The physiological characteristics and host plant relationships of 14 strains of rhizobia, isolated from the nodules of Caragana arborescens L., have been studied. The nodules from which these isolates were obtained came from six widely separated localities in North America. The strains differed markedly in rates of growth, in their ability to reduce nitrate, in their reactions in litmus milk and on 12 carbohydrate media, and in their abilities to fix nitrogen symbiotically. The strains showed uniformity in their abilities to produce nodules on Phaseolus vulgaris, Colutea arborescens, Lespedeza stipulacea, Trifolium pratense, and Lotus corniculatus. Because these species are now included in separate bacterial–plant groups, an assignment of C. arborescens to a cross-inoculation group appears unwise at this time. Thirty strains of rhizobia isolated from 20 species contained in 15 leguminous genera failed to nodulate C. arborescens in greenhouse tests. Various examples of nonreciprocal cross-inoculation were noted. Strains of caragana rhizobia reisolated from nodules formed by them on Trifolium pratense and Colutea arborescens retained their abilities to form nodules on Caragana arborescens.
Abstract The relative importance of diverse factors affecting aggregation in four Wisconsin soil types was studied by multiple regression analyses. Single and combined effects of pH and contents of organic matter, microbial gum, clay, and free iron oxide were considered. In general the most important single factor was the microbial gum. However, in the Kewaunee soil which has a relatively high content of clay and iron oxide, the iron oxide was of prime importance in aggregate formation. In all the soils, iron oxide showed a marked effect on aggregation with a tendency to be more important in the smaller aggregate size range. The effect of clay, however, was very small with the exception of the high clay soil (often > 30% clay), in which its effect was only exceeded by that of the free iron oxide. In the multiple correlation analysis the effect of organic matter was considered separately from that of microbial gums. By this statistical analysis only a slight positive relationship existed between organic matter and soil aggregation, i.e., the effect of organic matter was conditioned largely by its content of microbial gum; pH 6.5 was optimum. No relationship existed between soil aggregation and the rate of decomposition of organic matter as measured by CO 2 evolution from the soil.
Plaques of phages PR-1001, PS-192, PR-590a, and PsR-1012 were clear, without halos, and with definite edges. Phage heads were approximately 60 mμ in diameter with short blunt tails. Phage titers in nutrient broth were stable during 452 days storage at 4 °C but were decreased considerably at 25 °C. After 46 days at 37 °C no surviving particles of any of the phages were detected. Ultraviolet inactivation of particles was not a first-order reaction. The pH stability range was between pH 7 and 11. Inactivation at 50 °C and 55 °C was least in saline, more marked in nutrient broth, and most severe in distilled water. Protection against inactivation was a function of the mono- and di-valent cation concentration. Separately the cations provided insignificant protection. NH 4+ in combination with Ca ++ or Mg ++ inactivated phage particles as did also Cu ++ , Hg ++ , Al +++ , and Fe +++ in solutions of NaCl. Heat-inactivated particles showed collapsed heads that indicated a loss in DNA.
Abstract A method for evaluating the mechanisms and factors involved in soil aggregate stabilization by microorganisms is introduced and data are presented to illustrate its use. This method involves the incubation of artificial soil aggregates of uniform and controlled physical and chemical properties, so that the effect of one or more variables of soil composition or environment on aggregate stability can be investigated. The stabilities of differently‐amended artificial aggregates of two diverse soil materials, incubated for up to 20 weeks, were determined by wet‐sieving the dried aggregates. The two soil materials, one obtained from the Ap horizon of a Kewaunee silty clay loam (Gray‐Brown Podzolic) and the other from the Al horizon of a Waupun silt loam (Brunizem), differed markedly in their contents of clay, nitrogen and organic carbon. Microbial gum‐amended Kewaunee aggregates possessed a higher initial stability and were more resistant to degradation with continued incubation than corresponding Waupun aggregates. Added nitrogen was associated with accelerated degradation of Kewaunee but not Waupun gum‐amended aggregates. Incubated sucrose‐amended Kewaunee aggregates showed a slower increase to maximum stability but retained stability longer than sucrose + nitrogen‐amended Kewaunee and sucrose‐ and sucrose + nitrogen‐amended Waupun aggregates. Nonamended and nitrogen‐amended aggregates of both soils were completely unstable throughout the incubation period. The stability of moist aggregates was a function of wetting rate and moisture status prior to sieving; aggregates moistened slowly to saturation were 100% stable to wet‐sieving. Periodate treatment of the aggregates indicated that, with the exception of incubated sucrose‐amended Waupun aggregates, “periodate‐oxidizable” polysaccharides were important in aggregate stabilization. Four days of incubation under highly aerobic or anaerobic conditions caused a rapid increase in the stability of sucrose‐amended Waupun aggregates. Continued aerobic incubation resulted in a rapid decrease in aggregate stability; aerobically incubated aggregates were totally unstable after 12 days. In contrast, continued anaerobic incubation was associated with an increase in stability; 100% aggregate stability was reached after 15 days and was still present after 4 weeks. Anaerobically‐stabilized aggregates lost their stability completely after 9 days of aerobic incubation.
'/%3e%3cuse xlink:href='%23a' transform='rotate(72 0 0)'/%3e%3cuse xlink:href='%23a' transform='rotate(-72 0 0)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1) rotate(72)'/%3e%3c/g%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h1200v600H0z'/%3e%3cpath stroke='%23FFF' stroke-width='60' d='m0 0 600 300M0 300 600 0' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='40' d='m0 0 600 300M0 300 600 0' clip-path='url(%23c)'/%3e%3cpath stroke='%23FFF' stroke-width='100' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='60' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(45.4 0 0 45.4 900 120)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(30 0 0 30 900 120)'/%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 519.022 -457.666) scale(40.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 519.022 -457.666) scale(25)'/%3e%3c/g%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 668.57 -327.666) scale(45.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 668.57 -327.666) scale(30)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(50.4 0 0 50.4 900 480)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(35 0 0 35 900 480)'/%3e%3c/svg%3e)