Monday, September 3, 2018
This is the 50th of these science summaries, and somehow I am still not caught up with all the interesting papers I have! For that occasion, I'm tackling a number of papers that didn't group nicely but still looked relevant. So there are papers on GMOs, fertilization and nitrogen fixation, food security, and Chinese ag.
You've probably heard the debate about whether or not to label genetically engineered / modified food. On the one hand, consumers argue that food should be transparent about its origins to enable consumers to choose. The counterargument has been that since there isn't good evidence that GMOs can harm human health, it could result in consumers thinking it's being labeled b/c it's dangerous. But Kolodinsky and Lusk (2018) have some surprising good news - after GMO labels became mandatory in Vermont, consumer concern / opposition to GMOs actually went down (while in other states there was a non-significant small increase)! Figure 1 shows this graphically. However, after looking at Table 1 I'm less convinced as they have data for five points in time, and concern in the most recent data is about the same as the earliest data. So the finding could be an artifact of how they did their regression, but it's still a promising possibility worth looking into to better understand how these kinds of
labels may affect consumer beliefs.
Cheke 2018 is a commentary on how the use of pest control traits by GM crops can have unintended side-effects, just as traditional chemical pesticide use can. He presents brief interesting case studies, and essentially argues that we need long-term monitoring data to understand how novel forms of pest control impact target pests, non-target pests, and natural predators of pests. To be clear, this isn't an anti-GM paper, it just notes that switching from aerial sprays to toxins within the plant may have unintended consequences. See Zhang et al. 2018 under Chinese Agriculture below for more detail on Bt cotton as a case study.
Hasegawa et al. 2018 is an analysis of how climate mitigation may impact food security. They find that climate mitigation may cause more hunger than the direct effects of climate change. Livestock in particular would be more expensive (given higher GHG emissions and land use). The primary drivers would be food price increases driven by: tax on GHG emissions, higher land rents if ag expansion is slowed, and land competition by biofuel. However, this outcome isn't inevitable if policy is crafted more carefully. For example, a tax focused on animal products could drive a shift to producing more food consumable directly by humans, and revenue could be used for food aid. The key questions are thinking about who will bear the costs of implementation, and how it may affect food prices and thus food security for poor people in particular.
AGRICULTURE / NITROGEN FIXATION / FERTILIZER:
Van Deynze et al 2018 has been getting lots of press. The authors describe a landrace (local cultivar) of maize in Mexico that can fix its own nitrogen via microbes that hang out in aerial roots covered in goopy sugary mucilage. The authors did isotope tracing to verify that N was coming from the air and entering the aerial roots. Since corn uses tons of fertilizer, having it make its own sounds pretty exciting. In this case, it allows corn to be grown in nitrogen-depleted fields in Mexico, with ~30-80% of needed nitrogen coming from the air (varying by field and growth stage, but often ~50%). Unfortunately, this type of N-fixation is not as efficient as how legumes do it, and N-fixing maize is not brand new (Steve Wood sent me this paper from 1975 on the topic: http://www.pnas.org/content/72/6/2389.short). But (as Steve also pointed out) this is a pretty understudied area and there's a lot we don't know about the potential, which if nothing else shows the value in promoting crop diversity
so we don't lose potentially useful varities like this!
See https://news.wisc.edu/corn-that-acquires-its-own-nitrogen-identified-reducing-need-for-fertilizer/ for a blog post on this research.
Given how much research there is on trying to get crops to fix their own nitrogen, the finding by Griesmann et al. 2018 that many plants have lost the ability to fix N blew my mind. By comparing genomes of N-fixing plants to those that don't, they were able to find that ~3/4 of the species in their sample that didn't fix N had an ancestor that could! They suggest that the fact this ability has been lost multiple times reflects that plants spend a lot of energy to support N fixation, and that when N levels are adequate in the soil they eventually can lose the ability to fix it. In other words, as we try to engineer plants to fix their own N, it's worth reflecting on the costs that may have led plants in the past to reject this evolutionary path.
There's a blog on this one at http://www.sciencemag.org/news/2018/05/many-plants-need-bacterial-roommates-survive-so-why-do-some-kick-them-out
Chen et al. 2018 takes a different approach to improving fertilizer use in ag. They provide a review of possible ways to improve the environmental impacts of making chemical nitrogenous fertilizer. Currently the Haber-Bosch process relies on methane, so getting hydrogen from water instead could have many benefits if the higher energy cost can be overcome. This is a pretty technical paper but the first page is an overview that is much more accessible and introduces the possibilities.
Zuo et al 2018 is a nice dense analysis of how Chinese agriculture has changed over the past ~20 years, focusing on crop yield and five environmental indicators (irrigation water, excess N, excess P, land use, and GHGs). While total crop calories produced increased by 66% (with only 1% more net land use), and environmental intensity (impact per calorie) went down, the total environmental impact still worsened quite a bit. This is a common challenge in sustainable intensification - it's often relatively easy to boost yields, but challenging to reduce actual impacts to the environment at the same time. The authors also note that urbanization has pushed cropland to more marginal and water-scarce areas, and future shifts in cropland may pose further challenges. This is worth reading in its entirety for anyone working in Chinese ag, but Table 1 summarizes the changes well, and Figure 3 is an intriguing map with potential priority areas to focus on different indicators.
Zhang et al. 2018 is a look at the impacts of planting Bt cotton (genetically engineered to produce the Bt toxin) in China on three major pests, and how that relates to farm management and nearby land use. The good news is that the use of Bt cotton decreased spraying for bollworm, which in turn allowed natural predators of aphids to recover (which reduced aphid population). Unfortunately, the reduced spraying also allowed mirid bugs to increase. Figure 3 shows how the infestations and spraying changed over time as Bt cotton was rolled out. These trade-offs show the need to think holistically about pest control. Farms in landscapes with more non-farm land use (urban, water, forest, grass, 'unused') had less aphids and mirid bugs (but bollworms were unaffected, and other research has found diverse land use may promote mirid bugs). Finally, farmers were found to spray for aphids and bollwords at non-damaging levels, which represents a missed opportunity to increase their profits (via reduced
costs) and improve their sustainability.
Cheke, R. A. (2018). New pests for old as GMOs bring on substitute pests. Proceedings of the National Academy of Sciences, 201811261. https://doi.org/10.1073/pnas.1811261115
Chen, J. G., Crooks, R. M., Seefeldt, L. C., Bren, K. L., Bullock, R. M., Darensbourg, M. Y., … Schrock, R. R. (2018). Beyond fossil fuel–driven nitrogen transformations. Science, 360(6391), eaar6611. https://doi.org/10.1126/science.aar6611
Griesmann, M., Chang, Y., Liu, X., Song, Y., Haberer, G., Crook, M. B., … Cheng, S. (2018). Phylogenomics reveals multiple losses of nitrogen-fixing root nodule symbiosis. Science, 361(6398). https://doi.org/10.1126/science.aat1743
Hasegawa, T., Fujimori, S., Havlík, P., Valin, H., Bodirsky, B. L., Doelman, J. C., … Witzke, P. (2018). Risk of increased food insecurity under stringent global climate change mitigation policy. Nature Climate Change, 8(8), 699–703. https://doi.org/10.1038/s41558-018-0230-x
Kolodinsky, J., & Lusk, J. L. (2018). Mandatory Labels can Improve Attitudes toward Genetically Engineered Food. Science Advances, In press(June), 1–6. https://doi.org/10.1126/sciadv.aaq1413
Van Deynze, A., Zamora, P., Delaux, P., Heitmann, C., Jayaraman, D., Rajasekar, S., … Bennett, A. B. (2018). Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota. PLOS Biology, 16(8), e2006352. https://doi.org/10.1371/journal.pbio.2006352
Zhang, W., Lu, Y., Werf, W. Van Der, Huang, J., Wu, F., Zhou, K., & Deng, X. (2018). Multidecadal, county-level analysis of the effects of land use, Bt cotton, and weather on cotton pests in China. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1721436115
Zuo, L., Zhang, Z., Carlson, K. M., MacDonald, G. K., Brauman, K. A., Liu, Y., … West, P. C. (2018). Progress towards sustainable intensification in China challenged by land-use change. Nature Sustainability, 1(6), 304–313. https://doi.org/10.1038/s41893-018-0076-2