The greenhouse gas exchange responses of methane and nitrous oxide to forest change in Europe.

Climate change and air pollution, interact with altering forest management and landuse change to produce short and long-term changes to forest in Europe. The impact of these changes on the forest greenhouse gas (GHG) balance is currently difficult to predict. To improve the mechanistic understanding of the ongoing changes, we studied the response of GHG (N₂O, CH₄) exchange from forest soils at twelve experimental or natural gradient forest sites, representing anticipated future forest change. The experimental manipulations one or more per site included nitrogen (N) addition (4 sites), changes of climate (temperature, 1 site; precipitation, 2 sites), soil hydrology (3 sites), harvest intensity (1 site), wood ash fertilization (1 site), pH gradient in peat (1 site) and afforestation of cropland (1 site). In most of the investigated treatments N₂O emissions increased by 7±3 (range 0-30) µg N₂O-N m^-2 h^-1 across all treatments on mineral soils, but by up to 10 times the mineral soil maximum on 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 increased with N availability and decreased with soil C/N ratio, 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 reduced from 16±2 to 4±6 µg CH₄-C m^-2 h^-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 peat lands, (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). [ABSTRACT FROM AUTHOR]