Dynamics of dissolved organic 14C in throughfall and soil solution of a Norway spruce forest

Dissolved organic carbon (DOC) is an important component of the C cycle in forest ecosystems, but dynamics and origin of DOC in throughfall and soil solution are yet poorly understood. In a 2-year study, we analyzed the radiocarbon signature of DOC in throughfall and soil solution beneath the Oa horizon and at 90 cm depth in a Norway spruce forest on a Podzol soil. A two-pool mixing model revealed that throughfall DOC comprised mainly biogenic C, i.e. recently fixed C, from canopy leaching and possibly other sources. The contribution of fossil DOC from atmospheric deposition to throughfall DOC was on average 6% with maxima of 8-11% during the dormant season. In soil solution from the Oa horizon, (DOC)-C-14 signature was highly dynamic (range from -8aEuro degrees to +103aEuro degrees), but not correlated with DOC concentration. Radiocarbon signatures suggest that DOC beneath the Oa horizon originated mainly from occluded and mineral associated organic matter fractions of the Oa horizon rather than from the Oi or Oe horizon. Relatively old C was released in the rewetting phase following a drought period in the late summer of 2006. In contrast, the (DOC)-C-14 signature indicated the release of younger C throughout the humid year 2007. In soil solutions from 90 cm depth, (DOC)-C-14 signatures were also highly dynamic (-127aEuro degrees to +3aEuro degrees) despite constantly low DOC concentrations. Similar to the Oa horizon, the lowest (DOC)-C-14 signature at 90 cm depth was found after the rewetting phase in the late summer of 2006. Because of the variation in the (DOC)-C-14 signatures at this depth, we conclude that DOC was not equilibrated with the surrounding soil, but also originated from overlaying soil horizons. The dynamics of (DOC)-C-14 in throughfall and soil solution suggest that the sources of DOC are highly variable in time. Extended drought periods likely have a strong influence on release and translocation of DOC from relatively old and possibly stabilized soil organic matter fractions. Temporal variations as well as the input of fossil DOC needs to be considered when calibrating DOC models based on (DOC)-C-14 signatures.