Maria Viketoft

Long‐term effects of plant diversity and composition on soil nematode communities in model grasslands

An important component of plant-soil feedbacks is how plant species identity anddiversity influence soil organism communities. We examine the effects of grassland plant species growing alone and together up to a richness of 12 species on nematode diversity and feeding group composition, eight years after the establishment of experimental grassland plots at the BIODEPTH site in northern Sweden. This is a substantially longer time than most other experimental studies of plant effects on soil fauna. We address the hypotheses that (la) higher species or functional diversity of plants increases nematode diversity, as well as influences nematode community composition. Alternatively, (1b) individual plant species traits are most important for nematode diversity and community composition. (2) Plant effects on soil organisms will decrease with increasing number of trophic links between plants and soil fauna. Plant species identity was often more important than plant diversity for nematode community composition, supporting hypothesis 1b. There was a weak positive relation between plant and nematode richness;which could be attributed to the presence of the legume Trifolium pratense, but also to some other plant species, suggesting a selection or sampling effect. Several plant species in different functional groups affected nematode community composition. For example, we found that legumes increased bacterial-feeding nematodes, most notably r-selected Rhabditida, while fungal-feeding nematodes were enhanced by forbs. Other bacterial feeders and obligate root feeders were positively related to grasses. Plant effects were usually stronger on plant-, bacterial- and fungal-feeding nematodes than on omnivores/predators, which supports hypothesis 2. Our study suggests that plant identity has stronger effects than plant diversity on nematode community composition, but when comparing our results with similar previous studies the effects of particular plant species appear to vary. We also found that more productive plant species affected bacterial-feeding nematodes more than fungal feeders. Moreover, we observed stronger effects the fewer the number of trophic links there were between a nematode feeding group and plants. Although we found clear effects of plants on soil nematodes, these were probably not large enough to result in strong and persistent plant-soil-organism-plant feedback loops.

Soil and Freshwater and Marine Sediment Food Webs: Their Structure and Function

The food webs of terrestrial soils and of freshwater and marine sediments depend on adjacent aboveground or pelagic ecosystems for organic matter input that provides nutrients and energy. There are important similarities in the flow of organic matter through these food webs and how this flow feeds back to primary production. In both soils and sediments, trophic interactions occur in a cycle in which consumers stimulate nutrient cycling such that mineralized resources are made available to the primary producers. However, aquatic sediments and terrestrial soils differ greatly in the connectivity between the production and the consumption of organic matter. Terrestrial soils and shallow aquatic sediments can receive organic matter within hours of photosynthesis when roots leak carbon, whereas deep oceanic sediments receive organic matter possibly months after carbon assimilation by phytoplankton. This comparison has implications for the capacity of soils and sediments to affect the global carbon balance.

Effects of plant species and plant diversity on soil nematodes – a field experiment on grassland run for seven years

Plant species identity and diversity may greatly influence the composition of the nematode fauna. In this study the development of the nematode fauna was followed in a field experiment on arable soil with monocultures and mixtures of several plant species. Experimental plots were sown with one, four or 12 species of grass, legumes and forbs and were sampled four times in 7 years. Nematode diversity was little influenced by plant diversity. Due to a pronounced increase of Paratylenchus projectus populations, the Shannon diversity index decreased in several treatments towards the end of the experiment. Differences in nematode faunal composition among treatments increased with time. In spite of the rather long duration of the experiment, the faunal composition did not stabilise but changed continuously. The obligate plant feeders Tylenchorhynchus dubius , T. maximus and Pratylenchus spp. occurred in higher numbers in monocultures than in mixtures of several plant species. Among the microbivores, the abundance of some bacterial- and fungal-feeding nematode taxa seemed to be directly influenced by the plant species identity. In the plots with Trifolium spp. there was an increase of rapidly growing bacterial feeders belonging to Rhabditidae and Panagrolaimidae already after the first growing season. The abundances of some bacterial-feeding nematodes were correlated to total plant production. The numbers of the omnivorous Mesodorylaimus sp. appeared to be influenced by the degree of plant coverage. The abundance of several nematode taxa appeared to be little influenced by the composition of the vegetation. The plant feeder P. projectus and the bacterial feeder Prismatolaimus sp. reacted rather slowly and a distinct increase in numbers was only seen after 7 years, when P. projectus strongly dominated the fauna in several treatments. Among the bacterial feeders, some species with moderate growth rate belonging to Cephalobidae decreased with time in several treatments.

A test of the hierarchical model of litter decomposition

Our basic understanding of plant litter decomposition informs the assumptions underlying widely applied soil biogeochemical models, including those embedded in Earth system models. Confidence in projected carbon cycle–climate feedbacks therefore depends on accurate knowledge about the controls regulating the rate at which plant biomass is decomposed into products such as CO2. Here we test underlying assumptions of the dominant conceptual model of litter decomposition. The model posits that a primary control on the rate of decomposition at regional to global scales is climate (temperature and moisture), with the controlling effects of decomposers negligible at such broad spatial scales. Using a regional-scale litter decomposition experiment at six sites spanning from northern Sweden to southern France—and capturing both within and among site variation in putative controls—we find that contrary to predictions from the hierarchical model, decomposer (microbial) biomass strongly regulates decomposition at regional scales. Furthermore, the size of the microbial biomass dictates the absolute change in decomposition rates with changing climate variables. Our findings suggest the need for revision of the hierarchical model, with decomposers acting as both local- and broad-scale controls on litter decomposition rates, necessitating their explicit consideration in global biogeochemical models.Accurate understanding of plant litter decomposition is vital to inform Earth system modelling. Here the dominant hierarchical model for plant litter decomposition is found to be wanting, and revisions are suggested.

Plant induced spatial distribution of nematodes in a semi-natural grassland

This study describes the nematode community in a semi-natural grassland and investigates if certain individual plant species can cause a spatial structure in the nematode fauna. Nematode communities were analysed in soil under Trifolium repens, Festuca ovina and from randomly taken samples. Seventy-nine nematode genera were identified. Some of the species found have not previously been reported from Sweden. Multivariate analysis separated the nematode communities associated with the two selected plant species from each other, and several individual nematode genera differed in abundance between the plant species. Trifolium repens supported greater populations of the plant feeder Tylenchorhynchus and the bacterial feeders Eucephalobus, Chiloplacus, Eumonhystera and Panagrolaimus, but fewer numbers of the bacterial feeder Achromadora. Soil under F. ovina contained more nematodes from the family Alaimidae. A comparison is given with other studies from grassland systems in Sweden.

Aboveground insect herbivory increases plant competitive asymmetry, while belowground herbivory mitigates the effect

Insect herbivores can shift the composition of a plant community, but the mechanism underlying such shifts remains largely unexplored. A possibility is that insects alter the competitive symmetry between plant species. The effect of herbivory on competition likely depends on whether the plants are subjected to aboveground or belowground herbivory or both, and also depends on soil nitrogen levels. It is unclear how these biotic and abiotic factors interactively affect competition. In a greenhouse experiment, we measured competition between two coexisting grass species that respond differently to nitrogen deposition: Dactylis glomerata L., which is competitively favoured by nitrogen addition, and Festuca rubra L., which is competitively favoured on nitrogen-poor soils. We predicted: (1) that aboveground herbivory would reduce competitive asymmetry at high soil nitrogen by reducing the competitive advantage of D. glomerata; and (2), that belowground herbivory would relax competition at low soil nitrogen, by reducing the competitive advantage of F. rubra. Aboveground herbivory caused a 46% decrease in the competitive ability of F. rubra, and a 23% increase in that of D. glomerata, thus increasing competitive asymmetry, independently of soil nitrogen level. Belowground herbivory did not affect competitive symmetry, but the combined influence of above- and belowground herbivory was weaker than predicted from their individual effects. Belowground herbivory thus mitigated the increased competitive asymmetry caused by aboveground herbivory. D. glomerata remained competitively dominant after the cessation of aboveground herbivory, showing that the influence of herbivory continued beyond the feeding period. We showed that insect herbivory can strongly influence plant competitive interactions. In our experimental plant community, aboveground insect herbivory increased the risk of competitive exclusion of F. rubra. Belowground herbivory appeared to mitigate the influence of aboveground herbivory, and this mechanism may play a role for plant species coexistence.

Above‐ and belowground insect herbivory modifies the response of a grassland plant community to nitrogen eutrophication

Understanding the role that species interactions play in determining the rate and direction of ecosystem change due to nitrogen (N) eutrophication is important for predicting the consequences of global change. Insects might play a major role in this context. They consume substantial amounts of plant biomass and can alter competitive interactions among plants, indirectly shaping plant community composition. Nitrogen eutrophication affects plant communities globally, but there is limited experimental evidence of how insect herbivory modifies plant community response to raised N levels. Even less is known about the roles of above- and belowground herbivory in shaping plant communities, and how the interaction between the two might modify a plant communitys response to N eutrophication. We conducted a 3-yr field experiment where grassland plant communities were subjected to above- and belowground insect herbivory with and without N addition, in a full-factorial design. We found that herbivory modified plant community responses to N addition. Aboveground herbivory decreased aboveground plant community biomass by 21%, but only at elevated N. When combined, above- and belowground herbivory had a stronger negative effect on plant community biomass at ambient N (11% decrease) than at elevated N (4% decrease). In addition, herbivory shifted the functional composition of the plant community, and the magnitude of the shifts depended on the N level. The N and herbivory treatments synergistically conferred a competitive advantage to forbs, which benefited when both herbivory types were present at elevated N. Evenness among the plant species groups increased when aboveground herbivory was present, but N addition attenuated this increase. Our results demonstrate that a deeper understanding of how plant-herbivore interactions above and below ground shape the composition of a plant community is crucial for making reliable predictions about the ecological consequences of global change.

Free-Living Plant-Parasitic Nematodes do not Affect the Efficiency of Seed Tuber Fungicide Treatment against Rhizoctonia solani

Stem canker on germinating potato sprouts is often caused by seed-borne inoculum of the fungus Rhizoctonia solani. However, high amounts of free-living plant-parasitic nematodes have been found in field patches of potato plants with stem canker. Fungicide treatment of the seed tubers can be used to avoid stem canker caused by seed-borne inoculum but it is unknown if nematodes can affect this. To investigate whether free-living plant-parasitic nematodes, the root-lesion nematode Pratylenchus penetrans or a combination of several plant-parasitic nematode genera in a full nematode community, may have a negative effect on the fungicide seed treatment, a pot experiment with seed tubers inoculated with R. solani, half of which were treated with fungicides, was performed. The seed-borne inoculum caused severe damage to the plants, while no fungal damages were observed on the fungicide treated plants. This shows that the nematodes did not affect the fungicide treatment. The probability of black scurf decreased in treatments with a full nematode community, which may be due to the action of fungal-feeding nematodes.ResumenEl cáncer del tallo en brotes en germinación de la papa a menudo es causado por inóculo de la semilla del hongo Rhizoctonia solani. No obstante, se han encontrado grandes cantidades de nematodos fitoparásitos de vida libre en áreas del campo de plantas de papa con cáncer del tallo. Se pueden usar tratamientos de fungicidas en los tubérculos-semilla para evitar el cáncer del tallo causado por el inóculo de la semilla pero se desconoce si los nematodos pueden afectar esto. Para investigar si los nematodos fitoparásitos de vida libre, el nematodo lesionador Pratylenchus penetrans, o una combinación de varios géneros de nematodos fitoparasitos en una comunidad completa de nematodos, pudieran tener un efecto negativo en el tratamiento de fungicidas a la semilla, se desarrolló un experimento en macetas con tubérculos-semilla inoculados con R. solani, la mitad de los cuales se trataron con fungicidas. El inóculo en la semilla causó daño severo a las plantas, mientras que no se observaron daños en las plantas tratadas con fungicidas. Esto muestra que los nematodos no afectan al tratamiento con fungicidas. La probabilidad de costra negra disminuyó en los tratamientos con la comunidad completa de nematodos, lo cual pudo deberse a la acción de nematodos que se alimentan de hongos.

Cultivar Effects on the Interaction between Free-Living Plant-Parasitic Nematodes and the Fungal Pathogen Rhizoctonia solani in Potato

Crop damage is associated with infection by plant pathogens but can also arise through abiotic factors. However, the plant pathogens are involved in biotic interactions with other plant pathogens, and these interactions may differ depending of the cultivar of the crop. Here, the interaction between the fungus Rhizoctonia solani (AG3) and free-living plant-parasitic nematodes was investigated in a pot experiment with different potato cultivars. No synergistic interaction between R. solani and plant-parasitic nematodes was found, instead there was an effect of treatment with lower tuber yield when nematodes occurred alone. There were differences among the cultivars regarding incidence of black scurf, dry weight of stems and tubers, and there was interactive effects between treatment and cultivar regarding dry weight of stolons and roots. Therefore, results concerning incidence and damage of R. solani and/or plant-parasitic nematodes found for one cultivar may not be applicable to other cultivars.ResumenEl daño en el cultivo se asocia con infección por fitopatógenos, pero también puede surgir a través de factores abióticos. No obstante, los patógenos de plantas están involucrados en interacciones bióticas con otros patógenos, y estas interacciones pudieran diferir dependiendo de la variedad del cultivo. Aquí, se investigó la interacción entre el hongo Rhizoctonia solani (AG3) y nematodos fitoparásitos de vida libre en un experimento en macetas con diferentes variedades de papa. No se encontró una interacción sinergística entre R. solani y los nematodos fitoparásitos, en cambio, hubo un efecto de tratamiento con rendimiento más bajo de tubérculo cuando los nematodos estaban solos. Hubo diferencias entre variedades en relación a la incidencia de costra negra, peso seco de los tallos y los tubérculos, y hubo efectos interactivos entre tratamiento y variedad en relación al peso seco de los estolones y las raíces. De aquí que los resultados concernientes a la incidencia y daño de R. solani y/o nematodos fitoparásitos que se encontraron para una variedad pudieran no ser aplicables a otras.