Agricultural nitrogen input data
Estimated average total annual application of manufactured nitrogen (1km resolution, kg/km2/year) was extracted for England from the raster CEH Land Cover® plus Fertilisers (CEH, Wallingford, UK; https://www.ceh.ac.uk). CEH dataset used data from Defra British Survey of Fertiliser practice (2010-2015) to derive average annual application of manufactured fertilisers for each crop type and then derived total application at 1km resolution using crop areas from CEH Land Cover® plus: Crops (averaged 2015-2017) (Osório et al.2019). As changes in land use intensity in the UK were limited, with trends stable overall since 1994 (Martay et al. 2018), we assume that values based on these maps are representative for the sampling years (2011 and 2012).
For the Netherlands, mean values of nitrogen fertilizer application rate by crop were extracted from the database of the Netherlands Enterprise Agency (RVO) for 2016 (Gewascodelijst Stikstofgebruiksnormen ; https://english.rvo.nl). To estimate the mean value of nitrogen applied as fertilizer at 1km resolution (in kg/km2/year), we calculated a weighted average, taking into account the proportion of each crop in the landscape. Crop coverage per site were extracted for each 1km2 cell as an average of the BRP (Basisregistratie gewaspercelen ) shapefiles 2015 and 2016 obtained from the RVO (https://english.rvo.nl/).
\begin{equation} N\_Fer=\sum{(Proportion\ of\ each\ crop\ category\ \times\ mean\ annual\ application\ rate\ for\ the\ category)}\nonumber \\ \end{equation}
These years (2015-2016) correspond to the median of study years in the Netherlands included in the analyses, that are 2013, 2014, 2017 and 2018.