With global challenges such as diminishing environmental quality and public health, combined with accelerating climate change, we need more than ever to know how to confront many problems at once. Since plants inhale carbon dioxide and soils store carbon, there are numerous reasons to expect agriculture to play a significant role for improving the human prospect. Indeed, scientific research documents that we have tools to achieve ecological and climatic sustainability, but here’s the catch—we have to use them.
Photo taken on 8 August 2013 of instrument measuring carbon flux over a blue lupin plot with wheat and untilled plots in the background at the Maize and Wheat Improvement Center (CIMMYT) in Harare, Zimbabwe. Photo: D O’Dell
The case of cover crops and climate: insights from new science
Cover cropping is one agroecological practice that can bring many benefits, such as controlling pests, curbing erosion, and adding nitrogen to soils (using plants that fix nitrogen, legumes). Scientists suspect that cover cropping may also help fight climate change, since it protects or even adds to the carbon stored in soils. The only trouble is that changes in soil carbon are notoriously difficult to measure.
In a newly published paper in the Journal of Agricultural Sciences, Deb O’Dell from the University of Tennessee, Knoxville and colleagues report on the effects of winter cover cropping (O’Dell et al. 2015). Specifically, they compared two cover crops (winter wheat and blue lupin) with two alternative practices (leaving maize residue on the soil surface, or incorporating maize residue into soils). To see how the different practices affected carbon they measured the difference between the amount of carbon moving from the land to the atmosphere versus the amount moving from the atmosphere to the land. If you’re curious why and how scientists measure such “fluxes,” read on! But first, here are the punch lines:
- Cover crops can prevent carbon losses or even lead to gains. The land under both non-cover crop practices emitted relatively large amounts of carbon, which was not the case for either cover crop.
- Not all cover crops are equal. While the winter wheat plots accumulated large amounts of carbon, those planted with blue lupin only reduced losses relative to the other treatments. There are many variables to consider when selecting a cover crop, and these results serve as an important reminder that cover crops are not alike.
- Cover crops are crops. Cover crops are often used to improve soils or other aspects of farm management, but they are also crops in their own right that may have additional value for farmers. O’Dell and colleagues harvested several tons of wheat from the experiment, while still leaving residue on the soil surface, and noted a severe lack of research on cover crop yields and marketability.
- We need more systematic flux (carbon exchange) measurements. The method used to measure carbon fluxes (the Bowen Ratio method) was effective despite logistical and environmental obstacles (including rainfall and irrigation practices). Similar studies must be expanded to longer periods, more cover crops, additional practices, and an array of soil types and regions.
C is for…?: Why do scientists measure carbon fluxes anyway?
Everyone knows that C really stands for “Carbon”, right? Right??? Humor me for a moment and imagine C stands for “Cookie”, and let’s talk about fluxes.
- Counting cookies: Imagine that someone made it your responsibility to monitor the contents of a very important cookie jar. Would you (a) occasionally count the cookies (the content)? or (b) count the cookies that were added and removed (the flux)? If the jar were small, you’d probably just count the cookies. But—if the jar were large (think swimming pool), counting all the cookies would be tough (and taking a sample might not be reliable.) It might actually make more sense to monitor cookies as they are added or withdrawn to tally the contents of the jar.
- Counting carbon: Now back to the “real” C. Like our imaginary cookie jar, soil contains a big “pool” of carbon that is constantly added to or diminished. For decades, scientists have collected soil samples and measured carbon in the lab, comparing values over space and time. In the big pool of C, however, effectively measuring change over short periods, large areas, or complex studies can be tricky. This is why scientists developed tools to measure gas fluxes, such as the method used by O’Dell and colleagues. Among other advantages, these methods can measure the fluxes continuously, without disturbing the soil.
The average of 1 is… 1: an urgent need for more and longer studies
Studies like the one described above are providing critical insight into management practices that can be economically sustainable, ecologically regenerative, and useful in managing climate change. However, to understand which practices will work best, we sorely need more, and longer-term, experiments. As this research confirms, the potential benefits are significant. Let us not only develop new tools to understand and achieve sustainability, let us also use them!