Great question. One must measure the first question before the second! My guess is that even char has a half life ....

stable carbon in soil:

Paul Milham Thank you for your reply. It is difficult to understand that why soil organic carbon whether in humus or stable organic matter fraction is not stable.Are the soil microbes so powerful to decompose and consume every organic material? Even if they consume, what happens to their bodies and their decomposition products ? All plant materials are not equally decomposable . Some materials containing pectins and lignin are difficult to decompose. Who can stop interactions among the decomposition products of plant, animal and microbes? Will the interactions not produce new compound/molecules which are difficult to decompose? Several radiocarbon dating studies ,models and longterm experimental studies variously gave half life to humus and stable carbon from a tens, hundreds to a few thousand years.

Are the studies fiction or fact? What about the accumulated carbon in grasslands ,forests, peatlands etc.What is its nature? so dynamic or stable? Can we not characterize the carbon with presently available tools and techniques and answer those who are raising questions?

Thanks Subba Rao for sharing the intersting paper and asking valid questions. I think we have to look at soil as a dynamic system in the first place. Then we can investigate, estimate, model soil C. I understand mineral C, as mentioned in the link is supposed to be more stable and a candidate to look for. Agricultural soil scientists mostly focus on top soil; so never get a complete picture. Snitizer, Canada did good work on soil organic matter. We need 1. Methods to estimate all forms of carbo 2. Develop dynamical soil C models 3. Extend long term studies to deeper soil horizons. When we talk of soil science, chemistry and physics of it have to seen in tandem. I am excited by the debunking of earlier theories as much by quantum physics!

About 20 years ago I met a biogeochemist who'd had a go at dating SOM fractions and to his dismay found much younger ages than expected. In conversation he suggested that the system was indeed dynamic, with older material reacting with younger material. He didn't look at char. However if char is 'permanent' why are we not knee deep in it? Glaciation?

....among multiple publications your may check these: www.researchgate.net/publication/51699032_Persistence_of_Soil_Organic_Matter_as_an_Ecosystem_Property and www.publish.csiro.au/en/pdf/EN10006. Quantification is another issue; check for instance research published by Hammes et al (Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere - Hammes - 2007 - Global Biogeochemical Cycles - Wiley Online Library) and Roth et al (www.sciencedirect.com/science/article/abs/pii/S0146638012000277?via%3Dihub).

As I understand, we have to characterize at least four different pools of carbon in soil-1. Calcium carbonate form (from geogenic , pedogenic and biogenic sources) of inorganic carbon 2. Active or labile pool of organic carbon 3.Humic carbon /stable/resistant pool of organic carbon and 4. Inert/humin carbon

. Relatively calcium carbonate pool may be very stable pool, may be subjected to change under acidification processes/conditions.The inert carbon like charcoal or any fire derived carbon and also possibly the humin carbon in the clay matrix of soil may be very stable.The labile or active pool is very dynamic and soil microbes use it for their carbon, energy and nutrient source using decomposition and mineralization processes.The plants also use the nutrients derived from active fraction by the processes mentioned earlier. So among the organic pools, it is the humic/stable/resistant pool which is considered stable and important in soil-water-plant system.The stable fraction which plays role in soil structure, nutrient retention and release, interaction with pesticides etc.This fraction also complexes with clay and sesquioxides in soil.The complexing mechanisms also offer stability to this carbon.It is the stable or humic carbon which plays greater role in soil based ecosystems through several reactions and processes.This fraction needs thorough new investigation and re-examination of the previous work including its structure ,functional groups ,functions and processes.

Paul Milham Microbes may attack the stable pool of carbon if there is no much fresh carbon input in to soil or when there is a severe depletion of nutrient like N in soil under exhaustive cropping without fertilization. However , a small fraction of even the so called stable carbon may be decomposed and mineralized every year under the influence of soil microbes.But that may be compensated by formation of fresh stable carbon from the labile/active fraction on decomposition of easily decomposable portion.

My microbiologist colleagues also speak of new metabolisable C 'priming' the decomposition of older 'tougher' residues

I find the following paper has relevance to the present discussion.

Dynamic stability of soil carbon: Reassessing the" permanence" of soil carbon sequestration. Front.Environ.Sci.13 November 2020

https.//doi.org/10.3389/fenvs.2020.514701

Soil organic carbon improves the physical properties of the soil. It increases the cation exchange capacity (CEC) and the water-holding capacity and it contributes to the structural stability of clay soils by helping to bind particles into aggregates. Carbon is the main component of soil organic matter and helps give soil its water-retention capacity, its structure, and its fertility. On average, plants allocate 76% of carbon stocks to shoots and only 24% to the roots. However, this can vary considerably due to differing climates and environmental conditions and to understand that why soil organic carbon whether in humus or stable organic matter fraction is not stable. Are the soil microbes so powerful to decompose and consume every organic material? However, a small fraction of even the so called stable carbon may be decomposed and mineralized every year under the influence of soil microbes. Therefore, an integral to any approaches to promote carbon sequestering practices in managed soils are reliable, accurate and cost-effective means to quantify soil C stock changes and forecast soil C responses to different management, and edaphic conditions that may be compensated by formation of fresh stable carbon from the labile/active fraction on decomposition of easily decomposable portion

you can consult this site:

Factors controlling soil organic carbon stability...

https://www.nature.com › scientific reports › articles

by Q Tian · 2016