm).