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Contrary Science; Cover Crop Mixtures, Monocultures, and Mechanisms

Posted by Andrew McGuire | August 18, 2020
Planting diverse cover crop mixtures will increase soil microbial diversity and achieve increased benefits over monocultures because of the synergistic interactions of the different plant species. The ideas in this statement can be found in regenerative agriculture, agroecology, and as hypotheses in scientific journal articles. Many studies have looked at various aspects of this statement and now we have several reviews of the cumulative research. Let’s take a look at what they found.

Cover crop mixtures vs. monocultures

I have been writing about cover crop mixtures since 2013. At first I was enthusiastic. I planted a 22 species mixture in my garden and lesser mixtures in research plots. They do look better than monocultures…but then I started looking at the research results (2016, 2017, 2018). Most showed that mixtures were not better than monocultures, and that the best monoculture often beat the best mixture in terms of biomass production. Mixture believers gave me a lot of feedback and so I continued to watch the research. As more results were produced, mixtures continued to show infrequent benefits over monocultures. Then last year, Angela Florence (Florence et al., 2019) and I decided to review all the available research and summarize the results. Our paper was recently published (Florence and McGuire, 2020).

Close up of assorted plants.
Figure 1. A visually appealing cover crop mixture including Crimson clover, Medium red clover, Sweet clover, White clover, Hairy vetch, Berseem clover, Mustard, Arugula, Rapeseed, Annual ryegrass, Oats, Triticale, Teff, Foxtail millet, Sudangrass, Buckwheat, and Safflower. A. McGuire

We searched for all the published research on cover crop mixtures—you can read the details in the open access paper. We ended up with 27 studies from ten counties that gave us 243 full comparisons of the best mixture with the best monoculture for these measurements:

  • Cover crop biomass
  • Weed suppression
  • Nitrogen retention
  • Water conservation
  • Soil biology promotion
  • Following crop yield
  • Biomass stability over time

Although the results differed for each measurement, the best mixture did not outperform the best monoculture consistently in any of them (Figure 2; from the paper). Overall, we found that mixtures were better in 2% of the comparisons, monocultures in 10%, and for the remaining 88% of comparisons, the best mixture performed comparably to the best monoculture. For those cases where there was a statistical difference between the best mixture and best monoculture, the monoculture produced more biomass in 17 comparisons, the mixture in only two.

Cluster chart showing standardized difference between cover crop mixtures and monocultures separated by metric category.
Figure 2. Standardized difference between cover crop mixture and monoculture performance (N=285) separated by metric category. Positive values indicate where mixtures were superior to monocultures. Negative values indicate where monocultures were superior to mixtures. From Florence and McGuire, 2020 with permission from Wiley Online Library.

Contrary to expectations, the cumulative evidence thus far shows that cover crop mixtures do not give a clear advantage over monocultures. IF there are meaningful advantages of mixtures over monocultures, they are rare or difficult to attain. Why would this be?

Plant diversity and ecosystem function

Ecologists have long battled over what they call the biodiversity-ecological functioning question: whether higher biodiversity brings with it benefits in higher function, often measured in plants through biomass production. Much of the expectation of the benefits of mixtures is based on different species being able to access different resources which limits their competition with each other when living in the same space. This could also allow more complete use of the available resources (niche complementarity and niche partitioning). However, unlike with animals, plants all use the same basic resources, so to make this work ecologists have hypothesized that different plant species could access resources in different locations: “spatial resource partitioning across vertical resource gradients” (Barry et al. 2020). If this were true, tall and short plants, deep and shallow rooted plants, planted together would have an advantage over any single species.

A 2020 paper reviews past research regarding this idea (Barry et al. 2020). From 21 studies, they find that in grassland studies, where most of the biodiversity research has been done, the evidence in support of this idea is not strong; “we found little evidence for spatial resource partitioning along vertical resource gradients in more diverse grassland communities.” And they found this both for aboveground plant growth and belowground root growth. Contrary to expectations, diversity of form in plants does not consistently result in better efficiency or function.

Plant diversity benefits driven by soil fungal disease

Nevertheless, long-term grassland studies have shown that mixtures produce more biomass1 than monocultures. However, it often takes a year or up to 3-5 years for the advantages of mixtures to become apparent (Cardinale et al., 2007). A recent paper may help explain this. Van Ruijven et al. (2020) review the evidence for various mechanisms driving the plant diversity-productivity relationship and conclude that soil fungal disease is a likely cause.

Many fungal pathogens in the soil are plant-species specific; they affect one species and not many others. Disease from these fungi are much more serious when the susceptible species’ plants are densely planted than when they are spread out or when they are mixed with lots of other species not susceptible to the disease (called negative density dependence). With such a fungus in the soil, and there are many of them, a monoculture declines over time. It may outperform a mixture in the first year, as research has observed, but after 2, 3, 4 years, the mixture overtakes the monoculture because of the effects of the soilborne fungal disease.

Although this needs to be confirmed by further research, it would explain the lack of differences found between cover crop mixtures and monocultures. Unlike perennial grasslands, annual cover crops are not in the soil very long, just a couple months in many cases. This may not be enough time for the fungal disease to affect their growth sufficiently to give mixtures the advantage, especially when grown in diverse rotations. The same may hold for many intercrops with the exception of legumes+non-legumes where nitrogen effects from the legume dominate.

Soil diversity not linked to plant diversity

One last contrary finding. Another common belief is that a diverse mixture of plant species will drive greater diversity in the soil than a monoculture. This is often given as a reason to grow cover crop mixtures. Here again, we have a new analysis of the accumulated research investigating this mechanism. Zhou et al. (2020) analyzed 1235 experiments done around the world on the effects of what they call global change factors on soil microbial diversity. These change factors included land use change, such as occurs when a natural ecosystem is converted to agriculture, and nutrient fertilization. From this analysis they make several surprising conclusions.

First, most changes in the number of species (alpha diversity) can be explained by a change in pH. pH! It truly is the master variable in the soil. This is good news, as we can and often do manage soil pH.

Second, as they state it, “Conversion from highly diverse natural ecosystems to homogeneous agricultural monocultures has a positive effect on microbial alpha diversity.”

Read that again, a positive effect. The same goes for conversion to pasture.

Figure 3. Meta-analysis of the impacts of global change factors on soil microbial diversity and functionality2 Creative Commons CC BY license.
Supplementary Fig. 2. Responses of microbial alpha diversity to different land use change types. Weighted means and their 95% confidence intervals of response ratios (RR) are given. The numbers at the right side of the confidence intervals represent the sample siszes. NEtoAgr, conversion from native ecosystem to agriculture; NEtoPas, conversion from native ecosystem to pasture; NEtoPlant, conversion from native ecosystem to plantation; NEtoSec, conversion from native ecosystem to secondary ecosystem. Source data are provided as a Source Data file.

What is more surprisingly contrary is that this implies, and the authors state this, that changes in soil microbial diversity (number of species) are NOT linked to changes in plant diversity. I know, heresy, but thus sayeth the science.

This does not mean that the conversion to agriculture is all positive. The microbial biomass is reduced, probably due to the change from perennial to annual plants and the associated decrease in carbon flow to the soil3. Agriculture also changes the structure of the microbial community. What these structural changes mean in the crazily complex soil is a difficult question to answer, but they may all not be beneficial with respect to the soil or the environment. This does, however, remove another purported reason of growing cover crop mixtures.

Dealing with the contrary

So then, the current science tells us that cover crop mixtures are no better than monocultures, that the diversity in plant form (shoots and roots) does not lead to greater function in mixtures, that soilborne fungal pathogens may be the mechanism behind the advantage (in the long-term) of perennial plant mixtures, and that plant diversity is not linked to soil microbial diversity. All contrary to what we expected. Science is stubborn in resisting our ideals. Nevertheless, grow cover crops: grow mixtures, monocultures, in crops, before crops, after crops, summer, winter, spring and fall, grow cover crops.

 

 

1 Biomass is often used as an indicator of ecosystem functioning in ecological studies.

2 Copyright © 2020, Springer Nature. Zhou, Z., C. Wang, and Y. Luo. 2020. Meta-analysis of the impacts of global change factors on soil microbial diversity and functionality. Nature Communications 11(1): 3072. doi: 10.1038/s41467-020-16881-7.

3 I don’t think that that natural ecosystems should be the standard for agriculture unless they provide the same amount of food.

References

Barry, K.E., J. van Ruijven, L. Mommer, Y. Bai, C. Beierkuhnlein, et al. 2020. Limited evidence for spatial resource partitioning across temperate grassland biodiversity experiments. Ecology 101(1): e02905. doi: 10.1002/ecy.2905.

Cardinale, B.J., J.P. Wright, M.W. Cadotte, I.T. Carroll, A. Hector, et al. 2007. Impacts of plant diversity on biomass production increase through time because of species complementarity. PNAS 104(46): 18123–18128. doi: 10.1073/pnas.0709069104.

Florence, A.M., L.G. Higley, R.A. Drijber, C.A. Francis, and J.L. Lindquist. 2019. Cover crop mixture diversity, biomass productivity, weed suppression, and stability. PLOS ONE 14(3): e0206195. doi: 10.1371/journal.pone.0206195.

Florence, A.M., and A.M. McGuire. Do diverse cover crop mixtures perform better than monocultures? a systematic review. Agronomy Journal n/a(n/a). doi: 10.1002/agj2.20340.

van Ruijven, J., E. Ampt, D. Francioli, and L. Mommer. 2020. Do soil-borne fungal pathogens mediate plant diversity–productivity relationships? Evidence and future opportunities. Journal of Ecology.

Zhou, Z., C. Wang, and Y. Luo. 2020. Meta-analysis of the impacts of global change factors on soil microbial diversity and functionality. Nature Communications 11(1): 3072. doi: 10.1038/s41467-020-16881-7.

10 thoughts on "Contrary Science; Cover Crop Mixtures, Monocultures, and Mechanisms"

  1. Andrew McGuire says:

    Another recent review paper with evidence that plant diversity does not result in increased soil diversity.
    Plant Invasion Has Limited Impact on Soil Microbial α-Diversity: A Meta-Analysis. Open Access.
    https://www.mdpi.com/1424-2818/12/3/112

    1. Andrew McGuire says:

      This paper’s observations supports the view that disturbance (heat, drought, fertilization) lead to higher soil diversity, and as stated above, pH is a major factor in this.
      Not open access.

      https://www.sciencedirect.com/science/article/abs/pii/S0038071720302479

  2. Daniel says:

    Great stuff thanks

  3. Tony says:

    Interesting, excellent, thanks. Regarding the heresy, I would predict that microbial diversity dynamics would be very dependent on the specific case. Agronomically, the native ecosystem to ag system effect is interesting but probably of little relevance for cover crop “scenario” selection. For instance, no-till silage corn with interseeded overwintered rye versus say rye and a legume. Its questionable whether that would carry a useful cost/benefit ratio but I bet it would increase microbial diversity! Thanks again for the excellent work and sharing it.

    1. Andrew McGuire says:

      Thanks Tony, good observations.
      I am sure that there are exceptions – it’s soil. And your right, the native to ag change may not be fully applicable to this (see my other example in the comments)
      Regarding your example, where would the additional diversity in the mixture come from? And if it is all in the soil already, are we really talking about diversity or differences in abundance, and should we assume that the differences in abundance that we see are beneficial or not? Lot’s of questions.

  4. Paula A Thornton says:

    May seem silly, but seems as though the principles can be tested this way.

    I’m in a new residential lot that was stripped for the build. I’m not planting grass. The front yard will be a rock water feature (that well be expensive and awaiting budget) and I’m slowly converting the back yard into garden paths with edible plantings.

    I’m actually trying to preserve some of the alfalfa in the back yard to plant around (although from an edible perspective I prefer the purple flower variety and this is the yellow).

    Until I can use the soil, do I just let the weeds grow in? Do I cut them?

    How do I maintain/improve the soil until I can actually make use of it?

    1. Andrew McGuire says:

      Paula, interesting question. I think that having something growing on the soil will be better than not, but if the weeds go to seed, they could cause a bigger problem in the future. Planting some type of cover crop might be the best.
      Here some online references,
      http://pubs.cahnrs.wsu.edu/publications/pubs/fs111e/
      http://pubs.cahnrs.wsu.edu/publications/pubs/fs117e/
      https://www.sare.org/resources/managing-cover-crops-profitably-3rd-edition/

      1. Paula A Thornton says:

        Thanks Andrew. I ordered the book and will use it as a community reference.

  5. Roland Bunch says:

    I have worked in the tropics for 50 years. The first problem with your article is that the green manure/cover crops that we use are also mostly grain legumes. Therefore, by having mixtures of our cover crops, people get a more varied diet. Are you going to claim that a diet based on one species is the best? Furthermore, our gm/ccs, which include trees and bushes, can occupy much of the environment from ground level to 15 m above the soil, and bring up nutrients from as far as 15 m below the soil surface. There is no way you can create the biomass we can, or rescue the nutrients we can, using primarily annuals, or even perennials grown as annuals. Furthermore, since smallholders’ greatest limiting factor is land, we intercrop gm/ccs with food crops like maize, sorghum, cassava and even tomatoes, etc. That means we often have both food crops and gm/ccs on all the land all year long, producing biomass. Do you? Lastly, in a tropical forest, with the amazing amount of insect pests and plant diseases that exist in the lowland tropics, natural tropical forests virtually never have insect or disease attacks bad enough to kill off even one single species. I have asked a lot of African women (they spend a lot of time in the forests looking for food and firewood) if they have ever witnessed a plant in an intact forest that has died out because of insects or diseases. None of them has ever seen one single case of any such plant. Furthermore, virtually all the species in a tropical forest have been there for millions of years. If they died out one thousandth as fast as our crops in fields of limited biodiversity would if we didn’t use pesticides, the world wouldn’t have anything even approaching a tropical forest.

    I do not know under what conditions the experiments you cited were done, but their conclusions violate the conclusions that nature has come to over millions of years.

    I have also read, over the last 40 years, many peer-reviewed articles of scientists who have come to the conclusion that green manures don’t work (because they tried out three or four species when we have over 100 species presently being used, and never bothered to find out how best to grow them. Furthermore, just generalizing from three species to over a hundred is a logical fallacy in its own right). Yet some 25 million smallholder farmers are using gm/ccs as I write. Scientists for years said plowing was necessary, until they were proven wrong (and no one ever had to plow a tropical forest). I was taught as a child that protons and neutrons were the smallest units of matter in the universe, as well as all sorts of other nonsense that we now know is wrong about the extent and nature of the universe, as well as things much closer to home like the biodiversity in the soil and even on human skin.

    I am not against science. I read and use a lot of it. But scientists would be a lot more accurate and have a lot more credibility if you adopted a little bit of humility, and quit assuming time and time again that anything you have not yet tested must therefore not exist. Such an assumption is an obvious fallacy–and has been proven so in hundreds, if not thousands, of cases. Yet you go on committing the same error.

    Of course, you will likely excuse yourself by saying, “Well, we were not thinking of tropical habitats, or cover crops that doubled as human food, or cover crops grown as intercrops rather than rotations.”

    I rest my case: you have once again generalized from what a few people have tested to everything else that might exist out there–and you have therefore arrived at a false conclusion.

    Unfortunately, in this case we are not just talking about some abstract concept. Because of the advice of conventional scientists, smallholder farmers around the developing world have been taught that green manures and intercrops do not work (in spite of the fact that European and American farmers virtually all used green manures up until the 1940s–were they all mistaken?). As a result, many millions of smallholder farmers’ soils have become wastelands for lack of organic matter, they spent money on synthetic fertilizers instead of food (when gm/ccs could have helped their crops produce just as profitably as the fertilizers), and didn’t grow biodiverse grain legumes intercropped between their basic grains. As a result of this misguided advice, millions of children have grown up malnourished, and millions more have died. I know because I have lived among them. I don’t like saying it, because we have all done it at some point in our careers, but it nevertheless continues to be the truth: we all have a responsibility for the impacts of the errors we publicize when we should have known better.

    If you doubt anything of what I have said above, please read the second edition of my book, “Restoring the Soil, How to Use Green Manure/Cover Crops to Fertilize the Soil and Overcome Droughts.”

    1. Andrew McGuire says:

      Roland, thank you for your comments. I write for primarily an USA audience and so any generalizations that I make only apply to our conditions here; so yes, I am going to do as you guessed and say that this research does not apply everywhere to every situation. It does however apply to most cover crop use in the US and similar environments where similar annual cover cropping is being used. Also, nowhere did I say that cover cropping does not work. I quote myself in the blog: “…grow cover crops: grow mixtures, monocultures, in crops, before crops, after crops, summer, winter, spring and fall, grow cover crops.”

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