Rising atmospheric concentrations of nitrous oxide (N2O) contribute to global warming and associated climate change. It is often assumed that there is a linear relationship between nitrogen (N) input and direct N2O emission in managed ecosystems and, therefore, direct N2O emission for national greenhouse gas inventories use constant emission factors (EF). However, a growing body of studies shows that increases in direct N2O emission are related by a nonlinear relationship to increasing N input. We examined the dependency of direct N2O emission on N input using 26 published datasets where at least four different levels of N input had been applied. In 18 of these datasets the relationship of direct N2O emission to N input was nonlinear (exponential or hyperbolic) while the relationship was linear in four datasets. We also found that direct N2O EF remains constant or increases or decreases nonlinearly with changing N input. Studies show that direct N2O emissions increase abruptly at N input rates above plant uptake capacity. The remaining surplus N could serve as source of additional N2O production, and also indirectly promote N2O production by inhibiting biochemical N2O reduction. Accordingly, we propose a hypothetical relationship to conceptually describe in three steps the response of direct N2O emissions to increasing N input rates: (1) linear (N limited soil condition), (2) exponential, and (3) steady-state (carbon (C) limited soil condition). In this study, due to the limited availability of data, it was not possible to assess these hypothetical explanations fully. We recommend further comprehensive experimental examination and simulation using process-based models be conducted to address the issues reported in this review. (C) 2012 Elsevier B.V. All rights reserved.
