Whether denitrification in a given environment acts as a net source or sink for N2O depends on the relative abundance of oxygen, NO3- and suitable electron donors. Additional important factors include pH and temperature, as well as the possibility for uptake of N2O from the atmosphere. Generally, N2O reduction is highest when oxygen and NO3- are low, and when N2O and a suitable electron donor are available (Firestone and Davidson, 1989).
N2O reductases, the enzymes needed for the conversion of N2O to N2, are the most oxygen-sensitive of the enzymes involved in denitrification. A small amount of oxygen is sufficient to inhibit the conversion and will lead to accumulation instead of removal of N2O (Betlach and Tiedje, 1981; Blicher-Matthiesen and Hoffmann, 1999). Thus environments such as waterlogged soils, sediments, groundwater and riparian zones, where diffusive transport of oxygen is limited, can potentially be sites for N2O reduction.
High NO- concentrations also commonly inhibit N2O reductase activity (Blackmer and Bremner, 1978; Blicher-Mathiesen and Hoffmann, 1999). This explains the strong positive relationship between NO- availability and N2O accumulation generally observed for soils (e.g. Skiba et al., 1998). It also explains why net uptake of N2O is typically observed when dissolved NO3- concentrations in grassland and forest soils are low (e.g. <1-2 |ig NO--N/g soil; Ryden, 1981; Butterbach-Bahl et al., 1998). NO3- concentrations are expected to be lowest in environments where use of N-fertil-izers is limited and/or plant uptake of nitrogen is high, and where nitrification does not occur (e.g. due to a lack of oxygen and ammonia). In these environments, where electron acceptors are limiting, N2 will be the principal gas evolved.
In most anoxic soils and natural waters, sufficient organic carbon is present to support N2O reduction. In groundwater systems, where denitrification is frequently electron donor-limited, N2O leached from surface soils can be transported conservatively through the system, thus making N2O a sensitive tracer of plume movement (DeSimone and Howes, 1998). The electron donor limitation in groundwater may be alleviated when, prior to discharge, the groundwater passes through an organic carbon or pyrite-rich riparian zone. When the NO- concentration in groundwater is low, such a riparian zone can act as a net sink for groundwater N2O. When the NO3- concentration is high, particularly when compared to the availability of a reductant, N2O is expected to accumulate.
Denitrification rates under field conditions are lower in acidic soils when compared to neutral or slightly alkaline soils. The amount of N2O relative to N2 formed during denitrification is also pH-dependent and generally decreases with increasing pH (Simek and Cooper, 2002). With these data, highest rates of N2O reduction are expected in neutral to alkaline soils. The soil pH is also a major control of nitrification either directly or through its effect on soil cation exchange capacity (Robertson, 1989). Optimum values for nitrification range from 6.5 to 8 (Simek and Cooper, 2002). Thus, at neutral pH, enhanced NO3- production through nitrifica tion could limit the possibility of a soil to act as a net sink for atmospheric N2O.
Denitrification rates are very sensitive to temperature. Typical Q10 values determined for denitrification range from 5 to 16 (Ryden, 1983). At the same time, the ratio of N2O to N2 produced during denitrifica-tion decreases with increasing temperature (Firestone and Davidson, 1989). This indicates that the potential for removal of N2O through denitrification is enhanced at higher temperatures.
The nitrogen cycle includes some dynamic aspects that may be important for net removal of N2O from the atmosphere. For instance, Bakken and Bleken (1998) show that it can take decades to centuries before nitrogen is transported from soils to coastal waters depending on the route of transport (e.g. via subsoils or freshwaters). Ignoring these temporal characteristics in nitrogen budgets may lead to errors in estimates of nitrogen loads in aquatic systems, as well as in estimates of N2O fluxes associated with nitrogen leaching and runoff.
Net N2O uptake at the Earth's surface will occur only when in situ N2O concentrations in groundwater, soil and surface waters are lower than aqueous concentrations in equilibrium with the atmosphere. Thus, we conclude that such conditions most likely occur in soils and riparian zones with low nitrogen loading. Groundwater and most surface waters are expected to be less important as a sink for N2O because conditions are generally more conducive for N2O production due to either abundant presence of oxygen and/or NO3- or electron donor limitation. As indicated in Section 15.1, the atmospheric N2O concentration is currently increasing. Thus, the potential for N2O uptake by low oxygen and low NO- soil environments could increase in the future.
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