Prevention of aluminium toxicity with supplemental boron. II. Stimulation of root growth in an acidic, high-aluminium subsoil

Root growth inhibition is an early symptom of Al toxicity and B deficiency. Our hypothesis is that Al toxicity may induce B deficiency, and it was our objective to determine if incorporation of supplemental B would promote root penetration into an acidic, high-Al subsoil. Alfalfa (Medicago sativa L. cv. Hy-Phy) was grown in slanted tubes with a Plexiglas window along their length. The top half of the tub contained silt loam soil and the bottom half contained subsoil from the Bt1 horizon (26% Al saturation) of a Creldon silty clay loam. Both soils originally contained 0–9 kg B ha^?1. When root growth was measured in the bottom half of the high-A1 subsoil, all measurements—depth of rooting, total root growth, final root lengths and root dry weight—demonstrated greater root growth in treatments where additional B was incorporated into the high-Al subsoil. Results from this soil study extend those obtained in our hydroponic study in which supplemental B presented Al inhibition of root growth. Boron concentrations may need to he increased under acidic ‘high-Al’ soil conditions to promote root penetration into these soil zones, and this could be especially important during periods of drought stress.     

Root distribution,standing crop biomass and belowground productivity in a semidesert in México

In a semidesert community in México (Zapotitlán de las Salinas, Puebla) the vertical distribution of roots and root biomass was estimated at 0–100 cm depth on two sampling dates, November 1995 (wet season) and January 1998 (dry season). Root productivity at 7 to 14.5 cm depth was estimated with the in-growth core technique every two months from March 1996 to February 1998. The relationship between environmental factors and seasonal root productivity was analyzed. Finally, we tested the effect of an irrigation equivalent to 20 mm of rain on root production. Seventy four percent of the total number of roots were found at 0-40 cm depth. Very fine roots (<1 mm diameter) were found throughout the soil profile (0-100 cm). In contrast, fine roots (1-3 mm diameter) were found only from 0–90 cm depth, and coarse roots (>3 mm diameter) from 0–60 cm depth. The root biomass was 971.5 g m^–2 (S.D. = 557.39), the very fine and fine roots representing 62.9% of the total. Total root productivity, as estimated with the ingrowth core technique, was 0.031 Mg ha^–1 over the dry season and 0.315 Mg ha^–1 over the wet season. Only very fine roots were obtained at all sampling dates. Rainfall was significantly correlated with very fine root production. The difference between fine root production in non-watered (0.054 g m^–2) and watered (0.429 g m^–2) treatments was significant. The last value was the same as that predicted for a rain of 20 mm, according to the exponential model describing the relation between the production of very fine roots and rainfall at the site.     

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

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

Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content,and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to investigate whether differences in root growth among plant species are related to their ability to deplete the soil nitrate-N pool. Large differences were observed in root growth parameters. Monocot species had rooting depth penetration rates in the range of 1.0 to 1.2 mm d^–1 °C^–1, whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d^–1 °C^–1. Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum needed for the crops to reach a rooting depth of 1.0 m varied from 750 d °C for fodder radish to 1375 d °C for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m the non-legume dicot species generally had root intensities (number of root intersections m^–1 line on the minirhizotrons) 12 times as high as the monocot species.The amount of nitrate left in the topsoil (0–0.5 m) was only weakly correlated to a few of the measured plant and root parameters, whereas nitrate left in the subsoil (0.5–1.0 m) was clearly correlated to several root parameters. Subsoil nitrate residues were well correlated to root intensity, but showed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0–1.5 m), the soil water nitrate concentration was reduced from 119 g L^–1 without a catch crop to 61 g L^–1 under Italian ryegrass and to only 1.5 g L^–1 under fodder radish.The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In this study, none of the measurements made aboveground or in the upper soil layers were well related to subsoil nitrate depletion.     

等距开槽结合施用石膏改良苏打盐碱土

采用田间小区试验,研究了等距开槽结合施用石膏对东北松嫩平原西部苏打盐碱土的改良效果。结果表明:等距开槽(20～40cm心土层开槽,槽间距1m)能有效降低开槽点的土壤紧实度,对槽间土壤紧实度也有一定的降低作用,且与对照(不开槽)紧实度的差异达到极显著水平;等距开槽必须结合石膏才能改善土壤的化学性质,加石膏与未加石膏处理的pH和碱化度达到差异显著水平,而电导率值在处理前后差异不显著,开槽松土处理对槽内土壤化学性质改善效果不明显;磷石膏与脱硫石膏在改土效果上差异不显著,石膏配合开槽能有效改善土壤理化性质和植物生长状况,且对槽间土壤改良和植物生长均有一定的促进作用。     

Biomass and production of fine and coarse roots during regrowth of a disturbed subtropical humid forest in north-east India

Seasonal variation and depthwise distribution of dry matter in roots of different diameter classes and their annual production were studied using sequential core sampling. The investigations were carried out in three stands of a subtropical humid forest of north-east India representing different stages of regrowth after tree cutting. The mean annual standing crop of fine (<2 mm in diameter) and coarse (2–15 mm diameter) roots increased gradually from 5.4 Mg ha^-1 and 0.7 Mg ha^-1 in 7-yr old regrowth to 9.4 Mg ha^-1 and 2.8 Mg ha^-1 in 16-yr old regrowth, respectively. The contribution of fine roots to the total root mass declined from 88% in 7-yr old regrowth to 77% in both 13 and 16-yr old regrowths, while that of coarse roots increased from 12 to 23%. A major portion of fine roots (59–62%) was present in 0–10 cm soil layer, but the coarse roots were concentrated in 10–20 cm soil depth (38–48%). In all the three stands, biomass of both fine and coarse roots followed a unimodal growth curve by showing a gradual increase from spring/pre-rainy season to autumn/post-rainy season. Biomass to necromass ratio increased from 2.5 in the 7-yr old to 3.2 in the 16-yr old stand. The annual fine root production increased from 5.9 Mg ha^-1 to 7.7 Mg ha^-1 and total root production from 7.6 Mg ha^-1 to 14.7 Mg ha^-1 from 7-yr to 16-yr old regrowth.     

Effects of contrasting soil fertility on root mass, root growth, root decomposition and soil carbon under a New Zealand perennial ryegrass/white clover pasture

The contribution of below ground plant root tissue to soil carbon (C) pools is attracting considerable interest in the context of greenhouse gas mitigation options. A field experiment was conducted on a perennial ryegrass/white clover pasture in the Manawatu, New Zealand, to examine the effect of differing soil nitrogen (N) and phosphorus (P) fertility status on root dynamics. Root standing mass, shoot and root dry matter (DM) accumulation and root tissue decomposition were measured at 6–8 week intervals over one year at moderate (Olsen P?=?24, no added N) and high (Olsen P?=?49, 400 kgN ha^?1y^?1 added N) soil fertility levels. Shoot production was significantly greater in the high fertility treatment (2550 cf. 1890 gDM m^?2y^?1) but differences in root dynamics were confined to two periods in spring and winter. In late spring the pattern was for lower root mass (183 cf. 231 gDM m^?2 between 0–80 mm depth) and higher root production (0.71 cf. 0.52 gDM m^?2 d^?1 between 0–120 mm depth) under higher fertility. In winter the reverse was observed. There is some evidence that the soil type used in the root in-growth cores underestimated root production values for this site by a factor of approx. one third. Short-term differences between the two fertiity treatments in standing root mass and root production did not lead to treatment differences in topsoil C and N changes over four years. This may reflect insufficient separation in the two soil fertility treatments and a low overall root tissue input to soil organic matter.     

Above- and belowground organic matter storage and production in a tropical pine plantation and a paired broadleaf secondary forest

The distribution of tree biomass and the allocation of organic matter production were measured in an 11-yr-old Pinus caribaea plantation and a paired broadleaf secondary forest growing under the same climatic conditions. The pine plantation had significantly more mass aboveground than the secondary forest (94.9 vs 35.6 t ha^-1 for biomass and 10.5 vs 5.0 t ha^{-1 for litter}), whereas the secondary forest had significantly more fine roots (⩽2 mm diameter) than the pine plantation (10.5 and 1.0 t ha^-1, respectively). Standing stock of dead fine roots was higher than aboveground litter in the secondary forest. In contrast, aboveground litter in pine was more than ten times higher than the dead root fraction. Both pine and secondary forests had similar total organic matter productions (19.2 and 19.4 t ha^-1 yr^-1, respectively) but structural allocation of that production was significantly different between the two forests; 44% of total production was allocated belowground in the secondary forest, whereas 94% was allocated aboveground in pine. The growth strategies represented by fast growth and large structural allocation aboveground, as for pine, and almost half the production allocated belowground, as for the secondary forest, illustrate equally successful, but contrasting growth strategies under the same climate, regardless of soil characteristics. The patterns of accumulation of organic matter in the soil profile indicated contrasting nutrient immobilization and mineralization sites and sources for soil organic matter formation.     

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

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

Effects of heavy metal soil pollution and acid rain on growth and water use efficiency of a young model forest ecosystem

In a 4-year lysimeter experiment, we investigated the effects of topsoil heavy metal pollution (3,000 mg kg⁻¹ Zn, 640 mg kg⁻¹ Cu, 90 mg kg⁻¹ Pb and 10 mg kg⁻¹ Cd) and (synthetic) acid rain (pH 3.5) on tree growth and water use efficiency of young forest ecosystems consisting of Norway spruce (Picea abies), willow (Salix viminalis), poplar (Populus tremula) and birch (Betula pendula) trees and a variety of understorey plants. The treatments were applied in a Latin square factorial design (contaminated vs uncontaminated topsoil, acidified rain vs ambient rain) to 16 open-top chambers, with 4 replicates each. Each open-top chamber contained two lysimeters, one with a calcareous, and the other with acidic subsoil. The four tree species responded quite differently to heavy metal pollution and type of subsoil. The fine root mass, which was only sampled at the end of the experiment in 2003, was significantly reduced by heavy metal pollution in P. abies, P. tremula and B. pendula, but not in S. viminalis. The metal treatment responses of above-ground biomass and leaf area varied between years. In 2002, the heavy metal treatment reduced above-ground biomass and leaf area in P. tremula, but not in the other species. In 2003, metals did not reduce above-ground growth in P. tremula, but did so in the other species. It appears that the responses in above-ground biomass and leaf area, which paralleled each other, were related to changes in the relative competitive strength of the various species in the two experimental years. S. viminalis gained relative to P. tremula in absence of metal stress, in particular on calcareous subsoil, while P. abies showed the largest increases in growth rates in all treatments. Above- and below-ground growth was strongly inhibited by acidic subsoil in S. viminalis and to a lesser degree also in P. abies. In P. abies, this subsoil effect was enhanced by metal stress. Acid rain was not found to have any substantial effect. Whole-system water use efficiency was reduced by metal stress and higher on calcareous than on acidic subsoil.     

Macropore effects on phosphorus acquisition by wheat roots – a rhizotron study

Background and aims

Macropores may be preferential root pathways into the subsoil. We hypothesised that the presence of macropores promotes P-uptake from subsoil, particularly at limited water supply in surface soil. We tested this hypothesis in a rhizotron experiment with spring wheat (Triticum aestivum cv. Scirocco) under variation of fertilisation and irrigation.

Methods

Rhizotrons were filled with compacted subsoil (bulk density 1.4 g cm^?3), underneath a P-depleted topsoil. In half of these rhizotrons the subsoil contained artificial macropores. Spring wheat was grown for 41 days with and without irrigation and ³¹P–addition. Also, a ³³P–tracer was added at the soil surface to trace P-distribution in plants using liquid scintillation counting and radioactive imaging.

Results

Fertilisation and irrigation promoted biomass production and plant P-uptake. Improved growing conditions resulted in a higher proportion of subsoil roots, indicating that the topsoil root system additionally promoted subsoil nutrient acquisition. The presence of macropores did not improve plant growth but tended to increase translocation of ³³P into both above- and belowground biomass. ³³P–imaging confirmed that this plant-internal transport of topsoil-P extended into subsoil roots.

Conclusions

The lack of penetration resistance in macropores did not increase plant growth and nutrient uptake from subsoil here; however, wheat specifically re-allocated topsoil-P for subsoil root growth.

     

Nitrogen uptake in a Norway spruce stand following ammonium sulphate application,fertigation, irrigation,drought and nitrogen-free-fertilisation

The above-ground accumulation of N,N uptake and litter quality resulting from improved or deteriorated availability of water and nutrients in a 25 year old Norway spruce stand in SW Sweden (as part of the Skogaby project) is presented. Treatment include irrigation; artificial drought; ammonium sulphate addition; N-free-fertilisation and irrigation with liquid fertilisers including a complete set of nutrients according to the Ingested principle (fertigation). At start of the experiment the stand contained 86.5 t dry mass and 352 kg N ha⁻¹. The following three years the annual N uptake in untreated trees was 32 kg N ha⁻¹ to be compared with the annual N throughfall of 17 kg ha⁻¹. Simultaneously, the treatment with ammonium sulphate and liquid fertilisation resulted in 48 and 56 kg ha⁻¹ y⁻¹, respectively, in treatment specific N-uptake following an application of 100 kg N ha⁻¹ y⁻¹. Addition of a N-free fertiliser resulted in improved N-uptake by 19 kg N ha⁻¹ y⁻¹ and irrigation by 10 kg N ha⁻¹ y⁻¹, compared to control. A linear relation between total above-ground dry mass production and N-uptake was found for trees growing with similar water availability. Dry mass production increased with increased water availability given the same N-uptake. It is concluded that the studied stand this far is not N saturated', as N fertilisation resulted in both increased N uptake and increased growth. Addition of a N-free-fertiliser resulted in increased uptake of N compared to the control, indicating an increased mineralisation rate or uptake capacity of the root system. The linear relation between N uptake and biomass production shows that at this study site N is a highly limiting factor for growth.     

Is the simple auger coring method reliable for below-ground standing biomass estimation in Eucalyptus forest plantations?

Background and Aims

Despite their importance for plant production, estimations of below-ground biomass and its distribution in the soil are still difficult and time consuming, and no single reliable methodology is available for different root types. To identify the best method for root biomass estimations, four different methods, with labour requirements, were tested at the same location.

Methods

The four methods, applied in a 6-year-old Eucalyptus plantation in Congo, were based on different soil sampling volumes: auger (8 cm in diameter), monolith (25 × 25 cm quadrate), half Voronoi trench (1·5 m³) and a full Voronoi trench (3 m³), chosen as the reference method.

Key Results

With the reference method (0–1m deep), fine-root biomass (FRB, diameter <2 mm) was estimated at 1·8 t ha⁻¹, medium-root biomass (MRB diameter 2–10 mm) at 2·0 t ha⁻¹, coarse-root biomass (CRB, diameter >10 mm) at 5·6 t ha⁻¹ and stump biomass at 6·8 t ha⁻¹. Total below-ground biomass was estimated at 16·2 t ha⁻¹ (root : shoot ratio equal to 0·23) for this 800 tree ha⁻¹ eucalypt plantation density. The density of FRB was very high (0·56 t ha⁻¹) in the top soil horizon (0–3 cm layer) and decreased greatly (0·3 t ha⁻¹) with depth (50–100 cm). Without labour requirement considerations, no significant differences were found between the four methods for FRB and MRB; however, CRB was better estimated by the half and full Voronoi trenches. When labour requirements were considered, the most effective method was auger coring for FRB, whereas the half and full Voronoi trenches were the most appropriate methods for MRB and CRB, respectively.

Conclusions

As CRB combined with stumps amounted to 78 % of total below-ground biomass, a full Voronoi trench is strongly recommended when estimating total standing root biomass. Conversely, for FRB estimation, auger coring is recommended with a design pattern accounting for the spatial variability of fine-root distribution.     

Root and inorganic nitrogen distributions in sesbania fallow,natural fallow and maize fields

One hypothesis for a benefit of integrating trees with crops is that trees with deep root systems can capture and pump up nutrients from below the rooting zone of annual crops. Few studies have compared both root and nutrient distribution for planted trees, crops and grassland vegetation. A field study was conducted on a Kandiudalfic Eutrudox in the highlands of western Kenya to measure rooting characteristics and distribution of inorganic N and water in three land-use systems (LUS): (i) Sesbania sesban (L.) Merr. fallow, (ii) uncultivated natural weed fallow and (iii) unfertilized maize (Zea mays L.) monoculture. The maximum rooting depth was 1.2 m in the maize LUS, 2.25 m in a 13-month-old natural fallow, and > 4 m in a 15-month-old sesbania fallow. Total root length was 1.26 km m^-2 for the maize LUS, 5.98 km m^-2 for the natural fallow, and 4.56 km m^-2 to 4 m for the sesbania fallow. Root length to 1.2 m was greater (p < 0.01) for natural fallow than for maize and sesbania fallow. A considerable portion of the sesbania root length to 4 m was in the subsoil; 47% was at 1.2 to 4 m and 31% was at 2.25 to 4 m. Deep rooting of sesbania coincided with lower soil water below 2 m in the sesbania fallow than the natural fallow. Nitrate-N, but not ammonium-N, to 4 m was affected by LUS. Total nitrate to 4 m was 199 kg N ha^-1 for the maize LUS, 42 kg N ha^-1 for the natural fallow and 51 kg N ha^-1 for the sesbania fallow. Soil nitrate in the maize LUS was highest at 0.3 to 1.5-m depth on this Oxisol with anion sorption capacity. No such accumulation of subsoil nitrate was present under sesbania and natural fallow.     

Fine root vertical distribution and temporal dynamics in mature stands of two enset (Enset ventricosum Welw Cheesman) clones

Quantification of the role of fine roots in the biological cycle of nutrients necessitates understanding root distribution, estimating root biomass, turnover rate and nutrient concentrations, and the dynamics of these parameters in perennial systems. Temporal dynamics, vertical distribution, annual production and turnover, and nitrogen use of fine roots (≤2 mm in diameter) were studied in mature (5-year-old) stands of two enset (Ensete ventricosum) clones using the in-growth bag technique. Live fine root mass generally decreased with increasing depth across all seasons except the dry period. Except for the dry period, more than 70% of the fine root mass was in the above 0-20 cm depth, and the fine root mass in the upper 0–10 cm depth was significantly higher than in the lowest depth (20–30 cm). Live fine root mass showed a seasonal peak at the end of the major rainy season but fell to its lowest value during the dry or short rainy season. The difference between the peak and low periods were significant (p ≤ 0.05). Fine root nitrogen (N) use showed significant seasonal variation where the mean monthly fine root N use was highest during the major rainy season. There were significant effects on N use due to depths and in-growth periods, but not due to clones. Enset fine root production and turnover ranged from 2,339 to 2,451 kg ha⁻¹ year⁻¹ and from 1.55 to 1.80 year⁻¹, respectively. Root N return, calculated from fine root turnover, was estimated at 64–65 kg ha⁻¹ year⁻¹. Fine root production, vertical distribution and temporal dynamics may be related to moisture variations and nutrient (N) fluxes among seasons and along the soil depth. The study showed that fine root production and turnover can contribute considerably to the carbon and nitrogen economy of mature enset plots.     

Nitrate leaching stimulates subsurface root growth of wheat and increases rhizosphere alkalisation in a highly acidic soil

Subsurface acidity is a major factor limiting crop yield in some agricultural soils. The surface application of lime has limited effect on the subsurface acidity due to the slow downward movement, while deep incorporation of lime is costly. This paper tested the concept of biologically ameliorating subsurface acidity in a highly acidic soil through the net uptake of anions by plant roots. Nitrogen was supplied to the top soil (0–10 cm) as Ca(NO₃)₂ at rates equivalent to 30–240 kg N ha^?1. Four water levels were imposed (40, 60, 80 and 100% of field capacity). Aluminium-tolerant wheat was grown for 58 days. The high N and high water treatments stimulated root growth below 15 cm, which in turn increased N capture, resulting in a greater excess anion uptake over cations and thus alkalisation of subsurface soil layers. This study suggests that it is feasible to exploit the process of nitrate uptake by an aluminium-tolerant wheat genotype to increase pH in acidic subsoil.     

Root growth,macro-nutrient uptake dynamics and soil fertility requirements of a high-yielding winter oilseed rape crop

Shoot growth, root growth and macro-nutrient uptake by a high-yielding (5t/ha grain) winter oilseed rape crop have been measured. Maximum rooting density in the top 20cm of soil was 9.4 cm cm⁻³ and roots reached a depth of at least 1.8 m. Maximum nutrient uptakes were 364 kg ha⁻¹ for N, 43 kg ha⁻¹ for P, 308 kg ha⁻¹ for K, 287 kg ha⁻¹ for Ca and 16 kg ha⁻¹ for Mg. A 30-day drought coincided with the flowering period and root and shoot growth, as well as nutrient uptake rates, were reduced. Nutrient concentrations in the soil solution necessary to sustain the nutrient fluxes into the root system by diffusive supply have been calculated. Peak values were in the range 10 μM for P to 87 μM for N, lower than the observed concentrations, and it was concluded that nutrient transport to roots was not a limitation to uptake by this rape crop.     

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.     

多枝柽柳幼苗根系形态及生物量对不同灌溉处理的响应

多枝柽柳(Tamarix ramosissima)是塔里木河下游荒漠河岸林中的优势灌木, 对荒漠河岸植被群落的稳定起着重要作用。该文通过研究多枝柽柳幼苗根系形态对不同灌溉处理的响应, 分析人工水分干扰对多枝柽柳幼苗根系生长的影响。实验设计了侧渗分层和地表灌溉两种给水方式和高灌(50 L∙株 ^-1)、中灌(25 L∙株 ^-1)、低灌(12.5 L∙株 ^-1)三个给水水平, 并在整个生长季节监测每个植株的生物量及根系形态参数。结果显示: 与地表灌溉比较, 侧渗分层的灌溉方式显著提高了细根(0.5 mm < d < 2 mm)长、细根表面积和根系生物量, 并使根系生长至160 cm深度的土层, 大于地表灌溉深度(80-100 cm); 侧渗分层灌溉+高灌的组合促进根系生长的效果最显著(p < 0.05); 侧渗分层灌溉方式下总细根(d < 2 mm)的比根长随着给水量的增加显著增大, 而地表灌溉下比根长无显著变化; 侧渗分层灌溉方式下根冠比总体小于地表灌溉方式, 即侧渗分层灌溉使多枝柽柳地上部分发育较好。因此, 侧渗分层灌溉方式有显著促进多枝柽柳幼苗在生长早期快速发育的效果。     

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

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