Guntars O. Martinson

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.

Drying effects on archaeal community composition and methanogenesis in bromeliad tanks

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.

Effects of Nutrient Addition on the Productivity of Montane Forests and Implications for the Carbon Cycle

Both carbon storage and sequestration are major ecosystem services provided by forests. The NUMEX (Ecuadorian NUtrient Manipulation EXperiment) study aims to identify the underlying mechanisms for the variation of these services as affected by future changes in nutrient availability. The ongoing experiment is being conducted in southern Ecuador to improve our understanding of the effects of continuous moderate N and P addition to tropical montane forest ecosystems. This chapter summarizes the short-term effects of nutrient addition evident at the end of the experiment’s first year. The rapid responses of the studied Andean montane forests to N and P addition observed at this early stage of the experiment illustrate the vulnerability of the forests to higher nutrient deposition.

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.

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.

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.