Importance of root derived carbon for soil organic matter storage in a temperate old-growth beech forest – Evidence from C, N and 14C content

High carbon uptake of old-growth forests contradict the assumption that old-growth forests are carbon neutral and suggest their Kyoto relevance. As tree biomass is unlikely to store additional carbon in the long term we investigated carbon and nitrogen stock changes in the soil. Therefore we performed paired soil sampling in the years 2000 and 2004 at the National Park Hainich, Germany. In the upper 0–20 cm soil layer 498 g SOC m−2 was lost in 4 years, but interestingly in 20–60 cm soil depth 1155 g m−2 of SOC was stored, resulting in an accumulation rate of 164 g m−2 per year. In the same time nitrogen was accumulated at a rate of 32 g N m−2 per year. While in 0–20 cm preferential carbon was respired, in the deeper soil layers organic matter with a C/N ratio of microbial organisms was gained. Most interestingly 14C measurements indicate that gained and lost carbon is of recent carbon age.Our findings suggest that soil organic matter even in deeper soil horizons is very dynamic and that possibly root driven carbon input can significantly change soil organic matter stocks in all depth segments simultaneously. In consequence old growth forests still have a high soil organic matter storage potential as their roots sustain a long-lived permanent storage structure.

► High storage of C and N was found in the soil of an old-growth beech forest. ► C/N ratios and 14C contents explained the mechanisms of carbon sequestration. ► We suggest that SOM even in deeper soil horizons is very dynamic. ► Root/rhizosphere driven C and N input is most important for SOM storage. ► The drought period in summer 2003 most likely favored the subsoil SOM storage.