Introduction

Nitrous oxide (N2O) is one of the most important greenhouse gases. Although it is only a trace component in the atmosphere, it is the third most important contributor to the present radiative forcing. Furthermore, N2O is involved in the catalytic destruction of stratospheric ozone (Crutzen, 1970). Atmospheric concentrations of N2O have been increasing for at least a century from ~275 ppbv in pre-industrial times to more than 310 ppbv at present. Studies on the N2O budget have, so far, mainly focused on the sources of N2O in order to explain this increase. Relatively little is known about the soil sinks for N2O. Hence, it is difficult to balance the global N2O budget.

Initially, coal-generated power plants were considered to be the most important source of N2O. However, we now know that fossil fuel combustion is in fact a relatively small source of N2O and, since the late 1990s, it is widely accepted that the increase in atmospheric N2O is largely associated with agricultural activities. This anthropogenic source of N2O is estimated to be of the same order of magnitude as the natural release of N2O from soils and surface waters (Kroeze et al., 1999). Although the most important sources of N2O have thus been identified, the actual rates of emissions on a global scale are subject to large uncertainties. Current emission estimates typically have large uncertainty ranges. For instance, global emissions from agriculture as calculated using the Intergovernmental Panel on Climate Change (IPCC) guidelines are estimated at ~6 Tg N/year, with an uncertainty range of 1.2-17.9 Tg N/year (Mosier et al., 1998). These global budgets usually ignore the possibility of potential soil sinks or assume that the estimates for biogenic sources of N2O are in fact net fluxes that account for possible sinks. However, this may lead to errors in estimates of atmospheric budgets and trends in emissions as pointed out by Cicerone (1989), who showed that atmospheric mass balance calculations are relatively sensitive to uncertainties in the sink strength.

Most observations of N2O removal at the Earth's surface relate to soils. Here, both uptake and emission of N2O may occur simultaneously. The direction of the net flux, determining whether soils are a net source or sink, depends on the environmental factors regulating consumption and production. A useful framework for measuring and modelling N2O exchange between soils and the atmosphere is provided by the compensation point concept introduced by Conrad and Dentener (1999):

At the N2O compensation point the concentration of N2O in the soil gas phase for which there is no net exchange of N2O between ambient atmosphere and soil gas phase. Sink activity can occur when the ambient N2O concentration in the atmosphere exceeds that in the soil gas phase. The compensation point thus determines the direction of the flux at a given ambient atmospheric N2O concentration.

The compensation concentrations of N2O are typically significantly higher than the ambient atmospheric concentrations. This is in agreement with the observation that most soils are a source of atmospheric N2O. An increasing number of studies report on occasional consumption by soils, suggesting that during uptake the compensation concentrations are lower than ambient concentrations, i.e. <310 ppbv. Conrad and Dentener (1999) proposed that the compensation concentration for N2O increases along with temperature, soil moisture and nitrogen availability.

A further consideration is the fast increase in atmospheric nitrogen deposition as observed in many countries (Bouwman et al., 2002b). Even small deposition rates over prolonged periods of time may lead to changes in the nitrogen cycle of sensitive ecosystems (Bobbink et al., 1998). Nitrogen enrichment of ecosystems (partly) suppresses nitrogen limitation and thus may also lead to a reduced N2O sink strength. Even when sink activity is seasonal, this reduction of sinks for N2O may be important considering the vast areas of nitrogen-affected ecosystems and may also contribute to increasing atmospheric N2O concentrations.

The question arises as to what extent sinks for atmospheric N2O at the Earth's surface are quantitatively important. So far, very little attention has been paid to the sink strength, because the uptake of N2O by soils is generally considered to be small on a global scale. This uptake is driven by bacterial denitrification, which converts N2O into nitrogen gas (N2). In most global N2O budgets bacterial reduction of N2O is simply included in the total net flux of N2O from soils. However, its role may be important in reducing the net release of N2O through denitrification in the soil profile.

In this study, we will analyse the processes underlying bacterial reduction of N2O

to N2, and discuss the likeliness of N2O uptake in different systems. We will focus not only on soils but also on aquatic systems, including groundwater systems, riparian zones and surface waters. We will first define what we consider a sink for N2O (Section 15.2). Next, we will describe the processes involved and identify the most important factors influencing N2O uptake (Section 15.3). We will then discuss some observed fluxes as reported in the literature (Section 15.4). Finally, we will analyse the likeliness of N2O uptake in different systems (Section 15.5).

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