Methane and nitrous oxide emissions due to excreta returns from grazing cattle in Nasu, Japan

Abstract To quantify emissions of methane ( CH 4 ) and nitrous oxide (N 2 O) from the excreta returns of grazing cattle, we used a closed‐chamber method to quantify the fluxes of CH 4 and N 2 O from patches of dung, urine and untreated grassland (control) within an experimental site. Fresh dung or urine of beef cattle was applied five times at different times of year to simulate excreta deposition from year‐round grazing. CH 4 fluxes were greatest immediately after dung deposition and returned to control fluxes within 10–35 days. When precipitation occurred following dung deposition, CH 4 fluxes temporarily increased. Once CH 4 emission ceased, however, later rewetting did not lead to a resumption of CH 4 emission. CH 4 emission factors for dung ( CH 4 EF dung ) averaged 0.052%. During the normal grazing period from May to October, the mean CH 4 EF dung was estimated to be 0.076%. The CH 4 fluxes were not significantly affected by urine deposition. However, N 2 O fluxes increased only slightly following dung deposition. During the grass‐growing period, N 2 O fluxes increased after urine deposition, reached a maximum within 3–14 days, and then returned to the levels measured from untreated controls within 28–35 days. During the winter, N 2 O fluxes increased slightly after urine deposition. However, relatively small N 2 O fluxes, typically less than 100 μ g N m −2 h −1 , were observed intermittently following rainfall events. The N 2 O emission factors for dung (N 2 O EF dung ) averaged 0.013%, and those for urine (N 2 O EF urine ) averaged 0.570%. During the normal grazing period from May to October, the mean N 2 O EF dung and N 2 O EF urine were estimated to be 0.024% and 0.684%, respectively. These results suggest that CH 4 and N 2 O emissions from manure could be reduced by grazing cattle and leaving their manure in the field as opposed to housing cattle and composting their manure.