Rhizosphere Effect and Bacterial Community Structure in Horizons of Podzolic Soil under Spruce Plants (<i>Picea abies</i> L.)

Abstract

The relationships between the rhizosphere effects, allocation in soil horizons and bacterial community structure in the rhizosphere and the bulk soil of Retisol under spruce trees (Tver region, Russia) were studied. The rhizosphere factors (Rf) expressed as ratios of soil characteristics in the rhizosphere to that in the bulk soil were determined for the basic indices of microbial respiration, biomass and available nutrients pools in the top AEL (3–15 cm) and deep EL horizons (15–46 cm). The most prominent rhizosphere effects (Rf 1.6) were revealed for microbial biomass C, basal respiration, and SOM turnover rate. Rf value for the SOM turnover rate in humus AEL horizon was approximately 1.5, while in the EL horizon it reached 6. The Rhizosphere had higher microbial diversity, with a significant contribution of both Gram-positive and Gram-negative bacteria, including representatives of Acidobacteria, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Solibacteres and Spartobacteria. The Gram-positive orders Bacillales and Clostridiales predominated in the bulk soil, with the relative contributions of more than 80 and 50% for the AEL and EL horizons, respectively. Based on the number of microbial activity indices with high Rf values (three for the lower EL horizon and only one for the upper humus AEL horizon), the rhizosphere of the lower horizon is probably more pronounced “hot spot” of biological activity than that in the top soil layer.