Changes in the community composition and trophic structure of microarthropods in sporocarps of the wood decaying fungus Fomitopsis pinicola along an altitudinal gradient

Soil microarthropods colonize a wide range of habitats including microhabitats such as earthworm burrows, ant nests, tree trunks, moss mats and wood decaying fungi. While many of these microhabitats have been investigated intensively, the role of wood decaying fungi as a habitat and food resource for microarthropods found little attention. We investigated the density, community structure, reproductive mode and trophic structure of microarthropods, in particular oribatid mites, in the wood decaying fungus Fomitopsis pinicola (Schwarts: Fr) Karst. along an altitudinal gradient in Germany spanning from 350 m to 1160 m. Microarthropods were extracted from sporocarps, and stable isotope ratios (¹⁵N/¹⁴N; ¹³C/¹²C) of the fungus and the microarthropods were measured. Densities of most microarthropod taxa were highest at lower altitudes and decreased with increasing altitude. Oribatid mites were the dominant animal taxon. Their community structure gradually changed with altitude. Stable isotope ratios indicated that oribatid mite and other arthropod species occupy distinct trophic niches but most do not feed on F. pinicola. Notably, species of the same genus, e.g. Carabodes, occupied different trophic niches. Most oribatid mite species in F. pinicola reproduced sexually which is similar to the bark of trees but in contrast to the soil where most species reproduce via parthenogenesis. The findings indicate that (1) at high altitudes microarthropod density in fungal fruiting bodies is limited by low temperatures reducing animal metabolism and reproduction, and this also affects oribatid mite community structure, (2) despite the uniform habitat trophic niches of oribatid mite species differ and this also applies to morphologically similar species of the same genus, and (3) feeding on F. pinicola or associated resources facilitates the dominance of sexual reproducing species.     

Variation in the trophic position of common stream fishes and its relationship to the presence of a rare fish,northern leatherside chub (Lepidomeda copei)

Variation in trophic position can be caused by structural changes in food webs that may affect the presence of, or be affected by the presence of, individual species. We examined variation in the trophic position of fishes across 14 stream sites in the Bear River drainage, WY, USA. This drainage is the focus of ongoing conservation of northern leatherside chub (Lepidomeda copei). Our goals were (i) to describe variation in trophic position of individual species and (ii) to determine whether these measures differed between sites with and without northern leatherside chub. Mean trophic position of individual fish species varied between 0 and 3 trophic positions across sites. For two of these species, trophic position declined at sites without northern leatherside chub. Importantly, habitat surveys from a previous study at 10 of these sites revealed no differences in habitat suitability for northern leatherside chub. This suggests that trophic position revealed systematic differences among sites that were not apparent based on traditional species‐habitat modelling. We outline possible mechanisms behind these patterns and argue that monitoring variation in trophic position can complement traditional, habitat‐based methods for understanding species distributions.     

Contributions of salmon-derived nitrogen to riparian vegetation in the northwest Pacific region

We examined the relationship between the annual escapement of salmon and the δ ¹⁵N of willow (Salix spp.) leaves to evaluate the contribution of marine-derived nutrients (MDN) to riparian vegetation around the Pacific Northwest and Northeast regions. Foliar δ ¹⁵N values ranged from −3.42‰ to 4.65‰. The value increased with increasing density of carcasses up to 500 fish/km and 1500 fish/km. δ ¹⁵N values were variable at carcass densities below 500 fish/km. Possible factors affecting the fluctuation of δ ¹⁵N at reference sites are: (1) denitrification; (2) the presence of N₂-fixing trees, such as alder; and (3) agricultural runoff. δ ¹⁵N values at the sites with carcass densities over 500 fish/km were consistently high, while a value of δ ¹⁵N below zero was observed at only one site (Rusha River; δ ¹⁵N = −1.87‰). At this site, most adult pink salmon returned to limited locations near the estuary because steeper channel gradients acted as a migration barrier, resulting in the negative δ ¹⁵N value. Nevertheless, we concluded that our results showed evidence of the feedback of MDN to terrestrial vegetation, although the use of the δ ¹⁵N value as a terrestrial end member at spawning sites is limited. If the relationship between the enrichment index, which is expressed as the values using a mixing model, and salmon abundance was estimated, the availability of MDN in riparian ecosystems could possibly be evaluated and will lead to the establishment of escapement goals. An erratum to this article is available at .     

Land Use Effects on Mangrove Nutrient Status in Phang Nga Bay,Thailand

Tropical mangrove forests can play an important role in the functioning of adjacent marine ecosystems, by protecting them from an excess in land‐derived sediment and nutrients. The strength of this interaction may however depend on the nutrient status of the mangrove forest. This study related the nutrient status of eight mangrove forests in Phang Nga Bay (Thailand) to the land‐cover distributions in the upstream catchment areas. Nutrient status was assessed using indicators integrating over short (porewater and sediment nutrient composition) and long timespans (mangrove leaves and sesarmid crab tissue characteristics). Using multivariate statistics (PCA analysis), these nutrient status data were then related to the land cover data, which were obtained through the analysis of satellite imagery. Nutrient availability was lowest for mangroves in catchments with large natural vegetation cover and was elevated in catchments with increasing levels of anthropogenic influence. Furthermore, nutrient availability was significantly correlated with several forms of land use, including natural forest, rice paddies, cleared ground and urban areas. While all indicators supported these results, relationships were strongest for long‐term indicators. Information on the relationship between land use in the catchment area and mangrove nutrient status may be important for the effective management of this habitat, as well as adjacent marine systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Distribution and activity of hypolithic soil crusts in a hyperarid desert (Baja California, Mexico)

Widespread and ecologically important, biological soil crusts include those microbial communities living on the surface of the soil and those that live beneath semitranslucent rocks (a.k.a. hypolithic crusts). We examined the distribution, abundance, physiology, and potential soil N contributions of hypolithic, biological crusts in hyperarid ecosystems of the Baja California peninsula and islands in the midriff region of the Gulf of California, Mexico (Sonoran desert). Crusts were limited in distribution to areas with translucent quartz rocks less than 3 cm thick, were not found on areas of islands with seabird guano deposition, but covered as much as 1% (12,750 m²) of the surface area of one island. The percent of available rocks colonized by crusts was similar between the mainland (38%) and islands without seabird guano (26%). Carbon fixation rates in the field, which have not been previously reported, ranged between 0 and 1.23 μmol m⁻² s⁻¹, and in the lab ranged between 0.66 and 0.94 μmol m⁻² s⁻¹. Evidence of low rates of N fixation was inferred from δ ¹⁵N values of crust and soil. Hypolithic crusts were found to have minimal, if any, influence on soil salinity, pH, and , but may represent up to 14% of the biomass of primary producers on these islands and provide C and N to the belowground and possibly aboveground heterotrophic communities where crusts exist. The results of this study suggest a limited but potentially important contribution of hypolithic soil crusts to hyperarid ecosystems.     

Partitioning soil carbon responses to warming: Model-derived guidance for data interpretation

Parallel incubation at different temperatures combined with ¹³CO₂ efflux has been used to distinguish the temperature sensitivity of labile soil carbon (young soil carbon derived from newly-introduced vegetation) from that of resistant soil carbon (old, native vegetation-derived soil carbon). But we believe that this approach to assessing relative temperature sensitivities is confounded by differential rates of depletion of labile and resistant soil carbon at different temperatures. Here we employ a simple decomposition model to demonstrate potential pitfalls in interpreting ¹³CO₂ efflux data that inevitably, and potentially erroneously, lead to the conclusion that decomposition of resistant soil carbon pools is more temperature sensitive than labile pools. We conclude by offering a new approach for interpreting these data that eliminates this potential bias.     

Isotopic fractionation by saprotrophic microfungi: Effects of species, temperature and the age of colonies

Saprotrophic fungi represent an important resource for a number of fungivorous and omnivorous soil animals, but little is known about the patterns of isotopic fractionation by soil fungi. We grew five common species of saprotrophic microfungi in laboratory cultures on simple artificial substrate based on carbohydrates derived either from C3 or C4 plants. Fungal cultures were kept at 15, 20 or 25 °C. Isotopic composition of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) in bulk fungal tissue was determined after 11, 21 and 32 days. The fractionation of carbon and nitrogen stable isotopes was species-specific, but generally did not differ in C3- and C4-based growth media. The Zygomycete Mucor plumbeus did not differ in δ¹³C from the carbon source used, though Ascomycetes (Alternaria alternata, Cladosporium cladosporioides, Trichoderma harzianum and Ulocladium botrytis) were depleted in heavy carbon relative to the carbon source by 0.5-0.9‰. Three species were significantly depleted in ¹⁵N relative to the sodium nitrate that was used as a single source of nitrogen. In all species, δ¹⁵N but not δ¹³C tended to increase with the age of fungal colonies. The effect of temperature on δ¹⁵N was weak and inconsistent in different species. In contrast, all fungi except T. harzianum accumulated more ¹³С at 25 °C than at 15 °C. The overall variation in the isotopic signatures of saprotrophic fungi growing in identical conditions reached 8‰ for δ¹⁵N and 2.5‰ for δ¹³C due to species-specific differences in the isotopic fractionation and the age of individual fungal colonies. This variation should be incorporated into the interpretation of the isotopic composition of fungivorous soil animals.     

Plant species influence microbial diversity and carbon allocation in the rhizosphere

Plant species effects on microbial communities are attributed to changes in microbial community composition and biomass, and may depend on plant species specific differences in the quality of resources (carbon) inputs. We examined the idea that plant-soil feedbacks can be explained by a chance effect, which is the probability of a highly productive or keystone plant species is present in the community and will influence the functions more than the number of species per se. A ¹³C pulse labelling technique was applied to three plant species and a species mixture in a greenhouse experiment to examine the carbon flow from plants to soil microbial communities. The ¹³C label was given as CO₂ to shoots of a legume (Lotus corniculatus), a forb (Plantago lanceolata), a grass (Holcus lanatus) and a mixture of the three species. Microbial phospholipid fatty acids (PLFA) was analysed in order to determine the biomass and composition of the soil microbial community. The incorporation of the stable isotope into soil microorganisms was determined through GC-IRMS analyses of the microbial PLFAs. Plant species identity did not influence the microbial biomass when determined as total carbon of microbial phospholipid fatty acids. However, the labelled carbon showed that the grass monoculture (H. lanatus) and the plant mixture allocated more ¹³C into bacteria and actinomycete biomass than the other plant species. H. lanatus monocultures had also the highest amounts of ¹³C allocated to AM-fungi and saprophytic fungi. The carbon allocation from plants to soil microorganisms in a plant species mixture can thus be explained by the presence of a highly productive species that influence soil functions.     

Carbon input belowground is the major C flux contributing to leaf litter mass loss: Evidences from a C labelled-leaf litter experiment

Partitioning of the quantities of C lost by leaf litter through decomposition into (i) CO₂ efflux to the atmosphere and (ii) C input to soil organic matter (SOM) is essential in order to develop a deeper understanding of the litter-soil biogeochemical continuum. However, this is a challenging task due to the occurrence of many different processes contributing to litter biomass loss. With the aim of quantifying different fluxes of C lost by leaf litter decomposition, a field experiment was performed at a short rotation coppice poplar plantation in central Italy. Populus nigra leaf litter, enriched in ¹³C (δ¹³C ∼ +160‰) was placed within collars to decompose in direct contact with the soil (δ¹³C ∼ −26‰) for 11 months. CO₂ efflux from within the collars and its isotopic composition were determined at monthly intervals. After 11 months, remaining litter and soil profiles (0-20 cm) were sampled and analysed for their total C and ¹³C content. Gas chromatography (GC), GC-mass spectrometry (MS) and GC-combustion-isotope ratio (GC/C/IRMS) were used to analyse phospholipid fatty acids (PLFA) extracted from soil samples to identify the groups of soil micro-organisms that had incorporated litter-derived C and to determine the quantity of C incorporated by the soil microbial biomass (SMB). By the end of the experiment, the litter had lost about 80% of its original weight. The fraction of litter C lost as an input into the soil (67 ± 12% of the total C loss) was found to be twice as much as the fraction released as CO₂ to the atmosphere (30 ± 3%), thus demonstrating the importance of quantifying litter-derived C input to soils, in litter decomposition studies. The mean δ¹³C values of PLFAs in soil (δ¹³C = −12.5‰) showed sustained incorporation of litter-derived C after one year (7.8 ± 1.6% of total PLFA-C). Thus, through the application of stable ¹³C isotope analyses, we have quantified two major C fluxes contributing to litter decomposition, at macroscopic and microscopic levels.     

Evidence of Hierarchical Patch Dynamics in an East African savanna?

The Hierarchical Patch Dynamics Paradigm provides a conceptual framework for linking pattern, process and scale in ecosystems, but there have been few attempts to test this theory because most ecological studies focus on only one spatial scale, or are limited in their temporal scope. Here I use palaeoecological techniques (analysis of fossil pollen and stable carbon isotopes) to compare vegetation heterogeneity in an east African savanna at three spatial scales, over hundreds of years. The data show that patterns of vegetation change are different at the three spatial scales of observation, and suggest that different ecological processes dominate tree abundance at micro, local and landscape scales. Interactions between plants, disturbance (e.g., by fire and herbivores), climate and soil type may influence tree density at differing spatial and temporal scales. This hierarchical explanation of savanna vegetation dynamics could inform future biodiversity conservation and management in savannas. This revised version was published online in July 2006 with corrections to the Cover Date.