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

Abstract

<p class="1Body">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 <em>ca.</em> 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.</p>