Cattle play a major role in the nitrogen cycle of grasslands by redistributing approximately 80% of the N consumed in forage through excrement, urine and feces. However, a disproportionate amount of excrement-N is deposited in areas near pasture corners and water tanks as cattle spend approximately 25% of their time in these areas, which represent less than 2% of the total pasture area. These highly trafficked areas undergo nitrogen loading from excrement deposition and soil compaction from cattle trampling that are likely to impact soil atmosphere gas exchange of methane (CH4) and nitrous oxide (N2O). Our objective was to determine if there were significant differences in CH4 and N2O fluxes from adjacent pastures under moderate and adaptive grazing management. Moderate pasture was grazed annually with 21 head of cattle for 143 days, while the adaptive pasture was intensively grazed for 44 days with 221 head of cattle in 2015 and rested in 2016. Four replicates of static chambers were installed at a highly trafficked area near a water tank and at the center of each pasture. Weekly trace gas samples were collected over a two year period (April 2015 to April 2017) and were analyzed on a gas chromatograph.
Results/Conclusions
Cumulative N2O and CH4 emissions over the two year study were greater from the hotspot on the adaptive (2.03 ± 0.56 kg N2O-N ha-1 and 0.65 ± 2.57 kg CH4-C ha-1) compared to the moderate pasture (1.39 ± 0.14 kg N2O-N ha-1 and -2.00 ± 0.10 kg CH4-C ha-1). Over 90% of the cumulative N2O emissions from the adaptive hotspot occurred in 2015 (1.84 ± 0.53 kg N2O-N ha-1), when the pasture was intensively grazed. Furthermore, the adaptive hotspot was a substantial source of CH4 (1.77 ± 2.54 kg CH4-C ha-1) during this time due to feces deposition on one of the trace gas replicates. In 2016, N2O emissions were significantly greater from the hotspot on the moderate (0.72 ± 0.09 kg N2O-N ha-1) compared to the adaptive pasture (0.19 ± 0.03 kg N2O-N ha-1), whereas CH4 uptake rates were similar between the two pastures (adaptive: -1.12 ± 0.08 kg CH4-C ha-1 and moderate: -1.20 ± 0.06 kg CH4-C ha-1). While N2O and CH4 emissions were greater from the hotspot on the adaptive pasture on a per area basis, emissions per unit of beef production should be investigated to better determine the greenhouse gas impacts of these grazing managements.