The effect of different stocking densities and cattle urine on soil nitrous oxide emissions and compaction on a temperate pasture

Given the contribution from the livestock sector to greenhouse gas emissions (GHGe) and soil degradation via compaction and defecations it is imperative that best grazing management approaches are identified. Compacted soil from animal treading can lead to conditions that activate denitrifying bacteria and result in heightened nitrous oxide (N₂O) emissions. In addition, nitrogen input from cattle urination events amplify these emissions, however more data is required in this area. In this study we aimed to quantify soil N₂O emissions in parallel with soil compaction following a simulated grazing event using three stocking densities (control, low (LSD) and high (HSD) with 0, 10 and 100 cows/ha/day respectively) on a temperate Southwest UK pasture. In addition, data was collected to assess how cattle urinations affect soil GHGe under the different stocking densities. Following the grazing event measurements were taken for 12 weeks, soil GHGe were sampled via the use of static chambers and compaction was assessed via penetration resistance. Immediately after the simulated grazing the penetration resistance was 1379 kPa (SD ±368) for the control, 1544 kPa (SD ±429) for the LSD, and 1767 kPa (SD ±490) for the HSD. Differences in penetration resistance were found between the HSD and LSD (p=0.03), and HSD and control (p=0.0001), with a tendency between the control and LSD (p=0.07). Cumulative N₂O emissions were 493 g N₂O - N/ha for the control +urine, 804 g N₂O - N/ha for the LSD +urine and 1237 g N₂O - N/ha for the HSD +urine, with differences found between the control +urine and HSD +urine (p=0.0342). The N₂O emissions from the stocking densities without urine were however similar. This suggests that denitrification in soils is enhanced under high stocking densities from animal treading when there is a source of nitrogen present, such as cattle urine. The developing results are informative for improving pasture management under cattle grazing and provide data for models simulating soil mechanisms in these systems.