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.