Forest soil and deadwood CH4 fluxes in response to warming, increased precipitation, nitrogen fertilization and biochar addition across a Norwegian spruce age-management gradient

Methane (CH₄) is the second largest contributor to global warming and the importance of reducing net CH₄ emissions was recently highlighted through the 2021 Global Methane Pledge. Upland forest soils are most often acting as CH₄ sinks, but forest management – for example clear cutting and nitrogen (N) fertilization – carry the potential to turn CH₄ sinks into CH₄ sources. However, little is currently known about the underlying mechanisms and to what extent forest management affects the fluxes. Furthermore, the role of deadwood in the CH₄ cycle is poorly understood and quantified but has recently received increased attention. Deadwood, like soils, can act both as a sink and a source of CH₄ and the few available studies indicate that tree species, decay class and wood density are important regulators of CH₄ cycling in deadwood.

In the ForBioFunCtioN-project, we utilize state-of-the-art technology (LI-7810 Trace Gas Analyzer, LI-COR®) for in situ measurements of soil-atmosphere and deadwood-atmosphere exchange of CO₂ and CH₄ in an extensive climate and management manipulation experiment. Treatments include warming with open-top chambers, increased precipitation (on average 25 mm/year during the snow-free period), N fertilization (NH₄NO₃ 150 kg/ha) and biochar addition (10 t/ha) in a total of 12 treatment combinations (n = 144) across five Norwegian spruce dominated bilberry forest sites spanning from a recent clear-cut to mature managed (80 years) and old unmanaged (140 years) stands.

Here, we present the experimental setup of ForBioFunCtioN and soil and deadwood CH₄ flux measurements from the snow-free period in 2021 and 2022. Initial results showed that N fertilization decreased net soil CH₄ consumption and that, while at rare occasions functioning as a sink, the Norwegian spruce deadwood was almost exclusively a source of CH₄. The source strength of deadwood differed substantially between sites but CH₄ efflux from deadwood increased by biochar addition at all sites.