Impacts of biological nitrogen fixation on N2O emissions from global natural terrestrial ecosystem soils: An Analysis with a process-based biogeochemistry model

Abstract

Biological nitrogen fixation plays an important role in the global nitrogen cycle. However, the fixation rate has been usually measured or estimated at a particular observational site. To quantify the fixation amount at the global scale, a process-based model is needed. This study develops a biological nitrogen fixation model and couples it with an extant biogeochemistry model of N2O emissions to examine the fixation rate and its effects on N2O emissions. The revised N2O emission model better matches the observed data in comparison with our previous model that has not considered the fixation effects. The new model estimates that tropical forests have the highest fixation rate among all ecosystem types, and decrease from the equator to the polar region. The estimated nitrogen fixation in global terrestrial ecosystems is 61.5 Tg N yr−1 with a range of 19.8–107.9 Tg N yr−1 in the 1990s. Our estimates are relatively low compared to some early estimates using empirical approaches, but comparable to more recent estimates that involve more detailed processes in their modeling. Furthermore, we estimate that the fixation contributes to −5 % to 20 % changes in N2O emissions compared to our previous estimates, depending on ecosystem types and climatic conditions. This study highlights that there are relatively large effects of the biological nitrogen fixation on ecosystem nitrogen cycling and soil N2O emissions and calls for more comprehensive understanding of biological nitrogen fixation and more observational data for different ecosystems to improve future quantification of the fixation and its impacts.