Interactions between Nitrogen and Hydrological Cycles: Implications for River Nitrogen Responses to Climate and Land Use with the Model LM3-TAN

Abstract

Human activities, such as legume/rice cultivation and fossil fuel combustion, have dramatically increased reactive nitrogen (Nr), and its movement through ecosystems and environmental reservoirs. The substantial magnitude of this ‘new’ Nr production is problematic, as excess Nr can be extremely detrimental to the functioning of the various ecosystems. Inarguably, tracking the anthropogenic Nr movement is necessarily, but also challenging because of the complex nitrogen (N) cycle. A central contribution of this research is the development of a new process model LM3-TAN. The model captures key controls of the transport and fate of N in the vegetation-soil-river system in a comprehensive and consistent framework that is responsive to climate change and land-use and land-cover changes (LULCC). This dissertation is focused on investigating interactions between hydrological and N cycles in terrestrial and aquatic ecosystems, which have large implications for responses of river N and coastal eutrophication to changes in climate and land use, using novel applications of LM3-TAN.

The N supply via large rivers controls water quality in many of the world’s estuaries or coasts, where N limits biological productivity. Evidence has mounted that climate change is associated with more frequent and intense extreme weather events. This research reveals the critical role of increasing climatic variability and extremes, interacting with N storage, on Susquehanna River N loads which contribute about half of annual N loads to the largest estuary in the U. S., Chesapeake Bay. It was found that after 1-4 year dry spells, the likelihood to exceed a threshold N load increases by 31-86%, which is explained by flushing of accumulated soil N and by stimulated soil microbial processes. This memory effect is amplified when longer dry spells are followed by extreme precipitation. This research also quantifies downstream N-removal benefits with respect to ecosystem components (e.g., climate, basin location, land use) to prioritize sites for land-use management. In a case study for the Korean Peninsula, it was found that the greatest N-removal opportunities are given for sub-basins, with low precipitation, close to coasts, and with substantial Nr production. This result provides important implications for effective mitigation strategies to reduce coastal eutrophication.