Old organic carbon in soil solution DOC after afforestation -: evidence from 14C analysis

We analysed the C-14 signal in soil solution DOC from the transition zone between the A(p) and B horizon at 12 sites, representing an afforestation chronosequence ranging from agricultural land to 89-year-old first generation Norway spruce forests. Although DOC concentrations had high spatial and temporal variation, the C-14 signal in the DOC was consistent over two separate sampling periods for the individual plots. This indicates that the origin of DOC in the lower part of the A horizon is the same for each individual plot despite variations in DOC concentration in the soil solution. The DOC from the agricultural land had a C-14 signal close to contemporary atmospheric values; mean C-14 in DOC for 1999 and 2001 was 108 and 107% abs. modem, respectively. After establishment of forest, the C-14 values in DOC fall to around 100% abs. modem. This provides evidence that a large part of the DOC in the younger stands is derived from older, pre-bomb carbon. The C-14 enrichment gradually increased with stand age due to an increasing influence of decaying litter containing younger carbon, coming from that assimilated by the forest during the period with elevated C-14 concentrations in the atmosphere. The average C-14 concentration in DOC from the oldest stand was 111% abs. modem. Even if the younger carbon gradually influenced the proportion of C-14 in DOC when stand age increased, a substantial part of the DOC must still originate from SOM formed before afforestation. It is hypothesised that C-14 enrichment changes along the chronosequence is explained by two simultaneous processes, changes in input of litter and an increased mobilisation of older SOM from the A horizon due to the change in land-use. (c) 2005 Elsevier B.V. All rights reserved.