Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation, physico-chemical retention or preferential flow?

The cycling of dissolved organic matter (DOM) in soils is controversial. While DOM is believed to be a C source for soil microorganisms, DOM sorption to the mineral phase is regarded as a key stabilization mechanism of soil organic matter (SOM). In this study, we added C-14-labelled DOM derived from Leucanthemopsis alpina to undisturbed soil columns of a chronosequence ranging from initial unweathered soils of a glacier forefield to alpine soils with thick organic. layers. We traced the C-14 label in mineralized and leached DOM and quantified the spatial distribution of (DOC)-C-14 retained in soils using a new autoradiographic technique. Leaching of (DOC)-C-14 through the 10 cm-long soil columns amounted up to 28% of the added (DOC)-C-14 in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-(DOC)-C-14 as only 2-9% were mineralized, with the highest rates in mature soils. In line with the mass balance of C-14 fluxes, measured C-14.activities in soils indicated that the major part of litter (DOC)-C-14 was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all (DOC)-C-14 in the upper 3 cm of the soil columns. In the deeper soil, the C-14 label was concentrated along soil pores and textural discontinuities with similarly high C-14 activities than in the uppermost soil. These findings indicate DOM transport via preferential flow, although this was quantitatively less important than DOM retention in soils. The leaching of (DOC)-C-14 correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of (DOC)-C-14 immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization. (C) 2015 Elsevier Ltd. All rights reserved.