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Abstract: The trophic preferences of soil invertebrates such as Collembola are often determined by the analysis of gut contents, or through visual observations of the location of individuals. As an alternative approach, two species of Collembola, Folsomia candida and Proisotoma minuta, were offered a choice of the soil fungus Cladosporium cladosporioides or the bacterial feeding nematode Panagrellus redivivus; each exhibited distinct fatty acid profiles and stable carbon isotopic compositions. Over 21 days, the fatty acids i15:0, i17:0, 18:1(n-7) and 18:2(n-6) all increased in abundance in both collembolan species consistent with direct routing from the nematode dietary choice which contained a high concentration of these components. Collembolan fatty acid δ 13C values increased by between 5.7 and 21.6‰ over 21 days reflecting those of the nematode diet. Therefore, both fatty acid profiles and δ 13C values were consistent with a strong feeding preference of F. candida and P. minuta for the nematodes over the offered fungi. In fact, neither collembolan species consumed any detectable amount of C. cladosporioides. Comparison of the δ 13C values of the 16:0 and 18:0 fatty acids (which are biosynthesised by the Collembola as well as directly incorporated from the diet) and the 16:1(n-7) and 18:2(n-6) components (which are not biosynthesised by the Collembola) demonstrated that the input of distinct pools of C can lead to large shifts in δ 13C values between diet and consumer. The fatty acids that were not biosynthesised by Collembola better reflected the δ 13C values of the diet helping to differentiate between biosynthesised and directly incorporated compounds; an important prerequisite in the interpretation of compound-specific δ 13C values in trophic behaviour tests. The combination of fatty acid distributions and δ 13C values is a significant improvement on traditional methods of examining feeding preferences, since it determines directly the assimilated dietary carbon rather than relying on indirect observations, such as the proximity of individuals to a defined food source. [Copyright &y& Elsevier]

Abstract: Combined compound-specific stable carbon isotopic methods and fatty acid abundance determinations have been used to examine feeding preferences and C allocation in organisms where direct observation of feeding is difficult. In order to examine the effect of differing diets on the δ 13C values of fatty acids and sterols of Collembola, the diets of two collembolan species, Folsomia candida and Proisotoma minuta, were switched from a yeast diet to one of four isotopically distinct diets, and the δ 13C values of the lipids monitored over the next 39d. Cholesterol remained the only sterol detected in both collembolan species, despite the diets containing widely differing sterol compositions. The δ 13C values of collembolan lipids recorded after long term feeding were often different to those of the same components in the diet, indicating that fractionation or partitioning occurs during digestion, assimilation and biosynthesis within the Collembola, thereby shifting consumer lipid δ 13C values away from those of the corresponding dietary components. The rates of change of δ 13C values differed among compounds, with half-lives ranging between 29min and 14d. Some of these differences appear to be related to the abundance of dietary components, such that fatty acids present in high abundance in the diet (e.g. 18:2(n−6)) were rapidly assimilated in high proportions into collembolan lipids, leading to a rapid change in δ 13C values. Similarly, isotopic turnover in the 16:1(n−7) fatty acid, present in the newly presented diets in only low abundances, was significantly correlated to the rate of removal of this component from the consumer fatty acid pool. The rates of change of δ 13C values in P. minuta lipids did not vary significantly with diet, whilst the rates of change of δ 13C values of lipids in F. candida were affected by the diets the Collembola consumed. Results of an experiment providing F. candida and P. minuta with two diets of different quality demonstrated that F. candida responded to the high quality diet with increased growth and fecundity, whilst P. minuta responded with increased fecundity only. Thus, the abilities of the two species to respond to diets of varying quality, amongst other factors, is concluded to lead to differences in the rates of change of δ 13C values reflecting differences in lipid turnover. [Copyright &y& Elsevier]

Abstract: To assess the potential of fatty acid (FA) compositions to act as biomarkers in the soil food web, two species of Collembola, Folsomia candida and Proisotoma minuta, were switched to four possible diets: Cladosporium cladosporioides (a common soil fungus), Panagrellus redivivus (a bacteria feeding nematode), Zea mays (maize) and Alnus glutinosa (alder). The change in FA content of the Collembola was observed over the following 39 days. The four diets produced significant shifts in the FA compositions of the Collembola, with P. redivivus causing the most extreme changes; Collembola fed P. redivivus gained complex FA compositions similar to those of the nematode diet. Changes in the relative abundances of some FAs were found to follow negative exponential curves, as the components either accumulated in, or were removed from, the FA pool in the Collembola; abundance half-lives varied between 0.5 and 22.4 days, indicating that Collembolan FA compositions changed readily with the input of new exogenous components. The results demonstrate that Collembolan FA compositions are influenced by diet, and that the abundances of FAs such as i15:0, i17:0 and 18:1(n-7) may be used as biomarkers of nematode consumption by Collembola. In contrast, the C20 polyunsaturated FAs cannot be used as biomarkers for nematode predation as Collembola possess the ability to biosynthesise high abundances of these compounds when not provided by the diet. [Copyright &y& Elsevier]

Abstract: This paper describes the NERC Soil Biodiversity Programme''s integrated approach to data, information management and analysis, from field sampling through to modelling the soil ecosystem. It discusses the handling of meta-data from sampling in the field, construction of an integrated database to provide the framework whereby project data and meta-data can be linked and analysed together, and the development of the web-based data discovery and delivery system that fronts the database. The paper then describes how such data can be synthesised into mathematical models, concentrating initially on the Stable Isotope Modelling (SIM) framework before moving on to discuss models incorporating temporal and spatial dynamics. The paper concludes with a discussion of the future challenges facing soil modelling, and the potential of the technologies developed by the programme in helping to meet these challenges. [Copyright &y& Elsevier]