R.G.M. de Goede

A framework for soil food web diagnostics : extension of the nematode faunal analysis concept

Nematodes, the earth’s most abundant metazoa, are ubiquitous in the soil environment. They are sufficiently large to be identifiable by light microscopy and sufficiently small to inhabit water films surrounding soil particles. They aggregate around and in food sources. They include component taxa of the soil food web at several trophic levels. They can be categorized into functional guilds whose members respond similarly to food web enrichment and to environmental perturbation and recovery. Indices derived through nematode faunal analysis provide bioindicators for disturbance of the soil environment and condition of the soil food web. We enhance the resolution of faunal analyses by providing a weighting system for the indicator importance of the presence and abundance of each functional guild in relation to enrichment and structure of the food web. Graphical representations of food web structure, based on nematode faunal analyses, allow diagnostic interpretation of its condition. Simple ratios of the weighted abundance of representatives of specific functional guilds provide useful indicators of food web structure, enrichment, and decomposition channels.

Small-scale shifting mosaics of two dominant grassland species: the possible role of soil-borne pathogens

Abstract We analyzed the dynamics of dominant plant species in a grazed grassland over 17 years, and investigated whether local shifts in these dominant species, leading to vegetation mosaics, could be attributed to interactions between plants and soil-borne pathogens. We found that Festuca rubra and Carex arenaria locally alternated in abundance, with different sites close together behaving out of phase, resulting in a shifting mosaic. The net effect of killing all soil biota on the growth of these two species was investigated in a greenhouse experiment using gamma radiation, controlling for possible effects of sterilization on soil chemistry. Both plant species showed a strong net positive response to soil sterilization, indicating that pathogens (e.g., nematodes, pathogenic fungi) outweighed the effect of mutualists (e.g., mycorrhizae). This positive growth response towards soil sterilization appeared not be due to effects of sterilization on soil chemistry. Growth of Carex was strongly reduced by soil-borne pathogens (86% reduction relative to its growth on sterilized soil) on soil from a site where this species decreased during the last decade (and Festuca increased), while it was reduced much less (50%) on soil from a nearby site where it increased in abundance during the last decade. Similarly, Festuca was reduced more (67%) on soil from the site where it decreased (and Carex increased) than on soil from the site where it increased (55%, the site where Carex decreased). Plant-feeding nematodes showed high small-scale variation in densities, and we related this variation to the observed growth reductions in both plant species. Carex growth on unsterilized soil was significantly more reduced at higher densities of plant-feeding nematodes, while the growth reduction in Festuca was independent of plant-feeding nematode densities. At high plant-feeding nematode densities, growth of Carex was reduced more than Festuca, while at low nematode densities the opposite was found. Each plant species thus seems to be affected by different (groups of) soil-borne pathogens. The resulting interaction web of plants and soil-borne pathogens is discussed. We hypothesize that soil disturbances by digging ants and rabbits may explain the small-scale variation in nematode densities, by locally providing “fresh” sand. We conclude that soil-borne pathogens may contribute to plant diversity and spatial mosaics of plants in grasslands.

Changes in the composition of the plant-feeding nematode community in grasslands after cessation of fertiliser application

Abstract The community structure and abundance of plant-feeding nematodes were studied in four grasslands, which had not been fertilised for 6, 10, 23, and 28 years, respectively. We hypothesised that nutritional impoverishment of the grasslands would result in a decrease in the nematode density and biomass, and a shift in the nematode community structure towards species that are better adapted to nutrient-poor conditions. Furthermore, we expected that plant-specific nematode communities are present in the rhizosphere of different dominant plant species. Multivariate analyses showed gradual changes in the nematode fauna after the cessation of fertiliser application. Particularly, during the first 10 years of succession the density and species number of large endo- and ectoparasites strongly decreased resulting in a lower diversity of nematode genera. Root biomass, plant species, and season had a minor effect on the nematode numbers and community composition. However, within the rhizosphere of single plant species clear community changes were observed with time of non-fertilisation, indicating that qualitative changes within a plant species rather than qualitative differences between plant species affected the nematode community. Most likely, lower nutritional quality of the nematode food source, due to decreased nutrient concentrations and increased levels of defensive compounds in plant tissues under nutrient-poor conditions, was the major factor affecting the plant-feeding nematode community. Average nematode body size, which is constrained by the absolute amount of food necessary for growth and maintenance, decreased at lower nutrient availability. However, the results suggest that evolutionary adaptation of feeding strategies that reduces the energetic costs of feeding and movement, such as endoparasitism, a longer stylet, and the modification of feeding cells, enabled certain nematode species to support a relatively large body size under conditions of low nutrient availability.

Within-trophic group interactions of bacterivorous nematode species and their effects on the bacterial community and nitrogen mineralization

Knowledge of the interactions between organisms within trophic groups is important for an understanding of the role of biodiversity in ecosystem functioning. We hypothesised that interactions between bacterivorous nematodes of different life history strategies would affect nematode population development, bacterial community composition and activity, resulting in increased N mineralization. A microcosm experiment was conducted using three nematode species (Bursilla monhystera, Acrobeloides nanus and Plectus parvus). All the nematode species interacted with each other, but the nature and effects of these interactions depended on the specific species combination. The interaction between B. monhystera and A. nanus was asymmetrically competitive (0,−), whereas that between B. monhystera and P. parvus, and also A. nanus and P. parvus was contramensal (+, −). The interaction that affected microcosm properties the most was the interaction between B. monhystera and P. parvus. This interaction affected the bacterial community composition, increased the bacterial biomass and increased soil N mineralization. B. monhystera and P. parvus have the most different life history strategies, whereas A. nanus has a life history strategy intermediate to those of B. monhystera and P. parvus. We suggest that the difference in life history strategies between species of the same trophic group is of importance for their communal effect on soil ecosystem processes. Our results support the idiosyncrasy hypothesis on the role of biodiversity in ecosystem functioning.

On-farm impact of cattle slurry manure management on biological soil quality

The effects of dairy cattle slurry management on soil biota, soil respiration and nitrogen (N) mineralization were evaluated in a farm trial across 12 farms and a field experiment on 2 farms located in a dairy farming area in the north of the Netherlands. The slurry management consisted of slit injection or surface application of slurry; the use or no use of additives [Euromestmix® (MX) and Effective Microbes® (EM)] and the type and level of inorganic N fertilization. Slit injection negatively affected epigeic earthworms whereas its effect on anecic and endogeic earthworms was absent or even positive. Enchytraeids were not affected in a consistent way, whereas numbers of nematodes indicative of nutrient- enriched conditions increased. Inorganic N fertilizer had similar effects. Bacterial diversity was not different among the treatments. Nitrifier diversity, however, was high at one of the farms in the field experiment, and was negatively affected by inorganic N fertilizer. The use of MX was usually associated with higher numbers of earthworms. EM affected numbers of earthworms and numbers of bacterial and plant-feeding nematodes, but only in specific combinations of field history, slurry type and slurry application method. We found no effects of EM on the composition of the microbial community. Soil respiration was increased when slurry was surface-applied. The calculated N mineralization by earthworms was in the order of 70–200 kg N ha −¹ year −¹. It was highest under farm-characteristic surface application of slurry with MX and lowest under farm-characteristic slit injection of slurry without additives. Compared with the N mineralization by earthworms, that by enchytraeids and nematodes was quantitatively insignificant. Negative treatment effects on earthworms led to corresponding reductions in calculated N mineralization.

Do plant parasitic nematodes have differential effects on the productivity of a fast- and a slow-growing grass species?

We have examined the interaction between plant parasitic nematodes and plant species from different stages of grassland succession. In these grasslands, fertiliser application was stopped in order to restore the former nutrient-poor ecosystems. This management resulted in a reversed succession of high- to low-productivity. Nematodes isolated from a high-productive early-successional field and a low-productive late-successional field were inoculated to sterilised soil planted with seedlings of either Lolium perenne (a fast-growing early-successional species) or Festuca rubra (a slow-growing late-successional species). The experiment was performed at low and high supply rates of nutrients. We hypothesised that at a low nutrient supply rate the growth of L. perenne will be more reduced by nematode herbivory than the growth of F. rubra. Furthermore, we hypothesised that higher numbers of plant parasitic nematodes will develop under L. perenne. We found no support for our first hypothesis, because nematodes did not affect plant growth. Our results suggest that changes in the nutrient availability rather than plant parasitic nematodes affect plant succession in impoverished grasslands. On the other hand, plant species and nutrient supply rate significantly affected the density and composition of the plant parasitic nematode community. In line with our second hypothesis, plant parasitic nematodes reproduced better on the fast-growing L. perenne than on the slow-growing F. rubra. Our results, therefore, suggest that the succession of the plant parasitic nematode community is probably more affected by changes in the plant community than the other way round.

Effects of different treatments of cattle slurry manure on water-extractable phosphorus

Cattle slurry manure applied to land increases the risk of phosphorus (P) movement to surface waters, which may lead to eutrophication. The water-extractable fraction of P in slurry manure is correlated with P concentration in runoff from soils amended with slurry smanure, and thus is an effective indicator of environmental P loss. We evaluated the water-extractable P (WEP) and readily soluble P (RSP; i.e., P extractable in a single water extract) contents in slurry manure from nine farms. On some farms, the additive Euromestmix® (MX) is used to complex N-compounds in the slurry manure, but the effect of MX on P in the slurry manure is not known. We also focused on methodological factors affecting the measurement of P in slurry manure. Drying the slurry manure before analysis decreased WEP and RSP contents. Dilution of slurry manure by varying the dry matter-to-distilled water ratio increased the water-extractable fractions. Analysis of calcium and magnesium contents in water showed that these minerals are involved in the release of P in water. Total RSP content of slurry manures from the nine farms ranged from 1.83 to 4.06 mg P per g dry matter. A substantial portion of total P in the slurry manure occurred in the water-extractable (46–71%) and the readily soluble P fractions (24–51%). The addition of MX tended to decrease the readily soluble inorganic P content of slurry manures.

Establishment of carabid beetle and nematode populations in a nature restoration project after the abandonment of arable land

Abstract The colonization and establishment of carabid beetle and nematode populations was investigated during a 5-year period after the abandonment of formerly arable land. The management of the study area comprised the removal of the nutrient-rich topsoil and the introduction of differences in ground level. At the start of the project a major part of the soil fauna had been removed from the area. Both the carabid and the nematode fauna initially comprised predominantly opportunistic colonizers. The occurrence and numbers of the nematodes depended on soil type and soil moisture conditions.