Physical and biogeochemical controls over terrestrial ecosystem responses to nitrogen deposition

Anthropogenic perturbations to the global nitrogen (N) cycle now exceed those to any other major biogeochemical cycle on Earth, yet our ability to predict how ecosystems will respond to the rapidly changing N cycle is still poor. While northern temperate forest ecosystems have seen the greatest changes in N inputs from the atmosphere, other biomes, notably semi-arid and tropical regions of the globe, are now experiencing increases in N deposition. These systems are even less well understood than temperate forests, and are likely to respond to excess N in markedly different ways. Here, we present a new integrated terrestrial biophysics-biogeochemical process model, TerraFlux, and use this model to test the relative importance of factors that may strongly influence the productivity response of both humid tropical and semi-arid systems to anthropogenic N deposition. These include hydrological losses of dissolved inorganic and organic N, as well as multiple nutrient interactions with deposited inorganic N along the hydrological pathway. Our results suggest that N-rich tropical forests may have reduced productivity following excess N deposition. Our simulations of semi-arid systems show increases in productivity following N inputs if water availability is sufficient and water losses are moderate. The most important model controls over the carbon cycle response in each simulation were interactions that are not represented in the most common terrestrial ecosystem models. These include parameters that control soil solute transport and nutrient resorption by plants. Rather than attempt prognostic simulations, we use TerraFlux to highlight a variety of ecological and biogeochemical processes that are poorly understood but which appear central to understanding ecosystem response to excess N.