cover crop mixtures increase agroecosystem services

Discussion in 'Cropping' started by Great In Grass, Jan 7, 2017.

  1. Great In Grass


    First-of-kind study suggests cover crop mixtures increase agroecosystem services
    Jeff Mulhollem
    September 7, 2016
    College of Agricultural Sciences.

    That was the conclusion drawn from a two-year study of 18 cover-crop treatments, ranging in diversity from one to eight plant species. Cover crops were grown at the Penn State Russell E. Larson Agricultural Research Center preceding a corn crop. The researchers measured five benefits provided by cover crops — ecosystem services — in each cover crop system to assess the relationship between species.

    Those services included weed suppression and nitrogen retention during the cover-crop season, cover-crop aboveground biomass, inorganic nitrogen supply during the subsequent cash-crop season and subsequent corn yield.

    The study was the first field-based test of the relationship between cover-crop species and multifunctionality — the quality of cover crops to simultaneously provide multiple benefits — noted research team member Jason Kaye, professor of soil biogeochemistry. Never before had this relationship been examined and analyzed in a crop rotation.

    As continued research yields more precise information about optimal cover-crop seed mixtures and planting rates, Kaye predicted, farmers will deploy this strategy to enhance soil quality, control weed growth, manage critical nutrients such as nitrogen, improve crop yields and reduce nutrient runoff.

    "This kind of ecological study identifying a positive relationship between biodiversity and ecosystem services suggests that higher plant diversity will increase services from agroecosystems, and that has immediate implications for management practices and policies for sustainable agriculture, including Chesapeake Bay water quality," Kaye said. "In a corn production system, simply increasing cover-crop species richness will have a small impact on agroecosystem services, but designing mixtures that maximize functional diversity may lead to agroecosystems with greater multifunctionality."

    Cover-crop species grown in monocultures and in mixtures during the study included oats, canola, sunn hemp, soybean, barley, perennial ryegrass, forage radish, cereal rye, millet, sudangrass, red clover and hairy vetch.

    The research, which is published in the September issue of the Journal of Applied Ecology, shows that designing cover-crop mixes will involve trade-offs to achieve desired levels of ecosystem services, explained lead researcher Denise Finney, now an assistant professor of biology at Ursinus College, in Collegeville, Pennsylvania. She conducted the Penn State study while pursuing her doctoral degree, advised by Kaye.

    "For example, nitrogen cycling is an area where trade-offs can occur among services," she said. “In our research, we have found that cover-crop mixtures that excel at nitrogen retention can decrease soil nitrogen supply to cash crops and limit their yield. However, bi-cultures — correctly formulated to combine legume and nonlegume species — can both supply inorganic nitrogen and retain nitrogen."

    Finney said the researchers aimed to address two critical questions related to cover crops and multifunctionality: Does including more species in a cover crop system lead to greater multifunctionality? And are there guiding principles for cover-crop mixture assembly that will lead to increases in net multifunctionality? Cross-disciplinary, follow-on studies planned and underway on Pennsylvania and New York farms, led by Kaye, are intended to build on the knowledge gained through this study.

    The ultimate goal is to provide the agriculture industry with evidence-based information that will help farmers to design cover-crop mixtures that provide desired services using species combinations that augment biodiversity while minimizing economic or management constraints, Finney explained. Increasing ecosystem-service provision from agroecosystems is an emerging goal of contemporary agriculture.

    "Exploiting biodiversity to meet this goal is a promising approach," she said. "However, considerable research is needed to identify the functional traits that shape cover-crop community dynamics and to depict trade-offs among services in multifunctionality metrics."

    Northeast Sustainable Agriculture Research and Education, the U.S. Department of Agriculture and the ARCS Foundation supported this work.
    Bogweevil likes this.
  2. Great In Grass

    Cover crop best bet is monoculture, not mixture
    December 21, 2016
    By Andrew McGuire

    Can you see 17 species in this cover crop mix? Photo: A. McGuire.

    Cover crops are great. If I thought I could get away with it, I would just grow cover crops in my garden. They protect the soil, feed microbes, build soil structure, add root channels, and support beneficial insects. I think they look cool too. When cover crop mixtures got popular a few years ago, I got excited and grew a 17 species mix. It looked really cool, I mean, diverse, with all sorts of seeds that became all sorts of plants. I took pictures, showed my kids, and even had a neighborhood open garden event! (Well, maybe not that last one) Then I grew some vegetables after the cover crop. They did OK. Just OK. I wanted it to be the best tomato/squash/cucumber/lettuce crop ever, but I could not tell the difference between these vegetables and those I had grown after many previous un-biodiverse cover crops. Recent research results may explain this.

    Research thus far has consistently found that cover crop polycultures are not necessarily better than cover crop monocultures. This is now reaffirmed by a large study, done in Pennsylvania, published this year (Finney et al. 2016). The study had 18 treatments replicated four times (these studies are a lot of work). Eight species were grown as monocultures. Seven 4-species mixes, and two 8-species mixes, were the polyculture treatments, and they included a no cover crop check treatment. All this was planted in August, for two years (different field each year).

    The big idea behind cover crop mixtures is that the increased biodiversity will result in increased productivity, increased ecosystem services, or both. The Finney group tested both hypotheses. They found that the mixtures produced less biomass than the best monocultures (here, canola and cereal rye). They also found that mixtures did not provide increased ecosystem services (here, weed control, nitrogen scavenging, nitrogen storage, and effect on following crop) over the best monocultures. These findings are related. Finney et al. found that most of the ecosystem services which we want cover crops to provide are related to biomass production. Because a few of the monocultures produced the most biomass, they also provided more services. From this they concluded that “a mixture may not be necessary” and “a single cover crop species may be sufficient and more economical than a mixture.” (I attended several sessions on cover crop mixtures at this year’s meeting of the Agronomy, Crop Science and Soil Science Societies, and did not hear any results that went against these findings.)

    Mixtures do have one advantage, they can provide more services (multifunctionality) than a monoculture. However, in mixtures, the level of individual services provided is less than with a monoculture. For example, cereal rye is a great weed suppressor, but it does not fix nitrogen. If we mix hairy vetch, which fixes nitrogen from the air (an ecosystem service), with rye, we get both services, but the added vetch dilutes the weed suppression of the rye. There is a tradeoff in using mixtures to obtain multiple services (multifunctionality). In another strike against mixtures, Finney and Kaye (2016, same study, different paper) found that this multifunctionality was only weakly related to the number of species in a mixture. Their study, “does not support the hypothesis that increasing the number of species in a mix will lead to predictable increases in multifunctionality at levels that are agronomically or ecologically relevant.”

    Why don’t cover crop mixes work better than monocultures? Well, first, some ecological theory. The idea that biodiversity is better than monoculture comes from ecologists studying natural habitats. In nature, they observe niche differentiation (Connor et al. 2011). The idea is that a diverse mix of organisms can better use the available resources because of their different use characteristics. When their resource use does not overlap much, they are complementary. Take a field of wheat. If some plant species needed different resources than wheat, then we could expect that adding that species to the wheat field might result in more production in the same area. Better resource use means better productivity. That is the theory.

    Niche differentiation is seen most clearly with animals, but plants are different. First, they do not move. They are stuck where chance happens to put them. Second, plants require the same resources; sunshine, CO2, water, and nutrients (nitrogen for legumes is the exception). For these reasons, plants, especially annual crops, have much less opportunity for niche differentiation. And this is what we see in cover crop research results, little evidence of complementarity. Even in more natural systems, support for complementarity in plants is rare (Cardinale et al. 2011). The authors of this paper ask, “how can species be ‘complementary’ in their use of resources and production of biomass, and yet, a diverse community not perform processes any more efficiently than its most efficient species?” The simple answer is that there is no complementarity in these diverse mixes. Cardinale and colleagues cannot go this far, but rather think it “warrants more investigation.”

    Rather than complementarity, there are simple tradeoffs. When plant species compete for the same resources, there are winners and losers. Mixing a less productive species with a high productive species reduces total biomass production (with the exception of legumes in infertile soils) Winners dominate losers in mixed stands, to the point where the losers are suppressed by the canopy, or larger root system of the winner. This is what we see in cover crop polyculture research. Dominant species, which happen to be our most productive crops, tend to dominate. If we control them by reducing their seeding rate, the less extreme species grow better, but not enough to make up for the lower population of the dominant species.

    Finney and Kaye mention possible reasons why cover crop mixtures may not live up to ecological theory. Whereas polyculture advantages have been seen mainly in perennials growing together for several years, cover crops are annuals growing for just a short time. In natural systems, the number of species present is very much greater than in agriculture where we select dominant plant species for our crops.

    This demonstrates the basic problem with attempts to make agriculture more like nature; agriculture is not like nature. Whether it be crop rotation, cover crops, or the need to supply farm fields with inputs, agriculture is not like nature. Therefore, we should not be surprised when one of these principles, here the diversity-productivity relationship, does not apply.

    There are other reasons not to use mixtures in cover crops. First, because monoculture powers crop rotation benefits, planting a cover crop mixture increases the risk that a pest will find something in the mix it likes. Add to this the difficulty in seeding multiple species, finding the right timing for planting a diverse mix, and the increased cost of seed blends…if these mixtures do not give extra benefits, why grow them?

    What might be a feasible and possibly beneficial option is planting multiple varieties of one species. Wenatchee ARS researcher, Mark Mazzola (2002), has found significant differences in the effects of different wheat varieties on the soil microbial community. Similar effects may occur with varieties of other cover crop species. Adding this kind of diversity to a cover crop comes without all the problems of species mixes and may prove beneficial.

    I think that any cover crop can do some good. If you like planting polycultures, do it. But don’t let the appeal of the silver bullet, of the secret solution, cloud your judgement. Novelty entices the most sober-minded of us into thinking “this is it.” I still sometimes grow cover crop mixes, but also monocultures. Both are good, but as I found, and as science is confirming, cover crop mixes are not the restore-everything-to-as-it-should-be final solution we hope for.
    Bogweevil likes this.
  3. Dan Powell

    Dan Powell Member

    Hmmm? Monoculture better for cover crops. Depends what you're measuring I guess.

    @Andy Howard would probably have evidence to the contrary.
    Brisel likes this.
  4. shakerator

    shakerator Member

    Why the continual use of the word "services"?!

    Surely it's a farmer trying to improve his or her soil asset base to become more competitive at the market.....

    There seems this strange paradigm emerging of "natural capital" and my best guess is these "services" will be externally investable and susceptible to other flaws of Anglo American "market above all else-ism" reducing the ability of the farmer to reap their own benefits organically (no pun intended- or conventionally)
  5. so first decide what you want a cover to do then pick the best species to do that
    Rob E and Brisel like this.
  6. Brisel

    Brisel Member

    The "services" listed don't always correlate with those sought by the more vocal CC users in TFF, Nuffield scholars, no tillers etc.

    As above, it depends on what you want to achieve. The goals seem to be above ground, not much below it beyond mitigating nitrogen loss.
    Dan Powell likes this.
  7. Oakbank

    Oakbank New Member

    I think that the word "services" has just become the expression of choice, but it could easily be substituted with "function" or "process". The best way to understand what is going on is to listen to Joel Williams for a few hours, he is eloquent, informed and explains the topic in a way that is quite digestible (pun intended!) I really do believe that a better knowledge of your soil is the best investment a farmer can make in 2017 (or any other year for that matter)
    Brisel likes this.
  8. dontknowanything

    Below ground is what I want, that's where the SOM comes from.

    If I wanted the biggest amount of above ground biomass then I'd plant stubble turnips and dump on a load of N, as is the traditional method. But a mix gives me greater resilience to conditions, greater biodiversity, maybe an overyield (Crench trials showed this) and less chance of messing up a rotation.

    In my opinion.
    grommet, Dan Powell and Brisel like this.
  9. Jason

    Jason Member

    South West
    Was reading about this overyielding business the other week, lots of debate out there re conflicts within seed mixtures and genuine 'transgenic overyielding' is very hard to demonstrate even in trials it would seem.

    The point made by @dontknowanything about hedging your bets as it were is clearly valid though, we of course cannot possibly know which species or variety will perform to its optimum in advance.
    Brisel likes this.
  10. grommet

    grommet New Member

    The shire...
    good points well made, went to one of your talks at Rougham - interesting stuff & also that beef looked bloody marvelous!

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