Grassland Soil Organic Carbon Research Articles

Grasslands and Croplands Have Different Microbial Biomass Carbon Levels per Unit of Soil Organic Carbon

Primarily using cropped systems, previous studies have reported a positive linear relationship between microbial biomass carbon (MBC) and soil organic carbon (SOC). We conducted a meta-analysis to explore this relationship separately for grasslands and croplands using available literature. Studies were limited to those using fumigation–extraction for MBC for field samples. Trials were noted separately where records were distinct in space or time. Grasslands were naturally occurring, restored, or seeded. Cropping systems were typical of the temperate zone. MBC had a positive linear response to increasing SOC that was significant in both grasslands (p < 0.001; r2 = 0.76) and croplands (p < 0.001; r2 = 0.48). However, MBC increased 2.5-fold more steeply per unit of increasing SOC for grassland soils, as compared to the corresponding response in cropland soils. Expressing MBC as a proportion of SOC across the regression overall, slopes corresponded to 2.7% for grasslands and 1.1% for croplands. The slope of the linear relationship for grasslands was significantly (p = 0.0013) steeper than for croplands. The difference between the two systems is possibly caused by a greater proportion of SOC in grasslands being active rather than passive, relative to that in croplands, with that active fraction promoting the formation of MBC.

Increased uncertainty in soil carbon stock measurement with spatial scale and sampling profile depth in world grasslands: A systematic analysis

There is an important need to better understand how the soil organic carbon (SOC) stocks affect the global atmospheric C balance. Grasslands represent 40% of the earth’s land surface but efficiently and effectively quantifying their SOC stocks and their changes is a challenge. Although factors influencing the variability of grassland SOC stocks such as climate, management, or topography have been investigated, very few studies have quantified the uncertainty of SOC stock measurement. Quantifying this uncertainty is critical to determine our ability to detect changes in space or time, and ultimately to develop guidance to help designing appropriate measurement strategies for quantifying carbon stocks and stock changes. SOC stock measurement may be performed at various spatial scales, from local (≤1km2) to broader scale (≥100km2) applications. In addition, the recommended sampling depth for SOC measurement varies according to project purposes, national circumstances, and land use. To our knowledge, the assessment based on world literature of the effect of spatial scale (i.e. size of the measured area) and sampling depth on the uncertainty of SOC stock measurement in grasslands has never been performed. We quantified the uncertainty of SOC stock measurement from a global analysis of 51 research articles meeting strict requirements of rigour of SOC measurements, totaling 177 grasslands from 19 countries, to assess the effects of the spatial scale and the sampling depth of soil profile on this uncertainty, and to explore the implications for SOC stock change detection involving future sampling.We observed that spatial scale and soil profile depth combined to explain 44% of the variability of SOC stock uncertainty, as measured by the coefficient of variation (CV). More specifically, the CV increased with the spatial scale and soil profile depth measured. However, the sampling depth effect is less certain due to lack of data at deeper depths. This uncertainty associated with SOC stock measurements has ramifications for SOC stock change detection. For example, at a fixed 0–30cm soil profile depth, the minimum detectable change (MDC) of SOC stocks between two sampling dates (50 samples) was 14, 20 and 29% at 0.1, 10 and 10 000km2, respectively. At a fixed spatial scale of 0.1km2, the MDC was 12, 14 and 18% for soil profiles of 0–10, 0–30 and 0–100cm, respectively. Finally, this study provides global estimates of the uncertainty that will be useful for planning sampling strategies for SOC stock measurement in various projects across the world and to evaluate the feasibility of SOC stock measurement for different investment levels and timescales.

Overgrazing decreases soil organic carbon stocks the most under dry climates and low soil pH: A meta-analysis shows

Grasslands occupy about 40% of the world’s land surface and store approximately 10% of the global soil organic carbon (SOC) stock. This SOC pool, in which a larger proportion is held in the topsoil (0–0.3m), is strongly influenced by grassland management. Despite this, it is not yet fully understood how grassland SOC stocks respond to degradation, particularly for the different environmental conditions found globally. The objective of this review was to elucidate the impact of grassland degradation on changes in SOC stocks and the main environmental controls, worldwide, as a prerequisite for rehabilitation. A comprehensive meta-analysis was conducted using 55 studies with 628 soil profiles under temperate, humid, sub-humid, tropical and semi-arid conditions, to compare SOC stocks in the topsoil of non-degraded and degraded grassland soils. Grassland degradation significantly reduced SOC stocks by 16% in dry climates (<600mm) compared to 8% in wet climates (>1000mm) and Asia was the most affected continent (−23.7%). Moreover, the depletion of SOC stock induced by degradation was more pronounced in sandy (<20% clay) soils with a high SOC depletion of 10% compared to 1% in clayey (≥32% clay) soils. Furthermore, grassland degradation significantly reduced SOC by 14% in acidic soils (pH≤5), while SOC changes were negligible for higher pH. Assuming that 30% of grasslands worldwide are degraded, the amount of SOC likely to be lost would be 4.05Gt C, with a 95% confidence between 1.8 and 6.3Gt C (i.e. from 1.2 to 4.2% of the whole grassland soil stock). These results by pointing to greater SOC losses from grasslands under dry climates and sandy acidic soils allow identification of grassland soils for which SOC stocks are the most vulnerable, while also informing on rehabilitation measures.

SOC storage and potential of grasslands from 2000 to 2012 in central and eastern Inner Mongolia, China

Grassland ecosystem is an important component of the terrestrial carbon cycle system. Clear comprehension of soil organic carbon (SOC) storage and potential of grasslands is very important for the effective management of grassland ecosystems. Grasslands in Inner Mongolia have undergone evident impacts from human activities and natural factors in recent decades. To explore the changes of carbon sequestration capacity of grasslands from 2000 to 2012, we carried out studies on the estimation of SOC storage and potential of grasslands in central and eastern Inner Mongolia, China based on field investigations and MODIS image data. First, we calculated vegetation cover using the dimidiate pixel model based on MODIS-EVI images. Following field investigations of aboveground biomass and plant height, we used a grassland quality evaluation model to get the grassland evaluation index, which is typically used to represent grassland quality. Second, a correlation regression model was established between grassland evaluation index and SOC density. Finally, by this regression model, we calculated the SOC storage and potential of the studied grasslands. Results indicated that SOC storage increased with fluctuations in the study area, and the annual changes varied among different sub-regions. The SOC storage of grasslands in 2012 increased by 0.51×1012 kg C compared to that in 2000. The average carbon sequestration rate was 0.04×1012 kg C/a. The slope of the values of SOC storage showed that SOC storage exhibited an overall increase since 2000, particularly for the grasslands of Hulun Buir city and Xilin Gol League, where the typical grassland type was mainly distributed. Taking the SOC storage under the best grassland quality between 2000 and 2012 as a reference, this study predicted that the SOC potential of grasslands in central and eastern Inner Mongolia in 2012 is 1.38×1012 kg C. This study will contribute to researches on related methods and fundamental database, as well as provide a reference for the protection of grassland ecosystems and the formulation of local policies on sustainable grassland development.

Comparison of ingrowth core and sequential soil core methods for estimating belowground net primary production in grass–clover swards

AbstractEstimation of belowground net primary production (BNPP) is crucial for prediction of grassland soil organic carbon sequestration, but there are high coefficients of variation (CV) among methods of estimation. We compared accuracy and precision of two variations of the traditional soil core method and three of the ingrowth core method, over 2 years on grass–white clover swards in Germany. The ingrowth core method (5‐ to 9‐week ingrowth period) provided the most accurate BNPP estimate (471 g m−2) with the highest precision (CV: 31%), whereas the two soil core approaches resulted in underestimation and high error. Comparing results with other grassland studies, we found that reliability of the ingrowth core method was influenced by ingrowth period and sward rooting pattern related to grass phenological stages, while the precision was determined by ingrowth period and grassland productivity. We therefore recommend using the ingrowth core method for managed grasslands where belowground productivity is relatively high and installation disturbance is likely to be negligible. Using a 6‐ to 8‐week ingrowth period, the method delivers reliable and feasible BNPP estimates for temperate managed grasslands which can be applied to long‐term soil carbon budget simulation. Results from our work and other studies also indicated that standing root biomass masks seasonal pattern of rooting due to synchrony between root birth and death. The soil core method is likely to lead to underestimates and higher errors than the ingrowth core methods and thus should be avoided.

Addition of nitrogen fertiliser increases net ecosystem carbon dioxide uptake and the loss of soil organic carbon in grassland growing in mesocosms

Maintaining and increasing soil organic carbon stocks in grasslands is essential for sustainable productivity and to offset anthropogenic carbon emissions. Direct measurements of net ecosystem carbon dioxide exchange, FN, can be used to detect whether an ecosystem is a net sink or a source of carbon. However partitioning heterotrophic respiration, RH, from ecosystem respiration, RE, is needed to determine the impacts of land-use and global change on soil organic carbon stocks. We extracted intact soil cores from an intensively grazed dairy farm to establish mesocosms growing in controlled conditions and we subjected them to low and high nitrogen treatments (100 and 400kgNha1y−1, respectively). After concurrent clipping and addition of nitrogen, we measured the timing for the recovery of FN and its components, photosynthesis, A, and ecosystem respiration, RE, by measuring them daily. Subsequently, we measured RH from the same mesocosms seven days after the treatments were applied, using a non-disruptive, natural abundance carbon isotope technique. To test the significance of the presence of living roots when measuring RH, we compared the results obtained from the isotopic approach to those obtained from a root exclusion technique, which involved removing the roots from the mesocosms. As the plants grew after clipping, FN decreased (increasing net CO2 uptake) exponentially to mean (±standard error) steady-state values of 1.11±0.26μmolm−2s−1 (net source) and −0.19±0.33μmolm−2s−1 (near neutral) for the low and high nitrogen treatments, respectively. When measured using the isotopic approach, RH increased by 60%, from 1.26±0.29μmolm−2s−1 in the low, to 2.06±0.55μmolm−2s−1 in the high nitrogen treatment. Thus, addition of the high nitrogen resulted in an increase in soil organic carbon loss concurrently with an increase in net uptake of carbon by the ecosystem in the high nitrogen treatment compared with the low nitrogen treatment. When measured in the absence of living roots using the root exclusion technique, RH was overall much higher than the value obtained with the isotopic technique (4.34±0.13μmolm−2s−1), indicating an apparent negative rhizosphere priming effect. Furthermore, when using the root exclusion technique, no difference was found between the nitrogen treatments, suggesting that the presence of roots mediated the response of heterotrophic respiration to addition of nitrogen. These results highlight the need to include measurements of changes in RH alongside measurements of FN in non-disturbed ecosystems to interpret the processes regulating the effects of management practices on long-term changes in soil organic carbon stocks.

Soil organic carbon in playas and adjacent prairies, cropland, and Conservation Reserve Program land of the High Plains, USA

Soil organic carbon (SOC) is an important reservoir for atmospheric CO2 associated with climate warming. The High Plains, USA, lacks region-wide SOC estimates within playa wetlands and their adjacent watershed. Croplands often have less SOC than grasslands, and the Conservation Reserve Program (CRP; former croplands planted to grass) may return SOC. Our goals were to estimate SOC within playa wetlands and investigate small scale differences within wetland catchments across a broad agriculturally modified landscape. We estimated SOC (kgm−2) to 50cm depth from 4 soil cores/catchment (in playas and 10, 40 and 100m into uplands) at 56, 52, and 54 sites in native grassland, CRP and cropland, respectively. At a subset of sample locations within each land use type, we estimated SOC to 1m depth to characterize SOC missed by shallow sampling. In playa wetlands, CRP SOC from 0 to 50cm was 18% greater than croplands, but native grassland playa SOC did not differ from other land-uses. From 0 to 1m, SOC in native grassland wetlands and uplands was 20% greater than the same habitats within croplands, while CRP lands were intermediate. Native grassland playa SOC also was 16% greater than in surrounding native short grass prairie. Playas therefore represent an important SOC repository in the High Plains ecoregion. CRP playas and uplands may require an additional 10–30 years to resemble native grassland SOC. SOC increased with playa area throughout CRP and native grassland catchments, suggesting playa hydrogeomorphology influences adjacent upland SOC. High Plains playas store 20.8Tg C and cropland conversion caused a cumulative loss of 2.0Tg C from 82,000ha of playas. Currently, CRP enrollment on over 25,000ha of playas has returned 0.2Tg C (95% CI: 0.1–0.3), only half the historic SOC lost by cropland conversion within CRP playas. To promote SOC storage, native grasslands and large playas should be preserved and CRP enrollments should be maintained over long timescales.

草地土壤固碳潜力研究进展

PDF HTML阅读 XML下载 导出引用 引用提醒 草地土壤固碳潜力研究进展 DOI: 10.5846/stxb201310212541 作者: 作者单位: 中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所,中国科学院地理科学与资源研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家重点基础研究发展计划(973计划)(2011CB403206, 2012CB416906); 国家科技支撑计划(2013BAC03B04); 国家自然科学基金项目(41371196) Review on soil carbon sequestration potential in grassland ecosystems Author: Affiliation: Institute of Geographic Sciences and Natural Resources Research,-A Datun Road,Chaoyang District,Beijing,Institute of Geographic Sciences and Natural Resources Research,-A Datun Road,Chaoyang District,Beijing,Institute of Geographic Sciences and Natural Resources Research,-A Datun Road,Chaoyang District,Beijing 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:土壤固碳功能和固碳潜力已成为全球气候变化和陆地生态系统研究的重点。草地土壤有机碳库,作为陆地土壤有机碳库的重要组成部分,其较小幅度的波动,将会影响整个陆地生态系统碳循环,进而影响全球气候变化。因此,深入研究草地土壤固碳功能和固碳潜力对于适应和减缓气候变化具有重要意义。在土壤固碳潜力相关概念界定基础上,结合《2006年IPCC国家温室气体清单指南》,从样点及区域尺度上综述了目前关于草地土壤固碳潜力的一般估算方法,同时对各类方法的特点及适用性进行了评述,提出了草地生态系统固碳潜力研究概念模型。最后在对草地土壤固碳的影响因素及固碳措施总结的基础上,阐明了草地土壤有机碳固定研究中存在的问题和发展前景。 Abstract:Global climate change has aroused wide concern from the international community, and national or regional GHG (greenhouse gas) emission has become a focus of the intergovernmental diplomatic dispute and mediation. The stability, release or increment of the soil carbon stock is closely related to the change of the atmospheric carbon stock. Even a minor change in soil organic carbon (SOC) storage may result in a significant change in atmospheric CO2 concentration. Whether the soil carbon stock can increase becomes an essential theoretical basis for terrestrial ecosystems to absorb atmospheric CO2, and serve as carbon sink. Therefore, increasing soil carbon sink in terrestrial ecosystems has become an important strategy for controlling the rise in atmospheric CO2 concentration. As one of the world's most widespread terrestrial ecosystems, covering over 15% of the world's land surface, grassland ecosystems account for over 10% of the total carbon storage in terrestrial ecosystems, with nearly 90% of the carbon storing in top soil. Besides, grassland soil organic carbon could be strongly influenced by management practices. During the past decades, owing to the climate change, overgrazing and other unreasonable management practices, the grasslands in the world had been seriously degraded and the soil organic carbon had significantly decreased. However, improved management practices such as enclosure and recovery of degraded grassland and conversion of cropland to abandoned fields, may reverse the loss of soil organic carbon, and even help to sequestrate carbon in the atmosphere. Therefore, under the background of climate change, the function and potential of soil organic carbon sequestration in grassland ecosystems has become a key issue. In this essay, the recent research on soil carbon sequestration potential was reviewed, including quantitative methods for estimating soil carbon sequestration potential on both regional and sample scales, dominant affecting factors and management practices of carbon sequestration, and future research direction was also proposed lastly. Research on quantitative estimation of soil carbon sequestration in grassland ecosystems had been well developed in recent years, from simple statistics on sample scale to mechanism modeling coupled with GIS and RS on regional scale. IPCC also issued guidelines for national greenhouse gas inventories so as to provide a relatively uniform method for national carbon sequestration estimation, which contained three tiers on estimation of soil organic carbon storage in grassland: Tier1, Tier2, and Tier3. Among all the quantitative methods, mechanism modeling of soil carbon cycle became a major method to assess the carbon sequestration of terrestrial ecosystems and the reduction of greenhouse gas emission in the world. The dominant factors affecting soil organic carbon sequestration included natural factors (temperature, precipitation and CO2 concentration etc.) and human factors (reclamation, grazing, fire, enclosure, fertilization, irrigation and reseeding etc.), which held complicated influences on carbon sequestration with various time and intensity of management practices. There also existed many problems needed to be solved in the future, such as the methodology and adaptability of models and the uncertainty of estimation on regional scale. The potential and mechanism of carbon sequestration, and the relationship between carbon sequestration and environmental effects would be the key scientific problems. 参考文献 相似文献 引证文献

Impacts of management practices on soil organic carbon in degraded alpine meadows on the Tibetan Plateau

Abstract. Grassland soil organic carbon (SOC) is sensitive to anthropogenic activities. Increased anthropogenic disturbance related to overgrazing has led to widespread alpine grassland degradation on the Tibetan Plateau. The degraded grasslands are considered to have great potential for carbon sequestration after adoption of improved management practices. Here, we calibrated and employed the Century model to investigate the effects of overgrazing and improved managements on the SOC dynamics in alpine meadows. We calibrated the Century model against plant productivity at the Haibei Research Station. SOC stocks for validation were obtained in 2009–2010 from degraded alpine meadows in two communes. We found that Century model can successfully capture grassland SOC changes. Overall, our simulation suggests that degraded alpine meadow SOC significantly increased with the advent of restoration management from 2011 to 2030. Carbon sequestration rates ranged between 0.04 Mg C ha−1 yr−1 in lightly degraded winter grazing grasslands and 2.0 Mg C ha−1 yr−1 in moderately degraded summer grazing grasslands. Our modelling work also predicts that improve management in degraded Tibetan grasslands will contribute to an annual carbon sink of 0.022–0.059 Pg C yr−1. These results imply that restoration of degraded grasslands in the Tibetan Plateau has great potential for soil carbon sequestration to mitigate greenhouse gases.

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

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. 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. 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. 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.

Alpine grassland soil organic carbon stock and its uncertainty in the three rivers source region of the Tibetan Plateau.

Alpine grassland of the Tibetan Plateau is an important component of global soil organic carbon (SOC) stocks, but insufficient field observations and large spatial heterogeneity leads to great uncertainty in their estimation. In the Three Rivers Source Region (TRSR), alpine grasslands account for more than 75% of the total area. However, the regional carbon (C) stock estimate and their uncertainty have seldom been tested. Here we quantified the regional SOC stock and its uncertainty using 298 soil profiles surveyed from 35 sites across the TRSR during 2006–2008. We showed that the upper soil (0–30 cm depth) in alpine grasslands of the TRSR stores 2.03 Pg C, with a 95% confidence interval ranging from 1.25 to 2.81 Pg C. Alpine meadow soils comprised 73% (i.e. 1.48 Pg C) of the regional SOC estimate, but had the greatest uncertainty at 51%. The statistical power to detect a deviation of 10% uncertainty in grassland C stock was less than 0.50. The required sample size to detect this deviation at a power of 90% was about 6–7 times more than the number of sample sites surveyed. Comparison of our observed SOC density with the corresponding values from the dataset of Yang et al. indicates that these two datasets are comparable. The combined dataset did not reduce the uncertainty in the estimate of the regional grassland soil C stock. This result could be mainly explained by the underrepresentation of sampling sites in large areas with poor accessibility. Further research to improve the regional SOC stock estimate should optimize sampling strategy by considering the number of samples and their spatial distribution.

No general soil carbon sequestration under Central European short rotation coppices

AbstractWood from short rotation coppices (SRCs) is discussed as bioenergy feedstock with good climate mitigation potential inter alia because soil organic carbon (SOC) might be sequestered by a land‐use change (LUC) from cropland to SRC. To test if SOC is generally enhanced by SRC over the long term, we selected the oldest Central European SRC plantations for this study. Following the paired plot approach soils of the 21 SRCs were sampled to 80 cm depth and SOC stocks, C/N ratios, pH and bulk densities were compared to those of adjacent croplands or grasslands. There was no general trend to SOC stock change by SRC establishment on cropland or grassland, but differences were very site specific. The depth distribution of SOC did change. Compared to cropland soils, the SOC density in 0–10 cm was significantly higher under SRC (17 ± 2 in cropland and 21 ± 2 kg C m−3 in SRC). Under SRC established on grassland SOC density in 0–10 cm was significantly lower than under grassland. The change rates of total SOC stocks by LUC from cropland to SRC ranged from −1.3 to 1.4 Mg C ha−1 yr−1 and −0.6 Mg C ha−1 yr−1 to +0.1 Mg C ha−1 yr−1 for LUC from grassland to SRC, respectively. The accumulation of organic carbon in the litter layer was low (0.14 ± 0.08 Mg C ha−1 yr−1). SOC stocks of both cropland and SRC soils were correlated with the clay content. No correlation could be detected between SOC stock change and soil texture or other abiotic factors. In summary, we found no evidence of any general SOC stock change when cropland is converted to SRC and the identification of the factors determining whether carbon may be sequestered under SRC remains a major challenge.

Plant compensation to grazing and soil carbon dynamics in a tropical grassland

The effects of grazing on soil organic carbon (SOC) dynamics, particularly in the tropics, are still poorly understood. Plant compensation to grazing, whereby plants maintain leaf area (C input capacity) despite consumption (C removal) by grazers, has been demonstrated in tropical grasslands but its influence on SOC is largely unexplored. Here, the effect of grazing on plant leaf area index (LAI) was measured in a field experiment in Serengeti National Park, Tanzania. LAI changed little for grazing intensities up to 70%. The response curve of LAI versus grazing intensity was used in a mass balance model, called SNAP, of SOC dynamics based on previous data from the Serengeti. The model predicted SOC to increase at intermediate grazing intensity, but then to decline rapidly at the highest grazing intensities. The SNAP model predictions were compared with observed SOC stocks in the 24 grazed plots of a 10-year grazing exclosure experiment at eight sites across the park that varied in mean annual rainfall, soil texture, grazing intensity and plant lignin and cellulose. The model predicted current SOC stocks very well (R2 > 0.75), and suggests that compensatory plant responses to grazing are an important means of how herbivores might maintain or increase SOC in tropical grasslands.

Soil carbon stocks and their variability across the forests, shrublands and grasslands of peninsular Spain

Abstract. Accurate estimates of C stocks and fluxes of soil organic carbon (SOC) are needed to assess the impact of climate and land use change on soil C uptake and soil C emissions to the atmosphere. Here, we present an assessment of SOC stocks in forests, shrublands and grasslands of peninsular Spain based on field measurements in more than 900 soil profiles. SOC to a depth of 1 m was modelled as a function of vegetation cover, mean annual temperature, total annual precipitation, elevation and the interaction between temperature and elevation, while latitude and longitude were used to model the correlation structure of the errors. The resulting statistical model was used to estimate SOC in the ∼8 million pixels of the Spanish Forest Map (29.3 × 106 ha). We present what we believe is the most reliable estimation of current SOC in forests, shrublands and grasslands of peninsular Spain thus far, based on the use of spatial modelling, the high number of profiles and the validity and refinement of the data layers employed. Mean concentration of SOC was 8.7 kg m−2, ranging from 2.3 kg m−2 in dry Mediterranean areas to 20.4 kg m−2 in wetter northern locations. This value corresponds to a total stock of 2.544 Tg SOC, which is four times the amount of C estimated to be stored in the biomass of Spanish forests. Climate and vegetation cover were the main variables influencing SOC, with important ecological implications for peninsular Spanish ecosystems in the face of global change. The fact that SOC was positively related to annual precipitation and negatively related to mean annual temperature suggests that future climate change predictions of increased temperature and reduced precipitation may strongly reduce the potential of Spanish soils as C sinks. However, this may be mediated by changes in vegetation cover (e.g. by favouring the development of forests associated to higher SOC values) and exacerbated by perturbations such as fire. The estimations presented here provide a baseline to estimate future changes in soil C stocks and to assess their vulnerability to key global change drivers, and should inform future actions aimed at the conservation and management of C stocks.

Spatial and temporal variations of grassland soil organic carbon and total nitrogen following grazing exclusion in semiarid Loess Plateau, Northwest China

Grazing exclusion is an efficient management practice for preventing grassland degradation in semiarid Loess Plateau. However, little information was known about the spatial and temporal variations of soil organic carbon (SOC) and total nitrogen (TN) following grazing exclusion. In this study, grasslands with grazing exclusion for 11 years at 29 different locations were investigated. The results showed that SOC and TN concentrations decreased with increasing of soil depth. SOC and TN concentrations in 0–20 cm layer were 1.4–2.0 times of that in the other layers. The coefficients of variation of SOC and TN concentrations were about 20%, indicating that there was a moderate variation among different locations. The mean SOC and TN densities were 63.36 and 6.62 t ha−1 respectively which were significantly affected by altitude and the mean annual precipitation (MAP) gradients (P < 0.001). Grasslands with altitude of 500–2000 m or MAP of 450–570 mm were optimal for SOC and TN sequestrations. Otherwise, the effects of slope position and mean annual temperature on SOC and TN densities were insignificant (P > 0.05). To explore temporal variations of SOC and TN, grasslands with different grazing exclusion ages (5, 10, 15, 20, 25, and 30 years) were investigated. SOC and TN concentrations increased by 89–385% and 86–330% respectively along the chronosequence within 30 years. Based on the values of SOC and TN concentrations, the grasslands were divided into two groups, grazing grassland and grazing exclusion grasslands. Within the group of grazing exclusion grasslands, grazing exclusion for 25 and 30 years facilitated the grasslands to accumulate higher SOC and TN than that for 5, 15, and 20 years. To conclude, SOC and TN accumulations of grazing exclusion grassland were depended on attitude, MAP, and grazing exclusion years.

Herbivores regulate the sensitivity of soil organic carbon decomposition to warming

There is considerable interest in how the sensitivity of soil organic carbon (SOC) decomposition to warming is determined by the degree of recalcitrance (i.e., quality) of SOC. Although herbivores have widespread effects on the quality of plant litter, from which SOC is derived, herbivores are seldom considered in studies that examine the response of SOC degradation and greenhouse gas emissions to warming. In this study, we addressed the question: do herds of wild ungulates affect the temperature sensitivity of SOC decomposition in grasslands of Yellowstone National Park (YNP)? We examined the effects of ungulates on temperature sensitivity by comparing microbial respiration at different temperatures in incubated soils collected inside and outside long-term ungulate exclosures. Exclosure sites included grasslands on hilltops, slopes, and at the base of slopes that varied considerably in soil properties. Herbivores reduced the temperature sensitivity of SOC decomposition uniformly among sites by 20% (P = 0.002). The herbivore—induced decline in temperature sensitivity was probably governed by YNP grazers increasing the quality of grassland SOC. Results from this study suggest that herbivores regulate how global warming modifies grassland SOC decomposition, stability, and CO2 emissions.

Projections of changes in grassland soil organic carbon under climate change are relatively insensitive to methods of model initialization

SummaryModel initialization in soil organic carbon (SOC) turnover models has often been described as a crucial step in making future projections. Model initialization by the spin‐up of pools of SOC (model equilibrium run) has been questioned, because equilibrium has to be assumed. Measured SOC pools are independent of model assumptions and are thought to reflect better real site conditions. It has been suggested that model initialization with measured SOC fractions could provide an advantage over model spin‐up of SOC pools. In this study we tested this suggestion in relatively undisturbed native grasslands in Australia. We tested the Rothamsted SOC turnover model (RothC) under climate change at 12 sites with three different initialization methods, viz. model initialization with (i) spin‐up of model pools with inert organic matter (IOM) pool size calculated from a regression equation, (ii) spin‐up of model pools with measured IOM and (iii) all pools estimated from measured fractions. Averaged over the sites and initialization methods, maximum absolute variations (absolute differences in projected SOC stocks expressed as a percentage of initial 2008 SOC stocks) as well as averaged absolute variations throughout the projection period were very small (2.2 and 1.6%, respectively). Averaged across the sites, there were no significant differences in projected grassland SOC stocks under climate change after 93 years of simulation with model initialization by different methods and averaged absolute variation was only 1.6% across initialization methods. These findings suggest that in a relatively undisturbed land‐use system such as native grassland, projections of SOC under climate change are relatively insensitive to the model initialization method.

Long-term incubations of size and density separated soil fractions to inform soil organic carbon decay dynamics

Soil organic matter in coarse-textured soils is more vulnerable to environmental disturbances due to reduced potential for soil organic carbon (SOC) stabilization in aggregates or organo-mineral complexes. In sandy loam soils from the Rio Grande Plains region of southern Texas, woody encroachment has resulted in the rapid accrual of root and leaf tissues derived from trees and shrubs into poorly physically protected (macroaggregate >250 μm) and non-mineral associated (free light fraction <1.0 g cm−3) soil fractions. To determine the impact of changing plant input chemistry on potential degradability of accumulating SOC fractions, we measured the quantity and isotopic composition of respired CO2 from year-long incubations of the macroaggregate and free light soil fractions along a grassland to woodland successional chronosequence. During incubation of both fractions, the proportion of SOC respired from older woody stand soils (∼40–90 yrs) relative to recently established woody stands (<40 yrs) and remnant grassland soils decreased. We interpreted this decrease with woody stand age to result from a change to plant input chemistry with more lignin and aliphatic structures combined with a progressive shift to more non-hydrolyzable, poorly accessible forms of soil organic nitrogen, resulting in a system with slower short-term decay dynamics. The δ13C values of respired CO2 from all landscape elements indicated a selective release of older grassland-derived SOC in the first month of the macroaggregate incubation, possibly due to the disruption and rapid microbial utilization of grassland SOC after the soil fractionation process. Due to the sensitivity of these rapidly-cycling soil fractions to environmental disturbance and their capacity to influence longer-term SOC dynamics, understanding their decay dynamics is essential for understanding mechanisms of SOC stabilization. This is especially important in coarse-textured soils where large SOC stocks may be present in physical fractions that are relatively unprotected from decomposition.

Soil organic carbon stocks in southeast Germany (Bavaria) as affected by land use, soil type and sampling depth

AbstractPrecise estimations of soil organic carbon (SOC) stocks are of decided importance for the detection of C sequestration or emission potential induced by land use changes. For Germany, a comprehensive, land use–specificSOCdata set has not yet been compiled. We evaluated a unique data set of 1460 soil profiles in southeast Germany in order to calculate representativeSOCstocks to a depth of 1 m for the main land use types. The results showed that grassland soils stored the highest amount ofSOC, with a median value of 11.8 kg m−2, whereas considerably lower stocks of 9.8 and 9.0 kg m−2were found for forest and cropland soils, respectively. However, the differences between extensively used land (grassland, forest) and cropland were much lower compared with results from other studies in central European countries. The depth distribution ofSOCshowed that despite lowSOCconcentrations in A horizons of cropland soils, their stocks were not considerably lower compared with other land uses. This was due to a deepening of the topsoil compared with grassland soils. Higher grasslandSOCstocks were caused by an accumulation ofSOCin the B horizon which was attributable to a high proportion of C‐rich Gleysols within grassland soils. This demonstrates the relevance of pedogeneticSOCinventories instead of solely land use–based approaches. Our study indicated that cultivation‐inducedSOCdepletion was probably often overestimated since most studies use fixed depth increments. Moreover, the application of modelled parameters inSOCinventories is questioned because a calculation ofSOCstocks using different pedotransfer functions revealed considerably biased results. We recommendSOCstocks be determined by horizon for the entire soil profile in order to estimate the impact of land use changes precisely and to evaluate C sequestration potentials more accurately.

Characteristics of soil organic carbon and nitrogen distributed in different density fractions of mollisols under long-term continuous cropping and natural restoration

本文以海伦农田生态系统国家野外科学观测站长期定位试验的黑土为研究对象, 通过对不同作物连作(玉米、大豆、小麦)和自然恢复(草地、裸地)下土壤及其密度分组中有机碳、氮含量的测定, 比较分析了土壤总有机碳、全氮以及密度组分碳、氮分布的变化特征。结果表明不同作物连作下土壤总有机碳的含量差异不显著。0~10 cm和10~20 cm土层农田土壤游离态轻组有机碳含量具有显著差异(<i>P</i><0.05)。大豆连作的农田土壤游离态轻组有机碳主要分布在0~10 cm土层, 而小麦连作的土壤游离态轻组有机碳主要分布在10~20 cm土层。不同作物连作下土壤闭蓄态轻组有机碳含量差异不显著。不同组分中氮素具有与碳相似的分布特征。游离态轻组与闭蓄态轻组C/N比值之间呈显著负相关(<i>P</i><0.05)。草地与农田、裸地相比显著提高了土壤总有机碳和全氮的含量。草地土壤游离态轻组、闭蓄态轻组和重组有机碳含量显著高于农田和裸地。作物连作和自然恢复导致土壤碳、氮的重新分配, 改变了土壤碳、氮的赋存特征。