Abstract
Our knowledge of the rhizosphere bacterial communities in deep soils and the role of Eucalyptus and Acacia on the structure of these communities remains very limited. In this study, we targeted the bacterial community along a depth profile (0 to 800 cm) and compared community structure in monospecific or mixed plantations of Acacia mangium and Eucalyptus grandis. We applied quantitative PCR (qPCR) and sequence the V6 region of the 16S rRNA gene to characterize composition of bacterial communities. We identified a decrease in bacterial abundance with soil depth, and differences in community patterns between monospecific and mixed cultivations. Sequence analysis indicated a prevalent effect of soil depth on bacterial communities in the mixed plant cultivation system, and a remarkable differentiation of bacterial communities in areas solely cultivated with Eucalyptus. The groups most influenced by soil depth were Proteobacteria and Acidobacteria (more frequent in samples between 0 and 300 cm). The predominant bacterial groups differentially displayed in the monospecific stands of Eucalyptus were Firmicutes and Proteobacteria. Our results suggest that the addition of an N2-fixing tree in a monospecific cultivation system modulates bacterial community composition even at a great depth. We conclude that co-cultivation systems may represent a key strategy to improve soil resources and to establish more sustainable cultivation of Eucalyptus in Brazil.
Highlights
Eucalyptus is one of the most common genera in commercial forest plantations worldwide [1]
Soil samples from all layers were classified as medium-sandy except for samples from the A. mangium cultivation classified as sandy between 100 and 300 cm (S1 Table)
The subsurface environment might select for microbes that can survive under reduced energy sources, limiting the development of highly complex bacterial communities; alternatively, the subsurface soil life must have distinct substrate utilization, leading to the niche colonization of a more specialized and slow-growing bacterial community structure [17, 19, 23, 41]
Summary
Eucalyptus is one of the most common genera in commercial forest plantations worldwide [1]. A possible solution is the co-cultivation of Eucalyptus and leguminous trees in mixed systems [5,6,7] to provide an additional supply of nitrogen (N) for Eucalyptus [6, 8,9,10]. This strategy for N2 fixation is well described and is promoted by nodules found in Acacia mangium [11], the most commonly used legume in experimental stands of mixed plantations with E. grandis in Brazil [6, 12]. Co-cultivation benefits plants by creating more heterogeneous systems, providing more bio-diverse sources of microbes, and supporting efficient selection in the rhizosphere [13]
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