Dissolved organic carbon in seepage water - Production and transformation during soil passage

Dissolved organic carbon (DOC) in seepage water can combine with organic po llutants, with Al and heavy metal ions and transport them through the soil profile with a potential to contaminate groundwater. We studied the product ion of DOC in aerobic decomposition experiments at 8 degrees C and moisture close to field capacity in soils from two sites with different microbial a ctivities (spodic dystric Cambisols with moder (SLB) and mor-moder (SLS) la yers) using C-13-depleted plants of differing decomposability (Epilobium an gustifolium and Calamagrostis epigeios). Additionally, we investigated the DOC transformation during soil passage in decomposition experiments and in the field for the sites SLB and SLS. For SLS, decomposition of Epilobium re sulted in a cumulative CO2 production of 14% of the added C within 128 days . Priming effects were negligible. CO2: production for the experiments usin g Calamagrostis was less with 11% for SLB and 10% for SLS. Cumulative DOC p roduction was markedly high in the Epilobium decomposition experiment, bein g 25 g m(-2), out of which 11 g m(-2) were Epilobium-derived (2% of the add ed C). For the Calamagrostis experiments, cumulative productions of DOC and Calamagrostis-derived DOC (0.1% of the added C for SLS and SLB) were much less. During the soil passage, much of the DOC was removed by sorption or d ecomposition processes. Field studies at SLS and SLB using C-13 natural abu ndance showed that C-13 distribution of soil organic matter increased with depth, probably mainly due to a discrimination of C isotopes by decomposing microorganisms. DOG, however, showed a depletion of C-13 from -28 parts pe r thousand PDB to -29 parts per thousand (SLB at 40 cm) or --28 to -30 part s per thousand (SLS at 20 cm) with depth, owing to preferential decompositi on of C-13-enriched substances or preferential adsorption. This study indic ates that DOC production is strongly affected by litter composition and tha t significant changes in DOC composition may occur during its passage throu gh a soil depth of 40 cm.