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

We analysed the 14C signal in soil solution DOC from the transition zone between the Ap 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 14C 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 14C signal close to contemporary atmospheric values; mean 14C in DOC for 1999 and 2001 was 108 and 107% abs. modern, respectively. After establishment of forest, the 14C values in DOC fall to around 100% abs. modern. This provides evidence that a large part of the DOC in the younger stands is derived from older, pre-bomb carbon. The 14C 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 14C concentrations in the atmosphere. The average 14C concentration in DOC from the oldest stand was 111% abs. modern. Even if the younger carbon gradually influenced the proportion of 14C in DOC when stand age increased, a substantial part of the DOC must still originate from SOM formed before afforestation. It is hypothesised that 14C 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.