Abstract
Productivity and sustainability of tropical forest plantations greatly rely on regulation of ecosystem functioning and nutrient cycling, i.e., the link between plant growth, nutrient availability, and the microbial community structure. So far, these interactions have never been evaluated in the Acacia and Eucalyptus forest planted on infertile soils in the Congolese coastal plains. In the present work, the soil bacterial community has been investigated by metabarcoding of the 16S rRNA bacterial gene in different stands of monoculture and mixed-species plantation to evaluate the potential of nitrogen-fixing trees on nutrient and bacterial structure. At the phylum level, the soil bacterial community was dominated by Actinobacteria, followed by Proteobacteria, Firmicutes, and Acidobacteria. A principal coordinate analysis revealed that bacterial communities from pure Eucalyptus, compared to those from plantations containing Acacia in pure and mixed-species stands, showed different community composition (beta-diversity). Regardless of the large variability of the studied soils, the prevalence of Firmicutes phylum, and lower bacterial richness and phylogenic diversity were reported in stands containing Acacia relative to the pure Eucalyptus. Distance-based redundancy analysis revealed a positive correlation of available phosphorus (P) and carbon/nitrogen (C/N) ratio with bacterial community structure. However, the Spearman correlation test revealed a broad correlation between the relative abundance of bacterial taxa and soil attributes, in particular with sulfur (S) and carbon (C), suggesting the important role of soil bacterial community in nutrient cycling in this type of forest management. Concerning mixed plantations, a shift in bacterial community structure was observed, probably linked to other changes, i.e., improvement in soil fertility (enhanced P and C dynamics in forest floor and soil, and increase in soil N status), and C sequestration in both soil and stand wood biomass with the great potential impact to mitigate climate change. Overall, our findings highlight the role of soil attributes, especially C, S, available P, and C/N ratio at a lesser extent, in driving the soil bacterial community in mixed-species plantations and its potential to improve soil fertility and to sustain Eucalyptus plantations established on the infertile and sandy soils of the Congolese coastal plains.
Highlights
Introducing nitrogen-fixing trees (NFTs) such as Acacia mangium in Eucalyptus fast-growing plantations improves forest productivity [1,2,3], enhances C sequestration in both soil and biomass [4,5], and decreases N deficiency of inherently nutrient-poor soils previously beneath natural savannas in the Congolese coastal plains [5,6,7]
Soil phosphorus (P) status improves through increased soil available P in the coarse fraction of particulate organic matter POM of the plantation of Acacia or/and Eucalyptus compared to tropical savannas [6,8]
Even though the well-known high P demand of A. mangium as a NFT to sustain symbiotic root nodules and atmospheric N2 fixation processes [9,10] involves a decrease in soil available P beneath stands containing Acacia relative to Eucalyptus [5,11], P cycling in these soils is dominated by biological processes, i.e., organic mineralization [12], while forest floor and mineral soil contain most of the extractable P in inorganic form, reaching up to 70% in the mineral soil P [13]
Summary
Introducing nitrogen-fixing trees (NFTs) such as Acacia mangium in Eucalyptus fast-growing plantations improves forest productivity [1,2,3], enhances C sequestration in both soil and biomass [4,5], and decreases N deficiency of inherently nutrient-poor soils previously beneath natural savannas in the Congolese coastal plains [5,6,7]. Soil ecology of forest plantation plays an important role in several processes, e.g., improving nutrient cycling and soil fertility (N and P status) and forest productivity, and enhancing C sequestration, and has a potential impact in mitigating climate change [15,16,17]. Mixed-species plantations of Acacia and Eucalyptus stimulated microbial and bacterial activities in litter and soil, which may sustain nutrient availability in the long term [21], and enhanced both leaf litter accumulation and plant growth [20]
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