Legacy Effect of Long-Term Moderate Grazing on Methane and Nitrous Oxide Fluxes at Shortgrass Steppe.

Cattle redistribute up to 80% of the nitrogen consumed through forage via urine and feces patches. Cattle spend a considerable amount of time near pasture corners and water tanks, thus a disproportionate amount of nitrogen is redistributed in these areas forming hotspots. While hotspots make up a relatively small proportion (approximately 2.5%) of the total pasture area, these regions of the pasture are likely a significant source of greenhouse gas emissions due to soil compaction and high soil mineral levels. Our objective was to determine the legacy effect of 70 years of moderate grazing on nitrous oxide and methane fluxes from a pasture hotspot and center at the shortgrass steppe in Northern Colorado. Nitrous oxide and methane fluxes were measured using static chambers over a two year period beginning in April of 2015. Emissions peaked on 6 October 2015 from the pasture hotspot at 72 µg N₂O-N m^-2 hr^-1 following a series of precipitation events that followed a prolonged dry period. Cumulative nitrous oxide emissions from the pasture hotspot were 0.7 and 0.4 kg N₂O-N ha^-1 during years one and two, respectively, which was a factor of 10 greater than the pasture center during both years. Cumulative CH₄ uptake was 76% greater from the pasture center (-1.7 kg CH₄-C ha^-1) compared to the hotspot (-1.0 kg CH₄-C ha^-1) during the first year of the study, while CH₄ uptake during the second year was similar between the pasture center (-1.3 kg CH₄-C ha^-1) and hotspot (-1.2 kg CH₄-C ha^-1). While our results show a legacy effect of long term moderate grazing on greenhouse gas fluxes from the pasture hotspot, these fluxes did not have a significant effect on the pasture scale greenhouse gas budget since the hotspot area comprised such a small proportion of the total pasture area.