We modeled the biogeochemical nitrogen cycle with specific emphasis on N2O behavior to describe the global N2O cycle system quantitatively. Here all the major nitrogen cycle processes are included plus the processes directly related to N2O. The model includes 24 nitrogen reservoirs: 3 in the stratosphere (N2O, N2, and NOx), 4 in the troposphere (N2O, N2, NH3, and NOx), 7 on land (biospheric, N2O, NOx, PON, DON, NH4 +, and NO3 -), 6 in surface-ocean (biospheric, PON, DON, N2O, NH4 +, and NO3 -), 3 in the deep ocean (N2O, NH4 +, NO3 -), and sedimentary organic nitrogen. The exploration of the steady state flux combination, which produces the tropospheric N2O concentration profile consistent with observations, provided the most likely estimates of the N2O emissions from soils, rivers, and oceans, including contributions from nitrification and denitrification. Full incorporation of nitrogen species facilitated an analysis of the influences of various nitrogen cycle processes on tropospheric N2O concentration. Our sensitivity analyses revealed that the tropospheric N2O concentration was linked closely to other nitrogen species and was especially sensitive to terrestrial biological activities via nitrogen fixation, nitrogen assimilation, and ammonification. This result implies that the N2O budget in the troposphere can be constrained well by these terrestrial processes.