Physical and biogeochemical controls over terrestrial ecosystem responses to nitrogen deposition

Anthropogenic perturbations to the global nitrogen (N) cycle now exceed tho se to any other major biogeochemical cycle on Earth, yet our ability to pre dict how ecosystems will respond to the rapidly changing N cycle is still p oor. While northern temperate forest ecosystems have seen the greatest chan ges in N inputs from the atmosphere, other biomes, notably semi-arid and tr opical regions of the globe, are now experiencing increases in N deposition . These systems are even less well understood than temperate forests, and a re likely to respond to excess N in markedly different ways. Here, we prese nt a new integrated terrestrial biophysics-biogeochemical process model, Te rraFlux, and use this model to test the relative importance of factors that may strongly influence the productivity response of both humid tropical an d semi-arid systems to anthropogenic N deposition. These include hydrologic al losses of dissolved inorganic and organic N, as well as multiple nutrien t interactions with deposited inorganic N along the hydrological pathway. O ur results suggest that N-rich tropical forests may have reduced productivi ty following excess N deposition. Our simulations of semi-arid systems show increases in productivity following N inputs if water availability is suff icient and water losses are moderate. The most important model controls ove r the carbon cycle response in each simulation were interactions that are n ot represented in the most common terrestrial ecosystem models. These inclu de parameters that control soil solute transport and nutrient resorption by plants. Rather than attempt prognostic simulations, we use TerraFlux to hi ghlight a variety of ecological and biogeochemical processes that are poorl y understood but which appear central to understanding ecosystem response t o excess N.