Symbiosis of Arbuscular Mycorrhizal Fungi and Robinia pseudoacacia L. Improves Root Tensile Strength and Soil Aggregate Stability

Robinia pseudoacacia L. (black locust) is a widely planted tree species on Loess Plateau for revegetation. Due to its symbiosis forming capability with arbuscular mycorrhizal (AM) fungi, we explored the influence of arbuscular mycorrhizal fungi on plant biomass, root morphology, root tensile strength and soil aggregate stability in a pot experiment. We inoculated R. pseudoacacia with/without AM fungus (Rhizophagus irregularis or Glomus versiforme), and measured root colonization, plant growth, root morphological characters, root tensile force and tensile strength, and parameters for soil aggregate stability at twelve weeks after inoculation. AM fungi colonized more than 70% plant root, significantly improved plant growth. Meanwhile, AM fungi elevated root morphological parameters, root tensile force, root tensile strength, Glomalin-related soil protein (GRSP) content in soil, and parameters for soil aggregate stability such as water stable aggregate (WSA), mean weight diameter (MWD) and geometric mean diameter (GMD). Root length was highly correlated with WSA, MWD and GMD, while hyphae length was highly correlated with GRSP content. The improved R. pseudoacacia growth, root tensile strength and soil aggregate stability indicated that AM fungi could accelerate soil fixation and stabilization with R. pseudoacacia, and its function in revegetation on Loess Plateau deserves more attention.     

干旱区不同地下水埋深下胡杨细根空间分布特征

细根空间分布特征能够反映植物对环境的利用程度和适应性,这对评价植物适应逆境至关重要。为了探究胡杨细根空间分布在干旱环境下的适应性特征,以塔里木河下游极端干旱区不同地下水埋深条件下的成年胡杨(Populus euphratica Oliv.)为对象,采用人工挖掘法,对胡杨细根(D≤2 mm)空间分布及其与地下水埋深和土壤水分的关系进行了研究。结果显示:(1)在水平方向上(550 cm范围内),胡杨细根的根长密度(RLD)、表面积密度(SAD)、根质量密度(RMD)随水平距离的增加未发生显著变化;(2)在垂直方向上,土壤表层基本无细根分布,随土壤深度加深,胡杨细根RLD、RMD呈先增加后减少的分布特征,并且在地下水埋深较深处,胡杨细根在较深土壤层(280 cm)仍保持较高的比根长(SRL)和比表面积(SRA);(3)胡杨细根RLD、RMD与上层土壤(0~180 cm)含水量存在较高的正相关关系,而与深层土壤(180 cm以下)含水量存在空间差异。本研究表明生长在上层土壤(0~180 cm)的胡杨细根主要受水分的限制,而生长在土壤深层的细根很可能受地下水埋深的影响,同时为了应对干旱环境,胡杨根系不仅具有较强的水平扩展能力,也会向深层湿润的土壤发展。研究结果可为极端干旱环境下胡杨适应机制的研究提供参考。     

松嫩平原沙土区不同植被类型对土壤相关指标的影响

松嫩平原沙土区严重退化,不同植被对土壤影响的数据有助生态恢复,但相关数据匮乏严重。本研究以松嫩平原核心区新店林场的农田、草地、杨树林、落叶松林、樟子松林和榆树林等6种植被为研究对象,采集三层（0~20、20~40和40~60 cm）土样,测定土壤理化指标（容重、土壤含水量、土壤pH、土壤电导率、有机碳、碱解氮、全氮、速效磷、全磷、速效钾、全钾）及根系密度、根系含水量。结果如下：6种植被在整个土壤剖面上显著影响了13个指标中的8个指标（根系密度、容重、土壤含水量、土壤pH、土壤电导率、全氮、根系含水量、有机碳）,其中根系密度植被间差异在不同土层显著不同(交互作用,P<0.05),而其它7个指标在不同土层的种间差异基本一致（交互作用,P>0.05）。与林地和农田相比,草地具有最大的根系生物量,达到农田的1.8倍,是4种林分平均值的1.4倍。容重、土壤含水量、pH、土壤电导率值均草地最高,林地最小。土壤有机碳的累积受根系密度和容重影响明显,土壤氮除了受上述两个因素影响外,还受土壤含水量的影响。上述结果说明草地对于表层土壤生物固着明显,而人工造林能够改善土壤物理性质、降低盐碱但增加耗水,这些结果对于西部沙土区通过植被恢复来实现土壤修复提供数据支撑。     

Soil Anti-Scouribility Enhanced by Plant Roots

The magnitude of soil anti-scouribility depends on the physical condition of the soil. Plant roots can greatly enhance soil stability and anti-erodibility. A scouring experiment of undisturbed soil was conducted to investigate the effects of roots on soil anti-scouribility and its distribution in the soil profile. At the end of each erosion test, plant roots were collected from soil samples and root surface area was calculated by means of a computer image analysis system (CIAS). Root surface area density (RSAD), the surface area of the roots per unit of soil volume, was related to soil anti-scouribility. More than 83% of root surface area was concentrated in the 0-30 cm soil layer. Soil anti-scouribility increased with an increase in RSAD and the value of intensified soil anti-scouribility (ΔAS) can be expressed by exponential equations, depending on the plant species. These equations were ΔAS=9.578 6 RSAD^0.8321 (R^2=0.951) for afforested Pinus tabulaeformis Carr.ΔAS=7.8087 RSAD^0.7894(R^2=0.974) for afforested Robinia pseudoacacia L., and ΔAS=9.256 6 RSAD^0.8707(R^2=0.899) for Bothriochloa ischemum L.     

Soil Anti-Scouribility Enhanced by Plant Roots

The magnitude of soil anti-scouribility depends on the physical condition of the soil. Plant roots can greatly enhance soil stability and anti-erodibility. A scouring experiment of undisturbed soil was conducted to investigate the effects of roots on soil anti-scouribility and its distribution in the soil profile. At the end of each erosion test, plant roots were collected from soil samples and root surface area was calculated by means of a computer image analysis system (CIAS). Root surface area density (RSAD), the surface area of the roots per unit of soil volume, was related to soil anti-scouribility. More than 83% of root surface area was concentrated in the 0 - 30 cm soil layer. Soil anti-scouribility increased with an increase in RSAD and the value of intensified soil anti-scouribility (△AS) can be expressed by exponential equations, depending on the plant species. These equations were △AS = 9.578 6 RSAD0.8321 (R2 = 0.951) for afforested Pinus tabulaeformis Cart., △AS = 7.808 7 RSAD0.7894 (R2 = 0.974) for afforested Robinia pseudoacacia L., and △AS = 9.256 6 RSAD0.8707 (R2 = 0.899) for Bothriochloa ischemum L.     

子午岭植被恢复对土壤饱和导水率的影响

通过对子午岭林区不同植被的土壤性质进行实验室测定和野外调查,对饱和土壤水分运动的重要参数之一土壤饱和导水率(Ks)及其相关因子进行了多元分析和通径分析,揭示了植被恢复提高土壤水分传输性能的机理,主要结论如下:土壤有机质是子午岭林区九种植被下土壤饱和导水率提高的主要驱动因子.不同植被下的土壤饱和导水率均随深度的增加而迅速降低,尽管草地和先锋草地在5～10cm深度有一强透水层.土壤饱和导水率在剖面上的平均值,从辽东栎、早期森林、灌丛、先锋草地、弃耕地到草地依次降低.灌丛与草地、弃耕地的差异达到显著水平,辽东栎顶级群落的饱和导水率最高,植被的恢复明显提高了土壤饱和导水率.土壤容重、毛管孔隙度、>0.25mm团聚体含量及粘粒含量直接影响土壤饱和导水率.土壤有机质含量的提高能够改善容重、毛管孔隙度、团聚体含量等物理性质.     

黄土高原水蚀风蚀交错区植被地上生物量及其影响因素

采用野外调查的方法,于2009年9月下旬测定了六道沟小流域不同土地利用方式下的地上生物量以及土壤水分含量和养分含量,研究了水蚀风蚀交错区典型小流域植被地上生物量水平及其影响因素.结果表明:六道沟小流域主要植被地上干生物量在177～2207g·m-2;其中,玉米、谷子、弃耕地、人工草地、天然草地和灌木地的地上干生物量分别为2097～2207、518～775、248～578、280～545、177～396和372～680 g·m-2.农田平均土壤含水量(0～100 cm土层)最高,达14.2%,灌木地最低,为10.9%;弃耕地土壤水分含量的变异系数最大,为26.7%,说明弃耕地土壤水分有很强的空间异质性.土壤平均储水量大小顺序为:农田>人工草地>弃耕地>天然草地>灌木地,苜蓿地和柠条地出现土壤干化现象.植被地上干生物量与0～100 cm土层土壤储水量存在显著正相关关系(r=0.639,P<0.05),地上鲜生物量与植被的株高呈极显著正相关,较高植被的地上生物量可以间接控制水蚀风蚀交错区土壤侵蚀.植被地上生物量与土壤水分、养分具有很高的相关性,但与海拔、坡度、坡向、容重等的相关性不显著.     

Root seasonal pattern,spatial distribution,and C:N ratio of matgrass pasture (Nardus stricta L.) in the Lombardy Prealps

Abstract

The aim of the present study was to investigate carbon and nutrient cycling and the role of root dynamics in terrestrial ecosystems such as large abandoned pastures and natural grasslands present in the Prealps, for which below‐ground processes are currently enigmatic. In particular, we quantified root/leaf biomass and C:N ratio throughout two growing seasons. Additionally, root traits such as root length density (RLD), root mass density (RMD), and root diameter classes (RDC) were also investigated with the aim of understanding the spatial distribution of roots in the soil. In our samples, we found that the roots could be divided into three main diameter classes and hence quantified the presence of each class along the soil profile. With regard to total root biomass, we found the occurrence of two peaks of biomass accumulation during the growth season, and when biomass accumulation was compared with climatic data, it was impossible to obtain a clear indication of the root turnover rate. In fact, the strong influence of grazing on the above‐ground biomass could have affected, in turn, root biomass. In future, this possible complication will be avoided by repeating the measurements within enclosures to avoid grazing interference. We found that C:N ratio remained constant, with a single peak, suggesting a lower root decomposition during the warmest period (August 2006). The concentration of nitrogen in roots decreased with depth as a result of a decrease in roots with smaller diameters. The reverse was found for carbon content, which increased with depth, probably due to an increase in roots with larger diameters. This study represents the first attempt to estimate root turnover rates in this prealpine ecosystem, which have been analysed to date only for the above‐ground biomass.     

Implications of land-cover types for soil erosion on semiarid mountain slopes: Towards sustainable land use in problematic landscapes

The impact of land-cover types on soil erosion and runoff, as well as on physico-chemical soil properties, was monitored. The study area, an agroforestry landscape was located in Sierra Nevada Mountains in south-eastern Spain. Eight land-cover types were investigated: farmland planted with olive, almond, and cereals; forest with P. halepensis and P. sylvestris; shrubland; grassland; and abandoned farmland. The erosion plots replicated twice were located on hillslopes, where erosion and runoff were measured after 22 storm events. Forest dominated by Pinus stands exhibited significantly the lowest amounts of erosion and runoff, contrasting with abandoned farmland. Olive had higher erosion than did almond, cereals, or grasslands, but with the highest runoff rate under almond groves. The erosion and runoff response to shrubland showed an intermediate situation between forest and farmland–grassland uses. Under forest and shrubland, better soil properties were determined, especially higher organic C and total N, and lower soil-bulk density. Erosion was highly dependent on runoff, bulk density, soil organic C, and the degree of soil surface covered. Thus, the alteration in land cover is essential to an understanding of productivity of soil undergoing erosion, as sustainable planning can mitigate soil-degradation processes in the overall agroforestry landscape.     

不同植被防护措施对三峡库区土质道路边坡侵蚀的影响

道路建设形成的大量裸露边坡缺乏防护常常导致严重的水土流失,是山区流域重要的侵蚀源.本文以三峡库区新修土质道路边坡为研究对象,布设草本、草灌结合、草灌+梯坎组合、植生带、坡耕地、草本+坡耕地组合进行边坡防护,结合野外自然降雨观测,研究降雨及植被防护对边坡侵蚀的影响.结果表明:草灌结合措施的植被恢复速度最快,平均盖度由20.2％增加到91.6％,对土壤容重和导水率改善最明显,降低容重9.5％,增加导水率678.1％,其次是草本措施和植生带;梯坎+草灌结合的抗侵蚀能力最强,截流效率和阻沙效率分别为72.3％和80.2％,适用于坡度＞45°路堑,其次是草灌结合和草本,草本对路堤的防护效果高于路堑,草灌结合对路堑的防护效果更理想;在15°缓坡路堤,草本+坡耕地处理对径流的拦截效率很低,仅2.0％,但拦截泥沙效率与草本和草灌相当,为23.0％,当坡度＜15°路堤被用作耕地时,必须在路肩设置2 m长的草本缓冲带以满足侵蚀防护的要求.     

Effects of Revegetation on Soil Microbial Biomass,Enzyme Activities,and Nutrient Cycling on the Loess Plateau in China

Revegetation is a traditional practice widely used for soil and water conservation on the Loess Plateau in China. However, there has been a lack of reports on soil microbial–biochemical indices required for a comprehensive evaluation of the success of revegetation systems. In this study, we examined the effects of revegetation on major soil nutrients and microbial–biochemical properties in an artificial alfalfa grassland, an enclosed natural grassland, and an artificial shrubland (Caragana korshinskii), with an abandoned cropland as control. Results showed that at 0–5, 5–20, and 20–40 cm depths, soil organic carbon, alkaline extractable nitrogen and available potassium were higher in natural grassland and artificial shrubland compared with artificial grassland and abandoned cropland. Soil microbial biomass C (Cmic) and phosphorous (Pmic) substantially decreased with depth at all sites, and in abandoned cropland was significantly lower than those of natural grassland, artificial grassland, and artificial shrubland at the depth of 0–5 cm. Soil microbial biomass N (Nmic) was higher in artificial shrubland and abandoned cropland compared with that in natural and artificial grasslands. Both Cmic and Pmic were significantly different between the 23‐year‐old and the 13‐year‐old artificial shrublands at the 0–5 cm depth. The activities of soil invertase, urease, and alkaline phosphatase in natural grassland and artificial shrubland were higher than those in artificial grassland and abandoned cropland. This study demonstrated that the regeneration of both natural grassland and artificial shrubland effectively preserved and enhanced soil microbial biomass and major nutrient cycling, thus is an ecologically beneficial practice for recovery of degraded soils on the Loess Plateau.     

Soil microbial parameters and stability of soil aggregate fractions under different grassland communities on the Loess Plateau,China

Over-grazing and large-scale monocultures on the Loess plateau in China have caused serious soil erosion by water and wind. Grassland revegetation has been reported as one of the most effective counter measures. Therefore, we investigated soil aggregation, aggregate stability and soil microbial activities as key parameters for soil remediation through grassland revegetation. The results showed that soil microbial biomass carbon (Cmic) and microbial biomass nitrogen (Nmic) increased under revegetated grass communities compared to cropland and overgrazed pastures and were higher in surface layers (0–10 cm) than in the subsurface (10–20 cm). Although there are variations between the four investigated grassland communities, their values were 10 to 50 times higher in comparison to the cropland and overgrazed pastures, similar to the increase in soil enzyme activities, such as β-glucosidase and β-glucosaminidase. Soil aggregate stability (SAS) showed clear differences between the different land uses with two main soil aggregate fractions measured by ultra sound: < 63 μm and 100–250 μm, with approximately 70% and 10% of the total soil volume respectively. We also found positive correlations between SAS and soil microbial parameters, such as Cmic, Nmic, and soil enzyme activities. From this, we concluded that revegetation of eroded soils by grasses accelerates soil rehabilitation.     

No general soil carbon sequestration under Central European short rotation coppices

Wood 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.     

小麦和玉米根系取样位置优化确定及根系分布模拟

为了确定小麦(Triticum aestivum)、玉米(Zea mays)根系的最优取样位置和更准确地模拟根长密度在土壤剖面的分布, 在冬小麦和夏玉米的灌浆后期, 采用根钻法取样, 比较了不同取样位置对根系分布的影响; 采用Gerwitz和Page模型对根长密度的分布进行了模拟。结果表明, 冬小麦行间和行上取样在0-10 cm土层根长密度差异显著, 在10 cm以下土层差异减少。在确定根长密度分布的取样中, 在0-20 cm土层应考虑根长密度分布的空间差异, 即行上密度大于行间密度; 而在20-100 cm土层, 需要考虑行间根长密度大于行上的空间差异; 在1 m以下土层两个位置的差异逐渐消失, 可不考虑空间差异。夏玉米根长密度在上层土壤表现出距离植株不同位置差异显著的特征。植株位置(株上)、距植株10 cm和距植株20 cm位置根长密度在土壤中的分布特征是: 0-10 cm土层3个位置根长密度差异在50%以上, 根长密度大小是株上>距植株10 cm>距植株20 cm; 而在10-30 cm层次, 根长密度表现为距植株10 cm>株上>距植株20 cm, 30-50 cm土层株上位置的根长密度最小, 50 cm以下各位置根长密度差异不明显。对于玉米根系取样, 50 cm以上土层需要考虑根长密度的空间差异, 50 cm以下土层可不考虑。采用Gerwitz和Page模型, 结合华北平原机械化耕作下形成的土壤犁底层变厚及其犁底层容重增加对根系分布的影响, 在模型中加入土壤容重参数订正可以使模型更准确地模拟根长密度在土壤剖面的分布。     

稻草覆盖和香根草篱对红壤水稳性团聚体组成及有机碳含量的影响

基于植物篱和秸秆覆盖控制红壤坡耕地水土流失的长期定位试验,研究香根草篱(H)、稻草覆盖(M)、香根草篱+稻草覆盖(HM)水保措施下红壤水稳性团聚体组成及有机碳分布特征。结果表明:与常规等高农作模式(CK)相比,草篱、稻草覆盖、草篱+稻草覆盖模式下土壤总有机碳含量提高0.07—2.42 g/kg。草篱对土壤团聚体组成及其结合有机碳的影响在篱内效果显著,随着与草篱距离增大影响减弱。草篱和稻草覆盖对土壤团聚体组成和结合有机碳含量的影响不同,草篱主要增加2 mm水稳性团聚体含量及其结合有机碳含量,稻草覆盖增加0.25 mm水稳性团聚体含量及其结合有机碳含量。综合来看,草篱和稻草覆盖相结合对改善坡面土壤结构作用稳定。土壤有机碳含量较高时,土壤总有机碳含量与粒径2mm的大团聚体有机碳含量呈显著正相关(r=0.659);随着有机碳含量降低,土壤总有机碳含量与土壤0.25—0.053 mm和0.053 mm微小团聚体碳含量相关性逐渐增大。     

根区交替控制灌溉条件下玉米根系吸水规律

为了阐明根区交替控制灌溉(CRDAI)条件下玉米根系吸水规律,通过田间试验,在沟灌垄植模式下采用根区交替控制灌溉研究玉米根区不同点位(沟位、坡位和垄位)的根长密度(RLD)及根系吸水动态。研究表明,根区土壤水分的干湿交替引起玉米RLD的空间动态变化,在垄位两侧不对称分布,并存在层间差异;土壤水分和RLD是根区交替控制灌溉下根系吸水速率的主要限制因素。在同一土层,根系吸水贡献率以垄位最大,沟位最低;玉米营养生长阶段,10—30 cm土层的根系吸水速率最大;玉米生殖生长阶段,20—70 cm为根系吸水速率最大的土层,根系吸水贡献率为43.21%—55.48%。研究阐明了交替控制灌溉下根系吸水与土壤水分、RLD间相互作用的动态规律,对控制灌溉下水分调控机理研究具有理论意义。     

黄土丘陵区表层土壤有机碳沿降水梯度的分布

沿368～591 mm降水量梯度选取7个调查地点、共63个调查样点,在每个样点选择恢复年限相近的林地、草地和农地,调查表层（0～30 cm）土壤有机碳的分布特征,分析气候、土层深度和土地利用类型等因素对土壤有机碳分布的影响.结果表明： 在黄土丘陵区368～591 mm的降水量范围内,表层土壤有机碳含量表现为草地（8.70 g·kg^-1）>林地（7.88 g·kg^-1）>农地（7.73 g·kg^-1）,土壤有机碳密度表现为草地（20.28 kg·m^-2）>农地（19.34 kg·m^-2）>林地（17.14 kg·m^-2）.林地、草地、农地的土壤有机碳含量无显著差异,综合3种土地利用类型的数据分析表明,不同降雨梯度下土壤有机碳含量差异显著（P<0.001）,土壤有机碳含量（r=0.838,P<0.001）与年均降水量间存在显著线性正相关关系;由北向南（以最北端鄂尔多斯为起点）,土壤有机碳含量沿着368～591 mm的年均降水量梯度的递增速率为0.04 g·kg^-1·mm^-1,土壤有机碳密度的递增速率为0.08 kg·m^-2·mm^-1.年均降水量、土壤黏粒含量、林下枯落物蓄积量和农作物根系密度可较好地模拟表层土壤有机碳分布.     

Field performance of Solanum sisymbriifolium, a trap crop for potato cyst nematodes. II. Root characteristics

Hatching of potato cyst nematodes is induced by root exudates of Solanaceae, such as Solanum sisymbriifolium, and is therefore related to root length distribution of this crop. A mathematical model was derived to relate the hatching potential to root length density (RLD). A series of field experiments was carried out to study actual root length distribution of S. sisymbriifolium in relation to shoot properties and to provide input into the model. Using a modified Poisson distribution formula for the three‐dimensional distribution of roots in a volume of soil, the relation between the zone of influence of hatching agents and the RLD could be derived. On this basis, the minimal RLD was estimated, which is needed to expose 75%, 90% or 95% of cysts to root exudates, as a function of the length of the zone of influence of hatching agents on cysts. The logarithm of the total root length showed a linear relation with the logarithms of above‐ground biomass and with leaf area index. Root diameter distribution was the same for all crops examined and independent of soil depth. Fine roots (<0.4 mm in diameter) constituted around 50% of total root length. Using a zone of influence of 1.00, 0.75 and 0.50 cm around the centre of each root, a minimal RLD for sufficient soil exploration (75%) was estimated. Depth at which that minimal RLD was exceeded was linearly related to total root length (km m^?2) and to above‐ground crop biomass, enabling estimations being made of the potential hatching efficacy as related to measurable properties of S. sisymbriifolium crops. The proposed approach to derive potential hatching effects from crop properties needs further validation; particularly, the distance of influence of root exudates is a critical factor.     

Soil organic carbon stocks in China and changes from 1980s to 2000s

The estimation of the size and changes of soil organic carbon (SOC) stocks is of great importance for decision makers to adopt proper measures to protect soils and to develop strategies for mitigation of greenhouse gases. In this paper, soil data from the Second State Soil Survey of China (SSSSC) conducted in the early 1980s and data published in the last 5 years were used to estimate the size of SOC stocks over the whole profile and their changes in China in last 20 years. Soils were identified as paddy, upland, forest, grassland or waste‐land soils and an improved soil bulk density estimation method was used to estimate missing bulk density data. In the early 1980s, total SOC stocks were estimated at 89.61 Pg (1 Pg=10³ Tg=10¹⁵ g) in China's 870.94 Mha terrestrial areas covered by 2473 soil series. In the paddy, upland, forest and grassland soils the respective total SOC stocks were 2.91 Pg on 29.87 Mha, 10.07 Pg on 125.89 Mha, 34.23 Pg on 249.32 Mha and 37.71 Pg on 278.51 Mha, respectively. The SOC density of the surface layer ranged from 3.5 Mg ha⁻¹ in Gray Desery grassland soils to 252.6 Mg ha⁻¹ in Mountain Meadow forest soils. The average area‐weighted total SOC density in paddy soils (97.6 Mg ha⁻¹) was higher than that in upland soils (80 Mg ha⁻¹). Soils under forest (137.3 Mg ha⁻¹) had a similar average area‐weighted total SOC density as those under grassland (135.4 Mg ha⁻¹). The annual estimated SOC accumulation rates in farmland and forest soils in the last 20 years were 23.61 and 11.72 Tg, respectively, leading to increases of 0.472 and 0.234 Pg SOC in farmland and forest areas, respectively. In contrast, SOC under grassland declined by 3.56 Pg due to the grassland degradation over this period. The resulting estimated net SOC loss in China's soils over the last 20 years was 2.86 Pg. The documented SOC accumulation in farmland and forest soils could thus not compensate for the loss of SOC in grassland soils in the last 20 years. There were, however, large regional differences: Soils in China's South and Eastern parts acted mainly as C sinks, increasing their average topsoil SOC by 132 and 145 Tg, respectively. In contrast, in the Northwest, Northeast, Inner Mongolia and Tibet significant losses of 1.38, 0.21, 0.49 and 1.01 Pg of SOC, respectively, were estimated over the last 20 years. These results highlight the importance to take measures to protect grassland and to improve management practices to increase C sequestration in farmland and forest soils.     

宁南山区植被恢复对土壤团聚体水稳定及有机碳粒径分布的影响

土壤团聚作用和土壤有机碳固定之间密切相关.对宁南山区不同植被恢复措施和年限下土壤团聚体粒径分布及稳定性、土壤团聚体中有机碳及其组分分布进行了研究,探讨了有机碳及其组分对植被恢复的响应.结果表明,不同植被恢复措施下,土壤团聚体粒径分布表现为“V”字分布:＞5 mm和＜0.25 mm这两个粒径的团聚体含量最多,5-2 mm、1-0.25 mm团聚体的含量次之,2-1 mm粒径的团聚体含量最少.坡耕地的平均重量直径(MWD)最低,为1.4,其他植被恢复措施下土壤的平均重量直径MWD在1.9-3.1之间.不同的植被恢复措施下,0-20 cm土层和20-40 cm土层全土有机碳含量在7.4-17.7 g/kg之间、微生物碳含量分布在50.3-664.7 mg/kg之间、腐殖质碳含量在0.9-2.5g/kg之间.胡敏酸碳含量分布在0.2-0.6 g/kg,富里酸碳含量在0.6-1.9 g/kg之间.全土有机碳、微生物碳、腐殖质碳、富里酸碳均为坡耕地最低,其他植被恢复措施的有机碳、微生物碳、腐殖质碳、富里酸碳含量分别是坡耕地的1.1-2.3倍、2.0-8.4倍、1.0-2.0倍、1.2-2.4倍.不同粒径团聚体有机碳相比较,大多呈现中间高两边低的变化趋势,最大值出现在中间粒径,即5-2 mm、2-1 mm、1-0.25 mm这3个粒径.逐步回归表明,5-2 mm团聚体和1-0.25 mm团聚体有机碳含量的提高有助于土壤水稳性团聚体的形成.研究结果表明,植被恢复提高了土壤团聚体有机碳含量,在碳形态上,富里酸碳和微生物生物量碳对不同植被恢复措施的敏感度较高,胡敏酸碳含量则相对稳定.