Pig manure degradation and carbon emission: Measuring and modeling combined aerobic–anaerobic transformations

Greenhouse gas emissions from liquid livestock manure storage significantly contribute to global warming. Accurate farm‐scale models are essential for predicting these emissions and evaluating manure management strategies, but they rely on multiple parameters describing carbon loss dynamics. Surface respiration may significantly influence carbon loss and methane emission, yet it is not explicitly included in current models. We conducted experiments to measure pig manure surface respiration rate and its effect on organic matter degradation and methane and carbon dioxide emissions. Manure was incubated for 283 days at 10°C or 20°C under aerobic or anaerobic conditions, while measuring methane and carbon dioxide emission. This was followed by anaerobic digestion at 38°C. Surface respiration reduced the organic matter content, and the effect was temperature dependent. Methane emission was not affected by surface respiration, suggesting that substrate availability was not rate‐limiting for methanogenesis. Surface respiration rates were 18.1 ± 3.5 g CO2 m−2 day−1 at 10°C and 37.1 ± 13.1 g CO2 m−2 day−1 at 20°C (mean ± standard deviation) and were consistent with microsensor measurements of oxygen consumption in different manure surfaces. Based on these results, temperature‐ and surface area‐dependent respiration was incorporated in the existing anaerobic biodegradation model (ABM). Simulations showed that surface respiration accounts for 29% of carbon losses in a typical pig house and 8% for outdoor storage. Developing and refining algorithms for diverse carbon transformations, such as surface respiration, is crucial for evaluating the potential for methane emission and identification of variables that control emissions at the farm scale. Core Ideas Degradation of organic matter is much higher when manure is incubated with an aerobic atmosphere due to surface respiration. Carbon dioxide loss from surface respiration does not reduce methane emission. The fraction of carbon lost as methane during manure storage varies with season and management. Plain Language Summary Methane emission from livestock manure storage is a considerable contributor to global warming. Accurate models for predicting emission are needed for estimating effects of the local farmers management. Surface respiration may be an important process that affects methane and carbon dioxide emission from stored manure, and we conducted experiments to measure its rate and effect on these gases. Methane