关帝山华北落叶松人工林细根生物量空间分布及季节变化

利用根钻法研究了山西关帝山华北落叶松（Larix principis—rupprechtii Mayr）人工林细根生物量的空间分布和季节变化特征。结果表明,华北落叶松不同径级细根生物量随土层深度的增加而逐渐减少,土壤表层（0—10cm）中各径级细根的生物量最高,Ⅰ级细根（根直径0～1mm）的生物量在不同土层深度间差异显著（P〈0．05）;距树干不同水平距离处各径级的细根生物量差异均未达到显著水平（P〉0．05）。在0～10cm土层中,各径级细根生物量的季节变化差异显著（P〈0．05）,均表现为单峰型,峰值出现在9月份;在10～20cm和20-30cm土层中,Ⅰ级和Ⅱ级（根直径1～2mm）细根生物量季节变化差异显著,Ⅲ级细根（根直径2～5mm）和Ⅰ级死根（根直径0～2mm）生物量季节变化差异不显著。     

Root biomass distribution of planted <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> in a duplex soil in an oasis: desert boundary area

Duplex soils, consisting of a sandy surface soil (A-horizon) and silty-clay subsoil (B-horizon), occur in a boundary area between oasis and desert in northwestern China and create a challenging habitat for restoration of plant growth. We conducted an experiment in a 10-year-old H. ammodendron plantation forest to determine the influence of physical properties of duplex soil on water infiltration and plant root growth. We used a trenching method to assess root biomass, and classified roots into two diameter classes: fine (<2 mm) and coarse (>2 mm). Following a 26.7 mm rain event, water infiltrated to the B- horizon; further deep percolation was hindered by low hydraulic conductivity, so that B horizon remained at high available soil moisture for an extended period of time. Root biomass increased rapidly in, or very close to the B horizon, especially for coarse roots. The subsoil formed a barrier to root penetration, but may also reflect the accumulation of water resources at the boundary between the A- and B-horizon. Shoot growth and root distribution, shrub height and canopy area, and total root biomass were negatively correlation with depth to the B horizon, and that was reflected by quadratic functions. We conclude that the texture and structure of duplex soils influenced the soil environment for water infiltration and storage, indicating that the B-horizon underlying sand in duplex soils is advantageous for the growth, and development of planted sand-stabilizing vegetation. These results have important implications for sustainable development of sand-fixing plantations in desert ecosystems.     

Characterisation of hemp (Cannabis sativa L.) roots under different growing conditions

Hemp (Cannabis sativa L.) is mainly grown for its fibre and is considered a desirable crop for sustainable production systems. In a field trial carried out over two years in Northern Italy the root system of a hemp crop, cultivated at contrasting plant densities, was sampled and analysed with an image analysis software. Root length density (RLD) was highest in the first 10 cm of soil, almost 5 cm cm^?3; it decreased progressively until the depth of 130 cm, a part from a peak at 90–100 cm in response to a perched water table. Roots were found to 130 cm of depth in one year and to 200 cm in the other. Root diameter was finer (190 μm) in the upper soil layer, it increased with depth until 100 cm, and remained constant at 300 μm thereafter. Following the same trend of RLD, root biomass was highest in the first soil layer; 50% of the root biomass was found in the first 20 cm or 50 cm when taproot biomass was considered or not. Total root biomass was 3.21 t ha^?1 and 2.41 t ha^?1 in the two years of trial, but the ratio between aboveground and below ground biomass was constant at 5.46. None of the root parameters were significantly affected by plant population, which seems to confirm the plastic behaviour that hemp shows for aboveground development. The high root biomass production measured in this study, especially in deeper soil layers, provides additional evidence of the positive role that hemp can play in sustainable cropping systems.     

Comprehensive assessments of root biomass and production in a Kobresia humilis meadow on the Qinghai-Tibetan Plateau

Alpine meadow covers ca. 700,000 km² with an extreme altitude range from 3200 m to 5200 m. It is the most widely distributed vegetation on the vast Qinghai-Tibetan Plateau. Previous studies suggest that meadow ecosystems play the most important role in both uptake and storage of carbon in the plateau. The ecosystem has been considered currently as an active “CO₂ sink”, in which roots may contribute a very important part, because of the large root biomass, for storage and translocation of carbon to soil. To bridge the gap between the potential importance and few experimental data, root systems, root biomass, turnover rate, and net primary production were investigated in a Kobresia humilis meadow on the plateau during the growing season from May to September in 2008 and 2009. We hypothesized that BNPP/NPP of the alpine meadow would be more than 50%, and that small diameter roots sampled in ingrowth cores have a shorter lifespan than the lager diameter roots, moreover we expected that roots in surface soils would turn over more quickly than those in deeper soil layers. The mean root mass in the 0–20 cm soil layer, investigated by the sequential coring method, was 1995?±?479 g?m^?2 and 1595?±?254 g?m^?2 in growing season of 2008 and 2009, respectively. And the mean fine root biomass in ingrowth cores of the same soil layer was 119?±?37 g?m^?2 and 196?±?45 g?m^?2 in the 2 years. Annual total NPP was 12387 kg?ha^?1?year^?1, in which 53% was allocated to roots. In addition, fine roots accounted for 33% of belowground NPP and 18% of the total NPP, respectively. Root turnover rate was 0.52 year^?1 for bulk roots and 0.74 year^?1 for fine roots. Furthermore, roots turnover was faster in surface than in deeper soil layers. The results confirmed the important role of roots in carbon storage and turnover in the alpine meadow ecosystem. It also suggested the necessity of separating fine roots from the whole root system for a better understanding of root turnover rate and its response to environmental factors.     

Coarse root biomass for eucalypt plantations in Tasmania,Australia: sources of variation and methods for assessment

This study develops a feasible method for evaluating coarse root biomass (roots >2 mm diameter) of well established plantations of eucalypts and then examines coarse root biomass variability across tree age and size, fertilization treatment, species and site for Eucalyptus globulus and E. nitens in Tasmania, Australia. The most efficient sampling protocol consisted of rootball excavation and soil coring for bulk coarse roots, which when compared with total tree excavation estimated total coarse root biomass contained inside the sampled area to within 10%. Across all treatments, an average of 76% of the coarse root biomass was located within the rootball. The majority (>65%) of the coarse roots outside the rootball were located in the surface 20 cm of soil. When size class distribution was examined, 75% of coarse root biomass was found to occur in the larger (20+ mm) diameter size class, a size class that displayed considerable spatial heterogeneity. At the stand level, coarse root biomass ranged from 2.18 to 7.38 kg m ^-2 depending primarily on tree size but also on fertilization treatment, species and site. It is estimated that allocation to coarse root biomass production was around 0.2 kg m ^-2 year ^-1 (around 6% of estimated NPP) for the E. nitens stands examined in this study and around 1 kg m ^-2 year ^-1 (around 20% of estimated NPP) for the E. globulus stand examined. Robust relationships using above-ground parameters could be used to predict coarse root biomass regardless of fertilization or site, but species changed the relationship.     

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。     

三峡库区马尾松根系生物量的空间分布

以三峡库区主要植被马尾松人工林为研究对象,用内径为10 cm的根钻,分别在马尾松中龄林、近熟林和成熟林内,据树干0.5、1.0、1.5 m和2.0 m处设置取样点,各样点按0-10、10-20、20-30、30-40、40-60 cm将土壤分为5个垂直层次,对马尾松根系的空间分布格局进行调查。结果表明:(1)三峡库区马尾松总根系生物量(0-10 mm)为中龄林(4.72 t/hm²)显著高于成熟林(2.94 t/hm²)和近熟林(2.40 t/hm²)(P<0.05)。细根(0-2 mm)生物量随年龄增加而递减,差异不显著(P>0.05);(2)马尾松3个林龄中根系生物量表现出一定的水平分布特征,但具体趋势表现各异,细根生物量最大值均出现在距离样木1.0 m处;(3)细根主要分布在土壤上层,其中47.53%-71.73%的活细根集中在0-20 cm土壤深度内,且随土层的加深,其生物量明显减少。粗根(2-10 mm)则主要分布于20-60 cm土层范围内;(4)根系直径越小,受环境变化越明显。马尾松细根生物量分布主要受土壤深度的影响,树龄和不同水平距离对细根分布格局影响不显著(P>0.05),各因素对粗根生物量的影响均未达到显著水平(P>0.05)。     

Fine root dynamics and soil carbon accretion under thinned and un-thinned Cupressus lusitanica stands in, Southern Ethiopia

Background and aims

Forest management activities influences stand nutrient budgets, belowground carbon allocation and storage in the soil. A field experiment was carried out in Southern Ethiopia to investigate the effect of thinning on fine root dynamics and associated soil carbon accretion of 6-year old C. lusitanica stands.

Methods

Fine roots (≤2 mm in diameter) were sampled seasonally to a depth of 40 cm using sequential root coring method. Fine root biomass and necromass, vertical distribution, seasonal dynamics, annual turnover and soil carbon accretion were quantified.

Results

Fine root biomass and necromass showed vertical and temporal variations. More than 70 % of the fine root mass was concentrated in the top 20 cm soil depth. Fine root biomass showed significant seasonal variation with peaks at the end of the major rainy season and short rainy season. Thinning significantly increased fine root necromass, annual fine root production and turnover. Mean annual soil carbon accretion, through fine root necromass, in the thinned stand was 63 % higher than that in the un-thinned stand.

Conclusions

The temporal dynamics in fine roots is driven by the seasonality in precipitation. Thinning of C. lusitanica plantation would increase soil C accretion considerably through increased fine root necromass and turnover.     

Fine root biomass estimates from minirhizotron imagery in a shrub ecosystem exposed to elevated CO2

Fine root biomass and C content are critical components in ecosystem C models, but they cannot be directly determined by minirhizotron techniques, and indirect methods involve estimating 3-dimensional values (biomass/ soil volume) from 2-dimensional measurements. To estimate biomass from minirhizotron data, a conversion factor for length to biomass must be developed, and assumptions regarding depth of view must be made. In a scrub-oak ecosystem in central Florida, USA, root length density (RLD) was monitored for 10 years in a CO₂ manipulation experiment using minirhizotron tubes. In the seventh year of the study, soil cores were removed from both ambient and elevated CO₂ chambers. Roots from those cores were used to determine specific root length values (m/g) that were applied to the long-term RLD data for an estimation of root biomass over 10 years of CO₂ manipulation. Root length and biomass estimated from minirhizotron data were comparable to determinations from soil cores, suggesting that the minirhizotron biomass model is valid. Biomass estimates from minirhizotrons indicate the <0.25 mm diameter roots accounted for nearly 95% of the total root length in 2002. The long-term trends for this smallest size class (<0.25 mm diameter) mirrored the RLD trends closely, particularly in relation to suspected root closure in this system. Elevated CO₂ did not significantly affect specific root length as determined by the soil cores. A significant treatment effect indicated smallest diameter fine roots (<0.25 mm) were greater under elevated CO₂ during the early years of the study and the largest (2–10 mm) had greater biomass under elevated CO₂ during the later years of the study. Overall, this method permits long-term analysis of the effects of elevated CO₂ on fine root biomass accumulation and provides essential information for carbon models.     

Lolium perenne L. root systems are a collection of Gaussian curve shaped meso diameter class length distributions

Aims

Determine if the root system of Lolium perenne L. (L perenne) is a continuous distribution of diameters, or a collection of discrete diameters classes.

Methods

Plants from tillers of five clones were grown in a local soil amended with lime. Roots were excavated after they were grown in soil for 54 days, washed and imaged with both a commercial scanner (94 px mm^?1) and a high resolution, locally built, imager (204 px mm^?1). Images were converted to diameter class length data with WinRhizo.

Results

Scanned images did not have enough resolution to accurately measure fine roots diameters (<0.09 mm diam.). Therefore the high resolution images were used. The diameter class length distributions (DCLD) of these images demonstrated diameter class clusters (meso diameter classes) which could be modeled with a non-linear Gaussian (normal) curve model. Recreating the whole root system from a compilation of the DCLD, regenerated from the three parameters of each of the Gaussian curves for the root system, produced a distribution visually identical to the original whole root system curve.

Conclusions

L perenne root systems are a collection of meso diameter classes easily described by non-linear Gaussian models. The data set of the parameters from these models is much smaller than a WinRhizo data set, and can reconstruct the original whole system DCLD.     

Root biomass distribution and soil properties of an open woodland on a duplex soil

Data on the distribution of root biomass are critical to understanding the ecophysiology of vegetation communities. This is particularly true when models are applied to describe ecohydrology and vegetation function. However, there is a paucity of such information across continental Australia. We quantified vertical and horizontal root biomass distribution in a woodland dominated by Angophora bakeri and Eucalyptus sclerophylla on the Cumberland Plains near Richmond, New South Wales. The site was characterised by a duplex (texture contrast) soil with the A horizon (to 70 cm) consisting of loamy sand and the B horizon (to > 10 m) consisting of sandy clay. The topsoil had a smaller bulk density, a smaller water holding capacity but a larger organic component and a larger hydraulic conductivity in comparison to the subsoil. Root biomass was sampled to 1.5 m depth and declined through the soil profile. Whilst total biomass in the B horizon was relatively small, its contribution to the function of the trees was highly significant. Coarse roots accounted for approximately 82% of the root mass recovered. Lateral distribution of fine roots was generally even but coarse roots were more likely to occur closer to tree stems. Variation in tree diameter explained 75% of the variation in total below-ground biomass. The trench method suggested the belowground biomass was 6.03?±?1.21 kg m^?2 but this method created bias towards sampling close to tree stems. We found that approximately 68% of root material was within a 2 m radius of tree stems and this made up 54% of the total number of samples but in reality, only approximately 5 to 10% of the site is within a 2 m radius of tree stems. Based on these proportions, our recalculated belowground biomass was 2.93?±?0.59 kg m^?2. These measurements provide valuable data for modeling of ecosystem water use and productivity.     

Effect of aboveground intervention on fine root mass, production, and turnover rate in a Chinese cork oak (Quercus variabilis Blume) forest

Aims

Fine root is an important part of the forest carbon cycle. The growth of fine roots is usually affected by forest intervention. This study aims to investigate the fine root mass, production, and turnover in the disturbed forest.

Methods

The seasonal and vertical distributions of fine root (diameter ≤2 mm) were measured in a Chinese cork oak (Quercus variabilis Blume) forest. The biomass and necromass of roots with diameters ≤1 mm and 1-2 mm in 0-40 cm soil profiles were sampled by using a sequential soil coring method in the stands after clear cutting for 3 years, with the stands of the remaining intact trees as the control.

Results

The fine root biomass (FRB) and fine root necromass (FRN) varied during the growing season and reached their peak in August. Lower FRB and higher FRN were found in the clear cutting stands. The ratio between FRN and FRB increased after forest clear cutting compared with the control and was the highest in June. The root mass with diameter ≤1 mm was affected proportionately more than that of diameter 1-2 mm root. Clear cutting reduced FRB and increased FRN of roots both ≤1 mm and 1-2 mm in diameter along the soil depths. Compared with the control, the annual fine root production and the average turnover rate decreased by 30.7 % and 20.7 %, respectively, after clear cutting for 3 years. The decline of canopy cover contributed to the dramatic fluctuation of soil temperature and moisture from April to October. With redundancy discriminate analysis (RDA) analysis, the first axis was explained by soil temperature (positive) and moisture (negative) in the control stands. Aboveground stand structure, including canopy cover, sprout height, and basal area, influenced FRB and FRN primarily after forest clear cutting.

Conclusions

This study suggested that the reduction of fine root biomass, production, and turnover rate can be attributed to the complex changes that occur after forest intervention, including canopy damage, increased soil temperature, and degressive soil moisture.     

Growth, root and leaf structure, and biomass allocation in Leucanthemum vulgare Lam. (Asteraceae) as influenced by heavy-metal-containing slag

Effects of heavy metal contamination on growth, leaf turnover, biomass allocation and leaf and root structure of Leucanthemum vulgare Lam. were investigated. Plants were grown in two outdoor experiments, for 5 weeks or for 3 months, respectively, on sand with different additions of slag containing elevated levels of heavy metals, especially Cu and Ni. In the 3-month experiment nutrients were provided as composted manure, in the 5-week experiment as a solution. Slag contamination reduced plant growth, biomass allocation to roots, specific root length and specific leaf area, while root tissue density and leaf dry matter content increased. Fine root diameter increased, whereas coarse root diameters showed a non-significant decreasing trend. Toxicity of slag was lower in the 3-month experiment, probably due to organic matter in the substrate. We conclude that heavy metals in the soil around Cu–Ni smelters may, besides directly reducing growth of the plants, increase their susceptibility to other stresses such as drought, by reducing the root length to leaf area ratio. Fine and coarse roots show distinct responses, indicating that different root diameter classes should be regarded separately to fully understand stress responses of root systems.     

Effect of tillage system on the root growth of spring wheat

Little research has examined the influence of tillage system on root growth in wheat grown on rainfed Vertisols. A 3-year field study (2003, 2004 and 2005) was carried out on a typical Vertisol (southern Spain), to determine the effects of tillage system on root growth in spring wheat (Triticum aestivum L) grown in continuous rotation with faba bean (Vicia faba L), within the framework of the long-term “Malagón” experiment started in 1986. Tillage treatments were no-tillage (NT) and conventional tillage (CT), and the experiment was designed as a randomized complete block with three replications. The following parameters were measured: above-ground biomass, grain yield, root length density (RLD), root biomass (RB) and root N content. In the topmost 10 cm of soil, higher values were found under CT than under NT for RLD in the rainiest year (20.2 km m^?3 vs. 9.6 km m^?3 respectively) and for RB (512 kg ha^?1 vs. 261 kg ha^?1 respectively) in all study years. In deeper layers, no difference was recorded between the two tillage systems. Greater wheat root development in the upper soil layer under CT may reflect the greater soil penetration resistance found in the topmost 10 cm under NT. Root separation using a sieve with a 0.5 mm mesh screen led to a marked underestimation of RLD and RB, with values up to three times higher when using a 0.2 mm mesh screen. Mean wheat root N content in the topmost 30 cm of soil accounted for over 80% of total root N content. The highest grain yield was observed under NT, since this system provided greater water storage in the soil profile in the mostly dry study years.     

How deep must we dig? Sampling depth effects on root biomass assessments in Mediterranean riparian buffers

Abstract

Root turnover is a relevant process controlling carbon and nutrient cycles. Furthermore, root biomass could influence the effectiveness of riparian buffer systems with regard to immobilizing and processing soil water pollutants and improving soil quality. However, sampling root biomass presents many technical problems, because there is as yet no well-established method for assessment thereof. In the present study, we evaluated the effects of limited sampling depth on biomass assessment in four riparian buffers and on two different dates (January and June): a poplar forest, two mixed deciduous afforestations of different ages, and a grassy area. Although, for all systems, most root biomass was limited to the first 30 cm, detection of differences in root biomass among vegetation types depended strongly on sampling depth for both sampling dates. Furthermore, although the poplar forest presented one of the most stable root systems in terms of total biomass between the sampling dates, it was one of the most seasonally dynamic in terms of the amount and distribution with depth of root biomass. The results of this study suggest that a minimum sampling depth should be established for each study according to seasonal variation in vertical distribution of root biomass.     

Root distribution of different mature tree species growing on contrasting textured soils in temperate windbreaks

Aim

We studied the vertical and lateral root distribution of tree species from three genera (Populus spp. - poplar, Picea spp. - spruce, Salix spp. - willow) that were planted in temperate windbreaks and assessed the effects of soil texture on root density.

Methods

Root distribution to depths of up to 1 m was assessed using the trench-profile method at different distances from the tree rows (2, 6 and 9 m) in 18 mature (average age, 25 years-old) windbreak-sites that were located on light- or heavy-textured agricultural soils in southeastern Québec, Canada. Roots were classified into three diameter classes: fine (<1 mm), medium-size (1–5 mm), and coarse (>5 mm).

Results

Tree fine-root density in poplar and willow windbreaks was higher than in spruce windbreaks at 2 m from the tree row. Root densities were higher in light compared to heavy soils, but these differences were specific to poplar and spruce. Across species groups and soil types, 67 % of the roots occurred in the uppermost 30 cm. In this soil zone, different soil fertility variables (pH, clay content, CEC) were negatively correlated with root density. Densities of spruce and willow roots at 6 m from the tree row were much lower (and often unobserved) than that of poplar. At 9 m, low root densities were observed at only two sites.

Conclusions

We conclude that tree identity and soil type are important drivers of root distribution in temperate agroforestry systems. These results may have important implications for the management of tree competition in agroforestry systems and several ecosystem services that are provided by roots, including C-sequestration, erosion control and water infiltration.     

Response of Norway spruce root system to elevated atmospheric CO2 concentration

Root structure parameters, root biomass and allometric relationships between above- and belowground biomass were investigated in young Norway spruce (Picea abies [L.] Karst.) trees cultivated inside the glass domes with ambient (AC, 375 μmol(CO₂) mol^?1) and elevated (EC, A + 375 μmol(CO₂) mol^?1) atmospheric CO₂ concentrations ([CO₂]). After 8 years of fumigation, a mean EC tree in comparison with AC one exhibited about 37 % higher belowground biomass. The growth of primary root structure was unaffected by elevated [CO₂]; however, the biomass of secondary roots growing on the primary root structure and the biomass of secondary roots growing in the zone between the soil surface and the first primary root ramification were significantly higher in EC comparing with AC treatment about 58 and 70 %, respectively. The finest root’s (diameter up to 1 mm) biomass as well as length and surface area of both primary and secondary root structures showed the highest difference between the treatments; advancing EC to AC by 43 % on average. Therefore, Norway spruce trees cultivated under well-watered and rather nitrogen-poor soil conditions responded to the air elevated [CO₂] environment by the enhancement of the secondary root structure increment, by enlargement of root length and root absorbing area, and also by alternation of root to aboveground organ biomass proportion. Higher root to leaf and root to stem basal area ratios could be beneficial for Norway spruce trees to survive periods with limited soil water availability.     

Root distribution of poplar at varying densities on pastoral hill country

Spaced poplar (Populus spp.) trees are used widely in New Zealand for soil conservation on erodible pastoral hill country. Their root distribution in this environment, and factors that affect it, are poorly understood. Robust recommendations on effective tree spacing depend on knowledge of root systems. This study determined the effect of tree density, position between trees, and soil depth (0–90 cm) on root number, root diameter distribution, root area ratio (RAR), and cross sectional area per root for young trees on slopes. Data were collected for lateral roots using trenches. Greater than 80% of roots were < 5 mm diameter and root attributes were highest in shallow soil. Trees at 770 stems per hectare (sph) had 3–12 times more roots and 3–9 times greater RAR than those at densities of ≤ 237 sph, representative of most tree-pasture systems. Mean cross sectional area per root was similar across densities. Positions close to trees had twice as many roots (46 vs. 23/m²) and RAR (109 vs. 52 mm²/m²) as positions midway between trees. The study provided quantitative understanding of variation in root distribution with tree density and information useful for supporting and strengthening recommendations on densities for effective erosion control.     

Sieve size effects on root length and biomass measurements of maize (Zea mays) and Grevillea robusta

The purpose of this study was to investigate the effects of different mesh sizes on the recovery of root length and biomass and to determine whether the degree of recovery was influenced by plant species and sample location. Sieves of 2.0, 1.0, 0.5 and 0.25 mm (4.0, 1.0, 0.25 and 0.06 mm2) mesh sizes were used to recover and measure the root length and biomass of Zea mays L. (maize) at 0–15 cm and 30–45 cm depths and of Grevillea robusta A. Cunn. ex R. Br. (grevillea) at the same depths 1.0 m and 4.5 m from a line of grevillea trees. At 0–15 cm, the coarser sieves (sum collected with 2.0 and 1.0 mm sieves) recovered approximately 80% of the total root biomass measured, but only 60% of the root length. The proportion of total maize root length and biomass recovered by the coarser sieves decreased with soil depth. The proportion of total grevillea root length recovered by the coarser sieves was similar at the two soil depths, but increased slightly with distance from the tree line. The ≥ 0.5 mm sieves recovered between 93 and 96% of grevillea and maize root biomass and between 73 and 98% of their root length, depending on the sample location. Roots passing through the 0.5 mm sieve, but recovered by the 0.25 mm sieve were about 20% of total maize root length and grevillea root length at 1.0 m from the tree line but < 5% of the total grevillea root length at 4.5 m from the tree. Roots passing through the 0.5 mm sieve but recovered by the 0.25 mm sieve contributed only slightly to root biomass. Although the ≥ 0.5 mm sieves provided adequate measurements of root biomass, the ≥ 0.25 mm sieves were required for accurate measurement of fine root length. There was no universal correction for root length and biomass underestimation when large sieve sizes were used because the proportions of length and biomass recovered depended on the plant species and on soil depth and distance from the plant. This revised version was published online in August 2006 with corrections to the Cover Date.     

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。