Estimates of global N2O emissions from cattle, pig and chicken manure, including a discussion of CH4 emissions

Humans seem to have doubled the global rate of terrestrial nitrogen fixation. Globally 50-70% (85 Tg, 1 Tg = 10(12) g) of the nitrogen supplied in fertilizer (80 Tg N/a) and leguminous crops (40-80 Tg N/a) are used to feed cattle. The aim of the present study was to derive some estimates of global N2O production from animal manure. As the parameter giving the most stable numerical basis for regional and global extrapolation we adopted the molar emission ratios of N2O to NH3. These ratios were measured in cattle, pig and chicken housings with different manure handling systems, in dung-heaps and in liquid manure storage tanks. Individual molar emission ratios from outside manure piles varied over two orders of magnitude, strongly dependent on the treatment of the manure. A median emission ratio of 1.6 x 10(-2) (n = 65) was obtained in cow-sheds with slatted floors and liquid manure stored underneath and a median ratio of 24 x 10(-2) (n = 31) was measured in a beef cattle housing with a solid manure handling system. We next extrapolated to global NH3 emissions from those estimated for Europe, using N uptake by the animals as a scaling factor. Multiplication with observed N2O to NH3 ratios next provided some estimates of regional and global N2O emissions. To account for the great variability of the emission ratios of N2O/NH3, we developed upper and lower case emission scenarios, based on lower and upper quartiles of measured emission ratios. The global emission from cattle and swine manure is in the range of 0.2-2.5 Tg N-N2O/a, representing 44 +/- 39% of the annual atmospheric accumulation rate. This N2O emission arises from about 40 Tg N/a of cattle and pig manure stored in or at animal housings. We did not account for N2O emissions from another 50 Tg N/a excreted by grazing cattle, goats and sheep, and application of the manure to agricultural fields. Our study makes it clear that major anthropogenic N2O emissions may well arise from animal manure. The large uncertainty of emission ratios, which we encountered, show that much more intense research efforts are necessary to determine the factors that influence N2O emissions from domestic animal manure both in order to derive a more reliable global estimate of N2O release and to propose alternative waste treatment methods causing smaller N2O releases. In our studies we found large enhancements in N2O releases when straw was added to the manure, which is a rather common practice. In view of the ongoing discussion in Europe to re-install the traditional solid manure system (bed down cattle) for environmental and animal welfare reasons, it is noteworthy that our measurements indicate highest N2O release from this particulary system. In a similar manner, but based on a smaller data set, we also estimated the release of CH4 from cattle and swine manure and from liquid manure only to be about 9 Tg/year in good agreement with the estimate by the Environmental Protection Agency (1994) of 8.6 +/- 2.6 Tg/year. A total annual methane release as high as 34 Tg/a was derived for solid and liquid cattle and pig manure from animals in housings.