The role of soil conditions in fine root ecomorphology in Norway spruce (Picea abies (L.) Karst.)

The present study is an attempt to investigate the pattern of morphological variability of the short roots of Norway spruce (Picea abies (L.) Karst.) growing in different soils. Five root parameters – diameter, length and dry weight of the root tip, root density (dry weight per water-saturated volume) and specific root area (absorbing area of dry weight unit) were studied with respect to 11 soil characteristics using CANOCO RDA analysis. The investigation was conducted in seven study areas in Estonia differing in site quality class and soil type. Ten root samples per study area were collected randomly from the forest floor and from the 20 cm soil surface layer. Eleven soil parameters were included in the study: humus content, specific soil surface area, field capacity, soil bulk density, pH (KCl and H₂O dilution's), N and Ca concentrations, Ca/Al and C/N ratios, and the decomposition rate of fine roots (<2 mm dia.). Root morphological characteristics most strongly related to the measured soil characteristics in the different sites were specific root area, root density and diameter of the short roots, the means varying from 29 to 42 m² kg⁻¹, from 310 to 540 kg m⁻³ and from 0.26 to 0.32 mm, respectively; root density being most sensitive. The most favourable site and soil types resulting in fine roots with morphological characteristics for optimizing nutrient uptake (e.g. low short root density and high specific root area) were Umbric Luvisol (Oxalis), Dystric Gleysol (Oxalis) and Gleyic Luvisol (Hepatica). These soil types correspond to highly productive natural forest stands of Norway spruce in Estonia. All measured soil variables explained 28% of total variance of the root characteristics. The most important variables related to root morphology were the humus content, field capacity and specific soil surface area. This revised version was published online in June 2006 with corrections to the Cover Date.     

Decomposition of litter of three mudflat annual species in a northern prairie marsh during drawdown*

The loss of dry mass, nitrogen and phosphorus from shoot and root litter of mudflat annuals was examined in a series of experimental marshes in the Delta Marsh, Manitoba, Canada. Litter bags containing shoot material of three mudflat annuals (Aster laurentianus Fern., Atriplex patula L., and Chenopodium rubrum L.) were placed on the sediment surface of the marshes under drawdown conditions. In addition, litter bags containing root material of these three species were shallowly buried. Approximately 70% and 50% of both shoot and root litter, respectively, was still present after one year in the field. During the second year when the marshes were flooded, shoot and root litter lost an additional 20% and 0% of their mass, respectively. Except for Chenopodium roots, which accumulated nitrogen and phosphorus during both years, shoot and root litter lost from 0 to 50% of their nitrogen and phosphorus early in the first year, with levels generally remaining constant through the remainder of the study period. Our results indicate that mudflat annual litter decomposed slowly and would provide abundant habitat for aquatic invertebrates when these marshes were reflooded. However, most nutrient loss took place in the first year when the litter was unflooded, with little loss occurring in the second year when flooded.     

三峡库区马尾松不同直径细根分解动态及其影响因素

在三峡库区秭归县九岭头林场马尾松人工林进行一年的细根分解试验,研究马尾松直径<0.5、0.5～1和1～2 mm细根的分解动态及其影响因素.结果表明: 细根分解速率随直径增大而减小,直径<0.5、0.5～1和1～2 mm细根年分解率分别为34.0%、28.0% 和25.7%.直径<1 mm细根分解速率随时间增加而逐渐减小,直径1～2 mm细根分解速率随时间增加先逐渐增加再减小.在细根分解过程中,N、P和Ca浓度随时间增加而增加,K浓度呈先降低后上升再下降的趋势.细根分解速率与细根初始N、P、K和Ca浓度,以及C/N、C/P均显著相关,细根Ca浓度和土壤温度是影响细根分解的主导因子.     

Fine root decomposition of evergreen broadleaved and coniferous tree species in mid-subtropical China: dynamics of dry mass, nutrient and organic fractions

To gain insight into fine roots decomposition in subtropical China, the litter bag method was used to examine the decomposition dynamics of dry mass, N, P, K, and organic fractions in six natural forests and a Chinese fir plantation over a 2-year period in the Wanmulin Nature Reserve, Fujian. The seven tree species examined, representative of this area, differed significantly in their initial chemical quality and were used to determine the best substrate quality parameters to predict decomposition dynamics. Dry mass loss varied significantly among the different roots, which showed fast decomposition in the first year, with mass loss regulated by extractive and acid-soluble fraction, followed by a low rate in the second year, with mass loss dominated by acid-insoluble fraction. Net N release was constantly slower than the mass loss of acid-insoluble fractions, while K release was the other way around. Release of P seemed to be independent of disappearance of acid-insoluble fraction. Not all the very fine roots (0–1 mm) decomposed faster than the fine ones (1–2 mm), and decomposition rates of coniferous roots were not always lower than broadleaved species. Correlation analysis demonstrated that dry mass loss and net N and P release rates were not correlated with initial N concentration, but with acid-insoluble organic fraction and P related parameters at the end of a 2-year decomposition period. Our results suggest that N is a limiting factor of fine root decomposition. Additionally, P could also be an important driver of fine root decomposition and N and P dynamics in this low soil P availability area.     

Effects of elevated atmospheric CO2 on root decomposition in a scrub oak ecosystem

The effects of elevated atmospheric CO₂ on fine root decomposition over a 828‐day period were investigated using open top chambers with both ambient and elevated (700 ppm) CO₂ treatments in an oak–palmetto scrub ecosystem at Kennedy Space Center, Florida. Carbon dioxide enrichment of the chambers began 15 May 1996. The experiment included roots grown in ambient and elevated carbon dioxide. Vertical litterbags installed in September 1996 in each elevated and ambient chamber incubated from December 1996 to December 1998 showed no significant treatment effect on fine root or rhizome mass loss. Initial fine root percentage mass loss varied from 10.3% to 13.5% after three months; 55.5% to 38.3% of original mass had been lost after 828 days. A period of nitrogen immobilization occurred in both fine roots and rhizomes in the elevated CO₂ incubation, which is a potential mechanism for nitrogen conservation for this system in an elevated CO₂ world .     

高山森林三种细根分解初期微生物生物量动态

为了解川西高山森林凋落物分解过程的微生物生物量特征,采用凋落物分解袋法,测定了粗枝云杉(Picea asperata)、岷江冷杉(Abies faxoniana)和红桦(Betula albosinensi)细根分解几个关键时期微生物生物量碳(MBC)、氮(MBN)和磷(MBP)的动态特征。3个树种细根分解过程中的MBC均表现为在土壤深冻期下降至全年最低点后缓慢上升,至土壤融冻中期再次下降,到生长季节增长的趋势。然而,粗枝云杉与岷江冷杉细根分解过程中的MBC最大值出现在生长季节末期,红桦细根分解过程中的MBC最大值出现在土壤冻结初期。3个树种细根分解过程中的MBN表现出相似的动态规律:土壤深冻期急剧下降至全年最低,随后在冻融季节无显著变化,生长季节明显增加,到生长季节末期达到全年最大值。另外,粗枝云杉和岷江冷杉细根分解过程中MBP均随着分解的进行呈现增加趋势,而红桦细根分解过程中的MBP在土壤融冻末期出现最大值,在生长季节中期出现另一峰值,生长季节末期明显下降。这些结果表明冬季细根分解过程中仍存在一定的土壤微生物,但受到细根质量、温度及其驱动的环境因子的深刻影响。     

Root turnover in a beech and a spruce stand of the Belgian Ardennes

The theoretical basis of fine root turnover estimation in forest soils is discussed, in relation to appropriate experimental techniques of measurement. After sequential coring, the correct expression is the sum of significant positive increments of live and dead roots of the various diameter categories, to which the transfer of dead roots to organic matter derived from roots, OMDR, has to be added. This should not be confounded with dead root mineralization. The transfer rates should first be estimated in root dimensions and not in weight of dry matter. The measurements were carried out in a 120 year old beech (Fagus sylvatica L.) stand and a 35 year old Norway spruce (Picea abies Karst) stand, in the Eastern Ardennes, Belgium. The turnover rate of fine roots (diam. <5 mm) was 4393 kg ha⁻¹ year⁻¹ (root dry weight), including 711.2 kg ha⁻¹ year⁻¹ for dead root transfer to OMDR, for beech. For spruce, turnover rate was 7011 kg ha⁻¹ year⁻¹ (root dry weight), including 1498 kg ha⁻¹ year⁻¹ for dead root transfer to OMDR. Under beech, there was a slight root density increase in spring. No seasonal fluctuations were observed under spruce, but a strong irreversible drop in live root growth was found in the later season 1980–1981, corresponding to a decrease of tree height growth and trunk radius increment. Turnover rates were further expressed in dry weight and in amounts of elements (kg ha⁻¹ year⁻¹) (Ca, Mg, K, Na, Al, N, P, S). Correlative relations between root dimensions and dry weight and element concentrations show that the derived values, and in particular root specific density (dry weight volume⁻¹) vary according to species, root category, and seasonal sampling. Various schemes of seasonal variations of root growth, described in Europe, show that the major dependance on general climate is obscured by environmental factors (soil, exposure, species). It is suggested that root density fluctuation approach the steady state on an annual basis under mild Atlantic conditions.     

不同类型沙地上差巴嘎蒿细根的分布状态

 以生长于流动沙地和固定沙地上,处于植被演替不同阶段的半灌木差巴嘎蒿(Artemisia halodendron)种群为对象,用土钻取样法研究了生长季（2000年）降雨期前后差巴嘎蒿的根系随土壤深度的分布、生长动态及其与根际土壤含水量的动态关系,观察到：1)降雨期前各土层的根际土壤含水量随深度的增加而升高,增加的幅度为流动沙地>固定沙地;降雨期后根际土壤含水量随深度的增加而减少,减少的幅度为固定沙地>流动沙地。2)表土层（0～15 cm）中差巴嘎蒿的主根分布量在流动沙地显著高于固定沙地。3)降雨期前,差巴嘎蒿细根(直径<1 mm)分布比例在两种不同类型沙地上的差异表现为:在土层0～45cm中固定沙地(84.9%)极显著高于流动沙地(61.9%),而在深土层（>45 cm）中流动沙地（38.1%）显著高于固定沙地（22%）;降雨期后,不论是在固定沙地还是流动沙地细根多集中于0～15 cm的表土层中,流动沙地的细根分布比例由降雨期前33%增至降雨期后的78%,固定沙地由降雨期前的49%增至降雨期后的63%。表明流动沙地差巴嘎蒿种群细根的生长比固定沙地活跃,能够在生长季降雨期后迅速调整细根的分布比例,使细根分布适应降雨期后浅层土壤含水量高的特点。固定沙地的细根分布难以迅速适应土壤水分的变化,不利于差巴嘎蒿对水分的吸收,成为种群衰退的一个重要因素。     

Below‐ground biomass and productivity of a grazed site and a neighbouring ungrazed exclosure in a grassland in central Argentina

Abstract We estimated the below‐ground net plant productivity (BNPP) of different biomass components in an intensively and continuously 45‐ha grazed site and in a neighbouring exclosure ungrazed for 16 years for a natural mountain grassland in central Argentina. We measured approximately twice as much dead below‐ground biomass in the grazed site as in the ungrazed site, with a strong concentration of total below‐ground biomass towards the upper 10 cm of the soil layer in both sites. The main contribution to total live biomass was accounted for by very fine (<0.5 mm) and fine roots (0.5–1.0 mm) both at the grazed (79%) and at the ungrazed (81%) sites. We measured more dead biomass for almost all root components, more live biomass of rhizomes, tap roots and bulbs, and less live biomass of thicker roots (>1 mm) in the grazed site. The seasonal variation of total live below‐ground biomass mainly reflected climate, with the growing season being limited to the warmer and wetter portion of the year, but such variation was higher in the grazed site. Using different methods of estimation of BNPP, we estimated maximum values of 1241 and 723 g m^?2 year^?1 for the grazed and ungrazed sites, respectively. We estimated that very fine root productivity was almost twice as high at the grazed site as at the ungrazed one, despite the fact that both sites had similar total live biomass, and root turnover rate was twofold at the grazed site.     

Production and turnover rates of shallow fine roots in rangelands of the Patagonian Monte, Argentina

Selective sheep grazing in arid rangelands induces a decrease in total cover and grass cover and an increase in the dominance of shrubs. Both life forms differ in aboveground and belowground traits. We hypothesized that grazing disturbance leads to the replacement of grass by shrub fine roots in the upper soil, and this is reflected in changes in the seasonal dynamics of shallow fine roots at the community level. In two sites representative of non-grazed and grazed vegetation states in the Patagonian Monte, we assessed the canopy structure, and the fine root biomass, N concentration, production, and turnover during two consecutive years. The non-grazed site exhibited higher total, grass, and shrub cover than the grazed site. The grazed site had larger or equal fine root biomass than the non-grazed site except for late spring of the second year. This could be associated with the ability of shrubs to develop dimorphic-root systems occupying the soil freed by grasses at the grazed site, and with the larger contribution of grass than shrub fine roots in relation to an extraordinary precipitation event at the non-grazed site. This was consistent with the N concentration in fine roots. Fine root production was positively correlated to temperature at the grazed site and with precipitation at the non-grazed site. Fine root turnover did not differ between sites. Our results indicate that grazing leads to a shifting in the seasonality and main climatic controls of fine root production, while fine root turnover is mostly affected by changes in soil water conditions.     

Mass and nutrient loss of fresh plant biomass in a small black-water tributary of Caura river, Venezuelan Guayana

Decomposition rates and nutrient dynamic (N, P, K, Ca and Mg) were determined for green leaves and fine branches immersed in the water of a small tributary of Caura river (SE-Venezuela). 16% of the original dry weight of leaves and 11% of branches were lost at the end of the first sampling period: first month for leaves and second month for branches. This dry weight reduction was probably due to leaching of soluble material. After a 9-month period, the mass loss was 60% for leaves and 20% for fine branches. The pattern of dry weight and nutrient losses are in general agreement with previous studies of decomposition of leaf litter in both terrestrial and aquatic ecosystems. Potassium and magnesium are the elements most rapidly lost, showing the dominance of leaching processes; at the end of the first month 7% of the initial amount of K and 18% of the initial amount of Mg remained in leaves. The loss of calcium and phosphorus was much slower: 61% of Ca and 47% of P remained in the leaf material after the first sampling period. In contrast to K, Mg, Ca and P, the initial amount of nitrogen in leaves remained relatively unchanged during the first month of decomposition; in the subsequent sampling period, the amount of N decreased. The elements K and Mg in branches behaved similar to leaves: 4% of K and 22% of Mg were left at the end of the first sampling period. The initial amount of Ca and P in branches decreased slightly: 88% of Ca and 83% of P remained in branches at the end of this first sampling. Nitrogen behaved differently in branches than that in leaves. In branches the amount of N remained relatively unchanged during the first 5 months of decomposition; afterwards, N showed gradual increases, probably due to immobilization. At the end of the experiment the amount of N in branches was 16% higher than the initial amount.     

Phosphorus acquisition from soil by white lupin (Lupinus albus L.) and soybean (Glycine max L.), species with contrasting root development

White lupin and soybean have contrasting root morphologies: white lupin develops proteoid or cluster roots, roots with discreet clusters of short, determinate branch roots (rootlets) while soybean develops a more fibrous root system with evenly distributed, longer branch roots. Growth and P acquisition by white lupin and soybean were compared in a soil high in bound, total P, with or without additional inorganic P applied in solution. Additional P increased biomass by 25% and doubled total P in soybean. In contrast, white lupin did not respond to additional P in biomass or total P. However added P decreased cluster development on proteoid roots indicating that white lupin sensed the added P. The reduction in cluster weight per plant was exactly countered by an increase in dry weight of other roots. Soybean root development responded to P application, proliferating branch roots with active meristems in the upper portion of the soil profile where P was applied, and reducing root weight to plant weight by 13%. White lupin did not proliferate roots in response to P application. When P was not added to soil, soybean and lupin acquired similar P per unit root dry weight. However, white lupin accumulated 4.8 times more P per unit root length, suggesting that P acquisition in these plants involved other mechanisms such as the exudation of P solubilizing compounds. Soybean accessed P by developing more root length thus colonising more soil volume than white lupin and, therefore, was better able to take advantage of the added P. Pericycle and root tip meristem activities were critical to the differences in root development between white lupin and soybean, and therefore their responses to plant and soil P.     

Seasonal changes in hair roots and mycorrhizal colonization in Woollsia pungens (Cav.) F. Muell. (Epacridaceae)

The proportion of the root system comprising hair roots and their ericoid mycorrhizal colonization have been estimated in Woollsia pungens (Cav.) F. Muell. (Epacridaceae) at a site in New South Wales, Australia over a 12 month period. The technique used was a modification of the grid-line intercept method. Hair roots persisted and comprised at least about 50% of the root system all year round. The percentage of root length that was hair root varied with the season, being lowest in April/May (50%) and highest in October (70%). Hair root colonization differed significantly over the 12 month period, being highest over the winter to spring period (June–Oct) but there were infected hair roots present at all times of the year. There was a significant negative relationship between the percentage hair root length infected and log₁₀ transformations of both mean daily maximum and minimum temperatures, but no correlation with rainfall or soil moisture content. These findings contrast with those for south-west Australia where hair roots (and mycorrhizal colonization) are reported to disappear in summer.     

Decomposition of roots of black alder and hybrid poplar in short-rotation plantings: Nitrogen and lignin control

The decomposition of the roots (0–2 mm, 2–5 mm and 5–10 mm) of black alder (Alnus glutinosa (L.) Gaertn.) and hybrid poplar (Populus nigra L. X Populus trichocarpa Torr & Gray) was followed over a 462-day period in pure and mixed plantings in southern Quebec. Small roots of alder had the highest initial concentrations of nitrogen and lignin, and lost 9 and 10% less mass than medium and large roots, respectively. Large roots of poplar had the highest lignin-to-nitrogen ratio and showed the smallest loss of mass over the total incubation period. Slow root decomposition of black alder and hybrid poplar was characterized by a greater proportion of initial root nitrogen immobilized per unit of carbon respired. Lignin concentration in roots of alder and poplar increased rapidly at the beginning of the incubation. Our results suggest that high levels of nitrogen in roots of alder could contribute in slowing the rate of decomposition by allowing the formation of nitrogen-lignin derivatives and low levels of nitrogen in roots of poplar may limit the growth of microorganisms and the rate of root decomposition. A multiple regression was developed using initial nitrogen, lignin concentration and the ratio of lignin to nitrogen to produce an index of the rate of root decomposition. The correlation between the index values and the percentage of residual root mass was significant (r=0.98, p<0.01).     

Root exudation and rhizoplane bacterial abundance of barley (Hordeum vulgare L.) in relation to nitrogen fertilization and root growth

The abundance of bacteria in the rhizoplane of barley varieties was investigated at different soil nitrogen levels. Increased amendments of nitrogen resulted in higher bacterial numbers in the rhizoplane of barley seedlings of different varieties. A negative correlation was found between nitrogen level in the soil and the growth rate of the seedling roots. The effect of nitrogen on the bacterial abundances could be indirect through changed root growth and thereby changed exudation. The exudation of soluble organic carbon componds from barley seedling roots were measured in hydroponic culture. The effect of natural variation in root growth rate and of different concentrations of nitrogen in the nutrient solution was investigated. The amount of exudates consituted 2–66% of the dry weight increase in root biomass, depending on the root growth. Slower growing roots released considerably more organic carbon per unit root weight than faster growing roots. The variation in root exudation appeared to be mainly explained by differences in root growth, rather than of the nitrogen concentration in the nutrient solution. A significantly higher exudation rate was found during day time compared to night.     

水氮处理下不同品种水稻根系生长分布特征

为明确不同栽培条件下水稻(Oryza sativa)根系生长分布特征, 通过不同水氮处理和不同品种的水稻桶栽试验, 采用内置根架法, 于拔节期和抽穗期取样, 获取根系总干重(TRW)、不定根数(ARN)以及各类根(不定根、细分枝根和粗分枝根)的形态指标(长度、表面积和体积), 并分析植株根系生长状况和根系分布特征。结果显示: (1)各试验条件下抽穗期各项根系指标较拔节期均呈增长趋势。同一时期, 各项根系指标在3个施氮水平间均差异显著, 且随施氮量的增加而增加。不同水分处理下, 两个时期的ARN在湿润灌溉(W2)与保持水层(W1)之间差异均不显著, 而其他指标上W2处理均显著最高; 干旱处理 (W3)下, 仅拔节期的TRW和粗分枝形态指标与W1处理接近, 而在其他指标上均显著最低。不同品种间, ‘扬稻6号’ (V3)的各项根系指标均最高, 而‘日本晴’ (V1)和‘武香粳14’ (V2)间差异不显著。(2)各试验条件下, 抽穗期较拔节期根系下扎生长比例增加, 多分布于表层(0-5 cm)土中; 减少氮素和水分供应可提高根系在5 cm以下土层中的分布比例, 且分枝根反应最为明显; 品种V1和V2的深扎根性较V3明显。结果表明, 合理施氮与控水可优化水稻不同类型根的生长与分布特征, 但需考虑不同品种之间的差异。     

Clean rain promotes fine root growth and soil respiration in a Norway spruce forest

Soil acidification and N saturation are considered to affect the decomposition of soil organic matter as well as growth and mortality of fine roots in many forest soils. Here we report from a field experiment where ‘clean rain’ has been applied to the soil for about 10 years under a roofed plot of a 71‐year‐old Norway spruce plantation at Solling, Central Germany. Reduced amounts of protons (?78%), sulphate (?53%), ammonium (?86%), and nitrate (?49%) were sprayed on the soil surface of the clean rain plot between 1992 and 2001. In an adjacent roofed control plot, throughfall was collected and immediately re‐sprinkled below the roof construction without any chemical manipulation. One year before the clean rain treatment started, live and dead fine root masses (≤2 mm) were determined from undisturbed soil cores down to 40 cm mineral soil depth. Total live fine root mass was significantly lower in the clean rain plot than in the control plot. After the first sampling, the soil holes were refilled with quartz sand and repeatedly sampled in June 1992, June 1996, and October 2001. There were no differences in live and dead fine root masses between the plots in 1992 and 1996. In 2001, both live and dead fine root masses of the clean rain plot were about twice as high as in the control plot, indicating that fine root growth recovered in the mineral soil following 10 years of clean rain treatment. Moreover, the clean rain treatment significantly reduced the total N concentrations of live fine roots and 1‐year‐old needles. Our results suggest that the reduced N input promoted fine root growth to compensate N deficiency. Reduced Al concentration in soil solution may have contributed to the recovery of fine root growth, however, the toxicity of Al species is largely unknown. Mean annual soil respiration rate was 24% higher in the period from 2000 to 2001, indicating that the clean rain treatment increased respiration of roots and heterotrophic microorganisms within the rhizosphere. Laboratory incubation of samples from the organic horizon and the top mineral soil revealed no differences between the plots in the decay rate of soil organic matter. Our results suggest that strong reductions in atmospheric N deposition from about 30 to 10 kg N ha^?1 yr^?1 and decreasing acid stress can have beneficial effects on growth of fine roots in the mineral soil within a decade. We conclude that biological recovery under reduced atmospheric loads can affect the nutrient and carbon budget of spruce soils in the long run.     

Loss of dry matter and cell contents from fibrous roots of sugar beet due to sampling,storage and washing

To obtain correction factors for estimating root dry weight from washed samples and to test the efficiency of various procedures for storing root samples, dry matter losses were determined by simulating root washing methods with roots obtained from a nutrient culture. For sugar beet dry matter losses were higher than values previously found for wheat and ryegrass: about 30% for the procedure normally used and about 40% for samples pretreated with sodium pyrophosphate. The largest share of water-soluble sugars was lost from root samples within one day of storing roots. The N content of roots expressed on the basis of remaining dry matter rose first during handling of the root samples and decreased in samples stored for a longer period. In most cases no cell wall material (cellulose and lignin) is lost from the root samples; expressed on the basis of remaining dry weight the contents consequently rose.Communication no. 2 of the Dutch Programme on Soil Ecology of Arable Farming Systems     

Tree rooting patterns and soil water relations of healthy and damaged stands of mature oak (Quercus robur L. and Quercus petraea [Matt.] Liebl.)

At three sites in northwestern Germany, which represent the centres of the present oak damage, root distribution and biomass beneath healthy and damaged trees of mature pedunculate oak (Quercus robur L.; Neuenburg site) and sessile oak (Q. petraea [Matt.] Liebl.; Lappwald and Sprakensehl sites) were investigated, and soil texture, bulk density, duration of waterlogging periods and the water available in the mineral soil were determined. For Neuenburg and Sprakensehl, the available soil water was related to leaf water parameters determined in a separate investigation. At the clayey and hydromorphic sites of Neuenburg and Lappwald, the measurements were performed in each one healthy and one damaged part of the site, which differed in the number of oaks with crown damage. In the damaged stand of Neuenburg, the clay content of the subsoil was higher than in the healthy stand, and the soil water availability was reduced especially in dry periods. Compared to healthy oaks of the healthy stand, the density of finest plus fine roots as well as the biomasses of finest roots were lower beneath damaged oaks of the damaged stand. With decreasing relative available soil water (actually available water in relation to water available at the saturation state), the relative leaf water content decreased in damaged, but not in healthy oaks. At Lappwald, similar differences in soil water availability between the healthy and the damaged stand were found, but had no effect on the distribution or biomass of the roots. At the sandy site (Sprakensehl), the available soil water decreased drastically during a dry period, and predawn leaf water potentials of both healthy and damaged oaks declined with decreasing relative available soil water. However, the damaged oaks were not inferior to the healthy ones with respect to root density and biomass. It is concluded that, in the damaged stand of Neuenburg, the high clay content of the subsoil, which results in prolonged periods of waterlogging, in sharp changes from waterlogging to drought and decreased water availability in dry periods, is the reason for the reduced biomass and density of roots of the pedunculate oak. Thus, in northwestern Germany, unfavourable soil water relations are considered as a factor contributing to crown damage of pedunculate oak at hydromorphic sites, but not to damage of sessile oak.     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.