Widespread decreases in topsoil inorganic carbon stocks across China's grasslands during 1980s–2000s

Soil carbon (C) stocks consist of inorganic and organic components, ~1.7 times larger than the total of the C stored in vegetation and the atmosphere together. Significant soil C losses could thus offset any C sink in vegetation, creating a positive feedback to climate change. However, compared with the susceptible sensitivity of organic matter decay to climate warming, soil inorganic carbon (SIC) stocks are often assumed to be relatively stable. Here, we evaluated SIC changes across China's grasslands over the last two decades using data from a recent regional soil survey during 2001–2005 and historical national soil inventory during the 1980s. Our results showed that SIC stocks in the top 10 cm decreased significantly between the two sampling periods, with a mean rate of 26.8 (95% confidence interval: 15.8–41.7) g C m^?2 yr^?1. The larger decreases in SIC stocks were observed in those regions with stronger soil acidification and richer soil carbonates. The lost SIC could be released to the atmosphere as carbon dioxide, redistributed to the deeper soil layer, and transferred to the nearby regions. The fraction of soil carbonates entering into the atmosphere may diminish the strength of terrestrial C sequestration and amplify the positive C‐climate feedback.     

Selenium fertilization strategies for bio-fortification of food: an agro-ecosystem approach

Background and aims

Although numerous studies have quantified the effects of land-use changes on soil organic carbon (SOC) stocks, few have examined simultaneously the weight of carbon (C) inputs vs. outputs in shaping these changes. We quantified the relative importance of soil C inputs and outputs in determining SOC changes following the conversion of natural ecosystems to pastures or tree plantations, and evaluated them in light of variations in biomass production, its quality (C:N) and above/belowground allocation patterns.

Methods

We sampled soils up to one-meter depth under native grasslands or forests and compared them to adjacent sites with pastures or plantations to estimate the proportion of new SOC (SOC_new) retained in the soil and the decomposition rates of old SOC (k _SOC-old ) based on δ ¹³C shifts. We also analyzed these changes in the particulate organic matter fraction (POM) and estimated above and belowground net primary production (ANPP and BNPP) from satellite images, as well as changes in vegetation and soil’s C:N ratios.

Results

The conversion of grasslands to tree plantations decreased total SOC contents while the conversion of forests to pastures increased SOC contents in the topsoil but decreased them in deep layers, maintaining similar soil stocks up to 1 m. Changes in POM were less important and occurred only in the topsoil after cultivating pastures, following SOC changes. Surprisingly, both land-use trajectories showed similar decomposition rates in the topsoil and therefore overall SOC changes were not correlated with C outputs (k _SOC-old ) but were significantly correlated with C inputs and their stabilization as SOC_new (similar results were obtained for the POM fraction). Pastures although decreased ANPP (as compared to forest) they increased belowground allocation and C:N ratios of their inputs to the soil, probably favoring the retention and stabilization of their new C inputs. In contrast, tree plantations increased ANPP but decreased BNPP (as compared to grasslands) and scarcely accumulated SOC_new probably as a result of the high C retention in standing biomass.

Conclusions

Our results suggest that SOC changes are mainly controlled by the quantity and quality of C inputs and their retention in the soil, rather than by C outputs in these perennial subtropical ecosystems.

     

Changes of soil organic and inorganic carbon in relation to grassland degradation in Northern Tibet

The effect of livestock grazing on grassland degradation and the resulting impact on soil carbon concentration is an important factor in carbon estimation. We addressed this issue using field observations and laboratory analysis of samples from Tibetan grassland. Based on the field measurements, we investigated the soil organic carbon (SOC) and soil inorganic carbon (SIC) under two contrasting degradation states: lightly or non-degraded grasslands (LDG) and heavily degraded grasslands (HDG). We assessed their relationships with environmental factors using data collected from 99 sites across Northern Tibet during 2011–2012. Data were analyzed using a linear mixed-effects model and one-way ANOVA. The results showed that: (1) SOC concentration decreased and SIC concentration increased following grassland degradation, especially at soil depths in the range of 0–10 cm (P < 0.05); (2) the major environmental factors affecting SOC and SIC were soil pH and plant biomass; (3) spatially, the SOC density increased with the mean annual temperature and mean annual precipitation, whereas SIC exhibited the opposite trend; (4) the SOC density increased at first and then decreased with increasing grazing intensity, with an opposite trend in SIC; and (5) soil carbon storage in this region was 0.14 Pg smaller in the HDG than in the LDG. This study suggests that grassland degradation can significantly affect the vertical distribution and storage of SOC and SIC. The carbon sequestration capacity of the top 100 cm of soil in Northern Tibet was estimated as 0.14 Pg.     

Variation of soil carbon pools in Pinus sylvestris plantations of different ages in north China

Plantations play an important role in absorbing atmospheric CO₂ and plantation soil can serve as an important carbon (C) sink. However, the stocks and dynamics of soil C in differently aged plantation forests in north China remain uncertain. In this study, we measured soil inorganic carbon (SIC), soil organic carbon (SOC) and total nitrogen content (STN), the light (LF) and heavy fractions (HF) of soil organic matter (SOM) to a depth of 1 m in 3 different ages (10-, 30-, 40-year-old) of Pinus sylvestris var. mongolica (Mongolia pine) plantations in 2011 and 2012. Soil pH, texture and moisture were also measured to explore the causes of SOC dynamics for different stand ages. Our results showed that no significant difference in SIC content was observed at different soil depths. As forest age increases, SIC content as well as the C and N content in SOM, LF and HF initially rose and then decreased, while the LF in SOC initially decreased and then increased. Although the C:N ratio of SOC and HF did not significantly change, the C:N ratio of LF increased with depth. SOC dynamics at different stand ages were significantly correlated with soil moisture and clay content. Soil pH and moisture explained 58.63% of the overall variation of SOC at different depths. Moreover, the SOC increased during the early stage of afforestation, mostly because of the increase in recalcitrant C; however, the decrease of SOC with increasing stand age was also mainly affected by C loss in the recalcitrant C pool.     

Changes in topsoil carbon stock in the Tibetan grasslands between the 1980s and 2004

Climate warming is likely inducing carbon loss from soils of northern ecosystems, but little evidence comes from large-scale observations. Here we used data from a repeated soil survey and remote sensing vegetation index to explore changes in soil organic carbon (SOC) stock on the Tibetan Plateau during the past two decades. Our results showed that SOC stock in the top 30 cm depth in alpine grasslands on the plateau amounted to 4.4 Pg C (1 Pg=10¹⁵ g), with an overall average of 3.9 kg C m⁻². SOC changes during 1980s–2004 were estimated at −0.6 g C m⁻² yr⁻¹, ranging from −36.5 to 35.8 g C m⁻² yr⁻¹ at 95% confidence, indicating that SOC stock in the Tibetan alpine grasslands remained relatively stable over the sampling periods. Our findings are nonconsistent with previous reports of loss of soil C in grassland ecosystems due to the accelerated decomposition with warming. In the case of the alpine grasslands on the Tibetan Plateau studied here, we speculate that increased rates of decomposition as soils warmed during the last two decades may have been compensated by increased soil C inputs due to increased grass productivity. These results suggest that soil C stock in terrestrial ecosystems may respond differently to climate change depending on ecosystem type, regional climate pattern, and intensity of human disturbance.     

新疆天山中段巴音布鲁克高山草地碳含量及其垂直分布

对新疆天山中段巴音布鲁克高山草地(高山草原和高山草甸)的生物量和土壤有机碳进行了测定。结果表明积分和分层两种估算方法得到的土壤有机碳含量没有显著差异,但积分算法的优势在于能推算不同深度的土壤有机碳含量,便于与以往的研究进行比较;高山草甸的生物量和土壤有机碳含量均大于高山草原;其地上生物量分别为71.4和94.9 g C·m^-2,地下生物量分别为1 033.5和1 285.2 g C·m^-2; 1 m深度的土壤有机碳含量分别为25.7和38.8 kg·m^-2;地上生物量呈现较为明显的垂直分布格局,即随着海拔的增加,地上生物量先呈增加趋势,但当海拔超过一定界限后生物量突然下降;土壤含水率是导致南坡(阳坡)土壤有机碳含量空间分异的重要因素,但北坡(阴坡) 土壤有机碳含量还可能与地形、土壤质地等其它因素有关;两种高山草地(高山草原和高山草甸)的根系集中分布在40 cm以内,0~20 cm根系分别占其总量的76%和80%;土壤有机碳集中分布在60 cm以内,0~20 cm土壤有机碳分别占其总量的55%和49%;高山草原根系分布比高山草甸深,但较低的地下/地上比使得其有机碳分布比高山草甸浅。     

黄土丘陵区刺槐、辽东栎林地土壤碳、氮、磷生态化学计量特征

黄土高原中部的丘陵沟壑区位于半湿润、半干旱气候带,生态环境脆弱,水土流失严重,植被恢复是该地区水土保持与生态重建的重要措施。辽东栎天然次生林和刺槐人工林是该地区典型的森林植被类型。以黄土丘陵森林分布区边缘的两种主要森林类型为对象,通过采集林地不同深度土壤样品,对比分析两种林地土壤中碳、氮、磷含量的计量关系及垂直分布特征,旨在探明该区域土壤化学计量特征及主要影响因素。结果表明:(1)在两种林地类型中,土壤有机碳与全碳含量呈正相关关系,两种林地可用同一曲线进行拟合,说明特定土壤类型在同一区域其有机碳和无机碳相对含量基本稳定。(2)土壤有机碳与全氮比值在10左右,在不同土层深度无明显变化;而土壤全碳与全氮比值则随土壤深度的增加而增加,超过1 m以后呈现饱和曲线的变化趋势。(3)土壤氮磷比随土壤深度的增加呈幂次型降低。     

The microbe-mediated mechanisms affecting topsoil carbon stock in Tibetan grasslands

Warming has been shown to cause soil carbon (C) loss in northern grasslands owing to accelerated microbial decomposition that offsets increased grass productivity. Yet, a multi-decadal survey indicated that the surface soil C stock in Tibetan alpine grasslands remained relatively stable. To investigate this inconsistency, we analyzed the feedback responses of soil microbial communities to simulated warming by soil transplant in Tibetan grasslands. Whereas microbial functional diversity decreased in response to warming, microbial community structure did not correlate with changes in temperature. The relative abundance of catabolic genes associated with nitrogen (N) and C cycling decreased with warming, most notably in genes encoding enzymes associated with more recalcitrant C substrates. By contrast, genes associated with C fixation increased in relative abundance. The relative abundance of genes associated with urease, glutamate dehydrogenase and ammonia monoxygenase (ureC, gdh and amoA) were significantly correlated with N₂O efflux. These results suggest that unlike arid/semiarid grasslands, Tibetan grasslands maintain negative feedback mechanisms that preserve terrestrial C and N pools. To examine whether these trends were applicable to the whole plateau, we included these measurements in a model and verified that topsoil C stocks remained relatively stable. Thus, by establishing linkages between microbial metabolic potential and soil biogeochemical processes, we conclude that long-term C loss in Tibetan grasslands is ameliorated by a reduction in microbial decomposition of recalcitrant C substrates.     

Comparisons of three methods for organic and inorganic carbon in calcareous soils of northwestern china

With increasing interest in the carbon cycle on arid land, there is an urgent need to quantify both soil organic carbon (SOC) and inorganic carbon (SIC) thus to assess various methods. Here, we present a study employing three methods for determinations of SOC and SIC in the Yanqi Basin of northwest China. We use an elemental analyzer for both SOC and SIC, the Walkley-Black method for SOC, a modified pressure calcimeter method for SIC, and a simple loss-on-ignition (LOI) procedure for determinations of SOC and SIC. Our analyses show that all three approaches produce consistently low values for SOC (1-14 g kg(-1)) and high values for SIC (8-53 g kg(-1)). The Walkley-Black method provides an accurate estimate of SOC with 100% recovery for most soil samples. The pressure calcimeter method is as accurate as the elemental analysis for measuring SIC. In addition, SOC and SIC can be accurately estimated using a two-step LOI approach, i.e., (1) combustion at 375°C for 17 hours to estimate SOC, and (2) subsequent combustion at 800°C for 12 hours to estimate SIC. There are strong linear relationships for both SOC and SIC between the elemental analysis and LOI method, which demonstrates the capability of the two-step LOI technique for estimating SOC and SIC in this arid region.     

青藏高原高寒草地3米深度土壤无机碳库及分布特征

准确评估土壤无机碳库的大小及其分布特征有助于全面理解陆地生态系统碳循环与气候变暖之间的反馈关系.然而, 由于深层土壤剖面信息匮乏, 使得目前学术界对深层土壤无机碳库的了解十分有限.该研究基于342个3 m深度和177个50 cm深度的土壤剖面信息, 采用克里格插值方法估算了青藏高原高寒草地不同深度的土壤无机碳库大小, 并在此基础上分析了该地区土壤无机碳密度的分布特征.结果显示, 青藏高原高寒草地0-50 cm,0-1 m,0-2 m和0-3 m深度的土壤无机碳库大小分别为8.26,17.82,36.33和54.29 Pg C, 对应的土壤无机碳密度分别为7.22,15.58,31.76和47.46 kg C·m^-2.研究区土壤无机碳密度总体呈现由东南向西北增加的趋势; 高寒草原土壤的无机碳密度显著大于高寒草甸的无机碳密度.整体上, 不同深度的高寒草原无机碳库约占整个研究区无机碳库的63%-66%.此外, 深层土壤中储存了大量无机碳, 1 m以下土壤无机碳库是1 m以内无机碳库的2倍.两种草地类型土壤无机碳的垂直分布存在差异: 对高寒草原而言, 0-50 cm土壤无机碳所占的比例最大; 但对高寒草甸而言, 在100-150 cm深度土壤无机碳出现富集.这些结果表明青藏高原深层土壤是一个重要的无机碳库, 需在未来碳循环研究中予以重视.     

Soil inorganic carbon stock in alpine grasslands on the Qinghai-Xizang Plateau: An updated evaluation using deep cores

Aims To estimate the size and spatial patterns of 3-m-deep soil inorganic carbon (SIC) stock across alpine grasslands on the Qinghai-Xizang Plateau.Methods We conducted a comprehensive investigation and collected soil samples from 342 3-m-deep cores and 177 50-cm-deep pits across the study area. Using Kriging interpolation, we interpolated site-level observations to the regional level. The distribution of SIC density was then overlaid with the regional vegetation map at a scale of 1:1000000 to calculate SIC stock of the alpine steppe and alpine meadow. Kruskal-Wallis tests were further conducted to examine the differences of SIC density between the two grassland types and among soil depths with 50 cm-depth intervals.Important findings The total SIC stock at depths of 50 cm, 1 m, 2 m and 3 m were estimated at 8.26, 17.82, 36.33 and 54.29 Pg C, with SIC density being 7.22, 15.58, 31.76 and 47.46 kg C·m^-2, respectively. SIC density exhibited large spatial variability, with an increasing trend from the southeastern to the northwestern plateau. Much larger SIC stock was observed in the alpine steppe than alpine meadow, with the former accounting for 63%-66% of the total stock at depths of 50 cm, 1 m, 2 m and 3 m. A large amount of SIC stock was found in deep soils (1-3 m), amounting to approximately 2 times as much carbon stored in the top 1-m-deep soil layer. The vertical distributions of SIC density differed between the two grassland types. The highest proportions of SIC occurred in the upper 50 cm layer for the alpine steppe while the highest proportions occurred in 100-150 cm layer for the alpine meadow. These results highlight that a large amount of SIC is stored in deep soil layers, which should be considered in evaluating terrestrial carbon balance under global change scenario.     

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.     

Storage, patterns and controls of soil organic carbon in the Tibetan grasslands

The soils of the Qinghai-Tibetan Plateau store a large amount of organic carbon, but the magnitude, spatial patterns and environmental controls of the storage are little investigated. In this study, using data of soil organic carbon (SOC) in 405 profiles collected from 135 sites across the plateau and a satellite-based dataset of enhanced vegetation index (EVI) during 2001–2004, we estimated storage and spatial patterns of SOC in the alpine grasslands. We also explored the relationships between SOC density (soil carbon storage per area) and climatic variables and soil texture. Our results indicated that SOC storage in the top 1 m in the alpine grasslands was estimated at 7.4 Pg C (1 Pg=10¹⁵ g), with an average density of 6.5 kg m⁻². The density of SOC decreased from the southeastern to the northwestern areas, corresponding to the precipitation gradient. The SOC density increased significantly with soil moisture, clay and silt content, but weakly with mean annual temperature. These variables could together explain about 72% of total variation in SOC density, of which 54% was attributed to soil moisture, suggesting a key role of soil moisture in shaping spatial patterns of SOC density in the alpine grasslands.     

Moisture and vegetation controls on decadal‐scale accrual of soil organic carbon and total nitrogen in restored grasslands

Revitalization of degraded landscapes may provide sinks for rising atmospheric CO₂, especially in reconstructed prairies where substantial belowground productivity is coupled with large soil organic carbon (SOC) deficits after many decades of cultivation. The restoration process also provides opportunities to study the often‐elusive factors that regulate soil processes. Although the precise mechanisms that govern the rate of SOC accrual are unclear, factors such as soil moisture or vegetation type may influence the net accrual rate by affecting the balance between organic matter inputs and decomposition. A resampling approach was used to assess the control that soil moisture and plant community type each exert on SOC and total nitrogen (TN) accumulation in restored grasslands. Five plots that varied in drainage were sampled at least four times over two decades to assess SOC, TN, and C₄‐ and C₃‐derived C. We found that higher long‐term soil moisture, characterized by low soil magnetic susceptibility, promoted SOC and TN accrual, with twice the SOC and three times the TN gain in seasonally saturated prairies compared with mesic prairies. Vegetation also influenced SOC and TN recovery, as accrual was faster in the prairies compared with C₃‐only grassland, and C₄‐derived C accrual correlated strongly to total SOC accrual but C₃‐C did not. High SOC accumulation at the surface (0–10 cm) combined with losses at depth (10–20 cm) suggested these soils are recovering the highly stratified profiles typical of remnant prairies. Our results suggest that local hydrology and plant community are critical drivers of SOC and TN recovery in restored grasslands. Because these factors and the way they affect SOC are susceptible to modification by climate change, we contend that predictions of the C‐sequestration performance of restored grasslands must account for projected climatic changes on both soil moisture and the seasonal productivity of C₄ and C₃ plants.     

The effect of chemical fertilizer on soil organic carbon renewal and CO2 emission—a pot experiment with maize

Background and Aims

Previous studies have clearly shown substantial increases of soil organic carbon (SOC) in agricultural soils of Yellow River reaches. Those soils did not receive organic fertilizer input, but did receive chemical fertilizer inputs. Thus, to investigate the hypothesis that the observed SOC increases were driven by chemical fertilizer additions, a maize pot experiment was conducted using a Fluvisol that developed under C₃ vegetation in the Yellow River reaches.

Methods

Using the natural ¹³C abundance method we calculated the SOC renewal ratio (C _renewal), and separated total soil organic carbon (TOC) into maize-derived soil organic carbon (SOC_maize) and original soil organic carbon (SOC_original). Carbon dioxide fluxes and microbial biomass carbon (MBC) were determined by closed chamber method and fumigation-extraction method, respectively. The experiment included five treatments: (1) NPK: application of chemical fertilizer NPK; (2) NP, application of chemical fertilizer NP; (3) PK: application of chemical fertilizer PK; (4) NK, application of chemical fertilizer NK; and (5) CK: unfertilized control.

Results

Fertilization increased maize biomass (including grain, straw and root), TOC, C _renewal, SOC_maize, maize-derived carbon (MDC: including SOC_maize, and root and stubble biomass carbon) and MBC, and these values among the treatments ranked NPK>NP>PK>NK>CK. The C _renewal was 5.54–8.50% across the treatments. Fertilization also increased soil CO₂ emission (including root respiration and SOC_original decomposition), while the SOC_original decomposition during the maize growing season only amounted to 74.0–93.4 and 33.5–46.1% of SOC_maize and MDC among the treatments, respectively. Thus input was larger than export, and led to SOC increase. Maize grain and straw biomass were positively and significantly correlated with soil δ¹³C, TOC, C _renewal, SOC_maize, MDC and MBC.

Conclusions

The study suggests that chemical fertilizer application could increase C _renewal by increasing crop-derived C and accelerating original SOC decomposition, and that as long as a certain level of crop yield or aboveground biomass can be achieved, application of chemical fertilizer alone can maintain or increase SOC level in Fluvisol in the Yellow River reaches.     

Enhancement of Carbon Sequestration in Soil in the Temperature Grasslands of Northern China by Addition of Nitrogen and Phosphorus

Increased nitrogen (N) deposition is common worldwide. Questions of where, how, and if reactive N-input influences soil carbon (C) sequestration in terrestrial ecosystems are of great concern. To explore the potential for soil C sequestration in steppe region under N and phosphorus (P) addition, we conducted a field experiment between 2006 and 2012 in the temperate grasslands of northern China. The experiment examined 6 levels of N (0–56 g N m^-2 yr^-1), 6 levels of P (0–12.4 g P m^-2 yr^-1), and a control scenario. Our results showed that addition of both N and P enhanced soil total C storage in grasslands due to significant increases of C input from litter and roots. Compared with control plots, soil organic carbon (SOC) in the 0–100 cm soil layer varied quadratically, from 156.8 to 1352.9 g C m^-2 with N addition gradient (R² = 0.99, P < 0.001); and logarithmically, from 293.6 to 788.6 g C m^-2 with P addition gradient (R² = 0.56, P = 0.087). Soil inorganic carbon (SIC) decreased quadratically with N addition. The net C sequestration on grassland (including plant, roots, SIC, and SOC) increased linearly from -128.6 to 729.0 g C m^-2 under N addition (R² = 0.72, P = 0.023); and increased logarithmically, from 248.5 to 698 g C m^-2under P addition (R² = 0.82, P = 0.014). Our study implies that N addition has complex effects on soil carbon dynamics, and future studies of soil C sequestration on grasslands should include evaluations of both SOC and SIC under various scenarios.     

我国东北土壤有机碳、无机碳含量与土壤理化性质的相关性

根据黑龙江、吉林、辽宁省和内蒙古地区相关历史资料数据,分析了我国东北表层土壤(0-50 cm)土壤相关理化性质与有机碳、无机碳的相关性,得到如下结论:土壤全氮、碱解氮、全磷、速效磷、速效钾、K⁺离子交换量、Fe₂O₃、P₂O₅、总孔隙度均与土壤有机碳含量呈显著正相关(R²=0.10-0.94, n=38-345, P<0.0001),但与土壤无机碳含量则大多呈显著负相关(R²=0.11-0.30, n=37-122, P<0.01);与此相反,土壤pH值、容重与土壤有机碳呈负相关(R²=0.36-0.42,n=41-304, P<0.0001),而与无机碳呈显著正相关(R²=0.29-0.31,n=39-125, P <0.01)。表层土壤有机碳、无机碳与土壤理化性质呈相反变化趋势的结果说明,由于土壤利用方式变化所导致的土壤理化性质改变对土壤无机碳和有机碳可能具有相反影响。在研究土壤碳平衡过程中,应该充分考虑这种关系所导致的相互补偿作用,即有机碳的增加,可能意味着无机碳的减少,或者反之。目前研究中普遍忽略无机碳的变化,可能导致生态系统碳收支计算显著偏差,所获得的经验拟合方程有利于对我国东北地区土壤碳平衡研究产生的这种偏差进行粗略估计。     

中国温带草地土壤硫的分布特征及其与环境因子的关系

以内蒙古和青藏高原的6种主要草地类型为研究对象,分析了不同类型草地表层土壤硫(S)的分布特征及其与环境因子的关系。结果表明:1)青藏高原草原表层土壤(0—10 cm)的全硫含量(430.8 mg/kg)显著高于内蒙古草原(181.4 mg/kg;P0.01)。土壤硫素一般以有机硫的形式存在,具有植物有效性的土壤无机硫所占比例较少,内蒙古土壤这一比例为14.7%,青藏高原为24.5%。2)土壤硫的含量与土壤C、N的分布格局关系紧密,呈显著正相关关系;与土壤p H呈负相关关系。内蒙古与青藏高原研究区土壤的C/S和N/S值较低,这表明硫可能成为对草原生产力起限制性作用的营养元素。3)内蒙古草原表层土壤全硫、水溶性硫、吸附性硫均与年均温呈显著负相关(P0.05);土壤硫与年均降水呈显著正相关关系(P0.05)。青藏高原草地土壤硫中,除水溶性硫与年均降水呈显著正相关关系外,其余土壤硫含量均未与气候因子呈现显著相关关系。     

草原土壤有机碳含量的控制因素

基于374个高寒草原和温带草原土壤样品的测试结果,运用多元逐步回归分析模型定量评估了土壤环境因子对土壤有机碳(SOC)含量的影响.结果表明:高寒草原土壤有机碳含量(20.18 kg C/m2)高于温带草原(9.23 kg C/m2).土壤理化生物学因子对高寒草原和温带草原SOC含量(10 cm)变化的贡献分别是87.84％和75.00％.其中,土壤总氮含量和根系对高寒草原SOC含量变化的贡献均大于对温带草原SOC含量变化的相应贡献.土壤水分是温带草原SOC含量变化的主要限制性因素,其对SOC含量变化的贡献达33.27％.高寒草原土壤C/N比显著高于温带草原土壤的相应值,揭示了青藏高原高寒草原较高的SOC含量是由于较低的土壤微生物活性所导致.     

高寒草毡层基本属性与固碳能力沿水分和海拔梯度的变化

高寒草毡层是高原寒区自然植被下形成的松软而坚韧且耐搬运的表土层,认识其生态功能是促进草牧业生产休养保护和工程施工主动利用的前提。通过对青藏高原东部若尔盖高原植被的广泛调查,在布设沼泽、退化沼泽、沼泽化草甸、湿草甸、干草甸和退化草甸水分梯度群落样地,以及亚高山草甸、亚高山灌丛草甸、高山灌丛草甸和高山草甸海拔梯度群落样地的基础上,通过对不同类型群落样地草毡层容重、土壤颗粒组成和土壤有机碳(SOC)含量的测定分析,比较了水分和海拔梯度下草毡层固碳能力。结果表明,草毡层厚度平均为30cm,沼泽湿地草毡层容重最小,SOC含量在300g/kg以上;退化草甸容重最高,SOC含量显著下降。不同群落草毡层SOC密度在10—24kg C/m~2之间,随着土壤水分有效性的降低而降低;高山灌丛草甸草毡层SOC密度比草甸高15%。研究得出,保持草毡层稳定的质量含水量阈值为30%,SOC含量阈值为30g/kg;高寒植被草毡层在沼泽到草甸的退化演替中,容重、紧实度变大,有机碳含量减少,碳密度和碳储量下降;灌丛草甸的固碳能力大于草甸,但灌丛草甸的生产功能降低;保持可持续发展的草地生产能力,维护固碳生态功能,需要防止草毡层退化,抑制草甸向灌丛草甸演替。