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.     

Long-term Disturbance Effects in the Nematode Communities of South Mississippi Woodlands

The effects of soil disturbance on the nematode community were assessed at 30 sites on the outer coastal plain of Mississippi, representing four ages since soil disturbance plus a control group of six undisturbed sites. Thirty-five taxa were encountered, dominated in abundance and taxonomic richness by plant and bacterial feeders. Nematodes were more abundant and more taxonomically rich in sites with a low slope and deep litter cover, distant from trees. Plant feeders were more numerous at sites with a dense herb cover, suggesting limitation by food availability. When sites were arranged as a chronosequence, herb cover, litter depth, soil organic matter, soil moisture, and tree canopy cover increased through time consistent with succession to forest. The abundance of most trophic groups decreased in the 10 to 20 years following disturbance and increased thereafter, a pattern repeated in taxonomic richness of plant and bacterial feeders. Fifty years after disturbance, nematode abundance had not returned to levels observed in control sites. These results suggest that nematode succession following soil disturbance is a gradual process regulated by establishment of aboveground vegetation. There was no evidence of dispersal limitation or facilitation by colonist nematode species.     

Soil Nematode Diversity: Species Coexistence and Ecosystem Function

Soil nematode species diversity is often high, both at ecosystem and single soil-core scales. First, how can so many species coexist? There is evidence of niche partitioning, notably of physical space, but vast interspecific overlaps and trait plasticity seem equally common. It appears that coexistence of species with similar resource needs is made possible by small-scale disturbance and predation, which likely reduce local population sizes and interspecific competition. Regional processes such as dispersal, large-scale disturbance, and aggregation, which govern ecosystem level diversity, may also affect local species interactions and soil-core scale diversity. Second, what is the significance of having so many species, with so few trophic functions, for ecosystem processes? Focusing on bacterivore diversity, it is clear that species contributions to decomposition, likely to differ as a function of individual biologies, are concealed by the trophic group approach. However, considerable functional redundancy probably exists, which may explain why decomposition processes are maintained in highly disturbed soils despite the extinction of many species. Thus, soil nematode diversity is important for the long-term stability of soil functioning, and merits protection and further study.     

Sampling for Regional Monitoring of Nematode Communities in Agricultural Soils

Regional assessment of nematode communities to monitor the condition or ecological health of agricultural soils requires sampling programs with measures of known reliability and the ability to detect differences over time. Numbers of fields sampled in a region, samples taken per field, and subsamples assayed per sample must be balanced with cost to provide the best sampling scheme. We used components of variance from statewide surveys in North Carolina (1992) and Nebraska (1993) to estimate number of (i) fields to be sampled; (ii) 20-core, composite soil samples to be obtained for each field; and (iii) subsamples to be assayed for each composite sample to detect a specified amount of change in index values within a geographic region. Variances for these three components were used to estimate the degree of reliability for five ecologically based indices (four measures of maturity and one of diversity) of nematode communities. Total variance for maturity and diversity indices, based upon communities of free-living nematodes, was greater in North Carolina than in Nebraska; the opposite was true for indices based strictly upon maturity of communities of plant-parasitic nematodes or of all nematodes in soil. Variability within samples was greater in North Carolina than in Nebraska, especially for maturity indices based only upon free-living nematodes. We identified two possible sampling strategies for a regional survey: Option 1, with two independent samples per field and a single subsample assayed per sample, which would provide a reliability ratio value ≥0.6 for most indices; and Option 2, with three independent samples per field and two subsamples assayed per sample, which would provide a reliability ratio value ≥0.7 for several indices. When cost was considered, Option 1 was the better strategy. Number of fields to be sampled within a region or state varied with the index chosen; with specific indices, however, a 10% change in mean index value could be detected with a sample of 50 to 100 fields.     

Abiotic Soil Factors and Plant-parasitic Nematode Communities

A natural community of plant-parasitic nematodes is usually polyspecific. The host plant is the most important driving force in nematode populations, but abiotic factors are important in maintaining the steady state. Nematode communities often separate by abiotic soil factors. In any continuous habitat, including crop plants, generally there is a consistency of the most abundant species, which are largely predictable. Data on single species provide little information about community patterns. Although certain nematode species might be indicators of certain environments, only when we discuss such aspects as diversity and ordination do we relate to communities irrespective of any interactions among component species. Only if plant-parasitic nematodes act independently of each other do autecological studies have validity in polyspecific communities.     

Effects of Mixed Cropping on a Soil Nematode Community in Honduras

Nematode-resistant tropical legumes are effective in reducing populations of plant-parasitic nematodes when used in rotation systems. Mixed cropping is a common practice of many small farmers in Central America, but little is known about the effects of tropical legumes on nematode communities under these systems. To examine the effects of intercropping on the nematode fauna associated with squash (Cucurbita pepo) and cucumber (Cucumis sativa) in Honduras, two field experiments were conducted to compare nematode density and diversity in soil under cucurbits grown as a monocrop with that in soil under cucurbits intercropped with alfalfa (Medicago sativa) or hairy indigo (Indigofera hirsuta). A parallel series of field tests compared soil nematode communities associated with a cucurbit monocrop and a cucurbit intercropped with marigold (Tagetes patula), which may decrease nematode populations through the production of toxic root exudates. Among all four tests, over a period of 90 days, there were no consistent differences in densities of various nematode genera or trophic groups in intercropped versus monocropped plants, nor were there consistent differences in community diversities among treatments.     

荒漠草原典型群落土壤粒径和养分的分布特征及其关系研究

为了明确荒漠草原区土壤机械组成与养分的关系,以宁夏盐池荒漠草原4种典型群落为研究对象,通过对不同群落(柠条、沙蒿、蒙古冰草、短花针茅)表层(0~5cm)、亚表层(5~10cm)和深层(10~15cm)土壤粒径分布分形(PSD)、养分含量的动态变化分析,揭示荒漠草原区土壤结构与土壤养分的相关性。结果表明:(1)4种典型群落土壤PSD均呈正态分布,不同群落间的土壤PSD差异显著,粒径100~500μm颗粒含量对PSD影响最大,不同群落间的差异大于不同生境间或不同土层间。(2)4种典型群落除全磷(TP)外,其余土壤全肥均随土壤深度增加呈降低趋势,且冠下大于丛间,表现出荒漠草原区特殊的"肥岛"聚集效应,不同群落间分布特征均表现为:柠条短花针茅蒙古冰草沙蒿,速效养分含量相对较高,各群落均达到适宜水平。(3)土壤养分与土壤PSD显著相关,除速效磷(AP)外,其余土壤养分与土壤分形维数(D)均呈正相关关系,粒径100~250μm、250~500μm颗粒与土壤养分呈显著或极显著负相关关系,土壤中的黏粒、粉粒在有机无机胶结过程及土壤良好的结构维持中起主要作用。     

Effect of Agricultural Management on Nematode Communities in a Mediterranean Agroecosystem

The effects of agricultural management on the soil nematode community were investigated in a field study at depths of 0 to 10 cm and 10 to 20 cm during a peanut (Arachis hypogaea) growing season in Israel. Nineteen nematode families and 23 genera were observed. Rhabditidae, Cephalobus, Eucephalobus, Aphelenchus, Aphelenchoides, Tetylenchus, Tylenchus, Dorylaimus, and Discolaimus were the dominant family and genera. Ecological measures of soil nematode community structure, diversity, and maturity indices were assessed and compared between the managed (by fertilization, irrigation, and pesticide application) and unmanaged fields. The total number of nematodes at a 10-cm depth during peanut-sowing, mid-season, and harvest periods was higher in the treated (managed) plot than in the control (unmanaged) plot. Bacterivores and fungivores were the most abundant trophic groups in both plots and both depths. The relative abundance of each group averaged 60.8 to 67.3% and 11.5 to 19.6% of the nematode community, respectively. Plant parasites and omnivores-predators at the 0 to 10-cm depth were much less abundant than any other two groups in our experimental plots. During the growing season, except the harvest period, populations of plant parasites and omnivores-predators at the 10 to 20-cm depth were lower in the treated plot than in the control plot. Maturity index (MI), plant-parasite index (PPI), and ratio of fungivores and bacterivores to plant parasites (WI) were found to be more sensitive indicators than other ecological indices for assessing the response of nematode communities to agricultural management in an Israeli agroecosystem.     

西藏拉鲁湿地夏季土壤线虫群落特征

2010年6–7月,对拉鲁湿地5个样点的土壤动物进行了调查,共捕获土壤线虫10183条,隶属于2纲6目8科14属,其中真矛线属(Eudorylaimus)、似绕线属(Anaplectus)、地单宫属(Geomonhystera)为优势类群,占土壤线虫总个体数的62.23%。营养类群有食细菌线虫和捕食-杂食线虫。不同样点中,土壤线虫的类群数和个体数垂直分布差异显著(P<0.05),大小顺序为0–5 cm>5–15 cm。5个样点的土壤线虫Shannon-Weiner多样性指数(H′)依次为3#>5#>4#>1#>2#。5个样点间土壤线虫群落的相似程度为极不相似到中等不相似(S值在0.154–0.667)。     

Landscape Distribution of Microbial Activity in the McMurdo Dry Valleys: Linked Biotic Processes, Hydrology, and Geochemistry in a Cold Desert Ecosystem

In desert ecosystems, microbial activity and associated nutrient cycles are driven primarily by water availability and secondarily by nutrient availability. This is especially apparent in the extremely low productivity cold deserts of the McMurdo Dry Valleys, Antarctica. In this region, sediments near streams and lakes provide the seasonally wet conditions necessary for microbial activity and nutrient cycling and thus transfer energy to higher organisms. However, aside from a few studies of soil respiration, rates of microbial activity throughout the region remain unexplored. We measured extracellular enzyme activity potentials (alkaline phosphatase, leucine-aminopeptidase, beta-glucosidase, phenol oxidase, and peroxidase) in soils adjacent to lakes and streams, expecting activity to be primarily related to soil water content, as well as time of season and organic matter supply. Phosphatase and beta-glucosidase activities were higher in shoreline than upland soils; however, potential rates were not correlated with soil water content. Instead, soil organic matter, salinity, and pH were the best predictors of microbial activity. Microbial nutrient limitation metrics estimated from extracellular enzyme activity were correlated with pH and salinity and exhibited similar patterns to previously published trends in soil P and N content. Compared to other terrestrial ecosystems, organic matter specific rates for leucine-aminopeptidase and oxidative enzyme activities were high, typical of alkaline desert soils. Phosphatase activity was close to the global mean whereas beta-glucosidase activity was extremely low, which may reflect the lack of vascular plant derived organic matter in the Dry Valleys. In this cold desert ecosystem, water availability promotes microbial activity, and microbial nutrient cycling potentials are related to soil geochemistry. Author contributions:   LHZ performed research, analyzed data, and wrote the paper; RLS contributed new methods and wrote the paper; JEB conceived/designed study, performed research and analyzed data; MNG conceived/designed study and performed research; CTV conceived/designed study and performed research.     

荒漠草原3种典型群落类型下土壤粒径分布与微生物量碳氮的关系

为了探讨不同群落类型的土壤粒径分布（PSD）与土壤微生物生物量（MB）的关系,以宁夏盐池荒漠草原3种典型群落类型（冰草、沙蒿、短花针茅）为研究对象,测定了不同群落2种生境（冠下、丛间）0~5、5~10和10~15 cm表土层土壤PSD和微生物量碳（MBC）、微生物量氮（MBN）含量变化,并分析了土壤颗粒组成中砂粒、粉粒和黏粒体积分数变化与土壤有机碳（SOC）、全氮（TN）和MBC、MBN间的关系。结果表明：（1）不同群落类型土壤粒径都呈“倒V”型分布趋势,但土壤退化最严重的沙蒿群落中100~500 μm粒径颗粒含量相对较多,与其他两种群落形成显著差异。（2）不同群落类型SOC、MBC、MBN含量均随土层深度的增加而降低,同土层SOC、MBN含量均表现为冠下明显大于丛间,表现出“肥岛效应”,且0~5 cm土层差异显著（P＜0.05）。（3）对土壤粒径组成与土壤SOC、MBC、MBN间相关性研究表明,在土壤SOC、MBC和MBN含量较高的冰草、短花针茅群落类型中,0.01~2、2~50和50~100 μm土壤粒径的颗粒含量也高,SOC、MBC和MBN含量与＜100 μm的粉粒含量呈正相关关系;在沙蒿群落类型中粒径为100~250和250~500 μm的土粒含量增高,导致其SOC、MBC和MBN含量较低,表明不同群落类型对土壤理化结构产生影响的同时,对微生物生物量也有显著的影响。     

Potential for Nematode Control by Mycofloras Endemic in the Tropics

Results of mycological surveys of root-knot and cyst nematodes from tropical regions indicate that most fungal species associated with females or cysts of species of Globodera, Heterodera, and Meloidogyne are those found with nematodes from temperate areas. Some fungal species, however, were found in higher frequency in tropical regions than in temperate countries; e.g., Cylindrocarpon destructans and Ulocladium atrum were the most common species associated with G. pallida and G. rostochiensis cysts in Peru. These fungi are not so frequent in nematodes from temperate areas. Fungi associated with diseased nematodes in the tropics vary greatly in nutritional requirements and include thermophilic species as well as cold-tolerant fungi. Multi-cropping systems possible in most tropical regions may be designed to increase the frequency of occurrence of microbial species antagonistic to phytonematodes.     

民勤沙区几种荒漠植物群落的现实生态位研究

民勤沙区属于典型的干旱荒漠气候,植被以灌木荒漠为主,盖度一般只有5%~15%.植物群落内生态位宽度悬殊,植物种之间的生态位相似比例很小.生态位宽度和生态位相似比例小是受制于干旱荒漠生境条件的结果,是干旱荒漠生态环境中稀疏荒漠植物的主要特征之一.植物的现实生态位宽度与年际降水量关系密切,在地貌、土壤颗粒、土壤水分、土壤养分等影响因子中,土壤水分是主导因子.当地地下水位深20 m左右,植物无法利用,植物种之间对资源的竞争主要表现为对水分资源的竞争,降水资源是植物种之间竞争的关键资源.同一种植物在不同群落中的生态位宽度变幅较大,不同群落内部植物种竞争关系的差异是由物种的生态位和由相对稳定的地貌、土壤条件以及随年际变动的降水条件共同作用的结果.群落中的优势种植物的生态位最宽,当优势种生态位减小时,一年生短命植物往往会进入群落中.荒漠植物的冠层单一,以冠幅占样方面积比作为植物的特征值计算其生态位特征值比较准确可靠.     

氮沉降对土壤线虫群落影响的研究进展

综述了主要陆地生态系统(草原、农田和森林)土壤线虫群落对氮沉降增加的响应格局和机制。总体上,氮沉降增加对线虫数量一般无显著影响,但增加了土壤中富集机会主义者(即低营养级的r-策略者)数量,降低了线虫群落成熟度指数(MI),表明氮沉降增加可能会使土壤食物网简化。氮沉降增加主要通过改变土壤微环境(如增加含氮离子浓度、降低土壤pH)直接影响土壤线虫群落,或者改变植物地上地下资源的输入和线虫与其他土壤动物的关系,间接影响线虫群落。最后,根据目前研究现状,指出了当前研究存在的局限性,包括研究时间和空间尺度上以及研究技术手段上的局限。建议综合多个全球环境变化因子,并结合室内试验及分子手段的方法对土壤线虫群落进行研究。     

Vegetation-environment relationships in a Negev Desert erosion cirque

The relationship of desert vegetation to topographic and edaphic factors in Makhtesh Ramon, an erosional cirque in the Negev Desert of Israel, was analyzed using redundancy analysis ordination. Altitude and geological substrate had the most significant correlations with vegetation patterns. In particular, significant and monotonic increases in both total species richness and the percentage of annual species were found with increasing altitude. A comparison was made of the effectiveness of Whittaker's diversity sampling plots (Shmida 1984) versus the log-series survey method of McAuliffe (1990). We concluded that at small sample sizes, the Mc Auliffe plots were more likely to yield interpretable ordination results, even though they included only the perennial flora.     

民勤荒漠区主要植物群落的稳定性及生态效应

以民勤荒漠区5种主要植物群落10个样方12年的定位观测资料为基础,运用植被盖度和投影盖度2个指标对荒漠区植物群落的稳定性及其生态效应进行分析,以期阐释随着全球变暖荒漠植物群落的稳定性和生态效应的变化规律。观测结果表明,沙蒿(Artemisia arenaria)的植被盖度和麻黄(Ephedra przewalskii)的植被盖度以及沙蒿的投影盖度和梭梭(Haloxylon ammodendron)人工林的投影盖度显著下降,白刺(Nitraria tangutorum)的植被盖度亦下降。分析结果表明:(1)植被盖度相对较低的群落的稳定性较强,而植被盖度相对较高的群落的稳定性相对较差。(2)当年降水量是决定植被盖度及其投影盖度的关键因子。(3)单位面积的白刺的生态功能最大,其次是麻黄,再次是白刺+梭梭,单位面积的沙蒿的生态功能最低。荒漠区植被稀疏,植株枝条分布极不规则,以冠幅计算所得的植被盖度不能反映植被实际覆盖地表的状况。     

Nematode communities of Lake Tana and other inland water bodies of Ethiopia

Free-living nematodes from littoral benthic sediments of four lakes, two rivers and a hot spring in Ethiopia are studied. Populations of nematodes encountered are identified to the species level. The general nematode (generic and species) composition of the lakes, rivers and hot spring are appraised by giving special emphasis to the nematodes from L. Tana, i.e. three sites where different environmental factors come into play were considered. Gelda is a site close to a river inlet (River Gelda), Gedero is a site exposed to strong wind action while Zegie is a secluded site protected by a land mass from any strong wind action. Nematode communities were identified using classification and ordination techniques. The identified communities are then characterised with respect to species composition, biomass, density, diversity, size structure, feeding type and maturity index. Also, the vertical distribution of nematodes is examined at the community and species level. An attempt has been made to associate these variations with the considered environmental factors viz. site, water depth, sediment depth, median grain size and percentage of mud in the sediment.     

Influence of Poultry Litter Applications on Nematode Communities in Cotton Agroecosystems

The effects of the application of poultry litter at 0.0, 6.7, 13.4, and 20.1 tons/ha on population changes during the growing season on nematode communities were evaluated in two cotton production fields in North Carolina. Numbers of bactivorous nematodes increased at midseason in response to the rate at which litter was applied but decreased with increasing litter application rates at cotton harvest. Numbers of fungivores at cotton harvest were related positively to the rate of litter applied, and this affected a positive increase in the fungivore-to-bacterivore ratio at this sampling date. The rate at which poultry litter was applied resulted in an increase in the bacterivore to plant-parasite ratio, and this corresponded with increased cotton lint yield. Trophic diversity was increased by litter application rate at cotton harvest at one location but not at another. The plant-parasite maturity index was greater consistently at one site than at a second site where the Hoplolaimus columbus population density was above the damage threshold for cotton. The population density of H. columbus was suppressed with increasing rates of poultry litter application, but other plant-parasitic nematodes were affected marginally.