Nitrogen Cycles: Past, Present, and Future

Abstract

This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N 2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially disconnected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N 2 by denitrification.

The composition, spatial patterns, and influencing factors of atmospheric wet nitrogen deposition in Chinese terrestrial ecosystems

“…Agricultural activity is the main source of reactive N if the ratio is larger than 1, whereas, industrial activity is the main source if the ratio is lower than 1. The ratio of NH 4 + /NO 3 − in China was 1.22, which indicates that, as a developing country with a large population, both agricultural and industrial activities collectively influence the deposition of atmospheric N. Meanwhile, the deposition of NH 4 + -N and NO 3 − -N for China was higher than that for America or Europe (Table 3), in agreement with previous studies (Cape et al, 2012;Enzai et al, 2014;Galloway et al, 2004;Lü and Tian, 2007).…”

“…The deposition of atmospheric nitrogen (N) is integral to the global N cycle (Galloway et al, 2004;Liu et al, 2011). Anthropogenic activities over recent decades, such as the burning of fossil fuels and fertilizer application, resulted in a rapid increase in the emission of reactive N (Neff et al, 2002).…”

Evaluating anthropogenic N inputs to diverse lake basins: A case study of three Chinese lakes

“…Globally, mineral fertilizer use is the single largest source of anthropogenic N inputs (Galloway et al 2004;Howarth 2004). In China, mineral fertilizer data are compiled in yearbooks of single nutrient fertilizers (nitrogen, phosphorus, and potassium) and compound fertilizers.…”

Denitrification and indirect N2O emissions in groundwater: Hydrologic and biogeochemical influences

“…There is great uncertainty surrounding this indirect emission factor (EF 5 g) and it needs to be verified over a range of hydrogeological environments (IPCC, 2006). The amount of reactive N (N r ) that is converted back to N 2 is by far the largest uncertainty in the N cycle at all scales (Galloway et al, 2004), a fact that 5 restricts our ability to both manage and predict the consequences of an increasingly N-rich world (Townsend and Davidson, 2006). Denitrification of NO 3 --N migrating from cropped fields to groundwater along diverse hydrologic flow paths is particularly challenging to assess and model.…”