Technical Abstract: The transformation and transport processes of nitrogen (N) in agricultural systems are discussed and information is provided on overall reservoir sizes for N. Nitrogen is ubiquitous in the environment and is required for the survival of all living things. It is also one of the most important essential nutrients and is central to the production of all crop plants. The most abundant form of N in the environment is elemental dinitrogen (N2) gas that accounts for 78% of the atmosphere. N2 gas is inert and is not directly available for plant uptake and metabolism. The atmospheric reservoir is estimated to contain ~4x10+9 Tg N2 with a turnover time of 10+7 y. However, some of the most mobile substances found in the soil-plant-atmosphere system contain N and need to be understood. “Natural” fixation of atmospheric N2 is estimated to be ~100 Tg N y-1, globally primarily by lightning and biological processes. Once in fixed or ‘reactive’ form, N can be rapidly incorporated into living tissue. Conversion of relatively inert N2 gas to biologically available forms is limited by the microbially mediated rate of N-fixation. The estimate of the N contained in the terrestrial biomass reservoir is 3.5x10+4 Tg N with a turnover time of 50 y, while the soil reservoir is estimated at 9.5x10+4 Tg N with a turnover time of 2000 y. The estimated sizes of the Global reservoirs of dissolved N2 and inorganic N in the oceans are 2.2x10+7 and 6x10+5 Tg N, respectively. Sediments are estimated to contain 4x10+8 Tg N and marine biomass is estimated to contain 4.7x10+2 Tg N. Estimates of the N in soil show it to contain 2.7 times more N than does aboveground plant biomass, but only a fraction of the amount of N contained in the atmospheric reservoir. Microbially mediated denitrification (i.e. conversion back to N2 gas) completes the nitrogen cycle. Natural terrestrial and ocean denitrification amounts are estimated at 147 and 30 Tg N y-1, respectively.