Seasonal effects on microorganisms in the rhizosphere of two tropical plants in a polyculture agroforestry system in Central Amazonia, Brazil

Abstract.

Rhizosphere and bulk soil samples were taken from two perennial plant species (the palm Bactris gasipaes and the dicot Theobroma grandiflorum) growing in an experimental polyculture agroforestry system in Central Amazonia in the wet and the dry season. In the dry season, the soil water potential in the rhizosphere and the bulk soil was below pF 4.2. Microbial biomass and basal respiration were lower in the dry season than in the wet season. However, the lower metabolic quotient in the dry season indicated a higher metabolic efficiency. While the number of oligotrophic microorganisms was not affected by the season, the number of eutrophic microorganisms in the rhizosphere was lower in the dry season than in the wet season. The substrate utilisation pattern of the bacterial community was also affected by the season. Despite marked differences in root morphology there were no significant differences in any of the microbiological parameters between the two plant species. It is concluded that during the dry season, a large portion of the microbial biomass dies off or enters a dormant stage resulting in a change in community composition.