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United States Department of Agriculture

Agricultural Research Service

Title: Assessing Soil Microbial Populations and Enzyme Activity Following the Use of Microbial Inoculants

Authors
item Cadena-Cepeda, - AUBURN UNIVERSITY
item Burelle, Nancy
item Runion, George
item Kloepper, Joseph - AUBURN UNIVERSITY

Submitted to: International Plant Growth Promoting Rhizobacteria Workshop
Publication Type: Proceedings
Publication Acceptance Date: May 28, 2006
Publication Date: May 28, 2006
Citation: Cadena-Cepeda, Burelle, N.K., Runion, G.B., Kloepper, J.W. 2006. Assessing soil microbial populations and enzyme activity following the use of microbial inoculants. International Plant Growth Promoting Rhizobacteria Workshop.

Interpretive Summary: Soil enzyme assays were employed to study the population dynamics of several commercially available microbial inoculants. The research was undertaken in order to refine application methods and timing of biological products to enhance growth promotion and disease suppression effects on tomato and strawberry. Physiological activity of microbes was measured by assessing dehydrogenase activity, arylamidase activity, and fluorescein diacetate hydrolysis (FDA). In greenhouse trials, FDA was effective in measuring changes in microbial activity in the rhizosphere following inoculant application. Despite detecting changes in microbial activity, no changes in microbial populations were observed. It is possible that PGPR inoculants affected the structure of the microbial community, without changing population size. In strawberry field trials, none of the microbial procedures detected any change in microbial activity or population; however, the use of PGPR inoculants did promote growth and increased strawberry yield.

Technical Abstract: In recent years, use of microbial inoculants for promotion of plant growth has increased. Application frequency of PGPR is critical to achieving the maximum benefit from this technology. The study presented here, addresses the problem of developing methods for measuring soil microbial activity and population size in relation to the application of soil inoculants. Three commercial formulations of PGPR, containing bacilli strains (Equity®, Soil Builder®, and Ag Blend®) were used on tomato and strawberry in greenhouse and field experiments. Physiological activity of microbes was measured by assessing dehydrogenase activity, arylamidase activity, and fluorescein diacetate hydrolysis (FDA). Culturable microbial populations were determined by most probable number (MPN) and direct plate counting. In greenhouse experiments on tomato FDA was effective in measuring changes in microbial activity in the rhizosphere following inoculants application, while arylamidase and dehydrogenase procedures were not sensitive in detecting those changes. Despite detecting changes in microbial activity, no changes in microbial populations were observed. Thus, little or no correlation was detected between microbial enzymatic activity and bacterial population with the procedures used. It is possible that PGPR inoculants affected the structure of the microbial community, without changing population size. Therefore, these tests (based on total population counts) may not be adequate indicators for determining proper time intervals for re-inoculation. In strawberry field trials, none of the microbial procedures detected any change in microbial activity or population; however, the use of PGPR inoculants did promote growth and increased strawberry yield.

Last Modified: 12/19/2014
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