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Network complexity of rubber plantations is lower than tropical forests for soil bacteria but not for fungi

Abstract. Soil microbial communities play a crucial role in ecosystem functioning.
Past research has examined the effects of forest conversion on soil
microbial composition and diversity, but it remains unknown how networks
within these communities respond to forest conversion, including when
tropical rainforests are replaced with rubber plantations. Microbial
networks are viewed as critical indicators of soil health and quality. They
consist of two parts: nodes and edges. In this study, we used data from
Illumina sequencing and shotgun metagenome sequencing to analyze bacterial
and fungal community network structure in a large number of soil samples
from tropical rainforests and rubber plantation sites on Hainan Island,
China. Our results showed that only 5 %–10 % of shared network edges (i.e., links
between species A and B existing in both rubber plantations and rainforests)
were observed in both bacterial and fungal communities, which indicates that
forest conversion altered the soil microbial network structure. The identity of
keystone operational
taxonomic units (OTUs) differed entirely between rubber plantation and rainforest
sites, further underscoring the altered network structure. More edges and
more negative correlations within the soil bacterial–fungal networks were
observed at rubber plantation sites (dry season: 4284 total edges, 844
negative; rainy season: 7257 total edges, 1744 negative) than at rainforest
sites (dry season: 3650 total edges, 149 negative; rainy season: 6018 total edges, 489
negative), demonstrating that soil bacterial–fungal network structure was
more complex and stable in rubber plantations than in rainforests. For
bacteria, a larger number of network edges were observed among bacterial
networks in samples from tropical rainforest than in samples from rubber
plantations, indicating that rainforest bacterial networks were more complex
than those from rubber plantations. However, soil fungal networks from
rubber plantations showed more links, suggesting that forest conversion
increased fungal network complexity. More edges of network and more links
between species and functions were observed in the rainy season than in the
dry season, indicating that seasonal changes had a strong effect on network
structure and function. Further analysis shows that soil pH, potassium (AK), and
total nitrogen (TN) had more links with species of some phyla. In
conclusion, forest conversion results in an increase in soil pH as well as a
decrease in AK and TN, and these changes as well as seasonal variations had
a great impact on soil microbial composition, network structure and
function.
