Assessment of a rapid method,using soil cores,for estimating the amount and distribution of crop roots in the field

Summary A study was made of the relationship between the number of roots (N_r) observed on unit area of the freshly exposed, horizontal faces of soil cores, and the amounts of roots (per unit volume) present in the same cores. Soil cores, 7 cm diameter, were extracted to depths of 1 m from cereal crops in 1976 at three field sites located on clay soils. Sampling was either at the start of stem elongation, or at anthesis. Estimates of root length per unit soil volume (L) were derived from N_r by assuming random orientation of roots in the soil.Values of L were found to be highly correlated with the measured lengths of both the main roots (root axes) and the total roots (axes and laterals) washed from the soil at a given growth stage, for each of the soils. On average, L was 3.3 times the length of root axes washed from the soil, and was 0.42 times the length of total roots, but there was appreciable variation between different growth stages and field sites. Possible factors giving rise to differences between L and the measured lengths of roots are discussed. Estimates of root length from observation of soil cores may nonetheless provide a suitable basis for rapidly comparing therelative distribution of roots down the soil profile under field conditions.     

Effects of soil structural discontinuity on root and shoot growth and water use of maize

The role of roots penetrating various undisturbed soil horizons beneath loose layer in water use and shoot growth of maize was evaluated in greenhouse experiment. 18 undisturbed soil columns 20 cm in diameter and 20 cm in height were taken from the depths 30–50 cm and 50–70 cm from a Brown Lowland soil, a Pseudogley and a Brown Andosol (3 columns from each depth and soil). Initial resistance to penetration in undisturbed soil horizons varied from 2.5 to 8.9 MPa while that in the loose layer was 0.01 MPa. The undisturbed horizons had a major effect on vertical arrangement of roots. Root length density in loose layer varied from 96 to 126 km m^-3 while in adjacent stronger top layers of undisturbed horizons from 1.6 to 20.0 km m^-3 with higher values in upper horizons of each soil. For specific root length, the corresponding ranges were 79.4–107.7 m g^-1 (on dry basis) and 38.2–63.7 m g^-1, respectively. Ratios of root dry weight per unit volume of soil between loose and adjacent undisturbed layers were much lower than those of root length density indicating that roots in undisturbed horizons were produced with considerably higher partition of assimilates. Root size in undisturbed horizons relative to total roots was from 1.1 to 38.1% while water use from the horizons was from 54.1 to 74.0%. Total water use and shoot growth were positively correlated with root length in undisturbed soil horizons. There was no correlation between shoot growth and water use from the loose layers.     

Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils

Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.     

A comparison of penetrometer pressures and the pressures exerted by roots

Summary Previous work is reviewed in which the ratio of the pressures required for soil penetration by roots and penetrometers are compared. It appears that this ratio can vary from about 2 to 8 depending on conditions. However, there is very little experimental evidence and most of the work has been inferential.Direct measurements are reported for the stresses exerted by a 1 mm diameter penetrometer probe and by the roots of pea seedlings when penetrating Urrbrae fine sandy loam. Six soil conditions were used: (non-weathered remoulded soil cores + artificially weathered remoulded soil cores + undisturbed field clods) × (confined + unconfined cores or clods). The confinement treatment was to test for any effects of additional restraint to cylindrical root expansion. The weathering and field clod treatments were to test the hypothesis that root elongation is facilitated by tensile failure ahead of the root tip.The principal conclusions are as follows. The laboratory weathering treatment reduced the soil tensile strength by 25%. This resulted in a small but significant reduction in the pressure for root penetration into confined cores. Compared with remoulded non-weathered cores, field clods had a 2 to 3 fold greater penetrometer resistance and a 50% lower tensile strength. The force required for root penetration into unconfined field clods was only 10% greater than for unconfined non-weathered cores. For the former (which is closest to field conditions) the penetrometer had to exert a pressure 5.1 times greater than a root tip in order to penetrate the soil. Penetrometer penetration pressure was independent of probe diameter in the 1–2 mm range in the soil used. Core confinement restricts root radial expansion and modifies the penetration force of metal probes and plant roots.On the basis of the new results it is tentatively concluded that soil tensile failure can facilitate penetration by roots.     

棉花根系生长和空间分布特征

结合田间根钻取样和图像扫描分析方法, 研究了不同棉花品种根系的长度、直径和表面积动态及 0~ 10 0cm深和 0~ 4 0cm宽土壤范围内的空间分布特征。该方法与常规直尺测量结果相比相关系数R2 达到 0.899 (n =1318), 显示了较好的可靠性。研究结果表明, 棉花平均根长密度 (RLD) 在花铃期为 1.2 1~ 1.2 7mm·cm-3, 吐絮后降至 1.0 4~ 1.12mm·cm-3, 收花时为 0.76mm·cm-3 。棉花根平均直径在不同基因型间存在显著差异, 抗虫杂交棉的根直径最粗, 平均为 0.5 2mm ;早熟类型品种根直径较细, 平均为 0.36mm。在土壤深度上根直径的差异不显著, 但距棉行距离越远, 根的平均直径越小。在明确根系长度和直径动态规律的基础上, 提出了根表面积指数 (RAI) 的概念, 与地上部叶面积指数具有相似的含义和生物学意义, 且呈较好的指数相关关系 (R2 =0.779) 。RAI在生理发育时间 (PDT) 小于等于 4 0前, 其增长动态符合LOGISTIC生长规律 (R2 =0.84 9), 在PDT大于 4 0后, 呈线性递减趋势 (R2 =0.5 70~ 0.895 ), 且杂交抗虫棉的RAI在全生育期内均明显高于其它类型品种, 而早熟类型品种相对略低。RAI空间分布特征表现为, 开花前在浅根层内 (0~ 30cm) 分布最多, 花铃期以中层根系 (40~ 6 0cm) 为主, 吐絮后主要以深层 (70~ 10 0cm) 和距棉行较远的行间较多。研究结果为制定合理的施肥、灌溉措施提供了理论依据, 并量化了棉花根系的时空变化, 为进一步提高生长发育模拟模型的精度奠定了基础。     

Changes in arbuscular mycorrhizal associations and fine root traits in sites under different plant successional phases in southern Brazil

Fine root morphological traits and distribution, arbuscular mycorrhizal (AM) fungi, soil fertility, and nutrient concentration in fine root tissue were compared in sites under different successional phases: grass plants, secondary forest, and mature forest in Londrina county, Paraná state, southern Brazil. Soil cores were collected randomly at the 0-10- and 10-20-cm depths in three quadrants (50 m2) in each site. Plants from the different successional stages displayed high differences in fine root distribution, fine root traits, and mycorrhizal root colonization. There were increases in the concentration of nutrients both in soil and fine roots and decrease of bulk soil density along the succession. The fine root biomass and diameter increased with the succession progress. The total fine root length, specific root length, root hair length, and root hair incidence decreased with the succession advance. Similarly, the mycorrhizal root colonization and the density of AM fungi spores in the soil decreased along the succession. Mycorrhizal root colonization and spore density were positively correlated with fine root length, specific root length, root hair length, root hair incidence, and bulk density and negatively correlated with fine root diameter and concentration of some nutrients both in soil and root tissues. Nutrient concentration in root tissue and in soil was positively correlated with fine root diameter and negatively correlated with specific root length, root hair length, and root hair incidence. These results suggest different adaptation strategies of plant roots for soil exploration and mineral acquisition among the different successional stages. Early successional stages displayed plants with fine root morphology and AM fungi colonization to improve the root functional efficiencies for uptake of nutrients and faster soil resource exploration. Late successional stages displayed plants with fine root morphology and mycorrhizal symbiosis for both a lower rate of soil proliferation and soil exploration capacity to acquire nutrients.     

Root growth of subalpine and montane Eucalyptus seedlings at low soil temperatures

 This study examines the effect of different soil temperatures on root growth in seedlings of Eucalyptus pauciflora Sieber ex Sprengel subsp. pauciflora and Eucalyptus nitens (Deane & Maiden) Maiden. Seedlings were grown in a glasshouse in pots containing soil. Pots were held in water baths maintained at 3, 7 or 13°C, whilst shoots were exposed to ambient glasshouse temperatures. The experiments were designed to separate direct effects of soil temperature from effects due to differences in seedling size. In the first experiment, seedlings were grown to constant height (25 cm for both species), in the second to constant time (100 days for E. pauciflora and 64 days for E. nitens) and in the third experiment seedlings were transferred between soil temperatures. The rate of growth of both species increased with increasing soil temperature. E. nitens grew faster than E. pauciflora at 7 and 13°C, but E. pauciflora grew faster than E. nitens at 3°C. The rate of browning of roots increased with decreasing soil temperature and at a faster rate in E. nitens than E. pauciflora. Root length was highly correlated to root mass within diameter and colour classes (r² > 0.7). However, brown roots were heavier than white roots. Consequently, changes in root mass did not reflect changes in root length when the proportion of brown to white root also changed. For example, at a constant height of 25 cm at 3°C, E. nitens had greater root mass but lesser root length than E. pauciflora. E. pauciflora at 3°C grew faster, and had more root length and less brown roots than E. nitens. This supports the argument that E. pauciflora is better adapted than E. nitens to survive and grow at lower soil temperatures. Received: 16 December 1996 / Accepted: 2 April 1997     

A comparison of minirhizotron techniques for estimating root length density in soils of different bulk densities

Flexible- and rigid-walled minirhizotron techniques were compared for estimating root length density of 14- to 28-day-old Pinto bean (Phaseolus vulgaris L.) and spring whet (Triticum aestivum L.) plants in soil boxes under controlled environment conditions at three soil bulk densities (1.3, 1.5 and 1.7 g cm^–3). The flexible-tube system consisted of bicycle inner tubes inflated inside augered access holes and removed only when measurements were taken. Rigid tubes were constructed of extruded polybutyrate plastic. In both cases tubes were oriented horizontally. Despite similar root densities for wheat and beans based on measurements obtained from soil cores, root densities estimated from both types of minirhizotron were higher in bean than in wheat in uncompacted soil. Estimates of root density by the flexible tube minirhizotron were more closely correlated with soil core image analysis estimates than were those by the rigid minirhizotron system. At high soil bulk density, rigid tube measurements consistently overestimated actual rooting density of both wheat and bean. The relationship between estimated and actual rooting densities in the case of flexible tube measurements was not significantly influenced by soil bulk density. These findings were consistent with the theory that preferential root growth is induced by gaps at the soil-observation tube interface, inherent in the rigid tube technique, and was accentuated under conditions of high soil strength.     

Effects of fire,mowing and nitrogen addition on root characteristics in tall-grass prairie

Abstract. Root harvests and root windows were used to study the influence of fire, mowing and nitrogen additions on root lengths, biomass, and nitrogen content in tall-grass prairie. Four years of nitrogen additions (10 g m² yr^?1) increased below-ground mass by 15 % and nitrogen concentration in that mass by 77 %. In general, live roots and rhizomes exhibited greater increases in nitrogen concentrations than detrital roots and rhizomes. After four years of treatment, live roots and rhizomes immobilized an additional 1.5 to 5 g/m² of nitrogen, depending upon specific treatment, while dead roots and rhizomes immobilized an additional 3 to 3.5 g/m². Average root growth parameters, as measured with root windows, were positively correlated with above-ground peak foliage biomass; however, the only significant correlation was between average new root growth and above-ground peak foliage biomass (r = 0.73, p ≤ 0.04). Root growth and decay, as measured by annual mean values for eight root windows over a four year interval, were insensitive to climatic and treatment effects.     

Automated image analyses for separating plant roots from soil debris elutrated from soil cores

Historically, destructive root sampling has been labor intensive and requires manual separation of extraneous organic debris recovered along with the hydropneumatic elutriation method of separating plant roots from soils. Quantification of root system demographics by public domain National Institute of Health (NIH-Image) and Root Image Processing Laboratory (RIPL) image processing algorithms has eliminated much of the labor-intensive manual separation. This was accomplished by determining the best length to diameter ratio for each object during image analyses. Objects with a length to diameter ratio less than a given threshold are considered non-root materials and are rejected automatically by computer algorithms. Iterative analyses of length to diameter ratios showed that a 15:1 ratio was best for separating images of maize (Zea mays L.) roots from associated organic debris. Using this threshold ratio for a set of 24 soil cores, a highly significant correlation (r² = 0.89) was obtained between computer image processed total root length per core and actual root length. A linear relationship (r² = 0.80) was observed between root lengths determined by NIH-Image analyses and lengths determined independently by the RIPL imaging system, using the same maize root + debris samples. This correlation demonstrates that computer image processing provides opportunities for comparing root length parameters between different laboratories for samples containing debris.     

Distribution,length and weight of roots in young plantations ofEucalyptus grandis W. Hill ex Maiden irrigated with recycled water

Summary The root systems ofEucalyptus grandis W. Hill ex Maiden, irrigated with recycled municipal effluent at two sites in north-western Victoria, Australia, were studied by excavation and coring. Trees at Robinvale were four years-old and were irrigated using micro-sprays that covered only 70% of the ground surface area, whereas at Mildura, effuent was uniformly was uniformly applied to six years-old trees by flood and sprinkler irrigation. At Mildura where roots were excavated from a 2.80×2.80×1.20 m block of soil, a total root length of 1193 m.m⁻² and a total root weight of 3.1 kg m⁻² were estimated in the top metre. For roots >1 mm diameter, 77% of intercepts were at 0–30 cm, whereas only 50% were in the 50–100 cm soil horizon. At both sites where roots in the top 30 cm were studied by coring, the vertical distributions of root intercepts, length and weight were similar. Root length was greatest in the 0–10 cm soil horizon at both sites, and intercepts of roots <1 mm diameter comprised 73% and 81% of all roots at Mildura and Robinvale respectively. Roots <1 mm diameter contributed 85% of total length at both sites, but only 19% and 21% of total weight at Mildura and Robinvale respectively. The horizontal distribution of roots differed at the two sites. With uniform application of effuent at Mildura, root intercepts and length were concentrated in the centre of the irrigation bay, but at Robinvale, the concentration occurred closer to the tree row due mainly to the different method of irrigation. Root weight at both sites was highest within 50 cm of the tree row. Root densities of 0.11 to 0.57 cm cm⁻³ were estimated in the two plantations; these were similar to root densities measured inPinus radiata D. Don plantations up to 46 months old, but were considerably lower than those estimated for pastures. The implications of the results for the management of irrigated plantations of eucalypts are discussed.     

The growth,activity and distribution of the fruit tree root system

Summary The paper reviews information, much of it obtained from studies using the East Malling root observation laboratories, on the growth and development of the fruit tree root system. The production of new white root varies from year-to-year, generally being highest in the early years. As trees age, woody roots constitute an increasing fraction of total root length although the contribution made by new root growth to the total root length of established trees is also affected by soil management, being higher for trees under grass than under herbicide. Soil management also affects the balance of short (lateral) to long (extension) roots; under grass there are more lateral roots.Calculation of the rate of water uptake per unit root length needed at various times in the year to meet transpirational demand, suggests that woody roots, which recent experimental work has shown to be capable of absorbing water, must be responsible for much of total water supply.Measurements of VA mycorrhizal infection in field-grown trees indicated, for part of the season, higher per cent infection in trees grown under irrigated grass than under herbicide management. It is suggested that this, which is associated with raised leaf phosphorus levels, may be due at least partly to higher numbers of lateral roots, the root type which becomes infected. The growth and functioning of the root system under field conditions depend upon the production and integration of a range of root types.     

Root and shoot growth of semi-dwarf and taller winter wheats

Investigations are reported of root and shoot growth in semi-dwarf and taller winter wheat varieties grown in contrasting soils over three years. Comparisons were made of the distribution with depth of roots, estimated by injecting rubidium-86 into stem bases and counting the content in soil cores. The relative ability to absorb phosphate from different zones was measured from the recovery in aerial parts of ³²P injected into the soil at different depths. The distribution of dry matter in roots and aerial parts, and total root length, was measured using soil cores and samples of aerial parts taken during the growth of the crop. Relative growth rates of the aerial parts followed a sigmoid curve, but those of the roots showed little change between germination and anthesis. There was little evidence of varietal differences in root growth, though there was some indication that at depth the roots of the semi-dwarf varieties were more extensive and absorbed more phosphate than those of the taller varieties.     

Measuring Fine Root Turnover in Forest Ecosystems

Development of direct and indirect methods for measuring root turnover and the status of knowledge on fine root turnover in forest ecosystems are discussed. While soil and ingrowth cores give estimates of standing root biomass and relative growth, respectively, minirhizotrons provide estimates of median root longevity (turnover time) i.e., the time by which 50% of the roots are dead. Advanced minirhizotron and carbon tracer studies combined with demographic statistical methods and new models hold the promise of improving our fundamental understanding of the factors controlling root turnover. Using minirhizotron data, fine root turnover (y⁻¹) can be estimated in two ways: as the ratio of annual root length production to average live root length observed and as the inverse of median root longevity. Fine root production and mortality can be estimated by combining data from minirhizotrons and soil cores, provided that these data are based on roots of the same diameter class (e.g., < 1 mm in diameter) and changes in the same time steps. Fluxes of carbon and nutrients via fine root mortality can then be estimated by multiplying the amount of carbon and nutrients in fine root biomass by fine root turnover. It is suggested that the minirhizotron method is suitable for estimating median fine root longevity. In comparison to the minirhizotron method, the radio carbon technique favor larger fine roots that are less dynamics. We need to reconcile and improve both methods to develop a more complete understanding of root turnover.     

Longevity and turnover of roots in the shortgrass steppe: influence of diameter and depth

We used minirhizotrons to determine patterns of root longevity andturnover for the perennial bunchgrass Bouteloua gracilisinthe shortgrass steppe of eastern Colorado, USA. We hypothesized that rootlongevity would be partially controlled by root diameter, following previouslyobserved patterns in woody plants. In addition, we hypothesized that rootturnover would be greatest in surface soil horizons and decrease with depth dueto variation in soil moisture availability and temperature. Root longevity wascorrelated with root diameter. Median life span of roots > 0.4mm was approximately 320 days, while roots < 0.2mmhad a median life span of 180 days. There was approximately a 6%decreasein the likelihood of mortality with a 0.10-mm increase inroot diameter, controlling for the effect of depth in the soil profile. Rootlength production and mortality were highest in the upper20 cm of the soil profile and decreased with depth.However,because root length density also decreased with depth, there were nosignificantdifferences in turnover rate of root length among sampling intervals. Turnoverwas approximately 0.86 yr^–1 based on root length production,while turnover was 0.35 yr^–1 using root length mortality as ameasurement of flux. The imbalance between turnover estimates may be aconsequence of the time the minirhizotrons were in place prior to imaging or mayresult from our lack of over-winter measures of mortality. Our worksuggests that Bouteloua gracilis roots have complex lifehistory strategies, similar to woody species. Some portion of the root systemishighly ephemeral, while slightly larger roots persist much longer. Thesedifferences have implications for belowground carbon and nitrogen cycles in theshortgrass steppe.     

The effect of soil strength on the growth of pigeonpea radicles and seedlings

The effect of soil strength on the growth of pigeonpea radicles and seedlings was investigated in cores of three clay soils prepared at different water contents and bulk densities in the laboratory.Radicle elongation directly into soil cores was reduced from 50–70 mm d^-1 at strengths less than 0.5 MPa to 0 mm d^-1 at 3.5–3.7 MPa. The response to soil strength was affected by the water content of the soil, presumably as a result of reduced oxygen availability in wetter soil. This effect was apparent in soils wet to air-filled porosities less than 0.15 m³ m^-3.Radicles were more sensitive to high soil strength (>1.5 MPa) than were seedling roots which encountered the same conditions at 60 mm in the profile. Radicle growth ceased at 3.5 MPa which reduced seedling root growth by only 60%.Despite a 60% reduction in root length in the high strength zone, seedling roots compensated in zones of loose soil above and below the compacted layer, and total root length and shoot growth were unaffected. There was no evidence of a root signal response which results in reduced shoot growth in some species in response to high soil strength.The proliferation of roots in surface layers and the delayed penetration of the root system to depth in compacted soil are likely to expose seedlings to a greater risk of water-deficit in the field, particularly under dryland conditions where plants rely on stored subsoil water for growth.     

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.     

Genotypic Variation in Root Growth Angle in Rice (Oryza sativa L.) and its Association with Deep Root Development in Upland Fields with Different Water Regimes

Deep root development, which is important for the drought resistance in rice (Oryza sativa L.), is a complex trait combining various root morphologies. The objective of this study was to elucidate genotypic variation in deep root development in relation to morphological indicators such as vertical root distribution and root growth angle. Two experiments were conducted: one on upland fields, and one in pots and fields. In experiment 1, the root systems of six rice cultivars on upland fields were physio-morphologically analyzed under different water regimes (irrigated and intermittent drought conditions during panicle development). In experiment 2, cultivar differences in root growth angles were evaluated with 12 cultivars using the basket method under irrigated conditions. No cultivar × environment interactions were found for total root length or deep root length between irrigated and drought conditions in experiment 1. This suggests that constitutive root growth, which is genetically determined, is important for deep root development under intermittent drought conditions during reproductive stage. Among root traits, the deep root ratio (i.e., deep root weight divided by total root weight) was most closely related to deep root length under both water regimes. This suggested that vertical root distribution constitutively affects deep root length. Significant genotypic variation existed in the nodal root diameter and root growth angle of upland rice in experiment 2. It was considered that genotypes with thick roots allocated more assimilates to deep roots through root growth angles higher to the horizontal plane on upland fields. This is the first report on genotypic variation in the root growth angle of rice on upland fields. It should prove useful for rough estimations of genotypic variation in the vertical root distribution of upland rice because root growth angle is rapidly and easily measured.     

水曲柳根系生物量、比根长和根长密度的分布格局

采用连续钻取土芯法在生长季内对东北林业大学帽儿山实验林场17年生水曲柳人工林根系取样,研究水曲柳不同直径根系现存生物量、比根长和根长密度及垂直分布状况.结果表明,水曲柳人工林根系总生物量为1 637.6 g·m^-2,其中活根生物量占85%,死根占15%.在活根生物量当中,粗根(直径5～30 mm)占的比例最高（69.95%）,其次为活细根（直径<1 mm,13.53%）,小根(1～2 mm)和中等直径的根(2～5 mm)比例较小（分别为7.21%和9.31%）.直径<1 mm活细根的比根长为32.20 m·g^-1,直径5～30 mm粗根的比根长为0.08 m·g^-1.单位面积上活根的总长度为6 602.54 m·m^-2,其中直径<1 mm的细根占92.43%,其它直径等级则不到活根总长度的8%.直径<1 mm的细根生物量与根长密度具显著线性关系（R²=0.923）,但与比根长无显著相关关系（R²=0.134）.     

Spatial and temporal patterns of root distribution in developing stands of four woody crop species grown with drip irrigation and fertilization

In forest trees, roots mediate such significant carbon fluxes as primary production and soil CO₂ efflux. Despite the central role of roots in these critical processes, information on root distribution during stand establishment is limited, yet must be described to accurately predict how various forest types, which are growing with a range of resource limitations, might respond to environmental change. This study reports root length density and biomass development in young stands of eastern cottonwood (Populus deltoidies Bartr.) and American sycamore (Platanus occidentalis L.) that have narrow, high resource site requirements, and compares them with sweetgum (Liquidambar styraciflua L.) and loblolly pine (Pinus taeda L.), which have more robust site requirements. Fine roots (<1 mm), medium roots (1 to 5 mm) and coarse roots (>5 mm) were sampled to determine spatial distribution in response to fertilizer and irrigation treatments delivered through drip irrigation tubes. Root length density and biomass were predominately controlled by stand development, depth and proximity to drip tubes. After accounting for this spatial and temporal variation, there was a significant increase in RLD with fertilization and irrigation for all genotypes. The response to fertilization was greater than that of irrigation. Both fine and coarse roots responded positively to resources delivered through the drip tube, indicating a whole-root-system response to resource enrichment and not just a feeder root response. The plastic response to drip tube water and nutrient enrichment demonstrate the capability of root systems to respond to supply heterogeneity by increasing acquisition surface. Fine-root biomass, root density and specific root length were greater for broadleaved species than pine. Roots of all genotypes explored the rooting volume within 2 years, but this occurred faster and to higher root length densities in broadleaved species, indicating they had greater initial opportunity for resource acquisition than pine. Sweetgum’s root characteristics and its response to resource availability were similar to the other broadleaved species, despite its functional resemblance to pine regarding robust site requirements. It was concluded that genotypes, irrigation and fertilization significantly influenced tree root system development, which varied spatially in response to resource-supply heterogeneity created by drip tubes. Knowledge of spatial and temporal patterns of root distribution in these stands will be used to interpret nutrient acquisition and soil respiration measurements. The US Government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper. Mention of a commercial or proprietary product does not constitute endorsement or recommendation by the USDA Forest Service.