Peatlands are among the most important sources of the greenhouse gas methane. Climate warming has been assumed to further increase these emissions, which would be a positive feedback that would boost the warming. This assumption has been based on the overall positive response of methane production to increasing temperatures, and verification has been sought from measurements made during single, exceptionally warm summers.
This assumption is contrasted by novel results from a study where, for the first time, the whole methane cycle under global warming conditions was monitored by analyzing the community structures, abundances as well as the production and consumption potentials of the methane producing and consuming microbes, simultaneously with measuring the methane emissions in field conditions.
– The microbes in the anoxic peat layers that produce the methane and the microbes in the oxic peat layers that consume the methane are the two main factors that regulate the methane emissions from peatlands, says researcher Krista Peltoniemi of the Natural Resources Institute Finland (Luke).
Experimental warming decreased the methane production
Experimental warming of wet pristine sedge fen decreased the abundance and capacity to produce methane of the methane-producing microbes, methanogens. A reduction in the methanogen abundance was also observed under drier conditions that were induced by slightly draining the fen; this effect was not further amplified by warming. Warming did not affect the methane-consuming MOB (methane-oxidizing bacteria) community, or its capacity to consume methane in the wet conditions. However, warming in the drier conditions altered the MOB community.
All these responses resulted in decreased methane emissions as measured in field conditions. This implies that the methane emissions of sedge fens will decrease as a consequence of persistent warming, no matter whether warming takes place under wet or drier conditions.
– There are more uncertainties connected to the precipitation than to the warming predictions; and we do not yet know which scenario is the more likely to be realized. According to our results, sedge fens will not boost global warming by producing more methane in any case, which up till now has been a widely acknowledged hypothesis, says professor Raija Laiho of the Natural Resources Institute Finland (Luke).
Microbe communities adapt to global warming
The methane emission regulating microbe communities adapt to a warming climate. This is the reason why observations from single warm summers are not sufficient for predicting long-time changes. Methanogens and MOB may have different controlling patterns on methane fluxes when facing global warming. These patterns may further differ not only between moisture regimes, but within the same ecosystem type. Despite the differences in communities and their responses, the methane emissions still seem to respond similarly across sites, by decreasing.
Realization of the research
The research was a collaborative effort of Luke and the Universities of Helsinki and Eastern Finland. The field experiment with the warming and drying treatments were installed in two pristine fen sites that were located in the Lakkasuo mire in Orivesi (south Finland) and the Lompolojänkkä mire in Kittilä (north Finland). Warming was realized passively with open-top chambers, and drying with shallow ditches. The chosen study sites were sedge fens, treeless wet mires, from which the present methane emissions are relatively high. The methane emissions were measured on the spot for several years between spring and fall, while the microbial communities and their capacity to produce and consume methane were analyzed three years after the experiment started.
The study was funded by the Academy of Finland.