The effect of macro-invertebrates and plant litter of different quality on the release of N from litter to plant on alpine pastureland

The effect of ¹⁵N-labelled litter of different quality (Luzula sylvatica, a grass species, Vaccinium gaultheroides, a deciduous dwarf shrub, and Calluna vulgaris, a hardy dwarf shrub) and the presence of macro-decomposers (Lumbricus rubellus, Lumbricidae, and Enantiulus nanus, Diplopoda) on the growth of Dactylis glomerata (Poaceae), a grass species abundant on alpine pastureland, was investigated. After 4 months, the presence of soil animals significantly increased litter mass loss of L. sylvatica, V. gaultheroides and C. vulgaris by 27%, 11% and 40%, respectively. Soil animals generally reduced microbial biomass but significantly increased it in treatments where either L. sylvatica or C. vulgaris was present. The presence of soil animals significantly increased shoot and root biomass of D. glomerata by 48% and 64%, respectively. L. rubellus increased the transfer of ¹⁵N from the litter into plants. We conclude that macro-decomposers increased nutrient mobilization and plant uptake of nutrients mineralized from recalcitrant litter materials. Litter of L. sylvatica contributed most to the ¹⁵N uptake by D. glomerata, suggesting that litter quality is crucial for the cycling of nutrients on abandoned alpine pastureland.     

Microbial communities on litter of managed and abandoned alpine pastureland

Degradation of litter strongly depends on its chemical composition, which in turn affects the associated microorganisms. In the alpine region, the abandonment of pastures leads to a rigorous change in the composition of the litter layer, shifting from grass to highly recalcitrant dwarf shrub litter, thus affecting the food web and decomposition processes. Three species belonging to different functional groups (grasses, herbs, dwarf shrubs) and indigenous on managed and/or abandoned alpine pastureland were selected for this study, the annual grass Dactylis glomerata, characteristic for managed sites, Trollius europaeus as representative of a herb common on both managed and abandoned areas, and Vaccinium vitis-idea as typical dwarf shrub arising on abandoned alpine pastures. Litter bags containing litter material from either one of the three plant types were incubated on the soil surface for 9 weeks. DNA was extracted from the substrate, amplified and analysed using PCR–DGGE. Fingerprinting analyses of bacterial and fungal communities showed that the microbiota attached to the litter differed considerably depending on the plant type. Nonetheless, specific bacterial bands were present in the fingerprinting patterns of all three litter types. Identifying these organisms applying the 16S rRNA clone libraries revealed the dominance of bacteria from the class Bacteroidetes, representing two thirds of all identified band positions.     

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.     

Abundance and trophic structure of macro-decomposers on alpine pastureland (Central Alps, Tyrol): effects of abandonment of pasturing

On four differently managed and abandoned alpine meadows and pastures densities, biomasses and ¹⁵N signatures of the macrofauna were assessed to evaluate the structural and functional changes of the decomposer food webs. The composition of the macrofauna decomposer community changes remarkably after the abandonment of alpine meadows and pastures. Lumbricus rubellus functions as key primary decomposer on alpine meadows and pastures whereas on abandoned sites other primary decomposers including Dendrobaena octaedra, Cylindroiulus meinerti, C. fulviceps and diptera larvae become more important. Decomposer species, such as Enantiulus nanus, presumably function as both primary and secondary decomposers and endogeic earthworms, such as Octolasion lacteum and Aporrectodea rosea, uniformly function as secondary decomposers. Abandonment of pasturing causes a shift in the composition of the macrofauna and the newly established fauna is unable to process and translocate the litter materials produced by the plants of the abondoned sites.     

Compound specific plant amino acid δ15N values differ with functional plant strategies in temperate grassland

The distribution and natural abundance isotopic (δ¹⁵N) content of whole tissue and individual amino acids in plants in a temperate grassland were determined using ion chromatography (IC), continuous flow‐isotope ratio mass spectrometry (CF‐IRMS), and gas chromatography‐combustion‐isotope ratio mass spectrometry (GC‐C‐IRMS). The results showed that the selected plants (Lolium perenne, Juncus effusus, and Brachythecium rutabulum) differed in their amino acid content and distribution from the parent grassland soil. Bulk and individual amino acid δ¹⁵N isotope signatures were different between the plants, which concurred with their functional strategy in relation to the relative acquisition of available N sources. The individual amino acid δ¹⁵N values of histidine and phenylalanine could be used to differentiate between the three plant species.     

Seasonal variation in nitrogen isotopic composition of bog plant litter during 3 years of field decomposition

In this study, we describe the seasonal variation in ¹⁵N abundance in the litter of two Sphagnum species and four vascular plant species during 3 years of field decomposition in an Italian Alpine bog. Litter bags were periodically retrieved at the end of summer and winter periods, and the δ¹⁵N in residual litter was related to mass loss, litter chemistry, and climatic conditions. In Sphagnum litter, higher rates of decomposition during summer months were associated with an increase of δ¹⁵N probably due to the incorporation of microbial organic compounds rich in ¹⁵N. The litter of Eriophorum vaginatum and Carex rostrata was characterized by a decrease of δ¹⁵N, so that the final signature was significantly lower than in initial litter. On the other hand, the residual litter of Potentilla erecta and Calluna vulgaris was characterized by a final δ¹⁵N higher than in initial litter. Our data reported a seasonality of ¹⁵N abundance in the residual litter of Sphagnum species, but not in that of vascular plant species, thus highlighting the role of differences in litter chemistry.     

Species-dependent partitioning of C and N stable isotopes between arbuscular mycorrhizal fungi and their C3 and C4 hosts

Natural ¹³C and ¹⁵N abundances of mycorrhizal fungi are increasingly used in ecology but reference data on arbuscular mycorrhizal fungi (AMF) are scarce. In experiments with nine phylogenetically dispersed AMF strains inoculated on leek (C3 plant) and sorghum (C4) in pot cultures, we measured the ¹³C/¹²C and ¹⁵N/¹⁴N ratios in shoots, roots, AMF spores as well as carbon isotope signature of the C16:1ω5 fatty acid (FA), which is diagnostic for AMF. Spore δ¹³C values varied among AMF strains on any given host. They were significantly lower than shoot δ¹³C for sorghum (−2.5‰ on average) while for leek, no clear C isotope partitioning between spores and host shoots was observed. The FA C16:1ω5 fatty acids were more ¹³C-depleted than spores, without correlation with spore δ¹³C values. For both, sorghum and leek, spore δ¹⁵N was higher (+1–2‰ on average) than for shoots. We found no evidence that isotopic partitioning between the partners drives ¹³C and ¹⁵N abundances in plant–AMF symbiosis. Mycorrhizal roots displayed relatively high δ¹³C typical for heterotrophic organs, and not a mix between AMF and plant signatures. Interestingly, inoculation slightly decreased shoot δ¹³C on leek but not on sorghum, as compared with non-mycorrhizal plants, suggesting that AMF improved the plant's water status, a parameter affecting the δ¹³C of C3 but not C4 plants. Phylogenetically closer AMF displayed more similar spore δ¹³C and induced similar ¹³C and ¹⁵N abundances on leek shoots, but this observation was not confirmed for sorghum. Plant and AMF isotopic abundances hardly correlated with other parameters related to plant development, mineral nutrition or root mycorrhizal colonisation, and these correlations were never consistent between sorghum and leek. Thus, isotopic abundances in plant–AMF symbiosis were rather constrained by each AMF/plant interaction. Nevertheless, our data provide a valuable reference for future investigations of AMF communities and AM symbiosis in situ.     

Roots from beech (Fagus sylvatica L.) and ash (Fraxinus excelsior L.) differentially affect soil microorganisms and carbon dynamics

Knowledge about the influence of living roots on decomposition processes in soil is scarce but is needed to understand carbon dynamics in soil. We investigated the effect of dominant deciduous tree species of the Central European forest vegetation, European beech (Fagus sylvatica L.) and European ash (Fraxinus excelsior L.), on soil biota and carbon dynamics differentiating between root- and leaf litter-mediated effects. The influence of beech and ash seedlings on carbon and nitrogen flow was investigated using leaf litter enriched in ¹³C and ¹⁵N in double split-root rhizotrons planted with beech and ash seedlings as well as a mixture of both tree species and a control without plants. Stable isotope and compound-specific fatty acid analysis (¹³C-PLFA) were used to follow the incorporation of stable isotopes into microorganisms, soil animals and plants. Further, the bacterial community composition was analyzed using pyrosequencing of 16S rRNA gene amplicons. Although beech root biomass was significantly lower than that of ash only beech significantly decreased soil carbon and nitrogen concentrations after 475 days of incubation. In addition, beech significantly decreased microbial carbon use efficiency as indicated by higher specific respiration. Low soil pH probably increased specific respiration of bacteria suggesting that rhizodeposits of beech roots induced increased microbial respiration and therefore carbon loss from soil. Compared to beech δ¹³C and δ¹⁵N signatures of gamasid mites in ash rhizotrons were significantly higher indicating higher amounts of litter-derived carbon and nitrogen to reach higher trophic levels. Similar δ¹³C signatures of bacteria and fine roots indicate that mainly bacteria incorporated root-derived carbon in beech rhizotrons. The results suggest that beech and ash differentially impact soil processes with beech more strongly affecting the belowground system via root exudates and associated changes in rhizosphere microorganisms and carbon dynamics than ash.     

Does stable isotope analysis separate transgenic and traditional corn (Zea mays L.) detritus and their consumers?

Transgenic corn crops (including the Bt variety) are expanding rapidly worldwide, and the large amounts of cultural residues remaining after harvest pose questions about the fate of this novel source of plant detritus in soil. To verify whether transgenic and conventional corn litters were different in their isotopic signatures, the ¹³C and ¹⁵N stable isotopes of different portions of Bt and non-Bt Aristis corn plants after harvest were analysed. Laboratory feeding experiments were then conducted to assess the transfer of corn isotopic signals to detritivores using the isopod Trachelipus sp., reared on either Bt or non-Bt corn dead leaves as the only food source. δ¹⁵N differed significantly between Bt and non-Bt corn in kernels and stems, whereas both δ¹⁵N and δ¹³C were different in leaves before and after exposure to isopods. During feeding, the N and C isotopic signatures of isopods shifted towards the diet values. Significant differences existed both between sampling dates and corn treatments. The results suggest that detritus from transgenic and conventional corn crops may have different isotopic signatures and the isotopic differences can persist through the trophic levels, making corn detritus feeders suitable sentinel species for the Bt corn isotopic signal in agroecosystems.     

Do interactions with soil organisms mediate grass responses to defoliation?

Defoliation-induced changes in grass growth and C allocation are known to affect soil organisms, but how much these effects in turn mediate grass responses to defoliation is not fully understood. Here, we present results from a microcosm study that assessed the role of arbuscular mycorrhizal (AM) fungi and soil decomposers in the response of a common forage grass, Phleum pratense L., to defoliation at two nutrient availabilities (added inorganic nutrients or no added nutrients). We measured the growth and C and N allocations of P. pratense plants as well as the abundance of soil organisms in the plant rhizosphere 5 and 19 d after defoliation. To examine whether defoliation affected the availability of organic N to plants, we added ¹⁵N-labelled root litter to the soil and tracked the movement of mineralized ¹⁵N from the litter to the plant shoots.When inorganic nutrients were not added, defoliation reduced P. pratense growth and root C allocation, but increased the shoot N concentration, shoot N yield (amount of N in clipped plus harvested shoot mass) and relative shoot N allocation. Defoliation also reduced N uptake from the litter but did not affect total plant N uptake. Among soil organisms, defoliation reduced the root colonization rates of AM fungi but did not affect soil microbial respiration or the abundance of microbe-grazing nematodes. These results indicate that interactions with soil organisms were not responsible for the increased shoot N concentration and shoot N yield of defoliated P. pratense plants. Instead, these effects apparently reflect a higher efficiency in N uptake per unit plant mass and increased relative allocation of N to shoots in defoliated plants. The role of soil organisms did not change when additional nutrients were available at the moment of defoliation, but the effects of defoliation on shoot N concentration and yield became negative, apparently due to the reduced ability of defoliated plants to compete for the pulse of inorganic nutrients added at the moment of defoliation.Our results show that the typical grass responses to defoliation—increased shoot N concentration and shoot N yield—are not necessarily mediated by soil organisms. We also found that these responses followed defoliation even when the ability of plants to utilize N from organic sources, such as plant litter, was diminished, because defoliated plants showed higher N-uptake efficiency per unit plant mass and allocated relatively more N to shoots than non-defoliated plants.     

Influence of elevated CO2 and GM barley on a soil mesofauna community in a mesocosm test system

We hypothesized that the combined effect of rising levels of atmospheric carbon dioxide (CO₂) and increasing use of genetically modified (GM) crops in agriculture may affect soil food-webs. So we designed a study for the assessment of the effects of elevated CO₂ (eCO₂) concentrations and GM barley on a soil-mesofauna community employing a 2nd tier mesocosm test system. The GM barley, Hordeum vulgare cv. Golden Promise, had a modified content of amino acids and it was compared with three non-GM barley cultivated varieties including the isogenic line. Our mesocosm experiment was conducted in a greenhouse at ambient (aCO₂) and eCO₂ (+80 ppm) levels and included a multispecies assemblage of Collembola, Acari and Enchytraeidae with either a GM or conventional spring barley varieties. To detect food-web changes we added dried maize leaves naturally enriched in δ¹³C and δ¹⁵N relative to the soil substrate. Soil, plants and animals were collected after five and eleven weeks. We found that the eCO₂ concentration did not affect the plant biomass, but the predatory mite and two collembolan species showed significantly lower abundances at eCO₂. The densities of three collembolan species (Folsomia fimetaria, Proisotoma minuta and juveniles of Mesaphorura macrochaeta) was significantly lower in the GM treatment compared to some of the non-GM varieties. F. fimetaria was less abundant in presence of GM barley compared to the cultivated barley variety “Netto” at both CO₂ levels, while the density of P. minuta was significantly reduced with the GM barley compared to variety “Netto” at aCO₂ and the isogenic variety at eCO₂. Maize litter acted as a food source for the community, as it was revealed by δ¹³C values in microarthropods. Microarthropod δ¹³C decreased over time, which indicates a diet change of the species towards carbon derived from barley, due to maize litter decomposition. The industrially produced CO₂ gas also had a role as an isotopic marker, as the different δ¹³C values were reflected in the barley and in the collembolan species. GM barley did not affect δ¹³C and δ¹⁵N values of soil animals indicating that the overall trophic structure of the mesofauna community was not changed compared to the non-GM cultivated varieties. The mesocosm methodology integrating stable isotope analysis demonstrates the potential of the multi-species mesocosm as a tool to detect and track changes in the soil trophic interactions in response to environmental pressures, climate and novel agricultural crops.     

黑岱沟露天煤矿优势植物叶片及枯落物生态化学计量特征

以准格尔黑岱沟露天煤矿复垦区8种优势种植物的叶片及对应叶片枯落物为研究对象,通过对其碳、氮、磷、钾含量及生态化学计量学特征的研究,探讨退化生态系统植物内稳性、不同植被类型与NP限制率的关系,以期为黄土高原植被恢复与重建以及不同物种的合理配置提供合理建议,为完善生态化学计量学理论提供支撑。结果表明,不同植物叶片C、N、P、K的变化范围分别为418.2~564.1、7.19~33.21、0.82~2.37、4.98~32.77 g kg-1。不同植物叶片的生态化学计量学特征N∶P、P∶K、N∶K、C∶N、C∶P、C∶K比变化范围分别为7.16~22.45、0.036~0.23、0.70~4.55、14.92~64.44、218.54~557.36、12.78~121.14。该地区3种植被类型钾含量差异显著,草本植物叶片钾含量显著高于乔木林和灌木林。不同植被类型植物叶片N∶P比表现为灌木(18.86)草本(15.44)乔木(14.68),说明该区灌木林主要受P含量的限制,草本和乔木植物主要受N、P含量共同限制,该地区植被恢复应该以灌木为主。     

Symbiotic nitrogen fixation in the alpine community of a lichen heath of the Northwestern Caucasus Region (the Teberda Reserve)

The symbiotic fixation of atmospheric nitrogen by leguminous plants in the alpine community of a lichen heath at the Teberda State Biosphere Reserve is well adapted to low soil temperature characteristic for the altitude of 2800 m a.s.l. For the determination of the N fixation by isotopic methods (the method of the natural ¹⁵N abundance and the method of isotopic ¹⁵N dilution), Trifolium polyphyllum was taken as the control plant. This plant was used as it does not form symbiosis with the nitrogen-fixing bacteria in the highlands of the Northern Caucasus Region. The contribution of the N fixation to the N nutrition of different leguminous plant species as determined by the natural ¹⁵N abundance method amounted to 28–73% at δ¹⁵N₀ = 0‰ and 46–117% at δ¹⁵N₀ = −1‰; for the determination of the N fixation by the method of the isotopic label’s dilution, it was 34–97%. The best correlation of the results obtained by these two isotopic methods was observed for the natural fractionation of the N isotopes in the course of the N fixation in the range of −0.5 to −0.7‰. The determination of the nitrogenase activity of the roots by the acetylene method confirmed the absence of N fixation in T. polyphyllum and its different contribution to the N nutrition of different species of leguminous plants.     

Patterns of natural N in soils and plants from chemically and organically fertilized uplands

Diagnostic tests for organic production of crops would be useful. In this study, the difference in natural ¹⁵N abundances (δ¹⁵N) of soils and plants between fertilizer-applied upland (FU) and compost-applied upland (CU) fields was investigated to study using δ¹⁵N as a marker of organic produce. Twenty samples each of soils and plants were collected from each field in early summer after applying fertilizer or compost. The δ¹⁵N of fertilizers and composts was −1.6±1.5‰ (n=8) and 17.4±1.2‰ (n=10), respectively. The δ¹⁵N of total soil-N was significantly (P<0.05) higher in CU fields (8.8±2.0‰) than in FU fields (5.9±0.7‰) due to long-term continuous application of ¹⁵N-enriched compost, as indicated by a positive correlation (r=0.62) between N content and δ¹⁵N of total soil-N. The NO₃⁻ pool of CU soils (11.6±4.5‰) was also significantly (P<0.05) enriched in ¹⁵N compared to FU soils (4.7±1.1‰), while the ¹⁵N contents of NH₄⁺ pool were not different between both soils. Compost application resulted in ¹⁵N enrichment of plants; the δ¹⁵N values were 14.6±3.3‰ for CU and 4.1±1.7‰ for FU fields. These results showed that long-term application of compost resulted in a significant ¹⁵N-enrichment of soils and plants relative to fertilizer. Therefore, this study suggested that δ¹⁵N could serve as promising indicators of organic fertilizers application when used with other independent evidence. However, further studies under many conditions should be conducted to prepare reliable δ¹⁵N guidelines for organic produce, since the δ¹⁵N of inorganic soil-N and plant-N are influenced by various factors such as soil type, plant species, the rate of N application, and processes such as mineralization, nitrification, and denitrifcation.     

Transfer of N fixed by a legume tree to the associated grass in a tropical silvopastoral system

Below-ground transfer of nitrogen (N) fixed by legume trees to associated non-N₂-fixing crops has received little attention in agroforestry, although the importance of below-ground interactions is shown in other ecosystems. We used ¹⁵N natural abundance to estimate N transfer from the legume tree Gliricidia sepium (Jacq.) Kunth ex Walp. to C₄ grass Dichanthium aristatum (Poir.) C.E. Hubb. in a silvopastoral system, where N was recycled exclusively by below-ground processes and N₂ fixation by G. sepium was the sole N input to the system. Finding a suitable reference plant, a grass without contact with tree roots or litter, was problematic because tree roots invaded adjacent grass monocrop plots and soil isotopic signature in soil below distant grass monocrops differed significantly from the agroforestry plots. Thus, we used grass cultivated under greenhouse conditions in pots filled with agroforestry soil as the reference. A model of soil ¹⁵N fractionation during N mineralization was developed for testing the reliability of that estimate. Experimental and theoretical results indicated that 9 months after greenhouse transplanting, the percentage of fixed N in the grass decreased from 35% to <1%, due to N export in cut grass and dilution of fixed N with N taken up from the soil. The effect of soil ¹⁵N fractionation on the estimate of the reference value was negligible. This indicates that potted grass is a suitable reference N transfer studies using ¹⁵N natural abundance. About one third of N in field-grown grass was of atmospheric origin in agroforestry plots and in adjacent D. aristatum grassland invaded by G. sepium roots. The concentration of fixed N was correlated with fine root density of G. sepium but not with soil isotopic signature. This suggests a direct N transfer from trees to grass, e.g. via root exudates or common mycorrhizal networks.     

Carbon and nitrogen transfer in leaf litter mixtures

The decomposition rate of litter mixtures can differ from that expected on the basis of the decomposition rate of the individual components. This difference may be linked to nitrogen (N) transfer from high-N to low-N components. Transfer of N is probably also associated with transfer of C, but the extent and direction of this C transfer are unknown. This study examined transfer and loss in laboratory microcosms of C and N from two mixed litter species (Scots pine, Pinus sylvestris L. and maize, Zea mays L.), which have natural isotopic differences in ¹³C. Half the material was ¹⁵N-labelled and the plants were fertilised or unfertilised. Substantial bidirectional transfer of C and N occurred between the litters, with net transfer of C from pine to maize litter and net transfer of N from high-N to low-N litter. Mixtures of fertilised and unfertilised plant litter showed higher than expected C losses and net transfer of N. Mixtures with litters from the same fertilisation treatment had small or insignificant net transfer of N and their C losses did not differ from values estimated using the decomposition rates of the pure litters.     

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.     

Isotopic discrimination during long-term decomposition in an arid land ecosystem

Discrimination in carbon and nitrogen isotopes of decomposing plant litter in the northern Chihuahuan Desert was determined for a 5-year period. Factors influencing isotopic change were assessed from inter-species comparisons of litter chemistry, mass loss patterns, and isotope values of associated soil. Average δ¹⁵N_litter values of buried roots increased 1.2 and 2.6‰ for Big Blue Stem (Schizachyrium gerardi, grass) and Varital (Drypetes glauca, hardwood) during the study, respectively. Small but inconsistent variations were observed for Slash Pine (Pinus elliotii, conifer) roots. Average δ¹⁵N values of wooden dowels from Ramin (Gonystlylus bancanus, hardwood) increased ca. 2.0‰ during years 1–4, and then decreased slightly during year 5. Changes in δ¹⁵N_litter were independent of N content, and may reflect microbial fractionation or preferential retention of ¹⁵N enriched substrates. Surprisingly, there was no clear relationship between litter N dynamics and C/N ratios. There were no discernable changes in δ¹³C_litter values for Gonystlylus bancanus and Pinus elliotii. Average δ¹³C_litter values for Schizachyrium gerardi decreased ∼2.0‰ during years 0–2 and then increased slightly. In contrast, average δ¹³C_litter values for Drypetes glauca increased ∼0.5‰ from years 0–1 then remained relatively constant until decreasing slightly in year 5. δ¹³C_litter discrimination may have been masked by interfering δ¹³C fractionations or feedbacks between decomposers and litter chemistry. Our data indicate that isotopic discrimination is characteristic of early litter decay stages. These results may highlight aspects of isotope discrimination and nutrient cycling unique to arid land environments. Additional studies will be needed to confirm this.     

Trophic fractionation (ΔN) in Collembola depends on nutritional status: A laboratory experiment and mini-review

Isotopic studies in temperate forest soils suggested that collembolans occupy a broad range of trophic niches. This notion is mainly based on the wide (up to 8-9‰) range of δ¹⁵N signatures of different species. The interpretation of these data depends strongly on the correct estimation of nitrogen isotope fractionation per trophic level. It is usually assumed that a consumer is on average enriched in ¹⁵N by about 3.4‰ relative to its diet. However, trophic fractionation is not uniform across different animal taxa and different tissues, and varies with a range of factors, including quality of the diet and the nutritional status of animals. We performed a laboratory experiment aiming to compare the trophic fractionation of nitrogen isotopes (Δ¹⁵N) in different Collembola species fed with three fungi of different palatability. Collembolans were fed with a single-species fungal diet for 75 days. There was a positive correlation between the whole body C:N ratio and reproduction rates of collembolans. We therefore used the C:N ratio as a proxy of nutritional status. In all species of collembolans, the trophic fractionation decreased with increased whole body C:N ratio, and this factor explained most of the within-species variation in Δ¹⁵N. The analysis of published data on the trophic fractionation of collembolans in laboratory experiments confirmed this conclusion. However, the mean trophic fractionation in most collembolan species studied in the laboratory was fairly similar. In contrast, field studies have documented a consistent difference in δ¹⁵N among different families of collembolans. In particular, in nearly all published cases Onychiuridae had δ¹⁵N higher than Isotomidae or Entomobryidae. A sharp contrast between laboratory-based estimates of the trophic fractionation (generally uniform in different species and families) and field data (which show a consistent difference in δ¹⁵N among different species and even families of collembolans) confirms that δ¹⁵N values of field-collected animals bear important information on the trophic position occupied by a particular species, and are not heavily affected by the species-specific differences in trophic fractionation. As the Δ¹⁵N in collembolans depends on their nutritional status, we suggest that the C:N values should be reported along with isotopic data to allow cross-study comparisons.     

Niche differentiation among neotropical soldierless soil-feeding termites revealed by stable isotope ratios

Termites represent one of the most abundant belowground animal taxa in tropical rainforests, where their species richness is much higher than in any other ecosystem. This high diversity in soil ecosystems is however difficult to explain by classical Hutchinsonian niche theory, as there is little evidence for spatial or temporal separation between species. Using δ¹³C and δ¹⁵N isotopic ratios, we tested if resource partitioning along the humification gradient occurs in neotropical soldierless termites of the Anoplotermes-group. Two distinct sites were investigated to check if interspecific differences are transposable between sites. Significant differences in δ¹⁵N were found between species of the Anoplotermes-group. Although some species displayed higher intersite δ¹⁵N variation than others, species-average δ¹⁵N values for both sites were highly correlated, showing that sympatric soldierless soil-feeding termites feed on distinct components of the soil. Our data also suggest that some species are more likely to shift along this gradient than others, in response to overall habitat conditions or to the presence of competitors. Feeding niche differentiation can therefore account for the high species richness and diversity of soldierless soil-feeding termites in neotropical rainforests.