Spatial Characteristics of Soil Organic Carbon Storage in China＇s Croplands

The soil organic carbon （SOC） pool is the largest component of terrestrial carbon pools. With the construction of a geographically referenced database taken from the second national general soil survey materials and based on 1546 typical cropland soil profiles, the paddy field and dryland SOC storage among six regions of China were systematically quantified to characterize the spatial pattern of cropland SOC storage in China and to examine the relationship between mean annual temperature, precipitation, soil texture features and SOC content. In all regions, paddy soils had higher SOC storage than dryland soils, and cropland SOC content was the highest in Southwest China. Climate controlled the spatial distribution of SOC in both paddy and dryland soils, with SOC storage increasing with increasing precipitation and decreasing with increasing temperature.     

中国水稻土磷储量及其空间分异

Due to the growing concern about the agricultural phosphorus (P) losses pollution, an in-depth understanding of P in paddy soils of China would be helpful in providing a national perspective of the environmental impact of P cycling and fertility on China’s farms. In this study, we evaluated the P storage and the P density of paddy soils in China, characterized the spatial variations of P among the subgroups of paddy soils and soil regions in China, and evaluated the P data using GIS-based analysis, which included a newly compiled 1:1 M digital soil map of China, and using 1 490 soil profiles. The available and total P densities of paddy soils were 6.7 and 698.5 g m-3, respectively. Overall in China, the total P storage within 1 m of paddy soils was estimated to be 330.1 Tg. The P density of paddy soils varied substantially with subgroups due to the different soil water regimes such as groundwater table and soil drainage. The P availability in paddy soils, especially in surface layer, was higher in high temperature and precipitation areas. Further research is needed to examine more anthropogenic impact factors, such as increasing use of chemical fertilizer.     

基于土壤剖面测定数据计算中国土壤有机碳贮量

Soil organic carbon (SOC) storage under different types of vegetations in China were estimated using measured data of 2 440 soil profiles to compare SOC density distribution between different estimates, to map the soil organic carbon stocks under different types of vegetation in China, and to analyze the relationships between soil organic carbon stocks and environmental variables using stepwise regression analyses. Soil organic carbon storage in China was estimated at 69.38 Gt (1015 g). There was a big difference in SOC densities for various vegetation types, with SOC distribution closely related to climatic patterns in general. Stepwise regression analyses of SOC against environmental variables showed that SOC generally increased with increasing precipitation and elevation, while it decreased with increasing temperature. Furthermore, the important factor controlling SOC accumulation for forests was elevation, while for temperate steppes mean annual temperature dominated. The more specific the vegetation type used in the regression analysis, the greater was the effect of environmental variables on SOC. However, compared to native vegetation, cultivation activities in the croplands reduced the influence of environmental variables on SOC.     

基于Century模型和1:500 000土壤数据库的华东旱地土壤有机碳动态模拟

Changes in soil organic carbon (SOC) in agricultural soils influence soil quality and greenhouse gas concentrations in the atmosphere. Dry farmland covers more than 70% of the whole cropland area in China and plays an important role in mitigating carbon dioxide (CO2) emissions. In this study, 4109 dry farmland soil polygons were extracted using spatial overlay analysis of the soil layer (1:500000) and the land use layer (1:500000) to support Century model simulations of SOC dynamics for dry farmland in Anhui Province, East China from 1980 to 2008. Considering two field-validation sites, the Century model performed relatively well in modeling SOC dynamics for dry farmland in the province. The simulated results showed that the area-weighted mean soil organic carbon density (SOCD) of dry farmland increased from 18.77 Mg C ha1 in 1980 to 23.99 Mg C ha1 in 2008 with an average sequestration rate of 0.18 Mg C ha1 year?1. Approximately 94.9% of the total dry farmland area sequestered carbon while 5.1% had carbon lost. Over the past 29 years, the net SOC gain in dry farmland soils of the province was 19.37 Tg, with an average sequestration rate of 0.67 Tg C year1. Augmentation of SOC was primarily due to increased consumption of nitrogen fertilizer and farmyard manure. Moreover, SOC dynamics were highly differentiated among dry farmland soil groups. The integration of the Century model with a fine-scale soil database approach could be conveniently utilized as a tool for the accurate simulation of SOC dynamics at the regional scale.     

气候和土壤质地对土壤有机碳影响的区域差异研究

The agricultural soil carbon pool plays an important role in mitigating greenhouse gas emission ana unaerstanamg the son orgamc carbon-climate-soil texture relationship is of great significance for estimating cropland soil carbon pool responses to climate change. Using data from 900 soil profiles, obtained from the Second National Soil Survey of China, we investigated the soil organic carbon （SOC） depth distribution in relation to climate and soil texture under various climate regimes of the cold northeast region （NER） and the warmer Huang-Huai-Hai region （HHHR） of China. The results demonstrated that the SOC content was higher in NER than in HHHR. For both regions, the SOC content at all soil depths had significant negative relationships with mean annual temperature （MAT）, but was related to mean annual precipitation （MAP） just at the surface 0-20 cm. The climate effect on SOC content was more pronounced in NER than in HHHR. Regional differences in the effect of soil texture on SOC content were not found. However, the dominant texture factors were different. The effect of sand content on SOC was more pronounced than that of clay content in NER. Conversely, the effect of clay on SOC was more pronounced than sand in HHHR. Climate and soil texture jointly explained the greatest SOC variability of 49.0% （0-20 cm） and 33.5% （20-30 cm） in NER and HHHR, respectively. Moreover, regional differences occurred in the importance of climate vs. soil texture in explaining SOC variability. In NER, the SOC content of the shallow layers （0-30 cm） was mainly determined by climate factor, specifically MAT, but the SOC content of the deeper soil layers （30-100 cm） was more affected by texture factor, specifically sand content. In HHHR, all the SOC variability in all soil layers was predominantly best explained by clay content. Therefore, when temperature was colder, the climate effect became stronger and this trend was restricted by soil depth. The regional differences and soil depth influence underscored the importance of explicitly considering them in modeling long-term soil responses to climate change and predicting potential soil carbon sequestration.     

江苏省土壤有机碳空间差异性以及影响因素研究

Soil organic carbon (SOC) plays a key role in the global carbon cycle.In this study,we used statistical and geostatistical methods to characterize and compare the spatial heterogeneity of SOC in soils of Jiangsu Province,China,and investigate the factors that influence it,such as topography,soil type,and land use.Our study was based on 24 186 soil samples obtained from the surface soil layer (0-0.2 m) and covering the entire area of the province.Interpolated values of SOC density in the surface layer,obtained by kriging based on a spherical model,ranged between 3.25 and 32.43 kg m 3.The highest SOC densities tended to occur in the Taihu Plain,Lixia River Plain,along the Yangtze River,and in high-elevation hilly areas such as those in northern and southwest Jiangsu,while the lowest values were found in the coastal plain.Elevation,slope,soil type,and land use type significantly affected SOC densities.Steeper slope tended to result in SOC decline.Correlation between elevation and SOC densities was positive in the hill areas but negative in the low plain areas,probably due to the effect of different land cover types,temperature,and soil fertility.High SOC densities were usually found in limestone and paddy soils and low densities in coastal saline soils and alluvial soils,indicating that high clay and silt contents in the soils could lead to an increase,and high sand content to a decrease in the accumulation of SOC.SOC densities were sensitive to land use and usually increased in towns,woodland,paddy land,and shallow water areas,which were strongly affected by industrial and human activities,covered with highly productive vegetation,or subject to long-term use of organic fertilizers or flooding conditions.     

土壤矿物吸附和土壤团聚体对土壤有机碳含量的影响研究

Soil organic carbon (SOC) can act as a sink or source of atmospheric carbon dioxide; therefore, it is important to understand the amount and composition of SOC in terrestrial ecosystems, the spatial variation in SOC, and the underlying mechanisms that stabilize SOC. In this study, density fractionation and acid hydrolysis were used to assess the spatial variation in SOC, the heavy fraction of organic carbon (HFOC), and the resistant organic carbon (ROC) in soils of the southern Hulunbeier region, northeastern China, and to identify the major factors that contribute to this variation. The results showed that as the contents of clay and silt particles (0--50 μm) increased, both methylene blue (MB) adsorption by soil minerals and microaggregate contents increased in the 0--20 and 20--40 cm soil layers (P < 0.05). Although varying with vegetation types, SOC, HFOC, and ROC contents increased significantly with the content of clay and silt particles, MB adsorption by soil minerals, and microaggregate content (P < 0.05), suggesting that soil texture, the MB adsorption by soil minerals and microaggregate abundance might be important factors influencing the spatial heterogeneity of carbon contents in soils of the southern Hulunbeier region.     

Impacts of land use and salinization on soil inorganic and organic carbon in the middle-lower Yellow River Delta

Soil inorganic carbon (SIC) is an important reservoir of carbon (C) in arid, semi-arid, and semi-humid regions. However, knowledge is incomplete on the dynamics of SIC and its relationship with soil organic C (SOC) under different land use types in the semi-humid region, particularly in coastal zones impacted by soil salinization. We collected 170 soil samples from 34 profiles across various land use types (maize-wheat, cotton, paddy, and reed) in the middle-lower Yellow River Delta (YRD), China. We measured soil pH, electrical conductivity (EC), water-soluble salts, and SOC and SIC contents. Our results showed significant differences in both SOC and SIC among land use types. The dry cropland (maize-wheat and cotton) soils had significantly higher SOC and SIC densities (4.71 and 15.46 kg C m^-2, respectively) than the paddy soils (3.28 and 14.09 kg C m^-2, respectively) in the 0–100 cm layer. Compared with paddy soils, reed soils contained significantly higher SOC (4.68 kg C m^-2) and similar SIC (15.02 kg C m^-2) densities. There was a significant positive correlation between SOC and SIC densities over a 0–100 cm soil depth in dry cropland soils, but a negative relationship in the paddy soils. On average, SOC and SIC densities under maize-wheat cropping were 15% and 4% lower, respectively, in the salt-affected soils in the middle-lower YRD than the upper YRD. This study indicated that land use types had great influences on both SOC and SIC and their relationship, and salinization had adverse effect on soil C storage in the YRD.     

不同气候-土壤-耕作组合条件下水稻土有机碳长期演变模拟的不确定性和敏感性分析

Reporting modeling results with uncertainty information can benefit decision making by decreasing the extent that variability exerts a disproportionate influence on the options selected. For making decisions with more confidence, the uncertainty interval should be as narrow as possible. Here, the soil organic carbon(SOC) dynamics of the major paddy soil subgroup from 4 different paddy field regions of China(located in 4 counties under different climate-soil-management combinations) were modeled using the De NitrificationDe Composition(DNDC) model for the period from 1980 to 2008. Uncertainty intervals associated with the SOC dynamics for these 4 subgroups were estimated by a long-term global sensitivity and uncertainty analysis(i.e., the Sobol′method), and their sensitivities to 7 influential factors were quantified using the total effect sensitivity index. The results, modeled with high confidence, indicated that in the past 29 years, the studied paddy soils in Xinxing, Yixing, and Zhongjiang counties were carbon(C) sinks, while the paddy soil in Helong County was a C source. The 3 C sinks sequestered 12.2(5.4, 19.6), 17.1(8.9, 25.0), and 16.9(-1.2, 33.6) t C ha~(-1)(values in the parentheses are the 5th and 95th percentiles, respectively). Conversely, the C source had a loss of -5.4(-14.2, 0.06) t C ha~(-1) in the past 29 years. The 7 factors, which changed with the climate-soil-management context, exhibited variable influences on modeled SOC. Measures with potential to conserve or sequestrate more C into paddy soils, such as incorporating more crop residues into soils and reducing chemical fertilizer application rates, were recommended for specific soils based on the sensitivity analysis results.     

内蒙古荒漠化草地土壤碳、氮养分及土壤呼吸的空间异质性研究

There is a limited knowledge of spatial heterogeneity in soil nutrients and soil respiration in the semi-arid and arid grasslands of China. This study investigated the spatial differences in soil nutrients and soil respiration among three desertified grasslands and within two shrub-dominated communities on the Ordos Plateau of Inner Mongolia, China in 2006. Both soil organic carbon (SOC) and total nitrogen (TN) were significantly different (P < 0.01) among the three desertified grasslands along a degradation gradient. Within the two shrub-dominated communities, the SOC and TN contents decreased with increasing distance from the main stems of the shrub, and this “fertile island” effect was most pronounced in the surface soil. The total soil respirations during the growing season were 131.26, 95.95, and 118.66 g C m-2, respectively, for the steppe, shrub, and shrub-perennial grass communities. The coefficient of variability of soil respiration was the highest in the shrub community and lowest in the steppe community. CO2 effluxes from the soil under the canopy of shrub were significantly higher than those from the soil covered with biological crusts and the bare soil in the interplant spaces in the shrub community. However, soil respiration beneath the shrubs was not different from that of the soil in the inter-shrub of the shrub-perennial grass community. This is probably due to the smaller shrub size. In the two shrub-dominated communities, spatial variability in soil respiration was found to depend on soil water content and C:N ratio.     

中国农田表土有机碳含量变化特征——基于国家耕地土壤监测数据

通过对299个国家级耕地土壤监测点20余年数据的统计分析,评价了我国农田表土有机碳含量变化情况和固碳潜力。结果表明,全国约80％试验点有机碳年平均相对增长率（Average relative annual increment,ARAI）在-1．5％～7．5％。中国农田表土有机碳含量整体呈上升趋势。东北、华北等6个地理区域分析得出,华北、华东、西南农田表土有机碳含量显著增加;华东地区有机碳增加的农田面积占全国农田比例最大,东北最小。旱地和水田有机碳含量增加显著;水田有机碳增加的试验点所占比例大于旱地;对ARAI与初始有机碳含量进行相关分析得出,我国旱地和水田有机碳潜在储存能力估计值分别为17．2和27．7g·kg^-1。农田土壤类型中水稻土和褐土有机碳含量增加显著;黑土有机碳含量下降样本所占比例最高。对我国各典型种植制度分析得出,双季稻、麦-稻、麦-玉、单季小麦种植制度下农田有机碳有了显著增加;麦玉轮作较其他种植制度的农田有机碳年平均相对增长率高。     

农业开垦对中国土壤有机碳的影响

以中国土种志资料为基础,分析了耕地和非耕地表层土壤有机碳含量的变异性,以及各大区域土壤有机碳含量在耕地和非耕地、水田和旱地之间的差异;同时结合非耕地有机碳含量和耕地面积数据,估算了由于耕地的开垦而导致的土壤表层有机碳贮量的变化。结果表明,耕地土壤有机碳含量的变异性小于非耕地,水田小于旱地;耕地土壤有机碳含量明显低于非耕地土壤,平均减少了51.5%,水田却明显高于旱地;水田和旱地的开垦分别导致土壤表层有机碳贮量减少了0.67,3.63 Pg,共计4.3 Pg。最后还分析了土壤有机碳库减少量的空间分布格局,探讨了东北和华北这两大区域土壤有机碳贮量下降程度最大的原因,是由于黑土、黑钙土、暗棕壤等富含有机碳的草原草甸和湿地土壤大量开垦以及耕作管理措施粗放所致。     

中国稻田土壤有机碳汇特征与影响因素的研究进展

稻田是中国面积最大的耕地之一，稻田土壤有机碳（SOC）是重要的农业碳库，被认为在减缓大气二氧化碳（CO2）浓度上升和全球变暖中起着重要作用。明确中国稻田SOC汇特征与影响因素，有助于制定合理的农业管理措施，科学地增强稻田土壤固碳减排潜力。研究发现，在空间分布上，中国稻田SOC含量具有地域性差异，总体表现为华南、西南高于华北、西北，长江中游高于长江下游；且稻田SOC含量沿海拔升高而增加，随土壤深度增加而减少。在组成上，稻田土壤活性碳比例不超过5.3%，惰性碳比例远大于活性碳，高达60%以上，稻田固碳重点在于惰性组分。在影响因素上，人为管理措施是导致稻田碳汇变化的主要原因，并与自然因素密切相关。为充分发挥稻田土壤碳汇功能，未来研究应加强稻田SOC稳定机制研究，制定因地制宜的农业管理推广方案，为中国“双碳”目标的实现提供科学依据。     

Landuse impacts on soil organic carbon fractions in different rainfall areas of a subtropical dryland

Landuse can alter soil organic carbon (SOC) fractions by affecting carbon inflows and outflows. This study evaluated changes in SOC fractions in response to different landuses under variable rainfalls. We compared cropland, grassland and forest soils in high rainfall (Islamabad ~1142 mm) and low rainfall (Chakwal ~667 mm) areas of Pothwar dryland, Pakistan. Forest soils in both rainfall areas had highest SOC (11.32 g kg^?1), particulate organic carbon (POC, 1.70 g kg^?1), mineral-associated organic carbon (MOC, 7.17 g kg^?1) and aggregate-associated organic carbon (AOC, 7.86 g kg^?1). However, in rangeland and cropland soils, these varied with rainfall. Under high rainfall, SOC and MOC were 12% and 17% higher in rangeland than in cropland while POC and AOC were equal. Under low rainfall, SOC and MOC were higher in rangeland than in cropland by 7.21 and 1.79 g kg^?1 at 0–15 cm and equal at 15–30 cm depth. POC and AOC were higher in rangeland than in cropland, in both depths. Averagely, SOC, POC, MOC and AOC were 26%, 68%, 76% and 30% higher in high rainfall than in low rainfall soils. Sensitivity of SOC fractions to landuses observed under different rainfalls could provide useful information for soil management in subtropical drylands.     

1988―2019年中国农田表土有机碳库密度变化及其主要影响因素

  【目的】  研究中国农田土壤有机碳(SOC)密度时空变化特征及其主要驱动因素,为土壤肥力提升、固碳减排和粮食安全提供理论依据。  【方法】  基于1988至2019年农业农村部全国农田监测数据(1298个点位),分析水田、旱地和水旱轮作下SOC密度时空变化特征,利用方程拟合和提升回归树模型探究气候、施肥和土壤属性对SOC密度变化的影响。  【结果】  1988―2019年全国农田表层(0―20 cm) SOC密度平均为35.13 t/hm2,不同土地利用方式下表现为水田>水旱轮作>旱地,水田和水旱轮作较旱地分别高53.2%和24.9%。SOC密度随监测时间的延长呈先降低后增加的趋势,其中水田、旱地和水旱轮作分别在2000、1998和2004年之前呈下降趋势,之后呈上升趋势。不同利用方式水田SOC密度随监测时间的变化趋势在东北地区逐渐增加,华南地区逐渐降低,西南和长江中游地区则先降低后增加,转折点分别在1995和2002年;旱地SOC密度的变化趋势在西北、华北和华南地区逐渐增加,东北、长江中游和长江下游地区先降低后增加,转折点分别在2008、2004和2004年;水旱轮作下SOC密度的变化趋势在长江中游、下游地区先降低后增加,转折点分别在2001和2013年,在西南地区呈先上升后微弱下降趋势,转折点在2012年。提升回归树结果显示,水田SOC密度后期上升阶段最重要的解释变量在西南、东北和华南区是年均温,在长江中游和下游分别是钾肥用量和土壤速效钾含量;旱地SOC密度后期上升阶段最重要的解释变量在东北为年均温,华北和华南为年均降雨,长江中游、下游区为氮肥,西北为有效磷;水旱轮作SOC密度后期上升阶段最重要的解释变量,在西南、长江中、下游地区分别为年均降雨、有效磷、氮肥。  【结论】  1988―2019年全国农田表层SOC密度除华南水田外,整体上呈先降低后增加,水田、旱地和水旱轮作土壤的转折点分别在2000、1998和2004年,旱地SOC密度的提升快于水田和水旱轮作。影响表土SOC密度提升的主要因素,东北和西南地区水田和旱地为年均温,长江中、下游地区水田为钾肥投入和土壤速效钾含量,旱地为氮肥投入;华南地区水田为年均温,旱地为年降雨量;华北和西北地区旱地为年均降雨和土壤有效磷含量;西南和长江中游水旱轮作区为土壤有效磷含量,而长江下游为土壤速效钾含量。     

Soil organic carbon stocks in the Yunnan Plateau,southwest China: spatial variations and environmental controls

Purpose

Greater increases in temperature are expected in high mountain regions than the global average making mountain soils a potentially vulnerable component of the global carbon cycle. Little is known about the spatial distribution and environmental factors controlling soil carbon in mountainous regions. This study analyzed the spatial patterns of soil organic carbon (SOC) density in mountainous regions and evaluated the impact of various environmental factors on SOC density.

Materials and methods

This study focused on Yunnan Province, a typical high-elevation, mountainous region in southwest China. A soil database was constructed based on 684 soil profiles documenting 12 environmental factors.

Results and discussion

Significant differences in SOC density among the upper and subsoil layers were found. Forestland and grassland SOC density were significantly higher than that of cropland. The widely distributed forestland in Yunnan Province had a higher SOC storage capacity than non-forested areas. The forests of the northwestern mountainous region had higher SOC densities than those of the agricultural areas of the southeastern plateau region. Environmental factors affected SOC density more strongly in forestland than grassland and cropland, and those factors were better correlated to the 0–20 cm layer of soils than the 20–100 cm layer. Elevation is the major controlling factor controlling SOC density for the three land uses. Correlating topography to SOC density proved difficult. SOC densities were highest when annual temperatures ranged from 0–2 °C. SOC density decreased with increased precipitation up to 800 mm per year and then remained constant. The SOC density increased with elevation up to 4,000 m then decreased.

Conclusions

The results indicate that both the local land use development plans and regional climatic change scenarios may result in a decrease in SOC storage in this region and increase the CO₂ flux into the atmosphere. In consideration of the high sensitivity of the regional response to global warming, effective ecological compensation mechanisms and active forest protection should be undertaken in this region.     

Number of soil profiles needed to give a reliable overall estimate of soil organic carbon storage using profile carbon density data

Abstract

Soil profile data are the basis for estimating soil organic carbon (SOC) storage and any changes in SOC storage, which are of great significance in terms of global climate change. Estimates based on various profile data have been made for SOC storage in China. Modeling studies have given contrasting results on changes in SOC storage in Chinese croplands. A certain number of measured soil profile data are needed to validate the modeled results. In the present study, we examined the relationships between sample size, population variance and detection limit using the central limit theorem and the statistical properties of the normal distribution. Based on the profile dataset from the Second National Soil Survey in China, we calculated that to derive a reliable estimate of the overall mean SOC density for all the soils of China, a sample size of 4,000 soil profiles is needed. In this case, a reliable estimate is defined as having a 95% confidence interval and allowing a ±5% detection limit of SOC. The necessary sample size for cropland soils is 1,250. Our results indicate that approximately 100 samples only are needed to validate a modeled SOC loss of 20–30% in cropland soils in China. By aggregating the soil profiles in the dataset into soil orders and calculating the variance of each soil order, we show that the sample sizes in the dataset are insufficient to give reliable estimates on the carbon densities of most soil orders; thus, we conclude that there is considerable uncertainty in the SOC distribution maps resulting from the Second National Soil Survey.     

Soil organic carbon as affected by land use in young and old reclaimed regions of a coastal estuary wetland,China

Soil organic carbon (SOC) and selected soil properties were measured in fringe and ditch marshes and cropland of old and young reclaimed areas in a subtropical estuary in China in order to investigate the effects of land use and reclamation history on SOC. The results show that after the conversion of wetlands to cropland, a longer reclamation history (>20 yr) resulted in greater soil bulk density, salinity, clay and silt, and lower soil moisture, SOC and sand content, whereas a shorter reclamation history (<20 yr) induced smaller values for soil pH, moisture and sand. Ditch marshes had greater average SOC in the top 50 cm than fringe marshes and cropland. SOC decreased generally down soil profiles from 0 to 50 cm in depth, except for the obvious accumulation of SOC in deeper soils from old fringe and young ditch marshes. Ditch marshes had the greatest SOC densities in the top 50 cm in both regions compared to the other land uses. SOC densities in the top 50 cm were less in croplands than in fringe marshes in the young region, while there were no significant differences between them in the older one. Except for cropland, SOC densities in the top 50 cm of the fringe or ditch marshes in the old region were not significantly different from those in the young region. SOC in both regions was reduced by 13.53 × 10⁴ t (12.98%) in the top 50 cm of the marshes after conversion to cropland, whereas the regional SOC storage increased by 29.25 t when ditch marshes were included. The results from regression analysis show that bulk density and soil moisture significantly influenced SOC. The total SOC stored in both ditch marshes and croplands was higher compared to fringe marshes. The regional SOC storage in the top 50 cm was not reduced after reclamation due to C accumulation in the ditch marshes. The regional effects of cultural practices should be taken into account in devising strategies for managing soils in coastal wetlands, particularly in the developing world.