Testate amoebae (protista) of an elevational gradient in the tropical mountain rain forest of Ecuador

We investigated the species composition of testate amoebae at three altitudes (1000, 2000 and 3000 m) and two horizons (L and F/H/Ah) of a tropical mountain rain forest in southern Ecuador. A total of 135 species and intraspecific taxa of testate amoebae were found (36 samples). Rarefaction plots suggest that only few more species are to be expected. Species number in the L horizon was at a maximum at 2000 m (109 species) and similar at 1000 and 3000 m (75 and 71 species, respectively). Species numbers in the F/H/Ah horizon were 29, 67 and 48 at 1000, 2000 and 3000 m, respectively. The density of testate amoebae in the L horizon increased significantly in the order 1000<3000<2000 m with 3147±129, 9198±144 and 12,497±1317 ind. g⁻¹ dry matter and in the F/H/Ah horizon with 176±25, 3118±97 and 4986±102 ind. g⁻¹ dry matter, respectively. The significant Horizon×Elevation interaction reflects the exceptionally low abundance of testate amoebae in the Ah horizon at 1000 m. The results suggest that species richness of testate amoebae does not decrease continuously with elevation; rather, it peaks at an intermediate elevation. Further, the data suggest that diversity, but not density of testate amoebae in tropical forests exceeds that in temperate forests. Morphological features of testate amoebae reflected semiaquatic habitat conditions. The great majority of testate amoebae species of the studied tropical mountain rain forests are geographically widespread, including temperate regions; however 9 of the species (i.e. 6.7%) are considered tropical, some of these species likely represent Gondwana relicts.     

The influence of plant litter diversity on decomposer abundance and diversity

Although there has been much recent interest in the effect of litter mixing on decomposition processes, much remains unknown about how litter mixing and diversity affects the abundance and diversity of decomposer organisms. We conducted a litter mixing experiment using litterbags in a New Zealand rainforest, in which treatments consisted of litter monocultures of each of 8 forest canopy and understory plant species, as well as mixtures of 2, 4 and 8 species. We found litter mixing to have little effect on net decomposition rates after either 279 or 658 days, and for each species decomposition rates in mixture treatments were the same as in monoculture. Litter species identity had important effects on litter microfauna, mesofauna and macrofauna, with different litter types promoting different subsets of the fauna. Litter mixing had few effects on densities of mesofauna and macrofauna, but did have some important effects on components of the microfauna, notably microbe-feeding and predatory nematodes. At day 279, litter mixing also consistently reduced the ratio of bacterial-feeding to microbe-feeding (bacterial-feeding+fungal-feeding) nematodes, pointing to mixing causing a significant switch from the bacterial-based to the fungal-based energy channel. Litter mixing sometimes influenced the community composition and diversity of nematodes and macrofauna, but effects of litter mixing on diversity were not necessarily positive, and were much weaker than effects of litter species identity on diversity. We conclude that litter mixing effects on the abundance and diversity of decomposer biota, when they occur, are likely to be of secondary and generally minor significance when compared to the effects of litter species identity and composition.     

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.     

Effects of chitin amendment of soil on microorganisms,nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

Effects of soil amendment with crabshell chitin on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.), and on populations of soil bacteria, fungi, and plant-parasitic and free-living nematodes were investigated in a pot trial. Five soil samples were collected from Te Puke (Paengaroa Shallow Sand, a Typic Hapludand) and five from Hamilton (Bruntwood silt loam, an Aquic Hapludand), New Zealand. Subsamples of each soil were either amended with chitin or unamended and planted with white clover and ryegrass. The ryegrass shoot weight in amended soil was greater (P<0.01), most probably due to N mineralised from chitin. A significantly lower (P<0.01) root: shoot ratio of ryegrass in the amended soil also suggested improved N availability, and therefore less root mass was needed to support a given shoot mass. A reduction in nodulation was observed in 12-day-old white clover seedlings (P<0.05) and also in 6-week-old seedlings (P<0.01). The shoot weight of white clover was significantly lower (P<0.05) in amended soil, possibly due to phytotoxic effects of chitin. Chitin increased (P<0.01) the populations of bacteria and fungi by 13-fold and 2.5-fold, respectively. The cyst nematode of white clover, Heterodera trifolii, was significantly reduced in chitin-amended soil, possibly due to increased levels of chitinase produced by rhizosphere microorganisms. Two other plant-parasitic nematodes, Pratylenchus spp. and Tylenchus spp., were also reduced in ryegrass roots and in soil as a result of the chitin amendment. However, the total number of free-living nematodes increased 5.4-fold in amended soil.     

Identification and localization of food-source microbial nucleic acids inside soil nematodes

Microorganisms (e.g., prokaryotes, fungi) are food sources for soil nematodes, but they can also be potential mutualists or pathogens. Understanding the linkages between microorganism and invertebrate diversity in soils requires the ability to distinguish between these microbial roles. We tested the potential of a taxon-specific fluorescent in situ hybridization (FISH) procedure for identifying and localizing microbial rRNA within the bodies of soil nematodes. Our objective was to determine whether the rate of digestion permitted detection and identification of food-source nucleic acids within the nematode digestive system (i.e., pharynges, intestines) before their breakdown. First, using laboratory cultures of Caenorhabditis elegans maintained on Escherichia coli, we were able to localize bacterial rRNA throughout the nematode pharynx with the universal bacterial-probe EUB338, although never in the intestines. Second, we applied the fungal rRNA probe FR1 to Aphelenchus avenae cultured on the fungus Rhizoctonia solani. We were unable to detect fungal rRNA within these nematodes, and it appears that this material may be digested rapidly. Next, we applied our technique to nematodes extracted directly from soils. We were able to localize bacterial rRNA within the pharynges of bacterial-feeding species of nematodes from desert soils. We also localized archaeal rRNA using the probe ARC344. Finally, application of EUB338 to desert soil nematodes revealed the presence of bacteria in the intestines of some nematodes and within the ovary of a single nematode. This technique has great potential for use in understanding the feeding behavior of bacterial-feeding soil nematodes and in studies of nematode:bacterial relationships.     

Recolonization of methyl-bromide sterilized soils by plant and soil nematodes over 52 months

Recolonization of soils by organisms is dependent not only on biological factors but also on site factors including soil and vegetation. The total nematode populations in soil sterilized 52 months earlier with methyl bromide ranged from 77 to 123% of those in untreated soil; both vegetation and rainfall appear to have been important factors in the recolonization. In a high-rainfall pasture the dominant plant-feeding nematode Helicotylenchus did not recolonize and was replaced by Paratylenchus. Under high rainfall, Iotonchus failed to recolonize either pasture or forest, but Clarkus recolonized well. While indices such as the maturity index, the summed maturity index, PPI, the ratio of fungal: bacterial feeding nematodes, species richness, the Shannon-Weaver index, and indices of evenness, dominance, and diversity showed various effects, none gave consistent trends; rainfall and vegetation probably exerted too much influence. In the light of trends in our data, in data for other ecosystems, and the reality of r-K strategies in animals, it may not be reasonable to expect a global nematode index of soil health or ecosystem condition. While a nematode index may be useful in local or regional studies, research efforts may be better directed towards elucidating key populations of nematodes for investigating the impact of particular land management practices on ecosystem sustainability.     

桉树凋落物对土壤微生物群落的影响:基于控制实验研究

全面认识桉树种植对土壤微生物群落结构和功能的影响及机制,对于阐明单一物种对生态系统服务功能的影响具有重要意义。通过室内小盆模拟控制试验,采用随机区组设计,以土壤碳、氮含量有显著差异的3种天然次生林土壤为对象,以不添加凋落物的处理和添加天然次生林混合凋落物的处理为对照,研究桉树凋落物对土壤微生物群落结构及功能的影响。结果表明:(1)与天然次生林的混合凋落物相比,桉树凋落物具有较高的碳含量和较低的氮含量,其碳氮比也较高;(2)添加桉树凋落物的土壤中细菌、真菌、放线菌以及磷脂脂肪酸的总丰度显著高于不添加凋落物的土壤,但是显著低于添加天然次生林混合凋落物的土壤,并且不同凋落物处理下土壤微生物群落的磷脂脂肪酸组成存在显著差异;(3)不同凋落物处理下土壤微生物群落的碳代谢方式差异显著,添加桉树凋落物的土壤微生物群落的碳代谢功能优于未添加凋落物的处理,但是显著低于天然次生林混合凋落物处理的土壤,包括:碳代谢的活性和多样性。综上所述,与天然次生林本身的凋落物相比,桉树凋落物影响下的土壤微生物群落的生物量、多样性和代谢活性均较低,表明桉树凋落物为土壤微生物群落提供生境和食物的能力较弱。     

Effects of acidification and liming on feeding groups of nematodes in coniferous forest soils

Summary Nematodes were sampled in untreated, acidified, and limed plots in a Norway spruce (Fexboda) and a Scots pine (Norrliden) stand. At Fexboda, the total number of nematodes was significantly reduced after the acidification. This reduction was probably due to a shock effect, because the samples were taken only 5 months after an application of 200 kg H₂SO₄ ha^-1 to the forest floor. However, the root/fungal-feeding Aphelenchoides was not reduced, probably because it is more tolerant of high acid concentrations than most other nematodes. At Norrliden, where the samples were taken 7 years after the last application of H₂SO₄, no significant differences were found between the acidified and untreated plots. If the treatment with H₂SO₄ caused similar effects as at Fexboda, the results indicate a recovery of the nematode populations. Decreased predation from lumbricids rather than a recovery of microfloral populations probably allowed this recovery. No marked effect of lime, spread 2 (Fexboda) and 12 years (Norrliden) before the sampling on the numbers of any of the nematode feeding groups was found. This correlated with almost no change in bacterial biomass after liming, while the active fraction of fungal hyphae was unaffected by liming at Fexboda and reduced by liming at Norrliden. A tendency for decreasing numbers of all nematode feeding groups in the limed plots at Norrliden coincided with increasing numbers of lumbricids.     

Mixed-species legume fallows affect faunal abundance and richness and N cycling compared to single species in maize-fallow rotations

Rotation of nitrogen-fixing woody legumes with maize has been widely promoted to reduce the loss of soil organic matter and decline in soil biological fertility in maize cropping systems in Africa. The objective of this study was to determine the effect of maize-fallow rotations with pure stands, two-species legume mixtures and mixed vegetation fallows on the richness and abundance of soil macrofauna and mineral nitrogen (N) dynamics. Pure stands of sesbania (Sesbania sesban), pigeon pea (Cajanus cajan), tephrosia (Tephrosia vogelii), 1:1 mixtures of sesbania + pigeon pea and sesbania + tephrosia, and a mixed vegetation fallow were compared with a continuously cropped monoculture maize receiving the recommended fertilizer rate, which was used as the control. The legume mixtures did not differ from the respective pure stands in leaf, litter and recycled biomass, soil Ca, Mg and K. Sesbania + pigeon pea mixtures consistently increased richness in soil macrofauna, and abundance of earthworms and millipedes compared with the maize monoculture (control). The nitrate-N, ammonium-N and total mineral N concentration of the till layer soil (upper 20 cm) of pure stands and mixed-species legume plots were comparable with the control plots. Sesbania + pigeon pea mixtures also gave higher maize grain yield compared with the pure stands of legume species and mixed vegetation fallows. It is concluded that maize-legume rotations increase soil macrofaunal richness and abundance compared with continuously cropped maize, and that further research is needed to better understand the interaction effect of macrofauna and mixtures of organic resources from legumes on soil microbial communities and nutrient fluxes in such agro-ecosystems.     

红壤侵蚀区植被恢复对土壤呼吸及其温度敏感性的影响

土壤呼吸作为土壤与大气CO_2交换的重要环节,其排放量的大小在一定程度决定了土壤碳库的源与汇。研究红壤侵蚀区植被恢复对土壤呼吸及其温度敏感性(Q_(10))值的影响,对于理解土壤有机碳积累机制具有重要意义。本研究选取了福建省河田镇未治理地及邻近恢复13 a和31 a的马尾松人工林,对土壤呼吸进行监测。结果表明:未治理地土壤呼吸并无显著季节差异,恢复13 a和恢复31 a后土壤呼吸季节差异显著;恢复13 a和31 a土壤呼吸与Q_(10)显著高于未治理地,恢复13 a和31 a土壤呼吸与Q_(10)未见显著差异。季节尺度上,土壤呼吸速率与土壤水分相关性弱,而与土壤温显著相关,未治理地土壤温度仅能解释土壤呼吸速率的25.3%,而恢复后土壤温度则解释了土壤呼吸速率的48.8%～66.5%;结构方程模型表明,对土壤呼吸和温度敏感性影响最大的分别是凋落量和土壤微生物。研究为进一步认识退化生态系统土壤碳动态机制提供依据。     

黄土高原刺槐林不同组分生态化学计量关系研究

碳、氮、磷生态化学计量比是生态系统过程和功能的重要特征。刺槐具有生长快、适应性强、耐干旱贫瘠等特点,是黄土高原区水土保持造林的主要树种,以黄土高原刺槐林为研究对象,研究其不同坡向叶片-枯落物-土壤间的碳氮磷生态化学计量学特征,并对其相关性进行分析。结果表明,在阴坡和阳坡,C∶N表现为叶片枯落物土壤,C∶P,N∶P表现为枯落物叶片土壤;叶片、枯落物、土壤C∶N、C∶P、N∶P在阴阳坡均无显著性差异(p0.05),但叶片、枯落物、土壤在两两之间均有显著性差异(p0.05)。土壤的C∶N、C∶P、N∶P均表现为0~10cm10~20cm层,但差异性不显著。不论在阴坡或阳坡,叶片与枯落物的C∶N均为正相关(p0.05);在阳坡,叶片与枯落物的N∶P相关性显著;在阴坡,枯落物与0~10cm土壤的C∶N为显著正相关(p0.05)。     

祁连山青海云杉林叶片—枯落物—土壤的碳氮磷生态化学计量特征

以祁连山排露沟流域青海云杉(Picea crassifolia)林为研究对象,研究其不同海拔梯度叶片—枯落物—土壤间的碳、氮、磷生态化学计量学特征,并对其相关性进行分析。结果表明,在不同海拔梯度上,叶片、枯落物和土壤C∶N比的变化范围分别为22.95~36.72、21.41~41.61、12.41~20.70,均值大小依次为枯落物叶片土壤,C∶P和N∶P比的变化范围分别为510.2~739.8、398.6~698.1、134.1~219.7和18.13~26.86、6.71~26.28、7.96~16.56,均值大小依次均为叶片枯落物土壤。随海拔梯度的增加,除土壤C∶N比差异性不显著外(p0.05),叶片和枯落物的碳、氮、磷化学计量比在不同海拔间的差异显著性各不相同。叶片、枯落物和土壤C∶N比两两均具有显著正相关(p0.05),叶片与枯落物及土壤与枯落物C∶P比均具有显著负相关(p0.05),叶片与土壤C∶P比及不同组分N∶P比之间相关性均不显著(p0.05)。该研究结果有助于进一步了解青海云杉林碳、氮、磷在不同组分间的相互作用规律与机制。     

Vertical distribution of nematodes in arable soil under grass (Festuca pratensis) and barley (Hordeum distichum)

Summary The connection between faunal composition and soil factors is discussed in this study on vertical distribution of soil nematodes under grass and barley. The investigation was undertaken on the field site of a Swedish integrated research project Ecology of Arable Land. The Role of Organisms in Nitrogen Cycling. Higher nematode number (7.6 × 10⁶ m^–2) and biomass (340 mg dry wt. m^–2) were found under a 4-year-old grass ley than under barley (5.0 × 10⁶ m^–2; biomass, 136 mg dry wt. m^–2). Plant feeders dominated under the grass ley (3.2 × 10⁶ m^–2 whereas under barley the bacterial feeders (2.4 × 10⁶ m^–2) were the most abundant feeding group. Number, biomass, mean individual size and various community parameters indicated a much better nutritive situation for the nematodes under grass than under barley. The vertical changes in the various parameters, including proportion of egg-carrying females, indicated an increasing food shortage for the nematode populations towards greater depths. In the top soil, predation could be an important factor in regulating nematode number.Dedicated to the late Prof. Dr. M.S. Ghilarov     

Assessment of nematode biodiversity using DGGE of 18S rDNA following extraction of nematodes from soil

Soil nematodes are both taxonomically and functionally diverse, respond quickly to soil perturbation and have much potential as indicators of soil health. However, because of the perceived difficulty of identifying nematodes to species level morphologically, they are frequently neglected in soil ecological studies. Recently, extraction of soil DNA, amplification of 18S rDNA genes using nematode consensus primers and subsequent separation by denaturing gradient gel electrophoresis (DGGE) has been used to estimate nematode diversity in soil. Here, we investigate an alternative approach whereby nematodes are first extracted from the soil prior the 18S rDNA gene amplification using universal primers. We used this system to estimate nematode species richness in 10 soil samples—five from Scotland and five from the Netherlands. There was no direct correlation between species richness as estimated morphologically and by the PCR-DGGE method. However, inspection of the data suggested that the samples fell into two discrete groups, which was confirmed by canonical and stepwise discriminant function analysis; the values for the Shannon and equitability indices being important discriminators. Further analysis revealed a significant relationship between morphological species richness and DGGE estimates for species that represented greater than 1% of the sample biomass.     

Changes in soil organic matter and net nitrogen mineralization in heathland soils,after removal,addition or replacement of litter from Erica tetralix or Molinia caerulea

Summary The effects of different litter input rates and of different types of litter on soil organic matter accumulation and net N mineralization were investigated in plant communities dominated by Erica tetralix L. or Molinia caerulea (L.) Moench. Plots in which the litter on the soil had repeatedly been removed were compared with plots in the same plant community in which litter had been added to the soil. In another treatment, litter was removed and replaced by litter from the other plant community. Net N mineralization was measured in situ after 5 years. Less soil organic matter and soil N was found in plots in which litter had been removed, compared with control plots, or plots to which litter had been added, but these differences were significant for the Erica sp. soils only. Plots in which litter had been replaced and control plots did not differ significantly in the amount of soil organic matter. However, in both plant communities, the differences agreed with the faster decomposition rate of Molinia sp. litter compared with Erica sp. litter. The gravimetric soil moisture content was correlated positively with the amount of soil organic matter, both in the Erica sp. soils and the Molinia sp. soils. Net N mineralization rates (g N m^-2) differed significantly between treatments for Erica sp. soils but no for Molinia sp. soils. For Erica sp. soils, net N mineralization rates increased with increasing amounts of soil organic matter and soil N. Replacing the litter with Molinia sp. litter (which differs in chemical composition) had no clear additional effect on the net N mineralization rate.     

Effects of an arbuscular mycorrhizal fungus and two plant-parasitic nematodes on<Emphasis Type="Italic"> Musa</Emphasis> genotypes differing in root morphology

In this study, the effect of an arbuscular mycorrhizal fungus (AMF) and two migratory endoparasitic nematodes on Musa plant growth, including the root system, were examined. In addition, the AMF-nematode interaction was studied. Seven Musa genotypes with different root systems were selected. Based on their relative mycorrhizal dependency, two genotypes (Calcutta 4 and Obino l'Ewai) were selected for AMF-nematode interaction studies. The experiments were performed under greenhouse conditions. Mycorrhization with Glomus mosseae resulted in a significantly better plant growth even in the presence of nematodes. The effect of AMF on the root system was genotype-dependent and seemed to be related to the relative mycorrhizal dependency of the genotype. The nematodes also affected the root system, decreasing branching. Nematode population densities were significantly reduced in the presence of AMF, except for Pratylenchus coffeae in Obino l'Ewai. In the root system, it appeared that the decreased branching caused by the nematodes was counterbalanced by the increased branching caused by the AMF.     

Plant growth and microbial activity in a tropical soil amended with faecal pellets from millipedes and woodlice

The effect of soil fauna-mediated leaf litter (faecal pellets) versus mechanically fragmented (finely ground) leaf litter on biomass production of rice (Oryza sativa, var. Primavera) was assessed in pot tests. Rice seedlings were either grown in soil samples amended with faecal pellets of diplopods and isopods fed on leaf litter of a legume cover crop (Pueraria phaseoloides (Roxb.) Benth) and a peach palm (Bactris gasipaes) or in soil amended with finely ground leaf litter. The addition of faecal pellets caused a significant and dose-related increase in plant biomass compared to pure soil. Ground leaf litter induced a significantly smaller positive effect on plant biomass development with Pueraria litter > Bactris litter > mixed primary forest litter. In contrast, soil microbial biomass development during the 4 weeks plant test was higher in the soil amended with ground litter as compared to soil amended with feacal pellets. The results show a clear positive effect of the soil fauna on soil fertility and indicate differences in the availability of nutrients from the organic substrates to higher plants and soil microorganisms.     

黄土丘陵半干旱区人工柠条林土壤固碳特征及其影响因素

为了探讨黄土丘陵区不同生长年限的人工柠条林地土壤有机碳含量的变化特征及其影响因素,更好地阐明黄土丘陵区柠条林土壤的固碳机理,本文采用时空替代法,以撂荒2 a的坡耕地为对照,对黄土丘陵半干旱区不同林龄(10 a、17 a、26 a、34 a、40 a、50 a)人工柠条林地土壤有机碳(SOC)、全氮(STN)、全磷(STP)及柠条林的根系生物量和枯落物现存量进行了分析。结果表明:1)在0~60 cm的土层剖面上,0~20 cm土层SOC含量明显高于其他土层,并随土层深度的增加逐层递减,其中柠条林地0~20 cm土层SOC含量变化幅度为2.68~11.44 g·kg-1,而40~60 cm土层SOC含量仅在1.64~2.73 g·kg-1波动;与对照相比,随林龄增加柠条林地0~60 cm土层平均SOC含量先减小后增加最后趋于平稳:10 a和17 a柠条林SOC含量比对照显著降低了34.5%和26.9%,26 a柠条林的SOC含量显著升高,其值是对照的1.43倍,40 a和50 a柠条林SOC含量处于积累与消耗相对稳定的状态。2)对SOC含量与STN、STP含量及根系生物量和枯落物现存量进行相关性分析表明,SOC含量与STN含量、根系生物量及枯落物现存量之间存在极显著线性相关,但与STP含量相关性不明显,说明土壤中氮含量的增加能明显提高土壤的固碳能力,而根系生物量和枯落物现存量的多少能够决定土壤的固碳水平。     

Carbon transfer in a winter wheat (Triticum aestivum) ecosystem

Summary In a field experiment with ¹⁴C-labeled winter wheat conducted in the north-central region of the United States, crop-accumulated carbon (grain excluded) returned to the soil was found to be 542 g m^–2 year^–1. Almost half of the carbon from the underground compartment was released in the form of CO₂ during the first 3 months after harvest due to very favorable conditions for biological activity. After 18 months, no less than 80% of the carbon from the plant residues was mineralized. About 16% of straw carbon and 24% of root carbon was transferred into soil organic matter. The annual rate of soil organic matter decomposition was approximated as 1.7%.Contribution from the Missouri Agricultural Experiment Station, Journal Series Number 10134