Thomas E. Loynachan

Spore germination of Gigaspora margarita stimulated by volatiles of soil-isolated actinomycetes

Abstract Nineteen field isolates of actinomycetes and Streptomyces orientalis were evaluated for their production of volatile compounds that stimulate spore germination of Gigaspora margarita , an arbuscular mycorrhizal fungus. Some actinomycetes enhanced spore germination, whereas others did not. When placed in a common headspace with actinomycetes, but physically separated in a divided Petri dish, spores showed germination rates up to 73%. In control treatments lacking actinomycetes, spores showed 23% germination by the final day of observation, day 11. Actinomycetes having straight spore-bearing hyphae stimulated germination more than did actinomycetes having spiral spore-bearing hyphae ( P ⩽ 0.01 on day 11). Other traits of the actinomycetes that related ( P ⩽ 0.05) to increased spore germination included late sporulation and possession of dark vegetative hyphae but lacking melanin or other water-soluble pigments. Amounts of the volatile 2-methylisoborneol produced by actinomycetes were well-correlated with spore germination.

Extent of pyrolysis impacts on fast pyrolysis biochar properties.

A potential concern about the use of fast pyrolysis rather than slow pyrolysis biochars as soil amendments is that they may contain high levels of bioavailable C due to short particle residence times in the reactors, which could reduce the stability of biochar C and cause nutrient immobilization in soils. To investigate this concern, three corn ( L.) stover fast pyrolysis biochars prepared using different reactor conditions were chemically and physically characterized to determine their extent of pyrolysis. These biochars were also incubated in soil to assess their impact on soil CO emissions, nutrient availability, microorganism population growth, and water retention capacity. Elemental analysis and quantitative solid-state C nuclear magnetic resonance spectroscopy showed variation in O functional groups (associated primarily with carbohydrates) and aromatic C, which could be used to define extent of pyrolysis. A 24-wk incubation performed using a sandy soil amended with 0.5 wt% of corn stover biochar showed a small but significant decrease in soil CO emissions and a decrease in the bacteria:fungi ratios with extent of pyrolysis. Relative to the control soil, biochar-amended soils had small increases in CO emissions and extractable nutrients, but similar microorganism populations, extractable NO levels, and water retention capacities. Corn stover amendments, by contrast, significantly increased soil CO emissions and microbial populations, and reduced extractable NO. These results indicate that C in fast pyrolysis biochar is stable in soil environments and will not appreciably contribute to nutrient immobilization.

Desiccation tolerance of four strains of Rhizobium japonicum

Abstract The survival of different strains of Rhizobium japonicum was evaluated in three soils with two matric- and three osmotic-induced moisture potentials. Both drying and added NaCl significantly decreased populations. Strains CC 709 and USDA 110 were less affected by the matric- or osmotic-induced desiccation than strains CB 1809 and USDA 123. The survival of CC 709 and USDA 110 at 2 weeks with 0.7% added NaCI was 33 and 46% of initial counts in soils undergoing drying, and 70 and 69% in soils maintained at the 30kPa (0.3 bar) potential. Comparable survivals of CB 1809 and USDA 123 were 15 and 18% and 56 and 59%, respectively. The soil with the greatest clay and organic C contents maintained the highest populations during desiccation. Turbidity measurements indicated similar rates of growth of the four strains at a water activity ( A w ) of 0.999 in yeast mannitol broth (YMB). When the YMB was adjusted with glycerol to lower water activities, strains CC 709 and USDA 110 consistently showed greater growth than did strains CB 1809 and USDA 123. No growth of CB 1809 and USDA 123 was observed at an ( A w ) of 0.975. Water adsorption isotherms of freeze-dried cells showed that the more desiccation-susceptible strains (CC 709 and USDA 110) retained greater amounts of water at a given relative vapor pressure than did the two more tolerant strains.

Methane Flux in Cropland and Adjacent Riparian Buffers with Different Vegetation Covers

While water quality functions of conservation buffers established adjacent to cropped fields have been widely documented, the relative contribution of these re-established perennial plant systems to greenhouse gases has not been completely documented. In the case of methane (CH(4)), these systems have the potential to serve as sinks of CH(4) or may provide favorable conditions for CH(4) production. This study quantifies CH(4) flux from soils of riparian buffer systems comprised of three vegetation types and compares these fluxes with those of adjacent crop fields. We measured soil properties and diel and seasonal variations of CH(4) flux in 7 to 17 yr-old re-established riparian forest buffers, warm-season and cool-season grass filters, and an adjacent crop field located in the Bear Creek watershed in central Iowa. Forest buffer and grass filter soils had significantly lower bulk density (P < 0.01); and higher pH (P < 0.01), total carbon (TC) (P < 0.01), and total nitrogen (TN) (P < 0.01) than crop field soils. There was no significant relationship between CH(4) flux and soil moisture or soil temperature among sites within the range of conditions observed. Cumulative CH(4) flux was -0.80 kg CH(4)-C ha(-1) yr(-1) in the cropped field, -0.46 kg CH(4)-C ha(-1) yr(-1) within the forest buffers, and 0.04 kg CH(4)-C ha(-1) yr(-1) within grass filters, but difference among vegetation covers was not significant. Results suggest that CH(4) flux was not changed after establishment of perennial vegetation on cropped soils, despite significant changes in soil properties.

Soil drainage and distribution of VAM fungi in two toposequences

Much remains unknown about the distribution and abundance of vesicular-arbuscular mycorrhizal (VAM) fungi in natural ecosystems. Our objective was to determine whether soil drainage and position in the toposequence affect VAM fungal spore production and colonization of soybean (Glycine max L. Merr.) roots. Plant roots and rhizosphere soil from 24 soils, which consisted of sets of three soils each that varied in soil drainage class from four sites located in two toposequences [Clarion-Nicollet-Webster (CNW) and Sharpsburg-Macksburg-Winterset (SMW)], were sampled. Soils were analyzed for chemical properties, and numbers of VAM fungal spores per 50 g soil were determined by a wet-sieving-and-decanting technique. The percentages of root VAM fungal colonization were determined by the grid-line intersect method. The distribution of spores among locations, series and series within locations varied significantly (P < 0.001). Soil from the CNW toposequence averaged 417 spores (50 g)−1 soil, whereas soil from the SMW toposequence averaged 147 spores (50 g)−1 soil. Within the two toposequences, the poorly to somewhat poorly drained soils had higher spore counts than the well-drained to moderately well-drained soils. In the CNW toposequence, the mean spore counts of Nicollet and Webster soils (poorer drained) were 1.37-fold higher than those of Clarion soils (better drained). In the SMW toposequence, the mean spore counts of Macksburg and Winterset soils (poorer drained) were 2.06 fold higher than those of Sharpsburg soils (better drained). VAM fungal colonization of roots ranged from 74 to 88% in the CNW toposequence and from 68 to 86% in the SMW toposequence. No correlation was found between spore count and VAM fungal colonization.

Tn916-mediated genetic exchange in soil

Abstract In sterile soil, Tn916 was observed to transfer between Bacillus subtilis and Bacillus thuringiensis subsp. israelensis at an average frequency of 2.1 × 10 −5 conjugants per donor. Moreover, transfer of Tn916 between Enterococcus faecalis , from which Tn916 was originally isolated, and B. thuringiensis was demonstrated at an average frequency of 2.3 × 10 −6 . The ability of Tn916 to transfer between B. subtilis and B. thuringiensis was evaluated over varying conditions of soil moisture (2–13%), temperature (− 20–45°C), pH (3.8–9.2) and nutrient (0–2.5 mg g −1 brain heart infusion). Conjugal transfer of Tn916 was observed over the entire range of moisture conditions with maximum rates in the range of 2–5% whereas transfer was restricted to temperatures in the range 18–37°C (optimum of 30°C). Transfer of the transposon was restricted to pH values ranging from 6 to 8.9 but within this range, pH had minimal effect on transfer frequency. Increasing nutrients enhanced conjugal transfer of Tn916 throughout the range tested. Finally, besides conjugal transfer, mobilization of the non-conjugal plasmid pC194 by Tn916 was observed in a soil environment at an average frequency of 2.0 × 10 −6 . These results suggest that transposon-mediated genetic exchange between bacteria may facilitate transfer of antibiotic-resistance determinants as well as other non-conjugal DNA in soil.

Bacteria of the soybean rhizosphere and their effect on growth of Bradyrhizobium japonicum

Abstract We studied the rhizosphere populations of soybean ( Glycine max . L.) in two soils throughout the growing season and evaluated the activities of selected organisms on nodule occupancy of serogroup 123 (USDA 123), the dominant Bradyrhizobium japonicum found in mid-U.S.A., soils. Non-specific bacteria and actinomycetes. Gram-negative bacteria, enterobacteria and pseudomonads were enumerated approx. every 3 wk on six occasions during the growing season. Early in the season, 31% of the rhizosphere bacteria were inhibitory to USDA 123 as estimated by an agar-overlay technique, but only 8% were stimulatory. Nine of the stimulatory organisms were tested in the greenhouse in sterile soil for their ability to alter occupancy of soybean nodules by strain USDA 123 when challenged by USDA 110. Two of these rhizosphere organisms significantly enhanced nodulation of USDA 123, but further tests indicated that the results were not reproducible in non-sterile soil. In general, rhizosphere populations in the field increased throughout the season but varied with climatic variables. Populations were positively correlated with precipitation and temperature and negatively correlated with evaporative index.

ABUNDANCE AND CHARACTERIZATION OF COWPEA MISCELLANY RHIZOBIUM FROM SUDANESE SOILS

Abstract Sudan is the fourth largest exporter of groundnuts in the world, yet little is known concerning the plant-rhizobial symbiosis. A study was made on the abundance of groundnut-nodulating rhizobia in the soils of Sudan as related to soil properties and the duration since groundnuts were last planted. Also, physiological, serological and nitrogen-fixing characteristics of Sudanese rhizobia are reported. All but one of 32 sites contained more than 300 rhizobia g−1 soil capable of forming nodules on siratro (Macroptilium atropurpureum). Several of these soils had never been planted to groundnut. A correlation matrix indicated no relationship was present between soil rhizobial populations and any of the measured soil properties, or between soil rhizobial populations and the time since groundnuts were last planted in the rotation. Individual isolates of Rhizobium from six legumes: groundnut (Arachis hypogaea), mung bean (Vigna radiata), lubia (Dolichos lablab), cowpea (Vigna unguiculata), pigeonpea (Cajanus cajan) and bambara groundnut (Voandzeia subterranea) were obtained from four locations in Sudan. All isolates were able to nodulate each of the six legumes when grown in sterile vermiculite. The isolates grew in 0.1% NaCl-amended media, but growth was variable in 2.0% amended media. Most isolates grew after exposure to moist heat for 15 min at 50°C. Optimum pH for growth was, in general, between pH 6 and 8. Agglutination reactions indicated isolates from groundnuts, as well as isolates from other legumes, belonged to several serological groupings. Some isolates formed a large number of nodules on a Sudanese groundnut cultivar, whereas other isolates formed only few nodules.

Inhibition of Nodulation and Nitrogen Nutrition of Leguminous Crops by Selected Heavy Metals

This work studied the effects, under greenhouse conditions, of six heavy metals (Cd, Co, Cr, Cu, Ni, and Pb) on three leguminous crops representing food, feed, and forage crops commonly grown in Egypt. Metal concentrations ranged from 0 to as high as 4.8 mmol kg−1 soil. Results showed that all three plant parameters measured (dry matter yield, nodulation, and N uptake) decreased significantly with increasing heavy-metal concentrations. Plots of the natural log of each parameter against metal concentration were linear within the ranges studies. From the slopes of these regression lines, the concentration of each heavy metal required to achieve 50% reduction (R50) of each parameter was calculated. In general, the lowest metal concentrations for R50 were for Cd2+ and Pb2+ and the highest were for Cr3+ and Cu2+. Heavy-metal additions to soils should be closely monitored because they can negatively affect nodulation and N nutrition of leguminous crops.

Inhibition of Bradyrhizobium japonicum by diffusates from soybean seed

Rhizobia and bradyrhizobia are often added as seed coatings during inoculation, but leguminous seeds are reported to contain components toxic to the growth of these organisms. We evaluated different bradyrhizobial strains for tolerance to the toxic components of soybean (Glycine max L.) seed and sought the origin and characteristics of the toxins produced. Zones of growth inhibition of five strains of Bradyrhizobium japonicum (USDA 6, 110, 122, 123 and 135) were determined on laboratory media surrounding surface-sterilized whole seed, seed coat, cotyledon and dicotyledons-plus-embryo of three varieties of soybean. Disrupting the seed coat increased toxicity; the zone of inhibition surrounding the seed coat, cotyledon and dicotyledons-plus-embryo was double the zone of inhibition surrounding the intact, whole seed. Strains differed in their sensitivity to diffusates; strain USDA 6 (having a narrow zone of inhibition) was most tolerant to the toxins and USDA 123 (having a wide zone of inhibition) was the least tolerant. Autoclaving of seed, both wet and dry, was compared with surface sterilization by ethanol and HgCl2 treatment to determine the effect of different seed treatments on inhibition of growth; autoclaving, whether wet or dry, resulted in greater diffusion of seed toxins and larger zones of inhibition. Soaking seeds in water for 0, 2 and 4 h before placing them on the agar surface decreased toxicity; the decrease was linear with soaking time. The anti-bradyrhizobial substances were water soluble, thermostable and <2000 daltons in size.