1. 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), 095007.
2. Ponisio, L. C. et al. (2014). Diversification practices reduce organic to conventional yield gap. Proc. R. Soc. Lond. B Biol. Sci. 282, 20141396.
3. Seufert, V., Ramankutty, N. & Foley, J. A. (2012). Comparing the yields of organic and conventional agriculture. Nature 485, 229–232.
4. It is important to note too that these estimates are misleading. The 20-25% “yield gap” specifically measures yields for a harvest of a single crop, e.g., corn. However, organic production may have fewer harvests per year on average or may be nutrient-limited outside of field experiments and therefore have lower yields than is often reported.
5. 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
6. This is documented in the authors’ code: "04SetNewParameterValues_V2.gms":
*Z) FACTORS FOR FERTILIZER SUPPLY CALCULATIONS: ResiduesApplicationFactor(FAO_Countries,Crops,"Crop",Sys,"CurrentSituation")=1; 0.5; ResiduesNutrientUseEfficiencyFactor(FAO_Countries,Crops,"Crop",Nutrients,Sys,"CurrentSituation") = 1.
7. Peoples, M. B., Brockwell, J., Herridge, D. F., Rochester, I. J., Alves, B. J. R., Urquiaga, S., … Jensen, E. S. (2009). The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems. Symbiosis, 48(1–3), 1–17. https://doi.org/10.1007/BF03179980
8. The researchers estimate that a reduction in greenhouse gas emissions from synthetic fertilizer manufacturing and application would offset emissions from deforestation, but note that emissions could easily be higher.
9. Clark, M., & Tilman, D. (2017). Comparative analysis of environmental impacts of agricultural production systems, agricultural input efficiency, and food choice. Environmental Research Letters, 12(6), 64016. https://doi.org/10.1088/1748-9326/aa6cd5
10. Mueller, N. D., West, P. C., Gerber, J. S., MacDonald, G. K., Polasky, S., & Foley, J. A. (2014). A tradeoff frontier for global nitrogen use and cereal production. Environmental Research Letters, 9(5), 054002.
11. The authors do not clearly decompose emissions from deforestation and fertilizers. But based on the author’s statement that 100% conversion to organic would reduce GHG emissions 3-7% including deforestation, and 8-14% excluding deforestation (mostly from eliminating emissions from fertilizer production), I assume that improving conventional nitrogen efficiency (by assuming 50% reduction in synthetic fertilizers) would result in a 4-7% reduction in GHG while avoiding deforestation. Therefore net GHG emissions under the two scenarios are similar when using the authors’ land-use projections. Given that these are underestimated, emissions under organic would likely be greater.
12. As the study’s authors note.
13. Kniss, A. R., & Coburn, C. W. (2015). Quantitative evaluation of the environmental impact quotient (EIQ) for comparing herbicides. PloS One, 10(6), e0131200.
14. Bahlai, C. A., Xue, Y., McCreary, C. M., Schaafsma, A. W., & Hallett, R. H. (2010). Choosing organic pesticides over synthetic pesticides may not effectively mitigate environmental risk in soybeans. PloS One, 5(6), e11250.
15. Arandia, A., & Aldanondo-Ochoa, A. (2011). Pollution shadow prices in conventional and organic farming: An application in a Mediterranean context. Spanish Journal of Agricultural Research, 9(2), 363-376.