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

Andosols are among the most carbon rich soils, with an average of 254 Mg ha−1 organic carbon (OC) in the upper 100 cm. A current theory proposes an upper limit for OC stocks independent of increasing carbon input. This is assigned to finite binding capacities for organic matter (OM) of the soil mineral phase. We tested the possible limits in OC stocks for Andosols with already large OC concentrations and stocks (210 g kg−1 in the first horizon; 320 Mg ha−1 in the upper 100 cm). The soils received large inputs of 1800 Mg OC ha−1 as sawdust within a time period of 20 years. Adjacent soils without sawdust application served as controls. We determined total OC stocks as well as the storage forms of OM of five horizons down to 100 cm depth. Storage forms considered were pyrogenic carbon, OM of 2.0 g cm−3. The two fractions > 1.6 g cm−3 were also analyzed for Al-organic matter complexes (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 thus, contributed little to soil OM. In the two topsoil horizons, the fraction between > 1.6 and 2.0 g cm−3 had 65–86 wt % of bulk soil OC and were dominated by Al-OM complexes. In deeper horizons, the fraction > 2.0 g cm−3 contained 80–97 wt % of bulk soil's total OC and was characterized by a mixture of Al-OM complexes and imogolite-type phases, with proportions of imogolite-type phases increasing with depth. In response to the sawdust application, only the OC stock in 25–50 cm depth increased significantly (P = 0.05). The increase was entirely due to increased OC in the two fractions > 1.6 g cm−3. However, there was no significant increase in the total OC stocks within the upper 100 cm. We assume, the topsoil is saturated in terms of OC concentrations, and thus, added OC partly migrates downwards, where it becomes retained by OC-undersaturated minerals. This indicates the possibility to sustainably increase already large OC stocks further, given that the subsoil still has binding capacity and OC transport into deeper horizons is facilitated. The little additional OC accumulation despite the extremely large OC input over 20 years, however, shows that long time periods of high input are needed to promote the downward movement and deep soil storage of OC.