Flowering phenology of a herbaceous species (Poa annua) is regulated by soil Collembola

The role of soil organisms as possible driver of flowering has never been investigated. We hypothesized that Collembola (microarthropods) will change plant allocation to reproductive modes by changing soil nutrient availability. Individual seedlings of Poa annua were planted in microcosms, in the presence or absence of Collembola. Collembola affected biotic (fungal biomass) and abiotic ($\text{N}-{{\text{NO}}_3}^{-}$, P₂O₅) soil properties and some morphological (number of leaves, root biomass) and chemical (C:N, K, Mg, N) traits of P. annua. As a result, flowering of P. annua was promoted by the presence of Collembola. This provides experimental evidence that soil microarthropods can affect the reproduction strategy and phenology of a plant.     

Decreased biodiversity in soil springtail communities: the importance of dispersal and landuse history in heterogeneous landscapes

In previously published papers it had been demonstrated that at the local level the species richness of soil springtail communities was negatively influenced by landuse diversity. When the dispersal rate of soil animals was taken into account, quite opposite trends were displayed by species having poor or high dispersal capabilities. At the local level, species with short legs, non functional jumping apparatus and reduction of visual organs were distinguished against by landuse diversity, while species with long legs, functional jumping apparatus (furcula) and complete eyes, thus able to disperse at the soil surface, were not. It was verified, through aerial photographs taken 50 years ago, that landuse changes, expected to be more frequent in heterogeneous landscapes, may contribute to explain this phenomenon.     

Reactions to soil acidification in microarthropods: Is competition a key factor?

Summary Acidification of raw humus soil in coniferous forest areas leads to characteristic changes in the microarthropod community. Certain species are calciophilic and decrease in abundance, while others are acidophilic and increase in abundance. The simplest explanation for these changes is that population levels are directly related to soil pH. This hypothesis was tested by growing small populations of selected species in monoculture at different pH levels. Three acidophilic species were tested, the collembolan Mesaphorura yosii, the oribatid mite Nothrus silvestris, and the astigmatid mite Schwiebea cf. lebruni. A slightly calciophilic collembolan, Isotomiella minor, was also included. For all species, population growth was lowest in acidified raw humus. Even acidophilic species seem to have an optimum at a high pH. It is assumed that their success in low-pH soils is due to their ability to compete under these conditions. Competition may be a key factor in microarthropod reactions to soil acidification.Dedicated to the late Prof. Dr. W. Kühnelt     

Naphthalene addition to soil surfaces: A feasible method to reduce soil micro-arthropods with negligible direct effects on soil C dynamics

Soil fauna are a key component of soil biodiversity and a driver of soil functioning. While the importance of soil fauna is well recognized, quantitative estimates of the role of soil fauna on soil biogeochemical processes, such as plant litter decomposition, are limited by methodological constraints. The addition of naphthalene, a polycyclic aromatic hydrocarbon (C₁₀H₈), to suppress soil fauna has been used for decades in decomposition experiments, but its efficacy remains questioned. In fact, we lack a rigorous field assessment of the efficacy of naphthalene additions for soil fauna suppression and potential non-target effects on the soil microbial community and carbon cycling. We added naphthalene at a high rate (477 g m⁻²) monthly for 23 months on the bare soil surface of a tallgrass prairie. We determined the effect of such additions on the abundance of nematodes and micro-arthropods along the soil profile to a depth of 20 cm at 11, 16 and 23 months after initiating naphthalene application. We used the variation in the natural ¹³C abundance of the naphthalene (δ¹³C – 25.5‰) as compared to the native soil (δ¹³C ∼ −17‰) to quantify naphthalene contribution to soil CO₂ efflux and microbial biomarkers (PLFA). Naphthalene addition significantly reduced the abundance of oribatid mites (−45%), predatory mites (−52%) and springtails (−49%), but did not affect nematode abundance. The ¹³C abundance of a few Gram-negative (cy17:0, 18:1ω7c, 16:1ω7c), Gram-positive (a15:0, i15:0) and Actinobacteria (10Me-16:0, 10Me-18:0) PLFA markers decreased significantly in naphthalene treated plots, indicating bacterial utilization of naphthalene-derived C. Mixing models showed this contribution to be highly variable, with the highest naphthalene-C incorporation for Gram negative bacteria. Naphthalene-C was not incorporated in fungal PLFAs. This microbial utilization did not affect overall microbial abundance, community structure or activity, estimated as soil respiration. This experiment proves that naphthalene addition is a feasible method to reduce soil micro-arthropods in the field, with negligible direct effects on soil nematodes, microbial abundance and C dynamics.     

The effects of the insecticide chlorpyrifos on spider and Collembola communities

The effects of chlorpyrifos on aquatic systems are well documented. However, the consequences of the pesticide on soil food webs are poorly understood. In this field study, we hypothesised that the addition of a soil insecticide to an area of upland grassland would impact spider and Collembola communities by decreasing numbers of spiders, consequently, causing an increase in detritivore numbers and diversity.Chlorpyrifos was added to plots on an upland grassland in a randomised block design. Populations of Collembola and spiders were sampled by means of pitfall traps (activity density) and identified to species.Twelve species of Collembola were identified from the insecticide-treated and control plots. Species diversity, richness and evenness were all reduced in the chlorpyrifos plots, although the total number of Collembola increased ten-fold despite the abundance of some spider species being reduced. The dominant collembolan in the insecticide-treated plots was Ceratophysella denticulata, accounting for over 95% of the population.Forty-three species of spider were identified. There were a reduced number of spiders in insecticide-treated plots due mainly to a lower number of the linyphiid, Tiso vagans. However, there was no significant difference in spider diversity between the control and insecticide treatments.We discuss possible explanations for the increase in abundance of one collembolan species in response to chlorpyrifos and the consequences of this. The study emphasises the importance of understanding the effects of soil management practices on soil biodiversity, which is under increasing pressure from land development and food production. It also highlights the need for identification of soil invertebrates to an ‘appropriate’ taxonomic level for biodiversity estimates.     

Effects of rhizospheres on the community composition of Collembola in a temperate forest

Influence of living roots on the community composition of Collembola was investigated in a coniferous forest of Chamaecyparis obtusa. We conducted a buried pot experiment that constructed two different systems for carbon (C) availability. We used two types of pots with or without a C. obtusa seedling. The former pots were used to make a system with soil including living roots (i.e. a system based on root-derived and litter-derived C), while the latter was equivalent to soil system without living roots (i.e. a system based on litter-derived C). After 8 months, we harvested the pots and examined the collembolan community and environmental factors. The presence of living roots affected collembolan abundance and species-specific responses. These changes could be explained in terms of leaf biomass of the seedlings, indicating a possible linkage between above-ground productivity and collembolan community. Given the possibility that root-derived C is associated with aboveground plant mass, we speculate that collembolan community responds to root-derived C. The three dominant species, which are widespread in Japanese temperate forests, were more abundant in the presence of living roots. Moreover they were positively correlated with leaf biomass in the system with a seedling, again suggesting the fundamental importance of living roots for the organisation of the collembolan community.     

Decomposer animals (Lumbricidae, Collembola) and organic matter distribution affect the performance of Lolium perenne (Poaceae) and Trifolium repens (Fabaceae)

Decomposer animals stimulate plant growth by indirect effects such as increasing nutrient availability or by modifying microbial communities in the rhizosphere. In grasslands, the spatial distribution of organic matter (OM) rich in nutrients depends on agricultural practice and the bioturbation activities of large detritivores, such as earthworms. We hypothesized that plants of different functional groups with contrasting nutrient uptake and resource allocation strategies differentially benefit from sites in soil with OM accumulation and the presence of decomposer animals. In a greenhouse experiment we investigated effects of spatial distribution of ¹⁵N-labelled grass litter, earthworms and collembola on a simple grassland community consisting of Lolium perenne (grass) and Trifolium repens (legume). Litter aggregates (compared to homogeneous litter distribution) increased total shoot biomass, root biomass and ¹⁵N uptake by the plants. Earthworms and collembola did not affect total N uptake of T. repens; however, the presence of both increased ¹⁵N uptake by T. repens and L. perenne. Earthworms increased shoot biomass of T. repens 1.11-fold and that of L. perenne 2.50 fold. Biomass of L. perenne was at a maximum in the presence of earthworms, collembola and with litter concentrated in a single aggregate. Shoot biomass of T. repens increased in the presence of collembola, with L. perenne generally responding opposingly. The results indicate that the composition of the decomposer community and the distribution of OM in soil affect plant competition and therefore plant community composition.     

Collembola switch diet in presence of plant roots thereby functioning as herbivores

Collembola are abundant and ubiquitous soil decomposers, being particularly active in the rhizosphere of plants where they are assumed to be attracted by high microbial activity and biomass. While feeding on root associated microorganisms or organic matter they may also ingest plant roots, e.g. particularly root hairs and fine roots. Employing stable isotope analysis we investigated Collembola (Protaphorura fimata Gisin) feeding preferences and types of ingested resources. We offered Collembola two resources with distinct isotope signatures: a C4 plant (Zea mays L.) planted in soil mixed with ¹⁵N labelled litter of Lolium perenne L. (C3 plant). We hypothesised that Collembola obtain their nutrients (C and N) from different resources, with their carbon being mainly derived from resources that are closely associated to the plant root, e.g. root exudates, causing enrichment in ¹³C in Collembola tissue, while the incorporated nitrogen originating from litter resources. In contrast to our hypothesis, stable isotope analysis suggests that in absence of plant roots Collembola derived both the incorporated C and N predominantly from litter whereas in presence of plant roots they switched diet and obtained both C and N almost exclusively from plant roots.The results indicate that Collembola in the rhizosphere of plants, being assumed to be mainly decomposers, in fact predominately live on plant resources, presumably fine roots or root hairs, i.e. are herbivorous rather than detritivorous or fungivorous. These findings have major implications on the view how plants respond to decomposers in the rhizosphere.     

Effects of food quality, starvation and life stage on stable isotope fractionation in Collembola

Naturally occurring stable isotopes of carbon and nitrogen are powerful tools to investigate food webs, where the ratio of ¹⁵N/¹⁴N is used to assign trophic levels and of ¹³C/¹²C to determine the food source. A shift in δ¹⁵N value of 3‰ is generally suggested as mean difference between two trophic levels, whereas the carbon isotope composition of a consumer is assumed to reflect the signal of its diet. This study investigates the effects of food quality, starvation and life stage on the stable isotope fractionation in fungal feeding Collembola. The fractionation of nitrogen was strongly affected by food quality, i.e. the C/N ratio of the fungal diet. Collembola showed enrichment in the heavier isotope with increasing N concentration of the food source. Δ¹⁵N varied between 2.4‰, which assigns a shift in one trophic level, and 6.3‰, suggesting a shift in two trophic levels. Starvation up to 4 weeks resulted in an increase in the total δ¹⁵N value from 2.8‰ to 4.0‰. Different life stages significantly affected the isotope discrimination by Collembola with juveniles showing a stronger enrichment (Δ¹⁵N=4.9‰) compared to adults (Δ¹⁵N=3.5‰). Δ¹³C varied between −2.1‰ and −3.3‰ depending on the food quality, mainly due to compensational feeding on low quality diet. During starvation δ¹³C value decreased by 1.1‰, whereas the life stage of Collembola had no significant effect on isotopic ratios. The results indicate that the food resource and the physiological status of the consumer have important impact on stable isotope discrimination. They may cause differences in fractionation rate comparable to trophic level shifts, a fact to consider when analysing food web structure.     

The body-size distribution of arboreal collembolans in relation to the vertical structure of a Japanese cedar plantation

Collembolans are among the most abundant microarthropods in the aboveground parts of forest ecosystems, but little is known about their life-history traits compared with those of the soil-dwelling collembolans. The life-history traits of arboreal collembolans, as well as their abundances, are expected to be influenced by the vertical structure of a forest. We analyzed the body-size distributions of dominant arboreal species in relation to the vertical structure of the forest. The small individuals of the three arboreal species that inhabit both the canopy and soil litter (Xenylla brevispina, Tomocerus cuspidatus, and Sphyrotheca multifasciata) appeared continuously from spring to autumn in the soil litter, whereas those in the canopy litter were found during a particular period, only in July. These individuals tended to be larger in the canopy litter than in the soil litter, and in particular, T. cuspidatus grew to become significantly larger in the canopy. The mean body size of the most abundant migrating species (X. brevispina) increased gradually from the soil to the upper canopy, whereas strict arboreal species (Entomobrya sp.) had no significant trend in the relationship between body size and the height above ground level. We concluded that the life-history traits of arboreal collembolans were influenced by the vertical structure of the forest.