Soil organic carbon stocks of conifers, broadleaf and evergreen broadleaf forests of Spain

Forests represent an important resource for mitigating the greenhouse effect, but which is the contributions of the different forest types in sequestering and keeping soil C for a longer time is still uncertain, particularly in the Mediterranean area. The aim of this work is to quantify the soil organic C (SOC) stock in the 0–30 and 0–100?cm depths of mineral soil, according to the main forest types—conifers, broadleaf and evergreen broadleaf—and the different climatic zones of Spain, using a database comprising records of 1,974 pedons. Conifers and broadleaf forests show a trend in SOC stock distribution, with the stocks decreasing with increasing Mediterranean conditions. On average, in the 0–30?cm depth, the soils under broadleaf store the highest amount of SOC (5.9?±?0.1?kg?m^?2), followed by conifers (5.6?±?0.1?kg?m^?2) and evergreen broadleaf soils with an amount always lower (3.4?±?0.2?kg?m^?2). Climate and forest cover are the principal factors in determining the amount of SOC stored in Spanish forests. The significantly higher amount of SOC found in conifers and broadleaf forests than the evergreen broadleaf forests leads us to hypothesize a decrease in the SOC if climate change will increase drought periods with a consequent expansion of this latter forest type. Correlations between the SOC stocks under the different forest types, climate and soil features support the major role of climate and vegetation in controlling SOC sequestration in the Mediterranean area, while the effect of texture is less pronounced. Assigning a precise SOC stock to the different forest types, according to each climatic zone, would notably help to obtain an accurate SOC estimate at national level and for future assessments of the status of this large C reservoir.     

A multiple regression approach to assess the spatial distribution of Soil Organic Carbon (SOC) at the regional scale (Flanders,Belgium)

Estimates of the amount of Soil Organic Carbon (SOC) at the regional scale are important to better understand the role of the SOC reservoir in global climate and environmental issues. This study presents a method for estimating the total SOC stock using data from Flanders (Belgium). More than 6900 SOC measurements from the national soil survey (database ‘Aardewerk’) are combined with a digital land use map and a digital soil map of Flanders. The spatial distribution of the SOC stock is studied in its relation to factors such as soil texture, soil moisture (drainage class) and land use. The resulting map with a resolution of 15 m consists of different classes forming a combination of these environmental factors. The results show that the lowest SOC amount (kg m^? 2) is stored under cropland whereas the highest amount is found under grassland. Regarding the effect of soil properties, a significant correlation between SOC stock and depth of the ground water table is observed. Sandy loam soils stock the lowest SOC amount (kg m^? 2), whereas clay soils retain the highest SOC amount. First, the mean SOC amounts of the land use–soil type classes are calculated and assigned to the corresponding cells in order to obtain a total SOC stock with its spatial distribution for Flanders. Then, a multiple regression model is applied to predict the SOC value of a particular land use–soil type class on the map. This model is based on the observed relationships between SOC and land use–soil type characteristics, using the entire dataset. The first approach does not allow to obtain a (reliable) SOC value for all land use–soil type classes due to a lack of samples in some classes. A major advantage of the regression model approach is the attribution of class specific SOC values to each land use–soil type class, regardless of the number of observations in the classes. Consequently, by applying the model approach instead of the mean approach, the area for which a reliable SOC estimate could be obtained increased by 8.1% (from 9420 km² to 10179 km²) and the total predicted SOC stock increased by 10.1% (from 88.7 ± 5.6 Mt C to 97.6 ± 1.1 Mt C).     

Influence of estimation procedure on soil organic carbon stock assessment in Flanders, Belgium

Abstract. The purpose of the study was to determine the soil organic carbon (SOC) stock for Flanders, Belgium and to evaluate various methods for assessing SOC stock. The assessment methods first determined the SOC density (C mass per unit area) for pedons in a database of soil properties, and then spatially distributed the SOC density to soil and soil/land use categories on a map. The results showed that the pedon SOC density is influenced by drainage class, texture and land use/land cover. The SOC density estimation method significantly influences results and leads to differences of up to 6% in total estimated SOC stock for Flanders. Use of various spatial distributing methods creates differences of up to 2% in total estimated SOC stock. The largest difference in SOC stock estimate between any combination of assessment methods was 7% (125.6 Tg vs 134.9 Tg). These findings emphasize the importance of complete spatial soil databases of high quality that reduce uncertainty of estimates for use in research examining the role of soils in the C cycle. The results indicate that the need for these databases is greater than the need to standardize methods to determine the spatial distribution of SOC. A map of the distribution of SOC density shows that in Flanders a large proportion of SOC is stored in sandy soils in the north of the territory.     

A State‐Space Analysis of Soil Organic Carbon in China's Loess Plateau

An accurate estimation of soil organic carbon (SOC) is important for the evaluation and management of carbon (C) flux in terrestrial ecosystems. However, there is little work on the spatial variability of SOC in deep soils and its driving factors. Thus, the objective of the study was to derive the primary factors dominating the spatial distribution of SOC in different soil layers with the use of the autoregressive state‐space approach. The concentration of SOC was measured to the depth of 500 cm (n  = 86) along a south–north transect of China's Loess Plateau. The mean SOC of the 500‐cm soil profile generally decreased from south to north following the decreasing rainfall gradient. Based on the investigated factors, the state‐space model was able to capture 90.3–99.9% of the spatial variability of SOC in the various soil layers. According to the coefficients in the optimal state‐space model for each soil layer, climatic factors such as precipitation and temperature had a dominant control over the spatial distribution of SOC at shallow depths. However, both climatic and edaphic (e.g. soil texture) factors, and to a small extent land use, influenced the spatial behavior of SOC at the 40–200 cm soil depth. For soil layers below 200 cm, the importance of land use was revealed, and the spatial characteristics of SOC were together driven by land use, climatic and edaphic factors. This is critical for the management of soil C flux in deep soils and the C stock and cycle in terrestrial ecosystems. Table SI. Basic properties of soils and climate and elevation under three land uses along the south–north transect on the Loess Plateau (mean ± standard deviation). Note that SWC is gravimetric soil water content. Copyright © 2016 John Wiley & Sons, Ltd.     

Environmental and site factors controlling the vertical distribution and radiocarbon ages of organic carbon in a sandy soil

Soil organic carbon (SOC) content and radiocarbon concentration were measured in three particle-size fractions and charcoal fragments at four depths to bedrock in a sandy soil from SE Australia. SOC content declined with depth for all fractions. The enrichment factors of SOC showed that the finest particles are most important for SOC storage throughout the soil profile, and their importance for SOC storage increased with depth. In the topsoil, all particle-size fractions contained modern SOC. In contrast, charcoal from this depth gave radiocarbon ages of 85–165 years Before Present (BP). This difference was more pronounced at 30–60 cm, where the charcoal was dated at 2,540 years BP, over 12 times as old as the youngest fraction at that depth. These results confirm charcoal as a highly stable form of SOC. The radiocarbon data in the topsoil and near bedrock indicate that neither microaggregation nor mineral association is important for SOC stability in this soil. At intermediate sampling depths, the mid-sized fraction was the oldest. We believe that this is the result of charcoal accumulation in this fraction, inducing a shift in radiocarbon age. However, near bedrock (100–120 cm), radiocarbon concentration did not differ significantly between fractions, despite greater SOC retention in smaller fractions. In addition, radiocarbon ages at 100–120 cm indicate that charcoal is not present at this depth. We propose that environmental and soil conditions (substrate limitation, water and oxygen availability, and temperature) are responsible for the stabilization of SOC at this depth, where SOC concentrations were very low (0.1–0.3 %). Our results demonstrate that, although fine particles retain more SOC than coarse ones, they do not stabilize SOC in this sandy soil. Instead, environmental (bushfires and climate) and site factors (soil texture and soil mineralogy) control the distribution and stability of SOC throughout the soil profile.     

Land use and climate change impacts on soil organic carbon stocks in semi-arid Spain

Purpose

The sensitivity of soil organic carbon to global change drivers, according to the depth profile, is receiving increasing attention because of its importance in the global carbon cycle and its potential feedback to climate change. A better knowledge of the vertical distribution of SOC and its controlling factors—the aim of this study—will help scientists predict the consequences of global change.

Materials and methods

The study area was the Murcia Province (S.E. Spain) under semiarid Mediterranean conditions. The database used consists of 312 soil profiles collected in a systematic grid, each 12 km² covering a total area of 11,004 km². Statistical analysis to study the relationships between SOC concentration and control factors in different soil use scenarios was conducted at fixed depths of 0–20, 20–40, 40–60, and 60–100 cm.

Results and discussion

SOC concentration in the top 40 cm ranged between 6.1 and 31.5 g?kg^?1, with significant differences according to land use, soil type and lithology, while below this depth, no differences were observed (SOC concentration 2.1–6.8 g?kg^?1). The ANOVA showed that land use was the most important factor controlling SOC concentration in the 0–40 cm depth. Significant differences were found in the relative importance of environmental and textural factors according to land use and soil depth. In forestland, mean annual precipitation and texture were the main predictors of SOC, while in cropland and shrubland, the main predictors were mean annual temperature and lithology. Total SOC stored in the top 1 m in the region was about 79 Tg with a low mean density of 7.18 kg?Cm^?3. The vertical distribution of SOC was shallower in forestland and deeper in cropland. A reduction in rainfall would lead to SOC decrease in forestland and shrubland, and an increase of mean annual temperature would adversely affect SOC in croplands and shrubland. With increasing depth, the relative importance of climatic factors decreases and texture becomes more important in controlling SOC in all land uses.

Conclusions

Due to climate change, impacts will be much greater in surface SOC, the strategies for C sequestration should be focused on subsoil sequestration, which was hindered in forestland due to bedrock limitations to soil depth. In these conditions, sequestration in cropland through appropriate management practices is recommended.     

Soil organic carbon stock assessment for the different cropland land uses in Italy

The aim of this work was to quantify the soil organic C (SOC) stock in the top 30 cm of mineral soil for the whole Italian territory, according to the different land use types of the Intergovernmental Panel on Climate Change (IPCC) cropland category (arable land, agroforestry, vineyards, olive groves, orchards and rice fields), as a basis for future land use scenarios and to address mitigation policy at country level. A database for SOC stock was created with the data from the national project denominated SIAS and partly from regional map reports. All data were referred to the year 2000 since they were derived from surveys conducted from 1995 to 2005. The data were stratified according to the Italian climatic regions, the landscape position and the IPCC cropland subcategories. Taking into account the uncertainty in the estimate, the mean SOC stock values of the different subcategories show significant differences (p < 0.05) among climatic regions and landscapes, ranging from 41.9 ± 15.9 Mg C ha⁻¹ in the vineyards to 63.3 ± 27.9 Mg C ha⁻¹ in the rice fields. Generally, a small decrease of the SOC stock from the temperate regions toward the Mediterranean ones is observed. Taking into account the mean value of each subcategory and the country area they occupied in 2000, the total C stored in the upper 30 cm of soil was estimated at 490.0 ± 121.7 Tg C. The resulting estimate represents the 17% of the value reported by another study for the soil of the whole country down to 50 cm depth, suggesting the importance of preserving this large C pool. Considering the cropland category as a whole, the estimated mean SOC stock is 52.1 ± 17.4 Mg C ha⁻¹, similar to that reported for other European countries, 50–60 Mg C ha⁻¹. In conclusion, the assessment of the mean SOC stock of the different cropland land uses, landscape position and climate regions could notably help when assessing the impact of different agricultural practices and future stock change evaluation.     

藏东南色季拉山西坡不同植被土壤有机碳垂直分布特征及其影响因素

  【目的】  藏东南地区高山生态系统有巨大的土壤碳汇潜力，研究其不同生态系统下土壤有机碳 (SOC) 储存的变化特征及其影响因子，有助于深入了解青藏高原土壤碳循环及区域碳源汇平衡。  【方法】  本研究在西藏色季拉山西坡海拔3000～4600 m开展密集土壤采样，研究不同海拔高度下不同植被类型SOC的储存特征，并分析其关键影响因子。  【结果】  表层0—5 cm的SOC含量随海拔升高而增加，4个植被带SOC含量平均值表现为高寒草甸 (8.31% ± 0.77%) > 暗针叶林 (7.20% ± 0.90%) > 高寒灌丛草甸 (6.74% ± 0.80%) > 针阔混交林 (3.88% ± 0.46%)。在剖面5—10、10—15、15—20、20—30、30—40、40—60 cm各层SOC含量随海拔升高呈先增加后降低趋势，SOC含量在4种植被带的平均值表现为暗针叶林 > 高寒灌丛草甸 > 高寒草甸 > 针阔混交林。SOC含量随剖面深度增加而显著下降，高寒草甸和高寒灌丛草甸SOC垂直分布特征为表层聚集型，而针阔混交林和暗针叶林SOC垂直分布特征为普通递减型。剖面0—20、20—40、40—60 cm的SOC储量随海拔升高呈先增加后降低的特征。在表层0—20 cm高寒草甸SOC储量最高 (C 95.66 ± 4.81 t/hm2)；在剖面20—40和40—60 cm暗针叶林SOC储量最高，且其在整个0—60 cm剖面的SOC总储量在所有植被类型中最高 (C 199.14 ± 11.10 t/hm2)；针阔混交林SOC储量在剖面各层均为最低，且其在整个剖面的SOC总储量 (C 111.45 ± 10.30 t/hm2) 显著低于其他植被类型。剖面各层SOC储量与年平均温度、凋落物碳氮比呈显著负相关，而与海拔高度、年平均降水量和土壤含水量呈显著正相关。逐步回归显示土壤含水量是影响剖面各层以及整个剖面SOC储存的关键因子。随机森林模型对SOC储存的解释度为50.32%～65.82%，土壤含水量对表层土体SOC预测的相对贡献最高，年平均温度、年平均降水量和凋落物质量对各层SOC预测均有显著贡献，而植被类型对SOC预测的相对贡献随剖面加深而逐步增加。  【结论】  色季拉山西坡不同海拔高度下SOC的储存特征随不同植被类型和剖面深度而发生显著变化，环境因子(如土壤水分) 对表层土体SOC储存有关键影响，植被类型对深层土体SOC储量变化的预测有重要贡献。     

玉米秸秆覆盖与深翻两种还田方式对黑土有机碳固持的影响

秸秆还田是实现东北黑土肥力提升与保障区域生态环境安全的有效措施。明确玉米秸秆覆盖与深翻还田下土壤有机碳(SOC, Soil Organic Carbon)的变化及其在团聚体中的固持特征,对于揭示秸秆还田后黑土有机碳的稳定机制与固碳潜力具有重要意义。该研究基于黑土区中部6 a定位试验,选择常规种植(CK)、秸秆覆盖还田(SM, Stovers Mulching)和秸秆深翻还田(SI, Stovers Incorporation)3个处理,对0～10、10～20、20～30及30～40 cm土层SOC含量、容重、水稳性团聚体分布及团聚体中有机碳(OC, Organic Carbon)含量进行了分析与测定,并对各处理年均碳投入量、SOC储量与土壤固碳速率等进行了估算。与CK相比,SM处理显著增加了0～10 cm土层SOC含量,增幅为22.4%,但对10～40cm土层SOC含量无显著影响;SI处理显著增加了0～40cm土层SOC含量,增幅为18.1%～41.5%,以20～30cm的增幅最突出。与SM处理相比,SI处理0～10 cm土层SOC储量显著低于前者,而20～30 cm土层SOC储量反之。6 a间,SM处理耕层(0～20 cm)与亚耕层(20～40 cm)土壤固碳速率分别为1.34和0.77 Mg/(hm2·a),SI处理为0.85和1.74 Mg/(hm2·a)。秸秆不同还田方式显著改变了0～40 cm土层团聚体分布及其中OC含量。与CK相比,SM显著增加了耕层大团聚体(0.25 mm)比例与平均质量直径(MWD, Mean Weight Diameter),SI显著提高了0～40 cm土层团聚体MWD,且对10～40 cm土层团聚结构的改善作用优于SM;SM处理显著增加了0～10 cm土层2和0.053 mm粒级团聚体OC含量,SI处理不仅增加了0～10 cm土层2 mm粒级团聚体OC含量,也显著提高了10～40 cm土层各粒级团聚体OC含量。在黑土区,秸秆覆盖还田对SOC的提升主要集中于表层,秸秆深翻还田促进了0～40cm土层SOC积累与土壤团聚结构的改善。     

Impacts of Plant Community Changes on Soil Carbon Contents in Northeastern Illinois

Land-cover changes not only affect regional climates through alteration in surface energy and water balance, but also affect key ecological processes, such as carbon (C) cycling and sequestration in plant ecosystems. The object of this study was to investigate the effects of land-cover changes on the distribution of soil organic carbon (SOC) contents under four plant community types (deciduous forests, pine forests, mixed pine-deciduous forests, and prairies) in northeastern Illinois, USA. Soil samples were collected from incremental soil depths (0–10, 10–20, 20–30, and 30–50 cm) under the studied plant communities. The results showed that SOC concentration decreased with increases of soil depth in the studied forests and prairies. No significant differences of SOC concentrations were found at the upper soil layers (0–10 cm) among the four plant types. However, SOC concentrations were statistically higher at the lower soil depth (30–40 cm) in prairies than in other three forest types. The SOC storage (0–40 cm soil depth) was reduced in an order prairies (250.6) > mixed pine-deciduous forests (240.7) > pine forests (190.1) > deciduous forests (163.4 Mg/ha). The characteristics of relative short life cycle, restively high turnover rate of roots, and large partition of photosynthetic production allocated to belowground were likely attributed to the higher accumulation of C in soils in tallgrass prairies than in forests. Our data indicated the conversion of native tallgrass prairies to pure forest plantations resulted in a considerable decline of SOC storage. Results suggest that land-cover changes have a significant impact on SOC storage and sequestration in plant ecosystems.     

Assessing and mapping topsoil organic carbon stock at regional scale: A scorpan kriging approach conditional on soil map delineations and land use

In order to assess the potential of soils as C reservoir at regional scale, accurate estimates of soil organic carbon (SOC) are required, and different approaches can be used. This study presents a method to assess and map topsoil organic carbon stock (Mg ha⁻¹) at regional scale for the whole Emilia Romagna plain in Northern Italy (about 12 000 km²). A Scorpan Kriging approach is proposed, which combines the trend component of soil properties as derived from the 1:50 000 soil map with geostatistical modeling of the stochastic, locally varying but spatially correlated component. The trend component is described in terms of varying local means, calculated taking into account soil type and dominant land use. The resulting values of SOC, sand, silt, and clay contents are retained for calculating topsoil SOC stocks, using a set of locally calibrated pedotransfer functions (PTFs) to estimate bulk density. The maps of each soil attribute are validated over a subset of 2000 independent and randomly selected observations. As compared to the standard approach based on the mean values for delineation, results show lower standard errors for all the variables used for SOC stock assessment, with a relative improvement (RI) ranging from 4 per cent for SOC per cent to 24 per cent for silt. The total C stock (0–30 cm) in the study area is assessed as 73·24 ± 6·67 M t, with an average stock of 62·30 ± 5·55 Mg ha⁻¹. The SOC stock estimates are used to infer possible SOC stock changes in terms of carbon sequestration potential and potential carbon loss (PCL). Copyright © 2010 John Wiley & Sons, Ltd.     

耕作方式对华北农田土壤固碳效应的影响

研究不同耕作方式对华北农田土壤固碳及碳库管理指数的影响,可为探寻有利于农田固碳的耕作方式提供科学依据。该研究在中国农业大学吴桥实验站进行,试验于2008年设置了免耕秸秆不还田(NT0)、翻耕秸秆不还田(CT0)、免耕秸秆还田(NT)、翻耕秸秆还田(CT)和旋耕秸秆还田(RT)5个处理。研究测定分析了土壤容重、有机碳、易氧化有机碳含量及不同耕作方式下的碳库管理指数。通过对不同耕作方式下0～110cm土壤的分析,结果表明,随着土层的加深,土壤有机碳含量不断下降,NT显著增加了表层(0～10cm)土壤有机碳含量,而>10～50cm有机碳含量较其他处理(NT0除外)有所下降,深层(>50～110cm)处理间差异不明显;土壤容重与有机碳含量呈显著的负相关关系(P<0.01);0～30cm土层有机碳储量以NT最高,CT与其无明显差异,二者较CT0分别高出13.1%和11.0%,而至0～50cm土层,CT的碳储量最高,但与NT无显著差异(P<0.05);与CT0相比,NT0降低了各层土壤易氧化有机碳含量,而NT则在0～10cm土层表现为增加;RT、CT分别显著增加了0～10、>10～30cm土层的碳库管理指数。结果表明,秸秆还田可改善土壤质量,提高农田碳库管理指数,同时碳库管理指数受耕作方式的影响也较大,尤其是CT和RT;NT通过减少土壤扰动、增加有机质的输入,可提高上层土壤有机碳的储量。     

Vertical dynamics of soil organic carbon and its influence on Chengdu Plain paddy soils from the 1980s to the 2010s

ABSTRACT

The vertical dynamics of paddy soil organic carbon (SOC) play an important role in soil quality and carbon cycling. In this study, we used an exponential decay function to estimate the vertical dynamics of SOC content and determined its influencing factors in a typical paddy soil area of the Chengdu Plain from the 1980s to the 2010s. Our results show an overall increase in SOC stocks at 0–100 cm from 11.8 Tg in the 1980s to 13.7 Tg in the 2010s. SOC content increased at depths of 0–40 cm and declined at depths of 40–100 cm over the past three decades. The exponential decay function parameters C₀ and k significantly increased by 31.4% and 18.2% respectively, which suggests the vertical pattern of SOC distribution changed. The increase in nugget effects of C₀ and k and the decrease in the relative contributions of the parent material, subgroup and distance-to-river indicate that extrinsic factors played increasing roles in the vertical variation of SOC content. Our study concludes that rice planting has led to vertical variations of SOC content and decreased the effects of intrinsic factors on the vertical variation of SOC content of Chengdu Plain paddy soils over the past three decades.     

Soil organic carbon stocks in Veracruz State (Mexico) estimated using the 1:250,000 soil database of INEGI: biophysical contributions

Purpose

In this study, we quantified soil organic carbon (SOC) stocks and analyzed their relationship with biophysical factors and soil properties.

Materials and methods

The study region was Veracruz State, located in the eastern part of Mexico, covering an area of 72,410 km². A soil database that contains physicochemical analyses of soil horizons such as carbon concentration data was the source of information used in this study. The database consisted of 163 soil profiles representing 464 genetic horizons. Statistical analysis was used to investigate the effect of each factor (climate, altitude, slope) on SOC stock to 0.50 m depth and to assess differences in the distribution of SOC stock in terms of soil depth (0.0–0.20, 0.20–0.40, 0.40–0.60, 0.60–0.80, 0.80–1.0 m) and land use. In order to compute the spatial distribution of SOC stock to 0.50 m depth based on the soil sampling location, the kriging method was used.

Results and discussion

Results indicated that SOC stock (0.50 m depth) ranged between 0.44 and 41.2 kg C m^?2. Regression analysis showed that SOC stocks (0.50 m depth) are negatively correlated with temperature (r?=??0.38; P?<?0.001) and positively correlated with altitude (r?=?0.40; P?<?0.001) and slope (r?=?0.40; P?<?0.001). In addition, by multiple regression, temperature combined with precipitation explained more SOC stock variations (r?=?0.43; P?<?0.001) than the regression model with precipitation (r?=?0.13; P?=?0.16) alone. Also, slope combined with temperature and precipitation explained more SOC stock variations (r?=?0.46; P?<?0.001) than the regression model with slope alone. Forest lands, grasslands, and croplands have higher SOC stocks in the 0.0–0.20-m soil layer than in deeper layers. On average, forest lands, grasslands, croplands, and other lands (wetland and dunes) had a SOC stock of 13.6, 14.6, 15.1, and 8.5 kg C m^?2 at 1 m depth, respectively. Soil color correlated (?0.25 ≤ r ≤ ?0.89) with SOC content.

Conclusions

Overall, these results indicate the influence of major interactions between biophysical factors and SOC stocks. This research indicated that SOC stock decreased with soil depth, but with slight variations depending on land use. Thus, there remains a need for more SOC data that include an improved distribution of soil sampling points in order to entirely understand the contributions of biophysical factors to SOC stocks in Veracruz State.     

Soil organic carbon in some land uses of Costa Rica

The native vegetation in the Tropics is increasingly replaced by crops, pastures, tree plantations, or settlements with contradictory effects on soil organic carbon (SOC). Therefore, the general objective was to estimate the SOC stock depth distribution to 100-cm depth in soils of Costa Rica and to assess their theoretical carbon (C) sink capacity by different management practices. A study was established in three ecoregions of Costa Rica: the Isthmian-Atlantic Moist Forest (AM), the Pacific Dry Forest (PD), and the Montane Forest (MO) ecoregions. Within each ecoregion, three agricultural land uses and a mature forest were sampled to 100-cm depth. The SOC stock in 0–100 cm depth was 114–150 Mg C ha^?1 for AM, 76–165 Mg C ha^?1 for PD, and 166–246 Mg C ha^?1 for MO. Land use had only weak effects on SOC concentrations and stocks except at PD where both were lower for soils under mango (Mangifera indica) and pasture. This may indicate soil degradation which was also supported by data on SOC stratification. However, it was generally unclear whether differences among land uses within each ecoregion already existed particularly at deeper depths before land-use change, and whether the sampling approach was sufficient to investigate them. Nevertheless, about 26–71% of Costa Rica's total C emissions may be offset by SOC sequestration in agricultural and forest soils. However, ecoregion-specific practices must be implemented to realize this potential.     

Spatial patterns,storages and sources of black carbon in soils from the catchment of Qinghai Lake,China

Black carbon (BC), composed of char and soot, is an important component of soil organic carbon (SOC), and these materials are potentially important for the global carbon cycle and global climate. A thermal‐optical reflectance method was used to determine the spatial patterns of SOC, BC, char and soot in nine soil types collected from 152 sites in the Qinghai Lake catchment. All of the analytes showed large spatial variability: SOC, BC and char were most abundant in bog soils and least abundant in aeolian soils, while soot concentrations in alpine frost desert and in aeolian soils were about half of those in the other soils. The average BC concentration in the 0–20‐cm soil layers was 1.3 g kg^?1, and BC amounted to 5.6% of the SOC. Char, SOC and BC all decreased with soil depth, but soot showed little variation. The proportions of BC to SOC and char to BC showed contrasting trends in four soil profiles; the former increased and the latter decreased with depth. The quantity of SOC sequestered in topsoils of the catchment area was estimated to be 191 Tg; BC accounted for approximately 4.8% of this, and char made up approximately 85% of the total BC stock. The burning of animal dung for domestic cooking apparently was an important source of soil BC: combustion of other biofuels and fossil fuels was the other main source.     

Modelling soil organic carbon stocks and their changes in the northeast of Spain

Currently, there is little information about soil organic carbon (SOC) stocks and changes in Mediterranean areas at a regional scale. We modelled an area of 95 269 km² in northeast Spain using the Global Environmental Facility Soil Organic Carbon (GEFSOC) system to predict SOC stocks and changes in pasture, forest and agricultural soils. The spatial distribution of the different land‐use categories and their change over time was obtained by using the Corine database and official Spanish statistics on land use from 1926 to 2007. The model predicted the largest current SOC stock in forest soils at 578 Tg C. Agricultural soils were the second largest SOC reservoir, containing 244 Tg C. During the last 30 years, the model predicted a total SOC gain in the 0–30‐cm soil layer of 34 Tg C. Forest and grassland‐pasture soils had a decline in their stored SOC of 5 and 3 Tg C, respectively, because of the reduction in the soil surface occupied by both classes. The greatest SOC gain was predicted in agricultural soils with 42 Tg C caused by changes in management, which led to increases in C inputs. Although model uncertainty was not quantified, some hypothetical assumptions about the initialization and parameterization of the model could be potential sources of uncertainty. Our simulations predicted that in northeast Spain soil management has contributed to the sequestration of substantial amounts of atmospheric CO₂ during the last 30 years. More research is needed in order to study the potential role of soils as atmospheric CO₂ sinks under different managements and climatic conditions.     

Spatial variability of soil organic carbon in a catchment of the Loess Plateau

Abstract

The issue of soil organic carbon (SOC) is of increasing concern. Because SOC, as an important soil component in farming systems, is essential for improving soil quality, sustaining food production and quality, and maintaining water quality and as a major part of the terrestrial carbon reservoir, it plays an important role in the global carbon cycle. In this paper, a total of 665 soil samples from different depths were collected randomly in the autumn of 2007, and the spatial variability of SOC content at a small catchment of the Loess Plateau was analysed using classical statistics and geo-statistical analysis. In nonsampled areas classical kriging was utilized for interpolation of SOC estimation. The classic statistical analysis revealed moderate spatial variability with all five layers of SOC-content. In addition, the average SOC content decreased with soil depth and the relationship can be modelled by an exponential equation (y=3.1795x ^?1.2015, R ²=0.9866) and all of the SOC-content data in the different depth were normally distributed. The geo-statistical analysis indicated a moderate spatial dependence in 0–60 cm, while in the 60–80 cm depth spatial dependence was strong. The semi-variogram could be fitted by an exponential model for 0–10 cm depth; by a spherical model for 10–20 cm depth and 60–80 cm depth; and by a Gaussian model for 20–60 cm depth. The range increases with increasing depth. In addition, classical kriging could successfully interpolate SOC content in the catchment. In general, the geo-statistics method on a watershed scale could be accurately used to evaluate spatial variability of the SOC content in the Loess Plateau, China.     

Changes in carbon stocks of Danish agricultural mineral soils between 1986 and 2009

To establish a national inventory of soil organic carbon (SOC) stocks and their change over time, soil was sampled in 1986, 1997 and 2009 in a Danish nation‐wide 7‐km grid and analysed for SOC content. The average SOC stock in 0–100‐cm depth soil was 142 t C ha^?1, with 63, 41 and 38 t C ha^?1 in the 0–25, 25–50 and 50–100 cm depths, respectively. Changes at 0–25 cm were small. During 1986–97, SOC in the 25–50‐cm layer increased in sandy soils while SOC decreased in loam soils. In the subsequent period (1997–2009), most soils showed significant losses of SOC. From 1986 to 2009, SOC at 0–100 cm decreased in loam soils and tended to increase in sandy soils. This trend is ascribed to dairy farms with grass leys being abundant on sandy soils while cereal cropping dominates on loamy soils. A statistical model including soil type, land use and management was applied separately to 0–25, 25–50 and 50–100 cm depths to pinpoint drivers for SOC change. In the 0–25 cm layer, grass leys added 0.95 t C ha^?1 year^?1 and autumn‐sown crops with straw incorporation added 0.40 t C ha^?1 year^?1. Cattle manure added 0.21 t C ha^?1 year^?1. Most interestingly, grass leys contributed 0.58 t C ha^?1 year^?1 at 25–50 cm, confirming that inventories based only on top‐soils are incomplete. We found no significant effects in 50–100 cm. Our study indicates a small annual loss of 0.2 t C ha^?1 from the 0–100 cm soil layer between 1986 and 2009.     

Tillage effects on soil aggregation and soil organic carbon profile distribution under Mediterranean semi‐arid conditions

In rainfed semi‐arid agroecosystems, soil organic carbon (SOC) may increase with the adoption of alternative tillage systems (e.g. no‐tillage, NT). This study evaluated the effect of two tillage systems (conventional tillage, CT vs. NT) on total SOC content, SOC concentration, water stable aggregate‐size distribution and aggregate carbon concentration from 0 to 40 cm soil depth. Three tillage experiments were chosen, all located in northeast Spain and using contrasting tillage types but with different lengths of time since their establishment (20, 17, and 1‐yr). In the two fields with mouldboard ploughing as CT, NT sequestered more SOC in the 0–5 cm layer compared with CT. However, despite there being no significant differences, SOC tended to accumulate under CT compared with NT in the 20–30 and 30–40 cm depths in the AG‐17 field with 25–50% higher SOC content in CT compared with NT. Greater amounts of large and small macroaggregates under NT compared with CT were measured at 0–5 cm depth in AG‐17 and at 5–10 cm in both AG‐1 and AG‐17. Differences in macroaggregate C concentration between tillage treatments were only found in the AG‐17 field at the soil surface with 19.5 and 11.6 g C/kg macroaggregates in NT and CT, respectively. After 17 yr of experiment, CT with mouldboard ploughing resulted in a greater total SOC concentration and macroaggregate C concentration below 20 cm depth, but similar macroaggregate content compared with NT. This study emphasizes the need for adopting whole‐soil profile approaches when studying the suitability of NT versus CT for SOC sequestration and CO₂ offsetting.