Note: * Value amended from the '0.9Tg N' in Del Grosso et al (2008), which resulted from failure to convert from Tg of N2O to Tg of N (S. Del Grosso, personal communication to A. R. Mosier, 2009).
Source: Based on Del Grosso et al (2008), with world bottom-up calculations based on IPCC (2006) methodology; US calculations based on DAYCENT model simulations and IPCC (2006) methodology (US EPA, 2008). Top-down calculations based on Crutzen et al (2008).
Although the Del Grosso et al (2008) paper contains too large a value for US emissions modelled by DAYCENT (see note to Table 4.4), their conclusions about the agreement at the global scale are unaffected and encouraging. The numbers demonstrate that the 3-5 per cent EF relates to agriculture as a whole - i.e. it is not limited to crop-based biofuel production. Crutzen et al (2008) focused only on crop-based biofuels because of a logical inconsistency in one of the motivations for their production, i.e. the abatement of greenhouse gas emissions by replacing fossil fuel, while at the same time introducing another, much more potent, greenhouse gas. As food production for a still-growing world population will require further increases in fertilizer nitrogen application (Erisman et al, 2008), efforts to limit N2O emissions cannot be allowed to affect production. Instead, efforts need to focus on improvement of the nitrogen use efficiency throughout the production chain. This entails improvement of nitrogen use efficiency in the field, but also directs attention towards reducing wastage of food and prioritizing production of essential proteins for human consumptions in nitrogen-efficient pathways. This may in practice mean dietary changes towards more vegetarian food and less meat consumption.
Very recently, Davidson (2009) reported that although our calculation of a global EF of 4 ± 1 per cent (Crutzen et al, 2008) fits the data well for the two reference dates used in that paper (1860 and the 1990s), it underestimates the emissions in the late 19th and early 20th centuries, and Davidson argues that other sources of N2O were important in that period. Much of his paper is devoted to the calculation of alternative EFs for manure-N and fertilizer-N, and he achieves a good match to the rise in atmospheric concentration with EF values of 2 per cent and 2.5 per cent, respectively. However, although Davidson (2009) also discusses the 'mining' of soil N, i.e. the release of reactive N from inactive forms in old soil organic matter, under the influence of ploughing and overgrazing as agriculture expanded into new regions, and argues that this was a likely source of increasing N2O between 1860 and 1960, he does not make any calculation of the actual contribution. We (in Crutzen et al, 2008) did not attempt to match the global EF to the gradual rise in atmospheric N2O in the period between 1860 and the 1990s, and we readily accept that N mining would have been important, as the period coincided with the major expansion of crop and grazing land in, for example, the US, Canada, Argentina, South Africa, Australia and New Zealand, culminating with the massive Virgin Lands Programme in the former USSR in 1954-1964 (McCauley, 1976). Indeed, we hypothesize that this source is likely to have been responsible for much of the discrepancy that Davidson (2009) has noted. Mineral N arising from the decomposition of old soil organic matter (perhaps hundreds or even thousands of years old) in effect introduces additional reactive N into the terrestrial biosphere and its role in contributing to direct N2O emissions is now recognized (see above). Likewise, mineral nitrates (saltpetre) from dry Chilean deserts introduced into agricultural systems in the late 19th century (Smil, 2001) was another source of reactive N, just like any industrially produced nitrogen fertilizer compound. If the term we used, 'newly fixed N', were to be replaced by 'new reactive N', i.e. including mined soil N and saltpetre N as well as BNF-N and synthetic fertilizer N, then the apparent differences between Davidson's analysis and ours would be largely reconciled. Ojima et al (1993) estimated that 13.2-25.5Pg of carbon was released by land-use change to agriculture in the period 1800-1990. It seems reasonable to assume that at least 10Pg of this C was released between 1860 and 1960 - the period of greatest agricultural expansion - and with it around 0.8Pg of N. This is equivalent to, on average, 8Tg N yr_1, which is similar to Davidson's (2009) estimate of annual fertilizer N use in the mid-1950s. Obviously, the annual rate of soil C and N release will have fluctuated considerably during the 100-year period, and with it the associated N2O emissions. We consider that a detailed calculation of the global N release rates, on a decade-by-decade basis if not annually, would be a worthwhile project to undertake.
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