Size and Persistence of Nitrous Oxide Hot-Spots in Grazed and Ungrazed Grassland

Nitrous oxide (N2O) emissions from agriculture contributed an estimated 60% of the global total in 2005. In the UK, grassland soils account for 30% of total emissions, 22% of which are estimated to come from urine and dung patches. These patches are possible sources of ‘hot-spots’ (area ca. 1 m2) of N2O fluxes. Spatial and temporal heterogeneity of N2O hot-spot fluxes were investigated in three grassland fields (grazed with dairy cows (DG), grazed with young stock (YG) or cut for silage (SC)) using gas sampling chambers surrounding historic hot-spots to establish their size. Fluxes from old dung and urine patches were measured, as well as freshly applied dung and urine to simulate the creation of hot-spots. Potential chemical and physical drivers were also measured. Large spatial variability of N2O fluxes was seen in all three grassland fields. Mean N2O fluxes for the historic hot-spots in the grazed fields (DG and YG) were significantly greater than (SC). The mean N2O fluxes in DG and YG (117.9 and 243.5 ng N m-2 s-1) were 15 to 30% greater than for SC. Soil temperature (15 - 20 °C) was the most significant driver of N2O production with a 1°C rise in soil temperature increasing emissions under DG and YG. N2O fluxes were enhanced by the fresh dung but not by urine. However, in the urine treatment, the nutrient input increased the microbial respiration response for the CO2 flux. Hot-spot N2O
emissions from old urine and dung patches were persistent several months after application.