Tank bromeliads are highly abundant epiphytes in neotropical forests and form a unique canopy wetland ecosystem which is involved in the global methane cycle. Although the tropical climate is characterized by high annual precipitation, the plants can face periods of restricted water. Thus, we hypothesized that water is an important controller of the archaeal community composition and the pathway of methane formation in tank bromeliads. Greenhouse experiments were established to investigate the resident and active archaeal community targeting the 16S rDNA and 16S rRNA in the tank slurry of bromeliads at three different moisture levels. Archaeal community composition and abundance were determined using terminal restriction fragment length polymorphism and quantitative PCR. Release of methane and its stable carbon isotopic signature were determined in a further incubation experiment under two moisture levels. The relative abundance of aceticlastic Methanosaetaceae increased up to 34% and that of hydrogenotrophic Methanobacteriales decreased by more than half with decreasing moisture. Furthermore, at low moisture levels, methane production was up to 100-fold lower (≤0.1-1.1 nmol gdw(-1) d(-1)) than under high moisture levels (10-15 nmol gdw(-1) d(-1)). The rapid response of the archaeal community indicates that the pathway of methane formation in bromeliad tanks may indeed be strongly susceptible to periods of drought in neotropical forest canopies.

Background and aims Tank bromeliads collect organic matter and rainwater (= tank slurry) between their densely arranged leaf axils for their nutrient demand. Diverse communities of microorganisms inhabit these tanks and are responsible for the breakdown of organic matter. Anaerobic degradation results in the release of substantial amounts of methane. We hypothesized that each individual bromeliad harbors its own microbial community, which is affected by chemical tank-slurry properties. We further hypothesized that methanotrophic bacteria inhabit bromeliad tank slurries, potentially able to oxidize the produced CH 4 . Methods We investigated communities of Bacteria, Archaea, methanogenic and methanotrophic microorganisms measuring their abundance (qPCR) and composition (TRFLP) within eight bromeliad tanks of the species Werauhia gladioliflora sampled in a Costa Rican lowland forest. Tank slurries were analyzed for pH, carbon, nitrogen, oxygen and fatty acid concentrations. Methane oxidation rates were determined in five bromeliad tank slurries.Results Our results showed that microbial communities differed between plants and were affected by chemical tank slurry properties. Further, not only methanogenic archaea but also methanotrophic bacteria were detected in the tanks of all bromeliad plants, the latter being potentially able to aerobically oxidize between 25 and 62 μg CH 4 gdw. Conclusion Our results indicate that every bromeliad tank is a unique island with respect to its resident microbial community. The presence of methanogens and active methanotrophs in all tank slurries further indicates the potential for both methane formation and methane oxidation.

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Background and aims Microorganisms colonize plant roots for mutual benefits. Colonization is initiated by the soil microbial community but is also affected by soil conditions and plant type. Rice typically grows under wetland conditions that are anoxic, thus being supportive for an anaerobic methanogenic microbial community. Maize, however, grows under upland conditions that are oxic, thus being supportive for an aerobic microbial community. Crop rotation between wetland rice and upland maize is not uncommon, but the effect of this management on microbial colonization of plant roots is largely unknown and was the aim of our study. Methods We used the roots of rice and maize from a two-year study in the Philippines, where on the same soil wetland rice was cultivated either in both wet and dry season or was rotated with upland maize in the dry season. The microbial colonization of the root ecto-and endorhizosphere was assessed by using quantitative PCR and illumina sequencing of the bacterial and archaeal 16S rRNA genes. Results The data showed that maize roots had completely different microbial community structures than the rice roots from continuous wetland cultivation, while rice roots from crop rotation were in-between. These effects of management were seen for each of the different bacterial phyla. For example, among the most abundant operational taxonomic units (OTUs) Firmicutes, Deltaproteobacteria and the methanogenic Methanocella spp. were less abundant while Alphaproteobacteria and the methanogenic Methanobacterium spp. were more abundant on maize than on rice roots. Conclusions Our study showed that root colonization by Archaea and Bacteria was strongly affected by crop rotation between wetland rice and upland maize.

We studied potential denitrification activity and the underlying denitrifier communities in soils from a semiarid savanna ecosystem of the Kavango region in NE Namibia to help in predicting future changes in N(2)O emissions due to continuing changes of land use in this region. Soil type and land use (pristine, fallow, and cultivated soils) influenced physicochemical characteristics of the soils that are relevant to denitrification activity and N(2)O fluxes from soils and affected potential denitrification activity. Potential denitrification activity was assessed by using the denitrifier enzyme activity (DEA) assay as a proxy for denitrification activity in the soil. Soil type and land use influenced C and N contents of the soils. Pristine soils that had never been cultivated had a particularly high C content. Cultivation reduced soil C content and the abundance of denitrifiers and changed the composition of the denitrifier communities. DEA was strongly and positively correlated with soil C content and was higher in pristine than in fallow or recently cultivated soils. Soil type and the composition of both the nirK- and nirS-type denitrifier communities also influenced DEA. In contrast, other soil characteristics like N content, C:N ratio, and pH did not predict DEA. These findings suggest that due to greater availability of soil organic matter, and hence a more effective N cycling, the natural semiarid grasslands emit more N(2)O than managed lands in Namibia.