Using nitrogen fertilisation as indicator for nitrate contamination seems to be one of the most appropriate solutions for determining risk of nitrate leaching. Anyhow, individual farm data on mineral/organic fertiliser use often are not available (i. e. in the Netherlands and in Germany), and even sales of mineral fertilisers are only registered on the national level in some Member States. Consequently, in quite a few cases, data on nitrogen fertilisation are not accessible, at least not for the time span needed to study a certain groundwater contamination, e g. in France, very few data are available in databases before 1990.
On the assumption of high nitrogen efficiency (Klages et al., 2018), average crop yield may be used instead of data on nitrogen fertilisation. High fertilising efficiency can be found in arable, intensively managed cropping systems.
There are, however, a range of factors, which may lead to a reduced efficiency of the applied nitrogen (Klages et al. (2018):
- the effect of climate change, with little precipitation in the summer months, which might reduce the transformation of late mineral nitrogen application into yield and especially for wheat into raw protein of the grains,
- a high regional density of animal breeding farms; these farms on one side need to utilise the produced manure as compound fertiliser, on the other side, plant availability of the enclosed nitrogen is not as exact predictable as for mineral N-fertilisers,
- uncertainties on how to account for mineralisation of organic substance in soil biomass, e. g.. after intensive manure application, catch crop cultivation or cultivaton of nitrogen-fixing plants,
- the tendency of some farmers to overestimate their predicted harvest and in consequence to overfertilise.
The average crop yield therefore could be an acceptable indicator only for specific farming system. Average crop yields can be obtained on national level from the European statistical database (Eurostat, 2018e).