Distribution and dynamics of mycophagous and microbivorous nematodes in potato fields and their relationship to some food sources

Nematodes are not uniformly distributed in soil. The aim of this study was to investigate the distribution pattern of mycophagous and microbivorous nematodes and to develop sampling methods to detect them at different sites in the soil. Sampling of nematodes was carried out by three methods: using an auger, by collecting soil which was shaken from the roots (root shaking sample) and by collecting roots with the adhering rhizosphere soil (rhizosphere sample). Distribution patterns and population dynamics of mycophagous nematodes were similar to other microbivorous nematodes in three potato fields. Population densities in the rhizosphere samples were 4–50 times higher than those in auger samples. Samples obtained with the auger and root-shaking methods yielded more or less equal population densities. Numbers of the mycophagous nematodes Aphelenchus avenae, Aphelenchoides sp. and unidentified microbivorous nematodes increased several fold within a few days on suitable substrates in soil. These substrates could be the dying roots of plants of which the haulms were killed or flax straws that were previously colonised by the fungus Rhizoctonia solani.     

Protozoa,Nematoda and Lumbricidae in the rhizosphere of Hordelymus europeaus (Poaceae): faunal interactions,response of microorganisms and effects on plant growth

Interactions among protozoa (mixed cultures of ciliates, flagellates and naked amoebae), bacteria-feeding nematodes (Pellioditis pellio Schneider) and the endogeic earthworm species Aporrectodea caliginosa (Savigny) were investigated in experimental chambers with soil from a beechwood (Fagus sylvatica L.) on limestone. Experimental chambers were planted with the grass Hordelymus europeaus L. (Poaceae) and three compartments separated by 45-m mesh were established: rhizosphere, intermediate and non-rhizosphere. The experiment lasted for 16 weeks and the following parameters were measured at the end of the experiment: shoot and root mass of H. europaeus, carbon and nitrogen content in shoots and roots, density of ciliates, amoebae, flagellates and nematodes, microbial biomass (SIR), basal respiration, streptomycin sensitive respiration, ammonium and nitrate contents, phosphate content of soil compartments. In addition, leaching of nutrients (nitrogen and phosphorus) and leachate pH were measured at regular intervals in leachate obtained from suction cups in the experimental chambers. Protozoa stimulated the recovery of nitrifying bacteria following defaunation (by chloroform fumigation) and increased nitrogen losses as nitrate in leachate. In contrast, protozoa and nematodes reduced leaching of phosphate, an effect ascribed to stimulation of microbial growth early in the experiment. Earthworms strongly increased the amount of extractable mineral nitrogen whereas it was strongly reduced by protozoa and nematodes. Both protozoa and nematodes reduced the stimulatory effect of earthworms on nitrogen mineralization. Microbial biomass, basal respiration, and numbers of protozoa and nematodes increased in the vicinity of the root. Protozoa generally caused a decrease in microbial biomass whereas nematodes and earthworms reduced microbial biomass only in the absence of protozoa. None of the animals studied significantly affected basal respiration, but specific respiration of microorganisms (O₂ consumption per unit biomass) was generally higher in animal treatments. The stimulatory effect of nematodes and earthworms, however, occurred only in the absence of protozoa. The sensitivity of respiration to streptomycin suggested that protozoa selectively grazed on bacterial biomass but the bacterial/fungal ratio appeared to be unaffected by grazing of P. pellio. Earthworms reduced root biomass of H. europaeus, although shoot biomass remained unaffected, and concentrations of nitrogen in shoots and particularly in roots were strongly increased by earthworms. Both nematodes and protozoa increased plant biomass, particularly that of roots. This increase in plant biomass was accompanied by a marked decrease in nitrogen concentrations in roots and to a lesser extent in shoots. Generally, the effects of protozoa on plant growth considerably exceeded those of nematodes. It is concluded that nematodes and protozoa stimulated plant growth by non-nutritional effects, whereas the effects of earthworms were caused by an increase in nutrient supply to H. europaeus.     

Decomposer biomass in the rhizosphere to assess rhizodeposition

Quantification of the organic carbon released from plant roots is a challenge. These compounds of rhizodeposition are quickly transformed into CO₂ and eventually bacterial biomass to be consumed by bacterivores (protozoa and nematodes). Microbes stimulate rhizodeposition several-fold so assays under sterile conditions give an unrealistic value. Quantifying bacterial production from ³H-thymidine incorporation falls short in the rhizosphere and the use of isotopes does not allow clear distinction between labeled CO₂ released from roots or microbes. We reduced rhizodeposition in 3–5 week old barley with a 2 week leaf aphid attack and found that biomass of bacterivores but not bacteria in the rhizosphere correlated with plant–induced respiration activity belowground. This indicated top-down control of the bacteria. Moreover, at increasing density of aphids, bacterivore biomass in the rhizosphere decreased to the level in soil unaffected by roots. This suggests that difference in bacterivore biomass directly reflects variations in rhizodeposition. Rhizodeposition is estimated from plant-induced increases in bacterial and bacterivore biomass, and yield factors, maintenance requirements, and turnover rates from the literature. We use literature values that maximize requirements for organic carbon and still estimate the total organic rhizodeposition to be as little as 4–6% of the plant-induced respiration belowground.     

Salt distribution around roots of wheat under different transpiration rates

Summary Magnitude of Na and Cl accumulation around wheat roots was studied under different transpiration conditions in a loamy sand soil salinized with sodium chloride to an electrical conductivity of 4.1 mmho/cm in the saturation extract. A significant correlation was observed between rate of water loss per unit root length and Na and Cl content of the soil closely adhering to the roots. Under high transpiration condition, maximum ion accumulation occurred in the apparent free space of roots followed by the soil closely adhering to the roots. Results indicate that salt concentration at the root surface is markedly altered under the influence of transpiration rate such that under high transpiration conditions, the plant roots may be exposed to a much higher salt concentration than that anticipated from an analysis of the bulk soil samples.Department of Soil and Water EngineeringDepartment of Soils     

Effects of <i>Piriformospora indica</i> on the Respiration of <i>Taxus chinensis</i> var. <i>mairei</i> under Water Stress

Seedlings of Taxus chinensis var. mairei were used as experimental materials to study the adaptation of Piriformospora indica to this plant under water stress. The materials were divided into two groups, namely, with or without inoculation with P. indica. Each group was subjected to four different levels of water stress. Vitality and physiological and biochemical indexes of the roots of T. chinensis var. mairei were regularly measured. Under water stress, T. chinensis var. mairei had significantly decreased root vitality; root vitality was higher in inoculated roots than in uninoculated roots. Under intense water stress, the inoculated roots had a higher soluble sugar content than the uninoculated roots. Under water stress, T. chinensis var. mairei experienced decreased activity of aerobic respiratory metabolic enzymes. The activity of anaerobic respiratory metabolic enzymes and alcohol dehydrogenase initially increased and then decreased, whereas that of lactate dehydrogenase increased. The inoculated roots had a higher activity of respiratory metabolic enzymes than the uninoculated roots. As water stress was further intensified, the roots had significantly decreased activity of aerobic respiratory metabolic enzymes and significantly increased activity of anaerobic respiratory metabolic enzymes. The activity of respiratory metabolic enzymes decreased faster in the uninoculated roots than in the inoculated roots. This study demonstrated that Piriformospora indica plays a positive role in enhancing the antihypoxic ability of T. chinensis var. mairei, thereby alleviating plant damage due to water stress.     

Hirschmanniella spp. in Rice Fields of Vietnam

Root and soil samples from one-crop and two-crop rice fields were collected in a survey for Hirschmanniella spp. in Vietnam during 1978-80. Hirschmanniella spp. were found in 50-78% of the soil samples and 98-100% of the root samples collected. Population densities of nematodes in root systems were lowest at posttransplanting and highest at heading time. Numbers of nematodes inside roots increased 20-22 times from transplanting to heading in fields with both crop sequences. Population densities of Hirschmanniella spp. in two-crop rice fields were more than twice those in one-crop rice fields.     

Relationships between plant roots,proteolytic organisms and activity of protease

Proteolytic bacteria represented 18–58% of the bacterial population isolated from the rhizoplane of different crops. The activity of protease was considerably higher on roots of wheat growing in the soil than in the rhizosphere or free soil. However, only a slightly positive rhizosphere effect in the relative occurrence of casein-hydrolyzing bacteria could be observed. An indirect relationship between numbers of bacteria hydrolyzing casein and the activity of the enzyme could be found. The activity of protease related to a unit of culturable proteolytic bacteria was considerably higher on the root than in the rhizosphere and in the soil. A relationship between characteristics of the production of the enzyme by proteolytic bacteria and the protease activity on the surface of roots was demonstrated. The resulting enzyme activity on the surface of roots depended apparently on growth conditions of the plant and nature of root exudates and was influenced both by inactivation and protection due to adsorption of the enzyme by roots.     

Waterlogging responses in dune,swale and marsh populations of Spartina patens under field conditions

Summary Soil waterlogging responses were examined in three Spartina patens populations along a steep flooding gradient in coastal Louisiana. Root anatomy and physiological indicators of anaerobic metabolism were examined to identify and compare flooding responses in dune, swale and marsh populations, while soil physicochemical factors were measured to characterize the three habitats. Soil waterlogging increased along the gradient from dune to marsh habitats and was accompanied by increases in root porosity (aerenchyma). Aerenchyma in marsh roots was apparently insufficient to provide enough oxygen for aerobic respiratory demand, as indicated by high root alcohol dehydrogenase activities and low energy charge ratios. Patterns of root metabolic indicators suggest that dune and swale roots generally respired aerobically, while anaerobic metabolism was important in marsh roots. However, in each population, relatively greater soil waterloging was accompanied by differences in enzyme activities leading to malate accumulation. In dune and swale roots under these circumstances, depressed adenylate energy charge ratios may have been the result of an absence of increased ethanol fermentation. These trends suggest that: 1) Aerenchyma formation was an important, albeit incomplete, long-term adaptation to the prevalent degree of soil waterlogging. 2) All populations adjusted root metabolism in response to a relative (short-term) increase in soil waterlogging.     

Root-induced nitrogen mineralisation: A nitrogen balance model

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C:N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.     

Enhancement of Anaerobic Respiration in Root Tips of Zea mays following Low-Oxygen (Hypoxic) Acclimation

Root tips (10-millimeter length) were excised from hypoxically pretreated (HPT, 4% [v/v] oxygen at 25°C for 16 hours) or nonhypoxically pretreated (NHPT, 40% [v/v] oxygen) maize (Zea mays) plants, and their rates of respiration were compared by respirometry under aerobic and anaerobic conditions with exogenous glucose. The respiratory quotient under aerobic conditions with 50 millimolar glucose was approximately 1.0, which is consistent with glucose or other hexose sugars being utilized as the predominant carbon source in glycolysis. Under strictly anaerobic conditions (anoxia), glycolysis was accelerated appreciably in both HPT and NHPT root tips, but the rate of anaerobic respiration quickly declined in NHPT roots. [U-¹⁴C]Glucose supplied under anaerobic conditions was taken up and respired by HPT root tips up to five times more rapidly than by NHPT roots. When anaerobic ethanol production was measured with excised root tips in 50 millimolar glucose, HPT tissues consistently produced ethanol more rapidly than NHPT tissues. These data suggest that a period of low oxygen partial pressure is necessary to permit adequate acclimation of the root tip of maize to subsequent anoxia, resulting in more rapid rates of fermentation and generation of ATP.     

Phosphate-solubilizing peanut associated bacteria: screening for plant growth-promoting activities

Carbon and nitrogen are supplied by a variety of sources in the desert food web; both vascular and non-vascular plants and cyanobacteria supply carbon, and cyanobacteria and plant-associated rhizosphere bacteria are sources of biological nitrogen fixation. The objective of this study was to compare the relative influence of vascular plants and biological soil crusts on desert soil nematode and protozoan abundance and community composition. In the first experiment, biological soil crusts were removed by physical trampling. Treatments with crust removed had fewer nematodes and a greater relative ratio of bacterivores to microphytophages than treatments with intact crust. However, protozoa composition was similar with or without the presence of crusts. In a second experiment, nematode community composition was characterized along a spatial gradient away from stems of grasses or shrubs. Although nematodes generally occurred in increasing abundance nearer to plant stems, some genera (such as the enrichment-type Panagrolaimus) increased disproportionately more than others (such as the stress-tolerant Acromoldavicus). We propose that the impact of biological soil crusts and desert plants on soil microfauna, as reflected in the community composition of microbivorous nematodes, is a combination of carbon input, microclimate amelioration, and altered soil hydrology.     

添加葡萄糖和淀粉对盆栽甜樱桃根区土壤碳代谢及根功能的影响

在1年生盆栽甜樱桃土壤中添加葡萄糖和淀粉(4 g·kg^-1),以不添加外源碳为对照,处理后0~60 d内定期采根区土样测定土壤微生物生物量碳、蔗糖酶和淀粉酶活性以及微生物群落功能多样性,处理后第30天测定根系呼吸速率、呼吸途径和根系活力.结果表明： 添加葡萄糖后,土壤蔗糖酶活性及微生物生物量碳均表现为先升高再降低,峰值分别出现在处理后第15天及第7天,分别高于对照14.0%和13.1%,土壤有机质含量表现为先升高再降低再缓慢回升;添加淀粉后显著提高了土壤淀粉酶活性,第15天时为对照的8.5倍,土壤微生物生物量碳除在第7天低于对照外,其余时期均高于对照,土壤有机质含量表现为先升高再下降,处理后第60天高于对照19.8%.BIOLOG分析表明,处理后第15天平均吸光度（AWCD）值及微生物活性均达到最大值,表现为淀粉>葡萄糖>对照.处理后第30天,葡萄糖处理显著增加了土壤微生物对碳水化合物类、羧酸类、氨基酸类、酚酸类和胺类碳源的利用,淀粉处理显著增加了土壤微生物对碳水化合物类、羧酸类、聚合物类和酚酸类碳源的利用.处理后第30天,葡萄糖处理甜樱桃根系总呼吸速率分别较对照及淀粉处理提高21.4%和19.4%,根系活力分别提高65.5%和37.0%.添加葡萄糖和淀粉影响了甜樱桃根区土壤稳定碳源及不稳定碳源的代谢过程,整体上提高了土壤微生物活性,增强了甜樱桃根系呼吸速率及根系活力.     

Rapid modulation of nitrate reductase in pea roots

The regulatory properties of nitrate reductase (NR; EC 1.6.6.1) in root extracts from hydroponically grown pea (Pisum sativum L. cv. Kleine Rheinländerin) plants were examined and compared with known properties of NR from spinach and pea leaves. Nitrate-reductase activity (NRA) extracted from pea roots decreased slowly when plants were kept in the dark, or when illuminated plants were detopped, with a half-time of about 4 h (= slow modulation in vivo). In contrast, the half-time for the dark-inactivation of NR from pea leaves was only 10 min. However, when root tip segments were transferred from aerobic to anaerobic conditions or vice versa, changes in NRA were as rapid as in leaves (= rapid modulation in vivo). Nitrate-reductase activity was low when extracted from roots kept in solutions flushed with air or pure oxygen, and high in nitrogen. Okadaic acid, a specific inhibitor of type-1 and type-2A protein phosphatases, totally prevented the in vivo activation by anaerobiosis of NR, indicating that rapid activation of root NR involved protein dephosphorylation. Under aerobic conditions, the low NRA in roots was also rapidly increased by incubating the roots with either uncouplers or mannose. Under these conditions, and also under anaerobiosis, ATP levels in roots were much lower than in aerated control roots. Thus, whenever ATP levels in roots were artificially decreased, NRA increased rapidly. The highly active NR extracted from anaerobic roots could be partially inactivated in vitro by preincubation of desalted root extracts with MgATP (2 mM), with a half-time of about 20 min. It was reactivated by subsequently incubating the extracts with excess AMP (2 mM). Thus, pea root NR shares many of the previously described properties of NR from spinach leaves, suggesting that the root enzyme, like the leaf enzyme, can be rapidly modulated, probably by reversible protein phosphorylation/ dephosphorylation.     

Protozoan grazing of bacteria in soil—impact and importance

Interactions between bacteria and protozoa in soil were studied over 2-week periods in the field and in a pot experiment. Under natural conditions the total biological activity was temporarily synchronized by a large rainfall, and in the laboratory by the addition of water to dried-out soil, with or without plants. In the field, peaks in numbers and biomass of bacteria appeared after the rain, and a peak of naked amoebae quickly followed. Of the three investigated groups—flagellates, ciliates, and amoebae—only populations of the latter were large enough and fluctuated in a way that indicated a role as bacterial regulators. The bacterial increase was transient, and the amoebae alone were calculated to be able to cause 60% of the bacterial decrease. The same development of bacteria and protozoa was observed in the pot experiment: in the presence of roots, amoebic numbers increased 20 times and became 5 times higher than in the unplanted soil. In the planted pots, the amoebic increase was large enough to cause the whole bacterial decrease observed; but in the unplanted soil, consumption by the amoebae caused only one-third of the bacterial decrease.     

Spatial and temporal dynamics of hotspots of enzyme activity in soil as affected by living and dead roots—a soil zymography analysis

Aims

Hotspots of enzyme activity in soil strongly depend on carbon inputs such as rhizodeposits and root detritus. In this study, we compare the effect of living and dead Lupinus polyphyllus L. roots on the small-scale distribution of cellulase, chitinase and phosphatase activity in soil.

Methods

Soil zymography, a novel in situ method, was used to analyze extracellular cellulase, chitinase and phosphatase activity in the presence of i. living L. polyphyllus roots prior to shoot cutting and ii. dead/dying roots 10, 20 and 30 days after shoot cutting.

Results

After shoot cutting, cellulase and chitinase activities increased and were highest at the root tips. The areas of high cellulase and phosphatase activity extend up to 55 mm away from the root. Moreover, we observed microhotspots of cellulose, chitinase, and phosphatase activity up to 60 mm away from the next living root. The number and activity of microhotspots of chitinase activity was maximal 10 days after shoot cutting.

Conclusions

The study showed that young root detritus stimulates enzyme activities stronger than living roots. Soil zymography allowed identification of microhotspots of enzyme activity up to several cm away from living and dying roots, which most likely were caused by arbuscular mycorrhizal fungi.     

Effect of soil salinity, soil drought, and their combined action on the biochemical characteristics of cotton roots

Stress caused by soil salinity and soil drought limits cotton productivity in China. To determine the tolerance levels of cotton, we assessed the effects of soil salinity and soil drought on the biochemical characteristics of the roots of two cotton cultivars (CCRI-44, salt-tolerant; Sumian 12, salt-sensitive). Specifically, we analyzed root biomass, fatty acid composition, antioxidative enzyme activity, lipid peroxidation, H⁺-ATPase and Ca²⁺-ATPase activities. The cotton root biomass of the two cultivars declined significantly under conditions of soil salinity, soil drought, and the two stressors combined. The antioxidant enzyme activity of the roots also decreased markedly, which caused lipid peroxidation to increase, and changed the composition of the fatty acid membrane. H⁺-ATPase, Ca²⁺-ATPase and antioxidant enzyme activity decreased more under the two stressors combined. However, H₂O₂ content and O₂ ^? generation increased under the two stressors combined, compared to each stressor separately. Overall, the combination of soil salinity and drought has a greater inhibitory effect and more harmful impact on root growth than each stressor separately. The higher tolerance of CCRI-44 to soil salinity and drought stress than Sumian 12 might be explained by differences in cotton root antioxidative enzyme activity. The lipid peroxidation levels of cotton roots might represent an important biochemical trait for stress tolerance.     

Variations in ergosterol content and ornithine decarboxylase activity of ectomycorrhizal root-soil systems

Influences of soil P fertilization on temporal changes in ergosterol content and ornithine decarboxylase (E.C. 4.1.1.17, ODC) activity were monitored in rhizosphere soil, non-rhizosphere soil and Pinus contorta roots ectomycorrhizal with Hebeloma crustuliniforme grown in a loamy sand. With addition of mycorrhizal inoculum to loamy sand, ODC activity mg^-1 root increased between 10% and 2 fold within 21 weeks of pianting. Inoculation also decreased root mass per seedling. Inoculation increased mycelia mass per root mass by up to 2 fold but no differences were observed for total seedling mass until 35 weeks. Intramatrical mycelia were detrimental to early plant growth, but inoculated seedlings had 1.7 times more root mass and 1.3 times more shoot mass at 35 weeks. Rhizosphere soil contained up to 5 times more mycelia and up to 6 times greater ODC activity than non-rhizosphere soil. Inoculation increased rhizophere metabolic activity and intramatrical mycelia mass. Their sensitivity to fungal inoculation, P fertilization and temporal trends may make the methods useful in studies of rhizosphere ecology and root-microbe relationships.     

Relationship Between Paratrichadorus sp. Density, and Growth of Wheat in Pots

The effect of a Paratrichodorus sp. (close to P. tunisiensis) on the growth of wheat (Triticum durum Desf.) was investigated in pots containing different nematode densities and maintained in a growth chamber at 20 C for 40 days. The relation between fresh weight of tops and initial nematode density was according to the equation y = m + (1 - m)z^P⁻T. This suggests a tolerance limit of 1.4 nematodes/cm³ of soil under the conditions of the experiment; taking into account the effect of the great nematode mortality, it is estimated to be between 0.15 and 0.35 nematodes/cm³ soil. Models of the growth of the plants and the multiplication of the nematodes (assuming a constant mortality of the nematodes in the absence of roots) which explain the relation between initial and tinal nematodes densities at initial densities greater than 1 nematode/cm³ soil are described in an appendix. Sections of nematode infested roots showed disorganization of root structure clue to abnormal proliferation of lateral roots. Nematode feeding on the root cap and apical meristem caused cessation of root elongation and induced abnormal production of lateral root primordia.     

Optimized assay and storage conditions for enzyme activity profiling of ectomycorrhizae

The aim of a joint effort by different research teams was to provide an improved procedure for enzyme activity profiling of field-sampled ectomycorrhizae, including recommendations on the best conditions and maximum duration for storage of ectomycorrhizal samples. A more simplified and efficient protocol compared to formerly published procedures was achieved by using manufactured 96-filter plates in combination with a vacuum manifold and by optimizing incubation times. Major improvements were achieved by performing the series of eight enzyme assays with a single series of root samples instead of two series, reducing the time needed for sample preparation, minimizing error-prone steps such as pipetting and morphotyping, and facilitating subsequent DNA analyses due to the reduced sequencing effort. The best preservation of samples proved to be storage in soil at 4?C6°C in the form of undisturbed soil cores containing roots. Enzyme activities were maintained for up to 4?weeks under these conditions. Short-term storage of washed roots and ectomycorrhizal tips overnight in water did not cause substantial changes in enzyme activity profiles. No optimal means for longer-term storage by freezing at ?20°C or storage in 100% ethanol were recommended.     

Drip irrigation,soil characteristics and the root distribution and root activity of olive trees

A study was carried out on the root distribution and root activity of the olive tree (Olea Europaea, L., var. manzanillo) as influenced by drip irrigation and by several soil characteristics such as texture and depth. The experiments were conducted in two plots within a drip-irrigated grove of 20-year-old trees planted at 7×7 m spacing. One soil was a sandy loam, the other a clay-loam. Both cylinder and trench methods were used to determine root distribution. Labelling with ³²P was used to determine root activity. Under dryland conditions the adult tree adapted its rooting system, following the installation of a drip system, by concentrating the roots within the wet soil zones near the drippers. The highest root densities occur in those zones, down to a 0.6 m depth, the most abundant being the <0.5 mm diameter roots. The most intensive root activity was also found in that zone. For a given irrigation system, wet soil bulbs are more extensive and therefore root distribution expands to a larger soil volume when the soil is more clayey and with a hard calcareous pan present at about 0.8 m depth which prevents deep drainage.