The error begins when the author writes:
"DeAngelis knew that microbes often gain new abilities under stress. She found that the microbes in the warmed plots were better at digesting cellulose, hemicellulose, and lignin—tough, mealy molecules known as “recalcitrant carbon”...."
First off, either the author or (less likely) DeAngelis mistakenly term cellulose, hemicellulose, and lignin "recalcitrant carbon." I would call lignin labile but others consider it polymeric and therefore recalcitrant. Clearly a tree that falls in the forest contains all these things and will decay away at a rate determined by that specific ecosystem. I would not call that recalcitrant carbon. This misuse of the term recalcitrant has been under active discussion for at least ten years in the peer literature, such as here: DOI: 10.1071/EN10006
A better and more widely used standard for determining recalcitrance today is whether the carbon decomposes over time in any significant way, and that definition is not dependent on temperature or other soil variables, such as the desperate or mutant soil biota described in the article. Using that definition, which is straightforward, biochar would be termed recalcitrant but none of the carbon types described in the Harpers article would.
Two additional points about biochar should also be made. First, when photosynthetic origin carbon structures are taken to >500C they produce chain and ring structures as C binds to C (very few elements can do this). These bonds are too strong for quick microbial or fungal disintegration, although some very slow breakdown will occur over time, owing to a variety of factors. Second, examples such as the Amazonian Dark Earths have demonstrated man-made biochar's recalcitrance over some 8000 years in the equatorial tropics, and examples of naturally-occurring biochar from wildfire can be accurately dated hundreds of millions of years, over which timespan there have been a number of warmer earth events.
While the Harper's piece is interesting and informative, I am grateful it never mentions biochar, lest it lead to an unwarranted association of biochar's recalcitrance with the lignous material they were actually exploring.
My takeaway is that the Harvard forest studies have done nothing to diminish the hope for pyrolytic biomass-to-biochar carbon capture and soil sequestration (PyCCS) integrated into mixed-age mixed-species reafforestation and fire-prevention management at scale, as I and others have described since at least the mid-1970s, that for me hold the greatest promise for reversing climate change in the shortest time.