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。     

Evaluating minirhizotron estimates of fine root longevity and production in the forest floor of a temperate broadleaf forest

The minirhizotron technique (MR) for in situ measurement of fine root dynamics offers the opportunity to obtain accurate and unbiased estimates of root production in perennial vegetation only if MR tubes do not affect the longevity of fine roots. Assuming fine root biomass is near steady-state, fine root production (g m^–2 yr^–1) can be estimated as the ratio of fine root biomass (g m^–2) to median fine root longevity (yr). This study evaluates the critical question of whether MR access tubes affect the longevity of fine roots, by comparing fine root survivorship obtained using MR with those from a non-intrusive in situ screen method in the forest floor horizons of a northern hardwood forest in New Hampshire, USA. Fine root survivorship was measured in 380 root screens during 1993–1997 and in six horizontal minirhizotron tubes during 1996–1997. No statistically significant difference was found between estimates of survivorship of fine roots (<1 mm dia.) at this site from MR versus from in situ screens, suggesting that MR tubes do not substantially affect fine root longevity in the forest floor of this northern hardwood forest and providing greater confidence in measurements of fine root production using the MR technique. Furthermore, the methodology for estimating fine root production from MR longevity data was evaluated by comparison of fine root longevity and production estimates made using single vs. multiple root cohorts, and using root-number, root-length, and root-mass weighted methods. Our results indicate that fine root-length longevity estimates based on multiple root cohorts throughout the year can be used to approximate fine root biomass production. Using this method, we estimated fine root longevity and production in the forest floor at this site to be 314 days (or 0.86 yr) and 303 g m^–2 yr^–1, respectively. Fine root production in this northern hardwood forest is approximately equivalent to standing biomass and was previously underestimated by root in-growth cores. We conclude that the use of MR to estimate fine root longevity and production as outlined here may result in improved estimates of fine root production in perennial vegetation.     

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。     

水曲柳和落叶松细根形态及母根与子根比例关系

细根（直径〈2mm）的分枝是根系重要的结构特征,不同根序等级的细根在养分和水分吸收、C的消耗和寿命方面具有较大的差异,定量研究各根序等级之间的比例关系对认识细根死亡的顺序具有重要的理论意义。根据Pregitzer等2002年提供的方法,研究了17年生水曲柳（Fraxinus mandshurica Rupr．）和落叶松（Larix gmelinii Rupr．）人工纯林1-5级细根的直径、长度、比根长、生物量和数量。结果表明,两树种细根中1级根序的数量占总根系数量80％-90％,它们直径小、长度短、比根长高。随着根序等级（1级-5级）的增加细根直径增粗和长度增加、比根长减小。细根的数量和生物量在上下土层的分布受土壤资源有效性的影响。水曲柳5级根序-2级根序之间母根与子根的数量关系是1：3,落叶松是1：2-3。2级根序与1级根序之间母根与子根的数量关系,水曲柳是1：10—12,落叶松是1：8。如果当年生长的1级细根当中保持1：3的比例,将有65％-75％的1级细根死亡,占根系总数的55％～65％,总长度的40％-50％,以及总生物量的20％-30％。     

Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study

Efforts to characterize carbon (C) cycling among atmosphere, forest canopy, and soil C pools are hindered by poorly quantified fine root dynamics. We characterized the influence of free‐air‐CO₂‐enrichment (ambient +200 ppm) on fine roots for a period of 6 years (Autumn 1998 through Autumn 2004) in an 18‐year‐old loblolly pine (Pinus taeda) plantation near Durham, NC, USA using minirhizotrons. Root production and mortality were synchronous processes that peaked most years during spring and early summer. Seasonality of fine root production and mortality was not influenced by atmospheric CO₂ availability. Averaged over all 6 years of the study, CO₂ enrichment increased average fine root standing crop (+23%), annual root length production (+25%), and annual root length mortality (+36%). Larger increase in mortality compared with production with CO₂ enrichment is explained by shorter average fine root lifespans in elevated plots (500 days) compared with controls (574 days). The effects of CO₂‐enrichment on fine root proliferation tended to shift from shallow (0–15 cm) to deeper soil depths (15–30) with increasing duration of the study. Diameters of fine roots were initially increased by CO₂‐enrichment but this effect diminished over time. Averaged over 6 years, annual fine root NPP was estimated to be 163 g dw m^?2 yr^?1 in CO₂‐enriched plots and 130 g dw m^?2 yr^?1 in control plots (P= 0.13) corresponding to an average annual additional input of fine root biomass to soil of 33 g m^?2 yr^?1 in CO₂‐enriched plots. A lack of consistent CO₂× year effects suggest that the positive effects of CO₂ enrichment on fine root growth persisted 6 years following minirhizotron tube installation (8 years following initiation of the CO₂ fumigation). Although CO₂‐enrichment contributed to extra flow of C into soil in this experiment, the magnitude of the effect was small suggesting only modest potential for fine root processes to directly contribute to soil C storage in south‐eastern pine forests.     

细柄阿丁枫和米槠细根寿命影响因素

采用微根管技术对福建建瓯万木林自然保护区细柄阿丁枫(ALG)和米槠(CAC)细根进行了连续2 a的观测。估计细根寿命采用Kaplan-Meier方法,用对数秩检验(log-rank test)比较单一因素(细根直径、序级、出生年份、出生季节、土层以及邻近细根数量)对细根寿命的影响。同时采用Cox比例风险回归分析方法,分析上述因素对细根存活的影响程度。结果表明:细柄阿丁枫细根平均寿命和中值寿命分别为(286±8)d和(184±9)d,而米槠的则分别为(261±10)d和(212±8)d。仅考虑单一因素时,出生季节、径级、序级以及邻近细根数量对细柄阿丁枫和米槠细根寿命皆有极显著影响(P<0.01);出生年份对米槠细根寿命有极显著影响(P<0.01),但对细柄阿丁枫细根寿命的影响无统计学意义(P>0.05);土层深度对细柄阿丁枫细根寿命有极显著影响(P<0.01),而对米槠细根寿命的影响无统计学意义(P>0.05)。Cox比例风险回归分析则表明出生年份对二者细根寿命的影响皆无统计学意义(P>0.05),影响因素按照影响程度大小排列均依次是序级、出生季节、细根直径、邻近细根数量,而土层对细柄阿丁枫细根寿命的影响最弱,对米槠细根寿命的影响无统计学意义(P>0.05)。     

Uncertainties in interpretation of isotope signals for estimation of fine root longevity: theoretical considerations

This paper examines uncertainties in the interpretation of isotope signals when estimating fine root longevity, particularly in forests. The isotope signals are depleted δ¹³C values from elevated CO₂ experiments and enriched Δ¹⁴C values from bomb ¹⁴C in atmospheric CO₂. For the CO₂ experiments, I explored the effects of six root mortality patterns (on–off, proportional, constant, normal, left skew, and right skew distributions), five levels of nonstructural carbohydrate (NSC) reserves, and increased root growth on root δ¹³C values after CO₂ fumigation. My analysis indicates that fitting a linear equation to δ¹³C data provides unbiased estimates of longevity only if root mortality follows an on–off model, without dilution of isotope signals by pretreatment NSC reserves, and under a steady state between growth and death. If root mortality follows the other patterns, the linear extrapolation considerably overestimates root longevity. In contrast, fitting an exponential equation to δ¹³C data underestimates longevity with all the mortality patterns except the proportional one. With either linear or exponential extrapolation, dilution of isotope signals by pretreatment NSC reserves could result in overestimation of root longevity by several‐fold. Root longevity is underestimated if elevated CO₂ stimulates fine root growth. For the bomb ¹⁴C approach, I examined the effects of four mortality patterns (on–off, proportional, constant, and normal distribution) on root Δ¹⁴C values. For a given Δ¹⁴C value, the proportional pattern usually provides a shorter estimate of root longevity than the other patterns. Overall, we have to improve our understanding of root growth and mortality patterns and to measure NSC reserves in order to reduce uncertainties in estimated fine root longevity from isotope data.     

Leaf and fine root carbon stocks and turnover are coupled across Arctic ecosystems

Estimates of vegetation carbon pools and their turnover rates are central to understanding and modelling ecosystem responses to climate change and their feedbacks to climate. In the Arctic, a region containing globally important stores of soil carbon, and where the most rapid climate change is expected over the coming century, plant communities have on average sixfold more biomass below ground than above ground, but knowledge of the root carbon pool sizes and turnover rates is limited. Here, we show that across eight plant communities, there is a significant positive relationship between leaf and fine root turnover rates (r² = 0.68, P < 0.05), and that the turnover rates of both leaf (r² = 0.63, P < 0.05) and fine root (r² = 0.55, P < 0.05) pools are strongly correlated with leaf area index (LAI, leaf area per unit ground area). This coupling of root and leaf dynamics supports the theory of a whole‐plant economics spectrum. We also show that the size of the fine root carbon pool initially increases linearly with increasing LAI, and then levels off at LAI = 1 m² m^?2, suggesting a functional balance between investment in leaves and fine roots at the whole community scale. These ecological relationships not only demonstrate close links between above and below‐ground plant carbon dynamics but also allow plant carbon pool sizes and their turnover rates to be predicted from the single readily quantifiable (and remotely sensed) parameter of LAI, including the possibility of estimating root data from satellites.     

三峡库区马尾松细根生产和周转及其影响因子

采用连续根钻法、分解袋法、分室通量模型法计算三峡库区马尾松细根的年生产量和周转率,分析细根生产量和周转率与各影响因子的关系.结果表明: 马尾松<0.5、0.5～1和1～2 mm细根年均生物量分别为0.29、0.59、0.76 t·hm^-2,细根年生产量分别为0.13、0.49、0.37 t·hm^-2,细根年周转率分别为1.49、1.01、0.40 a^-1.各影响因子对不同径级细根生产与周转的影响不同.土壤温度、土壤钙含量显著影响<0.5 mm细根生产量与细根周转,且土壤温度解释生产量和周转率32.8%和25.0%的变异,土壤钙含量解释65.6%和73.1%的变异;细根生物量与细根生产量呈显著正相关,细根生物量分别解释<0.5、0.5～1和1～2 mm细根生产量41.0%、41.1%和54.5%的变异;细根P、K含量与<0.5 mm细根生产量具有显著相关性,分别解释<0.5 mm细根生产量32.2%、39.2%的变异.<0.5 mm细根与各影响因子的关系最为密切,土壤温度、土壤钙含量是细根生物量的主要影响因子.     

Fine root growth phenology,production, and turnover in a northern hardwood forest ecosystem

A large part of the nutrient flux in deciduous forests is through fine root turnover, yet this process is seldom measured. As part of a nutrient cycling study, fine root dynamics were studied for two years at Huntington Forest in the Adirondack Mountain region of New York, USA. Root growth phenology was characterized using field rhizotrons, three methods were used to estimate fine root production, two methods were used to estimate fine root mortality, and decomposition was estimated using the buried bag technique. During both 1986 and 1987, fine root elongation began in early April, peaked during July and August, and nearly ceased by mid-October. Mean fine root ( 3 mm diameter) biomass in the surface 28-cm was 2.5 t ha^–1 and necromass was 2.9 t ha^–1. Annual decomposition rates ranged from 17 to 30% beneath the litter and 27 to 52% at a depth of 10 cm. Depending on the method used for estimation, fine root production ranged from 2.0 to 2.9 t ha^–1, mortality ranged from 1.8 to 3.7 t ha^–1 yr^–1, and decomposition was 0.9 t ha^–1 yr^–1. Thus, turnover ranged from 0.8 to 1.2 yr^–1. The nutrients that cycled through fine roots annually were 4.5–6.1 kg Ca, 1.1–1.4 kg Mg, 0.3–0.4 kg K, 1.2–1.7 kg P, 20.3–27.3 kg N, and 1.8–2.4 kg S ha^–1. Fine root turnover was less important than leaf litterfall in the cycling of Ca and Mg and was similar to leaf litterfall in the amount of N, P, K and S cycled.     

扁刺栲不同根序细根形态和化学特征及其对短期氮添加的响应

研究川西亚热带次生常绿阔叶林优势树种扁刺栲1～5级细根形态和化学特征,及其对氮添加的响应.结果表明: 随根序等级的增加,扁刺栲根直径、根组织密度、K含量增加,而比根长、比表面积及N、P、Mg含量降低.氮添加显著增加了扁刺栲细根N含量,降低了Mg含量和C/N,使细根Ca含量呈下降趋势,对根序C、P、K、Na、Al、Mn、Fe含量无显著影响.氮添加未显著影响扁刺栲细根直径、比根长、比表面积和根组织密度.在所有处理中,细根P含量均与各形态特征呈显著线性回归关系.氮添加处理下,细根Mg含量与形态特征之间的线性关系由不显著变为显著,而细根N含量与形态特征之间的线性关系由显著变为不显著.氮添加会影响根系营养元素含量,并增强植物对P和Mg的需求.     

Effects of elevated atmospheric CO2, cutting frequency, and differential day/night atmospheric warming on root growth and turnover of Phalaris swards

We investigated seasonal root production and root turnover of fertilized and well‐watered monocultures of Phalaris for 2 years using minirhizotrons installed in six newly designed temperature gradient tunnels, combined with sequential soil coring. Elevated atmospheric CO₂ treatments were combined with two cutting frequencies and three warming scenarios: no warming, +3.0/+3.0 and +2.2/+4.0°C (day/night) atmospheric warming. The elevated CO₂ treatment increased both new and net root length production primarily when combined with atmospheric warming, where the constant warming treatment had a greater positive effect than the increased night‐time warming treatment. Responses to elevated CO₂ were greater when the swards were cut more frequently and responsiveness varied with season. For Phalaris swards, 17% of total net primary productivity went belowground. On account of root turnover, only one‐third of the new roots produced in the year following establishment could be expected, on average, to be recovered from soil cores. The interaction between the effects of CO₂ and warming, combined with the differential effects of the two warming treatments, has important implications for modelling belowground responses to projected climate change.     

三工河流域两种琵琶柴群落细根生物量、分解与周转

细根对植物群落功能的发挥和土壤碳库及全球碳循环具有重要意义。利用连续土钻取样法和分解袋法,于2010年5—10月整个生长季节内,对三工河流域两处长势不同的琵琶柴群落的细根(φ2mm)生物量、分解与周转规律及其与土壤环境的关系进行研究。结果表明,群落1和群落2土壤容重、土壤含水量、pH和电导率等土壤因子差异显著。两群落的细根生物量表现出相同的季节和垂直变化趋势,即在5—8月逐渐增加,8月达到最大值,9—10月份逐渐下降。平均月细根生物量分别为51.55g/m2和133.93 g/m2。群落1的活细根和死细根分别占总细根生物量的69.68%和30.32%,群落2活细根和死细根分别占总细根生物量的72.61%和27.39%。在垂直变化上,随土壤深度增加细根生物量先增加后逐渐降低,其中10—20cm土壤层次细根生物量比例最大,群落1和群落2分别占46.48%和29.15%。群落1和群落2的细根年分解率分别为34.82%、42.91%。达到半分解和95%分解时,群落1需要630 d和2933 d,群落2需要467 d和2238 d。群落1和群落2的细根净生产力分别为50.67 g/m2和178.15 g/m2,细根年周转率分别为1.41次、1.69次。逐步回归分析结果显示细根动态受土壤水分、pH值、电导度等土壤因子的显著影响,琵琶柴细根具有相对较低的分解速率和较高的周转速率。     

Fine root production and nutrient content in wet and moist arctic tundras as influenced by chronic fertilization

We used ingrowth cores to estimate fine root production in organic soils of wet sedge and moist tundra ecosystems near Toolik Lake on Alaska's North Slope. Root-free soil cores contained in nylon mesh tubes (5 cm diameter, 20–30 cm long) were placed in control and chronically fertilized (N plus P) plots in mid-August 1994 and were retrieved 1 year later. Estimated fine root production in control plots was 75 g m^–2 year^–1 in wet sedge and 56 g m^–2 year^–1 in moist tussock tundra. Fine root production in fertilized plots was 85 g m^–2 year^–1 in wet sedge and 67 g m^–2 year^–1 in moist tussock tundra. Although our estimates of fine root production were higher on fertilized than control plots, differences were not statistically significant within either tundra type. Comparisons between our estimates of fine root production and other estimates of aboveground (plus rhizome) production on the same (wet sedge tundra) or similar (moist tussock tundra) plots suggest that fine root production was about one-third of total net primary production (NPP) under non-fertilized conditions and about one-fifth of total NPP under chronic fertilization. Fine root N and P concentrations increased with fertilization in both tundra types, but P concentrations increased more than N concentrations in wet sedge tundra, whereas relative increases in N and P concentrations in moist tundra roots were similar. These data are consistent with other studies suggesting that NPP in wet sedge tundra is often P limited and that co-limitation by N and P is more important in moist tussock tundra.     

Fine Root Production and Turnover in a Norway Spruce Stand in Northern Sweden: Effects of Nitrogen and Water Manipulation

Fine root length production, biomass production, and turnover in forest floor and mineral soil (0–30 cm) layers were studied in relation to irrigated (I) and irrigated-fertilized (IL) treatments in a Norway spruce stand in northern Sweden over a 2-year period. Fine roots (<1 mm) of both spruce and understory vegetation were studied. Minirhizotrons were used to estimate fine root length production and turnover, and soil cores were used to estimate standing biomass. Turnover was estimated as both the inverse of root longevity (RTL) and the ratio of annual root length production to observed root length (RTR). RTR values of spruce roots in the forest floor in I and IL plots were 0.6 and 0.5 y⁻¹, respectively, whereas the corresponding values for RTL were 0.8 and 0.9 y⁻¹. In mineral soil, corresponding values for I, IL, and control (C) plots were 1.2, 1.2, and 0.9 y⁻¹ (RTR) and 0.9, 1.1, and 1 y⁻¹ (RTL). RTR and RTL values of understory vegetation roots were 1 and 1.1 y⁻¹, respectively. Spruce root length production in both the forest floor and the mineral soil in I plots was higher than in IL plots. The IL-treated plots gave the highest estimates of spruce fine root biomass production in the forest floor, but, for the mineral soil, the estimates obtained for the I plots were the highest. The understory vegetation fine root production in the I and IL plots was similar for both the forest floor and the mineral soil and higher (for both layers) than in C plots. Nitrogen (N) turnover in the forest floor and mineral soil layers (summed) via spruce roots in IL, I, and C plots amounted to 2.4, 2.1, and 1.3 g N m⁻² y⁻¹, and the corresponding values for field vegetation roots were 0.6, 0.5, and 0.3 g N m⁻² y⁻¹. It was concluded that fertilization increases standing root biomass, root production, and N turnover of spruce roots in both the forest floor and mineral soil. Data on understory vegetation roots are required for estimating carbon budgets in model studies.     

干旱区琵琶柴群落细根周转对土壤有机碳循环的贡献

尽管干旱区生态系统的脆弱性受到了广泛的关注, 但目前关于干旱区植物细根有机碳与土壤碳循环关系的研究还比较少见。在2010年整个生长季节内, 采用土钻法和内生长法, 对新疆干旱区的琵琶柴(Reaumuria soongorica)群落土壤特性、细根的生物量月动态、生产量和周转进行了研究。结果表明: 琵琶柴群落表层土壤含水量最低, 土壤含水量表现出从浅层到深层逐渐增加的趋势; 而表层土壤的有机碳含量最高, 随着土壤深度的加深, 有机碳含量逐渐降低。细根生物量的月平均值为54.51 g·m^-2, 群落细根生产量在82.76-136.21 g·m^-2·a^-1之间, 琵琶柴群落的细根周转率为2.08 times·a^-1, 通过细根死亡进入土壤中的有机碳为17 g·m^-2·a^-1。这些结果表明: 由于灌丛细根高的周转速率, 细根是干旱区土壤有机碳输入的重要部分。     

拉萨河谷杨树人工林细根的生产力及其周转

通过土钻取样和分解袋法对拉萨河谷杨树人工林细根的生长和周转进行了测定.结果表明,在该地区杨树人工林生态系统中,约80%的细根集中分布在0～30cm土壤表层中;接近树木一侧的活(死)细根生物量均高于外侧,但二者未达到显著的差异;在生长季期间,活细根生物量平均为2.576 t · hm-2,死细根生物量平均为1.566 t · hm-2,生长高峰出现在生长季初期.经估算,拉萨河谷杨树人工林细根年生长量为3.030 t · hm-2,年周转率为1.18次;但受高原低温的影响,细根分解缓慢,分解系数k平均为0.0007～0.0008.细根的这种生长特征是杨树对高原地区短暂生长季节和雨热同季气候条件的一种适应性表现.