Stefan Scheu

Bottom-up effects of plant diversity on multitrophic interactions in a biodiversity experiment

Biodiversity is rapidly declining, and this may negatively affect ecosystem processes, including economically important ecosystem services. Previous studies have shown that biodiversity has positive effects on organisms and processes across trophic levels. However, only a few studies have so far incorporated an explicit food-web perspective. In an eight-year biodiversity experiment, we studied an unprecedented range of above- and below-ground organisms and multitrophic interactions. A multitrophic data set originating from a single long-term experiment allows mechanistic insights that would not be gained from meta-analysis of different experiments. Here we show that plant diversity effects dampen with increasing trophic level and degree of omnivory. This was true both for abundance and species richness of organisms. Furthermore, we present comprehensive above-ground/below-ground biodiversity food webs. Both above ground and below ground, herbivores responded more strongly to changes in plant diversity than did carnivores or omnivores. Density and richness of carnivorous taxa was independent of vegetation structure. Below-ground responses to plant diversity were consistently weaker than above-ground responses. Responses to increasing plant diversity were generally positive, but were negative for biological invasion, pathogen infestation and hyperparasitism. Our results suggest that plant diversity has strong bottom-up effects on multitrophic interaction networks, with particularly strong effects on lower trophic levels. Effects on higher trophic levels are indirectly mediated through bottom-up trophic cascades.

An inter-laboratory comparison of ten different ways of measuring soil microbial biomass C

Abstract Ten versions of three methods for estimating soil microbial biomass, fumigation-incubation (FI), fumigation-extraction (FE) and substrate-induced respiration (SIR) were compared in seven different laboratories, using a variety of methodological and analytical procedures. Twenty different soil samples were analyzed from a range of arable and forest sites. The 10 different measurements gave almost identical ranking for microbial biomass C in the 20 soil samples. However, comparison of data obtained by different methods and by different laboratories is hampered by soil-to-soil variation between the methods and by systematic effects on the biomass measurements that cause over- or under-estimations. Each variant of the SIR method should have its own calibration factor. The relationship between the basic FI method and the other methods was mainly affected by the respiration rate of non-fumigated soil. The FI method is less suitable for the calibration of the FE and SIR methods in forest soils than in arable soils. Unintentional variation in the experimental procedures, such as in the duration of any conditioning incubation, may also contribute to the observed differences between the methods.

Effects of earthworms on plant growth: patterns and perspectives

Summary A total of 67 studies were located in which the response of plants to presence of earthworms was investigated. The number of studies increased strongly during the last decade. In most of the studies (79 %) shoot biomass of plants significanty increased in the presence of earthworms. However, knowledge on effects of earthworms on plant growth is very biased; most studies investigated crop plants, particularly cereals, and pastures; very little is known on plant species in more natural communities. Recently, interest in tropical plant species has increased considerably, however, the studies have considered almost exclusively agricultural plant species. Generally, experiments focused on the response of plant shoots but 45 % of the studies also considered roots. Most of the studies investigated European earthworms (Lumbricidae); very little is known on other earthworm species. Some early studies indicated that earthworms affect the composition of plant communities but only very recently has it been documented that earthworms affect plant competition. However, there is virtually no information on how earthworms affect plant performance in detail including fitness parameters such as flowering and seed production. It has been realized recently that earthworms not only modify plant growth and vegetation structure but also the susceptability of plants to herbivores. Herbivore performance might be stimulated but also reduced due to the presence of earthworms. Furthermore, earthworms function as subsidiary food resources to generalist predators when herbivore prey is scarce. The complex indirect interactions between earthworms and the aboveground system deserve further investigation in both natural and agricultural ecosystems. The imperative for future research is adopting an ecological rather than an agricultural perspective in studying earthworm-plant interrelationships and viewing earthworms as driving factors of the aboveground food web. It is suggested that studies on earthworm-plant interactions may contribute significantly to a more comprehensive understanding of terrestrial ecosystems and to the development of more environmentally friendly agricultural practices.

Non‐native invasive earthworms as agents of change in northern temperate forests

Exotic earthworms from Europe and Asia are invading many northern forests in North America that currently lack native earthworms, providing an opportunity to assess the role of this important group of invertebrates in forest ecosystems. Research on earthworm invasions has focused on changes in soil structure and carbon (C) and nitrogen (N) cycling that occur following invasion. These changes include the mixing of organic and mineral soil horizons, decreases in soil C storage, and equivocal effects on N cycling. Less well studied are changes in the soil foodwebs that accompany earthworm invasion. Soils of north temperate forests harbor a tremendous diversity of microorganisms and invertebrates, whose distribution and abundance can be substantially altered by earthworm invasion. Furthermore, invasive earthworms can affect understory plant communities, raising concerns over the loss of rare native herbs in some areas. The ecological consequences of earthworm invasion are mediated through physical, geochemica...

The soil food web of two beech forests (Fagus sylvatica) of contrasting humus type: stable isotope analysis of a macro- and a mesofauna-dominated community

Abstract The structure of the soil food web in two beech (Fagus sylvatica) forests, the Göttinger Wald and the Solling forest (Northern Germany), was investigated using variations in tissue 15N concentrations of animal species or taxa. The Göttinger Wald is located on a limestone plateau and characterized by mull humus with high macrofauna activity, particularly of Lumbricidae, Diplopoda and Isopoda. In contrast, the Solling forest is located on a sandstone mountain range and characterized by moder humus. The soil fauna of this forest is dominated by mesofauna, particularly by Collembola, Enchytraeidae and Oribatida. In June 1995 soil fauna was sampled using heat extraction. Three soil layers were analysed at each of the sites. 15N/14N ratios of bulk material increased strongly with soil depth in both forests. This also applied to the water-soluble fraction at the Göttinger Wald, but not at the Solling. Generally, the water-soluble fraction was more enriched in 15N than the bulk materials. For most animals studied 15N/14N ratios varied little with soil depth. In both forests soil animals could be classified either as saprophages, including microphytophages, or predators. On average, the δ15N of predatory taxa (Chilopoda, Araneida, Gamasina, Staphylinidae) exceeded that of saprophagous or microphytophagous taxa (Lumbricidae, Isopoda, Diplopoda, Collembola, Oribatida, Enchytraeidae) by 4.4 and 3.9‰ for the Göttinger Wald and the Solling, respectively. We assume that most of the saprophagous or microphytophagous taxa studied consist of primary and secondary decomposers and hypothesize that predators prey more on secondary than primary decomposers. Generally, average δ15N values differed little between saprophagous (Lumbricidae, Diplopoda, Isopoda) and microphytophagous taxa (Collembola, Oribatida). The variations in δ15N values of species within these taxa consistently exceeded the variation between them, indicating that the species of each of these taxa form a continuum from primary to secondary decomposers. Also, variations in δ15N values within predatory taxa in most cases exceeded that between taxa excluding top predators like Sorex. We conclude that using higher taxonomic units in soil food web analysis is problematic and in general not consistent with nature. Higher taxonomic units may only be useful for depicting very general trophic groupings such as predators or microbi-detritivores.

Earthworm invasion into previously earthworm-free temperate and boreal forests.

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.

Plant diversity effects on soil microorganisms support the singular hypothesis

The global decline in biodiversity has generated concern over the consequences for ecosystem functioning and services. Although ecosystem functions driven by soil microorganisms such as plant productivity, decomposition, and nutrient cycling are of particular importance, interrelationships between plant diversity and soil microorganisms are poorly understood. We analyzed the response of soil microorganisms to variations in plant species richness (1-60) and plant functional group richness (1-4) in an experimental grassland system over a period of six years. Major abiotic and biotic factors were considered for exploring the mechanisms responsible for diversity effects. Further, microbial growth characteristics were assessed following the addition of macronutrients. Effects of plant diversity on soil microorganisms were most pronounced in the most diverse plant communities though differences only became established after a time lag of four years. Differences in microbial growth characteristics indicate successional changes from a disturbed (zymogeneous) to an established (autochthonous) microbial community four years after establishment of the experiment. Supporting the singular hypothesis for plant diversity, the results suggest that plant species are unique, each contributing to the functioning of the belowground system. The results reinforce the need for long-term biodiversity experiments to fully appreciate consequences of current biodiversity loss for ecosystem functioning.

Automated measurement of the respiratory response of soil microcompartments: Active microbial biomass in earthworm faeces

Abstract A system to analyse the respiratory response of soil microcompartments by automated electrolytic microrespirometry is described. The system is computer controlled and allows simultaneous measurements of 16 samples. The minimum detection level is 0.83 μg O 2 and the capacity of the system can be extended up to about 2500 μg O 2 h − . The frequency of measurements is optional and ranges from about 10 s to 1 h. The system was used to determine the active and total microbial biomass in soil and earthworm [Aporrectodea caliginosa (Savigny)] faecal particles (mean age of 4 days) by the O 2 uptake method. Active microbial biomass was increased in earthworm faeces (+17%), but the percentages of active microorganisms were similar in faeces (5.0%) and soil (4.5%). Addition of diluted nutrients instead of dry matter resulted in very different shapes to O 2 uptake curves, indicating damage to microorganisms due to sudden moisture alterations. The usefulness of the microrespirometer system to investigate microbial properties of soil microcompartments is discussed.

Protozoa, Nematoda and Lumbricidae in the rhizosphere of Hordelymus europaeus (Poaceae): faunal interactions, response of microorganisms and effects on plant growth

Interactions among protozoa (mixed cultures of ciliates, flagellates and naked amoebae), bacteria-feeding nematodes (Pellioditis pellio Schneider) and the endogeic earthworm species Aporrectodea caliginosa (Savigny) were investigated in experimental chambers with soil from a beechwood (Fagus sylvatica L.) on limestone. Experimental chambers were planted with the grass Hordelymus europeaus L. (Poaceae) and three compartments separated by 45-μm mesh were established: rhizosphere, intermediate and non-rhizosphere. The experiment lasted for 16 weeks and the following parameters were measured at the end of the experiment: shoot and root mass of H. europaeus, carbon and nitrogen content in shoots and roots, density of ciliates, amoebae, flagellates and nematodes, microbial biomass (SIR), basal respiration, streptomycin sensitive respiration, ammonium and nitrate contents, phosphate content of soil compartments. In addition, leaching of nutrients (nitrogen and phosphorus) and leachate pH were measured at regular intervals in leachate obtained from suction cups in the experimental chambers. Protozoa stimulated the recovery of nitrifying bacteria following defaunation (by chloroform fumigation) and increased nitrogen losses as nitrate in leachate. In contrast, protozoa and nematodes reduced leaching of phosphate, an effect ascribed to stimulation of microbial growth early in the experiment. Earthworms strongly increased the amount of extractable mineral nitrogen whereas it was strongly reduced by protozoa and nematodes. Both protozoa and nematodes reduced the stimulatory effect of earthworms on nitrogen mineralization. Microbial biomass, basal respiration, and numbers of protozoa and nematodes increased in the vicinity of the root. Protozoa generally caused a decrease in microbial biomass whereas nematodes and earthworms reduced microbial biomass only in the absence of protozoa. None of the animals studied significantly affected basal respiration, but specific respiration of microorganisms (O2 consumption per unit biomass) was generally higher in animal treatments. The stimulatory effect of nematodes and earthworms, however, occurred only in the absence of protozoa. The sensitivity of respiration to streptomycin suggested that protozoa selectively grazed on bacterial biomass but the bacterial/fungal ratio appeared to be unaffected by grazing of P. pellio. Earthworms reduced root biomass of H. europaeus, although shoot biomass remained unaffected, and concentrations of nitrogen in shoots and particularly in roots were strongly increased by earthworms. Both nematodes and protozoa increased plant biomass, particularly that of roots. This increase in plant biomass was accompanied by a marked decrease in nitrogen concentrations in roots and to a lesser extent in shoots. Generally, the effects of protozoa on plant growth considerably exceeded those of nematodes. It is concluded that nematodes and protozoa stimulated plant growth by non-nutritional effects, whereas the effects of earthworms were caused by an increase in nutrient supply to H. europaeus.

Microbial activity and nutrient dynamics in earthworm casts (Lumbricidae)

SummaryMicrobial respiration, microbial biomass and nutrient requirements of the microflora (C, N, P) were studied in the food substrate (soil taken from the upper 3 cm of the mineral soil of a beech wood on limestone), the burrow walls and the casts of the earthworm Aporrectodea caliginosa (Savigny). The passage of the soil through the gut caused an increase in soil microbial respiration of about 90% over a 4-week period. Microbial biomass was increased only in freshly deposited casts and decreased in aging faeces to a level about 10% lower than in soil. Microbial respiration of the burrow walls was only increased over a shorter period (about 2 weeks). The microflora of the soil and the burrow walls was limited by P, whereas in earthworm casts, microbial growth was limited by the amount of available C. In aging faeces the P requirement of the microflora increased and approached that of the soil. Immobilization of phosphate in earthworm casts is probably caused by mainly abiotic processes. C mineralization by soil microflora fertilized with glucose and P was limited by N, except in freshly deposited casts. Ammonium, not nitrate, was responsible for this process. N dynamics in earthworm casts are discussed.