Aggregate-related changes in network patterns of nematodes and ammonia oxidizers in an acidic soil

Nitrification plays a central role in global nitrogen cycle, which is affected by biological interaction between soil microfauna and microorganisms. However, the complexity of soil biotic communities made it difficult to reveal organizational principles of the community and the interactions among species. Here, we used the network analysis to decipher the interactions between nematodes and ammonia oxidizers within aggregate fractions under 10-year manure application, and examine their associations with soil variables and potential nitrification activity (PNA). Three aggregate fractions included large macroaggregates (>2000 μm, LA), small macroaggregates (250–2000 μm, SA), and inter-aggregate soil and space (<250 μm, IA). Aggregate factions showed a remarkable effect on association networks of nematodes and ammonia oxidizers. The average connectivity (avgK) and the number of edges in overall networks increased with increasing aggregate sizes, while the average geodesic distance (GD) followed the opposite trend. The LA network could be viewed as a better organized or a better operational soil food web with more functional interrelated members than the SA and IA networks. The modules related to PNA were significantly correlated and clustered together as meta-modules in networks of aggregate fractions. The role-shifts prevailed among the network members such as significant module memberships (MMs) and generalist/specialist operational taxonomic units (OTUs). A half of shared nodes were further identified as shared MMs, dominated by ammonia-oxidizing bacteria (AOB) especially for Nitrosospira cluster 3a and 10. Soil pH could explain partly the shift of module hubs in different networks, while grazing by bacterivores might account for three exclusively connecters related to Nitrososphaera clusters 1.1. The strongly coupled modules correlated positively to pH and total carbon (TC), regardless of aggregate fractions. The network analysis approach provided new insights into potential importance of network interactions between nematodes and ammonia oxidizers in soil nitrogen cycling.     

A comparison of the FECPAK and Mini-FLOTAC faecal egg counting techniques

Faecal egg counts (FECs) are used for detecting and quantifying nematode infections and are the basis for determining drug efficacy and anthelmintic resistance in faecal egg count reduction tests (FECRTs). Currently, several FEC techniques are available for detecting and quantifying eggs of sheep nematodes. A comparison of the egg counts from the FECPAK (with a minimum detection limit of 30 eggs per gram (epg)) and Mini-FLOTAC (with a minimum detection limit of 5 epg) showed better diagnostic performance with Mini-FLOTAC in terms of measurement error (level of over- or under-estimation of FEC) and precision (variability in FEC). A tendency to under-estimate FEC was observed with the FECPAK particularly at egg densities of less than 500 epg. It is concluded that Mini-FLOTAC is a reliable diagnostic tool offering reduced measurement error and a higher level of precision.     

Dynamics of microbial biomass and soil fauna in two contrasting soils cropped to barley (Hordeum vulgare L.)

Summary This study compared the dynamics of shoots, roots, microbial biomass and faunal populations in two different soils cropped to barley. The dynamics of microbial C, protozoa, nematodes, acari, collembola, shoot and root mass were measured between July and October under barley at Ellerslie (Black Chernozem, Typic Cryoboroll) and Breton (Gray Luvisol, Typic Cryoboralf) in central Alberta. Very wet soil conditions in early July reduced the barley yield at Breton. The peak shoot mass was greater at Ellerslie (878 g m^–2) compared to Breton (582 g m^–2), but the root mass did not differ significantly between sites. Microbial C at 0–30 cm depth was greater at Ellerslie (127 g m^–2) than Breton (68 g m^–2). The average protozoa population (no. m^–2) did not differ significantly between sites. The average nematode population at 0–20 cm depth was greater at Ellerslie (5.1 × 10⁶ no. m^–2) compared to Breton (1.0 × 10⁶ no. m^–2) Acari and collembola populations at 0–10 cm depth at Ellerslie (43 × 10³ and 43 × 10² no. m^–2), respectively) were greater than at Breton (2 × 10⁴ and 9 × 10² no. m^–2) respectively). Tenday laboratory incubations of 0–10 cm soil samples from Ellerslie evolved more CO₂-C (120 g g^–1 soil) compared to samples from Breton (97 g g^–1 soil), but the CO₂-C evolution did not differ when expressed on an area basis (g m^–2) due to the greater soil bulk density at Breton. The soil from Breton respired twice as much CO₂-C when expressed as a proportion of soil C and 1.5 times as much CO₂-C when expressed as a proportion of microbial C, compared to the soil from Ellerslie. The greater CO₂-C: microbial C ratio, lower flush C:N ratio, and greater protozoa population: soil C ratio at Breton compared to Ellerslie suggest that the food web was relatively more active at Breton and was related to greater C availability and water availability at Breton.     

Microbial and microfaunal community structure in cropping systems with genetically modified plants

Soils from field sites at Foulum (DK), Narbons (FR) and Varois (FR) planted with genetically modified maize expressing either the insecticidal Bacillus thuringiensis protein (Bt) or herbicide tolerance (HT), as described elsewhere in this volume, were analysed for nematodes, protozoa and microbial community structure. These analyses were mirrored in single-species testing and in mesocosm experiments, and were coordinated with field samples taken for microarthropods, enchytraeids and earthworms so allowing for cross-comparison and a better understanding of the results observed in the field. Over the first 2 years of the field experiments (in 2002 and 2003), the effect of Bt-maize was within the normal variation expected in these agricultural systems. Sampling in 2004 and 2005 was expanded to include the effects of tillage (i.e. reduced tillage versus conventional tillage) and also the use of HT-maize. Tillage had major effects regardless of soil type (Varois or Foulum), with reduced-tillage plots having a greater abundance of microfauna and a different microbial community structure (measured both by phospholipid fatty-acid analysis (PLFA) and by community-level physiological profiling (CLPP)) from conventionally tilled plots. Grass, as a contrasting cropping system to maize, also had an effect regardless of soil type and resulted in greater microfaunal abundance and an altered microbial community structure. Differences in crop management, which for the Bt-maize was removal of the insecticide used to control European corn borer and for HT-maize was a change in herbicide formulation, were only tested at single sites. There were differences in microbial community structure (CLPP but not PLFA) and sporadic increases in protozoan abundance under the Bt-crop management. The HT-maize cropping system, which covered a shorter period and only one site, showed little change from the conventional system other than an altered microbial community structure (as measured by PLFA only) at the final harvest. The Bt-trait had a minimal impact, with fewer amoebae at Foulum in May 2003, fewer nematodes at Foulum in May 2004 but more protozoa at Varois in October 2002 and an altered microbial community structure (PLFA) at Foulum in August 2005. These were not persistent effects and could not be distinguished from varietal effects. Based on the field evaluations of microfauna and microorganisms, we conclude that there were no soil ecological consequences for these communities associated with the use of Bt- or HT-maize in place of conventional varieties. Other land management options, such as tillage, crop type and pest management regime, had significantly larger effects on the biology of the soil than the type of maize grown.     

Does plant growth phase determine the response of plants and soil organisms to defoliation?

To test a hypothesis that the effects of defoliation on plant ecophysiology and soil organisms depend on the timing of defoliation within a growing season, we established a greenhouse experiment using replicated grassland microcosms. Each microcosms was composed of three plant species, Trifolium repens, Plantago lanceolata and Phleum pratense, growing in grassland soil with a diverse soil community. The experiment consisted of two treatment factors—defoliation and plant growth phase (PGP)—in a fully factorial design. Defoliation had two categories, i.e. no trimming or trimming a total of four times at 2 week intervals. The PGP treatment had four categories, i.e. 1, 3, 7 or 13 weeks growth following planting before the first defoliation (subsequently referred to as PGP1, PGP2, PGP3 and PGP4, respectively). In each PGP treatment category, microcosms were harvested 1 week after the final defoliation. Harvested shoot and root mass and total shoot production (including trimmed and harvested shoot mass) increased with time and were lower in defoliated than in non-defoliated systems. The fraction of root biomass of harvested plant biomass decreased with time but was increased by defoliation at PGP3 and PGP4. The proportion of T. repens in total shoot production increased and those of P. lanceolata and P. pratense decreased with time. Defoliation increased the proportions of P. lanceolata and P. pratense in total shoot production at PGP3 and PGP4. Root N and C concentrations increased and root C-to-N ratio decreased with time in non-defoliated systems. Defoliation increased root N concentration by 38 and 33% at PGP1 and PGP2, respectively, but decreased the concentration by 22% at PGP4. In contrast, defoliation reduced root C concentration on average by 1.5% at each PGP. As with the effects on root N concentration, defoliation decreased the root C-to-N ratio at PGP1 and PGP2 but increased the ratio at PGP4. Among soil animal trophic groups, the abundance of herbivorous nematodes was higher at PGP4 than at PGP1-3 and that of predacious nematodes higher at PGP2-4 than at PGP1, while the abundance of bacterivorous, fungivorous and omnivorous nematodes and that of detritivorous enchytraeids did not differ between the PGP categories. Among bacterivorous nematodes, however, Acrobeloides, Chiloplacus and Protorhabditis species decreased and that of Plectus spp. increased with time. Defoliation did not affect the abundance of soil animal trophic groups, but reduced the abundance of herbivorous Coslenchus spp. at each PGP and raised the abundance of herbivorous Rotylenchus spp. and bacterivorous Eucephalobus spp. at PGP4. Confirming our hypothesis, the results suggest that the effects of defoliation on the attributes of grassland plants, such as biomass allocation between roots and shoots and root quality, may depend on the timing of defoliation within a growing season. However, contradicting our hypothesis, the results suggest that significant changes in plant attributes after defoliation may not always lead to substantial changes in the abundance of belowground organisms.     

四川桃树螺旋线虫的记述

记述了四川７种桃树螺旋线虫，其中除Ｈｅｌｉｃｏｔｙｌｅｎｃｈｕｓｄｉｇｉｔｉｆｏｒｍｉｓ已有报道外，其余６种国内未见正式报道。     

Characterization of nematode colonization and succession in disturbed soil using the Maturity Index

Nematode colonization and succession were studied over 60 weeks in field plots which had been disturbed by combinations of fumigation and manuring. It was expected that the Maturity Index, which is based on the composition of the nematode fauna, would decrease after disturbance, and subsequently increase with recovery and succession. The results showed that in all treatments, the initial colonization and growth were due to taxa scaled in colonizer-persister (c-p) group 1, resulting in a low Maturity Index. An increase in this index followed when these taxa were gradually replaced by c-p group 2 which thereafter remained the dominant group. C-p groups 3–5 developed only in the non-fumigated treatment. Manuring caused characteristic changes in the nematode fauna which could be described as a cycle, starting with an increase in enrichment opportunists (c-p 1), which were succeeded by general opportunists (c-p 2) which, in turn, were supplemented with persisters (c-p 3–5). It is expected that the same cycle will be observed as a result of other disturbances that cause temporary increase in food supply.     

How does litter quality and site heterogeneity interact on decomposer food webs of a semi-natural forest?

The relative importance of litter quality and site heterogeneity on population dynamics of decomposer food webs was investigated in a semi-natural mixed deciduous forest in Denmark. Litterbags containing beech or ash leaves were placed in four plots. Plots were located within gaps and under closed canopies at two topographically different sites, above and below a slope, respectively, to cover variable environmental conditions. Litter was collected after 2, 4 and 9 months of decomposition. Extensive decay prevented analysis of ash after 9 months. Density of bacteria (CFU), active fungal mycelium (FDA), protozoa (MPN) and nematodes were 4-15-fold higher in ash leaves than in beech leaves in accordance with the higher resource quality of ash. Similar effects of site on density of decomposers were evident in both litter types: with some exceptions, decomposers were higher at the low site and stimulated in gaps. Taxonomic diversity of nematodes increased during decomposition and functional diversity of nematodes followed a pattern often encountered, i.e. opportunistic bacterial-feeders were gradually replaced by fungal-feeders and slower growing bacterial-feeders while predators and omnivors peaked at the end of the study period. At the first sampling, where bacterial activity prevailed, the relative abundance of the two dominant bacterial-feeders, Rhabditidae (fast growing) and Plectus spp. (slower growing), depended more on site than litter type. At the second sampling where fungal activity became more important, the proportions of bacterial and fungal-feeding nematodes also depended more on site than on litter type. At the third sampling individual nematode taxa responded differently to site. In summary, we conclude that although litter quality had a major influence on the density of organisms in the decomposer food web, site effects were also detected and nematode functional groups responded more to site than to litter quality early on in the decomposition process.     

Influence of sewage sludge and heavy metals on nematodes in an arable soil

Summary The abundance of nematodes was investigated in agricultural plots treated in three different ways, the first with no treatment, the second with 300 m³ ha^-1 a^-1 raw sewage sludge and the third with 300 m³ ha^-1 a^-1 sewage sludge with the addition of heavy metals. The nematodes were determined down to the genus and were assigned to five feeding groups. Total nematode numbers were highest in the site treated with sewage sludge and heavy metals. The smallest total numbers were found in the control site. The plant-feeding nematode genera showed different patterns of abundance depending on the sludge treatment and heavy metal content. For the mycophagic and bacteriophagic nematodes, numbers increased with the amount of sludge, especially in the sites with a higher heavy metal content. The family Rhabditidae was the most numerous group in the sludge plus heavy metals treatment. In contrast to these findings, the omnivorous nematodes were very rare in the sludgetreated plots and were completely absent in plots treated with sludge plus heavy metals, whereas predatory nematodes were numerous only after the application of sludge alone.