Decomposition of 13C-labelled leaf and twig litter

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Biogeosciences Discussions This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Very few field studies have quantified the different pathways of C loss from decomposing litter even though this is essential to better understand long-term dynamics of C stocks in soils. Using 13 C-labelled leaf (isotope ratio (δ 13 C) = −40.8‰) and twig litter (δ 13 C = −38.4‰), we tracked down the litter-derived C in the soil respiration, in the 5 dissolved organic C (DOC) and in the soil organic matter of a beech forest in the Swiss Jura. After one year of decomposition, mass loss in the litter layer was almost twice as great for leaves as it was for twigs (75% vs. 40%). This difference was not the result of a slow mineralisation of the woody litter, but primarily of the only slight incorporation of twig-derived C into mineral soils. The C mineralisation rates of the twig litter were 10 only slightly lower than those of the leaf litter (10–35%), in particular after the loss of the readily available litter fraction. However, the leaching of DOC from twigs amounted only to half of that from leaves. Tracing the litter-derived DOC showed that DOC from both litter types was mostly retained (88–96%) and stabilised in the top centimetres of the mineral soil. In the soil organic C at 0–2 cm depth, we recovered 8% of the 15 initial leaf C, but only 4% of the twig C. Moreover, the 13 C mass balance suggested that a substantial fraction of the leaf material (∼30%) was transported via soil fauna to soil depths below 2 cm, while the twig litter mainly decomposed in situ on the soil surface, probably due to its rigid structure and low nutritional value. In summary, our study shows that decaying twigs are rapidly mineralised, but seem to be clearly less 20 important for the C storage in this beech forest soils than leaf litter.