Date Published: July 7, 2017
Publisher: Public Library of Science
Author(s): Arthur Prudêncio de Araujo Pereira, Pedro Avelino Maia de Andrade, Daniel Bini, Ademir Durrer, Agnès Robin, Jean Pierre Bouillet, Fernando Dini Andreote, Elke Jurandy Bran Nogueira Cardoso, Eiko Eurya Kuramae.
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.
Eucalyptus is one of the most common genera in commercial forest plantations worldwide . In Brazil there are 5.10 million hectares of planted Eucalyptus, and accordingly the species plays an important role in the country’s economy . Most of the Eucalyptus plantations in Brazil are monocultures , which may cause several problems in soils such as an imbalanced nutrient content [3, 4]. A possible solution is the co-cultivation of Eucalyptus and leguminous trees in mixed systems [5–7] to provide an additional supply of nitrogen (N) for Eucalyptus [6, 8–10]. This strategy for N2 fixation is well described and is promoted by nodules found in Acacia mangium , the most commonly used legume in experimental stands of mixed plantations with E. grandis in Brazil [6, 12]. Co-cultivation also benefits plants by creating more heterogeneous systems, providing more bio-diverse sources of microbes, and supporting efficient selection in the rhizosphere .
The bacterial community inhabiting the surface and subsurface of forest soils plays crucial roles in biogeochemical cycles [39, 40]. Although the physical-chemical soil attributes did not present significant differences among treatments (with the exception of the total mineral N), bacterial community structure presented a clear differentiation in the abundance and composition of the rRNA gene along soil layers in response to the established treatments. We did not find any published analysis of the abundance of the 16S rRNA gene in soil layers below a 300-cm depth and thus our inferences regarding deeper soil layers (down to 800 cm) rely exclusively on our own insights.
We verified the influence of the deep-soil and forest-tree composition on soil bacterial communities. In the co-cultivation of E. grandis and A. mangium, the legume exerted a greater influence on the soil bacterial community. Moreover, the soil bacterial community tends to be modulated by soil depth, and as a result it is possible to verify the formation of specific groups in each studied soil layer as a mechanism of plant recruitment. However, in deeper soil horizons, we could not detect typical and distinct rhizosphere communities. The major responsive bacterial groups were Proteobacteria and Acidobacteria (more frequent in samples between 0–100 and 100–300 cm). The major bacterial groups differentially displayed in the monospecific cultivation of Eucalyptus were Firmicutes and Proteobacteria. The use of mixed forests of Eucalyptus with leguminous trees emerges as a promising alternative for sustainability, and should be recognized as a highly important strategy for the conservation of soil biodiversity.