Hydrogenotrophic methanogenesis is the dominant methanogenic pathway in neotropical tank bromeliad wetlands

Several thousands of tank bromeliads per hectare of neotropical forest create a unique wetland ecosystem that emits substantial amounts of CH₄ . Tank bromeliads growing in the forest canopy (functional type-II tank bromeliads) were found to emit more CH₄ than tank bromeliads growing on the forest floor (functional type-I tank bromeliads) but the reasons for this difference and the underlying microbial CH₄ -cycling processes have not been studied. Therefore, we characterized archaeal communities in bromeliad tanks of the two different functional types in a neotropical montane forest of southern Ecuador using terminal-restriction fragment length polymorphism (T-RFLP) and performed tank-slurry incubations to measure CH₄ production potential, stable carbon isotope fractionation and pathway of CH₄ formation. The archaeal community composition was dominated by methanogens and differed between bromeliad functional types. Hydrogenotrophic Methanomicrobiales were the dominant methanogens and hydrogenotrophic methanogenesis was the dominant methanogenic pathway among all bromeliads. The relative abundance of aceticlastic Methanosaetaceae and the relative contribution of aceticlastic methanogenesis increased in type-I tank bromeliads probably due to more oxic conditions in type-I than in type-II bromeliads leading to the previously observed lower in situ CH₄ emissions from type-I tank bromeliads but to higher CH₄ production potentials in type-I tank bromeliad slurries.

Methane emission from feather moss stands

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH₄ . However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH₄ turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH₄ cycling. Therefore, we investigated methanogenic rates and pathways together with the methanogenic microbial community composition in feather moss stands from temperate and boreal forests. Potential rates of CH₄ emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH₄ h^-1 gdw^-1 . Temperature and water content positively influenced CH₄ emission. Methanogenic potentials determined under N₂ atmosphere in darkness ranged between 22 and 157 pmol CH₄ h^-1 gdw^-1 . Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH₄ was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 10⁴ and 10⁵ copies per gram dry weight moss of the mcrA gene coding for a subunit of the methyl CoM reductase. Archaeal 16S rRNA and mcrA gene sequences in the moss stands were characteristic for the archaeal families Methanobacteriaceae and Methanosarcinaceae. The potential methanogenic rates were similar in incubations with and without methyl fluoride, indicating that the CH₄ was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH₄ produced was depleted in ¹³ C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3-62 mM). The δ¹³ C of acetate was similar to that of the moss biomass, indicating acetate production by fermentation. Our study showed that the feather moss stands contained active methanogenic microbial communities producing CH₄ by hydrogenotrophic methanogenesis and causing net emission of CH₄ under ambient conditions, albeit at low rates.

Potential N2O Emissions from the Tanks of Bromeliads Suggest an Additional Source of N2O in the Neotropics

We studied the propensity of the tank bromeliad Werauhia gladioliflora to emit the greenhouse gas nitrous oxide (N₂O) at current and at increased N deposition levels in the range of predicted future scenarios. Potential production rates and net accumulation of N₂O from tank substrate corresponded to N availability. N₂O was produced in excess at all N levels due to a low level of N₂O reductase activity which agreed well with a low abundance of N₂O reducers compared to nitrite reducers. Transcriptional activation, however, indicated that expression of denitrification genes may be enhanced with increasing N supply eventually leading to more efficient N₂O turnover with potential for adaptation of denitrifier communities to higher N levels. Our findings indicate that tank bromeliads may constitute a novel source of N₂O in Neotropical forest canopies but further studies are required to understand the size and significance of in situ N₂O fluxes from tank bromeliads to the environment.

Tropical Andean forests are highly susceptible to nutrient inputs--rapid effects of experimental N and P addition to an Ecuadorian montane forest

Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha(-1) yr(-1)) and P (10 kg ha(-1) yr(-1)). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes-notably aboveground productivity--are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs.