Spatial differences
Different Amazonian habitats varied considerably in their biotic
composition10, 65. Habitat was the
most significant factor explaining community turnover in the 18S and COI
datasets. Although habitat was also significant for ITS, community
turnover was better explained by locality for this dataset. In the ITS
data, we found that campinas and igapós are dissimilar in their
communities (Fig. 4A). This can be explained by the physicochemical soil
properties (Fig. 3). When it comes to chemical properties, campinas and
igapós were placed at opposite extremes of PC1 and PC2 (Fig. 3B). With
respect to the physical properties, campinas have plots in both extremes
of PC1, but igapós were better explained by clay content (Fig. 3A). Clay
content was an important factor in explaining leaf litter in central
Amazon fungi56. For the 18S and COI, the similarity
between habitats is better explained by comparing seasonally flooded and
non-flooded habitats (Fig. 4). In both communities, igapós and várzeas
are similar to each other and distinct from terra-firme and campinas.
This is in agreement with results from studies of micro-organisms in
general in the same areas10. These results were
expected as the flooded period is a powerful factor that selects for a
very specific vegetation type 66, 67, 68,26. Igapós and várzeas are more restricted to a fine
soil texture, while in terra-firme and campinas the soil texture varies
more (Fig. 3A). However, regarding the chemical properties, terra-firme
and campinas have almost exclusively poor soils, while igapós and
várzeas present different gradients of soil fertility (Fig. 3B). These
distinct patterns among markers might be explained by the differences in
taxonomic coverage of each marker, since different species of fungi have
distinct habitat preferences12.
We were surprised to find that Campinas were, on average, the richest
habitat for fungi. This stands in contrast to patterns observed for
animals and plants69, 70, and fungi
in Colombian Amazonia14. One explanation for the
campinas being the richest environment may be the need for plants to
associate with micro-organisms that fix nutrients in the poor soil
habitats. For instance, some studies of campinas in Amazonia address the
diversity of ectomycorrhizal fungi71, 72, 73, 74, 75.
The general pattern is that the diversity of ectomycorrhizal fungal
diversity is the highest in temperate zones12,76,77,
but due to the poor soil in Campinas, the ectomycorrhizal fungi will be
more diverse.
The origin of the campinas in Amazonia is a debated
topic70, but the nature of their soil, caused by high
drainage and high acidity, is considered one of the poorest in the
world78. In this context, Singer et
al.73 hypothesized that the ectomycorrhizal fungi
increase the ability of their host plant to acquire nutrients and water
in these very stressful habitats. We found a high richness and number of
indicator OTUs in campinas (Fig. 5, Table 3), adding more evidence that
the campinas may be hotspots for the diversity of fungi and other
micro-organisms. However, we registered very few ectomycorrhizal
indicator OTUs, although these results could be biased by the lack of
representative DNA sequences from tropical areas79. It
is interesting that várzea areas have on average 43 to 53% fewer OTUs
of known mycorrhizal species for the three markers. Of the four habitats
analysed, várzea soils are of higher fertility as they are flooded by
nutrient-rich waters, decreasing the necessity for plants to associate
with mycorrhizal fungi, in accordance with the hypothesis proposed by
Singer et al.73.