Methane emissions from upland boreal forest soils

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Large uncertainties still exist in the magnitude of the sources and sinks that contribute to the global methane budget.  Boreal landscapes typically contain a mixture of forests, wetlands and peatlands, and the total catchment flux of methane will be determined by the source/sink behaviour of each ecosystem.  Upland forests are typically considered to be net sinks of methane, although periods of methane release have been observed after heavy rainfall.  Contrary to this, peatlands and wetlands are well known to emit large quantities of methane.  However, there is a still a lack of knowledge concerning how inter-annual variations in climate can affect the total catchment methane budget.

To address this knowledge gap, a group of researchers led by the Finnish Meteorological Institute conducted a twenty-eight month study in the Pallaslompolo catchment in northern Finland.  Methane fluxes were measured from a minerotrophic fen and a nearby upland forest.  The forest soil was observed to be a small methane sink (-0.18 to -2.3 mg CH4 m-2 d-1) from September 2010 to August 2011.  Following three months of heavy rainfall, the forest then became a large methane source until January 2012 (max = 92 mg m-2 d-1).  After this spike, the forest returned to acting as a methane sink for the remainder of the study (which ran until January 2013).

When upscaling flux measurements to the entire catchment, the forest consumed the equivalent of 10% of the methane emitted by the fen in the dry year.  During the wet year when the large forest methane spike was observed, the forest was a source equivalent to 57% of fen emissions. During August and September 2011, monthly methane fluxes from the forest were twice as large as the emissions from the fen.  The authors hypothesise that high forest methane emissions occur during wet conditions, but also require other factors which include: 1) high soil temperature; 2) an input of soil carbon from roots/litter; 3) high methanogenic activity.  This explains why high forest fluxes where not observed during spring snowmelt when soil moisture values peaked.

A signal of the high forest emission was also detected in the atmospheric methane concentration measured nearby. In the long-term data, the mean September concentration anomaly was well explained by the water level of the nearby lake, suggesting that the water level could act as a better proxy for soil methanogenic potential than a point measurement of soil moisture.

This paper shows that methane fluxes in boreal forests can show extreme variation between years according to differences in precipitation.  It demonstrates the importance of taking frequent, year-round flux measurements. Lead author, Annalea Lohila said: “We were surprised to see how differently the forest and wetland ecosystems responded to excess wetness, and how important the upland forests are for methane balances after upscaling to the landscape level.”

Lohila, A., Aalto, T., Aurela, M., et al. 2016. Large contribution of boreal upland forest soils to a catchment-scale CH4 balance in a wet year. Geophysical Research Letters, DOI: 10.1002/2016GL067718.

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