Introduction

The rapidly increasing human and animal population and the concomitant request for more food, feed and fibre have contributed to a rapid increase in available nitrogen in global agriculture during the last century (Galloway, 1998; Tilman et al., 2001). Similarly, rapid industrialization and traffic development have greatly increased fossil fuel consumption, and hence the amount of reactive nitrogen in the environment (Galloway et al., 2004). As a consequence, in industrialized areas with intensified agriculture, including large parts of Europe, there are increased atmospheric concentrations of nitrogen oxides (NOX: NO and NO2), mainly caused by traffic and industry, and of ammonia (NH3), mainly due to emissions from (intensive) agriculture. Even though the nitrogen deposition in Europe is likely to decrease in view of NOX emission reductions, recent estimates suggest that the global annual average nitrogen deposition over land will increase by a factor of 2.5, with the average nitrogen deposition over forests expected to increase from 10 to 20 Tg

N/year in the 21st century (Lamarque et al., 2005).

Increased atmospheric concentrations of NOX and NH3 and their subsequent deposition can have various effects on human health (Wolfe and Patz, 2002; Townsend et al., 2003), for example, due to the involvement of reactive nitrogen species in the production of tropospheric O3 (Crutzen, 1995) or the formation of secondary fine particles (e.g. Amann et al., 2001), and on the (nutrient) condition and diversity of natural ecosystems like forests (e.g. De Vries et al., 1995, 2003b, d; Bobbink et al., 1998). Increased depositions of atmospheric NOx and NH3 may also influence the exchange of the three main greenhouse gases (GHGs) - carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) - between biosphere and atmosphere.

The net effect of anthropogenic nitrogen deposition on the net GHG budget of natural ecosystems is still unclear. The exchange rates of CO2, N2O and CH4 between biosphere and atmosphere are the result of closely linked, but complex biologically mediated, production and consumption processes (Conrad, 1996). Increased nitrogen deposition usually increases net primary production (NPP) and the fixation of CO2 by vegetation (e.g. Townsend et al., 1996; Hungate et al., 2003). Increased nitrogen availability has probably contributed to the increase in forest growth rates observed across Europe and account for some of the 'missing global carbon sink' (e.g. Hunter and Schuck, 2002). Increased productivity may in turn also increase carbon sequestration in soil due to increased litterfall (e.g. Nadelhoffer et al., 1999; De Vries et al., 2005b). However, N2O emissions tend to increase when soil carbon increases, due to positive relationships between soil carbon contents, nitrogen turnover rates and N2O production (Six et al., 2004; Li et al., 2005). Moreover, increased nitrogen deposition may directly increase N2O emissions through increased availability of nitrogen for microbial processes directly involved in N2O production (Bowden et al., 1991; Butterbach-Bahl et al., 1997, 2002a). On the other hand, nitrogen deposition (especially NH4) may decrease the oxidation capacity of soils for atmospheric CH4, thereby decreasing the net influx of CH4 from atmosphere to biosphere (Steudler et al., 1989; Sitaula et al., 1995; Van den Pol-van Dasselaar et al., 1999), but inverse effects have also been found (Bodelier and Laanbroek, 2004). Evidently, the net effect of anthropogenic nitrogen deposition on the net GHG budget of natural ecosystems is the resultant of complex interactions and ecosystem feedbacks, and is highly dependent on local environmental conditions.

While CO2 exchange is measured in fairly extensive global networks (CarboEurope IP, FLUXNET), the effect of nitrogen inputs on carbon sequestration remains poorly quantified. Initiatives to model the global effect of nitrogen availability on carbon sequestration (e.g. Hudson et al., 1994; Holland et al., 1997; Asner et al., 2001) are limited by uncertainties in the mechanistic understanding of carbon-nitrogen interactions in plant and soil and a lack of field data for verification. Although a range of nitrogen fertilization experiments have been performed at the plot level, site networks that derive both net ecosystem exchange (NEE) and nitrogen uptake are extremely limited (Nadelhoffer et al., 1999). Unlike CO2, estimates on regional and continental (global) scales of the source strength of soils for N2O and CH4 remain much more uncertain, both in view of the complexity of the processes involved and the lack of extensive global networks. This holds even more strongly for the impact of nitrogen deposition on these emissions.

This chapter discusses the interactions between anthropogenic nitrogen deposition and exchanges of CO2, N2O and CH4 between forest ecosystems and the atmosphere in Europe. We focus on forest ecosystems in Europe because atmospheric nitrogen deposition is relatively high, while interactions between elevated anthropogenic nitrogen deposition and exchanges of CO2, N2O and CH4 for these forests have been explored in various European countries. Furthermore, a Pan-European Programme for Intensive and Continuous Monitoring of Forest Ecosystems has been carried out since 1994, which includes information on nitrogen deposition and on site factors that affect GHG emissions, thus allowing the possible upscaling of relationships to the European scale. Approximately 860 permanent observation plots, with more than 500 plots with atmospheric deposition data, have been selected in 30 participating countries (level I Monitoring Programme, e.g. De Vries et al., 2003d). Such upscaling is further enabled by a European Monitoring Programme on air pollution impacts since 1986, in which several forest and soil condition characteristics are monitored at a systematic 16 x 16 km grid at more than 6000 plots throughout Europe (level I Monitoring Programme, e.g. UN/ECE and EC, 2004). For these plots, relevant site and soil characteristics and modelled nitrogen deposition estimates are available (e.g. Nadelhoffer et al., 1999; De Vries et al., 2005b).

A quantitative assessment of this interaction requires a proper understanding of all major biotic and abiotic processes in ecosystems (Li et al., 2005). Therefore, we first discuss the processes and factors involved in the interactions and the effects of increased atmospheric nitrogen deposition on them (Section 17.2). In Section 17.3, we present an overview of published emission data of CO2, N2O and

CH4 from European forests and forest soils. In Section 17.4, we present the methods and data that were used to quantify the effects of nitrogen deposition on the net exchange of CO2, N2O and CH4 between European forests and atmosphere, and the results of our calculations. The calculations presented in this section are new and are all based on De Vries et al. (2006). Finally, in Section 17.5, we discuss the net CO2, N2O and CH4 emissions and the nitrogen deposition impacts on those emissions in terms of CO2 equivalents, by using the global warming potential (GWP) approach, i.e. 1 kg N2O is assumed to be 296 kg CO2 equivalents and 1 kg CH4 is 23 kg CO2 equivalents (Ramaswamy, 2001). Furthermore, the contribution of N2O and CH4 emissions from forests compared to agriculture, the reliability of the assessments and the research needs to improve the quantification are discussed.

due to the microbial processes of mineralization, nitrification, denitrification, methano-genesis and CH4 oxidation. The interactions between carbon and nitrogen, and the exchange of CO2, N2O and CH4 between biosphere and atmosphere are largely controlled by external drivers such as climate (radiation, rainfall, temperature), land use and management (forest type and its management), soil type and deposition of atmospheric nitrogen (Conrad, 1996; Groffman et al., 2000). This section summarizes the effects of atmospheric nitrogen deposition on the exchanges of CO2, N2O and CH4 between biosphere and atmosphere, using the relational diagram depicted in Fig. 17.1, while briefly describing the effect of other factors controlling the exchange of CO2, N2O and CH4 between forests and atmosphere.

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