Tolerance to acid soil conditions of the velvet beans Mucuna pruriens var. utilis and M. deeringiana

Velvet beans, fast growing leguminous cover crops used in the humid tropics, are shallow rooted on acid soils. This might be due to an inherent branching pattern, to an intrinsic toxicity of the acid subsoil or to a relative preference for root development in the topsoil. Such preference could be based on soil chemical factors in the subsoil or on physical factors such as penetration resistance or aeration. In a field experiment with two species of velvet bean (Mucuna pruriens var. utilis and M. deeringiana) all topsoil was removed and plants were sown directly into the acid subsoil. Root development was neither affected by this treatment nor by P fertilization or liming. In the absence of topsoil good root development in the exposed upper layer of subsoil was possible, so the hypothesis of a toxicity per se of the subsoil could be rejected. To test whether poor root development in the subsoil in the presence of topsoil is due to an inherent branching pattern of the plant or to a relative preference for topsoil, a modified in-growth core technique was used. Local topsoil and subsoil and an acid soil with a higher exchangeable Al content were placed in mesh bags at different depths and at different bulk densities, with and without lime and/or P fertilizer. A comparison of root development in mesh bags placed in the topsoil or subsoil showed that position and thus inherent branching pattern is not important. Root development in the subsoil was poor when this soil was placed in a mesh bag in the topsoil, but in an acid soil of much higher exchangeable Al content and higher percentage Al saturation more roots developed. In a second experiment in mesh bags, bulk density of the repacked soil in the range 1.0–1.5 g cm^-3 had no significant effect on root development. P fertilization and a high rate of liming of the soil placed in the mesh bag had a positive effect on root length density. It is concluded that poor root development in the acid subsoil under field conditions is due to a relative preference for topsoil. Al saturation and bulk density of the soil are not directly involved in this preference, but differences in availability of P and Mg or in Ca/Al ratios might play a role.     

The effects of lime and phosphorus on the function of wheat roots in acid top soils and subsoils

Two wheat varieties with differing aluminium tolerance were grown in pots of acid soil. Liming did not change significantly the amounts of chemically extractable P and K, but caused improved vegetative growth, increased inflow of P and K and reduced uptake of Al. Without lime, roots had a higher content and concentration of P than shoots; liming reversed this. Without lime the sensitive variety with a shorter root length had an Al inflow ten times that of the tolerant one: tolerance involves a mechanism for exlcuding Al. The inflow of P per unit inflow of Al (mol ratio) without lime was three times greater for the tolerant variety which therefore has more P to counteract the effects of Al. The same varieties were grown in two-layer soil columns, with a low P status and a limed topsoil and acid subsoil. Liming the subsoil improved plant growth but this was still restricted by low P availability. Addition of P to the topsoil caused good growth regardless of subsoil acidity: root growth increased in both layers and P (labelled with³²P) taken up from the topsoil was translocated to roots in the subsoil. This P inactivated root Al and allowed the roots to grow and take up more P from the acid subsoil with however a reduction in inflow. The sensitive variety was affected more by the acid subsoil and low P availability, had a similar ability to translocate P to subsoil roots but could not attain the growth rate of the tolerant wheat even with P and lime.     

The position of localized soil compaction determines root and subsequent shoot growth responses

Plants growing in soils typically experience a mixture of loose and compact soil. The hypothesis that the proportion of a root system exposed to compact soil and/or the timing at which this exposure occurs determines shoot growth responses was tested. Broccoli (Brassica oleracea var. italica cv. Greenbelt) seedlings were grown in pot experiments with compact, loose and localized soil compaction created by either horizontal (compact subsoils 75 or 150 mm below loose topsoil) or vertical (adjacent compact and loose columns of soil) configurations of loose (1.2 Mg m(-3)) and compact (1.8 Mg m(-3)) soil. Entirely compact soil reduced leaf area by up to 54%, relative to loose soil. When compaction was localized, only the vertical columns of compact and loose soil reduced leaf area (by 30%). Neither the proportion of roots in compact soil nor the timing of exposure could explain the differing shoot growth responses to localized soil compaction. Instead, the strong relationship between total root length and leaf area (r(2)=0.92) indicated that localized soil compaction reduced shoot growth only when it suppressed total root length. This occurred when isolated root axes of the same plant were exposed to vertical columns of compact and loose soil. When a single root axis grew through loose soil into either a shallow or deep compact subsoil, compensatory root growth in the loose soil maintained total root length and thus shoot growth was unaffected. These contrasting root systems responses to localized soil compaction may explain the variable shoot growth responses observed under heterogeneous conditions.     

Vertical Growth and Mycorrhizal Infection of Woody Plant Roots as Potential Limits to the Restoration of Woodlands on Landfills

We assessed the vertical growth and mycorrhizal infection of woody plant roots on a closed landfill, using tree and shrub clusters that had been previously installed in patches of increasing size to establish protocols for woodland restoration. The density of the fine roots of shrubs, which had poor-to-moderate mycorrhizal infection, decreased strongly with increasing depth. Oak ( Quercus ) seedlings planted within and outside patches were assessed for ectomycorrhizal infection. Oak root systems were mycorrhizal, but root-tip proliferation was improved and ectomycorrhizal composition was influenced by woody debris in the mineral soil. Most surviving oaks were found within patches, but all seedlings showed poor growth: most taproots were deflected horizontally above the boundary between surface soil and subsoil layers (−15 cm). Abrupt decreases in pH between surface and subsurface horizons (6.9 versus 5.3), together with poor drainage and aeration of the latter soil, were probably responsible for poor root growth. Root growth of greenhouse-grown pine and maple seedlings was similarly restricted in pots packed with topsoil over subsoil material. Our results suggest that many current specifications for the cover of closed landfills will not permit restoration of native woody plant communities because of physical limitations to root growth and infectivity. The structure of the engineered soil must address basic plant growth requirements as well as traditional concerns of drainage and barrier protection.     

Changes in root morphology of white lupin (Lupinus albus L.) and its adaptation to soils with heterogeneous alkaline/acid profiles

The ability of Lupinus albus L. to adapt to a heterogeneous soil profile containing acid subsoil below limed topsoil of the same type, and to utilize nutrients by significantly altering its root system structure, was investigated using specially constructed soil profile tubes. Plants grown in homogeneous acid profiles had the fastest growth while those grown in homogeneous limed-soil profiles showed the slowest growth and exhibited some chlorosis after 19 days. Limed topsoil combined with an acid subsoil profile initially retarded plant growth similar to that in a homogeneous limed soil. However, after 68 days significantly greater growth had occurred in the limed/acid soil treatment relative to the homogeneous limed soil, indicating plants had benefited from the acid subsoil stratum. Plants in the homogeneous limed soil profile had lower concentrations of P, Fe and Mn in shoots compared with those in heterogeneous soils. In contrast, the concentration of Ca increased by 74%, due mainly to an increase in the water-soluble Ca fraction. When grown in a heterogeneous limed/acid soil profile, concentrations of P, Ca, K, Mg, Fe, Mn and Zn in shoots were comparable to those grown in a soil with a homogeneous acid profile. Although total root production was lower in the homogeneous limed-soil profile compared to the acid-soil containing profiles, cluster root mass was maintained at a level comparable with that in acid soil. The roots in heterogeneous soil profiles exhibited extensive plasticity, demonstrating a root-type specific, morphological response to the soil conditions. Within the acid subsoil of a heterogeneous profile, there was a large increase in cluster root mass compared with non-cluster roots. The proliferation of cluster roots in acid soil below limed topsoil may enhance the plant's ability to exploit this soil and facilitate the cultivation of L. albus on limed soil. This revised version was published online in August 2006 with corrections to the Cover Date.     

Shrubland Restoration Following Woody Alien Invasion and Mining: Effects of Topsoil Depth, Seed Source, and Fertilizer Addition

Invasion by woody alien plants, construction, and mining operations are among the major disturbances degrading vegetation in the Cape Floristic Kingdom, South Africa. The aim of this study was to assess whether native fynbos shrubland vegetation could be restored following dense alien invasion and disturbance by mining. An area supporting dense alien trees was cleared and topsoil was stripped and stockpiled to simulate mining disturbance. A field trial investigated the effects of topsoil depth, seed mix application, and fertilizer on native species recruitment and vegetation development over a three‐year period. Soil‐stored seed banks contributed 60% of the species recruited, indicating that areas invaded for three decades have good restoration potential. The addition of a fynbos seed mix, which included serotinous overstory species, improved both the richness and structural composition of the vegetation. Most species sown in untopsoiled plots established, but survival and growth was low compared to topsoil plots. Poor growth in combination with a lack of soil seed bank species, indicate that restoring a diverse and functional cover of indigenous vegetation on subsoil is not possible in the short‐term. Soil amelioration is required to improve rooting conditions and initiate ecosystem processes. Shallow and deep topsoil treatments yielded high plant density, richness, and projected canopy cover, but canopy cover was higher in deep topsoil plots throughout the trial. Fertilizer addition increased canopy cover in untopsoiled and shallow topsoil plots via an increase in alien annual species. Fertilizer addition ultimately may lead to increased native vegetation cover in untopsoiled areas, but as it increased proteoid mortality on deep topsoil plots, it is not recommended for sites where topsoil is available. A species‐rich and structurally representative fynbos community may be restored on topsoiled areas provided that the native disturbance regime is simulated and seeds of major structural guilds not present in the soil seed bank are included in the seed mix.     

Effects of drought stress and arbuscular mycorrhiza on the growth of Gliricidia sepium (Jacq). Walp, and Leucaena leucocephala (Lam.) de Wit. in simulated eroded soil conditions

A greenhouse investigation was conducted to determine the effect of arbuscular mycorrhiza and drought on the growth of two tropical hedgerow legume trees (Gliricidia sepium and Leucaena leucocephala) under simulated eroded soil conditions. It was a factorial design with two levels of watering regime (adequate watering and drought), inoculation with Glomus deserticola (with and without), and two soil types (0-30 cm topsoil and 30-60 cm subsoil). Each treatment was replicated 3 times. After ten drought cycles, the growth of Gliricidia sepium in the subsoil was enhanced by mycorrhizal inoculation under both watering regimes whereas there was no significant contribution of mycorrhizal inoculation to the growth of L. leucocephala in both soil types under the two watering regimes. Drought stress significantly reduced most growth parameters for the two tree species in both soils with or without fungal inoculation. The N-fixing activity of Gliricidia sepium benefited from Glomus deserticola inoculation while that of L. leucocephala was not significantly affected in the topsoil. Mycorrhizal colonization was reduced for both tree species in the subsoil compared to the topsoil while it was significantly increased for both species in the subsoil when compared to the uninoculated subsoil counterpart. In the subsoil, inoculation of Gliricidia sepium with the mycorrhizal fungus increased root colonization by 89% and 73% under adequate watering and drought, respectively, whereas L. leucocephala had only a 38% and 42% increase in root colonization under comparative conditions in the subsoil. Thus Glomus deserticola inoculation may be beneficial to the growth of Gliricidia sepium in a badly eroded site where topsoil is missing.     

The distribution and abundance of wheat roots in a dense,structured subsoil – implications for water uptake

We analysed the abundance, spatial distribution and soil contact of wheat roots in dense, structured subsoil to determine whether incomplete extraction of subsoil water was due to root system limitations. Intact soil cores were collected to 1.6 m below wheat crops at maturity on a red Kandosol in southern Australia. Wheat roots, remnant roots, soil pores and root–soil contact were quantified at fresh breaks in the soil cores. In surface soil layers (<0.6 m) 30–40% of roots were clumped within pores and cracks in the soil, increasing to 85–100% in the subsoil (>0.6 m), where 44% of roots were in pores with at least three other roots. Most pores contained no roots, with occupancy declining from 20% in surface layers to 5% in subsoil. Wheat roots clumped into pores contacted the surrounding soil via numerous root hairs, whereas roots in cracks were appressed to the soil surface and had very few root hairs. Calculations assuming good root–soil contact indicated that root density was sufficient to extract available subsoil water, suggesting that uptake is constrained at the root–soil interface. To increase extraction of subsoil water, genetic targets could include increasing root–soil contact with denser root hairs, and increasing root proliferation to utilize existing soil pores.     

Soil strength influences wheat root interactions with soil macropores

Deep rooting is critical for access to water and nutrients found in subsoil. However, damage to soil structure and the natural increase in soil strength with depth, often impedes root penetration. Evidence suggests that roots use macropores (soil cavities greater than 75 μm) to bypass strong soil layers. If roots have to exploit structures, a key trait conferring deep rooting will be the ability to locate existing pore networks; a trait called trematotropism. In this study, artificial macropores were created in repacked soil columns at bulk densities of 1.6 g cm⁻³ and 1.2 g cm⁻³, representing compact and loose soil. Near isogenic lines of wheat, Rht-B1a and Rht-B1c, were planted and root–macropore interactions were visualized and quantified using X-ray computed tomography. In compact soil, 68.8% of root–macropore interactions resulted in pore colonization, compared with 12.5% in loose soil. Changes in root growth trajectory following pore interaction were also quantified, with 21.0% of roots changing direction (±3°) in loose soil compared with 76.0% in compact soil. These results indicate that colonization of macropores is an important strategy of wheat roots in compacted subsoil. Management practices to reduce subsoil compaction and encourage macropore formation could offer significant advantage in helping wheat roots penetrate deeper into subsoil.     

Model Trials for Phytoremediation of Hydrocarbon-Contaminated Sites by the Use of Different Plant Species

In order to further the development of plant-based remediation of sites contaminated by carbo-chemical and petro-chemical industries, the penetration of the roots of Phragmites australis in contaminated soil substrate was studied in model trials. The series of experiments contained model substrate with firm bitumen and tar. In terms of the level of root penetration, the roots and rhizomes penetrated equally through the middle of the pot and at the edges as well as coming up through the bottom. There were differences between the density of the root systems in the topsoil of the two variations, with the roots in the sample with a 3.5 -cm bitumen layer being more dense. The experiment also showed that shallow rooting plants can penetrate thick barriers and are suitable for planting in contaminated areas. In the subsoil zone, which contained many roots, a reduction of up to 85% in the MOH content was observed. In a second series of long-term pot experiments, the stimulation of hydrocarbon remediation by Phragmites australis, Alnus glutinosa, and Robinia pseudoacacia was studied. In the subsoil zone, which contained many roots, a reduction of up to 64% in the hydrocarbon content was determined. In the comparison between the cropped and noncropped treatments, the decontamination ratio was up to 40% higher in the cropped pots than in the pots without plants. For a determination of microbial activity, two enzymes (catalase, ß-glucosidase) and microbial biomass were measured. Variants with plants showed higher microbial activities than uncropped pots. By increasing “biostimulation,” pollution and also the leaching of pollutants can be reduced.     

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.

     

Morphological plasticity of wheat and barley roots in response to spatial variation in soil strength

Root systems of individual crop plants may encounter large variations in mechanical impedance to root penetration. Split-root experiments were conducted to compare the effects of spatial variation in soil strength on the morphological plasticity of wheat and barley roots, and its relationship to shoot growth. Plants of spring barley (Hordeum vulgare cv Prisma) and spring wheat (Triticum aestivum cv Alexandria) were grown for 12 days with their seminal roots divided between two halves of a cylinder packed with sandy loam soil. Three treatment combinations were imposed: loose soil where both halves of the cylinder were packed to 1.1 g cm^–3 (penetrometer resistance 0.3 MPa), dense soil where both halves were packed to 1.4 g cm^–3 (penetrometer resistance 1 MPa), and a split-root treatment where one half was packed to 1.1 and the other to 1.4 g cm^–3. In barley, uniform high soil strength restricted the extension of main seminal root axes more than laterals. In the split-root treatment, the length of laterals and the dry weight of main axes and laterals were increased in the loose soil half and reduced in the dense soil half compared with their respective loose and dense-soil controls. No such compensatory adjustments between main axis and laterals and between individual seminal roots were found in wheat. Variation in soil strength had no effect on the density of lateral roots (number per unit main axis length) in either barley or wheat. The nature and extent of wheat root plasticity in response to variation in soil strength was very different from that in response to changes in N-supply in previous experiments. In spite of the compensatory adjustments in growth between individual seminal roots of barley, the growth of barley shoots, as in wheat, was reduced when part of the root system was in compacted soil.     

生活污水慢渗生态处理对土壤及杨树生长的影响

为了观测生活污水杨树林地处理对土壤和林木生长的影响,2008—2009年在郑州市龙湖镇,采用不同水力负荷(0、3、6、9、12、15cm/周),进行了污水慢渗生态处理试验。测定了污水处理期间杨树地上部分生长量,对表层(0—40cm)和下层(40—100cm)土壤理化性质进行了分析。选用土壤容重、有机质、全氮、全磷、全钾、速效氮、速效磷、速效钾作为土壤质量因子,运用土壤质量综合指标评价不同处理表层土壤质量,对下层土壤的理化性状的变化进行了研究。结果表明:在3—9cm/周水力负荷时,表层土壤质量综合指标值(QI值)和杨树地上部分生长量均随着水力负荷的增加而增加,在9cm/周水力负荷时达到最大;水力负荷大于9cm/周,QI值和杨树地上部分生长量随着水力负荷增加而降低。在水力负荷较低时,污水处理对下层土壤性状影响较小,水力负荷大于9cm/周,污水处理对下层土壤性状产生了不良影响。生活污水杨树林地处理比较适宜的水力负荷是6—9cm/周。     

Performance of Anisantha (Bromus) tectorum and Rumex acetosella in sandy calcareous soil

Many plants are adapted to an eroded landscape with a large proportion of virgin soil. Open and disturbed soils are today almost only restricted to agricultural fields with high loads of fertilizers. We conducted a pot experiment in order to investigate growth and nutritional constraints of one calcicole species, Anisantha (syn. Bromus) tectorum, and one calcifuge species, Rumex acetosella, in decalcified topsoil and recently exposed calcareous subsoil from a field experiment in sandy grassland. In the pot experiment we implemented one treatment where we limed topsoil with CaCO₃ to the same amount as in subsoil.The subsoil had approximately 10% CaCO₃ and both species grew less in this soil compared to the topsoil, which had less than 1% CaCO₃. Germination rate of A. tectorum was higher in subsoil than in topsoil or limed topsoil. P fertilization of the limed topsoil counteracted the negative liming effect for A. tectorum, but only partly so for R. acetosella. P fertilization of subsoil increased the shoot biomass of A. tectorum, but not of R. acetosella. P concentration in plants was not reduced when growing on subsoil or limed topsoil compared to topsoil. The results show that lime addition may reduce the P availability also to calcicole species such as A. tectorum and we found indications for that Ca toxicity may be a causing factor for the calcifuge behavior of R. acetosella. The significance of the results for conservation management practices in sandy grasslands is discussed.     

Root distribution and morphology of maize seedlings as affected by tillage and fertilizer placement

Suboptimal soil conditions are known to result in poor early growth of maize (Zea mays L.) in no-tillage (NT) systems in contrast with conventional tillage (CT) systems. However, most studies have generally focused on maize roots at later growth stages and/or do not give details on root morphology. In a 2-year field study at two locations (silt loam and loam soils) in the Swiss midlands, we investigated the impacts of tillage intensity, NT vs. CT, and NP-fertilizer sidebanding on the morphology, vertical and horizontal distribution, and nutrient uptake of maize roots at the V6 growth stage. The length density (RLD) and the length per diameter-class distribution (LDD) of the roots were determined from soil cores taken to a depth of 0.5 m and at distances of 0.05 and 0.15 m from both sides of the maize row. The temperature of the topsoil was lower, and the bulk density and penetration resistance were greater in the topsoil of NT compared with CT. The growth and the development of the shoot were slower in NT. RLD was greater and the mean root diameter smaller in CT than in NT, while the vertical and horizontal distribution of roots did not differ between CT and NT. RLD increased in the zone enriched by the sidebanded fertilizer, independent of the tillage system, but LDD did not change. The poorer growth of the roots and shoots of maize seedlings was presumably caused by the lower topsoil temperature in NT rather than by mechanical impedance. The placement of a starter fertilizer at planting under NT is emphasized.     

Use of a microtensiometer technique to study hydraulic lift in a sandy soil planted with pearl millet (Pennisetum americanum [L.] Leeke)

A pot experiment was carried out with pearl millet (Pennisetum americanum [L.] Leeke) growing in a sandy soil in which the upper (topsoil) and lower (subsoil) parts of the pots were separated by a perlite layer to prevent capillary water movement. Using microtensiometers a study was made to establish whether it was possible to measure hydraulic lift by which the upper part of the soil was rewetted when water was supplied exclusively to the lower part of the soil.Hydraulic lift occurred during the first seven days of the period of measurement, with a maximum water release to the soil of 2.7 Vol. % during one night (equivalent to 10.8 mL water in the top 10 cm of the soil profile). This magnitude was obtained at very high root length densities, so that water release from the roots would be expected to be much smaller under field conditions.Hydraulic lift ceased when the soil matric potential in the topsoil dropped below-10 kPa at the end of the light period and could not be re-established, neither by extending the dark period, nor after rewatering the topsoil. The disappearance of hydraulic lift could be explained in part through osmotic adaptation of plant roots and, thus prevention of water release from the roots in the topsoil. It is concluded that hydraulic lift may affect nutrient uptake from drying topsoil by extending the time period favourable for uptake from the topsoil.     

The topsoil as the major store of the propagules of vesicular-arbuscular mycorrhizal fungi in southeast Australian sandstone soils

The formation of vesicular-arbuscular mycorrhizae (VAM) in intact soil profiles from two sites in southeastern Australia were measured at two depths using a bioassay grown in intact soil cores. Intact soil cores were taken from (1) topsoil (0–15 cm) and (2) subsoil (15–30 cm) four times during 1990. Seeds of Acacialinifolia (Vent.) Willd. (Mimosaceae) were sown into the cores and plants harvested 8 and 12 weeks after sowing. For 1990, at both sites and in all seasons, VAM most readily developed in the roots of seedlings of A. linifolia grown in topsoil. Limited VAM occurred in roots grown in subsoil cores. Most colonisation of roots by VAM occurred from cores collected during spring and summer. Spore numbers were quantified for each site and depth by wet-sieving 100-g samples of air-dried soil and counting turgid spores containing oil droplets. Three types of spores were found in the soils. Few spores were extracted from all soils sampled, and for the most abundant of the spore types at least twice as many spores occurred in the topsoil than in the subsoil for all seasons examined. As most of the propagules that initiate VAM infection were observed in the topsoil, disturbances which involve the removal and storage of the top 15 cm will adversely affected these fungi.     

土地利用变化对花岗岩红壤底土溶解性有机质数量和光谱特征的影响

了解底土溶解性有机质(DOM)的数量和化学结构对土地利用变化的响应,对科学评价区域土壤有机质动态和碳库稳定性具有重要意义。通过选取花岗岩红壤丘陵区同一景观单元的天然林地(常绿阔叶林)以及由此转变而来的杉木人工林、板栗园和坡耕地,采用化学分析结合光谱扫描(紫外光谱、二维荧光光谱和傅里叶变换红外光谱)技术,研究底土(0.2—1 m)和表土(0—0.2 m)DOM数量和结构对土地利用变化的响应差异,结果表明:58%—87%的DOM贮存在底土中。天然林地土壤的DOM数量最为丰富,底土DOM的宏观化学结构比表土更为简单,以碳水化合物、类蛋白为主。天然林转变为其他利用方式后,底土DOM的损失量(26%—41%)超过表土(12%—49%),冬季比夏季更为凸显;这反映底土DOM数量对人为干扰和植被变化的高度敏感性。同时,底土DOM宏观化学结构趋于复杂化,芳香类、烷烃类和烯烃类的化学抗性物质出现积累的现象。DOM光谱曲线形状、特定峰值、特征值对土地利用的响应敏感,对人为干扰后植被、土壤有机质的变化具有生态指示意义。研究显示,天然林地转变为其他利用方式后,不仅导致底土DOM的损失,也显著降低土壤有机质品质,长期上削弱底土的碳库稳定性和碳吸存能力。     

Tolerance to acid soil conditions of the velvet beans Mucuna pruriens var. utilis and M. deeringiana

Fast growing, climbing leguminous cover crops such as the velvet beans can be used to reclaim weed-infested, degraded soils in the humid tropics, especially land covered by the grass Imperata cylindrica; they climb over the grass leaves and shade the grass out if their cover lasts long enough. Tolerance of two species of velvet bean to eroded soils was investigated by removing topsoil and directly sowing into the subsoil; plots where topsoil was not removed were used as a control. The response to small amounts of P fertilizer and lime was also tested. Removal of the topsoil resulted in retarded growth of both species, in increased dry matter content of the shoot, in decreased specific leaf area and in increased leaf weight ratio, due to shorter internodes. Six weeks after planting the leaf area index (LAI) was about 1.2 where topsoil was retained, sufficient for a shading effect on Imperata. Where topsoil had been removed, the LAI was only 0.6. Mucuna pruriens var. utilis showed a faster aboveground growth than M. deeringiana; the species did not differ in tolerance to eroded soil. Small amounts of P fertilizer had no significant effect on the growth of both Mucuna species. Shoot: root ratios, on a dry weight basis, were much lower when topsoil had been removed, about 3.7 and 2.4 for M. p. utilis and M. deeringiana respectively, compared to 6.2 and 3.3 where topsoil was retained. Removal of topsoil led to reduced Mg and to increased Al concentrations in roots, and to increased levels of Mn and Al in shoots. In the second year no effect of lime or residual effect of P application was found on growth of Mucuna or Imperata. Removal of the topsoil had little effect on the growth of weeds after the cover crop had been harvested. Due to the high Al tolerance of Imperata, reclamation by Mucuna will be less effective if the topsoil has been lost by erosion.     

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.