Text S2. Comparison of data sets for nitrogen input from
agricultural
sources.
To estimate systematic differences between both national N surplus time
series, we selected the European estimation for croplands [kg] by
West et al. (2014) as reference. Because this data set covers only the
years around 2000 with a 5 arc minute resolution, we could not take it
as a consistent substitute for the national data.
A comparison of the national with the European excess N showed a good
fit (23 vs. 19 kg N ha-1 yr-1) for
the French data (Table S2.), but an underestimation of the national
estimations (34 vs. 47 kg N ha-1yr-1) for Germany by 28%. The estimates of retention
in Germany may therefore be slightly higher than assumed taking the
national N input data. But all in all, the methodological offset between
both national data sets can be assumed as acceptable due to the
comparison with the reference by West et al. (2014) and under
consideration of uncertainties in the national as well as European
estimations. Furthermore, a comparison of the national N diffuse input
in comparison to the European N excess data promised a more consistent
estimation for N input per hectare (Figure S2.).