Chapter 9 - Land-Based Nitrogen Sources and Their Delivery to Coastal Systems

Abstract

This chapter addresses land-based inputs of nitrogen (N) to the coastal zone. Biological N2-fixation occurring within coastal and open ocean regions can contribute substantial additional inputs of N at some times and locations. Of particular note is N from oceanic N2-fixation which is transported onto continental shelves from advection of open ocean waters and can considerably exceed land-based N sources to continental shelves. Local studies of patterns and controls of N export have been carried out and synthesized. These syntheses have, in turn, been used to develop spatially explicit regional and global models that can be used to estimate the patterns and magnitudes of N input to coastal ecosystems, as well as the N sources in watersheds and other factors controlling N export. Now reasonable, though uncertain, estimates of land-based N inputs to coastal waters globally via a number of different pathways, including rivers, submarine groundwater discharge, and direct atmospheric deposition are there. A better understanding of the magnitude of transport of different forms of N is also developing. Globally, dissolved inorganic nitrogen (DIN) and particulate nitrogen (PN) each account for approximately 40% of river transported N is in the form of DIN and PN, with dissolved organic nitrogen (DON) comprising approximately 20%. Additionally, a combination of high water runoff and anthropogenic activity leads to high yields of all N forms. Throughout most of the world, nonpoint sources (natural and/or anthropogenic) dominate N export, regardless of N form. This includes fertilizer which is estimated to be the single most important source of exported DIN in many north temperate latitude watersheds.