Impact of mixed-species forest plantations on soil mycobiota community structure and diversity in the Congolese coastal plains

Abstract

Mixed tree plantations containing nitrogen (N2)-fixing species have the potential to enhance C sequestration, soil biodiversity and forest productivity. Here, we investigated the impact of Acacia mangium and Eucalyptus urophilla x E. grandis mixed plantations in the Congolese coastal plains on soil mycobiota community structure and diversity by ITS metabarcoding sequencing and bioinformatic analysis. Higher Faith's phylogenetic diversity and Evenness' was found in Eucalyptus monoculture relative to stands containing Acacia. Differences in beta diversity were found among Eucalyptus and Acacia monoculture, and mixed-species stands highlight the effects of plant species on fungal community structure. Ascomycota, Basidiomycota and Rozellomycota phyla were predominant in all stands, with both Dikarya (Ascomycota and Basidiomycota) accounting for more than 70% in all stands. Correlation analysis revealed that sulfur (S) was the most correlated soil attribute with the three predominant phyla but also with Mucoromycota and Calcarisporiellomycota phyla, although mostly negatively correlated (4 out of 5). Phosphorus was mostly positively correlated to soil attributes (3 out of 4) and nitrogen was correlated twice, positively and negatively. Distance-based redundancy analysis revealed a positive correlation of nitrogen (p-value = 0.0019, contribution = 22%) and phosphorus (p-value = 0.0017, contribution = 19%) with soil mycobiota. A high prevalence of generalists (28% to 38%) than specialists (9% to 24%) were found among the different sites. In stands containing Acacia (pure and mixed species) the soil mycobiota harbor the prevalence of generalist strategies with the potential to withstand environmental stresses and utilize a higher number of resources against specialists in Eucalyptus stands. Stronger positive correlation between soil attributes and main fungal taxa, higher generalists' strategies and lower Faith's phylogenetic diversity and Evenness were reported in stands containing Acacia. This highlights the potential of mixed-species in preserving community stability following environmental disturbances and increasing the number of resources confirming their important ecological role in boosting the resilience of the forest ecosystems to climate and land-use (plant species as shown by PCA analysis) changes.

Fig 1

Alpha-(A) and Beta-diversity (B) of fungal communities in a sandy soil under mixed-species of Acacia and Eucalyptus plantations at 5 years into the second rotation in the Congolese coastal plains; 100% Acacia = Acacia monoculture stands; 100% Euca = Eucalyptus monoculture stands; 50A50E (near Euca) = soil sampled near Eucalyptus of mixed stands; 50A50E (near Acacia) = soil sampled near Acacia of mixed stands. a) The line inside the box represents the median, while the whiskers represent the lowest and highest values within the 1.5 interquartile range (IQR). The width of the distribution of points was proportionate to the number of points at that Y value. b) Principal component analysis (PCA) of fungal communities. Samples are colored according to the plant species and mixed plantations.

Fig 2

Relative abundance of phyla (A) and families (B) in a sandy soil under mixed-species of Acacia and Eucalyptus plantations at 5 years into the second rotation in the Congolese coastal plains. 100% Acacia = Acacia monoculture stands; 100% Euca = Eucalyptus monoculture stands; 50A50E (near Acacia) = soil sampled near Acacia in mixed plantations; 50A50E (near Euca) = soil sampled near Eucalyptus in mixed plantations. Other: unclassified taxa and other fungal phyla with low amplicon sequence variants (ASVs) abundance.

Fig 3

Correlation matrix of Spearman scores between soil attributes and dominant phyla (A) and families (B) that are significantly (P = 5%) correlated, either positively or negatively. The values of correlation coefficients are indicated according to the scale bar. Positive correlations are shown in blue while negative correlations are shown in red. The asterisks represent a significant correlation.

Fig 4. Correlation between soil chemical attributes (phosphorus, C/N, nitrogen, sulfur, and carbon) and soil mycobiota composition by distance-based Redundancy Analysis (db-RDA).

Fig 5

Niche occupancy (A) and networks (B) in sandy soil under mixed species of Acacia and Eucalyptus plantations at 5 years into the second rotation. 100% Acacia = Acacia monoculture stands; 100% Euca = Eucalyptus monoculture stands; 50A50E (near Acacia) = soil samples near Acacia (50A50E (near Acacia) in mixed plantations; 50A50E (near Euca) = soil samples near Eucalyptus in mixed plantation; Black and grey: rare and generalists, respectively. Blue, Orange, and Red: specialists in 100% Euca Green: specialists in 100% Acacia Purple: specialists in 50%A50%E near Euca Yellow: specialists in 50%A50%E near Acacia.