Associative symbiosis of Azotobacter chroococcum and higher plants.

The association between a selected strain of Azotobacter chroococcum and seven plants was investigated in water cultures under sterile conditions. Azotobacter population progressively increased in the nutrient solution and on the rhizoplane. Microbial propagation depends on the type of plant, being much higher in presence of wheat, followed by barley, maize, broad bean, and cotton, while in presence of fenugreek and lentil lower rates of multiplication were recorded. Inoculation increased the dry weight of plants by 5--12% and in length by 3--18% in addition to increased nitrogen content of plants and nutrient solution. Nitrogen balance showed no significant change in systems devoid of Azotobacter, but association between plants and the microorganism invariably showed positive results. The extent of N2-fixation depends on the type of plants; higher gains were recorded in presence of non-leguminous plants.

Effect of Azotobacter inoculation on plant growth and soil nitrogen.

The validity of seed bacterization with Azotobacter chroococcum in soils of variable densities of naturally present azotobacters was studied. Inoculation of barley grains had no effect on counts of total microflora, neither in rhizosphere nor in root-free soil, but significantly increased Azotobacter population, especially in the rhizosphere. The rate of colonization in the root region was much higher when soil initially harboured low Azotobacter densities. Bacterization improved plant growth and increased soil nitrogen through nitrogen fixation. Nitrogen balance in soils showed higher gains in the inoculated treatments over the uninoculated analogues of 30--98 ppm.

Effect of organic matter supplementation on nitrogen transformations in soils. I. Chemical and bacteriological changes.

The effect of supplementation with different organic materials on nitrogen transformations and on certain bacterial groups in soil was studied. Addition of wide C/N ratio organic matter, sawdust and maize stalks prevented NO3-N from being lost through leaching out or dentrification and favoured the development of Azotobacter and N2-fixing clostridia that in turn resulted in marked gains in nitrogen through N2-fixation. Nitrifying bacteria were adversely affected. Application of such materials together with high amounts of NH4NO3 lessened nitrogen losses in drainage water but increased losses through denitrification. Nitrogen-rich organic matter resulted in higher losses in nitrates from soils in comparison to those of wide C/N ratio organic materials.

Effect of organic matter supplementation on nitrogen transformations in soils. II. Nitrogen balance and yield.

Under controlled laboratory conditions, the effect of the application of uncomposted plant materials (sawdust and maize stalks) or horns and hooves (narrow C/N ratio) on soil nitrogen and plant growth was investigated. From nitrogen balance calculations it was found that the addition of the wide C/N ratio materials alone either increased soil nitrogen through N2-fixation or lessened nitrogen loss through denitrification. Although such materials resulted in the immobilisation of mineral nitrogen, no nitrogen starvation symptoms were noted on the growing plants. Negative nitrogen balances were usually obtained when soils received mineral nitrogen fertilizers either alone or together with maize stalks or supplemented with the narrow C/N ratio organic material.

Seed inoculation with Azotobacter chroococcum in sand cultures and its effect on nitrogen balance.

The effect of bacterization of barley grains with a selected strain of Azotobacter chroococcum was studied. In nitrogen-deficient sand, seed inoculation increased plant length, dry weight, nitrogen content in addition to a significant increase in soil nitrogen. The density of the organism on grains had a slight positive effect. Irrigation with KNO3-containing nutrient solution suppressed nitrogen fixation, but inoculation still had a pronounced stimulating effect on plants. In presence of mixed soil microflora the beneficial effect of bacterization was less than in monobacterial cultures. Azotobacters naturally present in soil also colonized in heavy densities on the rhizoplane, but their effect on plant growth and soil nitrogen were less if compared with that resulting from bacterization.

Association between Azotobacter and other soil bacteria and its effect on nitrogen fixation.

The association between A. vinelandii and either Agrobacterium sp. or Micrococcus sp., which are usually found as contaminants in Azotobacter cultures, was investigated. In comparison with pure cultures, association increased the microbiol counts in addition to increasing nitrogen fixation rates and efficiency. In liquid cultures higher Azotobacter densities were observed in the top 5 cm of the column concomitant with lowering the economic coefficient of utilization of carbonaceous compounds, which resulted in low efficiency of nitrogen fixation. In deep layers, lower amounts of nitrogen gain were obtained, but higher efficiencies of N2-fixation were recorded. In sand cultures, the biggest amounts of fixed nitrogen were in the 5--15 cm layer of the soil column and in deeper layers economic utilization of sugars occurred, but nitrogen gain sharply decreased.

Nitrogen-fixing capacity of Azotobacter as affected by the type and depth of substrate.

Sixty isolates, representing the different types of azotobacters in Egyptian soils, were studied for their N2-fixing capacity. When A. chroococcum and A. vinelandii were grown in shallow layers of N-deficient liquid medium, nitrogen gains ranged from 48 to 92 and from 20 to 120 ppm with efficiencies of N2-fixation from 15.8 to 50.0 and from 16.6 to 21.2 mg N fixed/g C oxidized, respectively. Culturing in deep layers generally lowered the nitrogen gains and the oxidation of carbon, but at different rates, resulting in the increase of the efficiency of N2-fixation by 2--78%, indicating that the depth of the liquid medium has a definite effect on the outcome of N2-fixation. In another experiment, A. vinelandii was grown in increasing depths of liquid or solid substrates and consequently at variable ratios of surface area: depth. In liquid medium, maximum counts, sugar consumption, and nitrogen gains were detected in the widest surface area: depth ratio, but the efficiency of N2-fixation increased with the narrowing of the ratio. In sand cultures an opposite trend was observed.

Studies on azotobacters prevailing in Egyptian soils.

Soil samples from different locations in Egypt, representing various types of soils, different degrees of fertility and under various standing crops, were used for isolating Azotobacter strains. Sixty Azotobacter isolates were obtained, and A. chroococcum was found to be the most predominant species (56 isolates), while A. vinclandii was sporadically found (4 isolates). Other species of the genus Azotobacter were not detected in the examined soil samples. Isolates were studied for their morphological, cultural, and physiological properties. Thirteen isolates, differing in morphological features, were selected for studying their pleomorphic character.

Studies on the aerobic non-symbiotic nitrogen-fixing bacteria, other than Azotobacter, in Egyptian soils.

Twenty isolates of micro-organisms capable of growing on nitrogen-deficient medium and found as contaminants in Azotobacter cultures were isolated from Egyptian soils and studied for their morphological, cultural, and physiological properties. These micro-organisms s are members of Rhizobiaceae, Pseudomonadaceae, Achromobacteriaceae, Enterobacteriaceae, Micrococcaceae, Bacillaceae, and Streptomycetaceae as well as some yeasts. In nitrogen-free medium the micro-organism fixed only small amounts of atmospheric nitrogen, hardly exceeding 3 ppm and because of their low sugar consumption rates, efficiences of N2-fixation sometimes reaching 18 mg nitrogen fixed/g carbon oxidized were recorded. Addition of 15 ppm combined nitrogen to the medium increased the amounts of nitrogen fixed to 3--9 ppm.

Evaluation of some biological tests as parameters for microbial activities in soils. I. Laboratory experiments.

The experiments were confined to the effects of the addition of different sources of carbon (glucose, wheat straw, and sawdust) on the microbial activities in soils: loamy sand, loam and saline clay were used. The parameters used for this purpose were total bacterial counts, dehydrogenase test, CO2 evolution, and oxidation of organic carbon. Salinity of soils had deleterious effects on all the parameters used. Addition of organic matter resulted in marked increases in bacterial counts, formazan production, and CO2 evolution. Glucose showed the most rapid effect and highest levels. Wheat straw yielded the highest results and sawdust always showed the least effect.

Evaluation of some biological tests as parameters for microbial activities in soils. II. Field investigations.

Investigations were designed to study the effect of certain factors on the microbial activities in soil. The parameters, used as an index of the microbial activities, were total bacterial counts, dehydrogenase activity, oxidation of organic carbon, and CO2 evolved/7 days. Bahteem Farm clay soil was examined for determining the effects of depth, type of fertilization, and crop rotation on the microbial activities. It appears that the microbial activities, as indicated by the tested parameters, were more pronounced in the surface 15 cm-layer than in the subsurface layer (15-30 cm). Results of all the parameters tested showed markedly higher increases with farmyard manure than with nitrogenous fertilizer and in the control, without significant differences between the latter two. Moreover, the time of sampling had no effect on the results obtained for all parameters. Different types of rotations did not exert significant variation in total bacterial counts, though more than one crop per year increased the organic carbon content of soil and mostly the dehydrogenase activity, whereas the evolution of CO2 tended to decrease. At Gabal el-Asfar Farm, the effect of irrigation with sewage effluent, for long periods, on the microbial activities of sandy soil was investigated. Sewage water stimulated the total bacteria, raised the dehydrogenase activity, the organic carbon, and the production of CO2. In North El Tahreer and Mariut Sectors, the effect of both the type and age of cultivation on the microbial activities in the calcareous soils were examined. Cultivation raised the figures of all the tested parameters progressively with time of cultivation. It was also noticed that crops exerted more beneficial effects on microbial activities than orchards, and the dehydrogenase test was the most reliable parameter to reveal this fact.

Biological activities during ripening of composts.

Three investigations were carried out. The first determined the relationships between dehydrogenase acitivty throughout the course of composting wheat straw and the changes in both organic carbon and total nitrogen. The second investigation tested the validity of the first investigation results. The third dealt with the effect of the degree of ripening of a compost, expressed by age and C/N ratio, on its dehydrogenase activity, and production of CO2. The results revealed continual decreases in the organic carbon, coincident with increases in total N and dehydrogenase activity. Noticeable chemical and biological changes occurred in the first and third month. Levels of dehydrogenase were highest in rice straw and lowest in cotton stalk compost, while the wheat straw compost occupied an intermediate position. Aging of the compost resulted in increases in the dehydrogenase figures and decreases in the amounts of CO2 produced.