Phosphorus is the main nutrient element for plant growth, whereas most of phosphate in soil is fixated by calcium, aluminum, iron and organic compounds. Available phosphate deficiency has been the main limiting factor that influences the yield and quality of agricultural products. P solubilizing microorganisms have been widely reported of solubilizing different forms of insoluble phosphates. P solubilizing Rhizobium, especially for strains with the ability of secreting 3-Indole acetic acid, which is also capable of fixating N and inoculation, is found excellent in promoting growth and yield of legume crops, which decreases the cost of agricultural production and the side effect of P application to the environment. The mechanism and amount range of P solubilization for Rhizobium are varied among strains. These results and phenomena from different studies are discussed in this paper.
As the most commonly used carrier for commercial inoculants, the development of peat has been limited because peat is a material of nonrenewable resource. Many other materials have been evaluated as alternatives to peat as carriers of rhizobia, yet seldom have been included in inoculants of phosphate dissolving rhizobia. As accessible and inexpensive carriers for rhizobial inoculants, corn stalk powder, loessal soil and vermiculite powder were used in the study to compare with peat carrier on the capacities of rhizobial solution absorption, pH value maintaining of microenvironment, viable rhizobial cells maintaining and the control of contamination. Completely randomized design and 4 replicates were used in the experiment. Twelve different compositions of selected inoculant carrier with different absorption volumes of rhizobial suspension, and were evaluated the ability of maintaining viable rhizobial cells and undesired microbes during the period of 120 days at room temperature. Thereafter, pH value, viable rhizobial cell number and undesired microbes of inoculants with selected absorption volume of rhizobial suspension that stored at 4 and room temperature respectively, after 1a storage, were evaluated. Viable rhizobial cells in inoculants were examined after 120d and 1a storage by plate counting method, and ratio of undesirable microbes was examined by antibiotic-carrying and normal plates counting method. The result indicated that: for a period of 120days at room temperature, maximum viable rhizobial cells were found in peat, vermiculite powder, corn stalk powder and loessal soil based inoculants when the absorption volume of rhizobial suspension of inoculants were 450, 500, 1000 and 200ml/kg, respectively; viable rhizobial cell numbers were better maintained in corn stalk powder than in peat, loessal soil and vermiculite, but undesired microbes contamination was a severe problem. In the study, viable rhizobial cell numbers in loessal soil was found the highest, followed by peat, while the most serious contamination was found in peat inoculants; corn stalk powder and vermiculite could not be used as inoculant carrier because fewer viable rhizobial cells existed in these inoculants. The greatest pH change was found in peat and loessal soil based inoculants during 1a storage because of enhanced acidification caused by metabolism of phosphate dissolving rhizobia; more viable rhizobial cells were found in the 4 carriers that stored at 4 than at room temperature after 1a storage. As carriers of phosphate dissolving Rhizobium inoculants, viable rhizobial cells of corn stalk powder after short time storage (120d) and of loessal soil after long time storage (1a) were found better than that of peat, and was also found more cost effective compared with peat, commercially. Both of the two carriers could be used as inoculant carriers at room temperature, but corn stalk powder could only be used as carrier with short shelf life.
The purpose of this study was to evaluate the effect of ampicillin as bacteriostats on the doubling time and relative survival rate of antibiotic resistantRhizobiumstrains and the ratio of undesirable microbes in artificially polluted inoculants during 60 days storage, and the nodule occupancy of two bacteriostat-resistant strains were also investigated. Fully grown Yeast-mannitol liquid media culture ofRhizobiummeliloti LW107 andRhizobiumsp. RSW 96 were added with ampicillin as bacteriostats at various concentrations in an artificial pollution test, and the relative survival rate of rhizobia and the counts of undesirable microbes were determined by the agar plate dilution method. The result shows that in the artificially polluted liquid inoculants, the survival rate of rhizobia was increased and the ratio of undesirable microbes declined significantly when the ampicillin was used as the bacteriostats. The growth of selected antibiotic-resistant strains was promoted at the concentration of less than 100 ug/ml and the doubling time was reduced, but inhibited and the doubling time was significantly prolonged at the concentration of more than 200 ug/ml. Inoculation tests also shows an increased nodulation competitiveness of two bacteriostat-resistant strains in five-fold dilutions of liquid inoculants containing ampicillin (at optimal concentration). According to comprehensive consideration, the optimum concentration for ampicillin as bacteriostat inRhizobiummeliloti LW107 andRhizobiumsp. RSW 96 inoculants appeared to be 100 ug/ml and 200 ug/ml, respectivly.
Two botanical antimicrobials: matrine and pyrethrin were used to study their wide spectrum inhibitory effect on microbes from air and soil, to compare their properties as effective inoculant additive. The result indicates that both the two antimicrobials have inhibited microbe number significantly as the increase of concentration contents, but stimulated microbe diameter. Matrine and pyrethrin have shown their superiority in inhibiting actinomycetes (completely inhibition concentration: 400 mg L-1 for air-oriented and 700mg L-1 for soil-oriented) and mould (completely inhibition concentration: 1000 mg L-1 for air-oriented and 1500mg L-1 for soil-oriented) growth, respectively, and could be chosen to conduct inhibition based on the specific situation of microbial inoculants.
Bud seedlings were used to study short-term effect of different saline and alkaline concentrations on alfalfa root and stem growth. Two neutral salts (NaCl and Na2 SO4) and 2 alkaline salts (NaHCO3 and Na2 CO3) were mixted at 9:1 mole ratio, respectively, to imitate typical saline and alkaline environments. 8 saline concentrations: 0, 10, 20, 40, 60, 80, 100, 120 mmol/L, and 7 alkaline concentrations: 0, 2, 5, 10, 20, 40, 50 mmol/L were selected to determine the stress effects on 5-day, 2-cm bud seedlings. The results indicated that bud seedlings’ root length increased firstly and then decreased as the increase of stress concentrations. On condition of neutral salt stress, 40 mmol/L treatment showed optimum root length, which was 102% times higher than CK, with significant difference (P<0.05). Under alkaline salt stress, 5 mmol/L treatment showed optimum root length, which was 156% times higher than the CK, with significant difference (P<0.05). The effects of the stresses on stem length surpassed root length. And the inhibitory effect of alkaline salt stress on seedlings’ growth in early stage surpassed neutral salt stress.
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