Four Materials as Carriers for Phosphate Dissolving <i>Rhizobium Sp.</i> Inoculants

Abstract

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.