Amendment of degraded desert soil with wastewater debris containing immobilized Chlorella sorokiniana and Azospirillum brasilense significantly modifies soil bacterial community structure, diversity, and richness

Abstract

The main goal of this study was to expand our knowledge of what happens to the soil bacterial community in an eroded desert soil when improvement of soil fertility is derived from the application of debris of tertiary wastewater treatment containing immobilized microalgae Chlorella sor- okiniana and the plant growth-promoting bacterium (PGPB) Azospirillum brasilense. We hypothesized that an "im- proved" non-agricultural desert soil will exhibit substantial changes in the structure of the bacterial community in a relatively short time after amendment. To assess the effect of the amendments, microalgae and PGPB alone or com- bined, on the structure of the rhizosphere bacterial commu- nity, changes in species richness and bacterial diversity over time were based on sequence differences in the 16S rRNA gene, performed with PCR-denaturing gradient gel electro- phoresis (DGGE) and then analyzed by similarity test and non-metric multidimensional scaling analysis. Root surface colonization and persistence in the rhizosphere of A. brasi- lense was monitored by fluorescent in situ hybridization and sequencing of DGGE bands. Application of waste debris significantly changed the rhizosphere bacterial population structure, whether comparisons were made over time, be- tween inoculated and non-inoculated soil, and among dif- ferent inoculated microorganisms. Species richness and diversity increased when the waste debris contained the microalgae-bacteria association and also over time. Even as its secondary role as an inoculant after wastewater treat- ment, A. brasilense colonized the root surface profusely and persisted within the rhizosphere bacterial community. This study demonstrated that small organic amendment to desert soil significantly changed soil bacterial community com- pared to the original soil and also 2 months after amend- ments were added.