Rhizospheric degradation of phenanthrene is a function of proximity to roots

Rhizodegradation of recalcitrant organic pollutants, such as polycyclic aromatic hydrocarbons (PAH), may benefit from the major role that root exudates have on rhizospheric microbial processes. We investigated the influence of the proximity to ryegrass (Lolium perenneL.) roots on microbial populations and their biodegradation of phenanthrene (PHE) using compartmented pots. PAH degrading bacteria, total heterotrophic bacteria and PHE biodegradation were quantified in three consecutive sections at different distance (0–3, 3–6, 6–9 mm) from a mat of actively exuding roots. A bacterial gradient was observed with higher numbers of heterotrophs and PAH degrading bacteria closest to the roots. In parallel, a PHE biodegradation gradient was evident in the presence of roots. The biodegradation reached 86%, 48% and 36% of initially added PHE, respectively, in the layers 0–3 mm, 3–6 and 6–9 mm from the roots. The biodegradation rate was similar throughout the three layers of the non planted control. The present experimental system seems well suited for spatial and dynamic studies of PAH rhizoremediation.     

Differential composition of bacterial communities as influenced by phenanthrene and dibenzo[a,h]anthracene in the rhizosphere of ryegrass (Lolium perenne L.)

Bioremediation technologies of Polycyclic Aromatic Hydrocarbons (PAH) are often limited by the recalcitrance to biodegradation of high molecular weight (HMW) PAH. Rhizosphere is known to increase the biodegradation of PAH but little is known about the biodegradability of these HMW compounds by␣rhizosphere bacteria. This study compared the effects of a 3 and a 5-ring PAH, phenanthrene (PHE) and␣dibenzo[a,h]anthracene (dBA) respectively, on the composition of bacterial community, the bacterial density and the biodegradation activity. Compartmentalized devices were designed to harvest three consecutive sections of the rhizosphere. Rhizosphere and non-rhizosphere compartments were filled with PHE or dBA spiked or unspiked sand and inoculated with a soil bacterial inoculum. Different bacterial communities and degradation values were found 5 weeks after spiking with PHE (41–76% biodegradation) and dBA (12–51% biodegradation). In sections closer to the root surface, bacterial populations differed as a function of the distance to roots and the PAH added, whereas in further rhizosphere sections, communities were closer to those of the non-planted treatments. Biodegradation of PHE was also a function of the distance to roots, and decreased from 76 to 42% within 9 mm from the roots. However, biodegradation of dBA was significantly higher in the middle section (3–6 mm from roots) than the others. Rhizosphere degradation of PAH varies with the nature of the PAH, and C fluxes from roots could limit the degradation of dBA.     

Spatial Distribution of Bacterial Communities and Phenanthrene Degradation in the Rhizosphere of Lolium perenne L.

Rhizodegradation of organic pollutants, such as polycyclic aromatic hydrocarbons, is based on the effect of root-produced compounds, known as exudates. These exudates constitute an important and constant carbon source that selects microbial populations in the plant rhizosphere, modifying global as well as specific microbial activities. We conducted an experiment in two-compartment devices to show the selection of bacterial communities by root exudates and phenanthrene as a function of distance to roots. Using direct DNA extraction, PCR amplification, and thermal gradient gel electrophoresis screening, bacterial population profiles were analyzed in parallel to bacterial counts and quantification of phenanthrene biodegradation in three layers (0 to 3, 3 to 6, and 6 to 9 mm from root mat) of unplanted-polluted (phenanthrene), planted-polluted, and planted-unpolluted treatments. Bacterial community differed as a function of the distance to roots, in both the presence and the absence of phenanthrene. In the planted and polluted treatment, biodegradation rates showed a strong gradient with higher values near the roots. In the nonplanted treatment, bacterial communities were comparable in the three layers and phenanthrene biodegradation was high. Surprisingly, no biodegradation was detected in the section of planted polluted treatment farthest from the roots, where the bacterial community structure was similar to those of the nonplanted treatment. We conclude that root exudates and phenanthrene induce modifications of bacterial communities in polluted environments and spatially modify the activity of degrading bacteria.     

Spatial distribution of bacterial communities and phenanthrene degradation in the rhizosphere of Lolium perenne L

Rhizodegradation of organic pollutants, such as polycyclic aromatic hydrocarbons, is based on the effect of root-produced compounds, known as exudates. These exudates constitute an important and constant carbon source that selects microbial populations in the plant rhizosphere, modifying global as well as specific microbial activities. We conducted an experiment in two-compartment devices to show the selection of bacterial communities by root exudates and phenanthrene as a function of distance to roots. Using direct DNA extraction, PCR amplification, and thermal gradient gel electrophoresis screening, bacterial population profiles were analyzed in parallel to bacterial counts and quantification of phenanthrene biodegradation in three layers (0 to 3, 3 to 6, and 6 to 9 mm from root mat) of unplanted-polluted (phenanthrene), planted-polluted, and planted-unpolluted treatments. Bacterial community differed as a function of the distance to roots, in both the presence and the absence of phenanthrene. In the planted and polluted treatment, biodegradation rates showed a strong gradient with higher values near the roots. In the nonplanted treatment, bacterial communities were comparable in the three layers and phenanthrene biodegradation was high. Surprisingly, no biodegradation was detected in the section of planted polluted treatment farthest from the roots, where the bacterial community structure was similar to those of the nonplanted treatment. We conclude that root exudates and phenanthrene induce modifications of bacterial communities in polluted environments and spatially modify the activity of degrading bacteria.     

Inoculation of PAH-degrading strains of Fusarium solani and Arthrobacter oxydans in rhizospheric sand and soil microcosms: microbial interactions and PAH dissipation

Very little is known about the influence of bacterial-fungal ecological interactions on polycyclic aromatic hydrocarbon (PAH) dissipation in soils. Fusarium solani MM1 and Arthrobacter oxydans MsHM11 can dissipate PAHs in vitro. We investigated their interactions and their effect on the dissipation of three PAHs—phenanthrene (PHE), pyrene (PYR) and dibenz(a,h)anthracene (DBA)—in planted microcosms, in sterile sand or non-sterile soil. In sterile sand microcosms planted with alfalfa, the two microbes survived and grew, without any significant effect of co-inoculation. Co-inoculation led to the dissipation of 46 % of PHE after 21 days. In soil microcosms, whether planted with alfalfa or not, both strains persisted throughout the 46 days of the experiment, without any effect of co-inoculation or of alfalfa, as assessed by real-time PCR targeting taxon-level indicators, i.e. Actinobacteria 16S rDNA and the intergenic transcribed spacer specific to the genus Fusarium. The microbial community was analyzed by temporal temperature gradient electrophoresis and real-time PCR targeting bacterial and fungal rDNA and PAH-ring hydroxylating dioxygenase genes. These communities were modified by PAH pollution, which selected PAH-degrading bacteria, by the presence of alfalfa and, concerning the bacterial community, by inoculation. PHE and PYR concentrations significantly decreased (91 and 46 %, respectively) whatever the treatment, but DBA concentration significantly decreased (30 %) in planted and co-inoculated microcosms only.     

Rhizosphere modelling reveals spatiotemporal distribution of daidzein shaping soybean rhizosphere bacterial community

Plant roots nurture a wide variety of microbes via exudation of metabolites, shaping the rhizosphere's microbial community. Despite the importance of plant specialized metabolites in the assemblage and function of microbial communities in the rhizosphere, little is known of how far the effects of these metabolites extend through the soil. We employed a fluid model to simulate the spatiotemporal distribution of daidzein, an isoflavone secreted from soybean roots, and validated using soybeans grown in a rhizobox. We then analysed how daidzein affects bacterial communities using soils artificially treated with daidzein. Simulation of daidzein distribution showed that it was only present within a few millimetres of root surfaces. After 14 days in a rhizobox, daidzein was only present within 2 mm of root surfaces. Soils with different concentrations of daidzein showed different community composition, with reduced α-diversity in daidzein-treated soils. Bacterial communities of daidzein-treated soils were closer to those of the soybean rhizosphere than those of bulk soils. This study highlighted the limited distribution of daidzein within a few millimetres of root surfaces and demonstrated a novel role of daidzein in assembling bacterial communities in the rhizosphere by acting as more of a repellant than an attractant.     

红壤区不同产量籼稻品种苗期根表和根际土壤硝化特征

以不同产量籼稻品种中旱22(ZH,高产品种)和禾盛10号(HS,低产品种)为材料,采用根际培养箱(三室)--速冻切片技术研究了红壤水稻土种植条件下,水稻苗期生长、氮素积累和氮素利用率(NUE)、根孔隙度(POR)、根际土壤矿质态氮含量和硝化强度.结果表明,ZH苗期生长、氮素吸收及NUE均显著优于HS,且ZH单株不定根数量及根系通气组织发育程度(用POR表示)均显著高于HS.ZH根际和土体土壤中铵(NH+4)含量始终低于HS,而硝(NO-3)含量则始终高于HS,但二者根表土壤NH+4和NO-3含量均无显著差异.ZH和HS硝化强度最大发生部位均是在距根表2 mm的根际土壤,分别为:0.48 μmol kg-1h-1和0.31 μmol kg-1h-1.随着距根表越远,硝化强度就越弱,直至距根表10～20 mm处土壤硝化强度就接近于土体土壤.ZH根际土壤硝化强度始终显著高于HS,但二者根表和土体土壤硝化强度均无显著差异.与不种水稻的CK相比,根际土壤硝化强度提高了约2～3.5倍.尽管红壤水稻土硝化作用很弱,但红壤区水稻根际硝化作用与水稻苗期生长和氮素营养密切相关.     

黄土丘陵半干旱区密植枣林随树龄变化的根系空间分布特征

该文研究了黄土丘陵半干旱区密植枣( Ziziphus jujuba ‘Lizao’)林群体根系随树龄变化的空间分布特征。对1年生、4年生、8年生和11年生4种树龄的密植枣林采用剖面法, 获得0-1 m土壤剖面上直径>3 mm、1-3 mm及<1 mm的根系数量和空间位置信息。利用方差分析, 评价了山地密植枣林林分根系随树龄变化的水平和垂直分布特征。结果表明: 3种直径的根系数量均随着树龄的增长而增加, 直径< 1 mm的根系增长速度最快; 随着土层加深, 根系数量递减, 1年生枣林的根系主要聚集在0-40 cm土层中, 4年生及以上树龄的根系主要分布在0-60 cm土层中; 0-1 m土层内, 1年生枣林(株距1.2 m)及4年以上树龄(株距2 m), 同树龄枣林中直径<1 mm的根系水平分布无差异; 同一土层中(0-20 cm, 20-40 cm, 40-60 cm), 无论树龄大小及离树干的水平位置如何, 不同直径根系的数量都无差异。研究表明: 在有水肥管理措施的条件下, 枣林根系垂直方向形成浅层型的适应模式; 在密植环境下, 枣林细根形成根网型的适应模式。     

Microbial community composition and functional diversity in the rhizosphere of maize

This study investigates the small-scale stratification of bacterial community composition and functional diversity in the rhizosphere of maize. Maize seedlings were grown in a microcosm with a horizontal mesh (53 M) creating a planar root mat and rhizosphere soil. An unplanted microcosm served as control. Thin slices of soil were cut at different distances from the mesh surface (0.2–5.0 mm) and analysed for bacterial community composition by PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) of 16S rDNA and tested for activities of different enzymes involved in C, N, P and S cycling. Bacterial community composition and microbial functional diversity were affected by the presence of the maize roots. The bacterial composition showed a clear gradient up to 2.2 mm from the root surface, while no such gradient was observed in the unplanted pot. Invertase and phosphatase activities were higher in the close vicinity of maize roots (0.2–0.8 mm), whereas xylanase activity was unaffected. This study shows that the changes in bacterial community composition and functional diversity induced by roots may extend several millimetres into the soil.     

Localization of sulfate reduction in planted and unplanted rice field soil

Rates of in situ sulfate reduction (SRR) in planted and unplanted rice fieldsoil were measured by the ³⁵SO^2– ₄-radiotracermethod using soil microcosms. The concentration of ³⁵SO^2– ₄ decreased exponentially with time.However, time course experiments indicated that incubation times of10–30 min were appropriate for measurements of SRRusing a single time point in routine assays. Unplanted microcosmsshowed high SRR of 177 nmol cm^-3 d^-1 inthe uppermost centimeter where average sulfate concentrations were<33 µM. Fine scaled measurements (1 mmresolution) localized highest SRR (<100 nmol cm^-3d^-1) at the oxic/anoxic interface at 2–5 mmdepth. In planted rice field soil, SRR of <310 nmolcm^-3 d^-1 were observed at 0–2cm depth. Sulfate reduction rates were determined at a millimeter-scalewith distance to a two dimensional root compartment. The SRR was highestat 0–1.5 mm distance to the root layer with rates up to500 nmol cm^-3 d^-1, indicating a highstimulation potential of the rice roots. SRR seemed to be mainlydependent on the in situ sulfate porewater concentrations. At thesoil surface of unplanted microcosms sulfate concentration decreasedfrom <150 µM to <10 µM within the first 8 mm of depth. In planted microcosmssulfate concentration varied from 87–99 µMsulfate at the 0–3 mm distance to the root layer to48–62 µM sulfate at a root distance>4 mm from the roots.The depth distribution of inorganic sulfur compounds was determinedfor planted and unplanted rice field soil. Sulfate, acid volatilesulfide (AVS) and chromium reducible sulfide (CRS) were up to 20 foldhigher in planted than in unplanted microcosms. CRS was the majorinsoluble sulfur fraction with concentrations >1.7µmol cm^-3. Organic sulfur accounted for25–46% of the total sulfurpresent (269 µg/g dw) in an unplanted microcosm.The biogeochemical role of sulfate reduction forshort-term accumulation of inorganic sulfur compounds(FeS, FeS_{_2} and S°) in rice soil wasdetermined in a time course experiment with incubationperiods of 5, 10, 20, 30 and 60 min. The relativedistribution of CRS and AVS formation showedlittle depth dependence, whereas the formation of³⁵S° seemed to be the highest in themore oxidized upper soil layers and near the root surface.AV³⁵S was the first major product of sulfatereduction after 20–30 min, whereas CR³⁵Swas formed, as AV³⁵S and ³⁵S°decreased, at longer incubation periods of >30 min.     

喀斯特峰丛洼地不同植被恢复阶段细根生物量、形态特征及其影响因素

以喀斯特峰丛洼地不同植被恢复阶段的草丛、灌丛、次生林和原生林为研究对象,采用土芯法,分0~10、10~20、20~30 cm等3层获取群落活细根(直径≤2 mm),分析其生物量、形态特征及其与土壤性状的关系.结果表明:各恢复阶段细根生物量为194.63~255.19g·m^-2,集中分布在0~10 cm表层土壤中,占0~30 cm土层总生物量60%以上,不同恢复阶段群落生物量的差异不显著;细根比根长和比表面积在不同恢复阶段差异显著,随着植被由草丛向原生林正向恢复而逐渐降低;超过66%的根长和64%的根面积分布在0~10 cm表层土壤中,多数细根根长和根面积均在0~0.5 mm和0.5~1 mm径级,这两级根长和根面积占其总量的87%和72%以上.冗余分析表明,喀斯特峰丛洼地植物群落细根特征与土壤性状之间存在着不同的相关性,其中土壤有机碳、速效钾和全氮对细根特征影响较大.这是植物长期适应生境条件形成的有效策略.     

Two biochemical forms of phenanthrene recovered in grassland plants (Lolium perenne L. and Trifolium pratense L.) grown in 3 spiked soils

Plant storage is a key component in the global cycling of persistent polycyclic aromatic hydrocarbons (PAHs) and constitutes a crucial step to understand environmental fate of such pollutants. This work aimed at quantifying biochemical forms of phenanthrene (PHE) stored in plants and determining the main parameter between the plant species and the soil characteristics involved in the PHE bioaccumulation. An experiment was conducted in a growth chamber to study the storage of PHE in ryegrass and clover in three distinct artificially contaminated soils (1,000 ??g g^?1 DW). A preliminary experiment was designed to develop an extraction method which distinguished easily extractable PHE (free-PHE) from less extractable PHE (bound-PHE). PHE was found to be mainly recovered in plant roots (around 90% of the total PHE recovered in plants) in its free form. PHE was transported upward from the roots into shoots. PHE recovered in shoots (around 1 ??g g^?1) was divided into its free (60%) and bound (40%) forms except for one treatment. Observed differences of root PHE storage were clearly related with the PHE dissipation in soil and then with the plant species. This work outlined the importance of quantifying the bound and free PAHs in plants in further researches.     

表施和混施污泥对团花根系生长的影响

林地利用被认为是污泥资源化利用的重要方式,但施用污泥对林木根系生长的影响报道较少。本研究通过根箱试验,分析表施和混施10%污泥对速生树种团花不同土层根系形态、土壤pH值和电导率动态变化及根系重金属含量的影响,并拟合土壤pH值、电导率和根系重金属含量与根长的关系。结果表明: 与不施污泥(对照)相比,混施污泥显著抑制了团花根长、根表面积和根体积增长,混施污泥120和240 d后,0～20 cm土层总根长分别为不施污泥的76.9%和67.4%;表施污泥对团花根长和根表面积的影响不显著,但显著提高了根体积。混施污泥显著提高了土壤pH值和电导率及根系重金属含量,混施污泥0～20和20～40 cm土层根系镉含量分别是不施污泥的11.5和10.0倍。线性回归拟合分析显示,不同处理0～20 cm土层的电导率与根长均呈显著负相关;表施和混施污泥根系镉含量与根长呈极显著负相关。上述结果表明,混施污泥抑制了团花根系生长,这可能是由于混施污泥提高了土壤电导率和根系镉含量所致,而表施污泥对团花根系生长的作用不明显。     

瓦屋山常绿阔叶次生林建群种扁刺栲的细根分布及其碳氮特征

四川瓦屋山国家森林公园是我国西部的中亚热带湿性常绿阔叶林的典型代表,具有四川现存的较为完好的扁刺栲(Castanopsis platyacantha)-华木荷(Schima sinensis)群系,该研究利用土钻法探讨了该群系内主要建群种扁刺栲标准木的细根分布及其碳氮特征。结果表明:(1)扁刺栲细根总生物量为173.62 g·m~(-2),其中活根生物量为135.29 g·m~(-2)。(2)随着土层深度的增加,扁刺栲细根生物量、根长密度、根系表面积和比根长呈下降趋势,0~30 cm土层所占比例分别为67.23%、69.53%、69.48%和57.20%;根长密度、根系表面积和比根长均随细根直径的增加而显著下降,直径小于1 mm的根系所占比例分别为58.84%、52.59%和51.36%。(3)扁刺栲细根生物量、根长和表面积消弱系数β均随根系直径的增加而增加。(4)根系C含量在第Ⅰ土层中随细根直径的增加而显著增加,在其他土层则无显著差异;直径小于2 mm的根系C含量在第Ⅰ土层中显著低于其他土层,大于2 mm的根系C含量在各土层间的差异不大。(5)根系N含量随根系直径和土层深度的增加而减少,C/N值则与之相反。该研究结果在一定程度上反映了该次生林地下细根的垂直分布及养分特征,为揭示该生态系统地下生态过程及今后在该生态系统研究环境变化对地下生态过程的影响提供了基础数据。     

Distinguishable root plaque on root surface of Potamogeton crispus grown in two sediments with different nutrient status

The properties of plaques were different on the root surface of Potamogeton crispus planted in sediments from two different shallow lakes. Lake Tangxunhu sediment, with low pH, contained low organic matter, whereas Lake Yuehu sediment, with high pH, had high calcium deposits mixed with high organic matter. The contents of mineral elements in sediment of Lake Tangxunhu was lower than that of Lake Yuehu, except for iron (Fe) content, but the contents of mineral elements extracted by sodium dithionite–sodium citrate–sodium bicarbonate (DCB) from root plaques were higher in Lake Tangxunhu than those in Lake Yuehu, except for Fe. These element distributions on P. crispus root plaques were characterized by scanning electron microscope combined with energy-dispersive X-ray spectrometer and were consistent with the contents of mineral elements in sediment. The root plaque of P. crispus planted in Lake Tangxunhu sediment mainly contained silicon (Si) and Fe, and the content of Si was greater than Fe, which may be contributed to the formation of poly-silicic-ferric in the natural conditions. However, the root plaque of P. crispus planted in the sediment with higher calcium content of Lake Yuehu was rich in Fe, Si, phosphorus (P), and calcium (Ca). Due to oxygen secretion by plant roots, the root plaque has more Fe₃(PO₄)₂ and a certain amount of Ca₃(PO₄)₂. The ratio of magnesium (Mn) to Fe extracted by DCB from root plaque in Lake Tangxunhu sediment was 0.031 and 0.010 in Lake Yuehu sediment. In Lake Tangxunhu sediment, lower content of organic matter results in weak reducibility. Enhanced oxidation ability by oxygen secretion of P. crispus root could oxidize low-valent Fe and Mn into iron–manganese oxide, which leads to formation of iron–manganese plaque on the root surface. However, this case is different in Lake Yuehu sediment, where Fe and Mn can be reduced in high organic sediment and low-valent Mn can precipitate in the sediment in which pH is >8. Thus, low-valent Fe in Lake Yuehu sediment moves to the root surface of P. crispus, where it oxidizes into Fe oxide, i.e., Fe plaque.     

利用发光酶基因标记技术跟踪棉花根圈中的绿针假单胞菌PL9L

采用细菌转化和杂交的方法,成功地将全套发光酶基因标记系统Tn7luxCDABE引入绿针假单胞菌(Pseudomonas chlororaphis)PL9,得到稳定的发光标记菌PL9L。采用发光菌落平板计数法和X射线胶片自显影法,通过盆栽试验和盒裁试验,研究了发光标记菌PL9L在棉花根圈的定殖动态和分布规律。盆栽试验结果表明,在灭菌土盆栽中,播种后6d左右PL9L在棉花根圈的定殖水平达最高(31×109cfu/g根土),播种后56d左右趋向稳定,PL9L数量为17×102cfu/g根土;未灭菌土盆载中,播种后8d左右PL9L的定殖水平达最高(11×109cfu/g根土),46d左右趋向稳定,菌数为14×102cfu/g根土。盒栽试验结果表明,PL9L可从种子向根尖方向扩散,但并不与根的伸长生长同步,播种后36d,灭菌土盒栽中PL9L可扩散至种子下方120cm以内,而未灭菌土盒栽中PL9L扩散至110cm以内。在棉花根尖区域均未检测到PL9L。     

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。     

云南松林的根系生物量及其分布规律的研究

利用平均标准木机械布点法测定了云南省永仁林业局云南松不同龄组林分的根系生物量及其沿土壤剖面深度的分布规律.结果表明,林分根系总生物量随林龄而增加,幼龄林(15～17年)的根系生物量为8.50 t·hm^-2,中龄林(30～32年)为11.70 t·hm^-2,成熟林为(>62年) 18.91 t·hm^-2.在不同龄组林分中,粗根(>10mm)生物量差异最大（1.5～12.3 t·hm^-2）,而中根（5～10 mm）（1.4～1.6 t·hm^-2）及小根（<5 mm）（5.3～6.2 t·hm^-2）的生物量差异最小.根系生物量沿土壤深度迅速减少,约93%的根系生物量集中分布在0～30 cm土层中,深土层(30～115 cm)的根系生物量仅占7%左右.     

A gradual rather than abrupt increase in soil strength gives better root penetration of strong layers

The purpose of this paper is to test the hypothesis that gradual rather than abrupt increases in strength lead to increased root penetration of strong layers. This was tested experimentally in a model system with multi-layered wax discs with a total thickness of up to 6 mm that either increased in strength with depth, or decreased in strength with depth. Strength was varied by altering the proportions of hard paraffin wax and white soft paraffin. Multi-layered wax discs consisted of three layers, each 1.5 or 2 mm thick, to give a disc thickness of either 4.5 or 6 mm. ‘Test’ wax discs had a 40% hard wax layer above a 60% wax layer, which was above an 80% wax layer. This led to a gradual increase in strength as roots encountered the disc from the sand growing medium. In ‘control’ discs, the order was reversed so that 80% wax was encountered immediately below the sand. These treatments were used to challenge roots of the rice cvs. Azucena and Bala. Compared with the control, the test layers increased root penetration of Azucena from a mean of 5.9 root axes to 12.4, while penetration of Bala was increased from 0.5 to 5.6. These results show that there was increased root penetration when roots encountered a gradual increase in strength rather than a sudden increase in strength. This was explored further with numerical simulations of stress distributions at the tips of roots. These simulations indicated that gradual rather than abrupt increases in strength decreased stress concentration at the root tip, consistent with the experimental results.     

模拟氮沉降对中亚热带杉木幼树根系生物量的影响

植物根系是全球陆地生态系统碳储量的重要组成部分,在全球生态系统碳循环中起着重要作用,日益加剧的氮沉降会影响根系生物量在空间和不同径级的分配,进而影响森林生态系统的生产力和土壤养分循环。以杉木幼树为研究对象,通过野外氮沉降模拟实验,研究氮沉降四年后对不同土层、不同径级根系生物量的影响。结果发现：（1）低氮和高氮处理总细根生物量较对照均无显著差异（P > 0.05）,高氮处理粗根生物量及总根系生物量较对照分别增加45%和40%（P < 0.05）;（2）与对照相比,施氮处理显著增加20-40 cm与40-60 cm土层细根和粗根生物量,且在低氮处理下,20-40 cm土层细根、粗根在总土层细根与粗根生物量的占比显著提高。（3）与对照相比,高氮处理显著增加了2-5 mm、5-10 mm及10-20 mm径级的根系生物量,低氮处理显著增加2-5 mm、5-10 mm径级根系生物量,且显著降低20-50 mm径级根系生物量。综上所述表明：氮沉降后杉木幼树通过增加较粗径级根系来增加对养分及水分的输送,同时通过增加深层根系生物量及其比例的策略来维持杉木幼树的快速生长;而根系生物量的增加,在一定程度上会增加根系碳源的输入,影响土壤碳循环过程。