Effects of Auxin at Different Concentrations on the Growth,Root Morphology and Cadmium Uptake of Maize (<i>Zea mays</i> L.)

Indoleacetic acid (IAA) is an important regulator that plays a crucial role in plant growth and responses to abiotic stresses. In the present study, a sand cultivation experiment was carried out to investigate the effects of IAA at different concentrations (0, 0.01, 0.1, 0.5, 1, and 2.5 mmol/L) on maize growth, root morphology, mineral elements (Ca, Mg) and Cd uptake under 20 mg/kg Cd stress. The results showed that 0.01 mmol/L is the optimal IAA concentration for enhancing the Cd tolerance of maize. Compared with the control treatment, 0.01 mmol/L IAA promoted maize growth, with significant increases in the height, shoot and root biomass by 34.6%, 25.0% and 16.3%; altered the root morphology, with increases in root length, root tip number, and root tip density by 8.9%, 31.4% and 20.7%, respectively; and enhanced the mineral element uptake of maize, resulting in signifi- cant increases in the Ca content in shoots and roots by 640.6% and 1036.4% and in the Mg content in shoots by 205.8%, respectively. In addition, 0.01 mmol/L IAA decreased the Cd content and uptake in the shoots by 51.9% and 39.6%, respectively. Furthermore, the Cd content and uptake exhibited a significant negative correlation with Ca content in roots and a significantly positive correlation with root morphology, and the Cd content in shoots was significantly and negatively correlated with root tip number. Thus, 0.01 mmol/L IAA was effective in enhancing the Cd tolerance and plant growth of maize.     

丛枝菌根真菌对砂培玉米幼苗根系特征、光合生理与镉累积的影响

【背景】丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)是一类重要的土壤微生物,能显著影响植物对镉(cadmium,Cd)的耐性与累积,但其对不同形态Cd胁迫的响应尚不清楚。【目的】探讨不同形态Cd胁迫下接种AMF对玉米(Zea mays L.)生长和Cd累积的影响。【方法】采用30 cm高的培养容器填装石英砂(0.2 mm),开展室内砂培玉米试验,研究溶解态和胶体态Cd (1 mg/kg)胁迫下,接种摩西斗管囊霉(Funneliformis mosseae)对玉米幼苗生长、根系特征、光合生理及Cd累积的影响。【结果】双因素分析表明,AMF和Cd形态均对玉米生长(株高和生物量)、根系特征、光合生理(叶绿素含量和光合速率)与Cd累积量存在显著的影响,但二者之间没有显著交互作用。与未接种处理相比,接种AMF显著降低玉米株高、生物量、叶片叶绿素含量和光合速率,抑制玉米根长、根表面积、根体积和根尖数;同时增加了玉米根系Cd含量,但减少玉米地上部Cd含量以及地上部与根系Cd累积量;与胶体态Cd处理相比,溶解态Cd显著降低玉米的根长、根表面积、平均根系直径、根尖数和地上部Cd累积量,但增加了植株叶片光合速率、根系Cd含量和累积量。相关分析发现,玉米根长、根表面积和根尖数与地上部Cd含量呈显著或极显著正相关,与根系Cd含量呈极显著负相关。【结论】溶解态Cd比胶体态Cd对砂培玉米幼苗的毒害效应严重,而且接种AMF加重溶解态和胶体态Cd对玉米幼苗的损伤,但降低了植株对Cd的累积。     

Alleviation of Cadmium Toxicity to <i>Medicago Truncatula</i> by AMF Involves the Changes of Cd Speciation in Rhizosphere Soil and Subcellular Distribution

Arbuscular mycorrhizal fungi (AMF) can improve plant tolerance to several abiotic stresses, including heavy metals, drought or salinity exposure. However, the role of AMF in alleviation of soil cadmium (Cd)-induced toxicity to plants is still largely unknown. In this study, Cd speciation in soil and subcellular distribution of Cd were used to characterize the roles of application AM fungi in the alleviation of Cd toxicity in alfalfa plants. Our results showed that the addition of Glomus mosseae in Cd contaminated soil (10 mg/Kg) significantly increased soil pH, cation exchange capacity (CEC) and organic matter in rhizosphere soil with Medicago truncatula L., and then account for significantly decreased contents of exchangeable and carbonate-bounded Cd speciation in rhizosphere soil, indicating alleviation of plant toxicity by reduction of bioavailable fractions of Cd. Although there is no significant difference found in Cd accumulation by roots and shoots respectively between Cd and AM-Cd treatments, more portion of Cd was recorded compartmentalization in cell wall fraction of both root and shoot in treatment of Cd with AM application, indicating alleviation of Cd toxicity to plant cell. Herein, application of AM fungi in Cd treatments performed to inhibit the appearance of Cd toxicity symptoms, including the improvement of leaf electrolyte leakage, root elongation, seedling growth and biomass. This information provides a clearer understanding of detoxification strategy of AM fungi on Cd behavior with development and stabilization of soil structure and subcellular distribution of plant.     

EFFECTS OF INOCULATION WITH ARBUSCULAR MYCORRHIZAL FUNGI ON MAIZE GROWN IN MULTI-METAL CONTAMINATED SOILS

Pot culture experiments were established to determine the effects of colonization by arbuscular mycorrhizal fungi (AMF) (Glomus mosseae and G. sp) on maize (Zea mays L.) grown in Pb, Zn, and Cd complex contaminated soils. AMF and non-AMF inoculated maize were grown in sterilized substrates and subjected to different soil heavy metal (Pb, Zn, Cd) concentrations. The root and shoot biomasses of inoculated maize were significantly higher than those of non-inoculated maize. Pb, Zn, and Cd concentrations in roots were significantly higher than those in shoots in both the inoculated and non-inoculated maize, indicating the heavy metals mostly accumulated in the roots of maize. The translocation rates of Pb, Zn, and Cd from roots to shoots were not significantly difference between inoculated and non-inoculated maize. However, at high soil heavy metal concentrations, Pb, Zn, and Cd in the shoots and Pb in the roots of inoculated maize were significantly reduced by about 50% compared to the non-inoculated maize. These results indicated that AMF could promote maize growth and decrease the uptake of these heavy metals at higher soil concentrations, thus protecting their hosts from the toxicity of heavy metals in Pb, Zn, and Cd complex contaminated soils.     

Integrated Fertilization Regimes Boost Heavy Metals Accumulation and Biomass of <i>Sedum alfredii</i> Hance

The hyperaccumulator Sedum alfredii Hance (S. alfredii) may be employed for zinc (Zn) and cadmium (Cd)-polluted soil remediation. However, the low phytoremediation efficiency, related to the low biomass production, limits its use with that purpose. In this experiment, nitrogen (N), phosphorus (P), and potassium (K) fertilizers, and organic manure were applied to investigate the phytoremediation ability of S. alfredii. Hydroponic and pot experiments were conducted using Zn-Cd polluted soil. The hydroponic experiment indicated that appropriate fertilizer application could increase (p < 0.05) the amount of accumulated Zn and Cd in S. alfredii. When N supply ranged from 0.5 to 2.5 mmol L⁻¹, it could improve growth and accumulation of Zn and Cd in whole plants of S. alfredii. The 1 mmol L^-1 N was an optimal N dosage for shoot biomass production and Cd accumulation in shoots, while the 2.5 mmol L^-1 was an optimal N dosage for Zn accumulation in shoots. Both low (<0.05 mmol L^-1) and high (>0.8 mmol L^-1) P supply decreased growth, and Zn/Cd accumulation in whole plants of the studied species. The 0.1 mmol L^-1 P was an optimal dosage for S. alfredii biomass production and Zn/Cd accumulation in shoots. The supply levels within the range from 0.3 to 1 mmol L^-1 K could significantly improve the biomass production of S. alfredii and its capability to accumulate Zn and Cd in the biomass. The 0.5 mmol L^-1 K was an optimal dosage for the whole biomass production and Zn accumulation in shoots, while the 1 mmol L^-1 was an optimal K dosage for Zn accumulation in shoots, which was 17.2% higher than the control. Moreover, the soil pot experiment showed that the combination of organic (fermented manure) and inorganic fertilizers made significant effects on the Zn and Cd-polluted soil remediation by S. alfredii. These effects varied, however, with the application of different proportions of N, P, K and organic matter. The Zn accumulation by S. alfredii reached the highest efficiency ability under the highest fertilizer mixing rate (N: 50 mg kg^-1, P: 40 mg kg^-1, K: 100 mg kg^-1, organic matter: 1%). Even more, S. alfredii showed the strongest ability to accumulate Cd with a lower fertilizer mixing rate (N: 25mg kg^-1, P: 20mg kg^-1, K: 50 mg kg^-1, organic matter: 0.5%).

     

Application of composted sewage sludge (CSS) as a soil amendment for turfgrass growth

We studied the growth of perennial ryegrass in soils amended with 5–100% composted sewage sludge (CSS), and the impacts of CSS amendment on soil physical and chemical properties. Soils amended with ≤20% CSS did not significantly affect the seedling emergence, while the contents of chlorophyll, nitrogen, phosphorous, and potassium of perennial ryegrass grown in such soils were greatly improved. Bulk density, water retention, and nutrient contents of the soil were also improved with the amendment of CSS, but high CSS contents introduced excessive amounts of heavy metals and soluble salts. Results show that Cu, Zn, and Pb accumulated slightly (up to ∼2.3 times) in clippings of perennial ryegrass grown in CSS-amended soils compared to those grown in the base and reference soils, while no significant Cd absorption in shoots of perennial ryegrass occurred. The detrimental effects on seedling emergence and turfgrass growth observed on substrates with high (≥40%) CSS contents were mainly attributed to the presence of high soluble salt concentrations. The findings suggest that addition of CSS at 10–20% levels can greatly improve the soil nutrient supply for turfgrass growth without significantly affecting heavy metal and soluble salt contents of the soil.     

丛枝菌根真菌促进湿地植物对污染水体中镉的吸收

丛枝菌根真菌(AMF)具有促进植物生长、增强植物耐重金属毒害及修复重金属污染生境的作用.本试验于模拟湿地系统中设0、5、10和20 mg·L^-1镉浓度,对芦苇和狼尾草分别进行接种和不接种摩西斗管囊霉(Fm)、根内根孢囊霉(Ri)处理.结果表明: 镉污染降低了AMF侵染;接种AMF提高了芦苇和狼尾草的株高、干质量、叶绿素含量、地上部与根内的氮和镉含量,增强了根系对镉的富集能力,降低了镉转移系数.在5 mg·L^-1镉处理下,接种Fm的芦苇处理的各项指标均高于其他处理,其AMF侵染率为60.6%,每毫米根长的侵入点和泡囊数分别为2.3和3.7;与不接种对照相比,接种Fm的芦苇地上部与根系干质量分别提高69.1%和75.0%,地上部与根系氮含量分别提高38.7%和27.8%,叶绿素含量和株高分别增加3.8%和11.1%.湿地系统镉浓度与植株地上部和根内的镉含量呈显著正相关.在20 mg·L^-1镉处理下,接种Fm的芦苇地上部和根内镉含量达到最大值182.4和663.3 mg·kg^-1,镉转移系数最低,为0.27,而富集系数最高,为0.55.Fm+芦苇是本试验条件下吸收富集污染水体中镉的最佳组合.     

Phytotoxic Effects of Cd, Zn, Pb, Cu and Fe on Sinapis Alba L. Seedlings and their Accumulation in Roots and Shoots

The inhibitory effects of Cd, Cu, Zn, Pb, and Fe on root elongation, contents of photosynthetic pigments, and metal accumulation in the roots and shoots of Sinapis alba were assessed. On the basis of growth inhibition metals can be arranged in a order Cu > Cd > Fe = Zn > Pb. All the metals, except Fe, were accumulated in significantly higher amount in the roots than in the shoots. Cd, Zn, Cu and Pb reduced chlorophyll a, and especially chlorophyll b content, and Zn and Pb reduced the carotenoid content, but less than that of chlorophyll a+b. The plants contained the highest concentration of Cd, and the lowest concentration of Zn.     

Effects of cadmium, nickel and lead on growth, chlorophyll content and proteins of weeds

The effects of Cd, Ni and Pb on the growth, chlorophyll (Chl) and protein contents, and content of proteases of potted weed plants Cyperus difformis, Chenopodium ambrosioides and Digitaria sanguinolis were determined. The three heavy metals inhibited the shoot growth but were less suppresive to root growth. They also lowered leaf Chl content. The changes in root and shoot protein and proteases contents of weeds were interrelated. The heavy metal additions to soil increased their contents in both roots and shoots, several times more in roots than in shoots. This revised version was published online in July 2006 with corrections to the Cover Date.     

黑麦草生长过程中有机酸对镉毒性的影响

研究了低分子量有机酸草酸、柠檬酸、乙酸及高分子量有机酸胡敏酸对黑麦草(Lolium Loinn)生长过程中Cd毒性的影响.结果表明,随着低分子量有机酸浓度增加,Cd毒性有所增强,致使黑麦草中的叶绿素含量降低及黑麦草的生物量降低,递降顺序是草酸＜乙酸＜柠檬酸.而施入胡敏酸后,Cd毒性逐渐减弱,黑麦草中的叶绿素含量及黑麦草生物量逐渐增加.对低分子量有机酸而言,无论迁移到黑麦草茎叶中,还是迁移到黑麦草根系中的Cd,随着施入的有机酸浓度增加,增加顺序为柠檬酸＞乙酸＞草酸.对胡敏酸而言,迁移到黑麦草茎叶和根系中的Cd,随着施入的胡敏酸浓度增加,Cd含量减少,说明其具有降低Cd毒性的作用.另外,根系中Cd含量明显高于茎叶中Cd含量,由此得知,黑麦草根系对Cd有较强的富集作用,并阻止Cd向茎叶中迁移.     

不同施氮量对乌龙茶生长和生理的影响

以武夷肉桂为研究对象,研究不同施氮量对乌龙茶幼龄茶树生长和生理的影响。结果表明,幼龄茶树对氮肥的需求不强烈,其新梢生物量、根生物量和总生物量以及茶叶产量随施氮量的增加而下降。茶树新梢全氮、叶绿素、游离氨基酸、茶多酚和咖啡碱的含量随施氮量的增加而增加,而茶树碳氮比随着施氮量增加而下降;但施氮并没有影响茶树总碳含量。老叶叶绿素含量、根全氮和硝态氮含量、新梢总糖含量与施氮量呈二次曲线回归关系,适度施氮促进根对氮的吸收、老叶叶绿素合成和新梢总糖代谢,过度施氮则相反。新梢生物量与其硝态氮含量和游离氨基酸总量显著负相关;根生物量与根碳氮比和新梢咖啡碱含量显著负相关;茎叶生物量和总生物量与根含氮量显著正相关,但与新梢硝态氮和氨基酸含量显著负相关。过度施氮造成茶树生产力下降的主要原因是因为过度施氮极显著提高了茶树氨基酸代谢水平,使用于茶树生长的碳代谢产物（如总糖）减少,进而影响茶树的生长。     

Evaluation of antioxidant bioindicators and growth responses in <Emphasis Type="Italic">Malva parviflora</Emphasis> L. exposed to cadmium

In this study, the effect of cadmium (Cd) uptake and concentration on some growth and biochemical responses were investigated in Malva parviflora under Cd treatments including 0, 10, 50 and 100 µM. The shoots and roots were able to accumulate Cd. However, increased Cd dose led to a considerable Cd content in the roots. Cd stress decreased growth, increased lipid peroxidation and also enhanced proline and ascorbic acid contents in both shoots and roots. Chlorophyll and carotenoid contents decreased in the plants with the increasing Cd concentration. While the activities of catalase (CAT) and superoxide dismutase (SOD) increased in the shoots under different Cd doses, these activities decreased in the roots as compared to the control. Both shoots and roots demonstrated a significant increase in guaiacol peroxidase activity in response to Cd stress. Contrary to the aboveground parts, the roots subjected to Cd doses showed a rise in protein content. Despite higher Cd content in the roots, it seems that CAT and SOD do not play a key role in detoxification of Cd-induced oxidative stress. These findings confirm that reduced biomass and growth under Cd stress can be due to an increase in oxidative stress and a decrease in photosynthetic pigment content. The present study clearly indicates that the shoots and roots exploit different tolerance behaviors to alleviate Cd-induced oxidative stress in M. parviflora.     

水培条件下四种植物对Cd、Pb富集特征

利用水培方法测定了不同浓度下向日葵、蓖麻、紫花苜蓿及芥菜的生物量和植物体内重金属Cd、Pb含量,分析了植物对重金属的富集特征。结果表明:经过5周培养后,4种植物根部与地上部对重金属的富集量随着浓度的增加而增加,Cd浓度为20mg·L-1时,向日葵的根部Cd含量最高,达到237.86mg·kg-1,地上部Cd含量为89.48mg·kg-1;而Pb浓度为200mg·L-1时,芥菜根部对Pb的吸收量较高,达到597.22mg·kg-1,地上部Pb含量最高的则出现在向日葵处理Pb100mg·L-1中,为318.33mg·kg-1。4种植物对Cd、Pb的富集系数随重金属浓度的增加而减小;根部及地上部富集系数与生物量和重金属浓度呈现出一定的相关性;另外,在Cd、Pb复合处理中,一种重金属的存在会在不同程度上影响植物对另一种重金属的吸收。通过比较4种植物根部与地上部的生物量和对Cd、Pb富集特征,认为相对于其他3种植物向日葵对Cd、Pb具有较强的吸收潜力,并可以作为Cd、Pb污染土壤植物修复的备选植物。     

丛枝菌根真菌对豆科作物生长和生物固氮及磷素吸收的影响

本研究以大豆为材料,采用盆栽和田间试验,探讨丛枝菌根真菌(AMF)对豆科作物生长的影响。盆栽试验设置了接种(+AMF)和不接种(-AMF)丛枝菌根真菌处理,田间试验设置了AMF菌丝非限制与限制处理。盆栽试验结果表明: 接种AMF显著提高了大豆地上部生物量(16.5%)和大豆根瘤数(131.4%),地上部磷含量、磷吸收量、氮含量和氮吸收量也显著增加。田间试验中,AMF菌丝非限制处理下大豆的地上部生物量、根系生物量、根瘤数量分别比限制处理下显著提高了123.6%、61.5%和212.5%,地上部和根系磷吸收量、氮含量、氮吸收量均显著高于限制处理,大豆根际土壤速效氮和有效磷含量也均显著高于限制处理。本研究可为进一步认识豆科作物与AMF的共生关系及田间磷肥高效利用提供理论参考。     

Phosphorus sources and availability modify growth and distribution of root clusters and nodules of native Australian legumes

A variety of native Western Australian legumes produced root clusters in sand culture confirming field and published observations. In general, these legumes grew equally well when supplied with organic or inorganic sources of phosphorus. The nitrogen content of shoots and roots varied little among treatments for all species, however, phosphorus content was always greater in plants supplied with inositol‐P. The plasticity of root growth in response to localized placement of organic and inorganic sources of phosphorus was demonstrated using a simple ‘split root’ technique. Total root dry weight was, on average, more than doubled in P‐amended sand when compared with non‐amended sand. Root clusters tended to be produced in areas of relatively high phosphorus concentration and nodules in areas of low phosphorus concentration. Levels of phosphorus in lateral roots grown in P‐amended sand were significantly different from lateral roots grown in the corresponding non‐amended sand. Growth increases averaging 70% for white sand to over 100% for yellow sand indicated a large degree of ‘plasticity’ in roots under conditions of heterogeneous supply of phosphorus. Spatially exclusive development of organs for the acquisition of nutrients is discussed in relation to requirements for carbon in organ production and maintenance.     

Functional diversity of arbuscular mycorrhizal fungal isolates in relation to extraradical mycelial networks

We investigated the functional significance of extraradical mycorrhizal networks produced by geographically different isolates of the arbuscular mycorrhizal fungal (AMF) species Glomus mosseae and Glomus intraradices. A two-dimensional experimental system was used to visualize and quantify intact extraradical mycelium (ERM) spreading from Medicago sativa roots. Growth, phosphorus (P) and nitrogen (N) nutrition were assessed in M. sativa plants grown in microcosms. The AMF isolates were characterized by differences in extent and interconnectedness of ERM. Phenotypic fungal variables, such as total hyphal length, hyphal density, hyphal length per mm of total or colonized root length, were positively correlated with M. sativa growth response variables, such as total shoot biomass and plant P content. The utilization of an experimental system in which size, growth rate, viability and interconnectedness of ERM extending from mycorrhizal roots are easily quantified under realistic conditions allows the simultaneous evaluation of different isolates and provides data with a predictive value for selection of efficient AMF.     

碳酸钙对水稻吸收重金属(Pb、Cd、Zn)和As的影响

选用重金属(Pb、Cd、Zn)和As复合污染土壤进行水稻盆栽试验,结果表明,碳酸钙的添加显著提高了土壤pH值,显著降低了土壤中交换态Pb、Cd、Zn和As的含量,与对照相比,交换态Pb、Cd、Zn和As含量分别最多降低了98.35%,93.72%,98.52%和69.48%。碳酸钙对水稻根、稻谷干重和总生物量没有显著影响,添加量过高时显著降低了水稻分蘖数和茎叶干重,说明过量施用碳酸钙对水稻生长会产生负面作用。因为碳酸钙的添加,水稻植株各部位重金属Zn含量显著降低,糙米中Zn含量最多减少了34.95%;根、谷壳中Pb、Cd含量显著降低,但糙米中含量却未显著降低;水稻各部位As含量均没有显著降低。参照《食品中污染物限量》(GB2762—2012),试验糙米中Pb、Cd、无机As含量均未达到限量标准。显然,碳酸钙的添加降低了Pb、Cd、Zn的生物有效性(水稻根系对Pb、Cd、Zn的吸收累积减少),但并未有效地抑制Pb、Cd向糙米转运;碳酸钙显著降低了土壤的交换态As含量,但并未使土壤中As的生物有效性明显降低(水稻植株各部位的As含量并未显著减少)。     

温度和施磷对石灰性潮土小麦苗期生长及磷形态的影响

以石灰性潮土为对象,通过盆栽试验研究温度和磷肥对小麦苗期生长和土壤无机磷形态转化的影响.结果表明: 温度和磷肥是影响小麦生长的重要因子,但二者交互作用影响不显著.温度对小麦生长的影响大于施用磷肥,15 ℃是小麦苗期的适宜生长温度.与不施磷肥(-P)处理相比,5 ℃下,施磷肥(＋P)处理显著促进了小麦生长,小麦地上部、根部生物量分别提高18.2%、33.3%,地上部、根部磷素积累量分别提高30.6%、13.3%,根冠比、株高、分蘖、根系活力分别提高3.5%、10.0%、10.5%、70.3%;15 ℃下,施用磷肥对小麦生物量、分蘖影响不显著,但小麦地上部、根部磷素积累量分别提高32.3%、23.8%,根冠比、株高、根系活力分别提高15.6%、2.5%、32.8%;25 ℃下施用磷肥对小麦生长没有促进作用. 3种温度下,施磷能显著增加各处理土壤有效磷(Olsen-P)、二钙磷(Ca₂-P)、八钙磷(Ca₈-P)、铝磷(Al-P)和铁磷(Fe-P)含量.-P和+P处理下,温度对Ca₂-P含量影响不显著,对Olsen-P、Ca₈-P、Fe-P、Al-P含量影响显著.Ca₈-P、Fe-P含量表现为5 ℃>15 ℃>25 ℃;Al-P含量表现为25 ℃>15 ℃>5 ℃.小麦苗期可以吸收利用根际土壤Ca₂-P、Ca₈-P、Al-P、Fe-P,而Al-P、Fe-P对小麦的有效性明显低于Ca₂-P、Ca₈-P.各处理pH、闭蓄态磷(O-P)和十钙磷(Ca₁₀-P)差异不显著.总之,温度主要通过影响小麦生长来影响磷素吸收,低温下施用磷肥能显著促进小麦生长,高温能加速石灰性土壤有效磷的固定,施磷能缓解这一过程.     

Effect of mineral nitrogen on nitrogen nutrition and biomass partitioning between the shoot and roots of pea (Pisum sativum L.).

The effect of mineral N availability on nitrogen nutrition and biomass partitioning between shoot and roots of pea (Pisum sativum L., cv Baccara) was investigated under adequately watered conditions in the field, using five levels of fertiliser N application at sowing (0, 50, 100, 200 and 400 kg N ha^–1). Although the presence of mineral N in the soil stimulated vegetative growth, resulting in a higher biomass accumulation in shoots in the fertilised treatments, neither seed yield nor seed nitrogen concentration was affected by soil mineral N availability. Symbiotic nitrogen fixation was inhibited by mineral N in the soil but it was replaced by root mineral N absorption, which resulted in optimum nitrogen nutrition for all treatments. However, the excessive nitrogen and biomass accumulation in the shoot of the 400 kg N ha^–1 treatment caused crop lodging and slightly depressed seed yield and seed nitrogen content. Thus, the presumed higher carbon costs of symbiotic nitrogen fixation, as compared to root mineral N absorption, affected neither seed yield nor the nitrogen nutrition level. However, biomass partitioning within the nodulated roots was changed. The more symbiotic nitrogen fixation was inhibited, the more root growth was enhanced. Root biomass was greater when soil mineral N availability was increased: root growth was greater and began earlier for plants that received mineral N at sowing. Rooting density was also promoted by increased mineral N availability, leading to more numerous but finer roots for the fertilised treatments. However, the maximum rooting depth and the distribution of roots with depth were unchanged. This suggested an additional direct promoting effect of mineral N on root proliferation.     

Root, mycorrhiza and earthworm interactions: their effects on soil structuring processes, plant and soil nutrient concentration and plant biomass

Earthworms, arbuscular mycorrhiza fungi (AMF) and roots are important components of the belowground part of terrestrial ecosystem. However, their interacting effects on soil properties and plant growth are still poorly understood. A compartmental experimental design was used in a climate chamber in order to investigate, without phosphorus (P) addition, the single and combined effects of earthworms (Allolobophora chlorotica), AMF (Glomus intraradices) and roots (Allium porrum) on soil structure, nutrient concentration and plant growth. In our experimental conditions, plant roots improved soil structure stability (at the level of macroaggregates) whereas earthworms decreased it. AMF had no effect on soil structure stability but increased P transfer from the soil to the plant and significantly increased plant biomass. Earthworms had no direct influence on P uptake or plant biomass, and the N/P ratio measured in the shoots indicated that P was limiting. Interactions between AMF and earthworms were also observed on total C and N content in the soil and on total root biomass. Their effects varied temporally and between the different soil compartments (bulk soil, rhizosphere and drilosphere). After comparison with other similar studies, we suggest that effects of earthworms and AMF on plant production may depend on the limiting factors in the soil, mainly N or P. Our experiment highlights the importance of measuring physical and chemical soil parameters when studying soil organism interactions and their influence on plant performance.