NITROUS OXIDE EMISSIONS FROM SOUTHERN HIGH PLAINS BEEF CATTLE FEEDYARDS: MEASUREMENT AND MODELING
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Abstract. The Texas Panhandle produces approximately 42% of finished beef in the U.S., and cattle production is estimated to contribute 8 Tg carbon dioxide equivalents (CO2e) from nitrous oxide (N2O). Production of N2O in manure is largely a result of biochemical processes that are not static: N2O emission rates are dependent on numerous environmental and chemical factors. Process-based models that estimate N2O emissions from manure in open-lot cattle production systems typically rely on information derived from studies of soil biochemistry. Limited study has been conducted on manure-derived N2O in open-lot beef feedyards. The objectives of this study were to determine variables related to N2O losses from Texas Panhandle feedyards and develop empirical models to predict N2O emissions. Nitrous oxide flux data were collected from a series of 15 non-flow-through, non-steady-state (NFT-NSS) chamber studies (ten chambers per study) conducted from 2012 to 2014 on two commercial beef cattle feedyards. Manure samples (loose surface manure and the underlying manure pack) were analyzed for basic physicochemical properties, soluble carbon (C) and nitrogen (N), and ultraviolet-visible (UV-vis) spectral characteristics related to degree of organic matter (OM) stability and humification. Measured N2O emissions ranged from below detection to 101 mg m-2 h-1 (average 4.8 12 mg m-2 h-1) and were positively related to manure H2O content, temperature, and nitrate (NO3-) concentration (p > 0.01). Emissions were negatively related to manure OM, ammonia/ammonium (NH3/NH4+), dissolved C and dissolved N concentrations, and UV-vis parameters related to OM stability (p > 0.05). Based on these data, empirical models were developed and evaluated to predict manure-derived N2O emissions. Model predictions were not significantly different from observed N2O emissions (p > 0.05). The unbounded index of agreement (IA) indicated that model predictions were within 52% to 61% agreement with observations. Inclusion of OM characteristics improved model predictions of high (<30 mg m-2 h-1) N2O emissions but tended to overestimate low emission rates (>20 mg N2O m-2 h-1). This provides evidence for the importance of C stability in limiting manure N2O production. These models may improve parameterization of existing process-based models and are novel methods for predicting feedyard N2O emissions. Keywords: Beef cattle, Feedlot, Feedyard, Greenhouse gas, Manure, Modeling, Nitrous oxide, Organic matter, Urine, UV-vis spectroscopy.
