Ron G.M. de Goede

Nematode community structure in relation to soil and vegetation characteristics

Abstract To test the hypothesis that nematodes can contribute to an ecological soil classification, the nematode fauna of several Dutch terrestrial habitats was studied. A total of 209 samples from 44 nature reserves or slightly managed sites ( n = 94) differing in vegetation (forest, shrubs, heathland, grassland) and soil type (clay, loam, sand) were studied. A selection of sites was studied over four seasons, and at one site variation in nematode fauna composition as a result of the selected sampling technique was studied. Nematodes extracted from bulk soil samples taken from the 0–10 cm depth mineral soil, and were identified to genus. Multivariate analysis techniques were used to classify the nematode samples into seven sample groups (SG) as described by soil characteristics in combination with the vegetation as follows: (1) SG A grasslands, dwarf-shrub vegetation and forest gaps on sandy soils; (2) SG B grasslands and forests on clayey soils; (3) SG C-D deciduous forests on sandy-loam soils; (4) SG E-F deciduous forests on sandy soils; (5) SG G coniferous forests on sandy soils. The nematode fauna of SG D-G were very similar, and were dominated by ten taxa: Acrobeloides, Aphelenchoides, Cephalenchus, Filenchus A, Filenchus B, Plectus A, Prismatolaimus, Rhabditidae, Tylolaimophorus and Wilsonema . Variation due to seasonal fluctuations and sampling technique, was small compared with differences in nematode fauna structure between different sites. The actual vegetation of some sites was not in agreement with the natural vegetation expected on ‘site characteristics’. Analyses of the nematode fauna supported the observed inconsistencies between actual and natural vegetation. It was shown that for a range of terrestrial habitats nematode communities could be defined, and that these communities could be related to soil characteristics and vegetation.

Seasonal dynamics and vertical distribution of plant-feeding nematode communities in grasslands

Summary The vertical distribution and seasonal dynamics of plant- and fungal-feeding nematode taxa in permanent grasslands were investigated. Dolichodoridae, Paratylenchus, Pratylenchus, Tylenchidae and Aphelenchoides dominated the upper 10 cm soil and their numbers strongly decreased with depth. The vertical distribution of these nematodes was correlated with the distribution of roots in the soil profile. Longidorus elongatus, Helicotylenchus pseudorobustus and Aphelenchus avenae were, however, also prevalent in the deeper soil layers. Trichodoridae and Hemicycliophora thornei seemed to prefer a depth of 30–40 cm. Most of the plant-feeding nematode taxa had an annual cycle in their abundance and population structures, which could largely be related to seasonal changes in the temperature and moisture contents of the soil. Although adult stages of sedentary endoparasitic genera were not counted, the peak number of juvenile Heterodera and Subanguina nematodes in spring indicated a distinct annual population cycle for these genera. The lowest densities of the semi-endoparasite Helicotylenchus pseudorobustus and the ectoparasites Geocenamus nanus, Tylenchorhynchus dubius, Paratylenchus nanus, Paratylenchus veruculatus and, to a lesser extent, Tylenchorhynchus maximus were found in winter/early spring, but their densities increased in summer/autumn. Generally, the population growth of these species in summer was preceded by a large proportion of adults followed by an increased number of juveniles. Taxa with short life cycles that could produce more generations per year, such as Tylenchidae and Aphelenchidae, and taxa with generation times of more than a year, such as Longidorus elongatus , generally did not show a distinct annual cycle.

On the relationships between nematodes, mycorrhizal fungi and plants: functional composition of species and plant performance

We analysed data from descriptive and experimental studies on the possible relationships between plants, nematodes and mycorrhizal fungi in (successional) plant communities in The Netherlands. A key role for pathogenic nematodes in cyclic succession in grazed grassland confirmed similar results in foredune succession. Plants with clonal growth forms appear to be particularly susceptible to pathogenic fungi. A large role for ectomycorrhiza at the species level (functional composition) was observed in determining the effects of acid rain and increased atmospheric CO2 on nutrient transfer to trees. Field studies showed a correlation between species richness of ectomycorrhizal fungi and hyphal-feeding nematodes in one case, but not in another. Population growth of hyphal-feeding nematodes on ectomycorrhizal hyphae was found to be strongly dependent on plant-mediated palatability of the fungus. We conclude that the limited available evidence suggests the importance of functional composition of species in determining interactions between plants, nematodes and mycorrhizal fungi, but the degree to which functional redundancy exists within the nematodes and within the mycorrhizal fungi warrants further investigation.

Soil Zoology : an indispensable component of integrated ecosystem studies

Abstract Society is faced with environmental issues that exceed the level of local governments, but which need to be dealt with at international or even global scales. These issues include nature conservation and management, development of a more sustainable agriculture, effects of environmental pollution, biodiversity, and effects of global change. The development of political and technical protocols necessary to control the future state of such environmental issues requires a detailed knowledge of the structure and functioning of the world’s ecosystems. Studies that integrate the interpretation of results from (detailed) community and ecosystem process and pattern studies and that, moreover, include social and economic aspects might be defined as ‘integrated ecosystem studies’. Four examples of integrated ecosystem studies are presented, and it is concluded that a full understanding of the contribution of soil zoology to such studies requires detailed analyses of interactions of fauna with other system components. If not, analyses can result in superficial conclusions that possibly underestimate the vulnerability of ecosystems to disturbances or their sensitivity to management.

Choice of Resolution by Functional Trait or Taxonomy Affects Allometric Scaling in Soil Food Webs

Belowground organisms often display a shift in their mass-abundance scaling relationships due to environmental factors such as soil chemistry and atmospheric deposition. Here we present new empirical data that show strong differences in allometric scaling according to whether the resolution at the local scale is based on a taxonomic or a functional classification, while only slight differences arise according to soil environmental conditions. For the first time, isometry (an inverse 1∶1 proportion) is recognized in mass-abundance relationships, providing a functional signal for constant biomass distribution in soil biota regardless of discrete trophic levels. Our findings are in contrast to those from aquatic ecosystems, in that higher trophic levels in soil biota are not a direct function of increasing body mass.

Effects of GM potato Modena on soil microbial activity and litter decomposition fall within the range of effects found for two conventional cultivars

Plant roots have a profound effect on soil microbial activity, particularly in the rhizosphere. Hence, it is important to understand the potential effects of genetically modified (GM) crops on soil microbial activity and related processes such as litter decomposition. In this study, we compared the effects of GM potato Modena on soil microbial activity and carbon (C) and nitrogen (N) mineralization to effects induced by Modena’s parental isoline (Karnico) and a conventional potato cultivar (Aventra). A field experiment was conducted at two sites to assess microbial catabolic diversity (using MicroRespTM) in the rhizosphere and in bulk soil, during flowering and senescence of the potato plants. In a laboratory experiment with soil and potato litter from the field experiment, we investigated whether the cultivars had modified the activity of soil microbial communities to such an extent that this affected C and N mineralization. Results of the field experiment showed no GM-induced effects on microbial catabolic diversity, while effects of field site location and sampling date were significant. Multivariate analysis including plant traits and soil characteristics revealed that microbial catabolic activities in rhizosphere soil were strongly correlated with soil organic matter and tuber sucrose content, whereas in bulk soil, they were primarily correlated with soil moisture. In the laboratory experiment, we found that Modena induced a “home-field advantage” in N mineralization, yet this effect was inconsistent across locations and was also observed for other cultivars. Based on our data and results from previous studies, we conclude that the effects of GM cultivar Modena on soil microbial activity and litter decomposition fall within the normal range of effects found for conventional potato cultivars.

Nematode-based risk assessment of mixture toxicity in a moderately polluted river floodplain in The Netherlands

Heavy metal polluted soils usually contain mixtures of different metals, whereas legislation is derived from concentrations of individual metals. The mixture toxicity of the Dutch floodplain Afferdensche and Deestsche Waarden was estimated to be high (msPAF ranged from 67-94%). Analyses of nematode community based bioindicators (Maturity Index, taxonomic diversity, trophic groups, multivariate analysis, DoFT-sentinels) were used to determine the ecological effects of the mixture toxicity in the floodplain soil. None of the indices indicated direct effects of heavy metals on the nematode community. This can be explained by the high adsorption of heavy metals on organic matter and clay particles resulting in a low bioavailability, and questions the estimation of the toxicity based on total concentrations of heavy metals in such environments. The nematode fauna showed great seasonal variation, which most probably was related to the temporal inundation of the floodplain.