西北地区赤霞珠葡萄根际土壤中AM真菌的多样性

对西北地区5个酿酒葡萄赤霞珠（Vitis vinifera L. cv. Cabernet Sauvignon）葡萄园根际土壤060 cm土层的AM真菌空间分布进行了研究。结果表明,葡萄根系可形成丛枝菌根,且侵染率较高,最高达79%; 在西北地区的5个样地中共分离出AM真菌4属22种,其中球囊霉属（Glomus）15种,无梗囊霉属（Acaulospora）4种,盾巨孢囊霉属（Scutellospora）2种,巨孢囊霉属（Gigaspora）1种。5个样地孢子密度大小顺序为: 陕西泾阳（JY）＞山西永济（YJ）＞陕西杨凌（YL）＞宁夏银川（YC）＞甘肃莫高（MG）。各样地葡萄根际土壤中AM真菌种的丰富度不同,陕西泾阳地区最高; 分布于葡萄根际的AM真菌按种类多少排序的属依次是: 球囊霉属无梗囊霉属盾巨孢囊霉属巨孢囊霉属,球囊霉属占据的比例保持着绝对优势; 根内球囊霉、摩西球囊霉、地表球囊霉在不同样地中均为优势菌株,副冠球囊霉,集球囊霉,细凹无梗囊霉是多数样地中的稀有种类。研究表明,葡萄与AM真菌具有良好的共生关系,二者协同进化产生了具有生态环境特异性的菌根真菌多样性; 葡萄根际存在较为丰富的丛枝菌根真菌资源,可供进一步开发利用。     

不同土壤类型下AM 真菌分布多样性及与土壤因子的关系

以禾本科植物群落为研究对象, 研究了宁夏六盘山林地、银川农耕地、暖泉农耕地、固原农耕地、盐池沙地、灵武沙地6 个采样地点5 种土壤类型(黑垆土、灌淤土、黄绵土、灰钙土、风沙土)下AM 真菌物种多样性及其与土壤因子的关系。结果表明: 5 种土壤类型采样点的植被根际土壤中共鉴定出5 属48 种AM真菌, 其中, 无梗囊霉属(Acaulospora)1 种, 巨孢囊霉属(Gigaspora)3 种, 球囊霉属(Glomus)37 种, 类球囊霉属(Paraglomus)1 种, 盾巨孢囊霉属(Scutellospora)6 种, 各采样点土壤均以球囊霉属为优势属。地球囊霉(G.geosporum)和木薯球囊霉(G. manihotis)是6 个采样地点中的优势种。不同土壤类型各采样点AM 真菌各属的频度存在明显差异, 球囊霉属在各点均有出现, 频度值最高。具有较高植被多样性的暖泉样点, AM 真菌的种属数量较多。土壤环境因子对AM 真菌孢子密度的影响因所处土壤、植被类型不同而异。pH、全盐、速效钾、速效磷等土壤肥力因子, 在PCA 轴上能最大程度地解释AM 真菌孢子密度与土壤环境因子之间相互关系的大部分信息。宁夏不同土壤类型区域中AM 真菌种类及分布一定程度上与该采样点的植被类型、植物多样性和土壤肥力特征相对应。     

贵州务川汞矿区土壤丛枝菌根真菌多样性研究

对贵州务川汞矿区不同水平汞污染植物根际土壤丛枝菌根(Arbuscular Mycorrhizal,AM)真菌资源分布及多样性进行分析，结果发现：本研究选取的务川汞矿区3个样地土壤汞含量在94.1～268.0 mg/kg，远高于国家规定的土壤汞含量，汞污染极其严重。随着汞含量增加，AM真菌侵染率、孢子密度显著降低，AM真菌群落丰富度、多样性及均匀度均有所降低，但差异未达显著水平，而仍有部分对汞污染耐受性较强的AM真菌有较高的分布。另外，管柄囊霉属AM真菌在5个样品中均有较高丰度，球囊霉属AM真菌在火棘根际的相对丰度随着汞含量升高而升高，因此，火棘–管柄囊霉属AM真菌或火棘–球囊霉属AM真菌联合进行汞污染修复，有一定的应用价值。     

内蒙古盐碱土中AM真菌的多样性与分布

在内蒙古盐碱土13种主要植物分离到3属26种AM真菌,其中球囊霉属(Glomus)22种,无梗囊霉属(Acaulospora)3种,原囊霉属(Archaeospora)1种。地球囊霉(Glomus geosporum)和地表球囊霉(Glomus ver-siforme)是该区域盐碱土中的优势种。13种主要植物均能被AM真菌侵染,其中玉米和马蔺的侵染率最高,达100%;根际土壤中AM真菌孢子密度范围为29~182个g-1烘干土,其中稻的孢子密度最高,达182个g-1烘干土;在不同土壤类型条件下植物的菌根侵染率具有明显的差异,其规律为草甸盐土>碱化盐土>盐化草甸土>碱化草甸土;根际土壤中孢子密度以碱化草甸土最高(101个g-1烘干土),其次为碱化盐土、草甸盐土和盐化草甸土。相关分析表明,根际土壤中AM真菌孢子密度与菌根侵染率无显著相关性。     

黄土高原柠条锦鸡儿根际AM真菌生态学研究

对陕西省延安、绥德、横山和榆林4个自然生境下柠条根际AM真菌空间分布和土壤成分进行测定分析。结果表明,AM真菌泡囊定殖率及孢子密度与样地生态条件密切相关,泡囊定殖率和孢子密度在绥德最高,丛枝定殖率在榆林最高。采样深度对孢子密度有显著影响,最大孢子密度发生在10～20cm土层,而AM真菌定殖率在0～10cm或20～30cm土层有最大值。孢子密度与泡囊定殖率呈正相关,与丛枝定殖率呈负相关。孢子密度与土壤有机质、速效磷、速效钾和Cl-含量呈负相关,泡囊定殖率与土壤pH呈正相关,而与土壤湿度、速效钾和Cl-含量呈负相关。土样中AM真菌多为球囊霉属(Glomus)种类,也有少数无梗囊霉属(Acaulospora)和巨孢囊霉属(Scutellospora)种类。建议孢子密度和泡囊定殖率可作为检测土壤环境状况的指标。     

河北安国7种中药材AM真菌遗传多样性研究

为探明药用植物根围AM真菌遗传多样性,于2010年8月在河北省安国市3个样地(中药材种植基地、霍庄、大户村)用随机抽样法采集7种中药材根围土壤样品,分离筛选双网无梗囊霉(Acaulospora bi-reticulata)为试验菌种,进行孢子DNA提取、PCR扩增、序列测定及聚类分析,研究了双网无梗囊霉遗传多样性与土壤因子的关系,并选取两条代表菌株所测序列,连同从NCBI数据库中下载的4属28种AM真菌相应序列构建系统发育树。结果表明,双网无梗囊霉可侵染不同样地不同宿主植物,3个样地7种中药材双网无梗囊霉DNA相似性达99.2%以上,其遗传特征保持了高度稳定性,双网无梗囊霉在样地和植物间具有广谱性。同一样地不同中药材根围土壤因子相似性很高,AM真菌DNA碱基序列相似系数也很高,而不同样地同一中药材根围AM真菌DNA碱基序列相似系数低于前者,可见遗传多样性与土壤因子密切相关。土壤因子对双网无梗囊霉基因序列的影响大于宿主植物的影响。     

黑土农田施加AM菌剂对大豆根际菌群结构的影响

为揭示在黑土农田条件下施加丛枝菌根(AM)菌剂对作物根际微生物群落的影响,试验以大豆为研究对象,田间播种时分别施加根内球囊霉(Glomus intraradices,GI)和摩西球囊霉(Glomus mosseae,GM)两种AM菌剂,以单施化肥处理(F)和不施加AM菌剂及化肥处理(CK)作为对照,采用传统与现代分子生物学手段,研究大豆根际土壤中菌群结构及根系内AM真菌多样性。结果表明:GI、GM处理的大豆菌根侵染率最高达到78.3%和86.6%;GI、GM、F处理的大豆根际土壤中可培养细菌、真菌和放线菌三大菌群的数量与CK处理相比显著提高(p0.05)。分离大豆结荚期根际土壤中AM真菌孢子,共获得Acaulospora属真菌3种,Glomus属真菌7种,孢子密度均较低,G.intraradices和G.mosseae均为各自处理的优势种群。对大豆结荚期根系和根际土壤PCR-DGGE图谱条带的丰度及优势条带测序分析,结果表明根际土壤中的AM真菌菌群数明显高于根系中AM真菌的菌群数量,GI处理的大豆根际土壤中AM真菌丰度值最大,GM处理大豆根系里的AM真菌丰度值最大,F处理的根际土壤中总AM真菌的数量最少;施加AM菌剂处理的大豆根系及根际土壤中的优势菌群分别为外源施加的两种AM真菌。     

丛枝菌根真菌对金叶六道木镉吸收及根际真菌群落结构的影响

为明确丛枝菌根(AM)真菌对促进绿化苗木镉(Cd)吸收的影响,通过盆栽试验比较接种不同AM真菌对12种绿化苗木Cd吸收的差异,并进一步分析接种对金叶六道木根际微生物数量和AM真菌群落结构的影响。结果表明,12种绿化苗木,加Cd处理6个月后,其叶片浓度的变化范围为0.25~2.59 mg·kg^-1。接种AM真菌处理组的叶片Cd含量均高于不接菌处理组。相比未接种,接种AM2摩西球囊霉(BGCAM00164)后金叶六道木叶片中Cd含量增加147.9%,故选择金叶六道木进行后续研究。接种AM212个月后,金叶六道木的根、枝、叶Cd浓度分别为164.7、22.86和10.57 mg·kg^-1,为不接菌处理的2.64倍、2.06倍和1.76倍,全株总Cd含量达5078μg·株^-1,显著高于不接菌对照(1745μg·株-1)。相比不接菌对照,接种AM2后降低了转移系数,将其更多吸收的Cd固定在根内,从而减少对植株的损害。接种AM真菌增加了AM真菌PLFA生物量,但降低了根际土壤微生物细菌和真菌PLFA生物量。所有样本中丰度较高的AM真菌为球囊霉科(Glomus)、类球囊霉科(Paraglomus)和原囊霉科(Archaeospora)。球囊霉科占总AM真菌的55%以上,是金叶六道木根际的主要优势种群。接种AM2后球囊霉丰度显著增加,由对照的61.8%上升至77.4%,但AM真菌的整体多样性和丰富度则表现为下降。接种AM1后类球囊霉科丰度显著增加,由对照的13.1%上升至17.8%,但球囊霉丰度无显著变化。主成分分析结果表明Cd和AM真菌接种可以改变金叶六道木根际AM真菌群落结构。总体而言,接种AM真菌(摩西球囊霉)能提高金叶六道木对土壤重金属Cd污染的修复效率,其联合修复技术可扩展Cd污染土壤植物修复的应用范围。     

长期保护性耕作对丛枝菌根真菌多样性的影响

为了明确我国北方干旱地区长期保护性耕作以及深松对丛枝菌根真菌(AMF)多样性的影响,笔者于2014年在山西省临汾市连续22年实施保护性耕作的长期定位试验基地,针对免耕覆盖(NTS)、深松免耕覆盖(SNTS)及传统耕作(TT)3种处理方式,进行了不同耕作条件下土壤AMF物种丰度、孢子密度、Shannon多样性指数以及AMF侵染率等因素的比较研究。结果显示,长期保护性耕作(NTS和SNTS)共分离鉴定出AMF 7属9种,其中根孢囊霉属(Rhizophagus)和斗管囊霉属(Funneliformis)各2种,球囊霉属(Glomus)、近明球囊霉属(Claroideoglomus)、无梗囊霉属(Acaulospora)、硬囊霉属(Sclerocystis)和隔球囊霉属(Septoglomus)各1种;而传统耕作(TT)共分离鉴定出AMF 6属8种,没有检测到无梗囊霉属。NTS、SNTS和TT处理在不同土层的AMF优势种基本一致,0~40 cm土层为摩西斗管囊霉(Fu.mosseae)和变形球囊霉(G.versiforme),40~80 cm土层为摩西斗管囊霉、变形球囊霉和聚丛根孢囊霉(Rh.aggregatum),80~120 cm土层为聚丛根孢囊霉,120 cm土层以下只有NTS和SNTS处理中存在聚丛根孢囊霉,说明保护性耕作措施促进了AMF向土壤深层发展。NTS和SNTS处理在同一土层的AMF物种丰度、孢子密度和Shannon多样性指数均高于TT处理,SNTS处理高于NTS处理。同一耕作措施不同土层的AMF物种丰度、孢子密度和Shannon多样性指数均随土层加深而逐渐降低;NTS和SNTS处理在小麦各生育期的丛枝侵染率和孢子密度均高于TT处理;各处理在小麦拔节期的AMF侵染率最高,分别为14.9%、16.1%和10.6%,而在收获期的土壤孢子密度最高,分别为111.7个·(100g)~(-1)、125.0个·(100g)~(-1)和90.3个·(100g)~(-1)。研究认为,长期免耕覆盖、尤其深松免耕覆盖,提高了AMF多样性。该研究结果可为中国北方旱作农田生态系统中AMF自然潜力的充分发挥,以及保护性耕作技术的合理应用提供科学依据。     

喀斯特不同植被恢复阶段土壤AMF组成及多样性研究

选择贵州花江、织金、花溪3个典型喀斯特地段的不同恢复阶段(乔木、灌木和草本)土壤丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)为研究对象,基于AMF孢子的形态鉴定,计算AMF孢子密度、分离频度、物种丰度、重要值、物种均匀度指数及Shannon-Wiener多样性指数,研究喀斯特不同恢复阶段土壤AMF物种组成及多样性。结果表明:3个采样地段共分离AMF菌种4目8科10属68种,其中球囊霉属(Glomus)数量最大达35种,其次是无梗囊霉属(Acaulospora)为17种,巨孢囊霉属(Gigspora)、类球囊霉属(Paraglomus)及原囊霉属(Archaeospora)各1种;不同采样地段各植被恢复阶段的优势种不同,所有样地中层状球囊霉Glomus lamellosum具有最大的相对丰度、分离频度和重要值;AMF种分类水平上,花江样地不同恢复阶段AMF丰度表现为乔木灌木草本,织金样地为灌木草本乔木,花溪样地为灌木乔木草本;不同植被恢复阶段土壤AMF均匀度指数和香农多样性指数在花江和织金多表现为乔木灌木草本,花溪样地Shannon-Wineer指数则是灌木显著高于乔木和灌木阶段。研究表明,喀斯特土壤中因异质生境条件的差异AMF菌种组成和物种多样性具有较大的差异。     

Tempo-spatial dynamics of arbuscular mycorrhizal fungi under clonal plant Psammochloa villosa Trin. Bor in Mu Us sandland

Tempo-spatial dynamics of AM fungi within the rhizome system of Psammochloa villosa (Poaceae) were investigated in Mu Us sandland, northwest China. Soil samples in the annual and perennial ramet rhizospheres of P. villosa were collected in 2007. AM fungal percent colonization reached maximal values in the rainy season and spore number in the dry season. Spore number exhibited positive correlation with soil pH and available phosphorous (P) (P < 0.01), and negative correlation with available nitrogen (N) (P < 0.05). Vesicular, arbuscular, hyphal and total colonization were positively correlated with soil organic matter and available P (P < 0.01), and negatively correlated with available N (P < 0.01). Fourteen species of AM fungi in four genera were isolated. The same AM fungal taxa were found in the annual and perennial ramet rhizospheres, although the last ones had higher fungal colonization and spore number. A high Shannon-Weiner diversity index of AM fungi was observed. Spore number and species richness indicated that Glomus was the predominant AM fungi, especially the small-spored taxa. AM fungal dynamics under P. villosa are highly seasonal: different aged ramets and nutrient availability have effects on AM fungal development and abundance in Mu Us sandland.     

毛乌素沙地1960—2018年气候变化特征及影响因子分析

为了解毛乌素沙地近年气候变化特征,以期为区域沙漠化防治和灾害预警提供参考,基于毛乌素沙地及其周围邻近地区10个气象站点1960—2018年的逐月气温和降水资料,采用线性倾向估计、Anusplin空间插值、Mann-Kendall突变检验、Morlet小波分析、R/S分析和Pearson相关分析等方法对毛乌素沙地近59 a的气候变化特征、未来气候变化趋势和主要影响因素进行了探究。结果表明:(1)毛乌素沙地近59 a的年平均气温呈显著上升趋势,增温速率达到0.35℃/10 a。其约在1996年发生突变,主要存在4 a左右的短变化周期,且增温速率随地区海拔的升高而加快。(2)毛乌素沙地近59 a的年降水变化速率为2.88 mm/10 a,夏季降水增速较快,但变化趋势均不显著且无明显的降水突变时间点和显著的振荡周期。(3)未来毛乌素沙地的年、季节平均气温和秋季降水均呈现持续性较强的上升趋势。总体而言,毛乌素沙地的气候呈现暖湿化趋向,厄尔尼诺—南方涛动(ENSO)事件对其冬季气温影响较大,北极涛动(AO)和北大西洋涛动(NAO)对其冬夏季节降水影响较显著。     

毛乌素沙地麻黄的蒸散耗水特征和水分供求关系

麻黄是毛乌苏沙地重要的药用植物,对麻黄蒸散量以及水分供求关系的研究将有助于了解其蒸散耗水以及水分生理生态的特征。本研究于2004年生长季(4月20日-9月20日),利用由涡度相关技术测算得到的麻黄蒸散通量数据,并结合自动气象观测系统同步观测得到的麻黄冠层微气象参数,分析了毛乌素沙地麻黄蒸散耗水特征及水分供求关系。结果表明:(1)在生长季,麻黄蒸散(ET)具有明显的时间变化特征。在晴和多云天气,ET变化为单峰曲线,在阴天呈多峰曲线变化趋势;在日际变化方面,日ET值的变化过程表现为,4月下旬逐渐升高、6-7月达到高峰、8月逐渐降低、9月中旬降至最低。生长季的ET日均值为0.60mm.d-1,总ET量为93.6mm。(2)主要生长期内,蒸散速率(ET)与冠层太阳总辐射(Ra)、空气温度(Ta)、相对湿度(RH)、风速(V)等气象要素有极显著相关关系(R2=0.731,P0.01),其中影响ET的主要微气象因子是太阳总辐射(Ra)。(3)就整个生长季而言,降水总量与麻黄蒸散耗水总量的比值为2.9,说明试验区降水可以满足麻黄生长的水分需求。但在麻黄返青初期的降水量较低,麻黄生长受到降水量与土壤含水量的制约。     

Dynamics of arbuscular mycorrhizal fungi and glomalin in the rhizosphere of Artemisia ordosica Krasch. in Mu Us sandland, China

To understand the ecological significance of arbuscular mycorrhizal (AM) associations in semi-arid and arid lands, the temporal and spatial dynamics of AM fungi and glomalin were surveyed in Mu Us sandland, northwest China. Soil samples in the rhizosphere of Artemisia ordosica Krasch. were collected in May, July and October 2007, respectively. Arbuscular, hyphal and total root infection and spore density of AM fungi peaked in summer. The mean contents of total Bradford-reactive soil proteins (T-BRSPs, TG) and easily extractable Bradford-reactive soil proteins (EE-BRSPs, EEG) reached maximal values in spring. Spore density and two BRSPs fractions were the highest in the 0-10 cm soil layer, but the ratios of two BRSPs fractions to soil organic carbon (SOC) were the highest in the 30-50 cm soil layer. Hyphal infection was negatively correlated with soil enzymatic activity (soil urease and acid phosphatase) (P < 0.05). Arbuscular infection was negatively correlated with soil acid phosphatase (P < 0.01). Spore density was positively correlated with edaphic factors (soil available N, Olsen P, and SOC) and soil enzymatic activity (soil acid and alkaline phosphatase) (P < 0.01). Two BRSPs fractions were positively correlated with edaphic factors (soil available N and SOC) and soil enzymatic activity (soil urease, acid and alkaline phosphatase) (P < 0.01). TG was positively correlated with soil Olsen P (P < 0.05). We concluded that the dynamics of AM fungi and glomalin have highly temporal and depth patterns, and influenced by nutrient availability and enzymatic activity in Mu Us sandland, and suggest that glomalin are useful indicators for evaluating soil quality and function of desert ecosystem on the basis of its relationship to AM fungal community, soil nutrient dynamics and carbon cycle.     

毛乌素沙地圪丑沟小流域沙柳水分利用来源研究

为探究毛乌素沙地典型治沙植物根系吸水来源及其影响因素,通过测定毛乌素沙地圪丑沟小流域沙柳(Salix psammophila)(18~20年)木质部水及其各种潜在水源(降水、土壤水和地下水)的氢氧同位素组成(δD和δ¹⁸ O),利用多元线性混合模型(IsoSource)研究沙柳水分来源的季节变化特征及其影响因素。结果表明,监测期间(6-11月)沙柳木质部水、降水、土壤水和地下水的δD值变化范围分别为-82.41‰~-52.91‰,-144.81‰~-6.60‰,-96.94‰~-42.65‰和-86.42‰~-71.82‰,δ¹⁸ O值变化范围分别为-10.77‰~-7.29‰,-18.86‰~-2.07‰,-12.85‰~-0.79‰和-10.86‰~-9.74‰。雨季(7-9月)降雨量和土壤含水量分别高于旱季(6,10,11月)24.80~90.10 mm和0.95%~1.84%,但6-9月地下水位却低于10-11月2~7 cm。沙柳根系在旱季6月主要利用深层土壤水(>200 cm)(33.70%)和地下水(26.20%),雨季(7-9月)逐渐转变为以吸收浅层土壤水(<200 cm)为主(50.70%~54.00%),10-11月由于气温降低、降水减少及沙柳生命活动减弱,浅层土壤水(<200 cm)对沙柳根系吸水的贡献高于雨季(7-9月)35.20%~40.00%,而地下水对沙柳根系吸水的贡献显著降低(<5.00%)。因此,沙柳根系对于毛乌素沙地季节性干旱具有较强的适应和调节能力,其吸水来源随降雨量、土壤含水量和地下水位季节波动而变化。     

Interactions of arbuscular-mycorrhizal fungi and Bacillus strains and their effects on plant growth,microbial rhizosphere activity (thymidine and leucine incorporation) and fungal biomass (ergosterol and chitin)

The effects of two Bacillus strains (Bacillus pumillus and B. licheniformis) on Medicago sativa plants were determined in single or dual inoculation with three arbuscular-mycorrhizal (AM) fungi and compared to P-fertilization. Shoot and root plant biomass, values of thymidine and leucine incorporation as well as ergosterol and chitin in rhizosphere soil were evaluated to estimate metabolic activity and fungal biomass, respectively, according to inoculation treatments. For most of the plant parameters determined, the effectiveness of AM fungal species was influenced by the bacterial strain associated. Dual inoculation of Bacillus spp. and AM fungi did not always significantly increase shoot biomass compared to single AM-colonized plants. The most efficient treatment in terms of dry matter production was the dual Glomus deserticola plus B. pumillus inoculation, which produced similar shoot biomass and longer roots than P-fertilization and a 715% (shoot) and 190% (root length) increase over uninoculated control. The mycorrhizas were more important for N use-efficiency than for P use-efficiency, which suggests a direct mycorrhizal effect on N nutrition not mediated by P uptake. Both chemical and biological treatments affected thymidine and leucine incorporation in the rhizosphere soil differently. Thymidine was greater in inoculated than in control rhizospheres and B. licheniformis was more effective than B. pumillus in increasing thymidine. Non-inoculated rhizospheres showed the lowest thymidine and leucine values, which shows that indigenous rhizosphere bacteria increased with introduced inocula. The highest thymidine and leucine values found in P-fertilized soils indicate that AM plants are better adapted to compete with saprophytic soil bacteria for nutrients than P-amended plants. Chitin was only increased by coinoculation of B. licheniformis and G. intraradices. B. pumillus increased ergosterol (indicative of active saprophyte fungal populations) in the rhizosphere of AM plants and particularly when colonized by G. mosseae. The different AM fungi have different effects on bacterial and/or fungal saprophytic populations and for each AM fungus, this effect was specifically stimulated or reduced by the same bacterium. This is an indication of ecological compatibilities between microorganisms. Particular Glomus–bacterium interactions (in terms of effect on plant growth responses or rhizosphere population) do not seem to be related to the percentage of AM colonization. The effect on plant growth and stimulation of rhizosphere populations, as a consequence of selected microbial groups, may be decisive for the plant establishment under limiting soil conditions.     

The bacterial community of tomato rhizosphere is modified by inoculation with arbuscular mycorrhizal fungi but unaffected by soil enrichment with mycorrhizal root exudates or inoculation with Phytophthora nicotianae

Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.     

Effects of hexachlorocyclohexane on rhizosphere fungal propagules and root colonization by arbuscular mycorrhizal fungi in Plantago lanceolata

Lindane ( γ‐hexachlorocyclohexane or γ‐HCH) is an organochlorine insecticide previously used extensively for the control of agricultural pests. We studied the effects of soil HCH contamination on vegetation and its associated arbuscular mycorrhizas (AM). The polluted and unpolluted plots had similar plant cover, with the same species richness and abundance. Plantago lanceolata plants were selected for mycorrhizal analysis because of their presence in both plots and known mycotrophy. The presence of HCH appeared to have no significant effect on the extent of colonization of Plantago roots by AM, suggesting a similar functionality of the fungal symbionts. However, infective AM propagules, the density of AM spores and viable AM hyphae in the rhizosphere were much less in the HCH‐polluted soil than in the unpolluted plot. Pre‐inoculation of four plant species with an isolate of Glomus deserticola obtained from the HCH‐contaminated soil resulted in increased growth and fungal colonization of roots compared with plants pre‐inoculated with the introduced fungus G. macrocarpum or colonized by the consortium of indigenous AM fungal species, when those plants were transplanted to an HCH‐contaminated soil. This suggests that the fungus increases the tolerance of plants to the toxic soil environment. We conclude that herbaceous and woody plants can grow in soil with little P contaminated with <100 mg HCH kg^?1 with the help of tolerant AM, despite the detrimental effect of HCH on AM fungal propagules in soil. The effects of AM fungi on plant growth and soil microbial community structure in HCH‐polluted sites could be important for remediation of the pollutant through the microbial activity in the rhizosphere.