应用微根管法测定细根指标方法评述

树木细根(直径<2mm)在森林生态系统能量流动和物质循环中起着重要的作用。原有的细根生产周转研究中常采用的土钻法、内生长法、挖掘法、根室法和土柱法等,均不能直接观察到细根的动态变化。微根管法是一种非破坏性、可定点直接观察和研究植物根系的方法,为研究细根的生长、衰老、死亡、分解和再生长的过程提供了有效的工具,尤其适用于细根周转、寿命和分解等方面的研究。但该技术不能直接测定单位面积的细根生物量、细根化学组成及细根周转对土壤碳和养分循环的影响,需要与土钻法结合。本文就运用微根管法对细根生物量、生产、周转和寿命等指标的研究方法进行了评述。     

关于“落叶松和水曲柳人工林细根生长、死亡和周转”一文中问题的讨论

植声：研究方法部分：应写出如何利用图片分析细根长度或直径来估计产量。从该文可看出,作者仅仅利用微根管中的数据来计算细根现存量、生产量和死亡量,进而推算出周转率。然而,国际上有很多研究发现,微根管中的细根现存量并不代表生态系统细根现存量（大大偏低）。因此当前国际研究大都通过传统的钻土芯法测定细根生物量（现存量）,并通过分析细根生物量与形态（根长或根直径）之间的关系,再利用微根管中的生长数据,计算出细根生产量。因此,该文目前存在这个严重问题,希望作这方面的改动,否则本文的周转率应运到生态系统上,就会明显偏估。另外,在研究方法中,也要说明如何测定的细根现存量。     

高寒草甸植被细根生产和周转的比较研究

植物根系是陆地生态系统重要的碳汇和养分库,细根周转过程是陆地生态系统地下部分碳氮循环的核心环节,在陆地生态系统如何响应全球变化中起着关键作用。在全球变化敏感地区之一的青藏高原,对该地区的主要植被类型矮嵩草草甸同时采用根钻法、内生长袋法和微根管法3种观测方法研究细根生产和周转速率,并探讨了极差法、积分法、矩阵法和Kaplan-Meier法等数据处理方法对计算值的影响。研究结果显示:在估算细根净初级生产力时,根钻法宜采用积分法,内生长袋法宜选用矩阵法;由此进一步以最大细根生物量为基础,根钻法和内生长袋法估测的细根年周转速率分别为0.36 a-1和0.52 a-1,内生长袋法的估算结果是根钻法的1.44倍。对于微根管法,将其观测得到的细根长度转换为单位面积的生物量值后,采用积分法计算出细根周转速率为0.84 a-1,远高于传统方法的估算结果;若采用Kaplan-Meier生存分析方法,则计算出的细根周转速率更高达3.41 a-1。     

温度升高对长江口芦苇湿地细根形态和生长的影响

全球变暖对滨海湿地植物细根的影响目前尚不十分清楚。以长江口崇明东滩芦苇(Phragmites australis)湿地为对象,利用开顶式生长箱法进行模拟升温。于2019年5-10月,结合微根管法和根钻法,对0-40 cm土层细根(直径≤2 mm芦苇须根)的总根长、总表面积、比根长、比表面积、平均直径和生物量等指标开展连续观测,并计算其净生长速率和周转速率,分析气温升高对芦苇湿地细根形态特征和生长动态的影响。结果表明:在整个生长季,升温显著提高了0-40 cm土层细根的总根长、总表面积和总生物量,且主要表现在0-20 cm土层,而对比根长、比表面积无显著影响。升温显著增强了0-40 cm土层细根的净生长速率,且使其季节变异性加大。升温显著提高了10-40 cm土层细根的周转速率,但在0-10 cm土层影响不显著。总之,升温显著提高了芦苇湿地细根的总量和生长速率,改变其在土壤中的垂直分布格局,但对其水分和养分吸收效率无显著影响。升温使细根周转速率加快,同时使参与周转的细根总量增加,导致各土层特别是0-20 cm土层根源有机碳输入显著增加,这可能会深刻影响芦苇湿地的土壤碳平衡。     

杨树人工林细根数量和形态特征的季节动态及代际差异

采集欧美杨107Ⅰ代和Ⅱ代人工林细根样品,分析杨树不同根序细根数量特征(根长度、表面积和生物量)和形态特征(比根长、根长密度、根组织密度)对季节波动的响应及其代际差异.结果表明： 杨树各根序细根数量特征(根长度、表面积和生物量)均呈明显的季节变化,且具有明显的根序差异性.低级根序细根数量特征季节差异显著,细根生物量在生长季显著增加而生长季后显著下降.高级根序细根比根长季节波动显著,而根长密度和根组织密度等形态特征波动较小.连作导致人工林杨树1～2级细根长度、生物量、比根长和根长密度在生长季显著增大.1级细根数量特征与土壤温湿度呈显著正相关,与土壤有机质和速效氮含量呈显著负相关;而2级细根数量特征仅与土壤养分显著相关.杨树人工林细根特征的季节动态及代际差异体现了杨树对细根的碳投入变化,因连作引发的土壤养分匮乏可能引发植株对根系的碳投入增加,这种碳分配格局与人工林地上部分生产力形成密切相关.     

落叶松和水曲柳人工林细根生长、死亡和周转

 细根周转是陆地生态系统碳分配格局与过程的核心环节,而细根周转估计的关键是了解细根的生长和死亡动态。该研究以18年生落叶松(Larix gmelinii)和水曲柳(Fraxi nus mandshurica)人工林为对象,采用微根管（Minirhizotron）技术对两树种0～40 cm深度的细根生长和死亡动态进行了为期1年的观测,研究了两树种细根在不同土层深度的生长与死亡动态、细根周转以及与土壤有效氮含量、土壤温度、大气温度和降水的关系。结果表明：1） 落叶松平均细根生长（Root length density production, RLDP）0.0045 mm•cm^－2•d^－1）明显低于水曲柳RLDP（0.0077 mm•cm^－2•d^－1）。两个树种细根平均RLDP在表层（0～10 cm）最大,而底层（30～40 cm）最小 ,两树种平均细根死亡（Root length density mortality, RLDM）也表现同样规律 。水曲柳春季生长的细根占41.7%,夏季占39.7%,而落叶松细根生长分别是24.0%和51.2%,水曲柳细根死亡主要发生在春季（34.3%） 和夏季（34.0%）,而落叶松细根死亡主要发生在夏季和秋季（分别占28.5%和32.3%）,两 树种细根生长与死亡在冬季均较小;2）落叶松细根年生长量（0.94 mm•cm^－2•a^－1）和年死亡量（0.72 mm•cm^－2•a^－1）明显低于水曲柳（1.52和1.21 mm•cm^－2•a^－1）,两树种细根表层年生长量和年死亡量均最高,底层最低。落叶松细根年周转为3.1次•a^－1（按年生长量计算）和2.4次•a^－1（按年死亡量计算）,相比较,水曲柳细根年周转分别为2.7次•a^－1和2.2次•a^－1;3）土壤有效氮含量、土壤温度、大气温度和降水综合作用影响细根生长和死亡动态,可以解释细根生长80%的变异和细根死亡95%以上的变异。     

微根管法和同位素法在细根寿命研究中的应用及比较

细根的生产和周转在陆地生态系统的碳和养分循环中起着重要作用,并且对全球环境变化具有一定的指示意义。细根寿命是估计细根周转的关键,其长短决定了养分和碳消耗与循环的速度。由于采用的研究方法不同,导致所得细根寿命估计值存在较大差异,目前最新的同位素和微根管2种方法之间寿命估计值差异可达10倍以上。本文对这2种研究方法的原理和优点进行了阐述,并从细根定义、细根寿命理论分布假设、细根取样误差等方面对导致这2种方法研究结果存在差异的原因进行分析,以期有助于今后根系研究的发展。     

微根窗技术及其在植物根系研究中的应用

植物根系对固定植株和获得水分和养分起重要作用,但是土壤不可观测性的限制,给根系生态学的研究带来一定的困难。因此,找到原位观察根系生长的方法对研究根系生态学就显得尤为重要。目前微根窗技术被认为是研究根系生态学最有前途的方法。从微根窗系统的组成、微根窗管的安装、微根窗图象的收集及微根窗数据的利用等几个方面进行了概述。阐述了在微根窗使用和操作过程中需要注意的几个问题,微根窗管与土壤之间的良好接触是获得高质量微根窗图像数据的前提和基础;图象收集的频率依赖于测定和计算的根系参数,如果想得到根系现存量、生产力、更新和寿命的信息,必须避免采样间隔时间过长。     

幼龄柠条细根现存量与环境因子的关系

 以晋西北黄土高原区柠条(Caragana korshinskii)幼龄人工林为研究对象, 应用微根管技术(Minirhizotron technique)对林地100 cm土层范围的柠条细根生长动态进行了观测。以2007年生长季(5～9月)的根长密度(RLD, mm·cm^–3)数据为基础, 对柠条细根现存量(RLDst, mm·cm^–3)及其与环境因子(≥10 ℃积温、同期土壤积温、积降雨量和土壤水分等)的关系作了研究。结果表明, 40～90 cm土层是柠条细根的主要分布区和生长活跃区, 其细根占细根总量的59.7%。柠条细根现存量的季节变化特征为: 5月至9月上旬RLDst持续增加, 9月下旬RLDst略有降低。柠条细根现存量季节变化与≥10 ℃积温、同期土壤积温和积降雨量均存在极显著正相关关系。     

基于土芯法的亚热带常绿阔叶林细根空间变异与取样数量估计

树木细根具有高度空间异质性,确定合理的细根取样策略是林木细根研究的前提。通过在福建省三明米槠天然常绿阔叶林内随机钻取96个土芯,分析细根生物量和形态特征的空间变异特征,并估计各指标所需的取样数量。结果表明:(1)随着径级增加,细根各指标变异系数增大,相应的取样数量增加;(2)随着土壤深度增加,单位面积细根生物量变异程度和相应的取样数量均增加。在置信水平为95%、精度为80%的条件下,直径为0-1 mm和1-2 mm的细根,分别采集16和42个样品可以满足测定单位面积细根生物量,采集17和31个样品可以满足测定单位面积细根长度,采集25和33个样品可以满足测定单位面积细根表面积。Shapiro-Wilk检验表明,除表层土壤0-1 mm细根单位面积生物量符合正态分布外,其余细根生物量和形态指标数据均不符合正态分布。研究结果为亚热带常绿阔叶林细根的合理取样提供了科学依据。     

Root sampling methods - applications and limitations of the minirhizotron technique

Applications and limitations of the minirhizotron technique (non-destructive) in relation to two frequently used destructive methods (soil coreing and ingrowth cores) is discussed. Sequential coreing provides data on standing crop but it is difficult to obtain data on root biomass production. Ingrowth cores can provide a quick estimate of relative fine-root growth when root growth is rapid. One limitation of the ingrowth core is that no information on the time of ingrowth and mortality is obtained.The minirhizotron method, in contrast to the destructive methods permits simultaneous calculation of fine-root length production and mortality and turnover. The same fine-root segment in the same soil space can be monitored for its life time, and stored in a database for processing. The methodological difficulties of separating excavated fine roots into living and dead vitality classes are avoided, since it is possible to judge directly the successive ageing of individual roots from the images. It is concluded that the minirhizotron technique is capable of quantifying root dynamics (root-length production, mortality and longevity) and fine-root decomposition. Additionally, by combining soil core data (biomass, root length and nutrient content) and minirhizotron data (length production and mortality), biomass production and nutrient input into the soil via root mortality and decomposition can be estimated.     

Advancing fine root research with minirhizotrons

Minirhizotrons provide a nondestructive, in situ method for directly viewing and studying fine roots. Although many insights into fine roots have been gained using minirhizotrons, a review of the literature indicates a wide variation in how minirhizotrons and minirhizotron data are used. Tube installation is critical, and steps must be taken to insure good soil/tube contact without compacting the soil. Ideally, soil adjacent to minirhizotrons will mimic bulk soil. Tube installation causes some degree of soil disturbance and has the potential to create artifacts in subsequent root data and analysis. We therefore recommend a waiting period between tube installation and image collection of 6-12 months to allow roots to recolonize the space around the tubes and to permit nutrients to return to pre-disturbance levels. To make repeated observations of individual roots for the purposes of quantifying their dynamic properties (e.g. root production, turnover or lifespan), tubes should be secured to prevent movement. The frequency of image collection depends upon the root parameters being measured or calculated and the time and resources available for collecting images and extracting data. However, long sampling intervals of 8 weeks or more can result in large underestimates of root dynamic properties because more fine roots will be born and die unobserved between sampling events. A sampling interval of 2 weeks or less reduces these underestimates to acceptable levels. While short sample intervals are desirable, they can lead to a potential trade-off between the number of minirhizotron tubes used and the number of frames analyzed per tube. Analyzing fewer frames per minirhizotron tube is one way to reduce costs with only minor effects on data variation. The quality of minirhizotron data should be assessed and reported; procedures for quantifying the quality of minirhizotron data are presented here. Root length is a more sensitive metric for dynamic root properties than the root number. To make minirhizotron data from separate experiments more easily comparable, idiosyncratic units should be avoided. Volumetric units compatible with aboveground plant measures make minirhizotron-based estimates of root standing crop, production and turnover more useful. Methods for calculating the volumetric root data are discussed and an example presented. Procedures for estimating fine root lifespan are discussed.     

Measuring Fine Root Turnover in Forest Ecosystems

Development of direct and indirect methods for measuring root turnover and the status of knowledge on fine root turnover in forest ecosystems are discussed. While soil and ingrowth cores give estimates of standing root biomass and relative growth, respectively, minirhizotrons provide estimates of median root longevity (turnover time) i.e., the time by which 50% of the roots are dead. Advanced minirhizotron and carbon tracer studies combined with demographic statistical methods and new models hold the promise of improving our fundamental understanding of the factors controlling root turnover. Using minirhizotron data, fine root turnover (y⁻¹) can be estimated in two ways: as the ratio of annual root length production to average live root length observed and as the inverse of median root longevity. Fine root production and mortality can be estimated by combining data from minirhizotrons and soil cores, provided that these data are based on roots of the same diameter class (e.g., < 1 mm in diameter) and changes in the same time steps. Fluxes of carbon and nutrients via fine root mortality can then be estimated by multiplying the amount of carbon and nutrients in fine root biomass by fine root turnover. It is suggested that the minirhizotron method is suitable for estimating median fine root longevity. In comparison to the minirhizotron method, the radio carbon technique favor larger fine roots that are less dynamics. We need to reconcile and improve both methods to develop a more complete understanding of root turnover.     

Root Dynamics in Restored and Naturally Regenerated Atlantic White Cedar Wetlands

This work addressed the seasonal and successional factors of root dynamics in natural and restoration Atlantic white cedar (AWC) wetlands. Using minirhizotrons and soil root cores, fine root dynamics were measured in a chronosequence of reference and restoration AWC wetlands to compare trends in ecosystem development after canopy harvest. Seasonal fine root abundance, production, and mortality were sampled during a 439‐day period in one restoration and three reference AWC wetlands. Soil cores were collected to measure fine root biomass and to determine allometric relationships between root length and biomass. Significant seasonal variation of root dynamics was observed in the young reference and restoration sites. The mature and intermediate‐aged sites exhibited little seasonal variability in root abundance and mortality. Root production was variable but not seasonally consistent. Results suggest that root dynamics become less seasonal as AWC communities shift from herbaceous to woody vegetation dominance. No trend in fine root abundance along the chronosequence was observed, suggesting that roots rapidly reestablish following tree harvest. Measurements of annual root length production suggest increasing annual production with decreasing stand age. However, a reversal of this trend was observed when using production estimates calculated from minirhizotron measurements and root length–mass relationships. These findings underscore the importance of supplementing minirhizotron data with root allometric relationships when analyzing vegetation gradients. Overall, results indicate substantial differences in the form and quantity of root contributions to soil organic matter in the restoration site compared to that in the reference chronosequence. Higher initial planting densities of AWC are recommended to achieve similar contributions of roots to soil organic matter accumulation in the restoration site.     

Seasonal patterns of fine root demography in a cool-temperate deciduous forest in central Japan

In forest ecosystems, fine roots have a considerable role in carbon cycling. To investigate the seasonal pattern of fine root demography, we observed the fine root production and decomposition processes using a minirhizotron system in a Betula-dominated forest with understory evergreen dwarf bamboo. The length density of fine roots decreased with increasing soil depth. The seasonal patterns of each fine root demographic parameter (length density of visible roots, rates of stand-total fine root production and decomposition) were almost the same at different soil depths. The peak seasons of the fine root demographic parameters were observed in the order: stand-total fine root production rate (late summer) > length density of the visible roots (early autumn) > stand-total fine root decomposition rate (autumn, and a second small peak in spring). The fine root production rate was high in the latter part of the plant growing season. Fine root production peaked in late summer and remained high until the end of the tree defoliation season. The higher stand-total fine root production rate in autumn suggests the effect of understory evergreen bamboo on the stand-total fine root demography. The stand-total fine root decomposition rate was high in late autumn. In the snow-cover period, the rates of both fine root production and decomposition were low. The fine root demographic parameters appeared to show seasonal patterns. The fine root production rate had a clearer seasonality than the fine root decomposition rate. The seasonal pattern of stand-total fine root production rate could be explained by both overstory and understory above-ground productivities.     

森林生态系统根系生物量研究进展

在森林生态系统功能过程研究中,特别是在生产力和生物地化学循环方面,根系的作用不容忽视,但是,由于研究方法和研究者的观念等方面的限制,对于根系的研究还远不及地上部分受到重视。而关于细根生物量,周转率和生产力等方面的是很少有人问津。为推动我国根系生物学研究的发展,本一面地介绍了９种典型的测度细根生物量的方法。包括：收获法、钻土芯法、内生长土芯法、平衡法、根观测实验室法、土壤碳平衡法、挖土块法、间接法和     

Estimating Fine Root Turnover in Tropical Forests along an Elevational Transect using Minirhizotrons

Growth and death of fine roots represent an important carbon sink in forests. Our understanding of the patterns of fine root turnover is limited, in particular in tropical forests, despite its acknowledged importance in the global carbon cycle. We used the minirhizotron technique for studying the changes in fine root longevity and turnover along a 2000-m-elevational transect in the tropical mountain forests of South Ecuador. Fine root growth and loss rates were monitored during a 5-mo period at intervals of four weeks with each 10 minirhizotron tubes in three stands at 1050, 1890, and 3060 m asl. Average root loss rate decreased from 1.07 to 0.72 g/g/yr from 1050 to 1890 m, indicating an increase in mean root longevity with increasing elevation. However average root loss rate increased again toward the uppermost stand at 3060 m (1.30 g/g/yr). Thus, root longevity increased from lower montane to mid-montane elevation as would be expected from an effect of low temperature on root turnover, but it decreased further upslope despite colder temperatures. We suggest that adverse soil conditions may reduce root longevity at high elevations in South Ecuador, and are thus additional factors besides temperature that control root dynamics in tropical mountain forests.     

Improved scaling of minirhizotron data using an empirically-derived depth of field and correcting for the underestimation of root diameters

Background and aims

Accurate data on the standing crop, production, and turnover of fine roots is essential to our understanding of major terrestrial ecological processes. Minirhizotrons offer a unique opportunity to study the dynamic processes of root systems, but are susceptible to several measurement biases.

Methods

We use roots extracted from minirhizotron tube surfaces to calculate the depth of field of a minirhizotron image and present a model to correct for the underestimation of root diameters obscured by soil in minirhizotron images.

Results

Non-linear regression analysis resulted in an estimated depth of field of 0.78 mm for minirhizotron images. Unadjusted minirhizotron data underestimated root net primary production and fine root standing crop by 61 % when compared to adjusted data using our depth of field and root diameter corrections. Changes in depth of field accounted for >99 % of standing crop adjustments with root diameter corrections accounting for <1 %.

Conclusions

Our results represent the first effort to empirically derive depth of field for minirhizotron images. This work may explain the commonly reported underestimation of fine roots using minirhizotrons, and stands to improve the ability of researchers to accurately scale minirhizotron data to large soil volumes.     

国外对树木细根的研究动态

森林生态系统的研究已经深入到了生态系统的各个层次,但无论是在哪一层次上的研究,其重点往往是树木的地上部分,而对地下部分——根系的研究相对较为薄弱。主要原因是对根系的生态学作用认识不足以及根系研究的困难。近来,随着对根系特别是细根在养分循环及能流中的重要作用的认识,细根的研究已成为森林生态系统研究的一个热点。根系中粗根(直径>5mm)虽然占总生物量的20%左