The aging of Japan's population coincided with the reductions in the per capita supply and consumption of food and energy. Reductions in runoff and leaching from land as well as the discharge of treated water were found, at −1.0% yr −1 for both. The distinct reduction in the atmospheric emissions of nitrogen oxides from transportation, at −4.3% yr −1, was attributed to both emission controls and a decrease in energy consumption. Nr loss decreased in both atmospheric emissions and loss to terrestrial water over time. With 420 Gg N yr −1 accumulating in human settlements, 5760 Gg N yr −1 was released from the human sector, of which 1960 Gg N yr −1 was lost to the environment as Nr (64% to air and 36% to waters), and the remainder assumed as dinitrogen. The net N inflow to Japanese human sectors in 20 Gg N yr −1 in total. In this study, the N budgets in Japan were estimated from 2000 to 2015 using available statistics, datasets, and literature. An N-budget approach at the national level would allow us to fully grasp the whole picture of Nr loss to the environment through the quantification of important N flows in the country. To achieve sustainable N use, Nr loss to the environment must be reduced. The benefits of the artificial fixation of reactive nitrogen (Nr, nitrogen compounds other than dinitrogen), in the form of N fertilizers and materials are huge, while at the same time posing substantial threats to human and ecosystem health by the release of Nr to the environment.
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