NITROUS OXIDE AND METHANE EMISSIONS FROM BEEF CATTLE FEEDYARD PENS FOLLOWING LARGE RAINFALL EVENTS

More than six million beef cattle are fed annually in feedyards on the semiarid Southern Great Plains (SGP). Manure deposited on the open-lot pen surfaces contributes to greenhouse gas (GHG) emissions. Nitrous oxide (N2O) and methane (CH4) are GHGs linked to climate change, and both have global warming potentials greater than carbon dioxide (CO2). Two sampling campaigns were conducted in 2019 to quantify N2O and CH4 emissions from open-lot pen surfaces. The occurrence of large, unforecast rainfall events during both campaigns provided an opportunity to compare GHG emissions from the dry manure before rainfall and from the wetted pen surface for one to two weeks following precipitation. Temporal variability was quantified by continuous sampling using six to eight automated flux chambers, a multiplexer system, and real-time analyzers. Spatial variability was quantified using a recirculating portable chamber on a 5 x 8 grid. Nitrous oxide emissions dropped below detection levels for ten days after the precipitation event. Nitrous oxide emissions were related to nitrification or other aerobic processes. Methane emissions dropped below detection levels for five days after the precipitation event and then increased to pre-rainfall levels by day 8. When present, N2O and CH4 emissions followed a diel pattern, with the highest emissions occurring during the afternoon when manure pack temperatures at the 25 mm depth were 36 degrees C to 38 degrees C and ambient temperatures were 31 degrees C to 32 degrees C. Average CH4 emissions from the feedyard pen surface were 96-fold lower than estimated enteric CH4 emissions. The results of this field research will be used to refine empirical models for predicting annual N2O and CH4 emissions from open-lot beef cattle feedyards on the semiarid SGP.