Relationship between root length density and soil microorganisms in the rhizospheres of white clover and perennial ryegrass

Abstract

Microbial parameters of rhizosphere soil, such as bacterial numbers or microbial activities, depend on the distance that microbes have to the root surface. In this study we show that the number of bacteria found in rhizosphere soil from white clover is highly correlated with root length density of the rhizosphere soil. In contrast, bacterial numbers, microbial activity (measured as fluorescein diacetate hydrolytic activity), and the amount of extractable carbon (C) in the rhizosphere of perennial ryegrass were independent of the amount of soil recovered from the roots. The missing rhizosphere effect in perennial ryegrass soils can be explained by the high rooting density of ryegrass, whereas the low rooting density of white clover results in gradients of microbial numbers and activities in soils. Results of these studies indicate that it is important to express microbial parameters on root length and soil weight bases, especially for less densely rooted soils.     

Effects of pine roots on microorganisms,fauna, and nitrogen availability in two soil horizons of a coniferous forest spodosol

Summary We investigated the effects of pitch pine seedling roots on extractable N, microbial growth rate, biomass C and N, and nematodes and microarthropods in microcosms with either organic (41% C, 1.14% N) or mineral (0.05% C, 0.01% N) horizon soils of a spondosol. Root quantity was manipulated by varying plant density (0, 1, 2, or 4 seedlings) and rhizosphere soil was separated from non-rhizosphere soil by a 1.2 m mesh fabric. In the rhizosphere of organic soil horizons, moisture, microbial growth rate, biomass C and N, and extractable N declined as root density was increased, but there was little effect on nematodes or microarthropods. High levels of extractable N remained after 5 months, suggesting that N mineralization was stimulated during the incubation. In the rhizosphere of mineral soil horizons, microbial growth rate, and nematode and microarthropod abundances increased at higher root density, and in the absence of roots faunal abundance approached zero. Faunal activity was concentrated in the rhizosphere compared to non-rhizosphere soil. In organic soil horizons, roots may limit microbial activity by reducing soil moisture and/or N availability. However, in mineral soil horizons, where nutrient levels are very low, root inputs can stimulate microbial growth and faunal abundance by providing important substrates for microbial growth. Our results demonstrate a rhizosphere effect for soil fauna in the mineral soil, and thus extends the rhizosphere concept to components of the soil community other than microbes for forest ecosystems. Although our results need to be verified by field manipulations, we suggest that the effects of pine roots on nutrient cycling processes in coniferous forests can vary with soil nutrient content and, therefore, position in the soil profile.     

Influence of grass species and soil type on rhizosphere microbial community structure in grassland soils

A number of studies have reported species specific selection of microbial communities in the rhizosphere by plants. It is hypothesised that plants influence microbial community structure in the rhizosphere through rhizodeposition. We examined to what extent the structure of bacterial and fungal communities in the rhizosphere of grasses is determined by the plant species and different soil types. Three grass species were planted in soil from one site, to identify plant-specific influences on rhizosphere microbial communities. To quantify the soil-specific effects on rhizosphere microbial community structure, we planted one grass species (Lolium perenne L.) into soils from three contrasting sites. Rhizosphere, non-rhizosphere (bulk) and control (non-planted) soil samples were collected at regular intervals, to examine the temporal changes in soil microbial communities. Rhizosphere soil samples were collected from both root bases and root tips, to investigate root associated spatial influences. Both fungal and bacterial communities were analysed by terminal restriction fragment length polymorphism (TRFLP). Both bacterial and fungal communities were influenced by the plant growth but there was no evidence for plant species selection of the soil microbial communities in the rhizosphere of the different grass species. For both fungal and bacterial communities, the major determinant of community structure in rhizospheres was soil type. This observation was confirmed by cloning and sequencing analysis of bacterial communities. In control soils, bacterial composition was dominated by Firmicutes and Actinobacteria but in the rhizosphere samples, the majority of bacteria belonged to Proteobacteria and Acidobacteria. Bacterial community compositions of rhizosphere soils from different plants were similar, indicating only a weak influence of plant species on rhizosphere microbial community structure.     

生草对渭北旱地苹果园土壤有机碳组分及微生物的影响

在渭北旱地苹果园行间播种毛苕子（Vicia villosa）、白三叶（Trifolium repens）、黑麦草（Lolium perenne）和早熟禾（Poa pratensis）,以清耕为对照,对0100 cm土层的土壤有机碳各组分及微生物群落功能多样性进行研究。结果表明,行间生草可显著增加040 cm土层土壤的总有机碳（TOC）、颗粒有机碳（POC）、轻质有机碳（LFOC）、易氧化有机碳（ROC）、可溶性有机碳（DOC）和微生物量碳（MBC）,豆科牧草毛苕子和白三叶的各有机碳含量总体上高于禾本科牧草黑麦草和早熟禾。其中在020 cm土层中,豆科牧草的TOC含量平均每年增加约1.2 g/kg,禾本科牧草每年增加约0.9 g/kg。生草处理的微生物群落碳源利用率（AWCD）、微生物群落Shannon 指数（H）和微生物群落丰富度指数（S）均高于清耕处理,其中豆科牧草的微生物活性更高。因此,生草可以提高土壤有机碳各组分的含量、土壤微生物群落碳源利用率、微生物群落的丰富度和功能多样性,豆科牧草毛苕子和白三叶提高效应更加明显。     

Soil organic carbon and nitrogen fractions and water‐stable aggregation as affected by cropping and grassland reclamation in an arid sub‐alpine soil

This paper investigates effects of cropping abandonment and perennial grass growing on soil organic C and N pools and aggregate stability, by comparing soils under native grassland, crop cultivation, perennial grass growing and cropping abandonment, in degraded cropland at a sub‐alpine site in north‐western China. The pools of total and particulate organic C (115 and 37 Mg ha⁻¹) in the 0–30 cm soil layer of native grassland were reduced by 31 and 54% after 30 years of crop cultivation. After 4 years of conversion from cropland to perennial grass growing total and particulate organic C pools were increased by 29 and 56%, whereas 4 year cropping abandonment increased particulate organic C by 36%. Rapid increases in total and particulate N were also found in perennial grass growing and cropping abandonment soils. The native grassland soil and soils of cropping abandonment and perennial grass growing had higher carbohydrate C concentrations in the 0–10 cm layer than the cropped soil. The rapid recovery of particulate organic fraction and carbohydrates in the re‐vegetated soils were probably due to higher plant biomass inputs and lower organic matter decomposition compared with those in the cropped soil. Aggregate stability of the 0–30 cm soil layer was significantly decreased by crop cultivation but showed a good recovery after 4 year re‐vegetations. This study suggests that reduction of soil organic matter and aggregate stability under crop cultivation may be remedied by cropping abandonment or perennial grass growing. Copyright © 2008 John Wiley & Sons, Ltd.     

不同肥力红壤水稻土根际团聚体组成和碳氮分布动态

研究水稻种植期间表层土壤团聚体数量及其有机碳、全氮含量的变化,对揭示人为耕作的影响、认知土壤肥力的演变规律具有重要意义。选择两种不同肥力的红壤性水稻土进行田间根袋试验,分别于水稻插秧前、分蘖期、孕穗期和成熟期采样,分析了水稻生长过程中根际和非根际土壤团聚体组成、稳定性以及有机碳、全氮分布的动态变化。结果表明,低肥力土壤团聚体以0.25 mm大团聚体为主(56.2%~64.0%),0.25~1 mm粒级团聚体含量最高;除1~2 mm粒级外,水稻生育期内根际土壤各粒级团聚体含量均有显著变化;取样时期、根际作用与取样时期的交互效应对0.25~1 mm和0.053~0.25 mm粒级含量有显著影响。高肥力土壤中以0.25 mm微团聚体为主(59.8%~72.0%),0.053~0.25 mm粒级团聚体比例最高,取样时期显著影响0.25 mm大团聚体含量,根际作用与取样时期的交互效应对2 mm粒级含量有极显著影响。与非根际相比,根际土壤大团聚体的破坏率较低,平均重量直径(MWD)较高,种植水稻有助于提高根际土壤的稳定性。两种肥力土壤团聚体中有机碳和全氮含量均表现为1~2 mm粒级最高,0.053~0.25 mm粒级最低,大团聚体中显著高于微团聚体。根际土壤中,水稻成熟期各粒级团聚体有机碳含量与插秧前无显著差异,而分蘖期和孕穗期有明显波动;水稻的生长降低了大团聚体中的全氮含量,对高肥力土壤的影响更为显著。总体而言,低肥力土壤中,根际作用主要影响团聚体组成和稳定性,取样时期影响团聚体碳氮含量;高肥力土壤中,团聚体组成和碳氮分布受根际作用和取样时期的共同影响。     

Carbohydrate composition in relation to structural stability, compactibility and plasticity of two soils in a long-term experiment

The effects of tillage on soil organic carbon content, carbohydrate content, monosaccharide composition, aggregate stability, compactibility and plasticity were investigated in a field experiment on a gleysol and on a cambisol under winter barley in South-East Scotland. Two long-term treatments (direct drilling and conventional mouldboard ploughing for 22 years) were compared with short-term direct drilling and broadcast sowing plus rotavation for 5 years. Carbohydrate released sequentially to cold water, hot water, 1.0 M HCl and 0.5 M NaOH was determined after hydrolysis as reducing sugar equivalent to glucose in both fresh and air-dried samples. All other measurements were made on dry soils only. About 3% of the soluble carbohydrate was extracted by cold water, 10% by hot water, 12% by HCl and 75% by NaOH from both the dry and fresh soils. The total reducing sugars of the fractions were proportional to the total organic carbon determined by dichromate oxidation or C analysis. Organic carbon and carbohydrates were concentrated near the surface of the direct drilled soil, but were more uniformly distributed with depth in the ploughed soil. The surface soil under direct drilling was more stable, less compactible and had greater plasticity limits than under ploughing. However, particle size distributions were unaffected by tillage so that differences in soil properties were attributed to differences in the quantity and quality of organic matter. Differences in compactibility, structural stability and plasticity limits between depths and tillage treatments correlated with total carbon and with total carbohydrates. The hot water extractable carbohydrate fraction correlated best with aggregate stability and the NaOH fraction correlated best with compactibility and plastic limit. Both fractions were greatest in the long-term direct drilled soil. The hot water fraction had a galactose plus mannose over arabinose plus xylose ratio of 1.0–1.6 in comparison to 0.4–0.7 in the NaOH fraction indicating that the microbial contribution within the hot water-soluble fraction was the greater. The hot-water fraction was likely to contain more exocellular microbial polysaccharides involved in the stabilizing of soil aggregates. The hot-water and NaOH carbohydrate fractions may be good indicators of soil organic matter quality relevant to the preservation of good soil physical conditions.     

磷肥减施对玉米根系生长及根际土壤磷组分的影响

【目的】 我国农业过量和不合理施用磷肥现象普遍存在,导致磷资源的浪费,对环境也造成潜在威胁。研究减少磷肥用量对玉米产量、根系形态及根际中磷转化特征的影响,为集约化农业生产体系中磷肥合理施用提供技术基础。 【方法】 在河北省衡水小麦玉米轮作体系下连续三年进行了田间试验,在冬小麦季设置4个P₂O₅用量处理:0、112.5、150.0、187.5 kg/hm2,收获后在原处理小区免耕播种夏玉米。利用WinRHIZO根系分析系统分析获取根长、直径等数据,测定玉米籽粒产量、生物量和地上部磷含量及根际土壤中磷形态等指标。 【结果】 与农民习惯磷肥用量(P₂O₅187.5 kg/hm2)相比,3年磷肥用量减施20%~40%处理(P₂O₅150和112.5 kg/hm2),玉米籽粒产量、根系长度与直径和土壤有效磷含量尚未发生明显变化。但3年不施磷处理,根际土壤有效形态磷含量和玉米籽粒产量开始出现下降趋势。2009年和2010年玉米收获期,不施磷肥处理根际土壤有机磷含量低于非根际土壤。2008年玉米苗期和收获期土壤有机磷分组中,中等活性有机磷含量最高;磷肥减施20%~40%处理苗期根际中中等活性有机磷含量显著低于非根际土壤。土壤无机磷形态分组研究发现:从玉米苗期到收获期,各磷肥处理根际和非根际土壤中Ca₂-P下降明显;而不同磷肥处理间土壤中Ca₁₀-P、Ca₈-P、O-P (闭蓄态磷)、Al-P和Fe-P含量差异不显著。减施磷肥处理2008年玉米苗期根际土壤微生物量P含量较非根际土壤高;与习惯施肥量相比,磷肥减施未明显降低根际土壤微生物量磷。 【结论】 在华北小麦玉米轮作种植体系下,在土壤肥力水平较高地区,连续3年将小麦季磷肥的习惯用量减少20%~40%,对夏玉米产量、根系形态以及根际土壤无机磷、有机磷、微生物量磷含量影响尚不明显,因此,该地区磷肥施用量可从习惯用量的P₂O₅180 kg/hm2减至112.5 kg/hm2。     

生草不同条件还田对桃园土壤微生物、酶活性及养分供应的影响

为探讨桃园生草不同条件还田对土壤微生物、酶活性及有效态养分的影响,以清耕为对照,设置生草自然还田、生草刈割还田、生草刈割配施有机物料腐熟剂还田3个处理,连续开展3年定位试验。采集根际和非根际土壤,研究桃园生草不同条件还田对土壤微生物数量、土壤酶活性以及土壤不同形态氮、钾含量的影响。结果表明:生草不同条件还田提高了根际土壤微生物的数量,生草刈割配施有机物料腐熟剂还田显著提高了根际和非根际土壤细菌和真菌的数量,较其他处理分别提高21.2%～48.2%和11.7%～17.0%,生草刈割后,配施有机物料腐熟剂能加速秸秆的腐熟与微生物繁殖;与清耕对照相比,桃园生草不同条件还田均能显著提高土壤酶活性,生草刈割配施有机物料腐熟剂的土壤脲酶活性、过氧化氢酶活性、蔗糖酶活性较其他处理分别提高10.2%～45.4%,26.8%～56.9%,20.5%～30.7%;桃园生草还田对土壤养分的积累具有正效应,以生草刈割配施有机物料腐熟剂还田效果明显,其不仅增加了土壤无机态氮和有机态氮含量,减少了氮素损失,同时还显著提高土壤速效钾和水溶性钾含量,较其他处理的土壤速效钾和水溶性钾含量分别提高12.6%～15.6%和11.4%～39.1%。综上,桃园生草刈割配施有机物料腐熟剂还田为提高土壤微生物数量、土壤酶活性及氮钾养分供应的较好途径,为果园生草精细化管理提供科学依据。     

Interactions between soil organic matter status, cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability

 The effects of sample pretreatment (field-moist, air-dried or tension rewetted) on aggregate stability measured by wet sieving or turbidimetry were compared for a group of soil samples ranging in organic C content from 20 to 40 g C kg^–1. Concentrations of total N, total and hot-water-extractable carbohydrate and microbial biomass C were linearly related to those of organic C. Aggregate stability measured by wet sieving using air-dried or field-moist samples and that measured by turbidimetry, regardless of sample pretreatment, increased curvilinearly with increasing soil organic C content. However, when tension-rewetted samples were used for wet sieving, aggregate stability was essentially unaffected by soil organic C content. Measurements of aggregate stability (apart from wet sieving using rewetted soils) were closely correlated with one another and with organic C, total and extractable carbohydrate and microbial biomass C content of the soils. The short-term effects of aggregate stability were also studied. Soils from under long-term arable management and those under long-term arable followed by 1 or 3 years under pasture had similar organic C contents, but aggregate stability measured by turbidimetry and by wet sieving using air-dried or field-moist samples increased with increasing years under pasture. Light fraction C, microbial biomass and hot-water-extractable carbohydrate concentrations also increased. It was concluded that both total and labile soil organic C content are important in relation to water-stable aggregation and that the use of tension-rewetted samples to measure stability by wet sieving is unsatisfactory since little separation of values is achieved. Received: 6 January 1999     

A comparison of aggregate stability and biological activity in earthworm casts and uningested soil as affected by amendment with wheat or lucerne straw

The mechanisms responsible for stabilization of earthworm casts were investigated in a laboratory study. Earthworms (Aporrectodea caliginosa) were fed soil or soil amended with either ground wheat straw or lucerne hay. Cast material and uningested soil material were incubated for 56 days, and changes in aggregate stability (measured by wet sieving), soil biological activity and macronutrient availability were measured periodically. In general, aggregate stability and microbial biomass C tended to increase during the incubation for both cast and soil material, whereas hot-water extractable carbohydrate content declined. For amended treatments, basal respiration rate and microbial metabolic quotient were large at the first sampling (7 days) but declined rapidly thereafter. There was a transitory increase in extractable P in fresh casts compared with uningested material, a sustained increase in mineral N concentrations but no change in exchangeable K content. For unamended treatments, the casts were less stable than soil material, but this difference diminished during incubation. Drying aggregates before analysing them did not reverse this trend. The casts contained more microbial biomass C than soil material did, but the basal respiratory rate, respiratory quotient and hot-water extractable carbohydrate content were less. By contrast, for wheat- and lucerne-amended treatments casts were more stable than soil material, and the microbial biomass was less at all sampling times. For wheat treatments, respiratory quotient and hot-water extractable carbohydrate content were larger for cast than soil material, but the opposite was the case for lucerne treatments. We attribute the stability of casts in amended treatments to the intimate mixing of part-decomposed organic fragments with comminuted soil particles, binding by microbial mucilage associated with the organic fragments and linking and binding by fungal hyphae.     

Continuous defoliation of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) and associated changes in the composition and activity of the microbial population of an upland grassland soil

A microcosm study was conducted to investigate the effect of continuons plant defoliation on the composition and activity of microbial populations in the rhizosphere of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Continuons defoliation of ryegrass and clover resulted in sigmficant (P <0.01) increases in soil microbial biomass, although whilst increases were measured from day 2 in soil sown with clover significant increases were only seen from day 21 in soil sown with ryegrass. These increases were paralleled, from day 10 onwards, by increases in the numbers of culturable bacteria. Numbers ofPsendomonas spp. also increased in the later stages of the study. No influence on culturable fungal populations was detected. Whilst shifts in the composition of the microbial populations were measured in response to defoliation there was little effect on microbial activity. No changes in either dehydrogenase activity or microbial respiration in the rhizosphere of ryegrass or clover were measured in response to defoliation, but both dehydrogenase activity and microbial respiration were greater in ryegrass than clover when values over the whole study were combined. Continuous defoliation resulted in significant (P <0.001) reductions in the root dry weight of ryegrass and clover, of the order 19% and 16%, respectively.     

Continuous defoliation of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) and associated changes in the composition and activity of the microbial population of an upland grassland soil

A microcosm study was conducted to investigate the effect of continuons plant defoliation on the composition and activity of microbial populations in the rhizosphere of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). Continuons defoliation of ryegrass and clover resulted in sigmficant (P <0.01) increases in soil microbial biomass, although whilst increases were measured from day 2 in soil sown with clover significant increases were only seen from day 21 in soil sown with ryegrass. These increases were paralleled, from day 10 onwards, by increases in the numbers of culturable bacteria. Numbers ofPsendomonas spp. also increased in the later stages of the study. No influence on culturable fungal populations was detected. Whilst shifts in the composition of the microbial populations were measured in response to defoliation there was little effect on microbial activity. No changes in either dehydrogenase activity or microbial respiration in the rhizosphere of ryegrass or clover were measured in response to defoliation, but both dehydrogenase activity and microbial respiration were greater in ryegrass than clover when values over the whole study were combined. Continuous defoliation resulted in significant (P <0.001) reductions in the root dry weight of ryegrass and clover, of the order 19% and 16%, respectively.     

Relationships between microbial indices in roots and silt loam soils forming a gradient in soil organic matter

Perennial rye grass (Lolium perenne) was grown in a greenhouse pot experiment on seven soils to answer the question whether the microbial colonisation of roots is related to existing differences in soil microbial indices. The soils were similar in texture, but differed considerably in soil organic matter, microbial biomass, and microbial community structure. Ergosterol and fungal glucosamine were significantly interrelated in the root material. This ergosterol was also significantly correlated with the average ergosterol content of bulk and rhizosphere soil. In addition, the sum of fungal C and bacterial C in the root material revealed a significant linear relationship with microbial biomass C in soil. The colonisation of roots with microorganisms increased apparently with an increase in soil microbial biomass. In the root material, microbial tissue consisted of 77% fungi and 23% bacteria. In soil, the fungal dominance was slightly, but significantly lower, with 70% fungi and 30% bacteria. Fungal glucosamine in the root material was significantly correlated with that in soil (r=0.65). This indicates a close relationship between the composition of dead microbial remains in soil and the living fraction in soil and root material for unknown reasons.     

半干旱农田生态系统作物根系和施肥对土壤微生物体氮的影响

盆栽和大田试验表明,作物根系显著影响土壤微生物体氮的含量。在田间试验条件下,根际土壤微生物体氮比非根际土壤平均高出N54.7μg/g;盆栽试验中,根际土壤微生物体氮平均含量为N77.1±13.6μg/g,而非根际土壤为N65.2±17.0μg/g,差异达显著水平,根际微生物体氮含量为非际根际土壤的1.10～2.04倍。施肥能明显增加土壤微生物体氮含量,但影响程度因肥料种类而不同。秸秆和富含有机物质的厩肥对土壤微生物体氮的影响远大于化学肥料,而且土壤微生物体氮含量随秸秆施用量增加而增加。在红油土上进行的20年长期田间定位试验结果表明,对不施肥和施氮磷处理,0—20cm土层的微生物体氮分别是N102.2和110.4μg/g;在施氮磷的基础上,每公顷配施新鲜玉米秸秆9375kg、18750kg、37500kg和厩肥37500kg时,相应土层微生物体氮分别是N147.5、163.2、286.4和265.3μg/g。培养条件下,当有效能源物质缺乏时,微生物对NH4+-N的同化固定能力远大于NO3--N,但在加入有效能源物质葡萄糖后,微生物对2种形态氮的固定量大幅度增加,且对2种形态氮的固定量趋于一致。     

Carbon Turnover in the Rhizosphere

Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as:

• –homogeneous labelling of whole plants with ¹⁴C
• –distinction between root and microbial respiration
• –separation of soil zones of known distances from the roots
• –determination of microbial soil biomass.

These methods were applied to study the following aspects:

• –release of organic plant carbon into the soil by growing roots
• –utilization of this plant carbon by the microbial biomass in the rhizosphere
• –related influence on the turnover of soil organic matter, and
• –spatial range of such root influence in the soil.

About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO₂, while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C-input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%). Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.     

根际土壤微生物量氮周转率的研究

用15N示踪法研究了不同植物对根际及非根际微生物量氮周转率的影响。结果表明 ,不同植物对微生物量氮周转率的影响不同。栽种豆科植物三叶草由于其根系分泌物中的含氮有机物含量较高 ,所以土壤中微生物量氮的周转率快于栽种黑麦草。两种植物根际土壤的微生物量氮的周转率都快于非根际土壤 ,与土壤蛋白酶活性的研究结果一致     

蔬菜根际环境钒的形态变化及植物有效性

通过盆栽试验和Tessier连续提取法,研究了蔬菜根际和非根际土壤中钒的形态分布与植物有效性。结果表明,钒在根际和非根际土壤中主要以残渣态和铁锰氧化物结合态存在,钒的含量随其形态不同而变化：当土壤中钒添加量从25mg·kg^-1增加到200mg·kg^-1时,根际土壤交换态钒的百分率增加了9．89％,碳酸盐结合态钒增加了9．38％,铁锰氧化物结合态钒增加了9．99％,残渣态钒的百分率下降了33．6％。土壤添加的钒量增加时,钒的迁移能力增强,使蔬菜的生物量显著降低,蔬菜对钒的吸收作用增强：钒添加量从0增至200mg·kg^-1时,蔬菜鲜质量降低了41．65％,而蔬菜中85％以上的钒积累在根部。根际土壤中碳酸盐结合态钒含量对植物的根、茎叶中钒含量有显著影响,碳酸盐结合态钒与蔬菜鲜质量呈显著负相关。     

乙草胺对玉米根际和非根际可培养微生物的影响

为探明土壤-植物系统中,田间施用量的乙草胺对玉米根际和非根际微生物数量的影响,采用田间试验及室内测试方法,在玉米苗期不同阶段测定了土壤中微生物量碳的变化,并进一步用平板稀释培养法研究了玉米根际和非根际土壤中细菌和真菌数量的变化。结果表明,乙草胺施用对玉米根际和非根际的土壤微生物群落均具有一定影响。可培养的根际细菌和真菌均呈现先抑制后刺激的变化,但与真菌不同,细菌受到的抑制作用时间较短,刺激作用时间较长;而本体土壤中可培养细菌和真菌则主要受到抑制作用,但是抑制作用的强度和持续时间差别很大。乙草胺对根际土壤微生物量碳可产生一定刺激作用,但影响并不显著;由于乙草胺施用对非根际土壤细菌和真菌的影响不同步并存在群落结构的补偿作用,从而维持了非根际土壤总体微生物生物量碳的基本稳定。     

Pb2+胁迫下黑麦草对外源有机酸的响应机制

为探讨根系分泌有机酸对植株吸收富集Pb2+的影响,土壤Pb2+含量为500mg/kg的条件下,设计外源有机酸类型为草酸、冰乙酸、丙二酸、酒石酸和苹果酸,其浓度均为1,3,5,6,7mmol/kg。在模拟日光温室中采用盆栽根袋法对黑麦草生长发育指标,根际与非根际土壤理化性质及黑麦草吸收富集效果进行分析。研究得出,外源有机酸能够在一定程度上促进黑麦草地上部分和根系干物质量的增加,黑麦草根系对Pb2+的吸收富集效果大于地上部分,有机酸的加入活化了土壤中的Pb2+,促进了植株对Pb2+的吸收富集。黑麦草根际土壤pH、Eh值和Pb2+残留量均小于非根际。酒石酸、苹果酸和丙二酸既不影响黑麦草植株生长,又可有效增加其对Pb2+的吸收富集,主要是增加地上部分对Pb2+的吸收富集;冰乙酸可显著活化土壤中的Pb2+。