The Technical Potential of Soil Carbon Sequestration
Here Darryl advocates plantation forests with regular planting and harvesting of trees after their vegetative growth phase, around 25 years in his example. The harvested material would then be made into biochar and amended into these forest soils to increase soil carbon and growth rates of subsequent plantings.
On one hand this sounds like rotational or holistic grazing where fields are divided and livestock intensively grazes (fells) and manures (biochars) the soil, and is then moved on. Thereby allowing enough time between grazing for fields to recover, and for plants to benefit from the manures and spend more time in the vegetative growth phase sequestering more carbon.
Only, this is basically clear felling whereas rotational grazing is more like forest thinning if I understand it correctly. If only we still had dinosaurs to thin and manage our forests for us, or alternatively robots that were economically viable.
One issue I see with the clear felling apart from the ecological, diversity and hydrological problems it creates – is the subsequent seedling growth stage where you aren’t maximising canopy area in order to maximise photosynthesis and carbon sequestration.
One solution may be thinning and forest management, and he does mention thinning but never delves into the details.
That brings me to one of the issues he mentions about the cost of making biochar commercially and that made me think about my field Terra Preta interpretation of how the Amazonians might have made it buried in soil, but again that’s clear felling, and probably wouldn’t pass the EPA…
I also wondered what a biochar retort might look like in place around a standing tree… just for fun.
I also wonder if something similar can this be done as a polyculture or as or in combination with a food forest at scale while increase ecosystem diversity and at the same time sequestering carbon through management.
But, no doubt “scale” and “performance” is the issue, and commercially it comes down to what is “economically viable” under the “carbon market” rules and can be done today.
His comment on subsoil carbon is interesting: “It’s more expensive to monitor it and measure it, than it’s worth.”
A new study titled Is the rate of mineralization of soil organic carbon under microbiological control? suggests that:
- the rate limiting step in SOC mineralization is abiotic (physical rather than biological).
- that mineralization of SOC may be a two-stage process: firstly, non-bioavailable forms are converted abiologically to bioavailable forms, which, only then, undergo a second process, biological mineralization.
I’ll speculate and say that mineralization is perhaps limited by dissolved organic matter in the form of complex carbon structures that are produced by weathering, oxidation, and detritivores that break apart plant litter. Liquid compounds that tend to be lost when composting.
Compounds that are probably why wet climates and seagrasses sequester the most carbon.
Why drying and wetting of these compounds leads to CO2-Bursts.
Compounds we should be trying to keep in the soil profile doing good, and not in aquifers or waterways creating algal blooms, like we really need to with this recently discovered underground molten lake the size of Mexico!
That depending on the complexity of the carbon molecule as it gets passed down the soil carbon continuum food web it may be more than a two-stage process, with more than one in both the physical and biological realms based upon chemical energy and chemisorption of the carbon compounds.
*shrugs* Just my guess, I suck at chemistry.
Add 1.5% biochar to a 3.5% (0-100mm sampled) soil organic carbon soil, wait 9 years and it begins to positively prime. Clearly they didn’t add enough to reach a tipping point faster, or compost it. I assume it was surface applied as the study is paywalled.
Paywalled: Biochar built soil carbon over a decade by stabilizing rhizodeposits : Nature Climate Change : Nature Research
114 site study.
- Grazing increased plant diversity
- Grazing increased introduced species in higher rainfall (>350mm May – Sept) areas
- Grazing increased aboveground biomass productivity in high rainfall areas, decreased it in lower rainfall areas.
- Grazing saw no change in SOC in the 6 study regions.
- Grazing increased root growth in the top 30cm of soil in high rainfall areas.
- Grazing increased decomposition of plant litter in high rainfall areas.
- Grazing lowers CO2/N20 flux.
- Rotational grazing (High intensity, low frequency) significantly lowered CH4 production.
- Native grasslands store most carbon.
Conclusion: Rotational grazing native grasslands in high rainfall areas FTW.
- Perennial grasslands produce higher below ground biomass than above
- Cultivation leads to a rapid loss 30-60% of soil C
- Continuous wheat cropping led to 19% loss of C
- Cropping saw 30-40% C loss after 5 years.
- Silvopasture (perennial pasture system) produced least CO2
- Naturally re-vegetated areas failed to recover even after 50 years.
Conclusion: Silvopasture intercropping FTW.
Basically what Colin Seis does with Pasture Cropping native grasses.