I may not be able to give an exact answer however what I know by my own experiments is that > 10k old SOC could mineralize in presence of plants. For me it implies the decomposition of existing "recalcitrant" SOC is possible with supply of external OM. So yes recalcitrant OM input would induce increased fertility sooner or later, just question of time.

Yes it will decompose slower but how that would affect soil fertility thinking of agriculture production? If you modify the original OM, even the labile one that stabilizes the SOM through microbial decomposition, you can modify the microorganisms linked to that SOM that together with the environmental and climatic conditions define the upper vegetation. I have serious doubts the concept of recalcitrance be ecological and I am not sure if we are really evaluating the impact of doing that in long term. The consequences could be harmful.

I wonder if an analogy to what happens with time in a no-till system might be appropriate? Initially as one goes from conventional to no-tillage, N fertilizer requirements are somewhat higher than with conventional tillage. This is because OM decay is slowed with no-till, thereby also slowing N mineralization from OM. However, the buildup of OM over time with no-till often more than compensates for the slower N mineralization rate, resulting in lowered N fertilizer requirement with time compared to conventional. This might also apply to addition of more recalcitrant materials. Even though per unit of OM, N mineralization is slower, the greater number of overall units compensates.

Recalcitrance is usually defined as the resistence of SOM to microbial degradation. It is supossed to be degraded at lower rates than labile SOM and to yield higher biological stabilization. What I am seeing is that even SOM considered recalcitrant has carbohydrates which are degraded by a less efficient metabolism. As a consequence the SOM turnover is more stable and does not reach a degree of reduction as high as the one given by fast decomposition of available carbohydrates. This response is obtained under a new method and makes me feel confuse. Even studying different chronosequences after land use change, I have found in all of them (from different climate regimes) to tend to sequester labile SOM accompanied with higher biological stabilization in terms of CO2 rates per unit C. Then I am finding also that those rates are just a function of the carbohydrates percentage in SOM. So, I do not think CO2 rate per unit soil C can be linked to biological stabilization but to higher or lower carbohydrate quantity. It seems that the labile SOM can reach a higher degree of reduction state with more afinity to create bonds with the soil mineral matrix enhancing physical protection than that considered recalcitrant.

I think so. Guess maybe chemical composition is not so important and physical protection is the main ruler at the end

Yes sure! That is the reason of my question. I see it can be really important to keep the labile SOM fractions to get SOM stabilization

It probably depends on the mineral composition of the soil since non-biological oxidation of humic acids is catalyzed by Fe(III) and Mn(IV).