Root growth as influenced by aggregate size

Summary This study examined the effects of aggregate size on root impedance and developed an equation to describe the root pressure necessary to avoid deflection around an aggregate. This critical root pressure was predicted to increase with increasing aggregate size, decreasing root diameter, and decreasing deflection angle. In growth chamber experiments, maize (Zea mays L.) seedlings were grown in A horizon material of Groseclose silt loam (Clayey, mixed, mesic, Typic Hapludult). The soil had been moist sieved into different aggregate sizes (0–1, 1–2, 2–3, and 3–6 mm diameter). The larger aggregates did constitute a slight root impedance as roots were deflected around them. Diameters of roots grown in 3–6 mm aggregates increased significantly, whereas root lengths were not always signficantly decreased. The smaller aggregates did not impede root growth and were readily displaced by roots. Large aggregates were more of an impedance to lateral roots than to main axes.     

水曲柳和落叶松不同根序之间细根直径的变异研究

细根直径大小和根序高低对细根寿命和周转估计具有重要的影响,研究不同根序之间的直径变异对认识细根直径与根序的关系具有重要意义。该文根据Pregitzer等(2002)提供的方法,研究了位于东北林业大学帽儿山实验林场尖砬沟森林培育实验站17年生水曲柳(Fraxinus mandshurica)和落叶松(Larix gmelinii)人工林细根1~5级根序的平均直径的变化、直径的最小值和最大值范围、直径的变异系数。结果表明,水曲柳和落叶松细根直径<2 mm时,包含5个根序,随着根序由小到大的增加,细根直径也在增大。各根序平均直径之间,存在较大的差异。在同一根序内,细根直径范围很大,水曲柳和落叶松一级根最小直径均<0.20 mm,最大直径分别<0.50 mm(水曲柳)和<0.70 mm(落叶松)左右。2~3级根序直径最小值在0.20~0.30 mm之间,最大值≤1.0 mm。5级根直径最小值<1.0 mm,最大值超过2.0 mm。随着根序等级增加,直径变异系数增大。一级根序的直径平均变异系数<10%,2~3级根序直径平均变异系数在10%~15%左右,4~5级根序直径的平均变异系数在20%~30%之间。因此,在细根寿命与周转研究过程中,必须同时考虑直径和根序对细根的寿命估计的影响。     

The effect of soil aggregate size on early shoot and root growth of maize (Zea mays L.)

Differences in plant growth arising from differences in aggregate size in the seedbed are normally atributed to limitations in nutrient or water supply during the early growth period. This study was initiated to determine if these were the only mechanisms by which aggregate size influences plant response. Four different aggregate size fractions (less than 1.6 mm, 1.6 to 3.2 mm, 3.2 to 6.4 mm and 6.4 to 12.8 mm diameter) were sieved from a silt loam soil. Nutrients were added to the soil and maize was grown in the aggregates for eighteen days after seedling emergence. Soil matric potential was maintained between — 3 and −20 kPa. Shoot dry weight declined by 18% as aggregate size increased from less than 1.6 mm to 1.6–3.2 mm. There was little further decline as aggregate size increased to 6.4–12.8 mm. Final leaf area showed a similar decline. The availability of nutrients or water were not limiting. Total root length in the coarsest aggregate system was less than 60% of that in the finest system. Main axes of seminal and nodal roots were longer in the coarser aggregate systems, the length of primary laterals was not affected, and length of secondary laterals was lower in the coarser systems. A greater proportion of the roots penetrated the larger aggregates than the smaller aggregates; however, the larger aggregates offered greater resistance to penetration by a rigid micropenetrometer (150 μ diameter probe). Diameter of the main axes roots were greatest in the largest two aggregate fractions. it is speculated that a combination of increased endogenous ethylene in roots in the finest aggregate system due to entrapment by water and increased mechanical resistance in the coarsest aggregate system accounts for the observed effects on root norphology.     

Soil aggregate sequestration of cover crop root and shoot-derived nitrogen

Cover crop roots and shoots release carbon (C) and nitrogen (N) compounds in situ during their decomposition. Depending upon the season, these C and N compounds may be sequestered, the C may be respired or the N may be leached below the root zone. A field study was established to identify the contributions of cover crop root and shoot N to different regions within aggregates in the A_p horizon of a Kalamazoo loam soil. Fall-planted rye plants (Secale cerealeL.) were labeled the next May with foliar applications of solutions containing 99% atom (¹⁵NH₄)₂SO₄. Isotopic enrichment of soil aggregates ranging from 2.0 to 4.0, 4.0–6.3 and 6.3–9.5 mm across was determined following plant residue applications. Concentric layers of aggregates were removed from each aggregate by newly designed meso soil aggregate erosion (SAE) chambers. Non-uniform distributions of total N and recently derived rye N in soil macroaggregates, across time, suggested that the formations and functions of macroaggregates are very dynamics processes and soil aggregates influence where N is deposited. Early in the season, more ¹⁵N migrated to the interior regions of the smallest aggregates, 2–4 mm across, but it was limited to only surfaces and transitional regions of the larger aggregates, 6.3–9.3 mm across. Exterior layers of aggregates between 6.0 and 9.5 mm retained 1.6% of the N_{derived from roots} in July 1999, which was three times more than their interior regions. This was slightly greater than the % N_{derived from shoot.} One month later, as the maize root absorption of N increased rapidly, % N_{derived from roots} and % N_{derived from shoot} were nearly equal in exterior layers and interior regions of soil aggregates. This equilibrium distribution may have been from either greater diffusion of N within the aggregates and/or maize root removal form aggregate exteriors. Results supported that most of roots grew preferentially around surfaces of soil aggregates rather than through aggregates. Cover crop roots contributed as much N as cover crop shoots to the total soil N pool. Subsequent crops use N from the most easily accessible zones of soil structure, which are surfaces of larger soil aggregates. Therefore maintaining active plant roots and aggregated soil structure in the soil enhances N sequestration and maximize soil N availability. These studies suggest that the rapid and perhaps bulk flow of soil N solutions may bypass many of the central regions of soil aggregates, resulting in greater leaching losses.     

陇东雨养苹果覆膜对土壤团聚体结构稳定性与细根分布的影响

不同的果园管理方式可影响果树根系生长、分布与土壤团聚体稳定性、有机碳固存,进而改变“根-土”复合体响应关系。对西北陇东旱塬不同覆膜年限(2 a、4 a和6 a)苹果园表层土壤(0—20 cm)细根生长进行调查,并采用干筛法和湿筛法相结合的方式对土壤团聚体进行分级(>2 mm, 0.5—2 mm, 0.25—0.5 mm和<0.25 mm)。计算团聚体稳定性参数[> 0.25 mm机械稳定性团聚体含量(DR_0.25)、> 0.25 mm水稳性团聚体含量(WR_0.25)、平均重量直径(MWD)、平均几何直径(GMD)、团聚体破坏率(PAD)、水稳系数(WSC)]和团聚体有机碳含量。分析细根生长与土壤物理结构对长期覆膜的响应,探明土壤团聚体稳定性与有机碳固持关系,揭示黄绵土物理结构稳定机制。结果表明：6 a处理通过增加表层土壤黏粒和物理性黏粒比例,改变孔隙结构,抑制细根生长,其根量、根长和根表面积仅为对照(CK)的20.97%、24.66%和41.25%;降低表层土壤团聚体力稳性,其DR_0.25、机械稳...     

An improved model of root growth in structured soil

Summary The growth of roots of maize, sorghum and soybean is modelled through beds of spherical aggregates. Effects of aggregate size and strength, and effects of the spread or distribution of aggregate strengths are investigated.This is achieved by a combination of a statistical model for soil structure with a statistical model for the penetration behaviour of a root at a void/aggregate interface. It is shown that the behaviour of a root at such an interfac is dependent on the previous history of the root in its passage through the soil.It is concluded that the smaller the aggregate size, the greater is the nutrient availability per unit length of root. The influence of aggregate size decreases with increasing soil strength.An increase in aggregate strength reduces the availability of nutrients per unit length of root. However, the rate of nutrient uptake per root axis goes through a minimum at a strength (for maize) of around 80 per cent of the maximum limiting aggregate strength for root penetration. An increase in the spread of aggregate strengths usually results in a proportional increase in nutrient availability. This effect is more pronounced with smaller aggregate sizes.     

Maximum axial growth pressures of the lateral roots of pea and eucalypt

Although lateral roots may contribute significantly towards growth and nourishment of plants, the mechanics of their elongation behaviour in strong soils is not well known. The aim of this study is to report maximum axial growth pressures (p) and maximum elongation rates (E) of the lateral roots of an annual herbaceous plant (pea) and a woody perennial (eucalypt). As such measurements have not been reported previously, measurements of P and E for lateral roots were compared with the primary roots of pea for which reports are widespread. Values of P were estimated from the measured maximum values of axial force and root diameter on single, intact roots of seedlings in the laboratory. Additional measurements of both P and E were made for the lateral roots of pea when the growth of the remaining root axes was stopped (with removal of tips) to determine the overall effects of root-growth-inhibition on P and E of single roots.Values of P and E for lateral roots of pea were significantly greater than those for the lateral roots of eucalypt. Although root diameter for the primary roots of pea were similar to those for the lateral roots of eucalypt, the former exerted nearly twice as much pressure as the latter. The lateral roots of pea elongated significantly slower than the primary roots; however, P of lateral roots was significantly lower than the primary roots when elongation of all other roots was inhibited during the measurements. Production and/or development of lateral roots increased when elongation of the remaining roots (both primary and lateral roots) of pea seedlings was restricted due to the removal of root tips and exposure of one of the lateral roots to high strength. In general, maximum axial force exerted by primary and lateral roots was similar for roots of <1 mm diameter. However, primary roots exerted greater maximum axial force than the lateral roots when root diameter was >1 mm. As axial pressure of lateral roots was independent of root diameter, thickening of root tips is less likely to assist penetration of lateral roots in strong soils.     

A comparison of penetrometer pressures and the pressures exerted by roots

Summary Previous work is reviewed in which the ratio of the pressures required for soil penetration by roots and penetrometers are compared. It appears that this ratio can vary from about 2 to 8 depending on conditions. However, there is very little experimental evidence and most of the work has been inferential.Direct measurements are reported for the stresses exerted by a 1 mm diameter penetrometer probe and by the roots of pea seedlings when penetrating Urrbrae fine sandy loam. Six soil conditions were used: (non-weathered remoulded soil cores + artificially weathered remoulded soil cores + undisturbed field clods) × (confined + unconfined cores or clods). The confinement treatment was to test for any effects of additional restraint to cylindrical root expansion. The weathering and field clod treatments were to test the hypothesis that root elongation is facilitated by tensile failure ahead of the root tip.The principal conclusions are as follows. The laboratory weathering treatment reduced the soil tensile strength by 25%. This resulted in a small but significant reduction in the pressure for root penetration into confined cores. Compared with remoulded non-weathered cores, field clods had a 2 to 3 fold greater penetrometer resistance and a 50% lower tensile strength. The force required for root penetration into unconfined field clods was only 10% greater than for unconfined non-weathered cores. For the former (which is closest to field conditions) the penetrometer had to exert a pressure 5.1 times greater than a root tip in order to penetrate the soil. Penetrometer penetration pressure was independent of probe diameter in the 1–2 mm range in the soil used. Core confinement restricts root radial expansion and modifies the penetration force of metal probes and plant roots.On the basis of the new results it is tentatively concluded that soil tensile failure can facilitate penetration by roots.     

棉花根系生长和空间分布特征

结合田间根钻取样和图像扫描分析方法, 研究了不同棉花品种根系的长度、直径和表面积动态及 0~ 10 0cm深和 0~ 4 0cm宽土壤范围内的空间分布特征。该方法与常规直尺测量结果相比相关系数R2 达到 0.899 (n =1318), 显示了较好的可靠性。研究结果表明, 棉花平均根长密度 (RLD) 在花铃期为 1.2 1~ 1.2 7mm·cm-3, 吐絮后降至 1.0 4~ 1.12mm·cm-3, 收花时为 0.76mm·cm-3 。棉花根平均直径在不同基因型间存在显著差异, 抗虫杂交棉的根直径最粗, 平均为 0.5 2mm ;早熟类型品种根直径较细, 平均为 0.36mm。在土壤深度上根直径的差异不显著, 但距棉行距离越远, 根的平均直径越小。在明确根系长度和直径动态规律的基础上, 提出了根表面积指数 (RAI) 的概念, 与地上部叶面积指数具有相似的含义和生物学意义, 且呈较好的指数相关关系 (R2 =0.779) 。RAI在生理发育时间 (PDT) 小于等于 4 0前, 其增长动态符合LOGISTIC生长规律 (R2 =0.84 9), 在PDT大于 4 0后, 呈线性递减趋势 (R2 =0.5 70~ 0.895 ), 且杂交抗虫棉的RAI在全生育期内均明显高于其它类型品种, 而早熟类型品种相对略低。RAI空间分布特征表现为, 开花前在浅根层内 (0~ 30cm) 分布最多, 花铃期以中层根系 (40~ 6 0cm) 为主, 吐絮后主要以深层 (70~ 10 0cm) 和距棉行较远的行间较多。研究结果为制定合理的施肥、灌溉措施提供了理论依据, 并量化了棉花根系的时空变化, 为进一步提高生长发育模拟模型的精度奠定了基础。     

Modification of soil aggregation by watering regime and roots growing through beds of large aggregates

The influence of root growth and soil watering regime on aggregation was studied under controlled conditions. The study examined the influence of pea (Pisum sativum cv Greenfeast), ryegrass (Lolium rigidum cv Wimmera) and wheat (Triticum aestivum cv Kite) roots on changes in aggregation and on the properties of the aggregates. The soil was a non swelling red-brown earth which was either kept wet or was allowed to wet and dry during the experiment. Root growth increased the percentage of small sized aggregates (<18 mm diameter), organic carbon, tensile strength and stability of aggregates in comparison with a non planted soil. Changes in aggregate size distribution and properties of the aggregates were related to root length density of the species and also to the soil watering regime. Root length density was in the order ryegrass>pea>wheat. Wetting and drying of soil increased the strength and stability of aggregates. Incubating aggregates allowed some roots to decompose but did not increase the strength or stability of aggregates compared with unincubated soil. The results of this experiment are of practical significance in soil structural management, and in studies of soil aggregation dynamics. It may be possible to use plant roots to alter the size and properties of aggregates.     

水曲柳根系生物量、比根长和根长密度的分布格局

采用连续钻取土芯法在生长季内对东北林业大学帽儿山实验林场17年生水曲柳人工林根系取样,研究水曲柳不同直径根系现存生物量、比根长和根长密度及垂直分布状况.结果表明,水曲柳人工林根系总生物量为1 637.6 g·m^-2,其中活根生物量占85%,死根占15%.在活根生物量当中,粗根(直径5～30 mm)占的比例最高（69.95%）,其次为活细根（直径<1 mm,13.53%）,小根(1～2 mm)和中等直径的根(2～5 mm)比例较小（分别为7.21%和9.31%）.直径<1 mm活细根的比根长为32.20 m·g^-1,直径5～30 mm粗根的比根长为0.08 m·g^-1.单位面积上活根的总长度为6 602.54 m·m^-2,其中直径<1 mm的细根占92.43%,其它直径等级则不到活根总长度的8%.直径<1 mm的细根生物量与根长密度具显著线性关系（R²=0.923）,但与比根长无显著相关关系（R²=0.134）.     

香果树根萌苗生长特性及影响因子分析

香果树(Emmenopterys henryi)为我国Ⅱ级野生保护植物,由于生境遭到破坏、过度开发利用以及种群自然更新能力差等原因,导致自然环境下香果树种群数量迅速衰减。运用方差分析、多重比较、主成分分析等方法,对自然环境下香果树种群的根萌苗生长状况及其主要影响因子进行了研究。结果表明:香果树根萌苗的数量随着苗龄的增长逐渐减少,1a根萌苗的死亡率最高;苗高和基径均随苗龄的增大呈指数增长;香果树根萌苗主要生长在母树东南方120°~150°(以正北方为0°按顺时针方向旋转,下同)位置、距母树树干2 m以内及直径2 cm、长度30 cm的露根上;根萌苗苗高受母树方位影响较小,受露根直径、露根长度以及与母树间距离的影响稍大;母树南向(168.75°)根萌苗的基径显著大于其它方向的根萌苗基径;与北向和南向相比,母树东向和西向根萌苗的冠幅较大;根萌苗苗高、基径和冠幅等形态指标均随着与母树树干间距离的增加而降低,但随着露根直径的增加各形态指标均呈先升高后降低的趋势(露根直径6.5 cm时苗高和基径达到最大值),随着露根长度的增加各形态指标呈先降低后升高的趋势;与树冠内根萌苗相比,香果树母树树冠外根萌苗的死亡率较高。主成分分析结果表明,随着林内光照条件的改善、土壤中有机质含量的增加、砾石覆盖率的增加以及适度的人为干扰均有利于香果树根萌苗的生长。因此,减小林冠郁闭度、增加人工抚育措施对维持香果树的种群发展、实现其种群自然更新具有重要作用。     

Non-destructive measurement of wheat roots in large undisturbed and repacked clay soil cores

Summary Non-destructive observations of root growth and distribution can be obtained from counting root intercepts with observation tubers inserted in the root zone. This paper describes the technique of inserting clear acrylic tubes horizontally into large undisturbed and repacked soil cores. Counts of roots intersecting scribed lines on the sides of the tubes were made with a fibrescope. Comparison was made between observation root tubes of different diameter (25 and 38.5 mm).The r² values for the relationships between root intercept counts and destructively determined values of root length density (RLD) ranged from 0.78 to 0.96. The larger diameter tubes had higher r² values. Theoretical calibration of the technique does not appear to be possible since analysis indicated that fewer roots intersected the scribed lines on the observation tube than would have been expected from a non-disturbed, randomly distributed root system. It is not known if this discrepancy is due to non-randomness or to an artifact associated with the insertion of the observation tube. Roots were not more prolific at the edge of the soil cores. Comparison of values of root length per unit soil surface area, rates of downward root growth and water uptake rates were within the ranges previously reported for wheat roots of field crops grown on clay soils. Observed root growth and distribution was found to be sensitive to four soil and water treatments imposed. It is concluded that the technique will allow quantitative analysis of root growth and distribution in undisturbed soil cores.     

Formation of aggregates by plant roots in homogenised soils

The influence of root growth and water regime on the formation of aggregates was studied in modified minirhizotrons under controlled conditions. Two soils, a black earth (67% clay) and a red-brown earth (19% clay) were ground and forced through a 0.5 mm sieve. Ryegrass, pea and wheat were grown for fifteen wetting and drying (wd) cycles for 5 months. Another set of minirhizotrons was not planted and served as a control. Measurements of aggregate size distribution (ASD), aggregate tensile strength (ATS), aggregate stability (AS), aggregate bulk density (ABD) and organic carbon (OC) were made on single aggregates of the 2–4 mm fraction. The results showed that aggregates of the black earth which has a high clay content and shrink/swell properties had more smaller aggregates with higher ATS, AS and ABD than those from the red-brown earth. It was also found that for both soils: (1) w/d cycles and higher root length density (RLD) increased the proportions of smaller aggregates and aggregate strength; (2) differences in the ability of the plant species to influence aggregation was evident and seemed to be related to the RLD. The RLD was in the order ryegrass > wheat > pea. Mechanisms likely to be involved in processes of aggregate formation and stabilization are discussed. They include cracking of soil due to tensile stresses generated during drying of a shrinking soil; changes in pore water pressure within the soil mass caused by water uptake by plant roots generating effective stresses; and biological processes associated with plant roots and root exudates.     

Root architecture and tree stability

Summary Root anchorage is discussed with a view to determining the optimum use of root material for enhanced stability. Field observations were made on Sitka spruce root systems while lateral forces were applied to the stem with a winch to pull the tree over. Measurements included the applied force, angles of inclination, soil and root movement, timing of the sound of root breakage using buried microphones, weight and shape of the root-soil plate and damage to the roots.Components of anchorage include the dimensions and mass of the root-soil plate levered from the ground by the displaced stem, and tensile strength of roots and soil beneath the plate; root and soil tensile strength and root/soil resistance on the windward perimeter; and on the lee side the stiffness of the hinge at the fulcrum.Strength properties of roots and soil are reviewed. Models devised for landslip are extended to consider behaviour under tension, of roots singly and in groups, and the concept is developed of a critical rooting density at which root/soil resistance exceeds soil strength, giving rise to the characteric root-soil plate on uprooted trees. The lee side part of the root-soil plate acts as a cantilevered beam and determines the distance of the fulcrum from the tree. Physical laws defining the reduced stiffness of beams as a result of subdivision, indicate the importance of the number/size distribution of roots and weakening effects of branching.On the windward side upward movement of the root-soil plate causes sequential breakage of soil and roots. Under an increasing applied load, failure occurs in parts of the soil-root system before the maximum force for uprooting is achieved. A preliminary approach is made to modelling where the changing contributions of the components of anchorage are allowed for throughout the uprooting process.     

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.     

Emergence of seedlings of sugar beet (Beta vulgaris L.) as affected by the size,roughness and position of aggregates in the seedbed

Laboratory and field experiments were carried out to quantify the effects of the size and roughness of aggregates placed in the seedling path of sugar beet, in order to help in decision making for soil tillage and sowing operations. Graded aggregates (10, 15, 20, 30, 40, 50, 60 and 70 mm longest axis) were either laid on the soil surface or included in the soil over the seeds. The percent emergence decreased exponentially with aggregate size over 10 mm when the aggregates were included in the seedbed. The result was the same with aggregates laid on the soil surface, but for aggregates over 30 mm (mass>10 g). Aggregates on the soil surface could be lifted by the seedlings until their weight exceeded the seedling emergence force. Larger aggregates or aggregates in the soil layer could not be moved. Seedlings which did not emerge remained blocked in small cavities in the aggregate surface. No seedlings were blocked under smooth aggregates or glass beads. The experimental results fits with a model giving the probability to meet a hole according the distance covered and the diameter and density of holes. The results obtained under controlled conditions were similar to those obtained in field experiments for a wide range of aggregate sizes. These results will be incorporated into a computerised seedbed generator to simulate the effects of seedbed structure on seedling emergence.     

Clustered root distribution in mature stands of <Emphasis Type="Italic">Fagus sylvatica</Emphasis> and <Emphasis Type="Italic">Picea abies</Emphasis>

Distribution of small roots (diameter between 2 mm and 5 mm) was studied in 19 pits with a total of 72 m² trench profile walls in pure stands of Fagus sylvatica and Picea abies. Root positions within the walls were marked and transformed into x-coordinates and y-coordinates. In a GIS-based evaluation, zones of potential influence around each root were calculated. The total potential influence produced isoline maps of relative root influence zones, thus indicating small root clustering. The questions studied were (1) whether there were marked clusters of small roots in the soil and (2) whether trees surrounding the pit (defined as tree density) correlate with the root abundance and distribution on the trench profile walls. Small roots of both species showed maximum abundance in the top 20 cm of the soil, where pronounced root clusters occurred next to areas with only low root accumulation. The area of root clusters did not differ significantly between the two stands. Weighted clumping, WC, calculated as a product of root class, and its area was used as an index of root clustering, which again did not differ between beech and spruce stands. However, evaluations on a single root level showed that beech achieved the same degree of clustering with lower number of roots. Regardless of soil properties related to root clusters, a significantly higher clustering acquired per root for beech than for spruce suggests beech to be more efficient in belowground acquisition of space. Because none of the parameters describing root clustering were correlated with tree density around the investigated soil profiles, clusters of small roots are inherently present within the tree stands.     

Using a fine root extraction device to quantify small diameter corn roots (≥0.025 mm) in field soils

The study of fine roots growing under field conditions is limited by the techniques currently available for separating these roots from soil. This study had two objectives: to measure the total root length of field grown corn (Zea mays L.) by root diameter class, and to develop an inexpensive and efficient root washing device that would effectively capture all of the roots in a field soil sample. An inexpensive Fine Root Extraction Device (FRED) was constructed from readily available materials and was successful at extracting all roots, including very fine diameter roots (0.025 mm), from field soil samples. Greater than 99.7% of marked roots introduced to the FRED were recaptured by the device. Soil samples from three depths, and on three dates, from field grown corn were placed in the FRED. We found that more than 56% of total root length occurred in roots whose diameters were smaller than 0.175 mm, and more than 35% of root length occurred in roots smaller than 0.125 mm in diameter. Corn roots of the diameters described here have not been reported in field soils prior to this study. Root researchers who fail to measure these very fine roots will significantly underestimate root length density. Widespread use of the FRED should improve our understanding of root distribution in field soils.     

不同植茶年限土壤团聚体及其有机碳分布特征

作为土壤结构的基本单元和土壤肥力的重要组成部分,土壤团聚体对土壤的物理、化学和生物特性均有重要影响。试验选取了雅安市名山区中峰乡生态茶园区12—15a、20—22a、30—33a和50a的茶园,研究其土壤团聚体及其有机碳总量、储量和活性组分的分布特征,探究植茶年限对土壤团聚体及其有机碳分布的影响。结果表明:(1)研究区土壤以2 mm粒级团聚体为主,约为70%—80%,且在0—20 cm土层植茶20—22a土壤团聚体含量最高;(2)茶园土壤团聚体有机碳含量随团聚体粒级的减小而增加,最大值出现在0.25 mm粒级团聚体,且在植茶50a时达最高值,0—20 cm土层团聚体有机碳含量均高于20—40 cm,土壤团聚体水溶性有机碳和微生物生物量碳随植茶年限的延长呈先增加后降低的变化趋势,植茶30—33a时含量最高,且小粒级团聚体水溶性有机碳含量较高而微生物量碳较低;(3)土壤团聚体对有机碳的贡献率约有70%来自2 mm粒级团聚体,团聚体有机碳储量随植茶年限延长呈增加的趋势,不同植茶年限0—20 cm土层各粒级团聚体有机碳储量均高于20—40 cm土层,且以0.25 mm粒级团聚体有机碳储量最高。研究结果在一定程度上揭示了不同植茶年限土壤团聚体及其有机碳的分布特征,可为改善区域土壤质量及实施退耕还茶工程提供理论指导。