Nitrogen availability and nematode populations in soil and litter after gap formation in a semi-natural beech-dominated forest

Small-scale spatial variation in N availability, substrate induced respiration (SIR) and population dynamics of nematodes in mineral soil and beech (Fagus sylvatica L.) litter were studied along two transects through a newly formed gap and surrounding forest in a semi-natural mixed deciduous forest in Denmark for three years after gap formation. In the litter, C/N ratio dropped, while decomposition rates and numbers of nematodes were stimulated in the gap as compared to under closed canopy. In contrast, the number of nematodes in soil were inhibited in the gap for two years after gap formation, but gradually recovered in the third year. Recovery was most marked in the northern part of the gap centre where dense regeneration of ash (Fraxinus excelsior L.) occurred. Omnivorous and predatory nematodes became more prevalent one year after gap formation. Effects on trophic groups of nematodes had disappeared by the end of the study period. The maturity index for nematodes was low one year after gap formation and was still lower in the gap than in the surrounding forest at the end of the study period. SIR in soil was lowest in the gap. Results thus indicate a stimulation of decomposers in the litter layer due to formation of the gap and a contrasting inhibition followed by recovery of decomposers in the soil, possibly governed by changes in C input from living roots. Soil C/N ratio, content of soil organic matter (SOM) and monthly nitrification and net N mineralization rates were not affected by a change in forest development phases caused by gap formation. However, seasonal trends and large spatial variation in net N mineralization and nitrification rates were apparent. Soil NH₄–N concentrations dropped markedly at the onset of the growing season in the second year and remained lower in the gap than in the closed forest throughout the growing season, possibly reflecting an increased N demand of the ash regeneration.     

黄瓜连作土壤高温处理对根结线虫和枯萎病的影响

[目的]设施黄瓜连作导致根结线虫和枯萎病发生普遍且严重,利用夏季温室休闲期高温闷棚是解决上述土传病害安全而有效的途径之一。本试验模拟高温闷棚,研究不同温度处理连作土壤根结线虫和枯萎病的变化,旨在为利用太阳光进行温室高温消毒提供理论支撑。[方法]本试验以连作1~19茬‘津优30’黄瓜品种的土壤为试材,首先探明不同连作茬次黄瓜的根结线虫和枯萎病发生程度以及土壤中二者病原物种群数量的动态变化,然后对发病较重的第17茬连作土壤分别进行45℃、50℃、55℃和60℃的高温处理,最后测定比较不同温度处理后的土壤中病原物数量变化,并对高温处理后的土壤进行栽培试验,测量和比较两种病害的发生程度及植株各项生长指标。[结果]随着黄瓜连作茬次的增加,土壤中根结线虫和枯萎病菌的种群数量均增加,两种病害也逐茬加重,至第17茬两种病害的发病程度及其对应病原物的数量均达最高(或次高)水平;对第17茬黄瓜连作土壤进行45℃~60℃的高温处理,随着温度的升高,土壤中的根结线虫和枯萎病菌的数量均减少,当处理温度达55℃时可完全杀灭土壤中根结线虫,达60℃时可同时杀灭枯萎病菌;用高温处理的连作土壤栽培黄瓜秧苗,其根结线虫病和枯萎病的发生程度均随着处理温度的升高而减轻,当处理土壤的温度达到50℃和60℃以上时,根结线虫病和枯萎病分别被完全控制;另外,用60℃处理的连作土壤定植黄瓜,其后期植株生长指标和壮苗指数也显著优于对照。[结论]土传病害随连作茬次增加而加重主要是缘于土壤中病原物积累,定植前对土壤55℃以上的高温处理,可有效减少乃至完全杀灭土壤中根结线虫和枯萎病菌,从而减轻或杜绝两种病害的发生。     

Nematode dispersion by runoff water: Case study of Radopholus similis (Cobb) Thorne on nitisol under humid tropical conditions

To minimize application of nematicides in banana fields, crop systems have been developed in the French West Indies that combine fallow or rotation crops and nematode-free in vitro plants. After two to four years, populations of the burrowing nematode Radopholus similis have developed enough to cause economic losses, leading banana growers to use nematicides. To understand how banana fields are recontaminated, we studied the dissemination of R. similis by water flow. At a 1-m scale, we analyzed the dispersion of R. similis under a rainfall simulator: we isolated a 1-m² study plot, placed a R. similis suspension on the upstream soil surface, and simulated a 60 mm/h rainfall for 72 min. We collected soil samples every 10 cm downstream after 12 min of rainfall, and subsequently at 20-min intervals, and extracted the nematodes using a Seinhorst elutriator and then a Baermann funnel. Our results showed that the nematode dissemination follows an inverse exponential law, and depends more on soil moisture at the beginning of rainfall than on the length of rainfall: in fresh soil, 69–80% of the R. similis recovered were found less than 10 cm downstream from the nematode inoculation line, whereas in wetted soil, 76–85% of the recovered individuals were collected in the outlet tub located downstream from the apparatus. This passive dissemination model partially explains the distance covered by individual nematodes but not the low percentage of motile nematodes recovered in the outlet tub (10% and 36% in fresh and wet soils) compared to the percentage of motile nematodes found in the soil (80% and 84% in fresh and wet soils). Indeed, water runoff is likely to disseminate R. similis over long distances only when soil moisture is close to field capacity.     

Tannic acid reduces recovery of water-soluble carbon and nitrogen from soil and affects the composition of Bradford-reactive soil protein

Tannins are plant-derived polyphenolic compounds that precipitate proteins, bind to metals and complex with other compounds. Solutions of tannic acid, or other phenolic compounds, were added to soil samples to determine if they would affect recovery of soluble soil carbon (WSC) or –nitrogen (WSN) or influence the extraction and composition of Bradford-reactive soil protein (BRSP), associated with glomalin. Tannic acid-C added with water was not completely recovered from samples and the amount of total net WSC and WSN recovered was reduced, suggesting formation of insoluble complexes. By comparison, non-tannin phenolics like gallic acid, or methyl gallate, had little effect on extraction of WSC or WSN while a simple gallotannin derived from tannic acid, 1,2,3,4,6-penta-O-galloyl-d-glucose (PGG), inhibited extraction most. The C and N concentrations in BRSP increased when soil samples were treated with tannic acid or PGG before extraction, a procedure that includes autoclaving. Increases were greatest in the 10–20 cm compared to 0–5 cm depth. Accompanying these were declines in the ratio of absorbance at 465 and 665 nm (E4/E6 ratio) of BRSP extracts suggesting formation of larger or heavier molecules. In contrast, C and N composition in lyophilized BRSP was unaffected or even slightly reduced when tannic acid or PGG were added to the BRSP extract solution after the extraction process. We conclude that some tannins can reduce the solubility of labile soil C and N, at least temporarily and given unpredictability of response associated with phenolic substances, the Bradford assay should not be relied on to quantify pools or composition of soil proteins like glomalin.     

小麦开花期适量灌溉提高水氮利用效率减少土壤硝态氮淋洗的机理

  【目的】  探究开花期土壤水分含量对小麦植株氮素积累转移、土壤硝态氮含量、小麦产量及氮素利用率的影响,为小麦氮素高效利用及节水高产栽培提供理论依据。  【方法】  于2018—2019和2019—2020年两个小麦生长季,在大田条件下,供试品种为济麦22,在开花期设置3个水分处理:不灌水 (W0)、将0—40 cm土层土壤相对含水量补灌至70% (W1) 和85% (W2)。测定了小麦开花期和成熟期氮素的积累和转运、小麦产量及氮素利用率,并对小麦成熟期0—200 cm土层土壤硝态氮含量进行分析。  【结果】  1) W1处理中,两个小麦生长季开花期营养器官贮存氮素转移量比W0和W2处理平均提高11.63%和7.27%,氮素转移率分别增加9.49%和6.11%;成熟期籽粒氮素分配量平均提高22.5%和12.9%,但叶片和穗轴+颖壳中的氮素分配量显著低于W0和W2处理,因而提高了氮素收获指数。2) 补灌至70% (W1) 处理降低了60—120 cm土层土壤硝态氮含量,小麦氮素吸收量比W0和W2处理平均提高11.4%和6.5%,土壤氮素表观盈余量平均降低51.0%和40.9%,W1处理减少硝态氮向深层土壤淋溶的风险,降低了0—200 cm土层土壤中无机氮的残留量和土壤氮素表观盈余量,有利于小麦根系对土壤氮素的吸收利用。3) W1处理的小麦千粒重比W0和W2处理平均增加11.0%和5.4%,籽粒产量提高25.9%和11.8%,水分利用效率平均提高17.0%和12.7%,氮素吸收效率提高了11.4%和6.5%,氮素利用效率增加了13.0%和4.9%。  【结论】  在小麦开花期,将0—40 cm土层土壤相对含水量补灌至70%,可以显著提高小麦灌浆中后期营养器官贮存氮素向籽粒的转移量和转移率,提高小麦成熟期籽粒中氮素的积累量和分配率,进而提高了产量、氮素收获指数、氮素利用率和水分利用效率,同时降低了60—120 cm土层土壤硝态氮含量,因而减少了环境风险。灌溉过量导致硝态氮过多向下移动,影响根系吸收,水分不足则降低氮素向籽粒的运转。     

Influence of bacterial-feeding nematodes on nitrification and the ammonia-oxidizing bacteria (AOB) community composition

The effects of bacterial-feeding nematodes on nitrification and the ammonia-oxidizing bacteria (AOB) community composition were studied in soil microcosms. Sterilized soils were inoculated with mixed soil bacteria (obtained by filtering) or with bacteria and bacterial-feeding nematodes, after which the dynamic inorganic nitrogen concentration was measured weekly. After 28 days of incubation, denaturing gradient gel electrophoresis (DGGE) based on PCR amplification of the amoA gene was used to analyze the AOB community composition. In addition, a clone library from the amoA gene fragments was established using clones randomly selected and sequenced from the two treatments. The results showed that the presence of bacterial-feeding nematodes led to significantly greater NH₄⁺ and NO₃⁻ contents over the entire incubation period, indicating that bacterial-feeding nematodes promoted both N mineralization and nitrification. The results of DGGE showed that the AOB community composition was significantly changed in the presence of bacterial-feeding nematodes. Furthermore, the sequencing results suggested that Nitrosospira sp. was the dominant species in the treatment without nematodes, while Nitrosomonas sp. and Nitrosospira sp. were the dominant species in the treatment with nematodes. Such changes in the AOB community may be one of explanation of the important role that nematodes play in promoting nitrification.     

水氮互作对小麦土壤硝态氮运移及水、氮利用效率的影响

为给强筋小麦(Triticum aeativum L.)高产优质栽培的水、氮合理运筹提供理论依据,在高产地力条件下,选用强筋小麦品种济麦20,设置不施氮（N0）、施氮180 kg/hm2 （N1）、240 kg/hm2 （N2）3个施氮水平,每个施氮水平下设置不灌水（W0）、底墒水+拔节水+开花水（W1）、底墒水+冬水+拔节水+开花水（W2）、底墒水+冬水+拔节水+开花水+灌浆水（W3）4个灌水处理,每次灌水量均为60 mm,研究了水氮互作对麦田耗水量、土壤硝态氮运移、氮素利用效率和水分利用效率的影响。结果表明,（1）增加施氮量,开花期和成熟期0—140 cm各土层的土壤硝态氮含量显著升高;增加灌水时期,土壤硝态氮向深层的运移加剧,成熟期0—80 cm各土层的土壤硝态氮含量降低,120—140 cm土层的土壤硝态氮含量升高。N1W1处理在开花期0—60 cm土层的土壤硝态氮含量较高,成熟期土壤硝态氮向100—140 cm土层运移少,有利于植株对氮素的吸收。（2）随施氮量的增加,子粒产量先升高后降低,以N1最高。N1水平下,W1处理获得了较高的子粒产量、子粒氮素积累量、氮素利用效率、氮肥农学利用率和氮肥偏生产力;在此基础上增加冬水(W2),上述指标无显著变化;再增加灌浆水(W3),上述指标显著降低。（3）施氮提高了小麦对土壤水的利用能力,随施氮量增加,土壤供水量及其占总耗水量的比例显著升高。N1水平下,W1处理获得了最高的水分利用效率;再增加灌水时期,水分利用效率显著降低,开花至成熟阶段的耗水模系数显著升高,灌水量占总耗水量的比例升高,降水量和土壤供水量占总耗水量的比例降低。本试验条件下,施氮为180 kg/hm2,灌底墒水＋拔节水＋开花水3水的N1W1处理,是兼顾高产、高效的水氮运筹模式。     

Effects of Pseudomonas aeruginosa on the diversity of culturable microfungi and nematodes associated with tomato: impact on root-knot disease and plant growth

The available information on Pseudomonas biocontrol inoculants on the non-target fungal and nematode community is scant. The current paper addresses this issue and investigates the effects of biocontrol agents Pseudomonas aeruginosa IE-6 and IE-6S⁺ (previously shown to suppress several soil-borne plant pathogens) on soil microfungi and plant-parasitic nematodes as well as on the root-knot development and growth of tomato (Lycopersicon esculentum). Furadan, a granular nematicide was included as a treatment for comparative purposes. Treatments were applied to soil at the start of each 52-day-long tomato growth cycle, and their effects on the composition and diversity of rhizosphere and endophytic microfungi and plant-parasitic nematodes were examined at the end of first and fourth growth cycle. Several diversity indices were employed to assess community diversity. A total of 16 genera comprising 23 microfungal species were isolated from the tomato rhizosphere. The most abundant fungal species belonged to the genera Aspergillus, Fusarium, and Penicillium. With a few exceptions, fungi were neither exclusively inhibited nor specifically promoted by the application of treatments at any of the growth cycles studied. However, Paecilomyces lilacinus, an egg and female parasite of root-knot nematode, though exclusively absent in the controls was isolated from the treatments. Both general diversity and equitability of rhizosphere microfungi were greater at first compared to the fourth growth cycle while species richness remained uninfluenced across the growth cycles and treatments. However, Furadan and IE-6S⁺ treatments considerably abated general diversity and equitability. Of the microfungal species isolated from the rhizosphere seven were also recovered from surface-sterilized root tissue of tomato suggesting that all the endophytes are primarily rhizosphere organisms. Diversity of endophytic fungi was consistently lower compared with that of the rhizosphere. Both general diversity and equitability declined in all three treatments relative to controls in the root tissue but species richness remained unaltered. Diversity and equitability of plant-parasitic nematodes in soil were reduced by all three treatments over the controls at fourth growth cycle whilst species richness did not change at either growth cycle. The biocontrol agents significantly reduced root-knot development and enhanced shoot growth of tomato over the controls. The possible implications of fungal composition and abundance because of biocontrol by Pseudomonas application are discussed.     

Effect of subsurface clay on nematode communities in a sandy soil

A variety of soil properties can directly or indirectly affect nematode community structure. The effects of subsurface clay content (at 20–40 cm depth) on nematodes in the surface layer (0–20 cm depth) of a sandy soil were examined in field experiments in Florida, USA. Plots were established in a site with a relatively uniform sandy upper soil layer (88–91% sand and 5–7% clay at 0–20 cm depth) but with varying levels of clay in the subsurface layer (3–35% clay at 20–40 cm depth). Nematode numbers in the surface soil layer were affected by the amount of clay in the subsurface layer. Population densities of a number of different nematode genera were greater in the surface layer of plots with 35% subsurface clay than in plots with 3% subsurface clay. Indices of nematode community structure were largely unaffected, since effects of subsurface clay were observed across all nematode groups. Most nematodes (70–80% of total numbers) occurred at 0–20 cm depth, although Teratocephalus was more common at 20–40 than at 0–20 cm. Subsurface clay content indirectly affected soil moisture and other environmental factors in the upper soil layer in which most nematodes reside.     

Paddy System with a Hybrid Rice Enhances Cyanobacteria Nostoc and Increases N2 Fixation

Biological nitrogen(N) fixation(BNF) plays a significant role in maintaining soil fertility in paddy field ecosystems. Rice variety influences BNF, but how different rice varieties regulate BNF and associated diazotroph communities has not been quantified. Airtight,field-based ~(15)N_2-labelling growth chamber experiments were used to assess the BNF capacity of different rice varieties. In addition,both the 16 S rRNA and nifH genes were sequenced to assess the influence of different rice varieties on bacterial and diazotrophic communities in paddy soils. After subjecting a rice-soil system to 74 d of continuous airtight, field-based ~(15)N_2 labelling in pots in a growth chamber, the amounts of fixed N were 22.3 and 38.9 kg ha~(-1) in inbred japonica(W23) and hybrid indica(IIY) rice cultivars planted in the rice-soil systems, respectively, and only 1%–2.5% of the fixed N was allocated to the rice plants and weeds. A greater abundance of diazotrophs was found in the surface soil(0–1 cm) under IIY than under W23. Sequencing of the 16 S rRNA gene showed significantly greater abundances of the cyanobacterial genera Nostoc, Anabaena, and Cylindrospermum under IIY than under W23.Sequencing of the nifH gene also showed a significantly greater abundance of Nostoc under IIY than under W23. These results indicate that the hybrid rice cultivar(IIY) promoted BNF to a greater extent than the inbred rice cultivar(W23) and that the increase in BNF might have been due to the enhanced heterocystous cyanobacteria Nostoc.     

Fungal-feeding habits of six nematode isolates in the genus Filenchus

The family Tylenchidae is a large group of soil nematodes but their feeding habits are not fully known. We studied the fungal-feeding abilities of nematodes in the genus Filenchus. We measured population growth rates (PGRs) of six nematode isolates, representing three Filenchus species, when feeding on seven fungal species on two types of culture media. On Potato Dextrose Agar (PDA) Filenchus misellus, Filenchus discrepans and an unidentified Filenchus sp. generally showed moderate to large PGRs on saprophytic fungi (Rhizoctonia solani, Chaetomium globosum, Coprinus cinereus, Flammulina velutipes) and low PGRs on plant-pathogenic fungi (Fusarium oxysporum, Pythium ultimum). In soil medium amended with chopped soybean plant material or wheat bran, the status of most of the fungi as food for the nematodes was similar to that on PDA, although PGRs tended to be lower in the soil medium. However, C. globosum, a good food on PDA, only supported low PGR in soil for each of the three nematodes. The PGRs of F. misellus on C. globosum in soil were still low even when types and amounts of organic matter amendments were varied. A nematophagous fungus, Pleurotus ostreatus (oyster mushroom), was determined to be a food for Filenchus on PDA or in soil, based on PGR measurements corrected for extraction efficiency. To determine whether fungal species and culture media affected nematode extraction efficiencies and, consequently, the apparent PGRs, we compared efficiencies between R. solani, C. globosum and C. cinereus, and between PDA and soil. The relatively low extraction efficiencies across fungal species in soil seemed responsible for the lower nematode PGRs in soil than on PDA. On PDA generally, fungal species did not affect the assessment. In soil, effects of fungal species on extraction were significant, but not consistent, across nematode species. Nevertheless, the extraction efficiency differences in soil were considered not to affect assessment of the three fungi as food for the nematodes. The confirmation that three Filenchus species reproduce by feeding on fungi in soil suggests that fungal-feeding is not an unusual habit in the field, in this genus. We believe that in community studies, nematodes in the genus Filenchus should be considered fungal feeders or root and fungal feeders, rather than only plant feeders. Our confirmation of fungal-feeding habits in the genus Filenchus supports the hypotheses that plant-feeding nematodes evolved from those feeding on fungi.     

Exotic plant influences soil nematode communities through litter input

The impact of exotic plant invasions on soil communities and nutrient cycling processes has received an increasing attention in recent years. To test whether the exotic plant invasions affect nematode communities through altering litter quality, we compared mass loss and nematode colonization during the stem litter decomposition of invasive Spartina alterniflora and native Phragmites australis in salt marshes of the Yangtze River estuary, China. Plastic drinking straws were synchronously used as controls. The addition of plant residues was found stimulating the growth of nematodes, particularly bacterial feeders on day 16 after burial. A top-down control of bacterivous nematodes by carnivores existed in nematode succession during the litter decomposition. With higher nitrogen content and lower C:N ratio, stem litter of the invasive S. alterniflora decayed faster and supported more abundant nematodes than the native P. australis. The greater nematode abundance in S. alterniflora was mainly due to two dominant genera of bacterial nematodes, namely Diplolaimelloides and Diplolaimella. Lower values of maturity index and structure index in S. alterniflora than in P. australis litter indicate that a more degraded food web condition resulted from the faster litter decay. A considerable difference in nematode community structures between two litter types only occurred in a certain period of the decomposition (from 8 to 32 days after burial), suggesting that the changes in faunal community structure are time dependent. In summary, this study confirmed the hypothesis that the invasion of S. alterniflora stimulates the growth of bacterial nematodes by producing higher quality of litter than the native P. australis. The results obtained here suggest that the invasion of exotic plant is likely to alter ecosystem functions indirectly through exerting its effect on soil decomposer communities such as nematodes.     

Wood, nematodes, and the nematode-trapping fungus Arthrobotrys oligospora

Researchers have proposed that Arthrobotrys oligospora and related fungi trap soil nematodes to obtain nitrogen and thereby compete saprophytically for carbon and energy in nitrogen-poor environments, including litter and wood. The current study tested two hypotheses concerning this model. The first was that wood decomposition would be enhanced if both large numbers of nematodes (a potential nitrogen supply) and A. oligospora (a cellulolytic organism that can use that N supply) were present. The second was that A. oligospora trapping activity would increase if large numbers of nematodes were added to soil containing abundant carbon (a wood dowel or chip). Although the first hypothesis was supported by an in vitro experiment on agar (A. oligospora degraded much more wood when nematodes were present), neither hypothesis was supported by an experiment in vials containing field soil. In soil, wood decomposition was unaffected by the addition of A. oligospora or large numbers of nematodes. Whereas A. oligospora trapped virtually all nematodes added to agar cultures, it trapped few or no nematodes added to soil. Given that the fungal isolate was obtained from the same soil and that the fungus increased to large numbers (>1×10³ propagules g⁻¹ soil), the failure of A. oligospora to trap nematodes in soil is difficult to explain. Soil nitrate levels, however, were high (71 mg kg⁻¹), and it is possible that with lower nitrate levels, trapping in soil might be stimulated by wood and nematodes.     

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.     

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.     

Soil microbial populations and activities as influenced by legume green fallow in a semiarid climate

Tilled fallow-wheat (Triticum aestivum L.) (F-W) is the most commonly used cropping system in the semiarid Canadian prairie. However, because frequent fallowing degrades soil, a partial fallow with annual legumes as green manure [i.e. ‘green fallow’ (GF)] has been proposed as a soil conserving and more bio-resource efficient alternative to bare fallow. We conducted a 6-year study to compare the influence of four GF-W, a F-W and a continuous W (Cont. W) system, on soil microbial communities, microbial biomass (MB) and activities in an Orthic Brown Chernozem, silt loam (Aridic Haploboroll) at Swift Current, Sask., Canada. The four GF legumes used were black lentil (Lens culinaris Medikus), Tangier flatpea (Lathyrus tingitanus L.), chickling vetch (Lathyrus sativus L.) and feedpea (Pisum sativum L.). They were grown to full bloom and then incorporated with a tandem disk. Analysis of soil taken from 0 to 10 cm depth after growing wheat in the sixth year of the experiment (i.e. 15 months after the most recent legume GF had been turned under) showed that most of the soil biochemical and microbiological attributes assessed were significantly improved (compared to F-W) by increasing cropping intensity (Cont. W), and even more by using the GF systems. The average improvement gained from the four GF legumes relative to F-W, was 385% for number of bacteria, 210% for filamentous fungi, 170% for MB-C, 191% for MB-N, 205% for cumulative C mineralization in 30 days at 21 °C, 202% for dehydrogenase, 171% for phosphatase, and 287% for arylsulfatase activity. The biologically active C and N, when expressed as ratios of MB-C or MB-N to total soil C or N, increased from 1.6 and 2.0% in F-W to 1.9 and 2.6% in Cont. W and to an average of 2.4 and 3.5% in GF-W. A sensitivity analysis (ratio of other treatment values to value for F-W) showed that Cont. W, and the GF-W systems even more, increased all major soil biological attributes tested. Among the GF systems, lentil-W consistently increased sensitivity the most, while Tangier flatpea-W usually increased it the least. The dynamic direct and indirect microbiological attributes were more sensitive indicators of changes in soil productivity than total organic C or N. The sensitivity of the attributes decreased in the sequence: Bacteria>Arylsulfatase>Filamentous fungi≈Cumulative C mineralization≈Dehydrogenase>MB-N>Phosphatase≈MB-C. Compared to the earlier published sensitivities of select physical and chemical attributes of soil quality to these same agronomic treatments, the microbiological attributes proved to be far more sensitive and more responsive to the beneficial influence of legume green fallowing in this semiarid loam.     

Dynamic changes of bacterial community under the influence of bacterial-feeding nematodes grazing in prometryne contaminated soil

Microcosm experiments were carried out to study the effects of bacterial-feeding nematodes and prometryne on soil bacterial communities in contaminated soil. Prometryne (5 or 10 mg kg⁻¹ dry soil, that is, P5 or P10) and bacterial-feeding nematodes (5 or 10 individuals g⁻¹ dry soil, that is, N5 or N10), singly and in combination (P5N5, P5N10, P10N5, P10N10), were added to a nematode-free soil. An uncontaminated nematode-free soil was studied for comparison (Control). Bacterial-feeding nematode grazing boosted soil enzyme activities in contaminated soils, thus speeding up prometryne degradation. In the initial stage of the experiment, prometryne enhanced the soil enzyme activities too, but served the opposite purpose later. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that prometryne contamination and nematode grazing over the incubation period exerted an obvious impact on Species richness (S), Shannon–Wiener index (H′) and Evenness (E_H) of soil bacteria, which increased initially, then decreased and increased again later. The cluster analysis of DGGE profiles showed that the similarity of soil bacterial communities in all treatments with indigenous microbes, P5, P5N5, P5N10, P10, P10N5, and P10N10 and the Control was 75%, 44%, 78% and 49% at Day 0, Day 8, Day 18 and Day 30, respectively. Compared to the Control, DGGE profiles displayed a varying characteristic bands pattern in all treatments over the incubation period with certain bands present in the treatments while not in the Control and vice versa, suggesting that bacterial-feeding nematode grazing and prometryne contamination affected soil bacterial communities evidently. Consequently, when added to contaminated soil, bacterial-feeding nematodes can contribute to restoration of contaminated sites by degrading toxic compounds like prometryne through enhanced microbial activity.     

Impact on soil fauna of sheep faeces containing a range of parasite control agents

To compare the impact of parasite control agents in sheep faeces, 1 kg quantities of fresh faeces were spread uniformly over 1 m² pasture plots in June 2001 (winter; a time of high earthworm activity). Faecal treatments applied to five replicate plots were C− (none), C+ (from untreated sheep), B (from sheep with an intra-ruminal bolus releasing a benzimidazole anthelmintic—‘albendazole’), ML (from sheep with a bolus releasing a macrocyclic lactone anthelmintic—‘ivermectin’), F (from sheep receiving a daily feed supplement containing chlamydospores of the nematophagous fungus, Duddingtonia flagrans). The disappearance of faeces was assessed visually over the 50 days following faecal application, then soil samples were taken to assess: (a) populations of earthworms and other soil macrofauna, (b) nematodes and other soil microfauna, and (c) the presence of D. flagrans in soil. Faecal disappearance was greatest in F and C+ plots and least in ML and B plots at 12 and 23 days (P < 0.05). Earthworm casting after 23 and 50 days was greater (P < 0.05) in plots with faeces (C+, ML, F, but not B) than in plots without faeces (C−). Greater earthworm activity in plots with faeces was reflected in greater numbers of earthworms, cocoons and greater biomass m⁻² than in C− plots. On the basis of faecal dry weight applied, F plots had most earthworms and ML plots the least. After 50 days total nematodes in 0–5 cm soil showed a treatment effect (P < 0.001), being more abundant in F, C+ and B than in C− and ML plots; enchytraeids, rotifers, tardigrades and copepods showed no treatment effects. A few nematode taxa (Acrobeles, Alaimus, Pungentus, Tylencholaimus) showed significant treatment effects. The greatest effect among nematodes was in nematode channel ratio (NCR) (P < 0.008), with a decrease in F plots; changes in NCR may reflect the impact of earthworm activity on soil processes rather than a direct effect of the fungal treatment on nematodes. D. flagrans did not become established in the soil. During the trial conditions were favourable for earthworms and their activity was high in all treatments receiving faeces, with F and ML plots being the extremes. There was an apparent shift towards fungal-mediated decomposition in F plots. At the end of the 50-day trial, in a period when earthworms were active, there was no evidence of differential effects of any of the anthelmintic treatments on environmental indicators.     

The abundance of nitrogen cycle genes amoA and nifH depends on land-uses and soil types in South-Eastern Australia

Nitrogen is a critical nutrient in plant-based primary production systems, therefore measurements of N cycling by microorganisms may add value to agricultural soil monitoring programs. Bacterial-mediated nitrogen cycling was investigated in soils from two broad land-uses (managed and remnant vegetation) across different Soil Orders from three geomorphic zones in Victoria, Australia, by examining the abundance of the genes amoA and nifH using quantitative polymerase chain reaction (qPCR). The aim of the study was to identify parameters influencing bacterial populations possessing the genes nifH and amoA, and examine their distribution at a regional scale across different management treatments. The gene amoA was most abundant in the neutral to slightly alkaline surface soils from Calcarosols in North-West Victoria. There was a highly significant (P < 0.001) interaction between land-use and geomorphic zones in terms of the abundance of amoA. Detection of the gene nifH was site specific with low copy number (less than 100 copies per nanogram of DNA) observed for some strongly acidic surface soil sites in North-East Victoria (Dermosols) and South-West Victoria (Sodosols/Chromosols), while nifH was more abundant in selected Calcarosols of North-West Victoria. The gene amoA was detected across more sites than nifH and was strongly influenced by land-use, with almost consistently greater abundance in managed compared to remnant sites, particularly for North-West and South-West Victoria. The abundance of nifH was not related to land-use, with similar copy numbers observed for both managed and remnant sites at some locations. For the gene nifH, there was no significant interaction between land-use and geomorphic zones, between managed and remnant sites or between the three geomorphic zones. Regression tree analysis revealed a number of likely soil chemical and microbial variables which may act as drivers of gene abundance of amoA and nifH. Variables identified as drivers for amoA included pH, Olsen P, microbial biomass carbon, nitrate and total nitrogen while for nifH the variables were microbial biomass carbon, electrical conductivity, microbial biomass nitrogen, total nitrogen and total potassium. Measures of N cycling genes could be used as an additional indicator of soil health to assess potential ecosystem functions. The spatial scale of the current study demonstrates that a landscape approach may assist soil health monitoring programs by evaluating N cycle gene abundance in the context of the different microbial and chemical conditions related to Soil Order and land-use management.     

Non-target effects of entomopathogenic nematodes on soil microbial community and nutrient cycling processes: A microcosm study

Under microcosm conditions, changes in the soil microbial biomass, respiration rates, and nitrogen pools as indicators of potential non-target effects of entomopathogenic nematodes on soil, were evaluated. Two tests were conducted using soil collected from the field with no history of entomopathogenic nematode application. Treatments consisted of applications of Steinernema carpocapsae All strain in the presence or absence of the wax moth Galleria mellonella larva as a target insect host, compared with the untreated control (soil only). In the second experiment an insecticide treatment (trichlorfon) was added. Microbial biomass (total nitrogen), and mineral nitrogen (NH₄-N, NO₃-N) were measured using standard methods up to 32 days and soil respiration up to 64 days in both experiments. No negative effect was detected in the soil microbial biomass, respiration and nitrogen pools after application of S. carpocapsae. However, a significant increase in ammonium was measured during almost the entire period of the test in the nematode plus larva treatment, not shown in the other treatments. This high level of ammonium in the nematode plus larva treatment showed that entomopathogenic nematodes can indirectly affect system-level processes in soil and adds evidence on the importance of indirect interactions affecting functions in soil food webs. The application of the insecticide trichlorfon significantly suppressed the microbial biomass and nitrification process.