Root growth dynamics linked to above-ground growth in walnut (Juglans regia)

Background and Aims Examination of plant growth below ground is relatively scant compared with that above ground, and is needed to understand whole-plant responses to the environment. This study examines whether the seasonal timing of fine root growth and the spatial distribution of this growth through the soil profile varies in response to canopy manipulation and soil temperature.Methods Plasticity in the seasonal timing and vertical distribution of root production in response to canopy and soil water manipulation was analysed in field-grown walnut (Juglans regia ‘Chandler’) using minirhizotron techniques.Key Results Root production in walnuts followed a unimodal curve, with one marked flush of root growth starting in mid-May, with a peak in mid-June. Root production declined later in the season, corresponding to increased soil temperature, as well as to the period of major carbohydrate allocation to reproduction. Canopy and soil moisture manipulation did not influence the timing of root production, but did influence the vertical distribution of roots through the soil profile. Water deficit appeared to promote root production in deeper soil layers for mining soil water. Canopy removal appeared to promote shallow root production.Conclusions The findings of this study add to growing evidence that root growth in many ecosystems follows a unimodal curve with one marked flush of root growth in coordination with the initial leaf flush of the season. Root vertical distribution appeared to have greater plasticity than timing of root production in this system, with temperature and/or carbohydrate competition constraining the timing of root growth. Effects on root distribution can have serious impacts on trees, with shallow rooting having negative impacts in years with limited soil water or positive impacts in years with wet springs, and deep rooting having positive impacts on soil water mining from deeper soil layers but negative impacts in years with wet springs.     

Relationships between fine root dynamics and nitrogen availability in Michigan northern hardwood forests

Minirhizotrons were used to observe fine root (Б mm) production, mortality, and longevity over 2 years in four sugar-maple-dominated northern hardwood forests located along a latitudinal temperature gradient. The sites also differed in N availability, allowing us to assess the relative influences of soil temperature and N availability in controlling fine root lifespans. Root production and mortality occurred throughout the year, with most production occurring in the early portion of the growing season (by mid-July). Mortality was distributed much more evenly throughout the year. For surface fine roots (0-10 cm deep), significant differences in root longevity existed among the sites, with median root lifespans for root cohorts produced in 1994 ranging from 405 to 540 days. Estimates of fine root turnover, based on the average of annual root production and mortality as a proportion of standing crop, ranged from 0.50 to 0.68 year^-1 for roots in the upper 30 cm of soil. The patterns across sites in root longevity and turnover did not follow the north to south temperature gradient, but rather corresponded to site differences in N availability, with longer average root lifespans and lower root turnover occurring where N availability was greater. This suggests the possibility that roots are maintained as long as the benefit (nutrients) they provide outweighs the C cost of keeping them alive. Root N concentrations and respiration rates (at a given temperature) were also higher at sites where N availability was greater. It is proposed that greater metabolic activity for roots in nitrogen-rich zones leads to greater carbohydrate allocation to those roots, and that a reduction in root C sink strength when local nutrients are depleted provides a mechanism through which root lifespan is regulated in these forests.     

林木细根寿命及其影响因子研究进展

 细根周转要消耗大量的C,它影响森林生态系统C分配格局与过程和养分循环,对生态系统生产力具有重要意义。细根的周转取决于细根的寿命,细根寿命越短,周转越快,根系对C的消耗也越多。大量研究表明,细根的寿命与地上部分C向根系供应的多少有密切关系,同时也与细根直径大小、土壤中N和水分的有效性、土壤温度以及根际周围的土壤动物和微生物的活动有关。本文综述了国外近年来在该领域里的研究进展,特别是对控制细根寿命的机理和主要影响因子进行了评述,目的是引起国内研究者的关注,促进我国根系生态学的研究与发展。     

Seasonal dynamics of fine root biomass,root length density,specific root length,and soil resource availability in a Larix gmelinii plantation

Fine root tumover is a major pathway for carbon and nutrient cycling in terrestrial ecosystems and is most likely sensitive to many global change factors.Despite the importance of fine root turnover in plant C allocation and nutrient cycling dynamics and the tremendous research efforts in the past,our understanding of it remains limited.This is because the dynamics processes associated with soil resources availability are still poorly understood.Soil moisture,temperature,and available nitrogen are the most important soil characteristics that impact fine root growth and mortality at both the individual root branch and at the ecosystem level.In temperate forest ecosystems,seasonal changes of soil resource availability will alter the pattern of carbon allocation to belowground.Therefore,fine root biomass,root length density(RLD)and specific root length(SRL)vary during the growing season.Studying seasonal changes of fine root biomass,RLD,and SRL associated with soil resource availability will help us understand the mechanistic controls of carbon to fine root longevity and turnover.The objective of this study was to understand whether seasonal variations of fine root biomass,RLD and SRL were associated with soil resource availability,such as moisture,temperature,and nitrogen,and to understand how these soil components impact fine root dynamics in Larix gmelinii plantation.We used a soil coring method to obtain fine root samples(≤2 mm in diameter)every month from Mav to October in 2002 from a 17-year-old L.gmelinii plantation in Maoershan Experiment Station,Northeast Forestry University,China.Seventy-two soil cores(inside diameter 60 mm;depth intervals:0-10 cm,10-20 cm,20-30 cm)were sampled randomly from three replicates 25 m×30 m plots to estimate fine root biomass(live and dead),and calculate RLD and SRL.Soil moisture,temperature,and nitrogen(ammonia and nitrates)at three depth intervals were also analyzed in these plots.Results showed that the average standing fine root biomass(live (32.2 g.m-2.a-1)in the middle(10-20 cm)and deep layer (20-30cm),respectively.Live and dead fine root biomass was the highest from May to July and in September,but lower in August and October.The live fine root biomass decreased and dead biomass increased during the growing soil layer.RLD and SRL in May were the highestthe other months,and RLD was the lowest in Septemberdynamics of fine root biomass,RLD,and SRL showed a close relationship with changes in soil moisture,temperature,and nitrogen availability.To a lesser extent,the temperature could be determined by regression analysis.Fine roots in the upper soil layer have a function of absorbing moisture and nutrients,while the main function of deeper soil may be moisture uptake rather than nutrient acquisition.Therefore,carbon allocation to roots in the upper soil layer and deeper soil layer was different.Multiple regression analysis showed that variation in soil resource availability could explain 71-73% of the seasonal variation of RLD and SRL and 58% of the variation in fine root biomass.These results suggested a greater metabolic activity of fine roots living in soil with higher resource availability,which resulted in an increased allocation of carbohydrate to these roots,but a lower allocation of carbohydrate to those in soil with lower resource availability.     

Interactive effects of soil temperature and moisture on Concord grape root respiration

Root respiration has important implications for understanding plant growth as well as terrestrial carbon flux with a changing climate. Although soil temperature and soil moisture often interact, rarely have these interactions on root respiration been studied. This report is on the individual and combined effects of soil moisture and temperature on respiratory responses of single branch roots of 1-year-old Concord grape (Vitis labruscana Bailey) vines grown in a greenhouse. Under moist soil conditions, root respiration increased exponentially to short-term (1 h) increases in temperature between 10 degrees C and 33 degrees C. Negligible increases in root respiration occurred between 33 degrees C and 38 degrees C. By contrast to a slowly decreasing Q10 from short-term temperature increases, when roots were exposed to constant temperatures for 3 d, the respiratory Q10 between 10 degrees C and 30 degrees C diminished steeply with an increase in temperature. Above 30 degrees C, respiration declined with an increase in temperature. Membrane leakage was 89-98% higher and nitrogen concentration was about 18% lower for roots exposed to 35 degrees C for 3 d than for those exposed to 25 degrees C and 15 degrees C. There was a strong interaction of respiration with a combination of elevated temperature and soil drying. At low soil temperatures (10 degrees C), respiration was little influenced by soil drying, while at moderate to high temperatures (20 degrees C and 30 degrees C), respiration exhibited rapid declines with decreases in soil moisture. Roots exposed to drying soil also exhibited increased membrane leakage and reduced N. These findings of acclimation of root respiration are important to modelling respiration under different moisture and temperature regimes.     

落叶松人工林细根动态与土壤资源有效性关系研究

树木细根在森林生态系统C和养分循环中具有重要的作用。由于温带土壤资源有效性具有明显的季节变化, 导致细根生物量、根长密度 (Rootlengthdensity, RLD) 和比根长 (Specificrootlength, SRL) 的季节性变化。以 17年生落叶松 (Larixgmelini) 人工林为研究对象, 采用根钻法从 5月到 10月连续取样, 研究了不同土层细根 (直径≤ 2mm) 生物量、RLD和SRL的季节动态, 以及这些根系指标动态与土壤水分、温度和N有效性的关系。结果表明 :1) 落叶松细根年平均生物量 (活根 +死根 ) 为 189.1g·m-2 ·a-1, 其中 5 0 %分布在表层 (0~ 10cm), 33%分布在亚表层 (11~ 2 0cm), 17%分布在底层 (2 1~ 30cm) 。活根和死根生物量在 5~ 7月以及 9月较高, 8月和 10月较低。从春季 (5月 ) 到秋季 (10月 ), 随着活细根生物量的减少, 死细根生物量增加 ;2 ) 土壤表层 (0~ 10cm) 具有较高的RLD和SRL, 而底层 (2 1~ 30cm) 最低。春季 (5月 ) 总RLD和SRL最高, 分别为 10 6 2 1.4 5m·m-3 和 14.83m·g-1, 到秋季 (9月 ) 树木生长结束后达到最低值, 分别为 2 198.2 0m·m-3 和 3.77m·g-1;3) 细根生物量、RLD和SRL与土壤水分、温度和有效N存在不同程度的相关性。从单因子分析来看, 土壤水分和有效N对细根的影响明显大于温度, 对活根的影响大于死根。由于土壤资源有效性的季节变化, 使得C的地下分配格局发生改变。各土层细根与有效性资源之间的相关性反映了细根功能季节性差异。细根 (生物量、RLD和SRL) 的季节动态 (5 8%~ 73%的变异 ) 主要由土壤资源有效性的季节变化引起。     

Environmental control of fine root dynamics in a northern hardwood forest

Understanding how exogenous and endogenous factors control the distribution, production and mortality of fine roots is fundamental to assessing the implications of global change, yet our knowledge of control over fine root dynamics remains rudimentary. To improve understanding of these processes, the present study developed regression relationships between environmental variables and fine root dynamics within a northern hardwood forest in New Hampshire, USA, which was experimentally manipulated with a snow removal treatment. Fine roots (< 1 mm diameter) were observed using minirhizotrons for 2 years in sugar maple and yellow birch stands and analyzed in relation to temperature, water and nutrient availability. Fine root dynamics at this site fluctuated seasonally, with growth and mortality peaking during warmer months. Monthly fine root production was strongly associated with mean monthly air temperature and neither soil moisture nor nutrient availability added additional predictive power to this relationship. This relationship exhibited a seasonal temperature hysteresis, which was altered by snow removal treatment. These results suggest that both exogenous and endogenous cues may be important in controlling fine root growth in this system. Proportional fine root mortality was directly associated with mean monthly soil temperature, and proportional fine root mortality during the over‐winter interval was strongly related to whether the soil froze. The strong relationship between fine root production and air temperature reported herein contrasts with findings from some hardwood forest sites and indicates that controls on fine root dynamics vary geographically. Future research must more clearly distinguish between endogenous and exogenous control over fine root dynamics in various ecosystems.     

Root respiration and its relation to nutrient contents in soil and root and EVI among 8 ecosystems, northern China

Root respiration is a critical and uncertain component of ecosystem carbon budgets. We assessed whether variation in root respiration at a reference temperature were associated with that of root and soil nutrient contents and enhanced vegetation index (EVI) among 8 ecosystems, including three forests, two shrublands, two meadow grasslands, and one meadow in a forest-steppe ecotone in northern China. Mass specific root respiration was positively related to root nitrogen content when data from all the different classes of root diameter were pooled. Area specific root respiration increased with soil available nitrogen through increasing fine root biomass. Beside the effect of soil or root nutrient contents, mass specific respiration of fine roots followed EVI patterns, suggesting that root respiration was strongly dependent on photosynthesis. A simple respiration–EVI relationship may provide a convenient way to estimate root respiration and has potential applications in quantitative spatial extrapolation.     

林木根呼吸及测定方法进展

 森林土壤呼吸的近2/3是由林木根呼吸产生的,林木根呼吸对估计森林C吸存及构建森林生态系统碳动态模型有重要意义,是全球碳循环研究的一个重要组成部分。林木根呼吸包括生长呼吸和维持呼吸,不同森林生态系统林木根呼吸对土壤呼吸的贡献大多在40%～60%范围内,林木根呼吸在生长季节较高而休眠季节较低。测定林木根呼吸的主要方法有排除根法、离体根法、同位素法和原位PVC管气室法,前两者相对简单、成本低,常用于森林生态系统中;同位素法可原位测定根呼吸,对土壤干扰较小,但不易操作,且成本高。根呼吸受土壤温度、根直径大小、根组织N浓度、环境CO2浓度、土壤湿度、养分有效性等因素的影响。今后的研究应集中在以下方面：1）探讨和比较不同条件下测定根呼吸组成（生长呼吸、维持呼吸）的最合适方法;2）加大在野外条件下使用有效方法分离根呼吸和根际微生物呼吸的力度;3）对森林生态系统根呼吸动态进行长期的定位研究;4）进一步加强研究不同气候带,不同森林类型林木根呼吸,并将研究尺度从气室扩大到区域或全球水平;5）加强林木根呼吸对全球变化的响应及机制的研究;6)对林木根呼吸进行多学科合作研究将为全球C循环做出新的贡献。     

Root production is determined by radiation flux in a temperate grassland community

Accurate knowledge of the response of root turnover to a changing climate is needed to predict growth and produce carbon cycle models. A soil warming system and shading were used to vary soil temperature and received radiation independently in a temperate grassland dominated by Holcus lanatus L. Minirhizotrons allowed root growth and turnover to be examined non‐destructively. In two short‐term (8 week) experiments, root responses to temperature were seasonally distinct. Root number increased when heating was applied during spring, but root death increased during autumnal heating. An experiment lasting 12 months demonstrated that any positive response to temperature was short‐lived and that over a full growing season, soil warming led to a reduction in root number and mass due to increased root death during autumn and winter. Root respiration was also insensitive to soil temperature over much of the year. In contrast, root growth was strongly affected by incident radiation. Root biomass, length, birth rate, number and turnover were all reduced by shading. Photosynthesis in H. lanatus exhibited some acclimation to shading, but assimilation rates at growth irradiance were still lower in shaded plants. The negative effects of shading and soil warming on roots were additive. Comparison of root data with environmental measurements demonstrated a number of positive relationships with photosynthetically active radiation, but not with soil temperature. This was true both across the entire data set and within a shade treatment. These results demonstrate that root growth is unlikely to be directly affected by increased soil temperatures as a result of global warming, at least in temperate areas, and that predictions of net primary productivity should not be based on a positive root growth response to temperature.     

Annual variation in soil respiration and its components in a coppice oak forest in Central Italy

In order to investigate the annual variation of soil respiration and its components in relation to seasonal changes in soil temperature and soil moisture in a Mediterranean mixed oak forest ecosystem, we set up a series of experimental treatments in May 1999 where litter (no litter), roots (no roots, by trenching) or both were excluded from plots of 4 m². Subsequently, we measured soil respiration, soil temperature and soil moisture in each plot over a year after the forest was coppiced. The treatments did not significantly affect soil temperature or soil moisture measured over 0–10 cm depth. Soil respiration varied markedly during the year with high rates in spring and autumn and low rates in summer, coinciding with summer drought, and in winter, with the lowest temperatures. Very high respiration rates, however, were observed during the summer immediately after rainfall events. The mean annual rate of soil respiration was 2.9 µ mol m^?2 s^?1, ranging from 1.35 to 7.03 µmol m^?2 s^?1. Soil respiration was highly correlated with temperature during winter and during spring and autumn whenever volumetric soil water content was above 20%. Below this threshold value, there was no correlation between soil respiration and soil temperature, but soil moisture was a good predictor of soil respiration. A simple empirical model that predicted soil respiration during the year, using both soil temperature and soil moisture accounted for more than 91% of the observed annual variation in soil respiration. All the components of soil respiration followed a similar seasonal trend and were affected by summer drought. The Q₁₀ value for soil respiration was 2.32, which is in agreement with other studies in forest ecosystems. However, we found a Q₁₀ value for root respiration of 2.20, which is lower than recent values reported for forest sites. The fact that the seasonal variation in root growth with temperature in Mediterranean ecosystems differs from that in temperate regions may explain this difference. In temperate regions, increases in size of root populations during the growing season, coinciding with high temperatures, may yield higher apparent Q₁₀ values than in Mediterranean regions where root growth is suppressed by summer drought. The decomposition of organic matter and belowground litter were the major components of soil respiration, accounting for almost 55% of the total soil respiration flux. This proportion is higher than has been reported for mature boreal and temperate forest and is probably the result of a short‐term C loss following recent logging at the site. The relationship proposed for soil respiration with soil temperature and soil moisture is useful for understanding and predicting potential changes in Mediterranean forest ecosystems in response to forest management and climate change.     

Soil Nematodes in Terrestrial Ecosystems

There has been much work on plant-feeding nematodes, and less on other soil nematodes, their distribution, abundance, intrinsic properties, and interactions with biotic and abiotic factors. Seasonal variation in nematode fauna as a whole is correlated with factors such as moisture, temperature, and plant growth; at each site nematode distribution generally reflects root distribution. There is a positive correlation between average nematode abundance and primary production as controlled by moisture, temperature, nutrients, etc. Soil nematodes, whether bacterial feeders, fungivores, plant feeders, omnivores, or predators, all influence the populations of the organisms they feed on. Although soil trematodes probably contribute less than 1% to soil respiration they may play an important role in nutrient cycling in the soil through their influence on bacterial growth and plant nutrient availability.     

Architectural Tradeoffs between Adventitious and Basal Roots for Phosphorus Acquisition

Adventitious rooting contributes to efficient phosphorus acquisition by enhancing topsoil foraging. However, metabolic investment in adventitious roots may retard the development of other root classes such as basal roots, which are also important for phosphorus acquisition. In this study we quantitatively assessed the potential effects of adventitious rooting on basal root growth and whole plant phosphorus acquisition in young bean plants. The geometric simulation model SimRoot was used to dynamically model root systems with varying architecture and C availability growing for 21 days at 3 planting depths in 3 soil types with contrasting nutrient mobility. Simulated root architectures, tradeoffs between adventitious and basal root growth, and phosphorus acquisition were validated with empirical measurements. Phosphorus acquisition and phosphorus acquisition efficiency (defined as mol phosphorus acquired per mol C allocated to roots) were estimated for plants growing in soil in which phosphorus availability was uniform with depth or was greatest in the topsoil, as occurs in most natural soils. Phosphorus acquisition and acquisition efficiency increased with increasing allocation to adventitious roots in stratified soil, due to increased phosphorus depletion of surface soil. In uniform soil, increased adventitious rooting decreased phosphorus acquisition by reducing the growth of lateral roots arising from the tap root and basal roots. The benefit of adventitious roots for phosphorus acquisition was dependent on the specific respiration rate of adventitious roots as well as on whether overall C allocation to root growth was increased, as occurs in plants under phosphorus stress, or was lower, as observed in unstressed plants. In stratified soil, adventitious rooting reduced the growth of tap and basal lateral roots, yet phosphorus acquisition increased by up to 10% when total C allocation to roots was high and adventitious root respiration was similar to that in basal roots. With C allocation to roots decreased by 38%, adventitious roots still increased phosphorus acquisition by 5%. Allocation to adventitious roots enhanced phosphorus acquisition and efficiency as long as the specific respiration of adventitious roots was similar to that of basal roots and less than twice that of tap roots. When adventitious roots were assigned greater specific respiration rates, increased adventitious rooting reduced phosphorus acquisition and efficiency by diverting carbohydrate from other root types. Varying the phosphorus diffusion coefficient to reflect varying mobilities in different soil types had little effect on the value of adventitious rooting for phosphorus acquisition. Adventitious roots benefited plants regardless of basal root growth angle. Seed planting depth only affected phosphorus uptake and efficiency when seed was planted below the high phosphorus surface stratum. Our results confirm the importance of root respiration in nutrient foraging strategies, and demonstrate functional tradeoffs among distinct components of the root system. These results will be useful in developing ideotypes for more nutrient efficient crops.     

The phenology of fine root growth in a maple-dominated ecosystem: relationships with some soil properties

A two-year study was undertaken in a maple-dominated watershed of southern Québec, Canada, to examine relationships between trends in fine root growth, stem diameter growth, soil moisture, soil temperature, mineralized-N and extractable-P. Until September, soil temperature was consistently higher in 1995 than in 1994. Apart from the first sampling in mid-May, soil moisture was higher in 1994 than in 1995. In 1994, most fine roots were produced before leaf expansion, whereas in 1995, fine root production peaked in July. Annual fine root production was estimated to be 2.7 times higher in 1994 than in 1995. Stem growth was strongly associated with the seasonal and annual variation in soil temperature. Root and diameter growth were asynchronous in 1994 but not in 1995. Fine root production was associated with two groups of variables: a soil fertility (mineralized-N and extractable-P) group and a physical soil environment (moisture and temperature) group. Our results are consistent with the negative effect of high soil-N fertility on fine root production but are inconclusive as to the positive effect of high soil-P fertility. Soil conditions that are detrimental to root growth such as high N availability and anaerobiosis could modify the normal dynamics of fine root growth.     

Clonal variation in above- and below-ground growth responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability

Trembling aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America making it important to terrestrial carbon and nutrient cycles. Due to anthropogenic climate change high latitude temperatures are expected to increase, making it necessary to assess the feedback between above- and below-ground carbon pools to increased temperature at sites of both high and low N-availability. We grew four clones of aspen at two levels of soil temperature and two levels of soil N-availability for 98 days and quantified photosynthesis, growth, biomass allocation, and root length production and mortality. High soil temperature increased rates of photosynthesis (65%), resulting in greater whole-plant growth (37%) through increases in roots, stems, and foliage; however these increases generally occurred only in soil of high N-availability. Root:shoot biomass allocation varied between clones but was unaffected by the soil temperature or N-availability treatments. Root length production and mortality increased at elevated soil temperature, but this response was modified by soil N-availability. At high soil temperature, soil N-availability had little effect on root dynamics, while at low soil temperature, high soil N-availability increased both the production and mortality (turnover) of roots. We conclude that trembling aspen has the potential for substantially greater growth and root turnover under conditions of warmer soil at sites of both high and low N-availability, but that allometric patterns of growth are under strong genetic, rather than environmental control. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root respiration in citrus acclimates to temperature and slows during drought

Citrus seedlings were grown in soil columns in which the root system was hydraulically separated into two equal layers; this enabled us to maintain roots in the upper layer without water for 110 d. The columns were placed into waterbaths modified so that soil temperatures in the top layer could be maintained at 25°C or at 35°C, while temperature in the bottom layer was maintained at 25°C. We hypothesized that, if citrus plants were grown in dry soil for an extended period, root mortality would increase if the cost of maintaining the roots was increased by elevating the soil temperature. However, during the drought period we did not observe any root mortality, even at the higher soil temperature. Moreover, we did not find that root respiration was increased by prolonged exposure to drought and higher soil temperature. We did find that root respiration rates slowed in dry soil. Furthermore, when the soil columns were switched from one temperature treatment to another, root respiration rates in wet soil rapidly increased when moved to a higher temperature or rapidly decreased when moved to a lower temperature. But after only 4 d, respiration rates returned to their original level; root respiration in dry soil was not affected by either short-or long-term shifts in soil temperature. Root respiration in citrus appears to acclimate rapidly to changes in soil temperature.     

东北地区落叶松人工林的根系呼吸

落叶松根系呼吸速率在6～9月期间逐渐升高,8月达到高峰,之后明显下降.幼林根系呼吸速率和根系呼吸占土壤总呼吸的比例均高于成熟林.根系呼吸速率与根生物量呈线性相关,与土温呈指数相关,与土壤含水量无明显相关关系,但温度较高时,土壤湿度的增加能促进根系呼吸.成熟林和幼林根系呼吸的Q10值分别为5.56和4.17.     

Effects of ectomycorrhizal colonization and nitrogen fertilization on morphology of root tips in a <Emphasis Type="Italic">Larix gmelinii</Emphasis> plantation in northeastern China

Root morphology is important in understanding root functions in forest ecosystems. However, the effects of ectomycorrhizal colonization and soil nutrient availability on root morphology is not clear. In this study, root morphology in relation to season, soil depth, soil nitrogen (N) availability, and mycorrhizal fungal colonization were investigated in a larch (Larix gmelinii) plantation in northeastern China. The first-order roots (or root tips) of larch were sampled four times in May, July, and September of 2005, and May of 2006 from two depths of upper soil layer (0–10 and 10–20 cm) in the control and the N-fertilized plots. The results showed that ectomycorrhizal (ECM) colonization rates for the first-order roots were reduced by 17% under N fertilization. The peak of root colonization rates occurred in summer and was positively correlated with soil temperature. ECM colonization significantly altered root morphology: root diameter was increased by 19 and 29%, root length shortened by 27 and 25%, and specific root length (SRL) reduced by 16 and 19% for the control and the N-fertilized plots, respectively. N fertilization led to decreased root length, but did not affect root diameter and SRL. In addition, effects of ECM colonization on root morphology varied with season and soil depth. The observed relationships among ECM fungal colonization, soil N availability, and root-tip morphology should improve our understanding of how root tips respond to environmental changes in soil in temperate forest ecosystems.     

Fine root distribution and persistence under field conditions of three co-occurring Great Basin species of different life form

Fine roots of an annual grass, a perennial grass and a perennial shrub were examined. Based on life histories and tissue composition, we expected the greatest root persistence for the shrub and shortest for the annual grass. Roots were observed with minirhizotrons over 2 yr for number, length and diameter changes. A Cox proportional hazard regression correlated root persistence with soil water, depth, diameter and date of production. In 2001, grass roots had similar persistence times, but shrub roots had the shortest. In 2002, the annual had the longest median root persistence, the perennial grass intermediate and the perennial shrub had the shortest. All species responded similarly to the magnitude of seasonal precipitation; root numbers increased with favorable soil moisture and disappeared with drying; fewer, thinner roots at greater soil depths were found in the drier year (2001). Root persistence increased with soil moisture, diameter and earlier appearance in the spring. Plasticity in root morphology and placement was influenced by water availability, yet persistence was surprisingly contrary to expectations.     

CO2浓度升高对森林土壤微生物呼吸与根（际）呼吸的影响

 根呼吸与微生物呼吸的作用底物不同,二者对高浓度CO₂的响应机理及敏感程度亦不同。在大气CO₂浓度升高的背景下,精确区分根呼吸与微生物呼吸是构建森林生态系统碳循环模型和预测森林生态系统碳源/汇关系所必需的。根（际）呼吸与微生物呼吸对高浓度CO₂的响应呈增加、降低或无明显变化等不同趋势,根（际）呼吸变化主要与根生物量明显相关,细根的作用大于粗根;土壤微生物呼吸变化存在较大的不确定性,微生物量和微生物活性与土壤微生物呼吸相关或不相关。根系统对高浓度CO₂的响应会潜在地影响微生物的代谢底物,进而影响微生物呼吸强度。凡影响土壤总呼吸的生物与非生物因子都会直接或间接地影响根呼吸与土壤微生物呼吸。