For my second public-facing journal roundup, I'm leading with this nice photo of a dried out lake because if you only have time to read one of these articles, it should be Brian Richter's excellent overview of how to address water scarcity driven by irrigated agriculture. As I'm always harping about, irrigation efficiency can actually increase water consumption and worsen scarcity, and this paper has some solutions.
Skip to the end for the obligatory "one of these things is not like the other" paper on olfactory perception so you know what's going on when you're sniffing all those spring flowers.
AGRICULTURE / WATER SCARCITY
/ IRRIGATION:
Richter
et al 2017 is a fantastic new overview (led by TNC's Brian Richter) of how to
address water scarcity driven by irrigated ag (which accounts for 90% of water
consumption globally). If you've been on this list for a while you've heard me
harp about how making irrigation more efficient can actually lead to more water
being consumed, and this paper tackles that very thorny issue (if you haven't
heard about it yet, just read this paper as it covers it quite well). There are
three key components to making this work: proper water budgeting, actual
changes in crop water use (via one of several strategies), and being able to
transfer water savings to other users or the environment (as opposed to just
shifting to more water-intensive crops or expanding irrigated cropland). One
key strategy they find as reliable to
reduce scarcity is changes in cropping (e.g. shift from rice to other grains,
or temporary fallowing). On the policy side, critical ingredients for success
are a formal water rights system (based on consumptive use rather than
withdrawal volumes) that allows for trading / selling water rights, as well as
capping total consumptive water use. The second page of the paper has a great
story about how ag in Arizona is repeating the mistakes of indigenous people in
the area who disappeared ~1450 AD when drought caused their irrigated ag to
collapse. You can also see slides related to this work here:
http://symposium.greenleafadvisors.net/wp-content/uploads/2017/02/NACD-Denver-Feb-2017_Richter_trimmed-and-secured.pdf
Scott et
al 2014 is another paper looking at the challenges of trying to reduce water
scarcity / depletion via irrigation efficiency. In addition to the well
described case of efficiency inreasing total water consumption, they also
describe a "scale paradox" (where water impacts are displaced in
space and time), and a "sectoral paradox" (where water
"saved" in agriculture is used by other sectors like urban or
industrial).
Bekchanov
et al 2016 is a case study of what increasing irrigation efficiency could look
like in the Aral Sea basin (Central Asia). They find that it could lead to
considerable economic benefits through boosting crop yields plus allowing
cropland expansion and a shift to more water-intensive crops. While only 3-4%
of irrigation comes from groundwater (so depletion is less of a concern), this
finding still raises questions for resilience: having crops that use more water
means more risk when water is scarce. To me this is a useful paper in showing the
need for policy to accompany irrigation changes to reduce those risks.
Dalin et
al 2017 explores the degree to which irrigation is driving the depletion of
groundwater in different countries around the world, and how that depletion
relates to agricultural trade. It's worth looking at Table 1 and Figures 2 and
3 which reveal interesting patterns. For example, the 42% of water depletion in
the US is for exports, while in China only 1% is. One way in which this could
be useful is in finding partners in advocating for better agricultural water
use and accompanying policies (e.g. in addition to working with Mexico on their
depletion, also pressuring US buyers of their products to advocate for reducing
water depletion).
AGRICULTURE / CLIMATE CHANGE
/ CARBON:
Carlson
et al 2016 is a nice summary of GHGs and emissions for row crops; they found
1.994 Gt CO2e / yr (although with a standard deviation of 2.172 Gt), and note
that other studies range from 2.294-3.102 Gt CO2e / yr. They find that the
major sources of crop emissions are methane from rice (48%), peatland drainage
(32%), and nitrogen fertilizer application (20%). You can get the paper and
supplementary info here:
http://www.nature.com/nclimate/journal/v7/n1/full/nclimate3158.html
I'm
getting a lot of questions about the suitability of cover crops for climate
mitigation / carbon sequestration lately, and Poeplau and Don 2015 is currently
my favorite reference on the topic. They find that on average cover crops
sequester 0.32 t C / ha /yr (=1.17 t CO2e/ha/yr), and did not find significant
impacts on this from tillage, climate, or cover crop type (which is
surprising).Two key notes on how to use and interpret this figure. First is
that this figure is about 50% higher than a few other studies (although it's
also more rigorous than them). More importantly is that this figure does NOT
account for changes in nitrous oxide; so for example if adding a leguminous
cover crop without reducing fertilizer, it is likely that nitrous oxide
emissions would be increased (and could offset the soil carbon gains). On the
other hand, in a precision ag context with regular soil testing, a nitrogen-fixing
cover crop could reduce fertilizer inputs which would boost the GHG benefits.
As always, the choice of cover crop and how it affects other management is key.
He et al
2016 is yet another paper challenging what we think we know about soil carbon.
The authors used radiocarbon dating to find that soil carbon was often much
older than most models assume them to be (thousands of years rather than
hundreds). This matters because it indicates that soil carbon is likely turning
over slower, and thus that soils will be slower to change in response to
management practices (reducing its efficacy for climate mitigation).
I'm only
including Esteves 2016 in my review to show the dangers of assuming that a
published journal article can be trusted as is. Figure 6 shows that the authors
consider Brazilian soy fields to act as fairly strong GHG sinks if you exclude
land cover change. The way they arrive at this unusual conclusion is by
treating corn grown in between no-till soy crops as a "byproduct,"
and then assigning credit for presumed land conversion avoided. A more
appropriate approach would have been to simple show that by producing more crop
on a given parcel of land, the emissions per unit of crop produced was lower.
Beware of results that look too good to be true!
AGRICULTURE / OTHER:
Miguez
and Bollero 2005 is a small (36 study) meta-analysis of how winter cover crops
affect corn yields. Some key findings: grass cover crops did NOT affect corn
yields, legume cover crops boosted yields as long as N fertilizer is <200 kg
N / ha (with bigger yield gains as N fertilizer is lower, e.g. 17% boost from
100-199 kg N / ha, vs. 34% boost for <99 kg N / ha), and biculture cover
crops (a mix of grass and legume crops) boosted yields especially at higher
fertilization rates (presumably to compensate for the nutrients used by the
cover crop). Note that some other studies have shown more mixed results for the
impact of cover crops on yields, but this provides some good clues about which
contexts they work well in. This study didn't look at "tillage
radish" or daikon, since that was pretty uncommon a decade ago.
This is
a blog rather than a paper, but it's a thought-provoking read. Essentially, the
author (Claire Kremen) argues that trying to intensify agriculture to meet
expected demands for food is the wrong approach. She advocates instead for a
focus on reducing demand (by reducing the amount of meat produceed and
consumed, better family planning to slow population growth, and sharply
reducing food waste), and also advocates for the resilience benefits of more
diverse agriculture. I personally have a hard time envisioning a world where we
won't need to intensify agriculture to some degree, but I also think Kremen
makes a compelling case for the need to also work on the demand side (which TNC
currently does very little on). It's a complicated issue but a great
conversation for conservationists to be having now. You can read the blog at
https://thebreakthrough.org/index.php/issues/the-future-of-food/responses-food-production-and-wildlife-on-farmland/demand-side-interventions
GRAZING / CLIMATE CHANGE:
Booker
et al 2013 argues that arid rangelands have limited potential for carbon
sequestration, and that since most rangelands in the U.S. are arid (if they
were wetter and more productive they would likely have been converted to
cropland) that we should focus on preventing conversion of rangelands to other
land uses (and avoiding soil erosion) rather than trying to significantly
increase soil C sequestration through changes in management. One key point is
that most C flux in arid rangelands is outside of the control of management,
driven by weather / climate and soil type. Unlike more mesic (wetter) systems,
arid rangelands typically do not have one "climax" vegetation
community that can serve as a management goal; rather, they tend to have
multiple possible states, with transitions among states controlled by weather
patterns and soil features in addition to potentially being influenced by
management. They recommend that work on shifting grazing management to improve
C should be focused on more mesic / wetter rangelands that allow a wider range
of management options and should respond more strongly to changes in
management. There is a nice overview of specific topics related to C on
rangelands including grazing management, woody shrubs, reforestation /
afforestation, soil erosion, restoration, and fire. They conclude with a
discussion of potential carbon policies and recommend that they a) not require
short-term accounting, b) don't assume management is the primary driver of C
storage, c) that they not allow sequestration to offset emissions without proof
of additionality, and d) focus on conserving rangelands and restoring degraded
cropland back to range.
REMOTE SENSING:
Liang et
al 2016 is an attempt to estimate grassland above ground biomass using remote
sensing, which highlights the challenges of doing so. They found that using a
single proxy for biomass didn't work well; the best one (NDVI) only explained
46% of the variation in biomass. A model relying on several variables performs
better, but even including data collected on the ground including grass cover
and height it only gets to 70% of the variance (63% if the ground data is only
used to train the remote sensing instead of being used directly). Some TNC
colleagues and I recently ran into similar challenges when trying to do
something similar in Peru (and others have hit the same issues in the US);
grassland remote sensing is hard!
MISCELLANEOUS:
Remember The Nature Conservancy's 2015 goal? Dinerstein et al 2017 presents an ambitious vision for nature that goes far beyond that with a catchy slogan ("nature needs half" meaning 50% of terrestrial ecoregions should be protected, http://natureneedshalf.org), along with an assessment of progress towards that vision, and a revised set of terrestrial ecoregions (available from http://ecoregions2017.appspot.com/). They don't get into the issue of how to manage protected areas effectively to meet conservation goals, and only briefly touch on the issue of conflicts with human needs (including indigenous communities). But one way or another, this paper is sure to prompt a lot of good discussion about conservation goals, and it's worth reading accordingly.
Spring
flowers have me thinking about odors, so I was fascinated by the Keller et al
2017 paper which evaluated how different people perceive and describe 476
different molecules, and built a model to predict how a molecule would be
perceived. The model did pretty well at predicting how pleasant and intense a
given odor would be, but only got <50% of the descriptors right
(unsurprisingly "fish" and "flower" were easy, but less
narrowly defined odors like "warn" or "wood" or
"musky" were harder). Honestly I find the paper to be pretty unclear,
but the topic was so interesting I still enjoyed reading it, especially once I
gave up on trying to decipher most of the diagrams.
For science,
Jon
p.s.
what do scientists like me do for earth day? Make a
soil
cake, of course!
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Booker K, Huntsinger L, Bartolome JW, Sayre N, Stewart W. What can ecological science tell us about opportunities for carbon sequestration on arid rangelands in the United States? Glob Environ Chang. 2013;23: 240–251. doi:10.1016/j.gloenvcha.2012.10.001
Carlson KM, Gerber JS, Mueller ND, Herrero M, MacDonald GK, Brauman KA, et al. Greenhouse gas emissions intensity of global croplands. Nat Clim Chang [Internet]. 2016;1(November). Available from: http://www.nature.com/doifinder/10.1038/nclimate3158
Dalin C, Wada Y, Kastner T, Puma MJ. Groundwater depletion embedded in international food trade. Nature [Internet]. 2017;543(7647):700–4. Available from: http://dx.doi.org/10.1038/nature21403
Dinerstein E, Olson D, Joshi A, Vynne C, Burgess ND, Wikramanayake E, et al. An Ecoregion-Based Approach to Protecting Half the Terrestrial Realm. Bioscience [Internet]. 2017;(April). Available from: https://academic.oup.com/bioscience/article-lookup/doi/10.1093/biosci/bix014
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