Gridded soil surface nitrogen surplus on grazing and agricultural land: Impact of land use maps

ExcessNapplication on agricultural land greatly impacts the environment in multiple ways, driven by
population growth and improving quality of human diets. Therefore, it is essential to quantify the
sources of the emissions ofNcompounds and their determinants (e.g. biologicalNfixation (BNF),
mineral fertilizer, manureNandNdeposition) to develop adequate mitigation measures. Here we aim
at comprehensively mapping and quantifyingNfluxes on agricultural land to analyze these sources on
different scales. As underlying grazing land maps used for such calculations are fairly different in terms
of methodology and definition and thus spatial extent and pattern, we investigate how this diversity in
grazing land maps affects quantification ofNindicators.Wecompared three different global grazing
land maps and analyzed the propagation of differences to discrepancies inNindicators calculated
from them.Wediscovered that (i) area differences propagated to high discrepancies inNsurplus
mostly in Asia, and to a minor extent also in Europe and Northern Africa. (ii) BNF constitutes an
important translator for differences on grazing land toNindicators, while also being a source of
further uncertainty, which warrants further scrutiny. (iii) A more inclusive definition of grazing land
results in overall lessNsurplus given the larger areas included but allows to provide a more
comprehensive estimate of the influence of human activity on theNcycle. This study is the first to
provide an in-depth analysis of the effect of grazing land and agricultural land area differences on
variousNbudget terms andNindicator calculation, highlighting opportunities for further research,
and the importance of a comprehensive accounting ofNsurplus when using an inclusive definition of
grazing land.