Achieving legislation requirements with different nitrogen fertilization strategies: Results from a long term experiment

• The N supply from SOM mineralization was the most relevant term in the N balance equation, followed by the effect of rotation with meadows, and incorporation of maize residues.
• The N use efficiency of well-managed FYM, in particular, but also of SLU was proved to be higher than expected and comparable to that of urea.
• A maximum allowed manure-N of 250 kg ha−1, as in the approved Derogation scheme in Italy, increases the value of manure-N and reduces the need for additional mineral N.
• A revised Nitrates Directives application scheme could be relaxed in its application limit of manure-N, but should be more ambitious in setting efficiency coefficients for manure fertilization.
• Long-Term Experiments can assist public authorities to reach agri-environmental legislative goals and provide practical, useful information to establish modern Action Programs.

The Nitrates Directive (91/676/EEC, Anonymous, 1991) was developed in Europe to limit environmental threats from intensive livestock farming and N fertilizer applications to crops. It imposed several rules on farmers and public bodies, one of which was nutrient fertilization plan adoption. Here we use results from the Tetto Frati (Northern Italy) Long-Term Experiment to verify the terms and coefficients in the official Italian guidelines and evaluate the limitations imposed to organic fertilization amounts. For this purpose, we mined long-term experimental data of crop yield, N uptake, N use efficiency, and soil organic matter content from miscellanea cropping systems fertilized with farmyard manure (FYM) and bovine slurry (SLU), typical of a dairy farm in Northern Italy. N fertilization efficiency indicators (Removal to Fertilizer ratio, Apparent Recovery and Nitrogen Fertilizer Replacement Value) indicated that in the long run, FYM behaved similarly to urea, and better than SLU. Even N supply rates as high as 250 kg N ha−1 were justified by high rates of crop removal. In fact, among the terms of the mass-balance equation, SOM mineralization was found to be most relevant, followed by meadow rotation residual effects. We conclude that a revised Nitrates Directives application scheme could be more relaxed in its application limit of manure-N, but should be more ambitious in setting efficiency coefficients for manure fertilization.