Assessing Soil Microbial Populations and Activity Following the Use of Microbial Inoculants: Effects on Disease Suppressiveness and Soil Health

Abstract

In recent years, assessing use of microbial inoculants for promotion of plant growth has increased. Optimizing 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. Commercial formulations of PGPR, containing bacilli strains (Equity®, Soil Builder®, Ag Blend®, PGA®, Bioyield® and FZB42®) 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 strawberry field trials, hydrolysis of FDA was significantly different among treatments at one out of four sampling times. Procedures to estimate population size (MPN) did not detect any change in microbial population; however, the use of PGPR inoculants promoted growth and increased strawberry yield. In greenhouse experiments on tomato, FDA was not always effective in measuring changes in microbial activity in the rhizosphere following of inoculants application, and arylamidase and dehydrogenase procedures were not sensitive at all. Despite detecting changes in microbial activity, no changes in microbial populations, estimated by MPN, were observed. Populations of total culturable and heat heat-tolerant bacteria were measured by plate counting. FZB42 and Bioyield treatments generally resulted in significantly greater total populations. Overall, population size measured by direct plate counts could be a useful procedure to study root colonization and persistence of introduced microorganisms in the rhizosphere. Knowing that introduced microorganisms are surviving, and their patterns of growth will help to determine when and how PGPR products should be applied. However, because of the lack of consistency the FDA procedure should not be used to decide frequency of application of PGPR products. Induction soil suppressiveness by PGPR and the relation to microbial activity and population size were also studied. The plant parasitic nematode, Meloidogyne incognita, and tomato were used as a model. Results showed significant reductions in number of nematode eggs per gram of root, number of juveniles per ml and number of galls in FZB42 and Bioyield treatments. Additionally, increases in population size were detected for those treatments by direct plate counting, although there was not a correlation between microbial activity and population size.