The effect of habitats
Habitat type was the strongest factor (p < 0.002) in
explaining community composition in the PERMANOVA analysis (18S –
R2 = 0.12, COI – R2 = 0.18, and ITS
– R2 = 0.08), with the exception of the ITS dataset.
In NMDS, the seasonally flooded forests, igapós, and várzeas were more
similar to each other than to campinas and terra-firmes, which were the
most similar to each other (Fig. 5). Campinas had the highest mean
number of OTUs and the highest number of OTUs considered indicators of
this habitat for all datasets (Table 3). Regarding the number of
exclusive OTUs, campinas had the highest number of OTUs in the COI
datasets (Fig. 5E). Terra-firme was the habitat with the highest number
of exclusive OTUs for the 18S and ITS datasets (Figs. 5B and 5H). In all
habitats, the majority of indicator OTUs were saprotrophs, followed by a
high proportion of OTUs that could not be classified by their functional
group (Table S2). Campinas have a moderate proportion of phytopathogen
indicator OTUs and terra-firme a moderate proportion of parasite
indicator OTUs (Table S2).
Discussion
Our results highlight the importance of habitat type for fungal
community composition in Amazonia and suggest that Amazonian fungi have
different diversity patterns for habitat and locality variables, with
the importance of each predictor varying between markers. By contrast,
community turnover shows a consistent pattern, with habitat being a
strong factor explaining community similarity between plots. This is
likely to be because different areas can have similar species richness
but different species composition due to historical, geographic, and
environmental factors. For instance, in a study of leaf litter fungi in
Central Amazonia, the abundance and richness of fungal morphospecies did
not change between low and high rainfall periods, but there was a low
proportion of shared morphospecies between periods56.
Our results also showed a low proportion of shared OTUs when compared
with a HTS study of micro-organisms in general in the same
area10 (Fig. 5).
Soil texture did not explain fungal diversity, while chemical soil
characteristics was of importance for COI and ITS soil communities,
indicating a high diversity in less fertile soil (Table 1). Although it
appears counter-intuitive, the habitat with lowest soil fertility was
the one with highest fungal and other microbial diversity: the
campinas10, 25. These results
suggest that factors other than soil properties explain a habitat’s
fungal diversity and community composition.
The soil diversity of the 18S dataset was negatively related with
carbon, while the specifics of the other datasets were not related to
carbon. This could be explained by taxonomic coverage of the 18S
dataset, which included the Chytridiomycota and Mucoromycota, which are
mostly saprobe groups57, 58.
Saprobes decompose matter into various constituent components, making
the nutrients available to other organisms. Saprobes are, in other
words, important agents in carbon cycling59. Hence, a
high fungal richness may lead to a faster carbon decomposition in soil
as well as a faster carbon assimilation in the above-ground biomass.
This is in agreement with Liu et al.60, who found that
phylotype richness and phylogenetic diversity of black soil fungi
responded negatively to total carbon content in China. Experiments
controlling the variables and quantifying the above-ground biomass are
necessary to further verify these observations.
Contrary to our expectations, pH had no effect on fungal richness. This
finding was surprising, since soils with more neutral pH generally have
a higher richness of micro-organisms11,61,62,63. Our
soil samples were all acidic, with the pH varying between 3.5 and 5.14.
Soil fungi studied by Liu et al.60 showed a similar
pattern as reported in this study – a higher relative influence of soil
carbon content than of soil pH. They also noted that fungi often have a
wider tolerance to pH variation than other micro-organisms, suggesting
that in soils with low pH variation such as presented here, the acidity
impact should be less striking60. On the other hand,
pH was important to explain community turnover for all datasets (Table
2). Furthermore, in tropical areas the relationship between fungal
communities and soil pH is affected by the fungal trophic
guilds64. It may indicate that in a highly diverse
area, such as Amazonia, fungal diversity will not be impacted by pH
variation but there will be a turnover of fungal species related with
the pH range.