J.M. Anderson

Spatiotemporal effects of invertebrates on soil processes

SummaryThe processes of C and N mineralization carried out by microorganisms are affected directly and indirectly by invertebrates over a wide range of spatial and temporal scales. Microfauna track temporal changes in bacterial and fungal populations in soil microsites, particularly in the rhizosphere, which alters the dynamic balance between N mobilization and immobilization. The feeding activities of mesofauna can determined the distribution, activities and composition of fungal communities. Macrofauna have major effects on fungal and bacterial activities, both directly, through feeding and gut passage, and indirectly, by affecting the microbial environment in litter and soil.Soil biological processes can therefore be considered a hierarchy of successive levels of organization where the macro-, meso- and microfauna influence microbial activities at different scales in the habitat mosaic. The spatial components of this hierarchy are integrated by plant roots; root morphology must therefore define the scales at which the system operates under different plant nutrient regimes.

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

Abstract 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.

Microbial biomass and activity in contrasting soil materials after passage through the gut of the earthworm Lumbricus rubellus hoffmeister

Earthworms (Lumbricus rubeltus) were fed on four different soils with light fraction organic material contents from 3.7 to 76.1% of the soil dry weightand soil water potentials standardized at — 8 kPa. Microbial biomass-C in the soils, as measured with a modified fumigation-extraction method, ranged from 0.4 to 7.5mg C g−1 soil. After gut passage (6–8 h) microbial biomass-C was not changed significantly but rates of CO2 production, bacterial plate counts, moisture content and concentrations of soluble organic-C were higher in casts compared with the standardized soils ingested by the earthworms.

PLFA profiles of microbial communities in decomposing conifer litters subject to moisture stress

Abstract The influence of moisture stress on microbial communities in decomposing coniferous litters was investigated using phospholipid fatty acid (PLFA) profiling. Studies were carried out in German and Greek forest plots under contrasting climatic conditions from the late summer to the early winter periods. Litterbags containing spruce (Germany) or pine (Greece) needles were subjected to different irrigation treatments over 4 months. The influences of climate and litter type on microbial community structure were larger than those imposed by irrigation or moisture stress treatments. In the German spruce litter, the PLFA signatures indicated that there was initially a larger bacterial than fungal biomass and both components decreased with time. Concentrations of individual PLFA, proportions of PLFA subgroups and principal component (PC) scores showed that, apart from sample date, mesh size was more important than irrigation treatment in determining microbial community structure; though treatment effects were less apparent in the third (winter) sample. Pine litter in the Greek site, with a Mediterranean climate, had a larger fungal than bacterial biomass. Little effect of treatment on individual PLFA concentrations or PC scores was measured, though both fungal and bacterial communities increased significantly with regular irrigation in the third (winter) sample. Effects of mesh size in the German spruce litter were related to differences in the abundance of microarthropods. This effect was absent from the Greek pine litter where there was a relatively low abundance of fauna. The final spruce litter sample, taken in winter, exhibited very different PC scores from other samples, suggesting marked changes in the microbial community in response to snow melt. Certain long chain fatty acids associated with eukaryotes were only found on this occasion. This study has shown that structure of bacterial communities associated with decomposing conifer litters is highly sensitive to changes in environmental conditions. There was, however, little indication that these differences in biota were functionally important for the initial phases of plant litter decomposition.

Effect of collembolan grazing upon nitrogen and cation leaching from decomposing leaf litter

Abstract Fragmented oak litter was incubated in the laboratory for 4 months with and without collembola. The effects of the animals upon fungal standing crop and leaching of inorganic nitrogen and cations was monitored over this period. The results showed that the fungal standing crop was higher in the presence of small numbers of animals than in litter lacking animals, yet at higher grazing intensities the fungal standing crop fell markedly. Significant increases in the leaching of ammonium, nitrate and calcium occurred as a consequence of animal grazing, but potassium and sodium losses from the litter were unaffected.

Long-term effects of temperature on carbon mineralisation processes

Abstract The long-term effects of temperature on soil C mineralisation were investigated in two experiments using 14 C labelled wheat straw incubated in organic soils from five coniferous forests located in different climate zones of Western Europe. In the first experiment, samples were incubated in the laboratory at 4, 10, 16, 23 or 30°C, with constant moisture, and the loss of 14 C was monitored for 550 days. Double negative exponential functions fitted to the 14 C loss data at different temperatures were used to define the relative proportions of labile and recalcitrant components in the original straw. The estimated proportions of these constituents were related to incubation temperatures with the amount of C reflecting the labile fraction increasing with increasing temperature. In the second experiment samples mixed with the labelled straw were incubated at 4, 16 or 30°C until the same percentage of 14 C loss was reached. The samples were then incubated again at a common temperature for 30 days and CO 2 production was measured to assess the lability of the remaining material. For all the soils, the amount of readily decomposed material was higher in samples conditioned at 4° than at 30°C. It was concluded that in addition to temperature controlling rates of C mineralisation in soil it also affects the processes of decomposition so that material produced at higher temperatures was more recalcitrant than at lower temperatures.

Influence of macroarthropod feeding activities on microflora in decomposing oak leaves

Abstract Fragmented (2–4 mm) and mechanically ground (0.1–0.2 mm) oak litter was inoculated with faeces of Oniscus asellus or Glomeris marginata and incubated in the laboratory. After 40 days microbial respiration rates were the same for both litter treatments. Woodlice or millipedes were then added in various numbers and CO2 evolution measured for a further 40 days. Microbial respiration in fragmented litter was initially increased to twice the control rates by four Oniscus and to 1.6 times control rates by six Glomeris but subsequently declined to rates slightly above controls. But in treatments with ten Oniscus respiration was depressed below the controls after 20 days. Cumulative CO2 measurements showed that microbial respiration was inhibited by grazing pressures higher than optimum values. Animals feeding on previously-ground litter produced a similar, but reduced response, suggesting that litter comminution by the animals was the main factor contributing to the enhancement of C mineralization. Disturbance of fragmented litter, either by Oniscus or by mechanical means, produced no significant changes in respiration. In ground-litter cultures there was a small enhancement of CO2 evolution for the first 10 days. Fungal standing crop was reduced to one-third of controls by Oniscus; the effects being particularly pronounced during the first 3 days of the experiment. Bacterial standing crop increased to a maximum of 10 times control levels.

Dynamics of reduction enzymes involved in the denitrification process in pasture soil

Abstract When oxygen is depleted in soil, reduction enzymes involved in the denitrification process are activated and de novo synthesis of enzymes starts within a few hours. The dynamics of these enzymes and the effect on the concentration of inorganic N formed were investigated for a soil from permanent pasture. Soil was incubated aerobically for 5 days and then amended with 100 mg NO3−-N kg−1. Treatments were with or without C2H2 and with or without chloramphenicol (found to inhibit de novo synthesis of reduction enzymes), purged of all O2, shaken and anaerobically incubated for 48 h while CO2 and N2O production and no3− and NO2− concentrations were monitored. Chloramphenicol was found to have no inhibitory effects on nitrate reduction indicating that nitrate reductase activity persisted in the absence of de novo synthesis. The persistence of nitrite reductase and nitrous oxide reductase was lower as the application of chloramphenicol increased NO2− concentrations and reduced N2 production. In the absence of chloramphenicol, de novo synthesis of nitrite reductase started 5 h and that of nitrous oxide 16 h after anaerobiosis was imposed. It is concluded that the dynamics of nitrite reductase have only a small effect on the N2O production as NO2− concentrations remained below l mg N kg−1 but the low persistence of N2O reductase in combination with its retarded de-repression results in a high N2O-to-N2 ratio when anaerobic conditions are rapidly induced.

Bacteria in the food, gut contents and faeces of the litter-feeding millipede glomeris marginata (villers)

Abstract Bacteria from the food leaves and faeces of millipedes maintained in group cultures (12 animals) for 24 h were counted. Dilution plating and direct observation both showed significant increases in bacterial populations (10–100 fold) after passage through the gut. In cultures of single animals the increases were primarily the result of rapid growth in the midgut, though there was some evidence of further proliferation in the hindgut and faeces. The ratio of viable counts to direct counts was lower in litter than in the gut and faeces, suggesting that the gut environment enhanced bacterial growth and viability. Correlation between counts from guts and food leaves further indicated that the growth response was by the litter microflora and not indigenous gut symbionts.

Effects of fungal inocula on the decomposition of lignin and structural polysaccharides in Pinus sylvestris litter

Abstract Litter bags containing sterile Scots pine (Pinus sylvestris) needles (19.8% lignin, 26.5% cellulose and 0.34% N) were inoculated with two species of fungi in the laboratory and then placed in the litter layer of a pine plantation. Marasmius androsaceus, which can degrade lignocellulose, was initially displaced by other fungal colonisers and was not detected in the litter after 2–3 months; but was re-isolated from the needles after 12 months. Trichoderma viride, which is a cellulolytic species and also antagonistic to other fungi, dominated the litter throughout the experiment. The control litter was naturally colonised by litter fungi. After 12 months, mass losses were similar at 52% for M. androsaceus and 48% for T. viride, compared with 36% for the control litter colonised by a more complex fungal community. Lignin concentrations increased with time in control litter and with T. viride because mass losses of carbohydrates were greater than those of lignin. Litter inoculated with M. androsaceus showed significant lignin decomposition throughout the experiment but cellulose concentrations showed a proportional increase in the first 6 months, suggesting that the fungus was preferentially exploiting hemicellulose and non-structural carbohydrates. Analysis of TFA-extractable sugars (mainly from hemicellulose) and CuO-derived phenylpropanoid moieties from lignin confirmed the differential patterns of resource decomposition which were not evident from total mass losses. During the initial stages of decomposition, T. viride was as effective in utilising structural polysaccharides as the complex fungal community in the control litter. Furthermore, M. androsaceus not only exhibited unexpectedly low cellulolytic activity but also facilitated lignin depolymerisation after the fungus was no longer detectable in the litter. The pre-inoculation of litter with these two fungal species therefore affected the overall dynamics of decomposition at a biochemical level. This study illustrates the importance of understanding the effects and interactions of specific fungi, rather than assumptions about the functional competence of diverse communities, on the processes of litter decomposition.