Isotopic detection of recent photosynthate carbon flow into grassland rhizosphere fauna

In this study, we measured the incorporation of recent photosynthate-C inputs into active rhizosphere fauna (earthworms, enchytraeids, mites and collembolans) in an upland grassland soil under natural environmental conditions. This was achieved by means of a ¹³CO₂ pulse-chase experiment made during the growing season, followed by a 20-day dynamic sampling of soil fauna for ¹²C/¹³C analysis by IRMS. The effect of post-¹³C labelling defoliation (cutting) on fauna ¹²C/¹³C ratios was also examined.Results showed that earthworms made up over 93% of the extracted fauna biomass, while mites, collembolans and enchytraeids together accounted for less than 7%. All fauna groups showed evidence of tracer ¹³C in their tissues within a week of ¹³CO₂ pulse labelling in both control and cut treatments. Cutting significantly increased the amount of tracer ¹³C entering the organisms (P=0.0002). Similarly, the fauna group also had a significant effect (P=0.0001). Time did not have any effect on fauna ¹³C content between groups as differences were only significant at the last sampling occasion. The interaction time×animal group, however, had a significant effect (P=0.0054).Collembolans accounted for most of the tracer ¹³C measured within the fauna biomass, i.e. mean±standard deviation of 44.78±12.75% and 44.29±14.69% of fauna ¹³C in control and cut treatments, respectively. Mites and earthworms contained between 22.13% and 28.45%, and enchytraeids less than 6% of the tracer ¹³C. We conclude that, during the growing season, there was a rapid incorporation of recent photosynthate-C into rhizosphere mesofauna. This carbon transfer was most significantly increased by defoliation in mites and collembolans (P<0.01). These results provide evidence that soil foodweb carbon dynamics are not solely underpinned by detrital decomposition but are also affected by short-term plant rhizodeposition patterns.     

Nitrogen and cation mobilization by soil fauna feeding on leaf litter and soil organic matter from deciduous woodlands

Decomposing oak litter was incubated in laboratory microcosms and the effects of adding a variety of soil animals upon nitrogen and cation release were monitored. Various groups of macrofauna caused a marked increase in ammonium release with smaller increases in calcium, potassium and sodium leaching. Microfauna also had significant but much less marked effects upon nitrogen and cation release.The effects of different grazing intensities of the millipede Glomeris marginata on a variety of forest organic substrates show that the animals amplified existing patterns of nutrient release.     

A comparison of subjective and quantitative methods for detection of physitis in Thoroughbred foals

AIM: To compare subjective and quantitative methods of assessing physeal swelling of the distal third metacarpal (Mc3) and metatarsal (Mt3) bones of Thoroughbred foals. METHODS: The fetlocks of 16 Thoroughbred foals were filmed on five occasions over a period of 5 months and the video images were transferred to computer and digitised. The rate of change of each of the medial and lateral contours of the distal Mc3 and Mt3 bones was determined by calculating values for the second derivative of the contour. The degree of physeal swelling at the same site was subjectively assessed and allocated a grade based on an ordinal scale of severity. The maximum value for the second derivative was compared with the grade of physeal swelling. RESULTS: The second derivative values for sites with slight to distinct physeal swelling (Grade >/=1.5) were significantly higher (p=0.001) than the second derivative values for sites with no apparent physeal swelling (Grade 0). CONCLUSION: This finding indicates that the value of the second derivative of the contour of a limb has potential as a diagnostic tool for quantitative detection of physitis.     

Identification of groups of metabolically-active rhizosphere microorganisms by stable isotope probing of PLFAs

We combined microbial community phospholipid fatty acid (PLFA) analyses with an in situ stable isotope ¹³CO₂ labelling approach to identify microbial groups actively involved in assimilation of root-derived C in limed grassland soils. We hypothesized that the application of lime would stimulate more rapid ¹³C assimilation and turnover in microbial PLFAs. Four and 8 d after label application, 18:1ω9, 18:2ω6,9 (fungal biomarkers) and 16:1ω7, 18:1ω7, 19:0cy (Gram-negative bacterial biomarkers) showed the most ¹³C enrichment and rapid turnover rates. This suggests that these microorganisms were assimilating recently-photosynthesized root C inputs to soils. Contrary to our hypothesis, liming did not affect assimilation or turnover rates of ¹³C-labelled C. ¹³C stable isotope pulse-labelling technique paired with analyses of PLFA microbial biomarkers shows promise for in situ investigations of microbial function in soils.     

Fatty acid composition and change in Collembola fed differing diets: identification of trophic biomarkers

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.     

Collembolan trophic preferences determined using fatty acid distributions and compound-specific stable carbon isotope values

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 δ¹³C values increased by between 5.7 and 21.6‰ over 21 days reflecting those of the nematode diet. Therefore, both fatty acid profiles and δ¹³C 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 δ¹³C 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 δ¹³C values between diet and consumer. The fatty acids that were not biosynthesised by Collembola better reflected the δ¹³C values of the diet helping to differentiate between biosynthesised and directly incorporated compounds; an important prerequisite in the interpretation of compound-specific δ¹³C values in trophic behaviour tests. The combination of fatty acid distributions and δ¹³C 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.     

Dissolved organic matter and its parent organic matter in grass upland soil horizons studied by analytical pyrolysis techniques

We have sought to understand the molecular mechanisms by which dissolved organic matter (DOM) forms and soil organic matter (SOM) degrades in upland peaty gley soil under grass. Pyrolysis mass spectrometry (Py-MS) and pyrolysis gas chromatography mass spectrometry (Py-GC/MS) were applied to characterize the DOM collected from lysimeters and its parent SOM. The macromolecular organic matter in the litter and fermentation (Lf) horizon of the soil consists primarily of little decomposed lignocellulose from grass, whereas the humus (Oh) horizon is characterized by an accumulation of selectively decomposed lignocellulose material, microbial metabolites and bound fatty acids. The mineral horizon produced a relative enrichment of furan structures derived from microbial reworking of plant polysaccharides but virtually no lignin signals. A series of exceptional long chain C₄₃ to C₅₃ fatty acids with odd over even predominance, probably derived from mycobacteria, were also identified in the Oh horizon. Side-chain oxidation and shortening, increase of carboxyl functionality and selective removal of syringyl (S) > guaiacyl (G) > p-hydroxyphenyl (P) lignin units were the main reactions when lignin degraded. Compared with SOM, the DOM shows a large accumulation of more oxidized lignin and aromatic structures, especially those containing carboxylic and dicarboxylic acid functionalities and with shorter side-chain length. The polysaccharide-type compounds in the DOM were more modified (greater abundance of furan structures in pyrolysis products), and had significantly lower molecular weight and more diverse polymeric structures than did those in soils. Increased temperature and rainfall appeared to result in greater relative abundance of lignin degradation products and aromatic compounds in DOM.     

Enchytraeid worms (Oligochaeta) enhance mineralization of carbon in organic upland soils

We investigated the functional role of enchytraeid worms (Oligochaeta) in organic upland soils experimentally, because that role of these animals is little known. We made microcosms of intact soil cores cut from two depths, 0–4 cm and 4–8 cm, of a Cambic Stagnohumic Gley from the Moor House National Nature Reserve (UK). Enchytraeids were added to half of the microcosms, resulting in four treatments: litter (L), litter + enchytraeids (L + E), soil (S) and soil + enchytraeids (S + E). Triplicates of each treatment were established, and all microcosms (60) were then incubated in the dark at 15°C, arranged in a fully randomized design. The experiment ran over 110 days, with five destructive harvests at days 10, 25, 50, 75 and 110, when microbial measurements (soil respiration and biomass C) as well as measures of decomposition (nutrient concentration in leachates) were made. Enchytraeids almost doubled the availability of organic carbon (measured as dissolved organic carbon in soil leachates) in the surface (0–4 cm) microcosms only. There were no effects of enchytraeids on the release of inorganic N or P from either soil horizon, although the release of ammonium and phosphate was correlated with the number of enchytraeids in the microcosms. The depth from which the soil was taken exerted a strong influence on nutrient leaching, with almost six times more ammonium and four times more carbon being leached from the surface (0–4 cm) layer than from the more decomposed (4–8 cm) horizon. There was little nitrate leaching from any of the treatments, with only one‐quarter as much nitrate leached from the surface (0–4 cm) as from the subsurface (4–8 cm) horizon. Enchytraeids had no detectable effect on microbial biomass, but they increased microbial respiration by 35% in the surface (0–4 cm) horizon. Because they enhanced microbial activity in this horizon we suggest that enchytraeids indirectly drive the processes of decomposition and nutrient mineralization in organic upland soils.