Annual agricultural N surplus in France over a 70-year period

Abstract

High levels of nitrogen (N) contamination of ground and surface water are still detected at European and national scales, despite the implementation of Directives, highlighting the need to improve understanding of changes in N pressure. Soil surface nitrogen balance was investigated at the county level in France over a 70-year period to identify areas with high N surpluses and trends in N pressure. Soil surface nitrogen balances were calculated for 90 NUTS3 (Nomenclature of Territorial Units for Statistics in the EU) called ‘departments’ (ranging from 611 to 10,145 km2, median surface area 6032 km2) and one NUTS2 entity. Over the whole period, the N surplus calculated for France as a whole averaged 37 kgN per ha of utilized agricultural area (UAA) and departmental N surpluses mean ranged from 10 to 86 kgN ha UAA−1. Imprecision, i.e. an 80% confidence interval in N surpluses, was calculated using Monte Carlo simulation. Average imprecision for the whole period ranged from 6 to 45 kgN ha UAA−1 across different departments. Analysis revealed that yearly and departmental imprecision values were mainly correlated with N export (R2 = 0.46). Despite this imprecision, the soil surface nitrogen balance was found to be a consistent and suitable tool to determine trends in N pressure at the department level. The model revealed an upward trend in N surplus until the 1990s for 82% of the area studied, and a downward or stable trend for more than 90% of the area since the European Nitrates Directive has been implemented.