Looks like I may need to redraw this graph. A new peer-reviewed paper points to biochar being permanent carbon storage with just a small part decomposing. Previous meta-studies have estimated that around 20% of the carbon in biochar is lost after 100 years and half after 500 years. It is challenging to prove empirically since the decay you see in the first years may be just the volatile parts being lost, it does not necessarily say anything about the fate of the bulk of the char. The new paper uses a novel method to test the stability and proposes a new test to determine permanence, "Random reflectance" (Ro).  It finds that most biochar is actually "inertinite", stable for millions of years. The transformation of biomass to inertinite is one of the two main natural pathways exerted by Earth to permanently store organic carbon, mineralization being the other. The paradigm shift has started, but I would be curious to hear what other biochar researchers have to say.