Topdown and bottomup estimates of global nitrous oxide emissions to the atmosphere

Nitrous oxide is produced in 'natural' and agricultural soils almost exclusively as a result of microbial processes (see Chapter 2). The main microbial reactions involved in the production of N2O are nitrification (oxidation of ammonium to nitrite) and denitrification (reduction of nitrate to dinitrogen; N2O is an obligate intermediate in this latter process). Although nitrification is basically an aerobic process and denitrification is essentially an anaerobic one, both can take place in the soil in close proximity under soil conditions that are amenable to upland crop and forage production. Because agricultural systems are typically 'leaky,' a considerable portion of new N additions is lost to the environment through leaching and runoff, ammonia volatilization and denitrification. Van der Hoek (1998) estimated that more than 60 per cent of the annual N input into food production was not converted into usable products. Globally, cereal crop N use efficiency remains near 40 per cent (Cassman et al, 2002; Balasubramanian et al, 2004).

Two general methods used to estimate soil N2O emissions can be broadly considered as either (1) bottom-up approaches based on soil surface gas flux measurements or models based on soil application of N, or (2) top-down approaches based on changes in atmospheric concentration of N2O and estimates of sink strength (Del Grosso et al, 2008). The bottom-up approaches considered in this chapter are (1) the IPCC (2006) methodology, (2) DAYCENT (daily version of Century) ecosystem model estimates of direct N2O emissions (Del Grosso et al, 2006), and (3) field-scale estimates based on soil surface gas flux measurements.

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