From under-barn to outdoor swine manure storage: Modelling the effect of frequent under-barn pit emptying on methane emissions in a cold climate

CONTEXT Swine manure is often stored temporarily in under-barn pits before being transferred to long-term outdoor storage. In cold climate, the temperature in these two storage locations is very different; significantly impacting CH 4 emission. Therefore, both temporary and long-term storage phase must be considered when estimating CH 4 emissions from manure management. Frequent emptying of an under-barn pit has been proposed as a CH 4 mitigation practice, but its consequential impact on CH 4 in the outdoor storage facilities needs to be considered to determine the impact on the total CH 4 emissions during the storage cycle. OBJECTIVES This study aims to investigate the potential effects of frequent manure emptying of an under-barn pit manure, and its management factors, including temperature, under-barn retention time, and the seasonal impact of emptying, on CH 4 mitigation using a process-based model, Manure-DNDC. METHODS Three scenario studies using the Manure-DNDC model were conducted to assess the effects of under-barn manure temperature, manure retention time, and the seasonal impact of emptying on CH 4 mitigation throughout the storage cycle. The Manure-DNDC used in this study was expanded to include an under-barn emission submodule and the ability to simulate manure temperature in earthen manure storage. The model was calibrated and validated using outdoor CH 4 emissions and manure temperature observations from a swine farm employing two different under-barn manure management practices over three years. RESULTS AND CONCLUSIONS Results using the verified model showed that shortening the retention time from 1 month to none (immediate emptying) could decrease CH 4 emissions by 18.1%. Lowering manure temperature by 2 °C in under-barn storage reduced total emission by 3.6–5.3% in the cold season, but only minimally by 0.5–1.1% in the summer. Compared to 2-week removal, frequent emptying in the cold season would achieve 3.9–9.7% annual CH 4 reduction, compared to 4.8% - 10.8% reduction with frequent emptying applied year-round. Based on the results, the optimal mitigation approach should target lower manure retention time (particularly during the cold season) and reduced under-barn manure temperature. However, future experimental data is needed to validate the model's capacity for simulating CH 4 emissions from under-barn pits. SIGNIFICANCE This study identified frequent manure emptying as a viable best management practice for mitigating CH 4 emissions throughout the storage cycle. We thoroughly investigates the mechanisms and management variables of frequent manure emptying, highlighting the greater importance of cold season management for under-barn manure. These findings provide valuable insights for manure storage system design and CH 4 mitigation practice development.