Nitrous Oxide and Ammonia Emissions from Cattle Excreta on Shortgrass Steppe

Grazing cattle redistribute nitrogen (N) consumed in forage through urine and feces patches. The high concentration of N in these patches often exceeds the uptake demands of the local plant community, thereby providing ideal conditions for losses of reactive N. However, knowledge on nitrous oxide (N2O) and ammonia (NH3) emissions from excretal patches on shortgrass steppe grassland is limited. We studied the effect of cattle urine (1002 kg N ha(-1)) and feces (1021 kg N ha(-1)) patches on N2O and NH3 emissions in two sites with contrasting vegetation: (i) coolseason (C3) 'Bozoisky-Select' Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski], pasture (C3Past) and (ii) C4-dominated native shortgrass steppe rangeland (C4SS). Nitrous oxide and NH3 were measured using semi-static and semi-open chambers, respectively. Cumulative N2O emissions were 217 and 173% greater and cumulative volatile NH3 emissions were 339 and 157% greater on C3Past compared with C4SS from the urine and feces treatments, respectively. Nitrous oxide emission factors were 0.20 and 0.05% for urine and 0.07 and 0.03% for feces on C3Past and C4SS, respectively. Our findings suggest that using the IPCC Tier 1 default emission factor (2%, 95% CI = 0.7-6%) to estimate N2O emissions from cattle excretal patches on shortgrass steppe grassland would result in a significant overestimation for these dryland systems. Ammonia emission factors were 35 and 10% for urine and 7 and 5% for feces on C3Past and C4SS, respectively. With the exception of the urine treatment on C3Past, observed NH3 emissions were consistent with the IPCC Tier 1 default assumption that 20% (95% CI = 5-50%) of excretal N is volatilized as NH3+NOx.