Modelling Nitrogen Balances of German Croplands: Advancing the MONICA Model for High-Resolution N Flux Estimates

Efficient management of nitrogen (N) in agricultural systems is crucial for mitigating greenhouse gas (GHG) emissions, reducing nutrient losses, and maintaining crop productivity. The aim of this study was to evaluate the N cycle on German cropland using the process-based MONICA model. In particular, the processes of denitrification, nitrification and N leaching, as these have a significant influence on N losses. The study relied on data from the German Agricultural Soil Survey (BZE-LW), which provides detailed crop sequence information, annual fertilization rates, and yields across 1235 sites. These data were supplemented with meteorological information from the German Weather Service (DWD) and environmental variables derived from remote sensing. An algorithm was developed to predict the timing of operations such as fertilization and tillage, to address the challenge of limited temporal resolution in management data, generating daily management information. This enhancement enabled high-temporal-resolution simulations for nine major crops cultivated in Germany between 2001 and 2018. Initial model evaluation applied MONICA to simulate crop yields, N leaching, and N₂O emissions, using large-scale plausibility checks based on emission factors and leaching loss estimates. While the model demonstrated reasonable performance in estimating nitrogen fluxes, challenges were identified in replicating reported yields. These were largely due to uncertainties in input data and unrepresented processes in the current model framework. Planned refinements to MONICA, in collaboration with project partners, aim to improve its representation of denitrification losses (N₂ and N₂O) using experimental data. Preliminary results underline the potential of MONICA for high-resolution simulation of agroecosystem N dynamics, though sensitivity analyses highlight the significant influence of uncertainties in soil properties and management inputs on model outputs. This work advances the MONICA model as a robust tool for simulating high-resolution N fluxes and evaluating mitigation strategies in agricultural systems. The insights gained provide a foundation for improving N management practices at regional scales, contributing to sustainable and climate-resilient agricultural systems in Germany.