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Litter bags are often used to determine the impact of soil animals on litter decomposition rates through the use of varied mesh sizes that exclude soil animals on the basis of body size. However, concerns have been raised regarding confounding factors that can co-vary with mesh size (e.g., microclimatic differences and leaching losses) and that might affect mass loss from litter bags independent of soil animal effects. To explore if these factors may interfere with the interpretation of studies that have used litter bags of varying mesh sizes, we quantified the effects of mesh size (50 μm, 0.4 mm and 1.0 mm aperture) on litter mass loss in the absence of soil animals. Further we measured potential microclimate (temperatures, light transmission and moisture) differences that may co-vary with mesh size. In addition, we quantified litter mass losses through litter fragmentation and leaching. Litter mass loss in the absence of soil animals did not differ between mesh sizes, suggesting that the use of the different meshes does not generate confounding factors that directly influence mass loss rates in the absence of animals. Microclimatic differences between litter bags of different mesh sizes were minimal and revealed slightly warmer (0.7 °C) temperatures and easier water entry for the fine than the coarse mesh bags, but no effect on litter evaporation rates. Light transmittance was largest for the fine mesh (79%) compared to the medium (60%) and coarse mesh (73%). The impact of UV on litter mass loss was 1% higher in the coarse mesh. Loss of fragmented litter was considerably higher from the coarse mesh (93%) than the fine mesh bags (17%). In conclusion, different mesh sizes can be used to reliably quantify the role of soil animals in litter mass loss from litter bags. The greater litter mass loss from litter bags with coarser mesh sizes is however caused by the soil animals both promoting litter decomposition and greater litter fragmentation (with the undecomposed fragments then being lost from the bags). Additional approaches are needed to determine the relative importance of these two effects of soil animals on litter mass loss from litter bags.
No abstract is provided for this article.
Elevational gradients are increasingly recognized as a valuable tool for understanding how community and ecosystem properties respond to climatic factors, but little is known about how plant traits and their effects on ecosystem processes respond to elevation. We studied the response of plant leaf and litter traits, and litter decomposability across a gradient of elevation, and thus temperature, in subarctic tundra in northern Sweden for each of two contrasting vegetation types, heath and meadow, dominated by dwarf shrubs and herbaceous plants respectively. This was done at each of three levels; across species, within individual species, and the plant community using a community weighted average approach. Several leaf and litter traits shifted with increasing elevation in a manner consistent with greater conservation of nutrients at all three levels, and the most consistent response was an increase in tissue N to P ratio. However, litter decomposition was less directly responsive to elevation because the leaf and litter traits which were most responsive to elevation were not necessarily those responsible for driving decomposition. At the community level, the response to elevation of foliar and litter traits, and decomposability, varied greatly among the two vegetation types, highlighting the importance of vegetation type in determining ecological responses to climatic factors such as temperature. Finally our results highlight how understanding the responses of leaf and litter characteristics of functionally distinct vegetation types, and the processes that they drive, to temperature helps provide insights about how future climate change could affect tundra ecosystems.
No abstract is provided for this article.
Nitrogen (N) and carbon (C) isotopic signatures (δ15N and δ13C) serve as powerful tools for understanding temporal changes in ecosystem processes
Bioassay experiments were used to assess the allelopathic potential of nodding thistle seeds on cumulative germination, speed of germination, and radicle elongation of five test species i.e., cocksfoot, perennial ryegrass, white clover, subterranean clover, and nodding thistle. Nodding thistle seeds inhibited germination of all species except subterranean clover in at least one of the bioassays used. Radicle elongation of all species was substantially inhibited in the presence of nodding thistle seeds both on filter paper and on soil. In soil systems, nodding thistle seeds inhibited radicle elongation of seeds over distances of up to 4-7 mm. A probability analysis demonstrated that a high density of nodding thistle seeds has the potential to exert a substantial negative effect on establishment of newly sown pasture. Nodding thistle seeds were either stimulated or largely unaffected by other seeds of their own species (except when seeds were touching) suggesting that the allelopathic potential of nodding thistle seeds may be an evolved mechanism to encourage establishment of thistle seedlings.
The influence of the herbicide glyphosate on two-way interactions between four fungal species was investigated on agar using direct opposition, volatile inhibitor and non-volatile inhibitor tests, and on barley straw using tests for competitive saprotrophic ability. The results indicated that glyphosate was often capable of shifting the direction of two-species interactions, but there was little agreement between the different tests. Reanalysis of particle colonization data presented earlier demonstrated that glyphosate frequently eliminated negative associations between the same four species, possibly by acting as a resource. It is concluded that interactions between pairs of fungi can be influenced directly by glyphosate (especially at high concentrations) and that this may influence soil fungal community structure.
An approach is presented for quantifying associations between species pairs over a scale of 0·5–1 mm by plating several 0·5–1 mm particles of washed organic matter on agar plates, and observing whether or not fungal species which emerge from the particles are distributed in a random manner with regard to each other. Most of the associations between the seven most abundant species in particles of organic matter from an agricultural soil were significantly negative. Co-occurrence of these species was weakly, but significantly, related to their correlation patterns across samples collected over a spatial scale of several metres.
Rooted cuttings of Kerner (<i>Vitis vinifera</i>), Michurinetz (<i>V. amurensis</i> X), and Okanagan Riesling (<i>V. labrus- cana</i> X), either virus-free or infected with <i>Rupestris</i> stem pitting virus (RSPV), were planted in 1987 in a randomized block experiment at Summerland, British Columbia (arid continental climate). Also, virus-free and RSPV-infected plants Madeleine Sylvaner, and Ortega (both <i>V. vinifera</i>) were planted in a completely randomized experiment at Saanichton Plant Quarantine Station, Sidney, BC (cool maritime climate). Yield components were not affected at the Summerland site, but weight of cane prunings were reduced in RSPV- infected Michurinetz vines in 1995-96. RSPV also lowered yield and clusters per vine of both cultivars in 1994 at Sidney, along with Ortega weight of cane prunings (1993 and 1994) and berry weight (1994). Berry titratable acidity (TA) at harvest was reduced at both sites in RSPV-infected vines, and pH was concomitantly increased. TA was consistently lower and the pH was always higher in the RSPV berries during veraison to harvest at Summerland. No apparent negative impact of RSPV was observed except delayed soluble solids (°Brix) accumulation in RSPV-infected Kerner and Okanagan Riesling vines at Summerland in 1991, and lower °Brix in Ortega RSPV berries at Sidney in 1993. In some seasons, fruit composition differences between virus-free and RSPV vines disappeared close to commercial maturity, but lower TA and higher pH persisted until harvest at the Summerland site in 1993. RSPV-infected vines at Sidney exhibited slightly delayed budburst in 1988-91 and 1994, but not 1993. These results suggest that RSPV has no major impact on grapevine growth and yield, and may advance fruit maturity in terms of TA and pH.
Root nodules from a variety of nodulated leguminous and non-leguminous plants were added to soil samples and the net release of mineral N was monitored periodically for 56 or 98 days. Over these periods, various nodules demonstrated either net N immobilisation, mineralisation, or immobilisation followed by mineralisation. No nodular tissue mineralised > 30% of its organic N content. Mineral N release from the nodules was not significantly related to their C to N ratios or organic N content (r = 0.410). Nitrogen fractionation data from these nodules indicated that the main forms of N present in combined form were α-amino N and hydrolysable unidentified N(HUN). Attempts to relate mineral N release to the forms of N present in the nodules indicated that nodule N mineralisation was significantly related to nodule concentration of NH4-N and hexosamine-N in combination (r =0.741). It was concluded that these two components are probably highly important in regulating mineral N release from decomposing root nodules.
Seabirds impose a high-nutrient, high-disturbance regime on the islands on which they nest, resulting in higher nutrient cycling rates, plant nutrient uptake and leaf nutrient content. On islands off the coast of New Zealand, seabird-dominated islands support greater densities of soil- and litter-dwelling consumer biota. We predicted that islands with high seabird densities would have higher levels of leaf damage as a result of higher densities of foliar consumers (herbivores and pathogens). Damage levels on leaves of six common tree species were compared between 9 islands with active seabird colonies and 10 islands with low seabird densities resulting from invasion by predatory rats. There were no consistent differences in leaf damage by chewing, mining, or phloem-feeding herbivores across plant species; pathogen damage was lower on islands with high seabird densities than on those with low densities, but this was driven by only two of the plant species. Instead, plant species differed in which of several possible damage types responded to seabird presence, and in which plant leaf traits responded to seabird-related environmental changes. Across plant species, those with more resource-acquisitive leaf traits such as high percent nitrogen and low structural investment experienced higher levels of chewing damage (which accounted for 66-100% of all damage), but not other damage types. We conclude that the fertilisation and disturbance regimes imposed by seabirds do not lead to consistent changes in consumer damage to plants, because of variable responses by both individual plant species and different consumer groups.
