A comparison of global spatial distributions of nitrogen inputs for nonpoint sources and effects on river nitrogen export

We compared four global data sets for the year 1995 for nonpoint N sources with 0.5° by 0.5° spatial resolution. Data were developed to drive models for assessing the river export of nitrogen (N) at the global scale. The data include annual N inputs (biological N fixation, atmospheric N deposition, N fertilizer, animal manure, and human emissions) and outputs (ammonia volatilization and N removed from agricultural fields by harvesting crops and grass consumption). There are important differences at the global, regional, country, and river-basin scales for all input and output terms in the data sets. The main differences are in the rates and the spatial allocation for biological N fixation and atmospheric N deposition, animal manure inputs and management, and N in harvested crops and grass consumption. Inputs of N fertilizer in agricultural systems are relatively well known at the country scale (and subnational scale for some large countries), but their spatial allocation also shows major differences between the four data sets. The level of disagreement between the different data sets increases with decreasing river basin size, which is related to the difficulty of spatial allocation when river basins cover only a few grid cells. Transport efficiencies to calculate river N export from the N surplus obtained from a regression approach and from a meta model derived from a conceptual model are in good agreement when aggregated to continents and receiving oceans.