Global-scale modelling of potential changes in terrestrial nitrogen cycle from a growing nitrogen deposition

Given the fact of growing deposition of atmospheric nitrogen (N) in terrestrial biosphere, it is critical to get a better understanding of potential changes in terrestrial N cycle causing from the increasing deposited N. In this study, a global scale process-based Terrestrial Biogeochemical Nitrogen Cycle (TBNC) model originally developed by Lin et al., (2000) has been improved and applied to quantify the changes of terrestrial N cycle under the scenarios of N deposition at conditions in 1993 and 2050 (Galloway et al., 2004). Sensitivity analysis and empirical validation indicated the reliability of the model for addressing the complexity of current N cycle changes and its capacity for investigating long-term scenarios in the future. Under the growing rates of 34% and 77% as in NOy (all oxidized forms of N including N2O) and NHx (NH3 and NH4+), depositions, the model results show that ammonium and nitrate in surface soil are predicted to increase about 10% and 23%, while other N pools have no obvious change. Major N fluxes in soil, i.e. denitrification, ammonium volatilization, nitrate leaching, gaseous losses (mainly N2O and NO) and nitrification, are predicted to increase about 10-25%. The responses of major biome classes show that an increase rate of 10% as in ammonium accumulation is predicted to occur in temperate forests, while temperate shrublands and grasslands are the most important nitrate reservoir with an increase rate of 20% in response to future N depositions. Generally, TBNC model could help us to quantitatively understand and explain the causes and consequences of spatiotemporal changes of global N cycle, and thereby provide a means of estimating the potential responses of terrestrial ecosystems to alteration of the global N cycle, especially from human impacts.