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

Spatial heterogeneity of fine roots in a subtropical evergreen broad-leaved forest and their sampling strategy based on soil coring method

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。

Fine roots, commonly defined as roots 〈2 mm in diameter, play an important role in nutrient cycle and energy flow in forest ecosystems.Tree fine roots are highly spatially heterogeneous, so only a large number of samples can be valid to study such heterogeneity. However, the minimum sample numbers and the proper sampling strategy is unknown yet, which results in uncertainties in fine-root studies.In this paper, by using the soil core sampling method, 96 soil cores were taken randomly in a subtropical evergreen broad-leaved forest located at Sanming, Fujian Province.Every soil core was divided into four parts according to soil depth： 0--10 cm, 10--20 cm, 20--40 cm, and 40--60 cm.Fine roots in different soil depth were washed and picked, and was further separated into different diameter classes of 0--1 mm and 1--2 mm. Fine roots were then scanned for measurement of morphological characteristics, and oven and weighed for biomass.The objectives of this research were to： （1） understand the spatial heterogeneity of fine root biomass and morphological characteristics; （2） estimate the sample sizes required for getting various fine root characteristic values. The results showed that, as diameter class increases, the coefficient of variation （CV） of fine root variables increases, and the required sampling sizes increase accordingly.The CVs for fine-root biomass, length and surface area per unit ground area of different diameter classes were unanimously between 40%--60%. At the same time, fine-root biomass, length and surface area at per unit ground areadeclined with the increases of fine root diameter class. With the increases of soil depth, fine-root biomass of different diameter classes per unit ground area decreased, and their CVs increased, while the required sampling sizes increased accordingly.Except for fine-root biomass of 0--1 mm per unit ground area in the surface soil which had a CV of 48.61%, the rest of fine roots from different diameter classes and soil depths had CVs of biomass per unit ground area consistently between 80%--90%.Under the condition with confidence level of 98% and precision of 80%, for fine roots of 0--1 mm and 1--2 mm, collecting 16 and 42 cores can meet the determination for fine root biomass per unit ground area, 17 and 31 cores for fine root length per unit ground area, and 25 and 33 cores for fine root surface area per unit ground area, respectively.By comparison, most reported root studies using soil coring were insufficient sampling sizes.The Shapiro-Wilk test showed that among different diameter classes, only fine-root biomass per unit ground area of 0--1 mm conforms to the normal distribution; among different soil depths, only fine-root biomass per unit ground area of 0--1 mm in the surface soil conforms to normal distribution.Thus, for field data on fine-root biomass and morphology by soil coring, data transformation were generally required to meet normality.These results provided a scientific basis for minimum sampling sizes and sampling strategy in subtropical evergreen broad-leaved forest and are therefore helpful to belowground studies.More studies should be conducted to further determine factors affecting the minimum sampling size of soil coring.