In as much as organic matter in the soil is the key to soil success and cover crop roots are an efficient way to put organic matter deep into the soil, it seems like we should focus on those legume cover crop with the highest amount of root biomass. Although that seems to be a key factor, I am surprised at the lack of info on root biomass comparing various legume cover crops.
If you have insight or URLs with info, I’d appreciate.
If you want an answer to carbon sequestration it depends rather on whether the solution is to be temporary or permanent!
If you grow legumes/ trees etc, when they die they put back CO2 into the atmosphere.
If you grow cactus when it dies it turns into chalk!
Cactus has many benefits including as a food!
Thanks Graham but carbon sequestration is not my goal. My goal is to enhance the microbial life within the soil so it is more productive. If the microbial life is dependent upon organic matter in the soil then it seems like growing cover crops that provide the maximum amount of root biomass is the fast-track way to enhance soil health.
Yes, tropical soil fertility correlates directly with organic matter content. Search for information on root:shoot ratio. Do not discount the value of using above ground growth as mulch. Probably best not to incorporate as this dramatically speeds up carbon metabolism.
See:
https://www.researchgate.net/publication/263722149_Root_Growth_Nutrient_Uptake_and_Use_Efficiency_by_Roots_of_Tropical_Legume_Cover_Crops_as_Influenced_by_Phosphorus_Fertilization/figures?lo=1
Tables 4 and 5
It is a strong function of soil phosphorus.
Also a function of level of shade.
See:
Shade tolerance of tropical forage legumes for use in agroforestry systems
Table 2.45 on pdf p. 182. (a low shoot:root ratio is a high root:shoot ratio)
Macroptilium atropurpureum has a high root:shoot ratio but can be weedy/invasive:
Siratro is its common name.
https://www.researchgate.net/publication/277622062_The_rise_and_fall_of_Siratro_Macroptilium_atropurpureum_what_went_wrong_and_some_implications_for_legume_breeding_evaluation_and_management
Thanks Robert for listing that reading material. It offers interesting information on topics related to my question.
I’ve decided to do a little testing on my own. While it may not be precise, hopefully it will offer some relative comparison between plants. Instead of planting into the ground, I will plant into buckets so when harvesting, i.e. measuring the roots, I am able to easily collect all of the roots. While this does not accurately replicate the natural setting, hopefully it will provide some comparison between the various plants we are growing.
Not sure how long will let them grow in the bucket but maybe 45 to 60 days because that would be sufficient to obtain a relative comparison.
It will be interesting to know what you find out. I imagine that, after letting a plant grow for a time in a bucket, you could recover most of the root system by carefully emptying the soil and washing off the soil. Then you could weigh the roots. A brief summary of your procedure and a table or graph showing your findings could make for a nice “ECHOs from Our Network” article in ECHO Development Notes 
See ECHO’s Author Instruction Page if that’s something you would be interested in.
Legumes could vary not only in how much root biomass they produce but the depth at which that biomass–and subsequent addition of soil organic matter–is grown. Pigeon pea, for example, will put its roots down deep, whereas something like sunn hemp will probably have a shallower root system. By rotating them you could be depositing organic matter at different depths.
All quite fascinating. Thanks for exploring this.
I want to revive this topic in order to point it back toward the carbon sequestration aspect. Somewhere recently I saw that 40% of carbon uptake finds its way to the above ground portion of a plant such as velvet bean. That leaves 60% in ground BUT half of that Carbon (30%) goes to feed the microbial life God created to do the digestion for plant life to go forward. My question is: If the climate change we are feeling is due to the CO2 released into the air then is it right to claim that gm/cc’s are sequestering Carbon in that lower biomass portion? Thanks for the conversation! Hope somebody notices. : :
Jeff, you are raising an interesting point but, I am not able to follow your conversation/ question on this topic. Could you restate your question in a different way and also avoid the abbreviations? Thanks, Glen
gm/cc is green manure/cover crop
I am just asking if the claim that green manure is sequestering carbon in the soil is a helpful way to say it based on all the multiple potential destinations of the biomass over time and space. This seems very related to the discussion at: How do roots contribute biomass to the soil?
Thanks Jeff for the clarification. In as much as my focus is not on sequestering carbon, I don’t have much to contribute to the conversation but given the various uses for cover crops, etc. your logic seems reasonable to me. My focus is on increasing organic matter in the soil for the purpose of feeding microorganisms and increasing soil moisture holding capacity.