Origin and Distribution of Nitrogen in Soil.

From the planet’s accretive formation ~4.6 billion yr ago to recent glaciations, tens of thousands of years ago, this chapter explores the early history of N on Earth; including the initial delivery of volatiles to the planet, the redox potential of the early atmosphere, and the conditions required for prebiotic organic synthesis. Recent phylogenetic work has greatly improved our understanding of the evolution of nitrifiers and denitrifiers, and of N₂ fixers and their hosts, and this has effected a reordering of the development of the biotically mediated processes of the global N cycle. Much of modern-day research on the global N cycle has focused on developing global budgets for emissions of the major nitrogenous gases, and these have helped to quantify the magnitude of human contributions. The nature and relative importance of N₂ fixation -by lightning, biological processes, high-temperature combustion, and industrially-is explored and related to gaseous and aqueous fluxes of N compounds. Given the importance of the ocean as a sink for terrestrial N and our increased understanding of N₂ fixation and denitrification rates, an oceanic budget is presented.