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It is increasingly acknowledged that climate change is influencing terrestrial ecosystems by increased drought and rainfall intensities. Soil microbes are key drivers of many processes in terrestrial systems and rely on water in soil pores to fulfill their life cycles and functions. However, little is known on how drought and rainfall fluctuations, which affect the composition and structure of microbial communities, persist once original moisture conditions have been restored. Here, we study how simulated short-term drying and re-wetting events shape the community composition of soil fungi and prokaryotes. In a mesocosm experiment, soil was exposed to an extreme drought, then re-wetted to optimal moisture (50% WHC, water holding capacity) or to saturation level (100% WHC). Composition, community structure and diversity of microbes were measured by sequencing ITS and 16S rRNA gene amplicons 3 weeks after original moisture content had been restored. Drying and extreme re-wetting decreased richness of microbial communities, but not evenness. Abundance changes were observed in only 8% of prokaryote OTUs, and 25% of fungal OTUs, whereas all other OTUs did not differ between drying and re-wetting treatments. Two specific legacy response groups (LRGs) were observed for both prokaryotes and fungi. OTUs belonging to the first LRG decreased in relative abundance in soil with a history of drought, whereas OTUs that increased in soil with a history of drought formed a second LRG. These microbial responses were spread among different phyla. Drought appeared to be more important for the microbial community composition than the following extreme re-wetting. 16S profiles were correlated with both inorganic N concentration and basal respiration and ITS profiles correlated with fungal biomass. We conclude that a drying and/or an extreme re-wetting history can persist in soil microbial communities via specific response groups composed of members with broad phylogenetic origins, with possible functional consequences on soil processes and plant species. As a large fraction of OTUs responding to drying and re-wetting belonged to the rare biosphere, our results suggest that low abundant microbial species are potentially important for ecosystem responses to extreme weather events.

It is increasingly acknowledged that climate change is influencing terrestrial ecosystems by increased drought and rainfall intensities. Soil microbes are key drivers of many processes in terrestrial systems and rely on water in soil pores to fulfill their life cycles and functions. However, little is known on how drought and rainfall fluctuations, which affect the composition and structure of microbial communities, persist once original moisture conditions have been restored. Here, we study how simulated short-term drying and re-wetting events shape the community composition of soil fungi and prokaryotes. In a mesocosm experiment, soil was exposed to an extreme drought, then re-wetted to optimal moisture (50% WHC, water holding capacity) or to saturation level (100% WHC). Composition, community structure and diversity of microbes were measured by sequencing ITS and 16S rRNA gene amplicons 3 weeks after original moisture content had been restored. Drying and extreme re-wetting decreased richness of microbial communities, but not evenness. Abundance changes were observed in only 8% of prokaryote OTUs, and 25% of fungal OTUs, whereas all other OTUs did not differ between drying and re-wetting treatments. Two specific legacy response groups (LRGs) were observed for both prokaryotes and fungi. OTUs belonging to the first LRG decreased in relative abundance in soil with a history of drought, whereas OTUs that increased in soil with a history of drought formed a second LRG. These microbial responses were spread among different phyla. Drought appeared to be more important for the microbial community composition than the following extreme re-wetting. 16S profiles were correlated with both inorganic N concentration and basal respiration and ITS profiles correlated with fungal biomass. We conclude that a drying and/or an extreme re-wetting history can persist in soil microbial communities via specific response groups composed of members with broad phylogenetic origins, with possible functional consequences on soil processes and plant species. As a large fraction of OTUs responding to drying and re-wetting belonged to the rare biosphere, our results suggest that low abundant microbial species are potentially important for ecosystem responses to extreme weather events.

Abstract Tropical mountain cloud forests (TMCF) harbour a high bryophyte (mosses and liverworts) biomass and diversity. Furthermore, the high air humidity makes these forests well suited for bryophyte‐associated nitrogen (N2) fixation by cyanobacteria, providing a potentially important source of N input to the ecosystem. However, few studies have assessed bryophyte‐associated N input in these ecosystems, and these have focused on epiphytic bryophytes, whereas abundant ground‐covering bryophytes have not been included. In this study, we quantified N2 fixation rates associated with bryophytes, focusing on ground‐covering mosses in a neotropical mountain cloud forest. Furthermore, we identified the effects of climate change (higher temperature 10 vs. 20° and lower bryophyte moisture level 50% vs. 100%) on N2 fixation across bryophyte species and groups (mosses and liverworts). Nitrogen fixation rates associated with ground‐covering moss species were up to 2 kg N ha−1 year−1, which is comparable to other N inputs (e.g. N deposition) in tropical cloud forests. Furthermore, changes in temperature showed little effect on N2 fixation, but low moisture levels significantly suppressed N2 fixation activity. We found low N2 fixation activity associated with the investigated liverworts. Our results demonstrate the importance of ground‐covering, moss‐associated N2 fixation as a N source in tropical cloud forests and suggest that predicted future declines in precipitation in these systems will reduce N inputs from bryophyte‐associated cyanobacteria. Read the free Plain Language Summary for this article on the Journal blog.

Abstract Tropical mountain cloud forests (TMCF) harbour a high bryophyte (mosses and liverworts) biomass and diversity. Furthermore, the high air humidity makes these forests well suited for bryophyte‐associated nitrogen (N2) fixation by cyanobacteria, providing a potentially important source of N input to the ecosystem. However, few studies have assessed bryophyte‐associated N input in these ecosystems, and these have focused on epiphytic bryophytes, whereas abundant ground‐covering bryophytes have not been included. In this study, we quantified N2 fixation rates associated with bryophytes, focusing on ground‐covering mosses in a neotropical mountain cloud forest. Furthermore, we identified the effects of climate change (higher temperature 10 vs. 20° and lower bryophyte moisture level 50% vs. 100%) on N2 fixation across bryophyte species and groups (mosses and liverworts). Nitrogen fixation rates associated with ground‐covering moss species were up to 2 kg N ha−1 year−1, which is comparable to other N inputs (e.g. N deposition) in tropical cloud forests. Furthermore, changes in temperature showed little effect on N2 fixation, but low moisture levels significantly suppressed N2 fixation activity. We found low N2 fixation activity associated with the investigated liverworts. Our results demonstrate the importance of ground‐covering, moss‐associated N2 fixation as a N source in tropical cloud forests and suggest that predicted future declines in precipitation in these systems will reduce N inputs from bryophyte‐associated cyanobacteria. Read the free Plain Language Summary for this article on the Journal blog.

Differences in bacterial community composition (BCC) between bulk and rhizosphere soil and between rhizospheres of different plant species are assumed to be strongly governed by quantitative and qualitative rhizodeposit differences. However, data on the relationship between rhizodeposit amounts and BCC are lacking. Other soil microorganisms, e.g. arbuscular mycorrhizal fungi (AMF), may also influence BCC. We simulated foliar herbivory (cutting) to reduce belowground carbon allocation and rhizodeposition of pea plants grown either with or without AMF. This reduced soil respiration, rhizosphere microbial biomass and bacteriovorous protozoan abundance, whereas none of these were affected by AMF. After labelling plants with (13)CO(2), root and rhizosphere soil (13)C enrichment of cut plants were reduced to a higher extent (24-46%) than shoot (13)C enrichment (10-24%). AMF did not affect (13)C enrichment. Despite these clear indications of reduced rhizosphere carbon-input, denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes PCR-amplified targeting DNA and RNA from rhizosphere soil did not reveal any effects of cutting on banding patterns. In contrast, AMF induced consistent differences in both DNA- and RNA-based DGGE profiles. These results show that a reduction in rhizosphere microbial activity is not necessarily accompanied by changes in BCC, whereas AMF presence inhibits proliferation of some bacterial taxa while stimulating others.