Abstract
Grassland soils managed with livestock grazing can act as sources and sinks for methane (CH 4 ) and nitrous oxide (N 2 O), but, in aggregate, are typically a source of N 2 O and a sink for CH 4 . Previously, we observed higher N 2 O losses from fresh patches of urine and manure in shortgrass steppe compared to controls receiving no excreta inputs while CH 4 was a short-term source from manure patches but remained a sink with urine application. Here, we investigate legacy impacts of > 75 years of cattle congregating in pasture hotspots near corners and water tanks under traditional, season-long (mid-May to October) and short term adaptive rotational grazing management on soil N 2 O and CH 4 fluxes. As expected, soil NO 3 and NH 4 were elevated in both long-term traditional grazed and adaptively grazed pasture hotspots. Similarly, N 2 O emissions over all three years of the study (2015–2017) were higher in hotspots for both traditional and adaptively grazed pastures. Contrary to our hypothesis, CH 4 uptake in hotspots was less than pasture centers only during 2015. Although hotspots make up a small portion (< 2%) of total pasture area, they were responsible for about 4–15% of pasture level N 2 O because emissions were 5–10 times higher than pasture centers on a unit area basis. When aggregating to the pasture level, the CO 2 equivalents of the CH 4 sink exceeded those of the N 2 O source implying a net soil GHG sink in this semiarid grazed grassland.