Cattle grazing increases microbial biomass and alters soil nematode communities in subtropical pastures

This study focused on examining the impacts of cattle grazing on belowground communities and soil processes in humid grasslands. Multiple components in the soil communities were examined in heavily grazed and ungrazed areas of unimproved and improved bahiagrass (Paspalum notatum Flugge) pastures in south-central Florida. By using small (1-m×1-m) sampling plots, we were able to detect critical differences in nematode communities, microbial biomass, and mineralized C and N, resulting from the patchy grazing pattern of cattle. Soil samples were collected on three occasions between June 2002 and June 2003. Microbial C and N were greater (P?0.01) in grazed than in ungrazed plots on all sampling dates. Effects of grazing varied among nematode genera. Most genera of colonizer bacterivores were decreased (P?0.10) by grazing, but more persistent bacterivores such as Euteratocephalus and Prismatolaimus were increased, as were omnivores and predators. Higher numbers of persisters indicated that grazing resulted in a more structured nematode community. Some herbivores, particularly Criconematidae, were decreased by grazing. Abundance of omnivores, predators, and especially fungivores were strongly associated with C mineralization potential. Strong correlation of microbial C and N with nematode canonical variables composed of five trophic groups illustrates important links between nematode community structure and soil microbial resources. Including the analysis of nematode trophic groups with soil microbial responses reveals detection of grazing impact deeper into the hierarchy of the decomposition process in soil, and illustrates the complexity of responses to grazing in the soil foodweb. Although highly sensitive to grazing impacts, small-scale sampling could not be used to generalize the overall impact of cattle grazing in large-scale pastures, which might be determined by the intensity and grazing patterns of various stocking densities at the whole pasture level.     

Nematode succession and microfauna-microorganism interactions during root residue decomposition

The quality of plant material affects the vigor of the decomposition process and composition of the decomposer biota. Root residues from hairy vetch (Vicia villosa Roth), rye (Secale cereale L.) and vetch+rye, packed in litterbags were placed in pots of soil at 15 C and the content of the bags was analyzed after 2, 4, 8 and 12 weeks. Bacterial biomass did not differ between residues with contrasting composition. Among bacterivores groups of nematodes that require high bacterial production dominated in fast decomposing resources whereas flagellates with smaller requirements prevail in slower decomposing resources. Biomass of bacterial feeding nematodes correlated positively with early phase (0-2 wk) decomposition that increased in the order: rye< vetch+rye<vetch. Bacterial biomass therefore seems to be under top-down (predation) control during early decomposition. In contrast, the fungal biomass differed between resources with highest values for rye. Moreover, this increase in fungal biomass occurred later during succession and was correlated with decomposition activity for rye in that period. Fungal biomass therefore seems to be under bottom-up (resource) control. The composition of the nematode assemblages (composed of 25 taxa) showed a clear relationship to initial plant resource quality as well as decomposition phase. Early successional microbivorous nematodes vary according to resource quality with demanding bacterivores+predators (Neodiplogasteridae) dominating in vetch and less demanding bacterivores (Rhabditidae) and fungivores (Aphelenchus) being equally common in vetch and rye. Later in the succession (2-4 wk) bacterivorous Cephalobidae and fungivorous Aphelenchoides prevailed similarly on the different root materials whereas bacterivorous protozoa and the amoebal fraction thereof dominated in rye. At week 12 no species dominated the nematode assemblages that were similar between the resources. The differences between nematode assemblages among plant resources at 2 week were similar to the results of a field study sampled after 6 weeks with the same soil and plant resources. This lends support to the relevance of the successional patterns observed in this incubation study.     

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.     

根结线虫数值分类方法的筛选研究

本研究对Melo■属60个OTUS和18项分类性状分别进行Q分类和R分类,结果表明;Q分类以在切比雪夫距离上用中间距离法进行聚类效果最佳,能概括和直观地表达根结线虫的表相关系;R分类在马氏距离上用中间距离法聚类能很好地表达数量性状之间、质量性状之间以及数量性状与质量性状之间的表相关系。Q分类结果还表明,60个OTUS的单位距离(种间)不存在属的差异,应取消H(?)属,原H(?)属中的种均应归属于Mel(?)属,M.(?)(?)并非是M.(?)的亚种,应提升为独立的一个种。     

南疆土壤线虫及微生物对枣树与绿豆间作种植方式的响应

通过研究新疆枣树与绿豆间作对枣园土壤线虫群落结构及土壤微生物数量的影响,以评价枣园间作绿豆的合理性。对新疆和田枣园枣树单作及间作条件下的枣树根际土壤线虫群落组成、营养结构及微生物数量进行分析。结果表明,与枣树单作相比,枣园行间间作绿豆,枣树根际土壤线虫群落组成发生改变,浅层及深层土壤食细菌线虫比例增高18.93%和21.45%,植物寄生线虫比例降低62.56%和61.62%;枣豆间作枣树根际浅层、深层土壤细菌数量显著增加,分别增加51.51%、41.13%,真菌数量显著降低,分别降低50.11%和37.53%,使得土壤由"真菌型"向"细菌型"转变;土壤浅层、深层pH降低至7.92和7.93,浅层、深层含水量上升至7.57%和5.80%。枣树间作绿豆有益于枣树根际土壤健康。     

Suppression of nematodes in a coastal grassland soil

The nematophagous fungi Arthrobotrys oligospora and Myzocytiopsis glutinospora increase to large numbers (>10³ propagules/g of soil) when moth larvae killed by entomopathogenic nematodes are added to soil microcosms. In spite of these increases, it is unclear how effective these nematophagous fungi are in suppressing nematodes. We measured nematode mortality in microcosms with small numbers of assay nematodes, and we examined assay nematodes recovered at the end of the experiment for signs of fungal parasites. Because the microcosms did not have a moat or other refuge, the assay nematodes remained vulnerable for the 3 days that they were in the soil. Mortality in this experiment was not substantially increased compared to a previous experiment, which measured the mortality of a larger number of assay nematodes in microcosms surrounded by a moat. Mortality, however, increased from 34 to 50% when recovered assay nematodes were examined and when those with conidia of the nematophagous fungus Hirsutella rhossiliensis were considered dead. The zoosporic fungus M. glutinospora was not detected, perhaps because the soil water potential was too low. Contrary to our expectations, there was no evidence of negative feedback on nematodes (i.e., no evidence of density-dependent mortality) because the addition of dead moth larvae greatly increased numbers of resident nematodes and A. oligospora but did not greatly affect the probability of nematode mortality.     

Nematodes and their relationship to forest dynamics: II. Abundance and morphometric variability of Monochida related to changes in humus forms

Mononchida nematode populations were analysed in parallel with stages of natural forest dynamics in a nature reserve within the state forest of Fontainebleau (La Tillaie, France). Changes in the relative abundance of species and morphometric variability of Clarkus papillatus provided additional information about the differentiation of sites on acid mull humus. Especially in clearings, the role of the herbaceous layer and dead fallen wood was demonstrated by the improvement of the humus form. Furthermore, although not typically representative of successional stages of forest development, Mononchida indicate the existence of changes of environmental conditions. These changes relate especially to a gradual differentiation between two groups of sites, one represented by the bare ground in clearing (located within the mature stage), growth and mature stages, and the other associated with the senescent stage and the clearing located near this site. Differences illustrated by the two types of clearing indicate the important role of windthrows in the forest regeneration process.     

The distribution of nematodes and soil microbial communities across soil aggregate fractions and farm management systems

We hypothesized that nematode and microbial communities vary between soil aggregate fractions due to variations in physical and/or resource constraints associated with each fraction and that this, in turn, contributes to management impacts on whole soil food webs. Nematode and microbial communities were examined within three soil fractions: large macroaggregates (LM; >1000 μm), small macroaggregates (SM; 250-1000 μm) and inter-aggregate soil and space (IS; <250 μm) isolated from soils of four agricultural management systems: conventional tomato (CON), organic tomato (ORG), a minimum till grain-legume intercrop with continuous cover (CC) and an unmanaged riparian corridor (RC). Aggregate fractions appeared to influence nematode assemblages more than did management system. In general the IS and LM fractions contained higher densities of all nematode trophic groups than did SM. Management × fraction interactions for bacterivores and fungivores, however; suggested a non uniform trend across management systems. The IS fraction exhibited stronger trophic links, per the nematode structure index (SI), while the LM and SM fractions had more active fungal decomposition channels as indicated by the channel index (CI). Higher adult to juvenile ratios in the LM and IS than the SM fraction, and a positive correlation between nematode density in the IS fraction and the proportion of macroaggregates in the soil, indicated an association between soil structure and nematode distribution. Microbial communities varied across both aggregate fractions and management systems. Phospholipid fatty acid (PLFA) analysis suggested that the LM fraction contained greater microbial biomass, gram positive bacteria, and eukaryotes than the IS fraction, while SM contained intermediate PLFA associated with these groups. Total PLFA was greater under RC and ORG than under CC or CON. Total PLFA was positively correlated with % C in soil fractions while nematode abundance exhibited no such relationship. Our findings suggest that microbial communities are more limited by resource availability than by habitable pore space or predation, while nematode communities, although clearly resource-dependent, are better associated with habitable pore space for the soil fractions studied here.     

闽南地区牛,羊消化道8种线虫第三期幼虫形态鉴别及生前诊断研究

取闽南地区牛、羊消化道常见的８种线虫第三期幼虫虫体，对它们进行形态鉴别，从而弄清它在生前诊断上的价值。     

Effects of dazomet on soil organisms and recolonisation of fumigated soil

Fumigation is a common practice to control soil pathogens, but little is known about the impacts of fumigation on other soil biota groups. The purpose of this study was to investigate the effects of fumigation on soil biota, including microorganisms, nematodes, and microarthropods. Bacteria were the most resistant group and some survived following treatment with 2000 mg kg⁻¹ dazomet. Some soil fungi survived 100 mg kg⁻¹ dazomet, although they were mainly Trichoderma. The fungi pathogenic to ginseng were all killed at 100 mg kg⁻¹, and showed both inter- and intra-species variation with respect to dazomet susceptibility. Among the nematodes, Aphelenchus was relatively resistant. The results suggested that susceptibility of soil organisms to dazomet differs between species, and that tolerant organisms may engage in recolonisation. In microcosm experiments, the microbial biomass and community were assessed using phospholipid fatty acid (PLFA) analysis while recolonisation of soil organisms was controlled by mesh size. The bacterial PLFA levels were changed little after fumigation, whereas the fungal PLFA levels gradually increased after fumigation. Principal analysis of the PLFA levels and the ratio of gram-negative to gram-positive bacteria showed that fumigation altered the microbial community. The number of nematodes did not recover even at 12 weeks after fumigation. The increased Collembolan numbers suggest that fumigated soil could be recolonised by specific organisms that have adapted to the conditions. In field experiments, we tested the ability of organic materials to enhance the recolonisation of fumigated soil by soil organisms. Bean powder and rice bran increased the microbial PLFA levels and nematode numbers at 6 weeks and 12 weeks after treatment, and the abundance of nematodes continued to increase 42 weeks after fumigation. The abundance of microarthropods was only slightly affected by the presence of the organic materials. We suggest that treating fumigated soils with organic materials is an effective technique to promote soil organism numbers. In addition, Trichoderma was observed to be relatively resistant to fumigation, and therefore, we propose that the fumigation effect can be improved by using a combination of resistant Trichoderma and dazomet.