Mismatch between a science-based decision tool and its use: The case of the balance-sheet method for nitrogen fertilization in France

For several decades in France, the balance-sheet method has been recommended and widely used to calculate N fertilizer rates. However, despite the scientific consensus on this method and its adoption as a regulatory tool, high N losses are still frequently occurring, suggesting limits in the implementation of this method. We assumed this consensus might hide discrepancies between some scientific concepts and the ways farmers use methods and tools. We combined a systematic analysis of official reports delivered by groups of experts (Nitrate Groups) from the 20 French regions concerned by the fifth reform of the Nitrate Directive, and interviews with experts, advisors and farmers. We identified principles of the method that reveal discrepancies between the theoretical model and its use, highlighting the gap between scientific concepts and their possible implementation. Here we show three frequent controversies that create uncertainties in calculating N fertilizer rates with the model. 75% of the Nitrate Groups debated about the estimation of the target yield, showing that there is no common understanding of the concept. In practice, farmers tend to fix the target yield as the value they desire more than the average value that can be reached in their fields. Although scientists emphasized the importance of measuring soil mineral content at the end of winter, sampling and uncertain extrapolation of the measurement lead to doubts and uncertainties, weakening the reliability of the N rates estimation. 45% of the interviewed advisors and seven Nitrate Groups put forward limits due to the regulatory implementation of the balance-sheet method, such as the reduced exploration of alternatives adapted to local specificities, or the prevention of an agronomical approach by an administrative one. These controversies among stakeholders showed that despite the rigor of the method and the scientific consensus on it, its implementation creates uncertainties, doubts and errors in the calculated N rates. Despite 40 years of agronomical and technological progress, major points of the method are still obstacles for its use. Considering these discrepancies between the model and its use, we suggest that, instead of persisting in improving incrementally each term of the balance-sheet method, we should switch to the innovative design of a completely new fertilizer calculation method, where users are taken into account from the beginning of the design process.