Phosphorus Rather than Nitrogen Addition Changed Soil Cyanobacterial Community in a Tropical Secondary Forest of South China

Soil cyanobacteria in tropical forests is understudied despite its important role in soil biochemical process and plant growth. Under a nitrogen (N) deposition background in tropical forests, it is important to learn how soil cyanobacterial communities respond to N deposition and whether phosphorus (P) mediated this response. A fully two-factor (N and P additions) factorial design with four blocks (replicates), each including a 12 x 12 m plot per treatment (Control, +N, +P, and +NP) were established in a tropical secondary forest in 2009. In July of 2022, soil cyanobacteria at 0-10 cm and 10-20 cm depths in the experimental site were collected and analyzed using a metagenomic method. The impact of N and P additions on soil cyanobacteria remained consistent across the different soil depths, even though there was a significant contrast between the two layers. The effect of N addition on soil cyanobacteria did not significantly interact with P addition. N addition increased soil N availability and decreased soil pH but did not significantly affect the soil cyanobacterial community. In contrast, P addition increased soil P availability and soil pH, but decreased soil N availability and substantially changed the soil cyanobacterial community. P addition significantly decreased the abundance of soil cyanobacteria, especially abundant ones. P addition also increased cyanobacterial species richness and Shannon's diversity, which might be explained by the decline in dominant species and the emergence of new species as nestedness and indicator species analyses suggest. We concluded that (1) soil cyanobacteria in tropical forests exhibits a greater sensitivity to elevated P availability compared to N; (2) an increase in soil P supply may mitigate the advantage held by dominant species, thus facilitating the growth of other species and leading to alterations in the soil cyanobacterial community. This study improves our understanding on how soil cyanobacterial communities in tropical forest responds to N and P addition.