第七届青年地学论坛

When models and data disagree the strong modeler seeks to revise the model whereas the strong experimentalist seeks to improve the dataset. The inverse modeler on the other hand recognizes that there is wisdom in both points of view and seeks the truth in the gap between models and observations. In this talk, I will demonstrate how to combine observations with models to estimate global-scale marine nitrogen fixation rates.
Fixed nitrogen (bioavailable nitrogen) is an essential nutrient that limits phytoplankton growth over most of the ocean. Each year a tremendous amount of fixed nitrogen is lost via denitrification and/or anammox in low oxygen environments. Compensating for this persistent removal of fixed nitrogen, special microorganisms known as diazotrophs can break up the triple-bond of N₂ gas to fix nitrogen and thus render it biologically available. An open question in oceanography is whether negative feedbacks couple denitrification and nitrogen fixation to prevent large swings in the ocean’s inventory of biologically available nitrogen. The leading hypothesis has been that there is a tight coupling because marine N₂ fixation was believed to occur in close geographic proximity to regions of marine denitrification. Support for this hypothesis originates from the pattern of N₂ fixation rates implied by the P^* method (Deutsch et al. 2007), which has the ocean’s largest N₂-fixation rates occurring directly above the oxygen minimum zones of the Eastern Tropical South Pacific (ETSP). The Deutsch et al. (2007) study was quite influential and led to research expeditions to make direct rate measurements of N₂ fixation in the ETSP. In contradiction to the P^* results, these in-situ rate measurements found consistently low rates of N₂ fixation. As a result, the field was left with little understanding of the global-scale spatial patterns of N₂ fixation and the status of the spatial-coupling hypothesis; and its potential for stabilizing the marine N cycle is uncertain. I will present results from a state-of-the-art Bayesian inverse modeling approach that combines hydrographic data, a global biogeochemical model and a data-constrained ocean circulation model, to infer global rates of marine nitrogen fixation and organic matter export. I will discuss whether denitrification and nitrogen fixation are coupled sufficiently through negative feedbacks to prevent large swings in the ocean’s inventory of biologically available nitrogen.

固氮,反硝化