Katarina Hedlund

Soil food web properties explain ecosystem services across European land use systems

Intensive land use reduces the diversity and abundance of many soil biota, with consequences for the processes that they govern and the ecosystem services that these processes underpin. Relationships between soil biota and ecosystem processes have mostly been found in laboratory experiments and rarely are found in the field. Here, we quantified, across four countries of contrasting climatic and soil conditions in Europe, how differences in soil food web composition resulting from land use systems (intensive wheat rotation, extensive rotation, and permanent grassland) influence the functioning of soils and the ecosystem services that they deliver. Intensive wheat rotation consistently reduced the biomass of all components of the soil food web across all countries. Soil food web properties strongly and consistently predicted processes of C and N cycling across land use systems and geographic locations, and they were a better predictor of these processes than land use. Processes of carbon loss increased with soil food web properties that correlated with soil C content, such as earthworm biomass and fungal/bacterial energy channel ratio, and were greatest in permanent grassland. In contrast, processes of N cycling were explained by soil food web properties independent of land use, such as arbuscular mycorrhizal fungi and bacterial channel biomass. Our quantification of the contribution of soil organisms to processes of C and N cycling across land use systems and geographic locations shows that soil biota need to be included in C and N cycling models and highlights the need to map and conserve soil biodiversity across the world.

Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach

Abstract Succession is one of the most studied processes in ecology and succession theory provides strong predictability. However, few attempts have been made to influence the course of succession thereby testing the hypothesis that passing through one stage is essential before entering the next one. At each stage of succession ecosystem processes may be affected by the diversity of species present, but there is little empirical evidence showing that plant species diversity may affect succession. On ex-arable land, a major constraint of vegetation succession is the dominance of perennial early-successional (arable weed) species. Our aim was to change the initial vegetation succession by the direct sowing of later-successional plant species. The hypothesis was tested that a diverse plant species mixture would be more successful in weed suppression than species-poor mixtures. In order to provide a robust test including a wide range of environmental conditions and plant species, experiments were carried out at five sites across Europe. At each site, an identical experiment was set up, albeit that the plant species composition of the sown mixtures differed from site to site. Results of the 2-year study showed that diverse plant species mixtures were more effective at reducing the number of natural colonisers (mainly weeds from the seed bank) than the average low-diversity treatment. However, the effect of the low-diversity treatment depended on the composition of the species mixture. Thus, the effect of enhanced species diversity strongly depended on the species composition of the low-diversity treatments used for comparison. The effects of high-diversity plant species mixtures on weed suppression differed between sites. Low-productivity sites gave the weakest response to the diversity treatments. These differences among sites did not change the general pattern. The present results have implications for understanding biological invasions. It has been hypothesised that alien species are more likely to invade species-poor communities than communities with high diversity. However, our results show that the identity of the local species matters. This may explain, at least partly, controversial results of studies on the relation between local diversity and the probability of being invaded by aliens.

Divergent composition but similar function of soil food webs of individual plants: plant species and community effects

Soils are extremely rich in biodiversity, and soil organisms play pivotal roles in supporting terrestrial life, but the role that individual plants and plant communities play in influencing the diversity and functioning of soil food webs remains highly debated. Plants, as primary producers and providers of resources to the soil food web, are of vital importance for the composition, structure, and functioning of soil communities. However, whether natural soil food webs that are completely open to immigration and emigration differ underneath individual plants remains unknown. In a biodiversity restoration experiment we first compared the soil nematode communities of 228 individual plants belonging to eight herbaceous species. We included grass, leguminous, and non-leguminous species. Each individual plant grew intermingled with other species, but all plant species had a different nematode community. Moreover, nematode communities were more similar when plant individuals were growing in the same as compared to different plant communities, and these effects were most apparent for the groups of bacterivorous, carnivorous, and omnivorous nematodes. Subsequently, we analyzed the composition, structure, and functioning of the complete soil food webs of 58 individual plants, belonging to two of the plant species, Lotus corniculatus (Fabaceae) and Plantago lanceolata (Plantaginaceae). We isolated and identified more than 150 taxa/groups of soil organisms. The soil community composition and structure of the entire food webs were influenced both by the species identity of the plant individual and the surrounding plant community. Unexpectedly, plant identity had the strongest effects on decomposing soil organisms, widely believed to be generalist feeders. In contrast, quantitative food web modeling showed that the composition of the plant community influenced nitrogen mineralization under individual plants, but that plant species identity did not affect nitrogen or carbon mineralization or food web stability. Hence, the composition and structure of entire soil food webs vary at the scale of individual plants and are strongly influenced by the species identity of the plant. However, the ecosystem functions these food webs provide are determined by the identity of the entire plant community.

Intensive agriculture reduces soil biodiversity across Europe

Soil biodiversity plays a key role in regulating the processes that underpin the delivery of ecosystem goods and services in terrestrial ecosystems. Agricultural intensification is known to change the diversity of individual groups of soil biota, but less is known about how intensification affects biodiversity of the soil food web as a whole, and whether or not these effects may be generalized across regions. We examined biodiversity in soil food webs from grasslands, extensive, and intensive rotations in four agricultural regions across Europe: in Sweden, the UK, the Czech Republic and Greece. Effects of land-use intensity were quantified based on structure and diversity among functional groups in the soil food web, as well as on community-weighted mean body mass of soil fauna. We also elucidate land-use intensity effects on diversity of taxonomic units within taxonomic groups of soil fauna. We found that between regions soil food web diversity measures were variable, but that increasing land-use intensity caused highly consistent responses. In particular, land-use intensification reduced the complexity in the soil food webs, as well as the community-weighted mean body mass of soil fauna. In all regions across Europe, species richness of earthworms, Collembolans, and oribatid mites was negatively affected by increased land-use intensity. The taxonomic distinctness, which is a measure of taxonomic relatedness of species in a community that is independent of species richness, was also reduced by land-use intensification. We conclude that intensive agriculture reduces soil biodiversity, making soil food webs less diverse and composed of smaller bodied organisms. Land-use intensification results in fewer functional groups of soil biota with fewer and taxonomically more closely related species. We discuss how these changes in soil biodiversity due to land-use intensification may threaten the functioning of soil in agricultural production systems.

Soil microbial community structure in relation to vegetation management on former agricultural land

Agricultural overproduction has led the European Union to encourage long-term abandonment of agricultural land. To enhance the transition of agricultural land to natural grasslands or forests different management practices in relation to vegetation can be used. The aim of this study was to understand the interactions between plant and associated soil microbial communities during the succession of agricultural land. A field experiment was established on newly abandoned agricultural land where the development of the soil microbial community was studied in plots after sowing a mixture of 15 plant species and in plots that were left to be naturally colonized by plants. The plants sown contained five species each of grasses, legumes and forbs representing three functional groups. A subset of these plots were inoculated with soil cores from a later successional stage. Adjacent soils with ongoing agricultural practices and deciduous beech forest were also studied. The microbial community composition and biomass were studied by determining fatty acid signatures (PLFA, NFLA) of soil microorganisms. The microbial community had changed within 2 years from that of the agricultural field diverging into one type present in sown plots and another in plots that were naturally colonized by plants. Sowing plant seed mixtures promoted growth of the bacterial community and sarophytic fungi. Respiration measurements showed higher microbial activity and biomass in sown plots. Bait plants for arbuscular mycorrhizal (AM) fungi and NLFA 16:1ω5 showed that the biomass of AM-fungi was reduced when seed mixtures were sown, which also decreased the formation of mycorrhizal associations on plant roots.

Mycelial responses of the soil fungus, Mortierella isabellina, to grazing by Onychiurus armatus (collembola)

Abstract Hyphal morphology and extracellular enzyme production of the fungus Mortierella isabellina were studied during grazing by a soil Collembola, Onychiurus armatus . Grazing induced switching from a “normal” hyphal mode, with appressed growth and sporulating hyphae, to fan shaped sectors of fast growing and nonsporulating mycelium which developed extensive aerial mycelium. Specific protease and α-amylase activities were several times higher in grazed cultures where switching occurred compared with plates without switching. Increased protease activity was localized to switched sectors within the mycelium. On the basis of inhibitor studies, the protease of “normal” mycelium was classified as a metalloprotease. Switching induced activity of an additional serine protease. Anion-exchange chromatography showed that the two proteases had similar charges and, according to SDS-gclatine-PAGE. the molecular weight of the serine protease was 80.000. Switching to a fast growing hyphal mode could be one explanation for compensatory growth of grazed fungi.

Food- and density-dependent dispersal: evidence from a soil collembolan

1. Dispersal in a fungivore species of Collembola, Onychiurus armatus, in a homo- geneous soil was theoretically approached as a deterministic compartmental process, in which the movement of animals between discrete patches could be described by a series of differential equations or by the discrete geometric probability distribution, assuming a constant probability to move from one patch to the next. 2. Experiments were designed for a release of collembolans in two types of soil in a physical arrangement of distinct patches represented by vials connected by tubings to cover a distance of 40 cm. Dispersal distances of individuals were determined, and theoretical and observed distributions compared. 3. Dispersal rates, estimated from transfer rate constants, ranged from 0 020 to 1 42 day-1, suggesting that an average 0. armatus moved less than 10 cm day -1. The probability to leave a patch varied between 0 10 and 0 50. 4. Dispersal was dependent on population density, soil type and length of fungal mycelium; it was almost twice as high at a high than at a low density (90 000 and 30000 individuals m-2) in a mor soil and four times as high in a sandy compared with a mor soil. Dispersal rate decreased as the mycelial length increased, especially in a sandy soil. 5. Collembolans in a feeding phase had a higher tendency to disperse than those that were moulting. 6. Enriching the soil patch at 40 cm distance from the release point with a favoured food item, the fungal species Mortierella isabellina, increased dispersal rate by more than four times in a mor soil, suggesting that fungal odour can attract collembolans from a large distance and enhance their rate of movement more than a three-fold increase of their population density does. (Less)

Effects of enchytraeid grazing on fungal growth and respiration

Abstract In view of the grazing optimization theory, we examined the effects of different intensities of grazing by the enchytraeid Cognettia sphagnetorum (Vejdovsky) on microbial respiration and hyphal length in soil. In a feeding experiment we established that C. sphagnetorum feeds on the fungus Mortierella isabellina (Oudem) and that the worms disperse fungal propagules. Grazing of C. sphagnetorum with densities ranging from 0 to 54,000 enchytraeids m −2 was studied in 2 experiments: (1) a multi-species system with C. sphagnetorum kept in soil with its natural microflora and fauna; and (2) a 2-species set up with C. sphagnetorum and M. isabellina inoculated into a sterillized soil. In the natural soil high densities of enchytraeids clecreased microbial respiration but no effects were found of enchytraeid grazing on the lengths of fungal hyphae. After 1 wk, in the 2-species soil, respiration of M. isabellina was higher in all experiments with enchytraeids present compared to controls with only the fungus. The hyphal lengths of M. isabellina in the inoculated soil increased about 2- to 3-fold at the lowest intensities of grazing but decreased to that of an ungrazed soil at the highest level of grazing.

Selective odor perception in the soil collembolaOnychiurus armatus.

The olfactorial response of the fungivorous soil collembolanOnychiurus armatus was examined in a bioassay covering volatile compounds identified in the odor blends of two of its preferred fungal speciesMonierella isabellina andVerticillium bulbillosum. The odor of the fungi was trapped using activated carbon filters, extracted with diethyl ether, and subjected to GC-MS analysis. About 50% of the compounds resolved by GC were identified by a combination of electron impact and chemical ionization mass spectrometry. In a Y-shaped olfactometer the collembolans were attracted to a variety of common odors, such as CO2 and 2-methyl-1-propanol, and a species-specific odor, such as 1-heptene, and arrested by, for example, decanal and 2-octene. The response was not improved by pairwise combinations of common and specific odors. An amount of 0.5 ng of ethyl acetate or 3 pg of 1-pentanol was sufficient to attract the collembolans. The specific compounds ofV. bulbillosum, 1-heptene and 1-octen-3-ol, may be key stimuli explaining whyO. armatus prefersV. bulbillosum.

Microarthropods Mediate Sperm Transfer in Mosses

Among flowering plants, animals commonly act as pollinators. We showed that fertile moss shoots attract springtails and mites, which in turn carry moss sperm, thereby enhancing the fertilization process. Previously, fertilization of mosses was thought to depend on the capacity of individual sperm to swim through a continuous water layer. The role of microarthropods in moss fertilization resembles the role of animals as pollinators of flowering plants but may be evolutionarily much older because of the antiquity of the organism groups involved.