Storm flow flushing in a structured soil changes the composition of dissolved organic matter leached into the subsoil

Dissolved organic matter increases typically in streams draining forested catchments during heavy rainstorms and snowmelt. Tracer methods and model calculations suggest that the storm flow flushing of dissolved organic matter is either due to lateral near-surface flow, i.e. within the organic forest floor, or preferential flow (funnelling) through the mineral soil. Both pathways should deliver forest floor-derived dissolved organic matter to streams that is hardly changed because of little to no interaction with mineral soil material and microorganisms. Here, we investigated the effect of rain storm induced vertical flushing through the mineral soil on the composition of dissolved organic matter in a structured Rendzic Leptosols under 90-year-old European beech (Fagus sylvatica L.). During two rainstorm periods in autumn 1998 with elevated transport of organic C, N, P and S from the forest floor into the subsoil, we sampled dissolved organic matter in forest floor leachates (sampled by zero-tension plate lysimeters), subsoil solutions (sampled by suction cups at 90 cm depth) and subsoil seepage (sampled by zero-tension plate lysimeters at 90 cm depth). The chemical composition of dissolved organic matter was characterised by fractionation with XAD-8 macroreticular resin, wet-chemical analyses of carbohydrates and lignin-derived phenols, and determination of the δ13C. During both rainstorm periods, all tested chemical features of dissolved organic matter in forest floor leachate and subsoil seepage matched each other greatly. In contrast, dissolved organic matter in soil solution contained smaller portions of XAD-8-adsorbable organic C, less lignin-derived phenols, more carbohydrates and showed smaller δ13C values than that in forest floor leachates and subsoil seepage. These results suggest a rather direct transfer of organic solutes from the forest floor into the subsoil and probably further to ground and surface waters during heavy rainstorms. Dissolved organic matter leaving the soil in heavy rainstorms by rapid water flow through macropores is likely less biodegradable, more UV-digestible and more reactive towards metals and organic pollutants than that released from soil at low rainfall intensity by matric flow.