Relationship between dehydrogenase activity and properties of soil aggregates

Studies of dehydrogenase activity in aggregates of chernozemic rendzina and lessivē soil proved that the diameter of aggregates significantly influences the activity of the above enzyme, subject to soil type. There is a significant positive correlation between the above activity and the percentage of particles 100 to 50 μm in aggregates of lessivē soil but not in those of chernozemic rendzina. Dehydrogenase activity is also affected by soil particles of other sizes, some pores in soil aggregates, the ratio of volume of air to water in them, content of total N and organic C as well as the total, external and internal surface areas, but the correlations in these cases are not significant at the level of 0.05.     

Is nitrification affected by the diameter and other properties of soil aggregates?

Nitrification was measured in fractions of chernozemic rendzina and lessivē soil differing in aggregate size. In both soils the maximum rates occurred in aggregates between 1 and 3 mm in diameter. The effects of structural and other properties (particle composition, pore-size distribution, surface area, organic C and total N content, ratio of air volume to water volume in aggregates) proved to be insignificant except for the nitrification rate in the lessivē soil, which positively correlated with the fraction of particles between 20 and 50 μm in diameter.     

Generic composition of fungi in soil aggregates

A structure of fungal flora was studied in aggregates of various diameter in a chernozemic rendzina and a lessivè soil. Aggregate diameter influenced strongly the distribution of fungi such asCephalosporium, Fusarium, Penicillium and sterile mycelia. The percentage of the above fungi changed with changing aggregate diameter as follows:Cephalosporium: 1–3>5–7 >0.5 in lessivè soil and ≤0.5>5–7>1–3 in chernozemic rendzina,Fusarium: ≤0.5>1–3>5–7 in lessivè soil and 5–7>1–3>0.5 in chernozemic rendzina,Penicillium: 5–7>0.5>1–3 in lessivè soil and 1–3>0.5>5–7 in chernozemic rendzina. The effect of aggregate size on the generic composition of fungal flora in the outer and inner parts of the aggregates was weaker than on that in the whole aggregates. This held especially forCephalosporium, Fusarium and sterile mycelia in the outer part of aggregates andPenicillum in the inner one. Moreover, the percentage of some fungi in whole aggregates of both soils was affected byV _a/V _w in the aggregates. Additional effect in lessivè soil aggregates was that of the percentage of pores measuring 0.1–0.05 μm. Some pores affected the contents of some fungi in the outer part of the aggregates of both soils and in the inner one of chernozemic rendzina aggregates.     

Distribution of microorganisms in soil aggregates: Effect of aggregate size

The distribution of microorganisms in soil aggregates with different diameters was determined using a “washing and sonic vibration” method. In humic rendzina the number of bacteria, actinomycetes and fungi located in aggregates measuring 3 to 1 and ≤0.5 mm was greater than in those of 7 to 5 mm.Pseudomonas were more numerous in aggregates of ≤0.5 mm than in those of 3 to 1 mm and spore-forming aerobic bacteria—in aggregates measuring 3 to 1 mm than in those of 7 to 5 mm. The number of microorganisms growing on asparagine agar andArthrobacter-Corynebacterium increased as 7-5<3-1<0.5 and 7-5<0.5<3-1 mm, respectively. In podzolic loess spore-forming aerobic bacteria inhabited preferentially aggregates measuring 7 to 5 mm,Arthrobacter-Corynebacterium aggregates of ≤0.5 mm. The number of bacteria was greater in aggregates of 3 to 1 than in those measuring ≤0.5 mm. Aggregates of various diameters differed also in the number of some microorganisms both in the outer and inner parts and the partition ratio of microorganisms between these parts. Differences were more numerous in the humic rendzina aggregates.     

Effects of forest conversion into grassland on soil aggregate structure and carbon storage in Panama: evidence from soil carbon fractionation and stable isotopes

Land-use and land-cover strongly influence soil properties such as the amount of soil organic carbon (SOC), aggregate structure and SOC turnover processes. We studied the effects of a vegetation shift from forest to grassland 90 years ago in soils derived from andesite material on Barro Colorado Island (BCI), Panama. We quantified the amount of carbon (C) and nitrogen (N) and determined the turnover of C in bulk soil, water stable aggregates (WSA) of different size classes (<53 μm, 53–250 μm, 250–2000 μm and 2000–8000 μm) and density fractions (free light fraction, intra-aggregate particulate organic matter and mineral associated soil organic C). Total SOC stocks (0–50 cm) under forest (84 Mg C ha⁻¹) and grassland (64 Mg C ha⁻¹) did not differ significantly. Our results revealed that vegetation type did not have an effect on aggregate structure and stability. The investigated soils at BCI did not show higher C and N concentrations in larger aggregates, indicating that organic material is not the major binding agent in these soils to form aggregates. Based on δ¹³C values and treating bulk soil as a single, homogenous C pool we estimated a mean residence time (MRT) of 69 years for the surface layer (0–5 cm). The MRT varied among the different SOC fractions and among depth. In 0–5 cm, MRT of intra-aggregate particulate organic matter (iPOM) was 29 years; whereas mineral associated soil organic C (mSOC) had a MRT of 124 years. These soils have substantial resilience to C and N losses because the >90% of C and N is associated with mSOC, which has a comparatively long MRT.     

Isolation and Characterization of Acetate-Utilizing Anaerobes from a Freshwater Sediment

Acetate-degrading anaerobic microorganisms in freshwater sediment were quantified by the most probable number technique. From the highest dilutions a methanogenic, a sulfate-reducing, and a nitrate-reducing microorganism were isolated with acetate as substrate. The methanogen (culture AMPB-Zg) was non-motile and rod-shaped with blunted ends (0.5–1 μm × 3–4 μm long). Doubling times with acetate at 30–35°C were 5.6–8.1 days. The methanogen grew only on acetate. Analysis of the 16S rRNA sequence showed that AMPB-Zg is closely related toMethanosaeta concilii. The isolated sulfate-reducing bacterium (strain ASRB-Zg) was rod-shaped with pointed ends (0.5–0.7 μm × 1.5–3.5 μm long), weakly motile, spore forming, and gram positive. At the optimum growth temperature of 30°C the doubling times with acetate were 3.9–5.3 days. The bacterium grew on a range of organic acids, such as acetate, butyrate, fumarate, and benzoate, but did not grow autotrophically with H₂, CO₂, and sulfate. The closest relative of strain ASRB-Zg isDesulfotomaculum acetoxidans. The nitrate-reducing bacterium (strain ANRB-Zg) was rod-shaped (0.5–0.7 μm × 0.7–1 μm long), weakly motile, and gram negative. Optimum growth with acetate occurred at 20–25°C. The bacterium grew on a range of organic substrates, such as acetate, butyrate, lactate, and glucose, and did grow autotrophically with H₂, CO₂, and oxygen but not with nitrate. In the presence of acetate and nitrate, thiosulfate was oxidized to sulfate. Phylogenetically, the closest relative of strain ANRB-Zg isVariovorax paradoxus.     

The interaction of surface and dust particle size on the pick-up and grooming behaviour of the German cockroachBlattella germanica

The relationship between the particle size of an inert silica dust, its up-take from different surfaces and the grooming behaviour of males, gravid females, and fifth and sixth instar nymphs of the German cockroachBlattella germanica (Dictyoptera: Blattellidae) (L.) was investigated. The normal grooming behaviour of gravid females, nymphs and males differed according to sex and age. The gravid females and nymphs exhibited greater grooming activity than the males, especially of the antennae and the legs. Gravid females, nymphs, and adult males exhibited increased grooming activity after exposure to dust in the size range 0.5–63 μm, but there was no significant difference in grooming behaviour from the control when cockroaches were exposed to dust sizes greater than 70 μm. Antennal grooming by males was greater than leg grooming when exposed to all dust sizes, except size particles ranging 4.5–7.5 μm. A dust pick-up experiment indicated that the average amount of dust transferred toB. germanica is affected by particle size, the porosity of the treated surface, and the sex and age of the cockroaches. Gravid females picked up greater amounts of dust than fifth and sixth instar nymphs, which in turn picked up more dust than males. Silica dust particles (0.5–7.5 μm) were picked up more effectively than larger particle sizes, by all three categories, males, females and fifth and sixth instar nymphs of cockroach on all three test surfaces plastic, ceramic and unpainted plywood. Plywood was the least effective surface for transfer of dust, of all sizes, to males, females and nymphs.     

Multiple shoot induction and callus regeneration in Sarcostemma brevistigma Wight & Arnott, a rare medicinal plant

An efficient micropropagation protocol based on multiple shoot induction and callus regeneration has been standardized in Sarcostemma brevistigma, a rare medicinal plant. The nodal cuttings were cultured on MS medium supplemented with BA (0.5–8 μM) or Kn (0.5–8 μM) alone or in combination with NAA (0.5–1.5 μM). Maximum multiple shoot induction was observed on MS medium supplemented with 4 μM BA. On this medium, 100% cultures responded with an average number of 11.3 shoots per explant. However, the average shoot length was limited to only 0.9 cm on this medium. The addition of 1 μM NAA along with 4 μM BA gave rise to an average number of 10.9 shoots with an average shoot length of 1.8 cm. Luxuriantly growing callus was obtained on MS medium supplemented with BA (5 μM) and 2,4-D (2 μM). The callus was subcultured on MS medium supplemented with BA (2–15 μM) or Kn (2–15 μM) alone or in combination with NAA (0.5–2 μM) for shoot organogenesis. Optimum callus regeneration was obtained on MS medium supplemented with 10 μM BA and 1 μM NAA. On this medium, 100% cultures responded with an average number of 13.4 shoots per culture. The shoots obtained via multiple shoot induction and organogenesis were rooted on half-strength MS medium supplemented with NAA (1–7 μM) or IBA (1–7 μM). IBA was better than NAA in terms of both the percentage of cultures that responded and the average number of roots per explant. The rooted shoots were successfully transplanted to soil with 86% success. This standardized protocol will help to conserve this rare medicinal plant.     

The response ofHelianthemum chamaecistus Mill. to mycorrhizal infection in two different types of soil

Summary The yield of mycorrhizal Helianthemum plants as affected by Cenococcum were assessed in two types of soils. 7–8 fold greater yield of mycorrhizal plants occurred in rendzina soil as compared to non-mycorrhizal plants. The increas ed yield of mycorrhizal plants in agricultural soils was not comparable to those grown in rendzina soil because of its high fertility status.     

Exopolysaccharide of Nostoc muscorum (Cyanobacteria) in the aggregation of soil particles

The effects on a saline-sodic soil of exopolysaccaride isolated from Nostoc Muscorum or the addition of a cyanobacterial mass proliferation were evaluated in a greenhouse experiment. By day 180 exopolysaccharide increased soluble C by 100%, microbial activity by 366% and the amount of water-stable aggregates larger than 250μm by 12 times. Inoculation with living cyanobacterial mass increased at the end of 365 oxidizable C by 11%, soluble C by 66%, microbial activity by 73% and aggregates larger than 250 μm by66%. A slimy film 3–5 mm thick, with N. Muscorum predominating, covered all the surface of inoculated soils. The higher soil aggregate stability produced by both treatments is a consequence of increased microbial activity and concentrating the soil polysaccharide. The high percentage of clays favours the creation of firm and long-lasting slime-mineral joints. Addition of isolated exopolysaccharide produces a faster and higher increase in soil aggregate stability than cyanobacterial mass inoculation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of heavy-metal additions on ammonification and nitrification in soils contaminated with cadmium,lead and zinc

Summary The sensitivity of the mineralization of nitrogen by a range of soils contaminated with heavy metals (up to 340 μg Cd g⁻¹, 7500 μg Pb g⁻¹ and 34000 μg Zn g⁻¹) to the addition of heavy metals in solution were studied using pot incubations (ammonification) and a soil perfusion technique (nitrification). The ammonification of peptone showed little correlation between treatments with Cd, Zn (1000 and 5000 μg g⁻¹) and Pb (10000 and 20000 μg g⁻¹) and origin of the soil. Nitrification was considerably more sensitive to heavy metals than ammonification. All the soils had active, often large, populations of ammonifying and nitrifying organisms which showed substantial similarities between the soils. The rate of nitrifying activity (NO₃−N production) was logrithmic in most cases. The presence of tolerant populations of nitrifying organisms in the contaminated soils was demonstrated. Tolerance was also eventually acquired after a longer lag phase, by the non-contaminated soil populations although the rate of activity was often reduced. Metals added in solution were adsorbed by the soil within 4 hours. Differences in toxicity between metal salts (chlorides, sulphates and acetate) were attributed to the amount left in solution. However, in many instances, acetate was found to stimulate all the stages in the mineralisation of nitrogen.     

Effects of forest management intensity on carbon and nitrogen content in different soil size fractions of a North Florida Spodosol

Pine plantations of the southeastern USA are regional carbon (C) sinks. In spite of large increases in woody biomass due to advanced growing systems, studies have shown little or even negative effects on the C content of the extremely sandy soils of this region. Hence, it is important to understand the mechanisms that determine the impact of intensive forest management on soil organic carbon (SOC) sequestration. This study was conducted to examine the C profile in a 4-year-old loblolly pine (Pinus taeda L.) plantation managed under two levels of management intensity (chemical understory control and fertilizer inputs). Soil organic C and nitrogen (N) pools were evaluated using two size fractionation methods, dry and wet sieving (2000–250 μm, 250–150 μm, 150–53 μm and <53 μm). Dry sieving was preferred over wet sieving for soil size fractionation, as it preserved more structure and water-soluble SOC components such as esters and amides and did not affect the N distribution. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were used to examine the chemical composition of the size fractions, which showed the presence of recently added organic matter in the largest sand fraction, as well as more decomposed organic matter in the <53 μm fraction. Intensive forest management reduced SOC in all three 2000–53 μm fractions, most likely due to reduced root input of understory plants that were controlled using herbicides. The 2000–250 μm fractions contained nearly half of the total SOC and showed a 23% decrease in C content due to the intensive management regime. Results from this study indicated the significance and responsiveness of sand size SOC fractions in Florida Spodosols. Results also showed that reductions in SOC due to intensive management occurred after four years and highlighted the need to understand the long-term impacts and the mechanisms responsible. Responsible Editor: Barbara Wick     

Nitrogen deposition effects on soil organic matter chemistry are linked to variation in enzymes,ecosystems and size fractions

Recent research has dramatically advanced our understanding of soil organic matter chemistry and the role of N in some organic matter transformations, but the effects of N deposition on soil C dynamics remain difficult to anticipate. We examined soil organic matter chemistry and enzyme kinetics in three size fractions (>250 μm, 63–250 μm, and <63 μm) following 6 years of simulated atmospheric N deposition in two ecosystems with contrasting litter biochemistry (sugar maple, Acer saccharum—basswood, Tilia americana and black oak, Quercus velutina—white oak, Q. alba). Ambient and simulated (80-kg NO₃ ⁻–N ha⁻¹ year⁻¹) atmospheric N deposition were studied in three replicate stands in each ecosystem. We found striking, ecosystem-specific effects of N deposition on soil organic matter chemistry using pyrolysis gas chromatography/mass spectrometry. First, furfural, the dominant pyrolysis product of polysaccharides, was significantly decreased by simulated N deposition in the sugar maple–basswood ecosystem (15.9 vs. 5.0%) but was increased by N deposition in the black oak–white oak ecosystem (8.8 vs. 24.0%). Second, simulated atmospheric N deposition increased the ratio of total lignin derivatives to total polysaccharides in the >250 μm fraction of the sugar maple–basswood ecosystem from 0.9 to 3.3 but there were no changes in other size classes or in the black oak–white oak ecosystem. Third, simulated N deposition increased the ratio of lignin derivatives to N-bearing compounds in the 63–250 and >250 μm fractions in both ecosystems but not in the <63 μm fraction. Relationships between enzyme kinetics and organic matter chemistry were strongest in the particulate fractions (>63 μm) where there were multiple correlations between oxidative enzyme activities and concentrations of lignin derivatives and between glycanolytic enzyme activities and concentrations of carbohydrates. Within silt-clay fractions (<63 μm), these enzyme-substrate correlations were attenuated by interactions with particle surfaces. Our results demonstrate that variation in enzyme activity resulting from atmospheric N deposition is directly linked to changes in soil organic matter chemistry, particularly those that occur within coarse soil size fractions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Microbial N Turnover and N-Oxide (N<Subscript>2</Subscript>O/NO/NO<Subscript>2</Subscript>) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg⁻¹ soil dry weight day⁻¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO _x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m⁻² h⁻¹, equivalent to 0.03–0.14 kg N₂O-N ha⁻¹ y⁻¹, and were considerably lower than previously reported for the same region. NO _x flux rates ranged between 0.16 and 0.48 μg NO _x -N m⁻² h⁻¹, equivalent to 0.01–0.04 kg NO _x -N ha⁻¹ y⁻¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.     

Distribution of Extensive nifH Gene Diversity Across Physical Soil Microenvironments

The diversity of nitrogen-fixing bacteria is well described for aquatic environments; however, terrestrial analyses remain mostly biased to rhizobial plant–microbe associations. We maximized the level of resolution for this study through the use of nucleotide sequence information extracted from a series of soil microenvironments, ranging from macroaggregates at 2000 μm to the clay fraction at <75 μm in diameter. In addition, we attempted to create an overview of the distribution of terrestrial nitrogen fixers across such microenvironments by combining culture-independent techniques with a suite of natural soil environments from uniquely different origins. Soil diazotroph diversity was analyzed phylogenetically for 600 terrestrial nifH sequences from 12 midsized clone libraries based on microenvironments of three separate soils across a global scale. Statistical analyses of nifH gene clone libraries were used to estimate coverage, establish degrees of sequence overlap, and compare cluster distributions. These analyses revealed an extensive diversity in a tropical (19 phylotypes) and an arctic soil (17 phylotypes), and moderate diversity in a temperate soil (11 phylotypes). Within each soil, comparisons across aggregate size fractions delineated nifH gene cluster shifts within populations and degrees of sequence overlap that ranged from significantly different (arctic, tropical) to significantly similar (temperate). We suggest that this is due to population separation across aggregates of different size classes, which results from differences in the temporal stability of aggregates as niches for microbial communities. This study not only provides new knowledge of the arrangement of diazotrophic communities at the soil microscale, but it also contributes to the underrepresented knowledge of soil nifH sequences in the public databases.     

Abundance of picophytoplankton in the halocline of a meromictic lake, Lake Suigetsu, Japan

Numerous (0.5 to 4.8 × 10⁵ cells/ml), small phytoplankton (smaller than 0.5–1 × 1–2 μm in cell size, picophytoplankton) were distributed in the halocline (depth 2–12 m, 4–14 practical salinity units) of the saline meromictic lake, Lake Suigetsu (35°35′ N, 135°52′ E), located in the central part of the coast of Wakasa Bay along the Japan Sea in Fukui Prefecture, Japan. Vertical distribution of phytoplankton revealed that the maximum number of picophytoplankton was always observed near or a little deeper than the oxic-anoxic boundary layer (depth 5–6 m); they were dominant phytoplankton in the water layer deeper than the oxic-anoxic boundary from July to late September 2005. Spectral analysis of autofluorescence emitted from the particle fractions smaller than 5 μm measured with a spectrofluorometer and from individual cells measured with a microscope photodiode array detector revealed that the major component of picophytoplankton was phycoerythrin-rich, unicellular cyanobacteria (picocyanobacteria). Eukaryotic phytoplankton about 2.5 μm in diameter were also found, but the numbers were low. Fluorescence intensity of chlorophyll a at 685 nm (room temperature) emitted from the particle fractions smaller than 5 μm was increased by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These observations indicated that at least some picophytoplankton had a functional photosystem II in the halocline where sulfide, the potential inhibitor of oxygenic photosynthesis, was always present. The large abundance together with their physiological potency suggest that picophytoplankton are one of the important primary producers in the halocline of Lake Suigetsu. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Aphanomyces infection in juvenile soft-shelled turtle,Pelodiscus sinensis,imported from Singapore

Aphanomyces sp. was isolated from the carapaces of two juvenile soft-shelled turtles with fungal infections imported from Singapore. Their sizes were 2.9–3.5 cm in carapace length. Lesions with integumental necrosis and ulceration looked like white cotton. The fungus exhibited slow growth, hyphae were 7.5–15 μm in diam, coarse, and abundantly branched. Zoosporangia observed in the isolate were complex, its entire thallus being converted into zoosporangial units, with short or long lateral evacuation tubes, and isodiametric, 100–500 μm in length. Clusters of zoospores were also produced at the terminals of hyphae. The production of the primary zoospores was achlyoid. The primary encysted zoospores were spherical, 10–15 μm in diam. No sexual stages were observed on a hemp seed incubated in sterile tap water. The optimal temperature for the fungus was 30° C.     

Degradation efficiency and molecular size alteration during the aerobic microbial treatment of lignite pyrolysis deposit water

 Investigations into aerobic biological degrada-tion were carried out as part of an extensive programme designed to facilitate the cheap remediation of a pyrolysis waste-water deposit. Attention was focused on the processes of carbon conversion by different populations. The susceptibility of a body of lignite-processing deposit water to microbiological degradation was examined in batch investigations in a Sapromat system and in continuous bench-scale fermenter cultivations, with respect to nutrient supply, inoculation culture and molecular size distribution. It was found that degradation best occurs with an adapted mixed culture. The autochthonous culture removes 30% less dissolved organic carbon (DOC) and has a 40% higher specific oxygen demand. A shortage of phosphorus, investigated with a view to avoiding additional eutrophication problems in the open water in the case of in situ remediation, causes reduced DOC degrada-tion and significantly higher specific oxygen demand. The biological process is overlapped by abiotic oxidation. During aerobic treatment, a concentration of colour-giving aromatic substances of between 0.5 kDa and 5 kDa was observed. This phenomenon is caused by the oxidation of low- and high-molecular-mass compounds. The removal of DOC is limited to 65% and mainly occurs in the range below 0.5 kDa (30%) and in the 0.5–1 kDa range (12%); the removal is negligible in the ranges 1–3 kDa (0.8%) and 3–5 kDa (2%) and a little higher in the ranges 5 kDa–0.3 μm (5%) and above 0.3 μm (6%). In the investigations it was discovered that DOC removal causes in the ranges below 0.5 kDa, 0.5–1 kDa and 5 kDa–0.3 μm mainly as a result of degradation, but the range above 0.3 μm is chiefly caused by bioadsorption. Aerobic microbiological treatment is able to remove most low-molecular-mass substances. In order to remove the macromolecular and colour-giving part of the deposit water, an additional treatment stage, e.g. flocculation, is required. Received: 10 October 1995/Received revision: 9 January 1996/Accepted: 15 January 1996     

Germination and seedling growth of carrots under salinity and moisture stress

Poor crop stand is a common problem in saline areas. Germination and seedling emergence may be depressed as a result of impeded aeration, saline or dry conditions. In this study, we examined the effects of salinity and moisture stress and their interactions on seed germination and seedling growth of carrots. Variable soil matric and osmotic potentials were either obtained by equilibrating soil salinized to different degrees on a 0.5 MPa ceramic plate soil moisture extractor or by adding different amounts of salt solutions to the same mass of air-dried soil, based on a previously determined soil moisture release curve, and allowing to equilibrate for 1 week. Germination decreased significantly in the investigated silty soil (Aquic Ustifluvent) at soil moisture potentials higher than −0.01 MPa, whereas osmotic potentials as low as −0.5 MPa did not influence germination. Matric potentials of −0.3 and −0.4 MPa, respectively, resulted in a strong decrease (35–95%) of germination and delayed germination by 2 to 5 days in the silty soil to which different amounts (18 and 36%, respectively) and sizes (0.8–1.2 mm and 1.5–2.2 mm, respectively) of sand particles had been added. No effect of sand and grain diameter was detected. Germination was not affected by comparable osmotic potentials. Seedling growth showed a much higher sensitivity than germination to decreasing matric potentials, but was not affected by osmotic potentials ranging from −0.05 to −0.5 MPa. Optimum shoot growth occurred at matric potentials between −0.025 and −0.1 MPa. Shoot and root growth decreased markedly at matric potentials higher than −0.01 MPa. Fresh weight of shoots decreased gradually at matric potentials lower than −0.2 MPa. Root growth was significantly increased at matric potentials of −0.1 to −0.3 MPa, whereas comparable osmotic potentials did not have equivalent effects. It is concluded that germination and seedling growth are differently affected by comparable matric and osmotic stresses and that water stress exerts a more negative effect than salt stress.     

Influence of Captan on some microorganisms and microbial processes related to the nitrogen cycle

Captan was applied to laboratory-incubated agricultural soil and to bacterial cultures to determine its effects on total counts of soil microorganisms, nitrification, ammonification of urea and asymbiotic dinitrogen fixation. In Captan-treated soils, total count of fungi, bacteria and actinomycetes decreased significantly only at a relatively high fungicide concentration (1000 μg.g⁻¹). Fungi and actinomycetes were more affected than bacteria. While oxidation of ammonia in an enriched, actively nitrifying culture was almost totally inhibited by Captan, ammonification of urea in incubated soil was only partly depressed. The depressing effect of Captan was more pronounced in cultures of Micrococcus than in those of Proteus. Asymbiotic dinitrogen fixation in nutrient-ammended soil was promoted during the first week and depressed on prolonged exposure to the fungicide depending on its first concentration. In autoclaved Azotobacter-inoculated soil a similar but less pronounced effect was noticed. Fixation by Azotobacter caltures was insensitive to Captan. In contrast, growth ofRhizobium phaseoli, R. leguminosarum andR. japonicum in yeast-extract-mannitol medium was adversly affected by Captan, particularly at 200 μg.ml⁻¹. Nodulation of pea and mung bean (1 month old potted plants) grown from surface-sterilized inoculated seeds in aptan-treated soil was also significantly depressed. Both total number of nodules decreased with increasing concentration of the fungicide, but the inhibitory effect was more pronounced in the number of effective nodules.