Forests in Europe are changing due to interactions between climate change, nitrogen (N) deposition and new forest management practices. The concurrent impact on the forest greenhouse gas (GHG) balance is at present difficult to predict due to a lack of knowledge on controlling factors of GHG fluxes and response to changes in these factors. To improve the mechanistic understanding of the ongoing changes, we studied the response of soil–atmosphere exchange of nitrous oxide (N₂O) and methane (CH₄) at twelve experimental or natural gradient forest sites, representing anticipated future forest change. The experimental manipulations, one or more per site, included N addition (4 sites), changes of climate (temperature, 1 site; precipitation, 2 sites), soil hydrology (3 sites), harvest intensity (1 site), wood ash fertilisation (1 site), pH gradient in organic soil (1 site) and afforestation of cropland (1 site).
On average, N₂O emissions increased by 0.06 ± 0.03 (range 0–0.3) g N₂O-N m⁻² yr⁻¹ across all treatments on mineral soils, but the increase was up to 10 times higher in an acidic organic soil. Soil moisture together with mineral soil C/N ratio and pH were found to significantly influence N₂O emissions across all treatments. Emissions were increased by elevated N deposition, especially in interaction with increased soil moisture. High pH reduced the formation of N₂O, even under otherwise favourable soil conditions.
Oxidation (uptake) of CH₄ was on average reduced from 0.16 ± 0.02 to 0.04 ± 0.05 g CH₄-C m⁻² yr⁻¹ by the investigated treatments. The CH₄ exchange was significantly influenced by soil moisture and soil C / N ratio across all treatments, and CH₄ emissions occurred only in wet or water-saturated conditions.
For most of the investigated forest manipulations or natural gradients, the response of both N₂O and CH₄ fluxes was towards reducing the overall GHG forest sink. The most resilient forests were dry Mediterranean forests, as well as forests with high soil C/N ratio or high soil pH. Mitigation strategies may focus on (i) sustainable management of wet forest areas and forested peatlands, (ii) continuous forest cover management, (iii) reducing atmospheric N input and, thus, N availability, and (iv) improving neutralisation capacity of acid soils (e.g. wood ash application).