Soil microorganisms control plant ectoparasitic nematodes in natural coastal foredunes

Belowground herbivores can exert important controls on the composition of natural plant communities. Until now, relatively few studies have investigated which factors may control the abundance of belowground herbivores. In Dutch coastal foredunes, the root-feeding nematode Tylenchorhynchus ventralis is capable of reducing the performance of the dominant grass Ammophila arenaria (Marram grass). However, field surveys show that populations of this nematode usually are controlled to nondamaging densities, but the control mechanism is unknown. In the present study, we first established that T. ventralis populations are top-down controlled by soil biota. Then, selective removal of soil fauna suggested that soil microorganisms play an important role in controlling T. ventralis. This result was confirmed by an experiment where selective inoculation of microarthropods, nematodes and microbes together with T. ventralis into sterilized dune soil resulted in nematode control when microbes were present. Adding nematodes had some effect, whereas microarthropods did not have a significant effect on T. ventralis. Our results have important implications for the appreciation of herbivore controls in natural soils. Soil food web models assume that herbivorous nematodes are controlled by predaceous invertebrates, whereas many biological control studies focus on managing nematode abundance by soil microorganisms. We propose that soil microorganisms play a more important role than do carnivorous soil invertebrates in the top-down control of herbivorous ectoparasitic nematodes in natural ecosystems. This is opposite to many studies on factors controlling root-feeding insects, which are supposed to be controlled by carnivorous invertebrates, parasitoids, or entomopathogenic nematodes. Our conclusion is that the ectoparasitic nematode T. ventralis is potentially able to limit productivity of the dune grass A. arenaria but that soil organisms, mostly microorganisms, usually prevent the development of growth-reducing population densities

Single introductions of soil biota and plants generate long-term legacies in soil and plant community assembly

Recent demonstrations of the role of plant-soil biota interactions have challenged the conventional view that vegetation changes are mainly driven by changing abiotic conditions. However, while this concept has been validated under natural conditions, our understanding of the long-term consequences of plant- soil interactions for above-belowground community assembly is restricted to mathematical and conceptual model projections. Here, we demonstrate experimentally that one-time additions of soil biota and plant seeds alter soil-borne nematode and plant community composition in semi-natural grassland for 20 years. Over time, aboveground and belowground community composition became increasingly correlated, suggesting an increasing connectedness of soil biota and plants. We conclude that the initial composition of not only plant communities, but also soil communities has a long-lasting impact on the trajectory of community assembly

First record of Helicotylenchus varicaudatus Yuen, 1964 (Nematoda: Hoplolaimidae) parasitizing Ammophila arenaria (L.) Link in Portuguese coastal sand dunes

A spiral nematode, Helicotylenchus varicaudatus Yuen, 1964, parasitizing Ammophila arenaria (L.) Link, the dominant grass in the Portuguese coastal sand dunes, is reported from Portugal for the first time and raises to seven the number of Helicotylenchus species detected in Portugal. A redescription of the species, with illustrations, and light and scanning electron microscope images of both female and male specimens, is presented. The rDNA containing the internal transcribed spacer regions (ITS) of H. varicaudatus was analysed with ITS-RFLP using the restriction endonuclease Hinf I. Molecular data from the ribosomal small subunit (SSU) (18S) confirmed the identification

Soil networks become more connected and take up more carbon as nature restoration progresses

Soil organisms have an important role in aboveground community dynamics and ecosystem functioning in terrestrial ecosystems. However, most studies have considered soil biota as a black box or focussed on specific groups, whereas little is known about entire soil networks. Here we show that during the course of nature restoration on abandoned arable land a compositional shift in soil biota, preceded by tightening of the belowground networks, corresponds with enhanced efficiency of carbon uptake. In mid- and long-term abandoned field soil, carbon uptake by fungi increases without an increase in fungal biomass or shift in bacterial-to-fungal ratio. The implication of our findings is that during nature restoration the efficiency of nutrient cycling and carbon uptake can increase by a shift in fungal composition and/or fungal activity. Therefore, we propose that relationships between soil food web structure and carbon cycling in soils need to be reconsidered

Soil microorganisms in coastal foredunes control the ectoparasitic root-feeding nematode Tylenchorchynchus ventralis by local interactions

1. In natural grassland ecosystems, root-feeding nematodes and insects are the dominant below-ground herbivores. In coastal foredunes, the ectoparasitic nematode Tylenchorhynchus ventralis would be a major root herbivore if not strongly controlled by soil microorganisms. Here, we examined if the suppressive effects of the microbial enemies of T. ventralis act by local interactions such as predation, parasitism or antagonism, or local induction of plant defence, or by non-local interactions, such as systemic effects when microorganisms in one section of the plant roots can affect nematode control in another section of the root system. We show that abundance of T. ventralis in the root zone of the grass Ammophila arenaria is suppressed by local interactions. 2. We compared local vs. non-local control of nematodes by a natural community of soil microorganisms in a split-root experiment, where nematodes and microbes were inoculated to the same, or to opposite root compartments. 3. The split-root experiment revealed that microorganisms affected T. ventralis numbers only when present in the same root compartment. Therefore, the effects of microorganisms on T. ventralis are due to local interactions and not due to induction of a systemic defence mechanism in the plant host. 4. When inoculated together with microorganisms, the nematodes were heavily infected with unknown bacteria and with fungi that resembled the genus Catenaria, suggesting that microorganisms control nematodes through parasitism. However, local defence induction cannot be completely excluded. 5. Besides microbial enemies of nematodes, the root zone of A. arenaria also contains plant pathogens. Root biomass was reduced by nematode infection, but also by the combination of nematodes and microorganisms, most likely because the soil pathogens overwhelmed the effects of nematode control on plant production. 6. We conclude that there may be a trade-off between beneficial effects of soil microorganisms on the plant host due to nematode control vs. pathogenic effects of soil microorganisms on the plant host. We propose that such trade-offs require more attention when studying below-ground multitrophic interaction

Ammonium-induced inhibition of ammonium-starved Nitrosomonas europaea cells in soil and sand slurries

Ammonia-oxidising bacteria are poor competitors for limiting amounts of ammonium. Hence, starvation for ammonium seems to be the regular condition for these bacteria in natural environments. Long-term survival in the absence of ammonium will be dependent on the ability to maintain large population sizes at the expense of endogenous energy sources and on the preservation of a relatively large capacity for ammonium oxidation. The effect of freshly added ammonium on the performance of ammonia-oxidising bacteria was studied in ammonium-enriched slurries consisting of samples taken from non-watersaturated soil and sand columns inoculated with Nitrosomonas europaea and Nitrobacter winogradskyi and continuously percolated with mineral medium containing ammonium. Immediately after introduction of the nitrifying bacteria to the columns, ammonium oxidation started and nitrate leached from the columns. After 6 weeks of incubation of the columns, 94% of the ammonium supplied was recovered as nitrate in the effluent and net cell growth had ceased. In slurries with freshly added ammonium, ammonium oxidation decreased after an initial period of relatively high oxidation rates, which lasted 6 at the most. This indicated that the cells had been starved for ammonium in the columns. After 3 days of slurry incubation the ammoniumoxidising activity restarted, but not in the presence of chloramphenicol, indicating de novo synthesis of enzyme systems. Restart of activity after 3 days could not be attributed to the release of free-living cells from the sand particles or to the presence of organotrophic bacteria in the slurries