Nematode trophic groups enhance nitrogen transformations in soil ecosystems

2011年5-11月,对西藏北部高寒草甸3种典型植物群落下0-30cm范围内不同深度土层的土壤线虫群落进行调查,采用浅盆法分离线虫,土壤性质指标,如pH、含水量、电导率分别采用电位法、烘干法、电导率仪法进行测定,应用营养类群组成、c-p类群结构及营养结构特征指数,以及营养类群、c-p类群与土壤性质之间的关系等特征值分析高寒环境下土壤线虫群落的功能结构特征,以了解高寒环境下植被对土壤线虫群落功能结构的影响。调查共分离得到33038条土壤线虫,隶属于2纲6目51科93属;线虫个体密度平均为847条/100g干土;表聚性明显。研究结果表明,高寒草甸不同植物群落的土壤线虫群落营养类群组成及分布特征均存在一定差异,植食性线虫和食细菌性线虫是调查区域的主要营养类群,不同植物群落间植食性线虫和杂食/捕食性线虫的相对多度差异明显。c-p类群组成结构特征结果表明:3种植物群落的土壤线虫cp2类群均为优势类群,生活策略以r-对策为主;高山嵩草植物群落土壤中的线虫食物资源在3种植物群落中最丰富;藏北嵩草群落土壤线虫数量低的可能原因是线虫食物资源的减少限制了cp1、cp2类群的增殖。<em>PPI</em>值表明:委陵菜群落受扰动的影响程度大于其余两种植被类型,而<em>MI、PPI/MI</em>值及cp5类群数量的结果则表明:委陵菜群落的稳定性较高,受到的干扰在3种植物群落中最少。<em>F/B</em>及<em>NCR</em>值均说明了3种植物群落的土壤有机质分解均主要依靠细菌分解途径。相关性分析结果表明:杂食/捕食性线虫在枯草期明显受到土壤含水量的影响;食真菌性线虫与土壤pH之间的关系密切,在盛长期则明显受到土壤电导率的影响;食细菌性线虫仅在返青期与pH有相关性。不同植物群落下土壤线虫群落功能结构特征的分异显示出线虫指示环境因子影响土壤生态系统的潜力。;Northern Tibet lies within the remote regions of the Tibetan Plateau, features a dry and cold climate, and forms a broad strip of land sensitive to global warming. Meadows are the largest and most important ecosystems in northern Tibet. Alpine meadows are extensively distributed in the northern Tibetan Plateau and are typical features of the region. The alpine meadow soil type in the study area is seasonally frozen. We chose three representative plant communities as samples areas, which we identified as <em>Potentilla</em> spp., <em>Kobresia pygmaea</em>, and <em>Kobresia littledalei</em> based on the dominant species of each community to study soil nematode communities. The plots are situated at Dejixiangdaga village at N31°31'38″-31°32'02″, E92°04'03″-92°04'16″ in Naqu County in the Naqu region of the Tibet Autonomous Region, China. Soil nematode communities in the upper 0-30 cm soil were investigated in three typical plant communities in alpine meadows of northern Tibet from May to November 2011. The soil nematodes were collected using the shallow basin method. The following soil chemical properties were tested: pH, potentiometry, soil water content (oven drying method), and electricity conductivity. The trophic composition, c-p group structure and functional structure index, as well as the characteristic values between trophic groups, c-p groups and soil properties, were analyzed to understand the effect of cold alpine conditions on soil nematode functional diversity. A total of 33038 nematodes, which were classified to nematode phylum, 2 classes, 6 orders, 51 families and 93 genera, were collected during the surveys. Average individual density was 847 nematodes per 100g dry soil. The results were mostly within the scope of previous reports, and nematodes obviously appeared to aggregate in the upper soil layers. The research indicated soil nematode tropic composition varied from one plant community to another in alpine meadows. Herbivorous and bacterivorous nematodes were the major nematode communities in the area. The relative abundance of herbivorous to omnivorous and/or predatory nematodes obviously varied in different plant communities. An analysis of the constitution of the c-p groups showed all cp2 groups were the predominant groups in all three plant communities, and all c-p groups were mainly <em>r</em>-strategists. <em>Kobresia pygmaea</em> was the most common food source for nematodes. <em>Kobresia littledalei</em> had the lowest nematode density, possibly because limited food availability limits the reproduction of cp1 and cp2 nematodes. The result of analysis of the plant parasite index indicated the <em>Potentilla</em> community was easier for nematodes to populate when compared to the other two vegetation types, whereas the data from the maturity index (<em>MI</em>), <em>PPI/MI</em> and cp5 demonstrated nematode stability in <em>Potentilla</em> was highest among three vegetation types. The fungal to bacterial feeder ratio and Nematode Channel Ratio showed bacterial decomposition was the major decomposition pathway in the soil organic substance decomposition system. Correlation of nutritional groups and environmental factors showed omnivorous/predatory nematodes were affected significantly by water content during dry periods, while a close relationship was found between soil pH and fungivorous nematodes, which was obviously affected by electrical conductivity (<em>EC</em>) during periods of rapid growth. The abundance of bacterivorous nematodes was only correlated to pH during the dry season. The presence of different plant communities leads to effects on water content, soil formation and the distribution of nutrients. The analysis related to water content, pH and <em>EC</em> reveals plant communities have important influences on the functional structure of nematode communities. Differences in nematode community structure in different meadows preliminarily indicate the potential for using nematodes as environmental indicator species.

Microbial community size is a potential predictor of nematode functional group in limed grasslands

Nematode community changes associated with decomposition of Crotalaria juncea amendment in litterbags

Summary Solidago gigantea is one of the most problematic species of invasive plants in Europe, where its rapid spread threatens the stability of ecosystems. The effect of invasion by S. gigantea on soil ecosystems and food webs can be evaluated using nematode communities as suitable and useful soil bioindicators. We compared the effect of S. gigantea on nematode communities in plots invaded with S. gigantea and non-invaded grasslands across three sites with varying soil and climatic conditions. Our findings indicated that invasion by S. gigantea did not strongly affect nematode taxonomic diversity but negatively affected functional diversity. This effect was evident in the increased abundance of bacterivore and fungivore nematodes and decrease in values of ecological and functional indices. The evaluation of the soil food web using nematode communities confirmed that soil in plots with S. gigantea were less mature and more disturbed with a degraded or depleted food web indicated by lower values of maturity, enrichment and structure indices, compared with non-invaded grasslands. On the other hand, taxonomic diversity, abundance, biomass and the functional indices differed between three study sites, but site had little or no effect on the values of the maturity indices. The interaction between site and invasion status confirmed the effect of invasion on nematode biomass, with the abundance of bacterivore and fungivore nematodes and the functional indices more prominent at sites characterised by cold and humid climates. These findings underscore the importance of considering both taxonomic and functional diversity and the characteristics of the plants and sites when evaluating the impact of plant invasion on soil biota.

Influences of organic and synthetic soil fertility amendments on nematode trophic groups and community dynamics under tomatoes

Cover crops alter the soil nematode food web in banana agroecosystems

Changes in diversity patterns and assembly processes of soil nematode communities during forest secondary succession on the Loess Plateau

A sequential extraction procedure reveals that water management affects soil nematode communities in paddy fields

This study aimed to investigate how great cormorants and grey herons affect the density and trophic diversity of soil nematodes under breeding colonies located in Stawy Raszyńskie Nature Reserve (central Poland). Soil samples from the colonies were compared to control samples from adjacent areas unaffected by birds. Samples were taken at each site (two colonies and two relevant control sites) to a depth of 20 cm, and the soil cores were split into topsoil (0–10 cm) and subsoil (10–20 cm). A modified Baermann method was used to extract nematodes from the soil. The soil under nests supported more abundant nematode communities, but with a lower trophic diversity compared to the control sites. The cormorants had a greater impact on nematodes than the herons. We found that the external nitrogen input, the higher organic matter content and abundance of ammonifying bacteria, as well as the lower soil pH under the colonies than in the control sites, affected the nematode trophic groups in different ways. Compared to the control sites, there were significantly more bacterivorous nematodes but fewer herbivorous nematodes under the colonies. No predatory nematodes were found under the bird colonies and, in the case of the cormorant colony, no omnivorous nematodes. No significant differences in the abundance of fungivorous nematodes between the impact and the control plots were noticed. The results indicate that allochthonous input under bird colonies promotes microbial activity and the most opportunistic trophic group of nematodes, which may at least temporarily enhance decomposition and mineralisation processes and consequently affect nutrient cycling in the wetland soil.

Based on the study of the structural and functional organization of nematode communities of primary ecosystems, it is possible to determine the degree of transformation of secondary ecosystems, because anthropogenic activity leads to changes in their formation. Nematode communities of indigenous ecosystems have preserved their evolutionary-formed structural and functional organization and the ratio of trophic groups. This ensures the integrity and stability of biogeocenoses. Therefore, such groups have a significant bioindication role. The research was conducted within the Skolivski Beskydy National Nature Park. A mixed beech forest was selected as the reference plot. For two years, we have been taking samples of litter and soil under the spruce canopy. Nematodes were separated from the soil using the Berman method on a Kempson device. De Man indices were used to determine species. Based on the division of nematodes into trophic groups according to G. Yeats, we calculated the part of each of them in the litter and soil. The number of nematodes in the litter increases from the upper horizon to the humus. In freshly fallen leaves the variety of nematodes (horizon L) is the least. In 2014 and 2015 in the upper horizon of the litter at different times of the year, the number of phytonematodes varied from 22 to 28 %, in the enzymatic horizon from 30 to 35 %, and in the humus horizon from 38 to 47 % of the total number of phytonematodes. The three litter horizons have the largest number of omnivorous nematodes, which is about two-thirds of the total number of the community with the largest quantity in summer. The part of predatory nematodes in spring and autumn is about 10 %, and in summer increases to 15 %. Bacteriovorus nematodes make about 18 % in spring, 25 % in summer, and 22 % in autumn, with a predominance in the F-horizon of litter. The part of fungivorous nematodes in all horizons of litter is approximately 5 %, with a predominance of the total number of this trophic group in the enzyme horizon. Omnivorous nematodes are represented mainly by species of the genus Eudorylaimus and Aporcelaimellus, carnivorous nematodes belong to the genera Prionchulus, Iotonchus, Tripyla, and bacteriophages are dominated by nematodes of the genera Plectus and Acrobeloides, and the group of fungivorous is mainly represented by species of the genus Aphelenchoides. Our results were later used to compare changes in nematode communities in derived ecosystems.

Soil performs numerous functions, which allow us to produce food and feed and provide us with clean freshwater. These functions rely on the high diversity of organisms residing in soils. Within the high complexity of the soil food web, nematodes, worm-shaped animals belonging to the phylum Nematoda, are an informative group for assessing the status of a soil-dwelling community due to their ubiquity, abundance and trophic diversity. Although nematodes also possess several other assets favourable for a biological indicator of soil ecosystems (e.g. easy extractability, differential sensitivities to disturbances, ecological interpretability), their microscopic identification demands a considerable amount of expertise and time because of their relatively conserved morphology. Hence, the use of a molecular method for the identification and quantification of nematode assemblages has the potential to lift practical limitations and allows for more intensive sampling schemes. The aim of the research described in this thesis was to assess the suitability of molecular taxon-specific assays, developed on the basis of a phylum-wide molecular framework of 2,400 full-length small subunit ribosomal DNA sequences, for the monitoring of nematode assemblages in field experiments. The method was applied to monitor the impact of different types of disturbances on the soil food web, i.e. agricultural practices, invasive plant species and the effects of genetically modified crop (potato). The second chapter of this thesis presents the background of the molecular method and the results of its first field application. It demonstrates the suitability of this method for use in extensive field experiments and the results of this study reveal distinct seasonal fluctuations between nematode genera classified to belong to the same feeding type group. A distinct response of nematode genera within trophic groups – taxonomically diverse groups of nematodes having the nature of their main food source as a common denominator – was also observed in the study described in Chapter 3. In this chapter, a study was conducted to investigate the impact of an invasive plant species called Giant goldenrod (Solidago gigantea, native to North America) on the plant community as well as on different trophic levels of the soil food web. In addition to monitoring the nematode community, pH and fungal biomass were measured in plots invaded or uninvaded by Giant goldenrod in two contrasting habitats. The results revealed that, in addition to outcompeting native plant species, this invader also reduced pH and increased fungal biomass in the soil of both habitats. Based on the results concerning the nematode community, the impact on the soil food web seemed to be selective since the local increase of fungal biomass appeared to benefit only one fungivorous nematode lineage of the three present in the field. This suggests that invasion by Giant goldenrod only stimulates one part of the fungal community. Contrary to the preceding chapters, the effects of different disturbances on the nematode community were studied in an arable setting in chapters 4 and 5. Chapter 4 describes a field experiment in which the impact of biofumigation, a pest control measure, on the nematode community was assessed. Biofumigation is considered as an alternative for the use of synthetic fumigants and entails the incorporation of mulched brassicaceous plant material, which, upon de-compartmentalisation, releases general biocides called isothiocyanates. In our experiment, these compounds as well as their precursors could not be related to the effects observed for the nematode community. Therefore, changes in nematode assemblages are more likely to be related to the intense mechanical disturbance and green manure – the addition of a large quantity of fresh plant material to the topsoil – rather than the release of isothiocyanates from the plant material Chapter 5 presents a field experiment in which the possible belowground side effects of a waxy starch GM potato, a genetically modified plant blocked in its amylose biosynthesis, were investigated. The nematode community was monitored during the growing season of this GM variety, its parental line and four other conventional potato cultivars in two experimental fields. Although we observed clear effects of location and time, no GM-related effects were observed on the nematode community. Our results, in line with previous studies concerning the microbial community, indicate there are no observable, non-transient effects related to this particular GM trait on the soil food web during the growing season. Overall, the results presented in this thesis demonstrate that, first of all, the developed molecular approach is suitable as a tool for the quantitative monitoring of nematode assemblages in field experiments, and, secondly, how a molecular monitoring method based on nematode taxon-specific DNA motifs can be exploited to get new insights into the ecology of terrestrial nematodes and – more in general – into the ecological functioning of this obscure, highly biodiverse and poorly understood habitat below our feet.

Impact of microplastic concentration on soil nematode communities on the Qinghai-Tibet Plateau: Evidence from a field-based microcosms experiment.

Effects of organic mulches on soil microfauna in the root zone of apple: implications for nutrient fluxes and functional diversity of the soil food web

Nematode communities in natural pristine forests composed of trees aged over 1000 years have not been thoroughly studied. Old-growth forests can provide diverse microhabitats for nematodes such as fallen trees with mossy conditions. This study aimed to characterize the community and trophic compositions of nematodes in relation to different microhabitats in old-growth Japanese cedar ( Cryptomeria japonica (L. f.) D. Don) forests in Yakushima Island, Japan. To this aim, nematodes retrieved from four different types of microhabitats were morphologically identified at the family/genus level and divided into five trophic groups. A nonmetric multidimensional scaling scatterplot showed that nematode communities clustered significantly in each microhabitat and their community compositions were significantly affected by C/N ratio. With respect to nematode trophic composition, bacterivorous nematodes dominated decaying barks of fallen trees, with a relative abundance of 45%, while fungivorous nematodes were dominant in living barks of cedar trunks (61%). Predatory–omnivorous nematodes predominated in mosses of fallen trees (51%). These results showed that nematode community and trophic composition were different among microhabitats characterized by nutrient conditions. The predatory–omnivorous nematodes were abundant in mosses with N-rich conditions. In contrast, fungivorous nematodes were abundant in cedar trunks with high C/N conditions. Thus, old-growth natural forests can provide diverse microhabitats where various nematode taxa coexist due to the presence of fallen trees and mosses.

Butyric acid, which is produced through fermentation of organic matter by anaerobic soil bacteria, possesses nematicidal properties. We investigated how the concentration of butyric acid in solution and gas phase affected the survival of 12 nematode species from four trophic groups. Our hypothesis was that survival of free-living and plant parasitic nematodes would differ, since free-living nematodes have shown some adaptation to survival in anaerobic soil environments. A 2-day incubation in sand amended with 0.88 mg butyric acid g −1 reduced plant parasitic and fungivorous nematodes by 84–100% as compared to untreated controls, whereas a concentration of 8.8 mg butyric acid g −1 was necessary to significantly reduce bacterivorous nematodes (70–98%). Sensitivity of entomogenous nematodes was variable, with Heterorhabditis adversely affected by 0.88 mg butyric acid g −1 sand, resulting in a 59% reduction, while Steinernema required a concentration of 8.8 mg butyric acid g −1 sand to see a significant decline (85%). Results were similar when nematodes were exposed to the gas phase of butyric acid for 7 days. The vapor from a 0.1 M solution reduced plant-parasitic and fungivorous nematodes by 89–96% while the vapor from a 1 M solution of butyric acid reduced entomogenous nematodes by 94–99%. Bacterivorous nematodes did not survive the 7-day incubation period in appreciable numbers in either controls or treated sand. A 2-day incubation of nematodes in sand acidified with HCl to achieve pH values of 3.4 and 3.0 (similar to sand amended with 0.88 and 8.8 mg butyric acid g −1 sand) had no effect on nematode survival in any of the trophic groups tested. A positive correlation was found between LC 50 values for butyric acid and nematode surface area-to-volume ratios for four out of five plant parasitic nematodes ( r=0.99; P=0.01) and a negative correlation was found for bacterivorous, entomogenous and fungivorous nematodes combined ( r=−0.77; P=0.07). The differentiation in chemical tolerances demonstrated here may hold a key to targeting plant parasitic nematodes without affecting free-living forms.