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. Andosols are among the most carbon-rich soils, with an average of254 Mg ha −1 organic carbon (OC) in the upper 100 cm. A currenttheory proposes an upper limit for OC stocks independent of increasing carboninput, because of finite binding capacities of the soil mineral phase. Wetested the possible limits in OC stocks for Andosols with already large OCconcentrations and stocks (212 g kg −1 in the first horizon,301 Mg ha −1 in the upper 100 cm). The soils received large inputsof 1800 Mg OC ha −1 as sawdust within a time period of 20 years.Adjacent soils without sawdust application served as controls. We determinedtotal OC stocks as well as the storage forms of organic matter (OM) of fivehorizons down to 100 cm depth. Storage forms considered were pyrogeniccarbon, OM of −3 density and with little to nointeraction with the mineral phase, and strongly mineral-bonded OM formingparticles of densities between 1.6 and 2.0 g cm −3 or> 2.0 g cm −3 . The two fractions> 1.6 g cm −3 were also analysed for aluminium-organic mattercomplexes (Al–OM complexes) and imogolite-type phases using ammonium-oxalate–oxalic-acid extraction and X-ray diffraction (XRD). Pyrogenic organic carbon represented only up to 5 wt % of OC, and thuscontributed little to soil OM. In the two topsoil horizons, the fractionbetween 1.6 and 2.0 g cm −3 had 65–86 wt % ofbulk soil OC and was dominated by Al–OM complexes.In deeper horizons, thefraction > 2.0 g cm −3 contained 80–97 wt % of thebulk soil's total OC and was characterized by a mixture of Al–OM complexesand imogolite-type phases, with proportions of imogolite-type phasesincreasing with depth. In response to the sawdust application, only the OCstock at 25–50 cm depth increased significantly ( α=0.05 , 1 - β = 0.8 ). The increase was entirely due to increased OC in the two fractions> 1.6 g cm −3 . However, there was no significant increasein the total OC stocks within the upper 100 cm. The results suggest that long-term large OC inputs cannot be taken up by theobviously OC-saturated topsoil but induce downward migration and graduallyincreasing storage of OC in subsurface soil layers. The small additional OCaccumulation despite the extremely large OC input over 20 years, however,shows that long time periods of high input are needed to promote the downwardmovement and deep soil storage of OC.

Massive carbon addition to an organic-rich Andosol increased the subsoil but not the topsoil carbon stock