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。     

Heterogeneity of soil and plant N and C associated with individual plants and openings in North American shortgrass steppe

Small-scale spatial heterogeneity of soil organic matter (SOM) associated with patterns of plant cover can strongly influence population and ecosystem dynamics in dry regions but is not well characterized for semiarid grasslands. We evaluated differences in plant and soil N and C between soil from under individual grass plants and from small openings in shortgrass steppe. In samples from 0 to 5 cm depth, root biomass, root N, total and mineralizable soil N, total and respirable organic C, C:N ratio, fraction of organic C respired, and ratio of respiration to N mineralization were significantly greater for soil under plants than soil from openings. These differences, which were consistent for two sites with contrasting soil textures, indicate strong differentiation of surface soil at the scale of individual plants, with relative enrichment of soil under plants in total and active SOM. Between-microsite differences were substantial relative to previously reported differences associated with landscape position and grazing intensity in shortgrass steppe. We conclude that microscale heterogeneity in shortgrass steppe deserves attention in investigation of controls on ecosystem and population processes and when sampling to estimate properties at plot or site scales.     

祁连山3种典型生态系统土壤微生物活性和生物量碳氮含量

 测定分析了祁连山高寒草甸、山地森林和干草原土壤中微生物活性、生物量碳氮含量。结果显示：就土壤微生物生物量碳含量,森林比干草原和高寒草甸中分别高60%和120%以上,干草原比高寒草甸中高40%以上(p<0.05)。就土壤微生物生物量氮含量,0～5 cm土层,森林比高寒草甸和干草原中分别高64%和111%以上,高寒草甸比干草原中高29%;5～15 cm土层,森林比干草原和高寒草甸中分别高7%和191%以上,干草原比高寒草甸中高171% 以上(p<0.05)。森林和干草原中土壤微生物生物量碳比例比高寒草甸中高32%以上,0～5和5～15 cm土层,森林和干草原中土壤微生物生物量氮比例比高寒草甸中高150%以上（p<0.05）。就土壤微生物活性,0～5和5～15 cm土层,森林和高寒草甸比干草原中高26%以上;15～35 cm土层,森林比干草原和高寒草甸中高28%以上 (p<0.05)。土壤微生物生物量碳氮含量与有机碳含量及微生物生物量氮含量和比例与微生物生物量碳含量和比例呈现正相关（r²>0.30,p<0.000 1）。土壤微生物生物量氮含量、微生物生物量碳氮含量比例、微生物活性与土壤pH值呈显著负相关,土壤微生物生物量碳氮含量及其比例、微生物活性与土壤湿度呈正相关。说明祁连山3种生态系统土壤中微生物生物量和活性受气候要素、植被、有机碳、pH值和湿度等因素 的共同影响。     

Spatial Variability of Soil Properties in the Shortgrass Steppe: The Relative Importance of Topography, Grazing, Microsite, and Plant Species in Controlling Spatial Patterns

We conducted a study to evaluate the relative importance of topography, grazing, the location of individual plants (microsite), and plant species in controlling the spatial variability of soil organic matter in shortgrass steppe ecosystems. We found that the largest spatial variation occurs in concert with topography and with microsite-scale heterogeneity, with relatively little spatial variability due to grazing or to plant species. Total soil C and N, coarse and fine particulate organic matter C and N, and potentially mineralizable C were significantly affected by topography, with higher levels in toeslope positions than in midslopes or summits. Soils beneath individual plants (Bouteloua gracilis and Opuntia polyacantha) were elevated by 2–3 cm relative to surrounding soils. All pools of soil organic matter were significantly higher in the raised hummocks directly beneath plants than in the soil surface of interspaces or this layer under plants. High levels of mineral material in the hummocks suggest that erosion is an important process in their formation, perhaps in addition to biotic accumulation of litter beneath individual plants. Over 50 y of heavy grazing by cattle did not have a significant effect on most of the soil organic matter pools we studied. This result was consistent with our hypothesis that this system, with its strong dominance of belowground organic matter, is minimally influenced by aboveground herbivory. In addition, soils beneath two of the important plant species of the shortgrass steppe, B. gracilis and O. polyacantha, differed little from one another. The processes that create spatial variability in shortgrass steppe ecosystems do not affect all soil organic matter pools equally. Topographic variability, developing over pedogenic time scales (centuries to thousands of years), has the largest effect on the most stable pools of soil organic matter. The influence of microsite is most evident in the pools of organic matter that turn over at time scales that approximate the life span of individual plants (years to decades and centuries).     

Influence of soil depth on the decomposition of Bouteloua gracilis roots in the shortgrass steppe

The distribution and turnover of plant litter contribute to soil structure, the availability of plant nutrients, and regional budgets of greenhouse gasses. Traditionally, studies of decomposition have focused on the upper soil profile. Other work has shown that temperature, precipitation, and soil texture are important determinates of patterns of decomposition. Since these factors all vary through a soil profile, it has been suggested that decomposition rates may vary with depth in a soil profile. In this work, we examine patterns of root decomposition through a shortgrass steppe soil profile. We buried fresh root litter from Bouteloua gracilis plants in litterbags at 10, 40, 70, and 100 cm. Litterbags were retrieved six times between July 1996 and May 1999. We found that the decomposition rate for fresh root litter was approximately 50% slower at 1 m than it was at 10 cm. After 33 months, 55% of the root mass buried at 10 cm remained, while 72% of the root mass buried at 1 m was still present. This corresponds to a 19-year residence time for roots at 10 cm and a 36-year residence time for roots at 1 m. Mass loss rates decreased linearly from 10 cm to 1 m. Patterns of total carbon and cellulose loss rates followed those of mass loss rates. Roots at 1 m tended to accumulate lignin-like compounds over the course of the experiment. Differences in the stabilization of lignin may be a consequence of differences in microbial community through a shortgrass steppe soil profile.     

Regional Patterns in Carbon Cycling Across the Great Plains of North America

The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m^–2 y^–1) to tallgrass prairie (180, 1221, and 208 g C m^–2 y^–1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.     

Conservation of nitrogen increases with precipitation across a major grassland gradient in the Central Great Plains of North America

Regional analyses and biogeochemical models predict that ecosystem N pools and N cycling rates must increase from the semi-arid shortgrass steppe to the sub-humid tallgrass prairie of the Central Great Plains, yet few field data exist to evaluate these predictions. In this paper, we measured rates of net N mineralization, N in above- and belowground primary production, total soil organic matter N pools, soil inorganic N pools and capture in resin bags, decomposition rates, foliar ¹⁵N, and N use efficiency (NUE) across a precipitation gradient. We found that net N mineralization did not increase across the gradient, despite more N generally being found in plant production, suggesting higher N uptake, in the wetter areas. NUE of plants increased with precipitation, and δ¹⁵N foliar values and resin-captured N in soils decreased, all of which are consistent with the hypothesis that N cycling is tighter at the wet end of the gradient. Litter decomposition appeared to play a role in maintaining this regional N cycling trend: litter decomposed more slowly and released less N at the wet end of the gradient. These results suggest that immobilization of N within the plant–soil system increases from semi-arid shortgrass steppe to sub-humid tallgrass prairie. Despite the fact that N pools increase along a bio-climatic gradient from shortgrass steppe to mixed grass and tallgrass prairie, this element becomes relatively more limiting and is therefore more tightly conserved at the wettest end of the gradient. Similar to findings from forested systems, our results suggest that grassland N cycling becomes more open to N loss with increasing aridity.     

芦芽山典型植被土壤有机碳剖面分布特征及碳储量

摘要: 基于芦芽山沿海拔梯度分布的灌丛草地、针阔混交林、寒温性针叶林和亚高山草甸四类典型植被下土壤剖面实测数据,分析了土壤有机碳的垂直分布特征及其与土壤理化因子的关系。结果表明,各植被类型下土壤剖面上层SOC含量最高,最大值往往出现在10—20 cm层,然后向下逐渐减小。土壤有机质含量由剖面上层最大值向下降低过程中,某深度土壤剖面层段有机质含量急剧减小。亚高山草甸剖面这一深度为20 cm,寒温性针叶林剖面为50 cm,针阔混交林剖面为20 cm,灌丛草地剖面为40 cm。0—10 cm层各植被类型间SOC含量差异不显著;10—20 cm层,亚高山草甸和寒温性针叶林SOC含量显著高于其他类型;20—50 cm层,亚高山草甸SOC含量与灌丛草地接近,显著高于针阔混交林,低于寒温性针叶林。植被类型对有机碳剖面分布影响较大。土壤剖面各层有机碳含量与容重呈显著负相关,与土壤含水量和全氮含量呈显著正相关,与土壤pH值呈弱的负相关,与深层黏粒和粉粒含量正相关,在30—50 cm正相关性显著。逐步回归分析结果表明,亚高山草甸SOC含量与土壤总氮含量、含水量和容重的显著相关,寒温性针叶林SOC含量与全氮含量显著相关,针阔混交林SOC含量则与总氮含量和土壤容重显著相关,而灌丛草地SOC含量与容重显著相关。在20 cm深度,四种植被土壤有机碳密度差异不显著;50 cm深度亚高山草甸、寒温性针叶林土壤有机碳储量显著高于针阔叶混交林和灌丛草地,50 cm深度土壤有机碳储量与海拔高度呈显著线性正相关（R2=0.299,P=0.01）。     

长期施肥条件下水稻土腐殖质组成及稳定性碳同位素特性

利用太湖地区26年水稻土长期定位施肥试验,分析了长期不同施肥处理对土壤剖面有机碳分布和稳定性碳同位素自然丰度(δ¹³C)的影响,以及土壤中不同结合态腐殖质组成的变化.结果表明：长期施肥使水稻土表层土壤有机碳含量显著升高,不同处理土壤剖面有机碳含量与土层深度呈极显著指数负相关(P<0.01);施化肥处理10～30 cm土层和施有机肥处理20～40 cm土层有机碳含量变化相对稳定;随土层深度增加,土壤δ¹³C值逐渐升高,其变化范围在-24‰～-28‰,不同处理土壤剖面有机碳含量与δ¹³C值呈显著线性负相关(P<0.05);0～20 cm土层,仅施有机肥处理(M0)、有机肥+氮+磷处理(MNP)、有机肥+氮+磷+钾处理(MNPK)、有机肥+秸秆+氮处理(MRN)以及秸秆+氮处理(CRN)的δ¹³C值明显降低;30～50 cm土层,除CRN外,有机肥和化肥处理土壤的δ¹³C值均明显升高;不同处理土壤中结合态腐殖质均以紧结合态腐殖质(胡敏素)为主,其含量在50%以上,其余部分为松结合态和稳结合态腐殖质;长期施肥使土壤松结合态腐殖质含量及富啡酸(FA)与胡敏酸(HA)比值(HA/FA)升高.     

Distribution of soil organic matter between fractions and aggregate size classes in grazed semiarid steppe soil profiles

Grazed steppe ecosystems are discussed as one of the big global carbon sinks that may have the potential to sequester large amounts of atmospheric CO₂ and mitigate the effects of global change if grazing is abandoned or management improved. But until today, little is known about sequestration potentials and stabilisation mechanisms in complete soil profiles of semiarid grasslands and how these systems react to grazing cessation. We applied a combined aggregate size, density and particle size fractionation procedure to sandy steppe soils under different grazing intensities (continuously grazed = Cg, winter grazing = Wg, ungrazed since 1999 = Ug99, ungrazed since 1979 = Ug79). Higher inputs of organic matter in ungrazed treatments led to higher amounts of OC in coarse aggregate size classes (ASC) and especially in particulate organic matter (POM) fractions across all depth. These processes started in the topsoil and took more than 5 years to reach deeper soil horizons (>10 cm). After 25 years of grazing cessation, subsoils showed clearly higher POM amounts. We found no grazing-induced changes of soil organic matter (SOM) quantity in fine ASC and particle size fractions. Current C-loading of fine particle size fractions was similar between differently grazed plots and decreased with depth, pointing towards free sequestration capacities in deeper horizons. Despite these free capacities, we found no increase in current C-loading on fine mineral soil fractions after 25 years of grazing exclusion. Silt and clay fractions appeared to be saturated. We suppose empirical estimations to overestimate sequestration potentials of particle size fractions or climatic conditions to delay the decomposition and incorporation of OM into these particle size fractions. POM quality was analysed using solid-state ¹³C NMR spectroscopy to clarify if grazing cessation changed chemical composition of POM in different ASC and soil depths via changing litter quality or changing decomposition dynamics. We found comparable POM compositions between different grazing intensities. POM is decomposed hierarchically from coarse to fine particles in all soil depths and grazing cessation has not affected the OM decomposition processes. The surplus of OM due to grazing cessation was predominately sequestered in readily decomposable POM fractions across all affected horizons. We question the long-term stabilisation of OM in these steppe soils during the first 25 years after grazing cessation and request more studies in the field of long-term OM stabilisation processes and assessment of carbon sequestration capacities to consider deeper soil horizons.     

森林土壤有机碳深度分布模型的构建与应用

了解森林土壤有机碳(SOC)的深度分布模式对正确估算森林碳储量,充分发挥森林碳汇功能,减缓全球气候变化有着重要意义。选取寒温带针叶林、温带落叶林、亚热带针阔混交林、热带常绿阔叶林等4类森林生物群系,建立SOC深度分布数据库,构建SOC质量密度的深度分布模型;使用Nash-Sutcliffe效率系数(E)、误差百分比(PE)、决定系数(R~2)等统计参量评定模型的模拟效果;利用构建的深度分布模型外推更深层SOC密度。研究结果表明:(1)本文所构建的森林SOC深度分布模型模拟值与观测值较为吻合,Nash-Sutcliffe效率E、误差百分比PE和决定系数R~2平均为0.74、6.95%、0.88(P0.05),模型模拟能力较高(E0.6),模拟误差值低于可接受的临界值(PE±15%),说明构建的模型可以对该地区森林SOC密度值进行估算;(2)寒温带针叶林0—20 cm层SOC质量密度较高,热带常绿阔叶林较低;20 cm以下则是寒温带针叶林较低,热带常绿阔叶林较高,热带常绿阔叶林具有更深层的SOC分布;用0—100 cm深度的SOC来表征区域SOC储量时结果偏低。若考虑0—200 cm深度,0—100 cm深度SOC值平均偏低约21.8%,在热带地区这种偏低趋势可能更加突出,误差可能更大。(3)模型对表层SOC密度有偏低预测趋势,对深层SOC密度预测值可能偏高;作为一个森林SOC深度分布模拟工具,模型可以在有限区域条件下估算不同深度SOC密度值。     

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

The dynamics of roots and soil organic carbon (SOC) in deeper soil layers are amongst the least well understood components of the global C cycle, but essential if soil C is to be managed effectively. This study utilized a unique set of land-use pairings of harvested tallgrass prairie grasslands (C₄) and annual wheat croplands (C₃) that were under continuous management for 75 years to investigate and compare the storage, turnover and allocation of SOC in the two systems to 1 m depth. Cropland soils contained 25 % less SOC than grassland soils (115  and 153 Mg C ha^?1, respectively) to 1 m depth, and had lower SOC contents in all particle size fractions (2000–250, 250–53, 53–2 and <2 μm), which nominally correspond to SOC pools with different stability. Soil bulk δ¹³C values also indicated the significant turnover of grassland-derived SOC up to 80 cm depth in cropland soils in all fractions, including deeper (>40 cm) layers and mineral-associated (<53 μm) SOC. Grassland soils had significantly more visible root biomass C than cropland soils (3.2 and 0.6 Mg ha^?1, respectively) and microbial biomass C (3.7 and 1.3 Mg ha^?1, respectively) up to 1 m depth. The outcomes of this study demonstrated that: (i) SOC pools that are perceived to be stable, i.e. subsoil and mineral-associated SOC, are affected by land-use change; and, (ii) managed perennial grasslands contained larger SOC stocks and exhibited much larger C allocations to root and microbial pools than annual croplands throughout the soil profile.     

云雾山典型草原火烧不同恢复年限土壤化学性质变化

云雾山典型草原处于黄土高原半干旱地区,也是草原火灾多发区,试验比较了未烧地与新烧地、火烧后3 a和火烧后11 a土壤有机碳(SOC)、全N、全P和速效K含量的变化过程。测量的土壤深度为50 cm,每10 cm一层,比较了4个样地0—10 cm、10—20 cm、20—30 cm、30—40 cm、40—50 cm土壤养分的变化。结果表明:(1)新烧地土壤剖面各层SOC、全N、全P和速效K含量都显著高于未烧地。(2)火烧后3 a样地土壤剖面各层SOC、全N、全P和速效K含量与未烧地差异不显著。(3)火烧后11 a样地土壤剖面各层全N含量都显著高于未烧地,SOC、全P和速效K含量除了0—10 cm层与未烧地差异不显著外,其它土层均显著增加。(4)4个样地的土壤剖面各层从上到下SOC、土壤全N、全P和速效K含量呈递减趋势。(5)3个火烧样地土壤表层(0—10 cm)的pH值和未烧地差异不显著。     

Controls on Soil Organic Carbon Stocks and Turnover Among North American Ecosystems

Despite efforts to understand the factors that determine soil organic carbon (SOC) stocks in terrestrial ecosystems, there remains little information on how SOC turnover time varies among ecosystems, and how SOC turnover time and C input, via plant production, differentially contribute to regional patterns of SOC stocks. In this study, we determined SOC stocks (gC m⁻²) and used soil radiocarbon measurements to derive mean SOC turnover time (years) for 0–10 cm mineral soil at ten sites across North America that included arctic tundra, northern boreal, northern and southern hardwood, subtropical, and tropical forests, tallgrass and shortgrass prairie, mountain grassland, and desert. SOC turnover time ranged 36-fold among ecosystems, and was much longer for cold tundra and northern boreal forest and dry desert (1277–2151 years) compared to other warmer and wetter habitats (59–353 years). Two measures of C input, net aboveground production (NAP), determined from the literature, and a radiocarbon-derived measure of C flowing to the 0–10 cm mineral pool, I, were positively and SOC turnover time was negatively associated with mean annual evapotranspiration (ET) among ecosystems. The best fit model generated from the independent variables NAP, I, annual mean temperature and precipitation, ET, and clay content revealed that SOC stock was best explained by the single variable I. Overall, these findings indicate the primary role that C input and the secondary role that C stabilization play in determining SOC stocks at large regional spatial scales and highlight the large vulnerability of the global SOC pool to climate change.     

高寒草原土壤有机碳及土壤碳库管理指数的变化

高寒草原对高寒生态系统的稳定具有重大意义。为探明高寒草原土壤有机碳（SOC）、土壤活性有机碳（ASOC）变化,以及草地退化对土壤碳库稳定性的影响,对藏北高原正常、轻度和严重退化高寒草原表层（0-10 cm）、亚表层（10-20 cm）土壤进行了初步研究。结果表明：（1）轻度、严重退化草地各土层SOC、ASOC均呈不同程度的下降。其中,退化草地SOC的降幅均以表层最大,且各土层降幅均随草地退化加剧而下降;退化草地ASOC的降幅则均以亚表层最大,但各土层ASOC的降幅随草地退化加剧而提高。（2）正常草地、轻度和严重退化草地表层ASOC比率分别为16.8%、21.3%、16.6%,亚表层分别为21.8%、18.1%和16.0%;土壤碳库活度与ASOC比率的变化趋势完全一致。因此,轻度退化草地SOC的不稳定性主要体现在表层土壤。（3）退化草地表层、亚表层碳库管理指数（CMI）均呈显著下降,但表层降幅相对较低;与严重退化草地比,轻度退化草地不同土层CMI明显提高。（4）高寒草原环境中,正常草地、轻度和严重退化草地各土层SOC、ASOC间则均呈一定程度的负相关,表明土壤微生物对SOC、ASOC的影响和作用可能不同。     

贵州喀斯特地区典型土壤有机碳垂直分布特征及其同位素组成

以贵州喀斯特地区两种主要土壤类型(石灰土和黄壤)为研究对象,通过测定土壤pH值、土壤有机碳(SOC)含量和植物优势种、枯枝落叶、土壤有机质的稳定同位素(δ13Csoc值)组成,探讨了该地区石灰土和黄壤剖面SOC垂直分布特征和δ13Csoc值组成差异。结果表明,与黄壤相比,石灰土剖面的SOC含量较高,石灰土剖面和黄壤剖面SOC含量变化范围分别在3.6～69.8和2.4～51.2g·kg-1。黄壤和黄色石灰土剖面SOC主要集中在0～20cm深度内,而黑色石灰土剖面从0～60cm逐步减少。黑色石灰土和黄壤剖面δ13Csoc值变化范围分别在-22.9‰～-21.5‰和-25.6‰～-22.4‰,前者较后者变化小。从剖面表土向下,黄壤剖面δ13Csoc值均出现逐步增加的趋势,而石灰土剖面δ13Csoc值从剖面表土向下出现上升-降低-不变的变化趋势。黄色石灰土剖面δ13Csoc值变幅较大,变化范围为-23.7‰～-18.2‰。在枯枝落叶转化为表层土壤有机质的过程中,石灰土剖面δ13Csoc值变幅高于黄壤。其中,黄壤剖面δ13Csoc值升高了2.6‰～3.0‰,石灰土剖面δ13Csoc值升高了5.5‰～6.3‰。上述结果揭示了SOC含量及其δ13C值随深度变化的差异,反映植物残体的输入及其在土壤中分解累积特征,有助于揭示SOC循环过程及规律和了解剖面土壤成土过程。     

Differential Effects of Conservational Management on SOC Accumulation in the Grasslands of China

Conservational management practices in grasslands have been considered one of the efficient options to enhance the soil organic carbon (SOC) accumulation. However, the SOC changes after the conservational management practices vary significantly under different grassland vegetation types and the environmental conditions. At present, it is not clear how the SOC accumulation changes along the soil profile if conservational management practice was adopted. In this study, we collected 663 paired observational data of SOC changes with and without conservational management practices in grasslands of China from 176 published literatures that has both the surface (0‒20 cm) and subsurface (to 40 cm depth) SOC measurements. The differences of SOC density (SOCD) between pre‒management and post‒management in the vertical soil layers were analyzed in order to establish a quantitative relationship of the SOC changes between the subsurface and the surface. The results revealed that in all grasslands, conservational management practices benefits the SOC accumulation by enhancing 0.43‒1.14 Mg C ha^–1 yr^–1. But the SOC increment weakened downwards along the soil profile. While the surface SOC was enhanced by 17% after conservational management, the subsurface SOC was enhanced by only 7%. The SOC accumulation was closely correlated with restoration duration, pre-management SOCD and the environmental factors and differed greatly among different grasslands and the practices adopted. The alpine and mountain grassland showed a higher annual SOC increment than the temperate grassland with the annual rate of 1.62 and 0.72 Mg C ha^-1 yr^-1, respectively. The SOC increment caused by the artificial plantation and the grazing exclusion conservational management was more than 2-fold that of the cropland abandonment and the extensive utilization. With the quantitative relationship of the SOC changes between soil layers, we provide a methodological option to estimate SOC changes to layers deeper than the recommendation of IPCC when only the surface layer SOC increment is available.     

Patterns of SOC and soil 13C and their relations to climatic factors and soil characteristics on the Qinghai–Tibetan Plateau

Background and aims

SOC inventory and soil δ¹³C were widely used to access the size of soil C pool and to indicate the dynamics of C input and output. The effects of climatic factors and soil physical characteristics and plant litter input on SOC inventory and soil δ¹³C were analyzed to better understand the dynamics of carbon cycling across ecosystems on the Qinghai-Tibetan Plateau.

Methods

Field investigation was carried out along the two transects with a total of 1,875 km in length and 200 km in width. Sixty-two soil profiles, distributed in forest, meadow, steppe, and cropland, were stratified sampled every 10 cm from 0 to 40 cm.

Results

Our result showed that SOC density in forest and meadows were much higher than in steppe and highland barley. In contrast, δ¹³C in forest and meadow were lower than in steppe and highland barley. Soil δ¹³C tended to enrich with increasing soil depth but SOC decline. SOC and δ¹³C (0–40 cm) were correlated with different climatic factors in different ecosystems, such that SOC correlated negatively with MAT in meadow and positively with MAP in steppe; δ¹³C correlated positively with MAT in meadow and steppe; and δ¹³C also tended to increase with increasing MAT in forest. Of the variation of SOC, 55.15 % was explained by MAP, pH and silt content and 4.63 % was explained by the interaction between MAT and pH across all the ecosystems except for the cropland. Meanwhile, SOC density explained 27.40 % of variation of soil δ¹³C.

Conclusions

It is suggested that different climatic factors controlled the size of the soil C pool in different ecosystems on the Tibetan Plateau. SOC density is a key contributor to the variation of soil δ¹³C.     

N fertilizer and harvest impacts on bioenergy crop contributions to SOC

Belowground root biomass is infrequently measured and simply represented in models that predict landscape‐level changes to soil carbon stocks and greenhouse gas balances. Yet, crop‐specific responses to N fertilizer and harvest treatments are known to impact both plant allocation and tissue chemistry, potentially altering decomposition rates and the direction and magnitude of soil C stock changes and greenhouse gas fluxes. We examined switchgrass (Panicum virgatum L.) and corn (Zea mays L.,) yields, belowground root biomass, C, N and soil particulate organic matter‐C (POM‐C) in a 9‐year rainfed study of N fertilizer rate (0, 60, 120 and 180 kg N ha^?1) and harvest management near Mead, NE, USA. Switchgrass was harvested with one pass in either August or postfrost, and for no‐till (NT) corn, either 50% or no stover was removed. Switchgrass had greater belowground root biomass C and N (6.39, 0.10 Mg ha^?1) throughout the soil profile compared to NT‐corn (1.30, 0.06 Mg ha^?1) and a higher belowground root biomass C:N ratio, indicating greater recalcitrant belowground root biomass C input beneath switchgrass. There was little difference between the two crops in soil POM‐C indicating substantially slower decomposition and incorporation into SOC under switchgrass, despite much greater root C. The highest N rate decreased POM‐C under both NT‐corn and switchgrass, indicating faster decomposition rates with added fertilizer. Residue removal reduced corn belowground root biomass C by 37% and N by 48% and subsequently reduced POM‐C by 22% compared to no‐residue removal. Developing productive bioenergy systems that also conserve the soil resource will require balancing fertilization that maximizes aboveground productivity but potentially reduces SOC sequestration by reducing belowground root biomass and increasing root and soil C decomposition.     

城市沿江土地覆被变化对土壤有机碳和轻组有机碳的影响

采用Vario EL III型元素分析仪,2007年7月分析了福州城市沿江3种土地覆被类型（芦苇湿地以及草坪和片林）土壤有机碳（SOC）与轻组有机碳（LFOC）的垂直分布特征.结果表明：3种土地覆被类型SOC和LFOC含量均表现为在土壤表层富集并向下层递减的趋势,且人工绿地在土壤剖面（0～60 cm）不同层次的SOC和LFOC含量均显著高于沿江芦苇湿地,其中,草坪0～20 cm土层的SOC含量显著高于片林;沿江芦苇湿地转为草坪和片林后,其SOC储量分别增加了94.8%和72.0%,LFOC储量分别增加了225%和93%;城市沿江湿地转变为城市人工绿地后,因植物物种、密度以及周期性人工管理等变化,使土壤质量得以改善、SOC和LFOC储量增加,其中LFOC对土地覆被变化的响应较SOC敏感,以表层(0～20 cm)土壤LFOC受到土地利用/覆地变化的影响最大.