George H. Wagner

Post-harvest residue input to cropland

SummaryTotal annual input of carbon with crop residues to the soils in Missouri (North Central Region of the USA) was found to be about 3.7 t ha−1 for wheat, 3.4 t ha−1 for soybeans and 9.2 t ha−1 for corn. Root biomass represented 40–50% of total residues and its accumulation appears to be influenced by climatic conditions. Estimates of carbon outflow from the ecosystem in the form of CO2 were in good agreement with the biomass input.

QUANTITATIVE AND QUALITATIVE EXAMINATION OF SOIL MICROFLORA ASSOCIATED WITH DIFFERENT MANAGEMENT SYSTEMS

Soil samples from field plots under long-term management systems were examined using standard plate count procedures. Samples collected at monthly intervals from the different plots showed a mean value for bacteria plus actinomycetes of 7 × 107/g soil. The mean for fungi was 4 × 105/g soil. Microbial populations were low for untreated soils, intermediate for soils treated with chemical fertilizers, and high for soils that received annual applications of manure. Soils cropped to wheat had greater numbers of microorganisms that those cropped to corn. Seasonal fluctuations in microbial numbers are discussed in relation to cultivation practices and to change in soil moisture and temperature. A total of 63 different fungal isolates representing 48 genera were identified; the Penicillium and Aspergilli predominated. The frequency of occurrence of some of the fungi is discussed in relation to the crop under cultivation, the soil treatment and/or the season of the year.

Soil microorganisms antagonistic towards Rhizobium japonicum

Abstract Success in introducing Rhizobium japonicum strains into soil is related to their interaction with native microorganisms including some that are antagonistic. Actinomycetes, bacteria, fungi and rhizobiophages antagonistic towards strains of R. japonicum were counted directly using soil samples from field plots under different crop and soil management systems. The antagonistic actinomycete population varied from 1.3 × 103 to 4.5 × 105 g−1 dry soil and ranged up to 90% of total actinomycetes. Soybean rhizosphere soil samples included antagonistic actinomycetes ranging up to 70% of total actinomycetes. The antagonistic bacterial population was less than 10% of total bacteria and the proportion did not vary significantly with crop or soil management practices. Antagonistic fungi were observed for many of the soils examined but they could not be counted. There were few rhizobiophages and they were found most frequently in soybean rhizospheres. Occasional bacterial and actinomycete colonies that stimulated growth of R. japonicum were randomly observed among the soil samples tested.

Aflatoxin B1 effects on soil microorganisms

Abstract Corn contaminated with aflatoxin is unfit for consumption by animals and is most often disposed of by plowing it into the soil. The effect of aflatoxin B1 on the population and activity of soil microorganisms was determined at concentrations of 1, 100 and 10,000 ng ml−1 of agar media or g−1 of soil. Aflatoxin B1 at 10,000 ng ml−1 of medium reduced the number of viable fungi by 38% and the number of bacteria and actinomycetes by 34%. Soil amended to 10,000 ng aflatoxin B1 demonstrated a slight, yet significant reduction in the population of fungi and bacteria plus actinomycetes. At this rate the antagonistic effect on soil microorganisms began at 14 days after aflatoxin B1 was added and lasted nearly 6 weeks. Subsequently no significant differences were observed among any of the treatments. When the soil was amended with alfalfa to provide a substrate for microbial growth, the population showed a more significant reduction due to aflatoxin B1, but the duration of the effect was reduced. The evolution of CO2 from soil amended with aflatoxin B1 showed little if any diminution. Similarly, aflatoxin B1 failed to demonstrate a significant effect on nitrifying bacteria. Aflatoxin B1 was found to be slightly deleterious to Rhizobium japonicum, resulting in a 30% reduction in numbers at the highest treatment rate. Using auxotrophic cultures of R. japonicum, aflatoxin B1 was also shown to induce the formation of mutants.

Detection of soluble Rhizobium japonicum antigens in soil by immunodiffusion

Abstract Innovative techniques for quantifying rhizobia in soil are needed. Immunodiffusion was adapted for use in evaluating four strains of Rhizobium japonicum incubated in soil. Reproducible precipitin bands were obtained using uniform soil cores 5 mm dia × 4 mm length collected with a cork borer. A flocculent of Ca(OH)2 and MgCO3 added to the microcores enhanced band formation. Clear distinct bands developed from populations in soil of 1 × 108 cells g−1 or greater. The complete set of bands which characterized cells cultured on artificial media did not develop from cells inoculated into soil, but at least one detectable band was always observed. Parameters of intensity value and band positioning were related to the population in the soil. Detection by immunodiffusion of a series of rhizobial populations in soil decreased with time almost linearly as the initial population decreased. Direct diffusion of antigens from soil and the development of specific precipitin bands provides a new approach to studying soil microorganisms.

Use of gas sampling tubes for direct measurement of 14CO2 in soil air

Abstract Commercially available chromatographic tubes for detection of gases in industrial air were utilized in experiments where CO 2 from soil respiration carried a 14 C label. Using a series of different gas mixtures, the equipment was examined in the laboratory and demonstrated to be accurate in measuring CO 2 contents over the range from 0.03 to 5%. Activity measurements were made by transferring the chromatographic packing, with absorbed 14 C, from the tubes into scintillation solution for counting. In field experiments, 14 CO 2 from soil air was examined over time after labeling wheat plants and monitoring their maturation and decomposition.

Survival and multiplication ofRhizobium japonicum strains in silt loam

SummaryAntibiotic resistant mutants 8-0 StrR, 110 TetR and 138 KanR derived from wild typeRhizobium japonicum strains were inoculated into silt loam soil to cell concentrations greater than 2×108/g of soil. Population changes were monitored using antibiotic media and strain identification was done using immunodiffusion assay on microcores of soil. Immunodiffusion bands formed by the mutant strains with homologous antisera essentially duplicated bands formed by the parent strain. Strains 110 TetR and 8-0 StrR had cross reacting antigens whereas antigens of strain 138 KanR reacted only with the homologous antiserum. Populations ofR. japonicum strains introduced into sterile soil increased over a period of four weeks under both single and mixed culture inoculations. All populations decreased by the end of six weeks and thereafter remained constant. When theseR. japonicum strains were introduced into non-sterile soil, the population did not increase over the initial population added. Population decreased gradually for two weeks and then maintained thereafter. It was possible to recover very low populations of antibiotic resistantR. japonicum strains from both sterile and unsterile soils using media containing specific antibiotics. Detection ofR. japonicum strains by immunodiffusion was accomplished only when the population was ≥109 cells/g of soil. The method using antibiotic resistant mutants permitted an evaluation of the interactions of variousR. japonicum strains in soil with respect to their survival and multiplication.

Observations of fungal growth in soil using a capillary pedoscope

Abstract A simple technique for constructing glass pedoscopes from standard glass slide and cover slip material is described. When inserted into soil, the rectangular glass capillaries are colonized by soil microorganisms and can be removed for microscopic examination. The pedoscopes were used to observe fungal growth in soil incubated in the laboratory. Rate of growth and pattern of development were influenced by organic amendments. A very dense mycelium colonized a capillary containing straw, whereas mycelial density was intermediate and sparse when dextran and glucose were used respectively. Morphological differences associated with age and substrate were noted. Fungal growth was observed periodically as it penetrated the length of a dextran treated capillary. The average growth rate was 40 μm h−1. Photomicrographs taken at intervals of 24 h showed a combined mycelial length of 820 μm arising from elongation and branching from a single hyphal tip.

Soybean root respiration assessed from short-term14C-activity changes

During and immediately after labelling of soybeans (Glycine max. L.) in the field by exposure to14CO2, its respiratory deposit into the soil atmosphere, and its liberation from the soil were used in conjunction with estimates of below-ground plant biomass to apportion total soil respiration. Root respiration of soybean plants at stage V6 was estimated at 4 mg CO2.(g root)−1.h−1. Soil biota, during the same time, contributed 35% of total soil respiration.

Interactions ofRhizobium japonicum strains in soybean nodules and their contribution to nitrogen fixation

SummaryRhizobium japonicum strain 8-0 StrR applied as inoculum to Clark 63 soybeans formed small ineffective nodules which had very low nitrogenase activity compared to nodules formed by two effective strains, 110 TetR and 138 KanR. Mean numbers of cells per milligram of nodule tissue for plants up to 34 days old were 7.7×106 for 8-0 StrR, 4.1×108 for 110 TetR and 7.6×108 for 138 KanR. Cell counts per unit mass of nodule were independent of plant age for strains 110 TetR and 138 KanR, however, for strain 8-0 StrR, 25 and 34 days old plants had fewer viable cells per nodule mass than 18 day old plants. When a mixture of two effective strains was used, the nodules of individual plants were predominantly caused by either 110 TetR or 138 KanR. In one experiment the predominance was random, but in another, strain 110 TetR clearly dominated. Strain 138 KanR was absent in some nodules on 18 day old plants, and in others, less than 102 cells per nodule were found. When strains 8-0 StrR and 138 KanR were used as mixed inoculum, most of the nodules had strain 8-0 StrR but strain 138 KanR was detected in many nodules and was generally evident in the largest nodules. Nitrogenase activity by many individual nodules was low except for nodules which had cells of 138 KanR. Nitrogenase activity by whole root systems of these plants was relatively high and similar to plants that had only nodules of strain 138 KanR. Similar relationships were observed for a mixed inoculum of 8-0 StrR and 110 TetR. In general, mixed inoculations resulted in nodules with a particular strain being dominant for each individual plant. Double infections within individual nodules were not uncommon and such nodules often had disproportionate numbers of cells of two competingR. japonicum strains.