Value of process-based Models compared to Tier 2 Adoption to achieve case-specific Greenhouse Gas

Process-based models are available for the farm components animal, field/soil and stored manure. These models are used to unravel the key mechanisms involved with variation in farm greenhouse gas (GHG) emissions in the farm components. However, while process-based models exist for each of the key categories of farm emissions, linking the models within a common framework offers a unique insight into the biotic and abiotic mechanistic drivers of emissions. All process-based models describe microbial activity and decomposition of organic matter, giving rise to the methane, nitrous oxide or carbon dioxide emissions. Apart from the representation of in situ conditions, every model adopts the concept of microbial activity being limited by both degradable organic matter and available nitrogen. As a consequence, inputs and outputs of the individual models could be related in the present project AnimalChange (FP7-266018). This enabled the use of the models in combination to study GHG emissions for a specific farm case or management option. The models covered emissions from enteric fermentation in dairy cattle and from soils and stored manure on dairy farms. Farm cases under varying climatic conditions were selected for availability of data to allow realistic model simulations. Predictions of GHG emissions by process-based models were compared to the values obtained when adopting the more generic Tier 2 approach. The extent to which adoption of concepts represented by process-based models may aid in obtaining a more accurate and farm specific survey of GHG emissions is discussed. This includes the need to delineate any potential trade-offs between individual GHGs as well as the need to explore any potential impact of future climate change.