Table 1.
Comparison of the main N fluxes in China and in the United States and the EU27
| N fluxes and socioeconomy | China | United States | EU27 | ||
| Ti et al. (18) (2007) | Cui et al. (19) (2010) | This study | |||
| Nr creations, Tg N⋅yr−1 | |||||
| NBNF | 10.3* | 8.4 | 7.1 | 6.4 | 0.5 |
| CBNF | — | 4.6 | 4.6 | 7.7 | 1.0 |
| HBNF-SF | 29.1 | 29 | 32.0 | 10.9 | 11.4 |
| HBNF-IN | — | 5.1 | 5.1 | 4.2 | 6.0 |
| NOx-FF | — | 8.5 | 6.6 | 5.7 | 3.4 |
| Nr losses, Tg N⋅yr−1 | |||||
| N2 | 20† | 10.6 | 24.3 | — | 11 |
| NH3 | 10.3 | 10 | 17.2 | 3.1 | 3.2 |
| NOx | — | 10 | 7.4 | 6.2 | 3.5 |
| N2O | — | 0.4 | 1.4 | 0.8 | 0.7 |
| Runoff | 9.7 | 12 | 12.2 | 4.8 | 10.5 |
| Leaching | — | 2.1 | 5.9 | 4.2 | — |
| Export by air | — | −1.6 | 8.7 | — | 2.3 |
| Export by water | — | 7.7 | 5.5 | 1.9 | 4.5 |
| Accumulation, Tg N⋅yr−1 | 7.9‡ | 32.4§ | 22.7 | ||
| Cropland | — | — | 7.5 | 0.8 | −3.4 |
| Forest | — | — | 6.0 | 0.4 | 0.7 |
| Grassland | — | — | 0.6 | 0.3 | −0.03 |
| Human | — | — | 2.2 | — | — |
| Groundwater | — | — | 5.9 | — | — |
| Aquaculture | — | — | 0.3 | — | — |
| Urban green land | — | — | 0.2 | — | — |
| Socioeconomic factors | |||||
| Land area, million km2 | 9.6 | 9.8 | 4.3 | ||
| Population, million | 1,341.3 | 310.4 | 500.4 | ||
| GDP, trillion, PPP, $2005 | 9.1 | 13.1 | 13.8 | ||
Data year for China was 2010 in this study; the years listed for each study represent the data years for previous studies within China. Data year for the United States was 2002, and data year for EU27 was 2000. The data for the United States and the EU27 were adopted from Doering et al. (13) and Leip et al. (32), respectively. NBNF, natural biological N fixation; CBNF, cultivated biological N fixation; HBNF-SF, Haber–Bosch N fixation for synthetic fertilizer use; HBNF-IN, Haber–Bosch N fixation for industrial N use; NOx-FF, NOx emission via fossil fuel combustion. Nr losses include both natural and anthropogenic sources; these are difficult to separate because a large fraction of natural emissions derive ultimately from anthropogenic sources; for example, much of the N2O emission from forest derives from anthropogenic Nr deposition. In this study, we listed the detailed sources of Nr losses from 14 subsystems in SI Appendix, Matrix, to illustrate these issues. The socioeconomic factors are all on a 2010 basis. GDP is at 2005 prices and adjusted for purchasing power parity (PPP). Units are in Tg N⋅yr1. We cannot compare our results with Ti et al. (18) and Cui et al. (19) on the subsystem level; they did not report these data.
Including CBNF.
Including N2O.
Only including organic N accumulating in soil and biomass.
Including 17 Tg N⋅yr−1 in soil and biomass, 5.8 Tg N⋅yr−1 in solid waste, and 9.6 Tg N⋅yr−1 in inland water.