In the Soil

In addition to their relatively well known role as a source of various atmospheric trace gases, soils can often act as a sink, too. Nitrous oxide consumption in the soil is primarily related to denitrification. It is supposed that most of denitrifiers are equipped with nitrous oxide reductase encoded by nosZ gene (Philippot et al., 2007). Thus for example Okereke (1993) found the ability to reduce N2O to N2 in 59 out of 71 denitrifying bacteria. In biochemical experiments, N2O is also substrate of nitrogenase, enzyme responsible for di-nitrogen reduction, which non-specifically reduces molecules with triple bonds. It has not been proved whether this side reaction of nitrogenase plays significant ecological role in any habitat. Other possible N2O-consuming pathways include nitrifier denitrification and absorption of N2O in soil liquid phase and its subsequent transportation out of the soil profile. However, these pathways are difficult to quantify and the experimental data are very limited (entrapment of N2O in the soil and subsoil and associated dissolution of the N2O into soil water has been extensively reviewed by Clough et al., 2005). Nevertheless reduction of N2O to N2 by denitrifying microorganisms represents probably the most important process of N2O uptake from the soil gaseous phase. Chapuis-Lardy et al. (2007) summarized in their recent excellent review data on negative fluxes of N2O from various soils. They came to conclusion that soils can often act as a net sink for N2O. Negative N2O fluxes were typically connected to low mineral N and low pO2. Crucial controlling factors for N2O uptake (either by reduction to N2 and absorption by water) represented soil properties: in addition to availability of mineral

N and aeration conditions, mainly soil temperature, pH, redox conditions and lability of soil organic C and N may play the role. In general, factors decreasing diffusion of N2O in soil matrix should increase the probability of N2O consumption in the soil. The authors also concluded that (logically) in soils with high N2O production substantial rates of N2O consumption might take place, but these are masked by larger N2O production (Chapuis-Lardy et al., 2007). Although it seems probable that in soils exhibiting high denitrification rates and thus producing net positive N2O fluxes also relatively large N2O consumption occurs (due to either reduction to N2 or solubility in soil water), it is not known whether soils with low and/or no production of N2O could consume significant amounts of atmospheric N2O. Theoretically any soil containing water represent a net N2O sink consuming N2O from the above-ground air due to movement of N2O molecules first to the soil pores and then to the soil liquid phase. Assuming no reduction, the soil liquid phase becomes a temporary storage body for N2O (Clough et al., 2005). Moreover the presence of denitrifiers specialized in N2O reduction to N2 can not be omitted. Denitrifying microbes can grow with N2O as sole respiratory electron acceptor when all others such as oxygen, nitrate or nitrite are missing. However, information on these processes possibly occurring in upland soils (analogically to methane oxidation consuming traces of methane from the troposphere) is scarce. As a better understanding of N2O consumption in the soil is fundamental for better understanding of the whole N2O budget, more research in this direction is needed.

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