Abstract
Microbiota are essential components of the soil, driving biogeochemical cycles. Fungi affect decomposition and biotic interactions with plants across scales. Climate projections suggest that extended dry seasons may transform sensitive rain forests into savanna-like vegetation, with consequent changes in biogeochemistry. Here we compare the impacts of natural seasonality with 14 years of partial throughfall exclusion in an Amazonian rain forest, focussing on soil fungal functional diversity, extracellular soil enzyme activities (EEA) and their implications for nutrient dynamics. Large changes in fungal diversity and functional group composition occur in response to drought, with a conspicuous increase in the abundance of dark-septate fungi and a decrease in fungal pathogens. The high seasonality of EEA in the control (non droughted) and suppression of seasonality in the drought treatment, together with an increased implied nitrogen demand in the dry season induced by experimental drought, suggest that the changed soil microbiota activity may signal a pending shift in the biogeochemical functioning of the forest.
Highlights
Microbiota are essential components of the soil, driving biogeochemical cycles
We found that responses to natural seasonal drought in fungal community richness/composition, functional groups and soil extracellular enzyme activity (EEA) contrasted with those elicited by extreme drought
There was a high richness of dark-septate fungi (DSF), a fungal group that has not been reported quantitatively before in a tropical evergreen rain forest[48]
Summary
Microbiota are essential components of the soil, driving biogeochemical cycles. Fungi affect decomposition and biotic interactions with plants across scales. Drought frequency, duration and severity are predicted to increase pan-tropically, in Amazonia, and impact rain forests as a result of global climate change this century[2,3,4] These regional and global changes in climate could, in the long term, transform large portions of the remaining evergreen Amazonian rain forests into a seasonal savanna-type vegetation or into seasonal and dry forests[5,6,7], modifying biogeochemical cycles substantially[8,9] and compromising the provisioning of ecosystem services[10]. While additional studies have focused on litterfall[23] and soil respiration[17,24], the effects of long-term drought on microbe-driven biogeochemical cycles have not yet been investigated How such impacts on nutrient cycling may be offset by novel soil microbial communities that are able to elicit functional changes is a little explored subject, especially in tropical rain forests. Soil microbial communities produce extracellular enzymes involved in the turnover of C and nutrients from organic compounds, and ratios of potential soil extracellular enzyme activity (EEA) are used to infer microbial C, nitrogen (N) and phosphorus (P) demand[28], and provide important information on biogeochemical cycles, for which information is relatively limited in tropical soils[29,30]
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