Global and regional estimation of net anthropogenic nitrogen inputs (NANI)
Since the Industrial Revolution, human activity has greatly altered the Earth's reactive nitrogen (N) cycle, leading to widespread N pollution that can have adverse effects on human health, ecological functions, and biodiversity. Quantifying the net anthropogenic nitrogen input (NANI) has proven to be a powerful method of evaluating the extent of human alteration of the N cycle on watershed and regional scales. In this study, NANI was estimated in 98% of the land area, covering 99% of the population of countries and regions around the world during the years of 1961, 1980, 1990, 2000 and 2009. The NANI estimation methodology considered atmospheric N deposition, N fertilizer consumption, the net import or export of N in agricultural commodities (human food, livestock feed, etc), and N fixation in agriculture and planted forests as its main sources. The global NANI level has increased from 1961 to 2009 (from 378 to 1226 kg N/km2/yr). Asia, the Caribbean region, Europe and North America yield higher-than-average NANI values. The increase in the net N inputs of Asia is the main reason for this increase in global NANI. In 1961, the main sources of NANI are net oxidized N deposition (42%) and N fixation (34%). Since 1961, the largest component of global NANI has been N fertilizer consumption (59%–64%), followed by net atmospheric N deposition (16%–23%) and N fixation (18%–20%). Globally, replacing fossil fuels with clean energy, limiting population growth, reducing the excess consumption of proteins per capita, reducing the proportion of animal product protein in the human diet, and balancing the application of N fertilizers between regions are all potential ways to reduce NANI. However, each of these practices have drawbacks. For any country or region, in addition to the above practices, NANI can also be decreased by reducing synthetic fertilizer consumption, decreasing food import, and sustainably leasing or cocultivating agricultural land in other countries or regions. In addition, the application of highly efficient organic fertilizers and high-yielding crop species, as well as the promotion of precision agriculture, can to some extent also reduce NANI.