BIOGEOGRAPHIC VARIATION AND RESPONSES TO DRYING IN LOWLAND TROPICAL FOREST SOIL MICROBIAL COMMUNITIES

Abstract

Soil contains more carbon (C) than terrestrial vegetation and the atmosphere combined, with some of the largest terrestrial C stocks in tropical rainforests. Soil microbes decompose organic matter, playing a vital role in the storage or loss of soil C. With climate change, drought conditions are predicted to increase in many tropical regions, including both chronic drying and extended drought, potentially influencing these processes. This project explored the effects of chronic and seasonal drying on soil microbial communities across four distinct tropical forests in a long-term drying experiment. In this thesis, I investigate the effects of a chronic drying manipulation on soil microbial community abundance and variation across different forests and seasons. I also compared my findings with previously published data from these forests after short-term drying. This project used soils from a long-term drying experiment established in 2018 across four seasonal lowland forests in Panama. Soils were collected from 0 – 10 cm depths during three seasonal periods in control and drying plots in 2024 and 2025 from a total of 32 plots (n = 4 per forest per treatment). The forests varied in baseline rainfall and soil fertility. For microbial community assessment, the 16S V4 rRNA region was amplified to identify bacteria, and the ITS1 rRNA region was used for fungi. I calculated alpha and beta diversity indices, and compared taxonomic community composition. I found significant biogeographic variation in microbial diversity and taxonomy, with significant differences across the forests and significant effects of the drying treatment. For bacteria, some trends were consistent with prior findings after shorter-term drying, and I also identified new trends, such as a robustly significant increase in Actinomycetota and marginally significant increase of Pseudomondata within the drying plots across the forests, as well as more differences within the forests individually, suggesting the development of a drought microbiome. While fungal communities were resilient under shorter-term drying, I observed a longer-term shift toward a robust significant increase in relative abundance of drought-tolerant communities such as Agaricomycetes and Eurotiomycetes, and marginal increases of Dothideomycetes and Sordariomycetes. To balance these increases, I also observed marginally significant declines in the fungal classes Glomeromycota Incertae sedis and Rhizophydiomycetes. These results suggest that drying in tropical forests will have a significant long-term impact on soil microbial diversity, with potential downstream effects on soil C cycling and storage.