Nitrous oxide uptake by soil

Based on average emission factors of N2O and N2 following addition of N, Vieten et al (2009) estimate the microbial N2O sink by reduction of N2O to N2 to be 0.8 to 0.9 times the soil N2O source. However, under certain conditions the capacity of soil to act as a sink for N2O is thought to be greater than its capacity to emit N2O, resulting in negative fluxes of N2O (Blackmer and Bremner, 1976; Freney et al, 1978; Ryden, 1981). Interpretation of such findings, and a consensus of environmental and soil conditions conducive to such strong reduction, are confounded by our limited knowledge of the regulation of the denitrifier N2O reductase (Richardson et al, 2009). Net negative fluxes indicative of a N2O sink have been reported in several systems and controlled environment experiments (reviewed by Chapuis-Lardy et al, 2007). Since these net negative fluxes can be significant in magnitude (Flechard et al, 2005) they should never be ignored when calculating mean fluxes, or seasonal or annual emissions. However, the underlying reasons for these negative fluxes are rarely explored, with them often being attributed to experimental, sampling or analysis artefacts.

Uptake of N2O in soil is often attributed to reduction of overlying atmospheric N2O to N2 during denitrification, the only proven terrestrial biological N2O sink, and most likely occurs where N2O production is low (Chapuis-Lardy et al, 2007). However, there is no direct evidence of atmospheric N2O being drawn down into the soil pore space. The range of conditions under which negative fluxes have been reported, including those not typically associated with anaerobic denitrification (Khalil et al, 2002), suggest that abiotic mechanisms, or as yet unidentifed biological sinks, may also be operating. N2O consumption appears to be negatively correlated with O2 availability and pH, and is greater under low N availability (Chapuis-Lardy et al, 2007). The latter poses problems when relying on application of 15N-labelled substrates or 15N-N2O to quantify N2O reduction, as application of N may lower a soil's sink potential. Clough et al (1999) showed that two-thirds of pore space 15N-N2O was reduced to 15N-N2 before it had diffused upwards to the soil surface, and conversely it is possible for N2O produced within the surface layers of soil to move downward by passive diffusion, convective movement with rainfall, or transport in solution in soil leachates (Clough et al, 2005). The longer N2O remains in the soil, either due to production in deep soil layers or due to factors slowing diffusion, the more N2O is consumed. The potential and rate for this consumption will depend on the proportion of N2O-reducing micro-sites as predominantly controlled by O2 concentration, O2 demand, aggregate size, water content and soil pH (Simek and Cooper, 2002; Fujita and Dooley, 2007; Vieten et al, 2009). Michaelis-Menten kinetics indicates the potential for N2O consumption to account for 1.6 per cent of total respiration in soil at pH 7, and 0.9 per cent at pH 2.9 (Vieten et al, 2009). Greater understanding of the regulation of microbial N2O reduction and abiotic mechanisms of N2O sinks is urgently required before this N2O uptake can be included in the budgets of the Intergovernmental Panel on Climate Change (IPCC) (Mosier et al, 1998), or for us to propose management strategies to mitigate N2O emission by increasing its consumption.

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