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

Abstract

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.