Modelling the production and transport of dissolved organic carbon in forest soils B. MICHALZIK 1,2,7 , E. TIPPING 1, *, J. MULDER 3 , J.F. GALLARDO LANCHO 4 , E. MATZNER 2 , C.L. BRYANT 5 , N. CLARKE 6 , S. LOFTS 1 and M.A. VICENTE ESTEBAN 4 1 Centre for Ecology and Hydrology (Windermere), Ambleside, LA22 0LP, Cumbria, UK; 2 Bayreuth In- stitute of Terrestrial Ecosystem Research, University of Bayreuth, Germany; 3 Department of Soil and Water Sciences, Agricultural University of Norway, Ås, Norway; 4 Consejo Superior de Investigaciones Cientificas, Salamanca, Spain; 5 Natural Environment Research Council Radiocarbon Laboratory, East Kilbride, Scotland, UK; 6 Norwegian Forest Research Institute, Ås, Norway; 7 Current address: Institute for Waste Management and Contaminated Site Treatment, Dresden Technical University, Pratzschwitzer Str. 15, Pirna, 01796, Germany; *Author for correspondence (e-mail: et@ceh.ac.uk; phone: ++ 44 (0)15394 42468; fax: ++44 (0)15394 46914) Received 5 September 2001; accepted in revised form 29 November 2002 Key words: Dissolved organic carbon, Fluxes, Forest, Humic substances, Isotopes, Soil Abstract. DyDOC describes soil carbon dynamics, with a focus on dissolved organic carbon (DOC). The model treats the soil as a three-horizon profile, and simulates metabolic carbon transformations, sorption reactions and water transport. Humic substances are partitioned into three fractions, one of which is immobile, while the other two (hydrophilic and hydrophobic) can pass into solution as DOC. DyDOC requires site-specific soil characteristics, and is driven by inputs of litter and water, and air and soil temperatures. The model operates on hourly and daily time steps, and can simulate carbon cycling over both long (hundreds-to-thousands of years) and short (daily) time scales. An important feature of DyDOC is the tracking of 14 C, from its entry in litter to its loss as DO 14 C in drainage water, enabling information about C dynamics to be obtained from both long-term radioactive decay, and the charac- teristic 14 C pulse caused by thermonuclear weapon testing during the 1960s (bomb carbon). Param- eterisation is performed by assuming a current steady state. Values of a range of variables, including C pools, annual DOC fluxes, and 14 C signals, are combined into objective functions for least-squares minimisation. DyDOC has been applied successfully to spruce forest sites at Birkenes (Norway) and Waldstein (Germany), and most of the parameters have similar values at the two sites. The results in- dicate that the supply of DOC from the surface soil horizon to percolating water depends upon the con- tinual metabolic production of easily leached humic material. In contrast, concentrations and fluxes of DOC in the deeper soil horizons are controlled by sorption processes, involving comparatively large pools of leachable organic matter. Times to reach steady state are calculated to be several hundred years in the organic layer, and hundreds-to-thousands of years in the deeper mineral layers. It is estimated that DOC supplies 89% of the mineral soil carbon at Birkenes, and 73% at Waldstein. The model, param- eterised with steady state data, simulates short-term variations in DOC concentrations and fluxes, and in DO 14 C, which are in approximate agreement with observations. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. Biogeochemistry 66: 241–264, 2003.