Spatial distribution and uncertainties of nitrogen budgets for agriculture in the Tagus river basin in Portugal - Implications for effectiveness of mitigation measures

The present study describes a methodology to quantify the gross soil nitrogen balance (SNB) for agricultural land use in the Tagus Nitrate Vulnerable Zone (TVZ) between 1989 and 2016, including effects of implementation of the EC Nitrates Directive (ND, 91/676/EEC) since 2004. The study uses decadal information from National Agricultural Census at parish level and is supported by a Geographical Information System (GIS). The average SNB of the TVZ decreased significantly (p < 0.001) from 125 kg ha(-1) in 1989 to 63 kg ha(-1) in 2016 (on average 1.8 kg ha(-1) yr(-1)). The N input via irrigation water was particularly high in seven municipalities where it represents up to 35% of the total inputs. Using a Monte Carlo approach, the average uncertainty of the SNB was calculated at 12%. The most important sources of uncertainty were mineral fertilization, nitrate concentration in groundwater used for irrigation and the percentage of farmers complying with ND measures. GIS analysis shows that, at the scale of municipalities, the SNB has decreased since the ND implementation in regions dominated by irrigated intensive crop production, while the SNB has stabilized in municipalities in the south of TVZ with intensive livestock production (75% of area). We conclude that the implementation of ND and national measures to reduce the SNB was not equally effective across the TVZ. Our results indicate that the most effective measures to reduce the SNB in the future are increasing compliance of farmers with the ND related measures and measures increasing the nitrogen use efficiency of livestock manures.