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How do roots contribute biomass to the soil?

Perennial plants periodically shed their leaves and grow new leaves. Do the roots do the same? In other words, do the large roots function like branches on the tree that have small roots that grow and die periodically replaced by new young roots? What I’m getting at is, if a plant stays in the field growing does it shed a significant amount of root biomass adding to the organic matter within the soil or does that not happen until the plant dies?

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Good question. I have heard that when cut (pruned, coppiced, pollarded, etc.) perennials will shed roots but not in the mere process of growth. There are reasons for/consequences of shedding leaves - such as removal of accumulated toxins, so not sure if this applies to the roots. The matter of connections to mycorrhizal networks may also be significant - roots are connected to mycelia in a way that vastly expands the way roots can feed from and to the soil. So will be interested to hear of any research/wisdom on this.

I’m sure someone in this world has a sure answer. Two people I can think of who I suppose would be confident in answering the question are
Elaine Ingham Contact Us - Soil Food Web School - EN
and Matt Powers https://www.youtube.com/results?search_query=Matt+Powers+roots&sp=EgIIBA%3D%3D.

Thanks Troy. I reached out to Soil Food Web School. Will report back what they say. Glen

Adam Cobb of Soil Food Web responded. Here is his response. I posted a follow-up question to him and will post that when received.

There are a few different “pathways” by which roots add organic matter to soil ecosystems. You mentioned that when roots die, they are eaten by all kinds of creatures in the soil. These organisms incorporate the carbon and other elements into their bodies, and when those organisms die the elements continue to cycle, unless removed. One of the most common ways carbon is removed from soil is respiration of bacteria and other organisms (where CO2 escapes into the air).

Roots also directly feed carbon to mycorrhizal fungi, in exchange for scarce nutrients. You can read more about that here (an article I wrote for ECHO Asia): Mycorrhizal Fungi - Our Tiny Underground Allies | ECHOcommunity.org

One of the other KEY pathways for carbon to become part of the soil organic matter pool is via root exudates. These are compounds (mostly simple sugars) that plants leak into the soil around their roots, in order to feed and stimulate the growth of bacteria, fungi, and other organisms that can gather resources from the surrounding soil. The real “show” of the soil ecosystem is happening right near the roots of plants, as these (carbon containing) exudates are eaten by the organisms attracted to the area.

In terms of long-term carbon “storage” in the soil, not all organisms are created equal. Fungi, for example have an amazing capacity to transform carbon into forms that are not easy to decompose. In general, the organic microbial pool is where we want carbon and other elements to be stored. As long as the food web is healthy, predatory organisms can eat bacteria and fungi and slowly release those resources back into the root zone, where the plants can be nourished and the organic matter can accumulate.

Please let me know if that helps clarify the dynamics of soil carbon. Thanks

Sincerely,

Adam Cobb, PhD

Here are the notes from Adam Cobb as he responded to my follow questions on the topic.

If you are talking about the roots of perennial plants, we do often see a seasonal shift in root biomass, but that is not likely to be much from shedding of dead root material. As perennials go dormant, they tend to shuttle a lot of their reserve resources from above- to belowground. When they reawaken, those resources (including carbon) are pulled from the roots back up to create new aboveground tissue. As a result, the root systems seem to shrink.

One of the reasons so many regenerative agriculture advocates are focused on multi-species annual cover crops is the large amount of root and shoot biomass that can accumulate quickly and then become part of the soil organic matter. This can be part of an effective land transition strategy; however, increasingly research is finding that the soil organic material from dead plants is not as effective for long term soil carbon accumulation as the root exudates of a diverse perennial plant community. This is likely one of the reasons why diverse perennial pasture, with grazing that is well managed, seems viable for restoring soil organic matter: Emerging land use practices rapidly increase soil organic matter | Nature Communications

Sincerely,

Adam Cobb, PhD

As we’ve watched this conversation, we have added resources to an ECHOcommunity collection named Root Biomass to facilitate discovery and additional conversation.

edn.link/rootbiomass

Thank you to all who have been contributing. We love the interaction and contributions. How does the discussion impact your thinking on the subject?

That is an excellent question and well-articulated and it affords an opportunity to talk about a confusing subject. One that is often misunderstood.
There is a process called sloughing-off of roots. Plants/trees store nutrients in their root systems, the products of photosynthesis (generally sugars) and also the products of removing minerals through ground water extraction or selective action of roots to acquire minerals. All of these are available for spurts of growth following the dry season or the winter season when there is sufficient moisture and/or warmth in the soil to start growing. Or if you cut back a tree (coppice, pollard, prune, etc.) this will occur to the extent that you have cut it back and will delay if it in a cold winter or a dry season. The effect is even greater with a sloughing-off effect that deletes a good portion of the smaller storage roots and depending on the severity can be very hard on a tree and limits its long-run productivity.
Some of the greatest foregone opportunities are in how this underground reserve of nutrients is utilized.
Perennial plants provide the most flexibility for management. Annuals the least. You can manipulate the system through ripping or root pruning to intentionally remove roots at the beginning of the rainy season when you have intercropping or living mulch/green manure cover crops. This potential is very seldom used and has great potential, particularly for perennial tree and herbaceous legumes to #1 control competition of the perennial intercrop and #2 at the same time make that nodulated nitrogen and other gathered nutrients in the roots, severed by the ripping, available to your crop.
You can spray weeds or grass weeds with RoundUp or another chemical and then you have a dead mulch which is very efficient for cropping. This is perhaps the most efficient non-mechanized system I have used because with this system I did not have to weed and used little water due to the mulch being in place. And I did not have to gather mulch and the roots breaking down fed the crop. I have actually seen the edge of where I have sprayed RoundUp, the plants are very green on the edge because they have taken up additional nutrients from the underground root system breaking down. I am very saddened by the neglect of this method by those who are averse to chemical use because it has such a great potential to reduce labor and add great benefit, greatly contributing to human flourishing. This is perhaps the best method of conservation farming, in my opinion.

Thanks Dan for the great information.

A couple of clarifying questions because I’m not sure I followed you correctly and I want to be sure to clearly understand what you’re saying.

Are you saying that sloughing off of perennial roots is a normal natural process that happens during the climate cycles? In other words, there’s nothing we need to do to initiate the sloughing off.

Secondly, are you also saying that a very similar process to sloughing off happens whenever trees are pruned, coppiced, etc?

In other words are these two different but similar “events“ that happen in the life of perennial roots?

Now about the use of round up. Would the same affect have occurred had a herbicide not been used but instead the vegetation cut low to the ground mechanically with a hoe, machete, etc.?

Great questions. You are really getting into the throes of figuring this out. I am saying that sloughing off mainly occurs when the plant regrows after being cut back. We are talking about perennial crops. There may be some occurring in the northern latitudes with the onset of spring to move nutrients from storage to growth but you can manage to get a great amount of sloughing off when you cut the perennial back. The more severe the cutting the more sloughing off occurs. That can be a good thing or a bad thing depending on what “service” and “production” functions you need from the perennial you are cutting. Cutting back resulting in sloughing off results in less long-term productivity. Grasses are designed to be more tolerant but other species may not be as tolerant and can result in not only less long-run productivity but a shorter lifespan of the perennial. Generally, you will find on google the “coppicability” of species and how well they “bounce” back. Most species used for agroforestry have good coppicability. Roland Bunch’s work on perennial herbaceous legumes are good to consider here for cutting back to release nitrogen or reduce competition with shorter crops or to slow down the growth to give a taller crop a head start. In addition, these are great for ripping on both sides and intercropping because the ripping forces the release of nutrients. A grass slasher is the best tool for slashing back herbaceous material if you don’t have a mechanical mower. There would be a big difference between the amount of sloughing off occurring from cutting low and the amount of nutrients released from a RoundUp spray but if you want about 1/3 or 1/4 of the effect (I am using an educated guess based on experience) then slash back. A herbicide-killed grass or thick weed mulch is one of the greatest unsung hero techniques that we have (in my opinion).

Dan can you or others comment on the potential downside of using Roundup in the manner you described? For example, what effect does the short term occasional use have on the soil micro-organisms, earth worms, etc. It obviously has some upside as you mentioned but wondering about the downside and if there are downsides, are they short lived or what? Thanks for your excellent comments/informations.