Keywords: nitrogen, fertilisation, recommendations, mass balance, innovative approaches, regulation, harmonisation
Nitrogen (N) budget at farm level is influenced by the fertilisation rates applied by farmers. At larger scales, it is closely linked to multiple environmental concerns relating to air, water and biodiversity. In this study, we analysed the way the N fertilisation recommendations are calculated in ten West European countries, in order to detect innovative algorithms, original calculations and references that could be shared among countries having equivalent crop practices.
Our first result relates to published national official documents governing N fertilisation within each country. A detailed analysis of these documents reveals three categories of calculation methods: (i) ‘N mass balance’ (France, Italy, Spain), (ii) ‘Corrected standard’ (Germany, the Netherlands, Switzerland, Luxembourg), and (iii) ‘Pre-parameterised calculation’, relying on a soil N supply typology (United Kingdom, Ireland) or on a model parameterisation (Belgium).
Sixteen parameters were identified throughout the 10 national methods. The more complex algorithms use 10 parameters (Italy, France), while the simplest only rely on 3 (Luxembourg). The most common parameters considered include the direct effect of N concentration in manure, the total N uptake by crop and the N released by crop residues. At the opposite end, very few countries explicitly take N leaching, the residual mineral N in soil at harvest and atmospheric losses into consideration.
In addition to the previous theoretical approach, we have tested the extent to which the different methods converge or not when practically used. We therefore applied the ten national methods to two contrasting crop scenarios chosen in order to erase as much as possible the cultural and pedoclimatic specificities between the ten countries. The two case studies display large discrepancies in their recommendations, ranging from no fertilisation to 135 kg N/ha, and from 111 to 210 kg N/ha for a wheat crop fertilisation grown in a livestock and polyculture-farm scenario, respectively. The differences in the recommended rate are not accounted for by the complexity of the equations used. For the same conclusions, we identified differences in the consideration of manure’s N availability, N uptake by crop (even for the same yield) and in the leaching of N calculation. The degree of regulatory status of the calculation methods was a more interesting parameter.
Two countries exhibit original tools that we consider worth highlighting: parallel to a simplified field N mass balance, Germany performs a second compulsory mass-balance at the farm scale considered to be a safeguard against risks of excess N budget. Another effective tool, implemented by Belgium, consists of controlling the residual soil mineral N at harvest in comparison to regional references.