Nitrogen (N) deposition is one of the major inputs to cropland and consequently important for the estimation of N Use Efficiency (NUE) for crop production. However, the estimates for N deposition carry large uncertainty, and existing assessments of N budgets and NUE on agricultural land use different estimates of N deposition. To evaluate the uncertainties in existing methods for national scale N deposition estimation and assess their impacts on the resulting NUE estimation for countries around the world, we 1) reviewed existing methods and related data sources for quantifying N deposition inputs to crop production on a national scale; 2) identified the most up–to–date data sources and designed methods to quantify N deposition input to crop production on a national scale; 3) collected N deposition data from observation sites in major countries (e.g., UK, US, and China) to validate the estimated N deposition input; and 4) conducted sensitivity analysis to evaluate how the uncertainties in N deposition affect crop NUE assessment. As a result, we established four estimates for N deposition inputs on cropland for 251 countries around the world during 1961–2020 as combinations of two sets of N deposition maps (ACCMIP¹ and Wang et al.^2-4) and two sets of cropland maps (HYDE⁵ and LUH2⁶). The four products (1. AH: ACCMIP and HYDE, 2. AL: ACCMIP and LUH2, 3. WH: Wang et al. and HYDE, and 4. WL: Wang et al. and LUH2) show good agreement in N deposition estimates for the majority of countries, but have large differences in several Asian countries (e.g., China, India, and Pakistan), and the differences are mostly caused by the use of different N deposition maps. According to the comparison with the observation records in China, the deposition estimates based on Wang et al. show a better agreement with the observations. Hence, the authors recommend using product #4 WL (Wang et al. and LUH2) as the reference dataset for N deposition in the global assessments of N budgets by countries. 

References:

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The data contain N deposition (kg N ha^-1 yr^-1)  in cropland. Each file has rows for countries and columns for years. The NaN stands for "Not a Number".