Aggregation of activity data on crop management can induce large uncertainties in estimates of regional nitrogen budgets

Comprehensive assessment of regional nitrogen budgets (NBs) is critical to understand the productivity–sustainability nexus in agriculture. Using detailed data from six Danish catchments in the LOOP monitoring program (2013–2019), the study inter-compares three ways to represent farm management in the regional process-based model LandscapeDNDC: A) detailed field-level management, B) sequential mono-cropping of six major crops with area-weighted averaging, and C) simulation of 20 dominant crop rotations. Approach A best reproduces observed yields (r² = 0.93 long-term; yearly r² = 0.77). Approaches B and C show lower correlations (long-term r² = 0.92 and 0.77, respectively; yearly r² = 0.72 for B) but remain significant (p < 0.05). Aggregation strongly affects NB components: relative to A, B overestimates total gaseous N losses by 31.4% (+7.6 kg N ha⁻¹ yr⁻¹), while C overestimates by 17.6% (+4.3 kg N ha⁻¹ yr⁻¹). Nitrate leaching is similar on average for B but with a different pattern, whereas C overestimates leaching by 204.9% (+30.2 kg N ha⁻¹ yr⁻¹). Results underscore that management representation can induce large uncertainties in regional NB and environmental impact assessments, with implications for national and international nutrient budgets and their comparability.