Movement of Five Nematode Species through Sand Subjected to Natural Temperature Gradient Fluctuations

Temperature gradient fluctuations that occur naturally as a result of heating and cooling of the soil surface were reproduced within 15-cm-d, 15-cm-long acrylic tubes filled with moist sand. Sunny and rainy periods during the late summer in eastern Texas were simulated. Five ecologically different nematode species were adapted to fluctuating temperatures for 20-36 hours at a simulated depth of 12.5 cm before being injected simultaneously into the centers of tubes at that depth. When heat waves were propagated horizontally to eliminate gravitational effects, the movement of Ditylenchus phyllobius, Steinernema glaseri, and Heterorhabditis bacteriophora relative to the thermal surface was rapid and largely random. However, Rotylenchulus reniformis moved away from and Meloidogyne incognita moved toward the thermal surface. When heat waves were propagated upward or downward, responses to temperature were the same as when propagated horizontally, irrespective of gravity. The initial direction of movement 1.5 hours after introduction to 20-era-long tubes at five depths at five intervals within a 24-hour cycle indicated that M. incognita moved away from and R. reniformis moved toward the temperature to which last exposed. Differences in movement of the five species tested relative to gravity appeared related to body length, with the smallest nematodes moving downward and the largest moving upward.     

Use of Nematodes as Biomonitors of Nonfumigant Nematicide Movement through Field Soil

Three field experiments were established in a loamy sand soil in the Coastal Plain of North Carolina to determine downward movement of aldicarb and fenamiphos with a nematode bioassay. Penetration of bioassay plant roots by Meloidogyne incognita was measured at 1, 3, 7, 14, 21, and 28 days after treatment in the greenhouse as a means of determining nematicide effectiveness. Chemical movement was similar in planted and fallow soil. Nematicidal activity was greater in soil collected from the 0 to 10 cm depth than from the 10 to 20 cm depth. Fenamiphos suppressed host penetration by the nematode more than aldicarb under the high rainfall (19 cm) and low soil temperatures that occurred soon after application in the spring. During the summer, which had 13 cm precipitation and warmer soil temperatures, both chemicals performed equally well at the 0 to 10 cm depth. At the lower soil level (10 to 20 cm), aldicarb limited nematode penetration of host roots more quickly than fenamiphos. Both of these chemicals moved readily in the sandy soil in concentrations sufficient to control M. incognita. Although some variability was encountered in similar experiments, nematodes such as M. incognita have considerable potential as biomonitors of nematicide movement in soil.     

Vertical Distribution of Rotylenchulus reniformis in Cotton Fields

The possible impact of Rotylenchulus reniformis below plow depth was evaluated by measuring the vertical distribution of R. reniformis and soil texture in 20 symptomatic fields on 17 farms across six states. The mean nematode population density per field, 0 to 122 cm deep, ranged from 0.4 to 63 nematodes/g soil, and in 15 fields more than half of the R. reniformis present were below 30.5 cm, which is the greatest depth usually plowed by farmers or sampled by consultants. In 11 fields measured, root density was greatest in the top 15 cm of soil; however, roots consistently penetrated 92 to 122 cm deep by midseason, and in five fields in Texas and Louisiana the ratio of nematodes to root-length density within soil increased with depth. Repeated sampling during the year in Texas indicated that up to 20% of the nematodes in soil below 60 cm in the fall survived the winter. Differences between Baermann funnel and sugar flotation extraction methods were not important when compared with field-to-field differences in nematode populations and field-specific vertical distribution patterns. The results support the interpretation that R. reniformis below plow depth can significantly impact diagnosis and treatment of cotton fields infested with R. reniformis.     

稻田不同水分管理方式对土壤线虫群落的影响

在下辽河平原地区就稻田不同水分管理方式对土壤线虫多度、营养类群、群落组成的影响进行了研究．结果表明,0～10cm土层不同水分管理处理的线虫总数在耙耕前和黄熟期显著低于对照,10～20cm土层各时期处理间线虫总数的差异不显著,20～30cm土层线虫总数在耙耕前和黄熟期差异极显著．北方单季稻水田试验共观察到土壤线虫16科22属．绕线属(Plectus)、垫刃属(Tylenchus)、单宫属(Monhystera)是优势属,绕线属和垫刃属对不同的水分管理比较敏感．在耙耕前和黄熟期不同水分管理方式对0～10cm土壤食细菌线虫能够产生显著影响．稻田土壤中食细菌线虫和植物寄生线虫是优势营养类群,而捕食／杂食性线虫的相对多度最低．     

根际效应对大豆田土壤线虫群落组成及多样性的影响

根际作为重要的环境界面是植物与环境之间物质能量交换的场所,关于根际效应的研究已成为土壤生态学的新兴热点领域,然而有关大豆根际效应对土壤动物多样性影响的研究报道并不多见。在三江平原选择连续耕作15a的大豆田,对大豆根际区与非根际区土壤线虫群落结构组成进行了对比分析。结果表明:大豆根际区土壤线虫总数、辛普森多样性指数(Dom)显著高于非根际区,根际区的物种数(S)、物种丰富度指数(SR)显著低于非根际区。说明大豆根际效应增加土壤线虫的丰度,但降低了线虫群落结构的复杂性。大豆根际区植物寄生线虫(PP)、食真菌线虫(FF)和食细菌线虫(BF)数量显著高于非根际区,而PP类群的比例在根际区却显著低于非根际区。这一研究结果表明食微线虫(FF和BF)类群在大豆根际区的比例增加更显著。食真菌与食细菌线虫数量比值(F/B)指示大豆根际区细菌生物量相对高于真菌生物量。研究结果丰富了农田土壤线虫多样性的研究内容,并为我国东北大豆田线虫病害的防治及定制相应的农业管理措施提供参考。     

Nematode communities indicate diverse soil functioning across a fog gradient in the Namib Desert gravel plains

Soil nematodes are fundamentally aquatic animals, requiring water to move, feed, and reproduce. Nonetheless, they are ubiquitous in desert soils because they can enter an anhydrobiotic state that allows them to persist when water is biologically unavailable. In the hyper‐arid Namib Desert of Namibia, rain is rare, but fog routinely moves inland from the coast and supports plant and animal life. Very little is understood about how this fog may affect soil organisms. We investigated the role of fog moisture in the ecology of free‐living, soil nematodes across an 87‐km fog gradient in the gravel plains of the Namib Desert. We found that nematodes emerged from anhydrobiosis and became active during a fog event, suggesting that they can utilize fog moisture to survive. Nematode abundance did not differ significantly across the fog gradient and was similar under shrubs and in interplant spaces. Interplant soils harbor biological soil crusts that may sustain nematode communities. As fog declined along the gradient, nematode diversity increased in interplant soils. In areas where fog is rare, sporadic rainfall events can stimulate the germination and growth of desert ephemerals that may have a lasting effect on nematode diversity. In a 30‐day incubation experiment, nematode abundance increased when soils were amended with water and organic matter. However, these responses were not evident in field samples, which show no correlations among nematode abundance, location in the fog gradient, and soil organic matter content. Soil nematodes are found throughout the Namib Desert gravel plains under a variety of conditions. Although shown to be moisture‐ and organic matter‐limited and able to use moisture from the fog for activity, variation in fog frequency and soil organic matter across this unique ecosystem may be biologically irrelevant to soil nematodes in situ.     

Seasonal fluctuation in nematode population associated with mango,Mangifera indica L.

Abstract

Mango, Mangifera indica L. Family – Anacardiaceae, an economically important fruit tree, was selected to study the effect of seasonal changes of the population of plant parasitic nematodes viz., Hoplolaimus indicus, Helicotylenchus indicus, Rotylenchulus reniformis, Tylenchorhynchus mashoodi, Tylenchus filiformis and Hemicriconemoides mangiferae around the roots. The population was investigated at 10, 20 and 40 cm depths. It was observed that seasonal fluctuations have a direct effect on the nematode population. The population was larger at 10 cm depth followed by 20 and 40 cm depths. The largest nematode population was observed when the percent soil moisture was high. Both soil temperature and soil moisture were equally important. The soil pH also affected indirectly the nematode population densities.     

Effects of Seasonal and Site Factors on Xiphinema index Populations in Two California Vineyards

Sampling of Xiphinema index for 2 years (1993-95) in two California vineyards indicated that a greater number of nematodes occurred during the winter months. The number of juveniles increased four-fold from December 1993 to January 1994, indicating a high reproductive rate during this time. Extremely high or low soil temperatures corresponded to low nematode numbers. Samples were taken from 0 to 31 cm and 31 to 62 cm deep both within and between the vine rows. Numbers of nematodes were greatest at the 0- to 31-cm depth in one vineyard with a loamy sand soil, and at a depth of 31 to 62 cm in the second vineyard, which had a silt loam soil. In both vineyards, X. index population densities were greater within the vine row.     

基于高通量测序分析西藏北部高寒草甸不同深度土壤线虫群落分布特征

为了研究藏北高寒草甸土壤线虫多样性,于2017年8月,采用Illumina MiSeq测序技术,研究了西藏北部高寒草甸0—25 cm范围内不同深度土层的土壤线虫群落组成与结构特征。结果表明:5个不同深度共获得OTU 990个,隶属于3个纲,7个目,25个科,30个属,刺嘴纲Enoplea为共有优势土壤线虫群落;对样品进行Alpha多样性评价,计算Chao 1指数、Shannon指数和Ace指数,发现5—10 cm的土壤样品群落有较高的丰富度;属水平Heatmap图分析可知20—25、15—20、0—5、5—10、10—15 cm土壤线虫群落的组成相似性有一个递增的趋势。与不同深度藏北高山嵩草(盛长期)线虫群落结构相关性较大的土壤化学指标是K~+、含水率、有机质和Zn~(2+)。研究发现不同深度土壤线虫种类及丰度存在一定的差异,可为研究藏北高寒草地土壤线虫群落特点提供依据。     

Nematodes as Sentinels of Heavy Metals and Organic Toxicants in the Soil

Field and laboratory research has repeatedly shown that free-living soil nematodes differ in their sensitivity to soil pollution. In this paper, we analyze whether nematode genera proved sensitive or tolerant toward heavy metals and organic pollutants in six long-term field experiments. We discuss overlaps between nematode physiological responses to heavy metals and to organic pollutants, which may explain why nematodes can exhibit co-tolerance toward several contaminants. We propose a simple method for separating direct effects of soil contamination on nematode populations from indirect effects mediated through the food chain. Finally, we analyze the extent to which nematodes exhibited consistent responses across the experiments analyzed. Our results show that (a) indirect effects of pollution were generally strong; (b) fewer nematode genera were tolerant than sensitive; (c) many genera, including practically all Adenophorea, exhibited a common response pattern to contaminants; and (d) several genera of the Secernentea exhibited differential tolerance toward particular pollutants. We conclude that bioindication of soil contamination should preferentially be based on tolerant, and less on sensitive, nematodes. We provide a list of nematode genera that may potentially serve as differential bioindicators for specific soil contaminants.     

A Neoaplectanid nematode in the larch sawfly Cephalcia lariciphila (Hymenoptera: Pamphiliidae)

A nematode parasitic on prepupae of larch sawfly (Cephalcia lariciphila) appears to be indistinguishable from Neoaplectana carpocapsae. Of three temperatures tested the optimum for development was 25 °C at which most eggs were produced in both the first and second generations. Infective nematodes entered sawfly prepupae through the anus and mouth, but the preferred mode of entry was through the spiracles; prepupal hosts were extremely attractive to infective nematodes. Nematodes overwintered in prepupal hosts and in the soil. Dauerlarvae penetrated 10 cm of packed moist soil to infect prepupae constrained at the bottom of a vertical tube, sawfly mortality decreasing with depth. Dauerlarvae may also migrate 8 cm horizontally, and 5 cm upwards, to invade the host. It is suggested that the nematode could be used to supplement biological control of Cephalcia lariciphila.     

Effect of Soil Texture on the Distribution and Infectivity of Neoaplectana glaseri (Nematoda: Steinernematidae)

The vertical migration of infective juveniles of Neoaplectana glaseri applied to the soil surface or introduced 16 cm below the soil surface was studied in pure silica sand, coarse sandy loam, silty clay loam, and clay. The number of juveniles that migrated and infected wax moth pupae placed in the soil decreased as the proportion of clay and silt increased. The majority of nematodes moved downwards 2-6 cm from the surface, but some penetrated to a depth of 14 cm in pure silica sand and coarse sandy loam. In pure silica sand and coarse sandy loam, nematodes introduced 16 cm below the soil surface were able to infect wax moth pupae located at depths of 0-4 cm and 28-32 cm. Nematodes showed a greater tendency to disperse downwards from the point of application. Movement of the nematode was least in clay soil and limited in silty clay loam soil. The number of migrating nematodes was greatest when wax moth pupae were present.     

宁夏枸杞根际土壤线虫群落特征

宁夏枸杞具有很高的药用价值和营养价值,在宁夏大面积种植,带来了巨大的经济效益.研究不同条件下宁夏枸杞根际土壤线虫群落特征,对防止枸杞园土壤退化具有重要意义.以宁夏枸杞之乡——中宁为研究区,系统分析了季节、树龄和土层对枸杞根际土壤线虫数量和生态指数的影响.结果表明: 夏季宁夏枸杞根际土壤线虫数量最多,小杆属和拟丽突属为优势属;春季线虫数量最少,小杆属、针属和盘旋属为优势属;从春季到秋季,线虫稀有属种类和个体密度逐渐减少.随着树龄的增加,夏季根际土壤线虫总数逐渐减少;春季和秋季枸杞根际线虫总数先增加后减少,其中9年树龄根际土壤线虫总数最多.各树龄根际均为食细菌线虫所占比例最大,食真菌线虫和捕食-杂食类线虫比例很小,树龄<3年和>9年时植物寄生线虫比例相对较大.夏季枸杞根际20～40 cm比0～20 cm土壤线虫数量增加49.4%.随着树龄的增加,枸杞根际土壤线虫多样性指数、均匀度指数和丰富度指数均呈减小-增大-减小的趋势,而优势度指数先减小后增大.土壤pH值与土壤线虫优势度指数呈显著正相关;土壤有机质和速效磷与植物寄生线虫分别呈显著正相关和负相关;土壤碱解氮与线虫总数呈显著正相关;速效钾与线虫多个生态指数均呈显著负相关.整体上,随着树龄的增加,春季和秋季土壤线虫数量先增加后减少,夏季土壤线虫数量呈逐渐减少的趋势;线虫多样性下降,土壤逐渐退化.此外,在幼林和树龄>9年后需要抑制土壤植物寄生线虫的繁殖,以减少后期因线虫引起的病害.     

江苏省不同农业区土壤线虫群落分布特征

调查了江苏省不同农业区农田土壤线虫群落多样性,分析了土壤线虫数量和群落结构与土壤环境因子的关系,并探讨了土壤线虫对土壤健康的生物指示作用.结果表明： 农田土壤线虫共鉴定出2纲7目19科41属.6个农业区的土壤线虫的密度、群落组成均具有一定的差异性.沿海农业区的线虫数量最多(每100 g干土400条),显著高于徐淮、宁镇扬和沿江农业区(P＜0.05),而沿江农业区的土壤线虫数量最少(每100 g干土232条),这可能是由于土壤质地、年均降雨量和年均气温等因素的差异造成的.地理位置相近的农业区线虫优势属相似.相关性分析结果显示,土壤线虫数量与土壤有机质、全氮、速效氮、速效钾和速效磷均呈显著正相关关系;RDA分析表明,土壤全氮含量、速效磷及pH对线虫群落种属组成影响较大.分析江苏省农田土壤线虫群落空间分布特征,可为农田土壤生态系统健康状况评价提供数据支撑.     

Vertical distribution of soil nematodes in an age sequence of <Emphasis Type="Italic">Caragana microphylla</Emphasis> plantations in the Horqin Sandy Land,Northeast China

The vertical distribution of soil nematodes down to a depth of 50 cm was studied in an age sequence of 0-, 5-, 10-, and 22-year-old Caragana microphylla plantations (treatments) in the Horqin Sandy Land, Northeast China. The abundances and generic compositions of nematode fauna in five soil layers (0–10, 10–20, 20–30, 30–40, and 40–50 cm) were analyzed. 42 genera were observed in the nematode suspensions, and Acrobeles was the dominant genus in all treatments. The results showed that the total number of nematodes and the generic diversity in an age sequence of C. microphylla plantations decreased with increasing soil depth. Significant differences in the numbers of total nematodes, bacterivores (BF), plant parasites (PP), and omnivores–predators (OP) were observed between treatments and depths. BF was the most abundant trophic group in our study, followed by OP. The numbers of OP showed an obviously increasing trend with increasing age of C. microphylla plantation. The vertical distribution of the soil nematode communities was related to gradual changes in soil chemical properties, and it indicated that C. microphylla plantations have played positive roles in improving soil environmental conditions and restoring desertified ecosystems in the Horqin Sandy Land. The ecological indices selected were influenced by plantation chronosequence but not by soil depth.     

Effects of the nematicide imicyafos on soil nematode community structure and damage to radish caused by Pratylenchus penetrans

The effects of the non-fumigant nematicide imicyafos on soil nematode community structure and damage to radish caused by Pratylenchus penetrans were evaluated in two field experiments in consecutive years (2007 and 2008). Nematode densities in soil at 0 - 10 cm (the depth of nematicide incorporation) and 10 - 30 cm were measured. The application of imicyafos had a significant impact on the density of P. penetrans at 0 - 10 cm but had no effect on free-living nematode density. PCR-DGGE analysis conducted using extracted nematodes showed that the nematode community structure 12 d after application in 2007 was altered by the application of imicyafos at the 0 - 10 cm depth, but not at 10 - 30 cm. No significant differences were observed in the diversity of the nematode community at harvest (89 and 91 d after application) between the control and imicyafos treatments in both depths and both years. In both years, the damage to radish caused by P. penetrans was markedly suppressed by the nematicide. Overall, the nematicide imicyafos decreased populations of P. penetrans in soil and thereby decreased damage to radish, while having little impact on the soil nematode community.     

Observations on the population dynamics, life cycle and ecology of the plant parasitic nematode Rotylenchus robustus

In a forest nursery growing Sitka spruce Rotylenchus robustus was most numerous at a soil depth of 10–19 cm, the vertical distribution being associated with that of the tree roots. Although no consistent seasonal fluctuations in nematode numbers were obvious there was a seasonal pattern in the relative frequency with which adult and larval nematodes were recovered.
Experimentally, the life cycle of R. robustus was completed in 14–18 months depending upon the time of year when nematodes were added to the trees. Soil moisture content of less than 8.7% (pF 3.8) restricted the movement of 50% of the population. The overall mean basal temperature for the completion of the life cycle of R. robustus was calculated to be 6.5°C.
Soil temperature was considered to be the dominant factor controlling the rate of development under field conditions in Scotland; only occasionally did soil moisture become limiting.     

The soil as an environment for plant parasitic nematodes

British arable soils are uniform in texture to plough depth but vary in structure and the content of new organic matter according to recent cultivations. Subsoils are less uniform and little influenced by cultivations. Although remarkably uniform in structure and outward appearance nematodes vary greatly in length and girth. Root ectoparasites live wholly in the soil but root endoparasites may spend only brief periods there. Nematodes that spend much time in the soil surface are subject to a harsher microclimate than those inhabiting deeper layers. The movement and activity of nematodes in soil is influenced by the thickness of water films, the amount of pore space with dimensions the nematodes can traverse, the stability and packing of aggregates, the oxygen consumption of competing organisms and the rate of supply. Other factors of which little is known are the degree of continuity of the usable spaces and their tortuosity. The moisture characteristic curve which relates water withdrawn to the suction pressure (matric potential) applied is a useful tool which enables the space available to a given nematode to be related to its cross-sectional diameter. Although total pore space and usable pore space are correlated in a general way with each other and with texture (e.g.% sand), and bulk density is correlated with pore space within textural classes, none of these measures can be substituted for an estimate of the usable pore space appropriate to the size of the nematode being studied. For most purposes the solid matrix of arable soils can be regarded as an inert skeleton supporting the pore space. The stability of the matrix is therefore an important parameter, determining changes of space and time. Little is known about the distribution of macro-voids in soils and rapid methods of assessing them are needed. These may be more important for and more easily used by long nematodes than short ones. For nematodes living near the soil surface diurnal temperature fluctuations may be large. Their movement and activity should be related to temperature fluctuations via the Q₁₀-curve. For nematodes living deeper in the soil diurnal fluctuations are unimportant. So development can be mirrored by curves of accumulated temperature in day degrees above basal development temperature. Unlike the pattern of rainfall accumulation, which varies from place to place and year to year, and must greatly affect nematode multiplication, crop damage and activity, the pattern of temperature accumulation is remarkably stable. Differences from place to place and year to year are moderated by the changes in sowing date they impose. Consequently a nematode species and the host plant it infests usually develop together under similar soil climatic conditions from planting date onwards. In real soils the spaces in which nematodes live are partly filled by water in winter but are progressively drained as the season advances and become air filled. Then water films are too thin to permit movement except while the soil is draining after rainfall. Therefore the duration of activity is proportional to rainfall, and total activity throughout development is proportional to accumulated rainfall after discounting amounts too small to penetrate the soil. The activity of root ectoparasitic nematodes could be modelled by multiplying the duration of activity by temperature, using the Q₁₀ curve if necessary (i.e. by the rate of activity). The soil environment imposes constraints on the animals that live in it. Populations are relatively static and inbred. The pore space also limits the properties of substances that link parasite with parasite (pheromones) and parasite with host (phytomones). The many additional complications that arise when plants are grown are depicted diagrammatically.     

青藏高原不同牧草人工草地对土壤线虫群落的影响

为缓解草蓄矛盾,青藏高原人工草地得到快速发展。土壤线虫对环境变化敏感,是草地生态系统的重要组成部分。然而,不同牧草人工草地对土壤线虫群落的影响尚不明确。2016年7月,对多年生禾本科(垂穗披碱草Elymus nutans Griseb.、老芒麦Elymus sibiricus L.、早熟禾Poa annua L.和羊茅Festuca ovina L.)、一年生禾本科燕麦Avena sativa L.和多年生豆科紫花苜蓿Medicago sativa L.等6种单播牧草人工草地(建植期4年)和天然草地(对照)的土壤线虫群落进行了调查。结果表明：(1)土壤线虫隶属于2纲8目32科58属,平均密度为1754个/100 g干土;紫花苜蓿样地的线虫密度最低,为949个/100 g干土;燕麦样地最高,为3267个/100 g干土;(2)与天然草地相比,燕麦样地的线虫群落总密度、多样性以及植食性和杂食-捕食性线虫密度显著增加,而其他人工草地的线虫群落密度、多样性以及植食性、食真菌和食细菌线虫密度均无显著变化;(3)土壤线虫总密度以及各营养类群密度在不同人工草地间差异显著,且均在燕麦样地最高;(4)建...     

中国两个气候过渡区森林土壤线虫群落的比较研究

为比较不同气候过渡区地带性森林土壤线虫群落结构和多样性,以南亚热带向中亚热带气候过渡区的石门台常绿阔叶林和北亚热带向暖温带气候过渡区的鸡公山落叶阔叶林为样地,探讨不同气候区土壤微食物网能流方式的差异。结果表明,石门台土壤线虫群落属数(S)、生物量(Biomass)、多样性指数(H')、丰富度指数(SR)、成熟度指数(MI)和结构指数(SI)在表层土壤(0~10 cm)均显著高于鸡公山。石门台的土壤线虫通路指数(NCR)均值高于0.5,而鸡公山的NCR均值小于0.5,说明前者土壤食物网可能偏向于细菌能流通道,而后者偏向于真菌能流通道,这也与食细菌线虫、食真菌线虫生物量的计算结果一致。可见线虫群落结构确实存在明显的南北差异,并较好地指示了土壤能流和养分循环状况。