Abstract
Abstract. 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).
Highlights
European forests sequester atmospheric carbon (C) in biomass and soil at an estimated annual net rate of 109 Tg (Luyssaert et al, 2010), which is equivalent to 10 % of the European fossil fuel emissions
Results of the annual greenhouse gas (GHG) exchange were compiled from 12 sites with manipulation experiments or observational gradients (Table 1) selected to represent multiple aspects of forest change across Europe (Fig. 1)
Field measurements of soil GHG exchange were established at all sites and for all treatments as part the NitroEurope project (C2 sites) and designed to address the objectives of this study
Summary
European forests sequester atmospheric carbon (C) in biomass and soil at an estimated annual net rate of 109 Tg (Luyssaert et al, 2010), which is equivalent to 10 % of the European fossil fuel emissions. The atmospheric concentration of both gases has constantly risen since the late 1890s, amounting presently to a 158 % and 19 % increase, respectively, compared to pre-industrial levels (WMO, 2010) At present, these two greenhouse gases constitute 18 % (CH4) and 6 % (N2O) of the global radiative forcing in the atmosphere (not accounting for water vapour), and their role in the global warming is expected to increase in the future (WMO, 2010). The first European Nitrogen Assessment (Sutton et al, 2011) stated that European forests were responsible for a net cooling effect, the magnitude of this phenomenon is still associated with considerable uncertainty (Schulze et al, 2009; Butterbach-Bahl et al, 2011a). The importance of forests for the European GHG balance will increase
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