Defining Sinks

When do we consider a system a sink for atmospheric N2O? This question is not easily answered.

Many terrestrial and aquatic systems exchange N2O with the atmosphere (Fig. 15.1). Most N2O is formed during bacterial denitrification, during which nitrate (NO-) is reduced to the gaseous nitrogen compounds nitric oxide (NO) and N2O. In some cases, the denitrification process stops here, so that the N2O can escape to the atmosphere (fluxes F1 and F3 in Fig. 15.1).

One of the characteristics of N2O is that it easily dissolves in water. As a result, atmospheric N2O may enter soil pores, and if the soil is wet, N2O may be dissolved in soil water (fluxes F2 and F4 in Fig. 15.1). Consecutively, it may be denitrified by bacteria to N2. This way, soils and aquatic systems can remove N2O from the atmosphere.

Since N2O and NO- may be lost from top soils through leaching and runoff (fluxes F5 and F7 in Fig. 15.1), groundwater and riparian zones can also act as potential important sinks for atmospheric N2O. There may also be some reverse flows of N2O and NO3- from subsoils to the top soils in case of lateral water flows (fluxes F6 and F8 in Fig. 15.1). However, these fluxes are usually much smaller than the leaching fluxes (F6 << F5 and F8 << F7). Through leaching (and runoff ), N2O and NO3- may be transported not only to subsoils but also to deeper ground-water, riparian zones and, eventually, to

F4

F3 F2

J

F1

(S k

Riparian zone/ aquatic system

F7

J

Dg —► NO2 —► NO —► (N ' F8 F5

)—*N2 kF6

F10

NC

—► NO2 —► NO —► (f

^ ground water i)— N2

F9 no3 *

Fig. 15.1. Sources and sinks for nitrous oxide (N2O) at the Earth's surface.

Fig. 15.1. Sources and sinks for nitrous oxide (N2O) at the Earth's surface.

aquatic systems such as rivers and coastal waters (fluxes F9 and F10 in Fig. 15.1). In the course of this transport, denitrification may take place, during which N2O is formed from NO- and/or reduced to N2. However, we suspect that subsoils and groundwater systems do not play an important role as a sink for atmospheric N2O.

Whether a system is considered a source or a sink depends on the strengths of the fluxes F1, F2, F3 and F4. Net N2O uptake may occur when N2O uptake exceeds emission. A complicating factor is the temporal and spatial variability of all these fluxes. Both the emissions (F1 and F3) and uptake (F2 and F4) of N2O are extremely variable in time and space. However, in most soils and aquatic systems overall emissions largely exceed overall uptake.

In this chapter, we concentrate on systems in which net N2O uptake occurs (F2 > F1 or F4 > F3) over a relatively large area (e.g. 1 ha) and prolonged period of time (e.g. 1 year) because it is very difficult to assess the importance of temporal sink activity in systems with overall net emissions. Our aim is to deduce some general characteristics of systems that act as an N2O sink and to identify their likely importance on a global scale.

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