Estimate of changes in agricultural terrestrial nitrogen pathways and ammonia emissions from 1850 to present in the Community Earth System Model

Abstract

Abstract. Nitrogen applied to the surface of the land for agricultural purposes represents a significant source of reactive nitrogen (Nr) that can be emitted as a gaseous Nr species, be denitrified to atmospheric nitrogen (N2), run-off during rain events or form plant useable nitrogen in the soil. To investigate the magnitude, temporal variability and spatial heterogeneity of nitrogen pathways on a global scale from sources of animal manure and synthetic fertilizer, we developed a mechanistic parameterization of these pathways within a global terrestrial model. The parameterization uses a climate dependent approach whereby the relationships between meteorological variables and biogeochemical processes are used to calculate the volatilization of ammonia (NH3), nitrification and run-off of Nr following manure or fertilizer application. For the year 2000, we estimate global NH3 emission and Nr dissolved during rain events from manure at 21 and 11 Tg N yr−1, respectively; for synthetic fertilizer we estimate the NH3 emission and Nr run-off during rain events at 12 and 5 Tg N yr−1, respectively. The parameterization was implemented in the Community Land Model from 1850 to 2000 using a transient simulation which predicted that, even though absolute values of all nitrogen pathways are increasing with increased manure and synthetic fertilizer application, partitioning of nitrogen to NH3 emissions from manure is increasing on a percentage basis, from 14 % of nitrogen applied (3 Tg NH3 yr−1) in 1850 to 18 % of nitrogen applied in 2000 (22 Tg NH3 yr−1). While the model confirms earlier estimates of nitrogen fluxes made in a range of studies, its key purpose is to provide a theoretical framework that can be employed within a biogeochemical model, that can explicitly respond to climate and that can evolve and improve with further observation.