Effect of Land Cover Type on Soil Bacterial Diversity
We show that soil bacterial alpha diversity was generally higher in the
cleared land cover plots but that these plots had less heterogeneous
bacterial communities – results that are consistent with previous work
on the effects of urban versus natural land cover on soil bacteria. The
global study by Delgado-Baquerizo et al. (2021) showed that soil
bacterial communities from human-impacted locations had consistently
higher alpha diversity compared to their paired natural ecosystem
locations. Similar results were found by Han et al. (2021) comparing
different vegetation communities in more urban and more natural areas.
Delgado-Baquerizo et al. (2021) also showed that bacterial communities
were more homogenous across urban and/or more human-impacted soils. They
suggested this was primarily driven by similarities in urban soil
management practices and land cover changes. In our case, both cleared
land cover plots have been historically impacted by humans through the
clearing of all non-grass vegetation and continued mowing.
Similarly, our network analysis provided supporting evidence that soil
bacterial community complexity was affected by land cover type. We saw
fewer bacterial interactions and network complexity (lower connectance)
in our native plots. Moreover, we found more positive interactions in
our cleared plots and more negative interactions in the native plots.
The presence of more positive associations in the cleared plots suggests
these bacteria may undergo greater cooperation for resources or a lack
of competition among the interacting bacteria. The results of our ‘hub
taxa’ analysis showed that the bacteria with the highest node degree at
the genus level were different at each land cover type. This suggests
that the hub taxa have only a fleeting influence on the network
structure across time. However, when examining the hub taxa at the
phylum level, we found that Gram-negative Acidobacteriota groups were
most prevalent, featuring in the top position (highest node degree for
positive interactions) twice for both land cover types. This suggests
that Acidobacteriota may have a fundamental mutualistic role to play in
the rhizosphere. This is corroborated by the literature, which confirms
Acidobacteriota roles in C- and N-cycling and plant health, amongst
other functions (Kalam et al., 2020; Huber et al., 2022). These
ecological functions could conceivably influence rhizosphere bacterial
interactions. Regarding negative hub taxa (bacterial groups with an
antagonistic association), Bdellovibrionota had the highest node degree
for negative interactions, featuring twice in each land cover class.
These bacteria are often obligate aerobic predators (Ortiz et al.,
2021), consuming Gram-negative bacteria, which could potentially help
explain their negative association in our networks.
Additionally, we found that soil temperatures and moisture were greater
in the cleared plots than paired native plots, a pattern that was
similarly noted by Delgado-Baquierizo et al. (2021). Soil bacteria are
generally sensitive to changes in soil temperature and moisture (Wu et
al., 2015), which may influence the soil bacterial communities
(Delgado-Baquerizo et al. 2021). The presence of distinct soil bacterial
communities in different land cover types highlights the need to not
only conserve soil bacterial biodiversity present in our natural areas
but also to support the development of ways to restore soil bacteria in
modified areas for soil biodiversity conservation.