Growing season CH₄ and N₂O fluxes from a subarctic landscape in northern Finland; from chamber to landscape scale

Abstract. Subarctic and boreal emissions of CH₄ are important contributors to the atmospheric greenhouse gas (GHG) balance and subsequently the global radiative forcing. Whilst N₂O emissions may be lower, the much greater radiative forcing they produce justifies their inclusion in GHG studies. In addition to the quantification of flux magnitude, it is essential that we understand the drivers of emissions to be able to accurately predict climate-driven changes and potential feedback mechanisms. Hence this study aims to increase our understanding of what drives fluxes of CH₄ and N₂O in a subarctic forest/wetland landscape during peak summer conditions and into the shoulder season, exploring both spatial and temporal variability, and uses satellite-derived spectral data to extrapolate from chamber-scale fluxes to a 2 km  ×  2 km landscape area.

From static chamber measurements made during summer and autumn campaigns in 2012 in the Sodankylä region of northern Finland, we concluded that wetlands represent a significant source of CH₄ (3.35 ± 0.44 mg C m⁻² h⁻¹ during the summer campaign and 0.62 ± 0.09 mg C m⁻² h⁻¹ during the autumn campaign), whilst the surrounding forests represent a small sink (−0.06 ± < 0.01 mg C m⁻² h⁻¹ during the summer campaign and −0.03 ± < 0.01 mg C m⁻² h⁻¹ during the autumn campaign). N₂O fluxes were near-zero across both ecosystems.

We found a weak negative relationship between CH₄ emissions and water table depth in the wetland, with emissions decreasing as the water table approached and flooded the soil surface and a positive relationship between CH₄ emissions and the presence of Sphagnum mosses. Temperature was also an important driver of CH₄ with emissions increasing to a peak at approximately 12 °C. Little could be determined about the drivers of N₂O emissions given the small magnitude of the fluxes.

A multiple regression modelling approach was used to describe CH₄ emissions based on spectral data from PLEIADES PA1 satellite imagery across a 2 km  ×  2 km landscape. When applied across the whole image domain we calculated a CH₄ source of 2.05 ± 0.61 mg C m⁻² h⁻¹. This was significantly higher than landscape estimates based on either a simple mean or weighted by forest/wetland proportion (0.99 ± 0.16, 0.93 ± 0.12 mg C m⁻² h⁻¹, respectively). Hence we conclude that ignoring the detailed spatial variability in CH₄ emissions within a landscape leads to a potentially significant underestimation of landscape-scale fluxes. Given the small magnitude of measured N₂O fluxes a similar level of detailed upscaling was not needed; we conclude that N₂O fluxes do not currently comprise an important component of the landscape-scale GHG budget at this site.