Shortgrass Steppe for Greenhouse gas Reduction through Agricultural Carbon Enhancement network in Nunn, Colorado
Cattle play a major role in nutrient cycling of grassland ecosystems through biomass removal and excrement deposition (urine and feces). We studied the effects of cattle excrement patches (urine at 430 and feces at 940 kg N ha-1) on nitrous oxide (N2O) and methane (CH4) fluxes using semi-static chambers on cool-season (C3), Bozoisky-select (*Psathyrostachys juncea*) pasture, and warm-season (C4)-dominated native rangeland of the shortgrass steppe (SGS) in northeastern Colorado. Nitrous oxide emission factors (EF; i.e., percent of added N emitted as N2O-N) did not differ between urine and feces on the C4-dominated native rangeland (0.11 and 0.10%) and C3 pasture (0.13 and 0.10%). These EFs are substantially less than the Intergovernmental Panel on Climate Change (IPCC) Tier 1 Default EF (2%) for manure deposited on pasture, indicating that during dry years the IPCC Tier 1 Default EF would result in a significant overestimation of emissions from excrement patches deposited on SGS C4-dominated native rangeland and C3 pasture. Over the first year of the study (19 June 2012 to 18 June 2013), cumulative CH4 uptake was 38% greater for urine (-1.49 vs. -1.08 kg CH4-C ha-1) and 28% greater for control plots (-2.09 vs. -1.63 kg CH4-C ha-1) on C4-dominated native rangeland compared to C3 pasture. In contrast, feces patches were net sources of CH4 with emissions from the C3 pasture (0.64 kg CH4-C ha-1) 113% greater than the C4-dominated native rangeland (0.30 kg CH4-C ha-1). Conversion of C4-dominated native rangeland to C3 pasture can have short and long term effects on CH4 uptake; therefore consideration should be taken before implementing this management practice.