Lin et al., 2009

Lin X, Wang S, Ma X, Xu G, Luo C, Li Y, Jiang G, Xie Z. 2009. Fluxes of CO₂, CH₄, and N₂O in an alpine meadow affected by yak excreta on the Qinghai-Tibetan plateau during summer grazing periods. Soil Biology and Biochemistry 41: 718-725.

I read this paper in an attempt to gain a better understanding of the methods used to compare greenhouse gas fluxes between ecosystems or treatments, and between gases, particularly the use of CO₂-equivalents when estimating total global warming potential contributions by ecosystems that may be simultaneously sources and sinks for the greenhouse gases CO₂, CH₄, and N₂O. In addition, this is one of a small number of studies I have been able to find that draw conclusions about global warming potentials based only on growing season measurements, rather than whole-year or growing season plus “cold season” (often what the tourist industry might call the shoulder seasons, spring and fall, though sometimes including winter as well).
These authors studied the effects of yak (Bos grunniens) excreta, dung and urine, on soil emissions of the three greenhouse gases. Excreta were hypothesized to increase GHG emissions because both are rich in nitrogen, especially inorganic forms of nitrogen such as urea, ammonia, and nitrate, contain sufficient water to stimulate microbial activity in dry soils, and, in the case of dung, are rich sources of labile organic carbon compounds and large microbial populations already present in the material. Furthermore, because production of CH₄ by grazing mammals is strongly linked to their digestive systems, fresh dung may contain considerable dissolved CH₄ that will be emitted quickly upon excretion.
The main results of this study were that while fresh dung did significantly shift a patch of meadow from a weak sink for CH₄ to a source, this difference was not sufficient to render the larger meadow area a net source because the spatial distribution of dung patches, as well as the duration of the CH₄ emission from dung, were relatively small. Urine application did not significantly increase CH₄ emission, which is surprising considering the high N concentration and rapid, large addition of water represented by urination by a yak; both factors are expected to increase methanogenesis.
Emissions of CO₂ were increased by dung, but not by urine when considering a longer, cumulative set of emissions. Interestingly, urine produced a significant pulse of CO₂ nearly immediately upon application to the soil, though whether this CO₂ is the result of hydrolysis of urea ((NH₂)₂CO + H₂O --- 2NH₃ + CO₂) or increased microbial respiration is not clear.
Emissions of N₂O were increased by both dung and urine application relative to untreated controls. However, the magnitude of the increase was less than predicted by IPCC (2001) guidelines for calculating the effects of grazing mammals on grasslands; those guidelines were based primarily on temperate low-altitude grasslands, not the high-altitude alpine meadows studied here. In general, patches of yak excreta accounted for an increase in N₂O emissions of up to about 10% compared to ungrazed and untreated control meadow, while total CO₂-equivalents emissions increased by about 1%, largely due to the small total areal extent of excreta patches.