Reconstruction of spatially detailed global map of NH₄⁺ and NO₃⁻ application in synthetic nitrogen fertilizer

Abstract. Currently, available historical global N fertilizer map as an input data to global biogeochemical model is still limited and existing maps were not considered NH₄⁺ and NO₃⁻ in the fertilizer application rates. This paper provides a method for constructing a new historical global nitrogen fertilizer application map (0.5°  ×  0.5° resolution) for the period 1961–2010 based on country-specific information from Food and Agriculture Organization statistics (FAOSTAT) and various global datasets. This new map incorporates the fraction of NH₄⁺ (and NO₃⁻) in N fertilizer inputs by utilizing fertilizer species information in FAOSTAT, in which species can be categorized as NH₄⁺- and/or NO₃⁻-forming N fertilizers. During data processing, we applied a statistical data imputation method for the missing data (19 % of national N fertilizer consumption) in FAOSTAT. The multiple imputation method enabled us to fill gaps in the time-series data using plausible values using covariates information (year, population, GDP, and crop area). After the imputation, we downscaled the national consumption data to a gridded cropland map. Also, we applied the multiple imputation method to the available chemical fertilizer species consumption, allowing for the estimation of the NH₄⁺ ∕ NO₃⁻ ratio in national fertilizer consumption. In this study, the synthetic N fertilizer inputs in 2000 showed a general consistency with the existing N fertilizer map (Potter et al., 2010) in relation to the ranges of N fertilizer inputs. Globally, the estimated N fertilizer inputs based on the sum of filled data increased from 15 to 110 Tg-N during 1961–2010. On the other hand, the global NO₃⁻ input started to decline after the late 1980s and the fraction of NO₃⁻ in global N fertilizer decreased consistently from 35 to 13 % over a 50-year period. NH₄⁺-forming fertilizers are dominant in most countries; however, the NH₄⁺ ∕ NO₃⁻ ratio in N fertilizer inputs shows clear differences temporally and geographically. This new map can be utilized as input data to global model studies and bring new insights for the assessment of historical terrestrial N cycling changes. Datasets available at doi:10.1594/PANGAEA.861203.