References

1. https://www.mp.se/politik/mat-och-jordbruk

2. Ponisio, L. C., M’Gonigle, L. K., Mace, K. C., Palomino, J., de Valpine, P., & Kremen, C. (2014). Diversification practices reduce organic to conventional yield gap. Proceedings of the Royal Society of London B: Biological Sciences, 282(1799). Abstract. Retrieved from http://rspb.royalsocietypublishing.org/content/282/1799/20141396

3. This is an average of the gap observed in many studies of different crops in different environments. The gap varies by crop as well as location, management practices and many other factors.

4. See, for example, Ponisio et al. (2014). p. 5

5. For simplicity and ease of reading, We refer to chemical compounds containing nitrogen, such as ammonium and nitrate, as nitrogen, except when discussing forms of nitrogen pollution.

6. Smil, V. (2001). Enriching the earth : Fritz Haber, Carl Bosch, and the transformation of world food production. MIT Press. p.xiii

7. Smil, V. (2001) p.22

8. Smil, V. (2001) p.29

9. Savio, Hannah L. (2011) "Sustainable Agriculture in Ancient Rome." Senior Capstone Projects. http://digitalwindow.vassar.edu/senior_capstone/2 p.31-32

10. Smil, V. (2001) p.32-33

11. Smil, V. (2001) p.27

12. Bruns, H. A. (2012). Concepts in crop rotations. In Agricultural Science. InTech. http://cdn.intechopen.com/pdfs/36492/InTech-Concepts_in_crop_rotations.pdf p. 25

13. Smil, V. (2001) p.32-33

14. Max Roser and Esteban Ortiz-Ospina (2017)—‘World Population Growth.’ Published online at OurWorldInData.org. Retrieved from: https://ourworldindata.org/world-population-growth/

15. Totten, Kimberly. (2016). “What a load of guano: 5 facts you didn't know about bird poop.” Smithsonian National Museum of History Blog. http://americanhistory.si.edu/blog/what-load-guano-5-facts-you-didnt-know-about-bird-poop

16. Smil, V. (2001). p.46-48

17. Smil, V. (2001). p.58

18. Erisman, J. W., Sutton, M. A., Galloway, J., Klimont, Z., & Winiwarter, W. (2008). How a century of ammonia synthesis changed the world. Nature Geoscience, 1(10), 636–639. https://doi.org/10.1038/ngeo325 p. 637.

Calculated: The article states the number of number of humans supported per hectare of arable land has increased from 1.9 to 4.3 persons between 1908 and 2008 (126%) and that 30-50% of this yield increase was due to mineral fertilizer application. 30 to 50% of the yield increase is 38 to 63%.

19. Erisman et al. (2008). p. 637

20. Ponisio, L. C., M’Gonigle, L. K., Mace, K. C., Palomino, J., de Valpine, P., & Kremen, C. (2014). Diversification practices reduce organic to conventional yield gap. Proceedings of the Royal Society B: Biological Sciences, 282(1799), 20141396–20141396. https://doi.org/10.1098/rspb.2014.1396. p. 4

21. Seufert, V., Ramankutty, N., & Foley, J. A. (2012). Comparing the yields of organic and conventional agriculture. Nature, 485(7397), 229–232 https://doi.org/10.1038/nature11069 p. 230.

22. Ponisio et al. (2014). p. 45

23. Berry, P. et al. Is the productivity of organic farms restricted by the supply of available nitrogen? Soil Use Manage. 18, 248–255 (2002) in Seufert, V., Ramankutty, N., & Foley, J. A. (2012). Comparing the yields of organic and conventional agriculture. Nature, 485(7397), 229–232 https://doi.org/10.1038/nature11069 p. 230.

24. https://thebreakthrough.org/archive/how_landefficient_is_organic_a

25. Smil, V. (2001) and Crews, T.E., (1993). Phosphorus regulation of nitrogen fixation in a traditional Mexican agroecosystem. Biogeochemistry 21, 141–166 in Crews, T. E., & Peoples, M. B. (2004). Legume versus fertilizer sources of nitrogen: ecological tradeoffs and human needs. Agriculture, Ecosystems & Environment, 102(3), 279–297. https://doi.org/10.1016/j.agee.2003.09.018 p. 280

26. Klein Goldewijk, K., A. Beusen, J.Doelman and E. Stehfest, New anthropogenic land use estimates for the Holocene; HYDE 3.2. In Max Roser and Hannah Ritchie (2017)—‘Yields and Land Use in Agriculture’. Published online at OurWorldInData.org. Retrieved from: https://ourworldindata.org/yields-and-land-use-in-agriculture/

27. Although more research is warranted to examine whether demand for manure and revenue from selling manure leads to increases in animal production.

28. Nowak, B., Nesme, T., David, C., & Pellerin, S. (2013). To what extent does organic farming rely on nutrient inflows from conventional farming? Environmental Research Letters, 8(4), 44045. https://doi.org/10.1088/1748-9326/8/4/044045 p. 3

29. e.g. Robertson, G. P., Vitousek, P. M., & Kellogg, W. K. (2009). Nitrogen in Agriculture: Balancing the Cost of an Essential Resource. Annual Review of Environment and Resources, 34(1), 97–125. https://doi.org/10.1146/annurev.environ.032108.105046

30. Lassaletta, L., Billen, G., Garnier, J., Bouwman, L., Velazquez, E., Mueller, N. D., & Gerber, J. S. (2016). Nitrogen use in the global food system: past trends and future trajectories of agronomic performance, pollution, trade, and dietary demand. Environmental Research Letters, 11(9), 95007. https://doi.org/10.1088/1748-9326/11/9/095007 Supplementary Data 7.

31. There would also be indirect reductions since some synthetic fertilizer usually escapes to the atmosphere upon application (through volatilization) and then falls back onto cropland through a process known as atmospheric deposition. However, this deposition provides a small portion of the nitrogen crops use so eliminating it would likely not have a large impact. Additionally, if non-synthetic sources are unable to fully replace synthetic nitrogen, there would likely be less feed available for livestock, and less manure available. We omit these second-order effects given the challenges in modeling them.

32. Approximately 5.5 billion acres, compared to Russia’s land area of 4.2 billion acres. https://data.mongabay.com/igapo/world_statistics_by_area.htm

33. This assumes that a 60% cut in nitrogen leads to a similar decrease in yields and that each new acre of farmland could provide the same amount of nitrogen as current farmland. In reality, yields would decrease slower than N applications in some areas (e.g. where N is currently overapplied) and faster in other areas (e.g. where there is little fertilizer applied). However, even if average global yields fall by 30% when N falls by 60%, billions more acres of cropland would be needed to maintain current production, holding all else constant.

34. Lassaletta et al. 2014 Supplemental Material 1 Methods Table 3

35. This is based on estimates of current average green manure fixation of 110 kgN/ha/yr, area planted with green manure crops of 24 million hectares and observations of potential clover fixation rates of ~220kgN/ha/yr. See excel workbook for calculations, assumptions and sources.

36. See excel workbook for calculations, assumptions and sources.

37. https://www.ers.usda.gov/topics/farm-practices-management/crop-livestock-practices/soil-tillage-and-crop-rotation/

38. Poeplau, C., & Don, A. (2015). Carbon sequestration in agricultural soils via cultivation of cover crops—A meta-analysis. Agriculture, Ecosystems & Environment, 200, 33–41. https://doi.org/10.1016/j.agee.2014.10.024. P. 36.

39. Huber, V., Neher, I., Bodirsky, B. L., Höfner, K., & Schellnhuber, H. J. (2014). Will the world run out of land? A Kaya-type decomposition to study past trends of cropland expansion. Environmental Research Letters, 9(2), 024011. http://iopscience.iop.org/article/10.1088/1748-9326/9/2/024011 p.3, Fig 1.

40. Wu, W., You, L., & Chen, K. Z. (2015). Cropping Intensity Gaps: The Potential for Expanded Global Harvest Areas (No. 1459). Retrieved from https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2685300. p. 21. Note: The study refers to cover crops, a term which is interchangeable with green manure in this case. Calculation: converted from ha to % using Wu et al's estimate of global cropland of 2.1 billion ha: 2,710,000 ha / 2,100,000,000 ha.

41. This assumes that the 13% of global cropland that can support an additional crop for harvest overlaps entirely with the area that could support a cover crop. This doesn’t explicitly account for residual N from leguminous fixation left after the first harvest. See excel workbook for calculations, assumptions and sources

42. Webb, J., Sørensen, P., Velthof, G., Amon, B., Pinto, M., Rodhe, L., ... & Reid, J. (2013). An assessment of the variation of manure nitrogen efficiency throughout Europe and an appraisal of means to increase manure-N efficiency. In Advances in agronomy (Vol. 119, pp. 371-442). Academic Press. p. 380-381 & 407-408

43. Such as applying manure to crops more often, storing it in modern systems, and incorporating it into soil shortly after applying it, rather than waiting or leaving it on the soil.

44. Oenema, O., & Tamminga, S. (n.d.). Nitrogen in global animal production and management options for improving nitrogen use efficiency. Science in China Series C: Life Sciences, 48(2), 871–887. https://doi.org/10.1007/bf03187126 p. 884

45. The contribution would likely be less or the land footprint of crop production would be greater. Since other sources cannot completely replace synthetic fertilizer, overall crop production would be lower and less feed would be available (or more land would be needed to maintain feed levels). This would likely reduce the amount of manure available. See excel workbook for calculations, assumptions and sources.

46. Bodirsky, B. L., Popp, A., Lotze-Campen, H., Dietrich, J. P., Rolinski, S., Weindl, I., … Stevanovic, M. (2014). Reactive nitrogen requirements to feed the world in 2050 and potential to mitigate nitrogen pollution. Nature Communications, 5(May), 3858. https://doi.org/10.1038/ncomms4858 p. 6 and Supplementary Data 1

47. Calculation: (66 TgN non-synthetic fertilizer current inputs + 36 TgN from additional nitrogen fixation + 20 TgN from manure improvements + 16.5 TgN from food waste & sewage improvements) / 163 TgN current inputs. See excel workbook for calculations, assumptions and sources.

48. See excel workbook for calculations, assumptions and sources.

49. Calculation based on assumption that average yield gap estimates apply globally

50. The lower bound estimates assumes 12% of land can add cover crops but cannot add harvest crops, and assumes a 20% yield gap.

The upper bound estimate assumes 0% of land can add cover crops but cannot add harvest crops, and assumes a 25% yield gap.

51. Assuming 10% of global cropland could add a cover crop/green manure, that the average green manure B fixation rate rose from 110 kgN/ha to 166 kgN/ha (representing a switch of some to clover for example), that 10 TgN of losses were eliminated from improving manure management and use, and that 25% of food waste and sewage N is recycled as fertilizer. See excel workbook for calculations, assumptions and sources.

52. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_012568.pdf

53. http://csanr.wsu.edu/can-manure-supply-n-and-p-to-ag/