Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation, physico-chemical retention or preferential flow?. (September 2015) - Hagedorn, Frank
Abstract: 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 14 C-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 14 C label in mineralized and leached DOM and quantified the spatial distribution of DO 14 C retained in soils using a new autoradiographic technique. Leaching of DO 14 C through the 10 cm-long soil columns amounted up to 28% of the added DO 14 C in the initial soils, but to less than 5% in the developed soils. Biodegradation hardly contributed to the removal of litter-DO 14 C as only 2–9% were mineralized, with the highest rates in mature soils. In line with the mass balance of 14 C fluxes, measured 14 C activities in soils indicated that the major part of litter DO 14 C was retained in soils (>80% on average). Autoradiographic images showed an effective retention of almost all DO 14 C in the upper 3 cm of the soil columns. In the deeper soil, the 14 C label was concentrated along soil pores and textural discontinuities with similarly high 14 C 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 DO 14 C correlated negatively with oxalate-extractable Al, Fe, and Mn. In conjunction with the rapidity of DO 14 C immobilization, this strongly suggests that sorptive retention DOM was the dominating pathway of litter-derived DOM in topsoils, thereby contributing to SOM stabilization. Graphical abstract: Highlights: Addition of 14 C-labelled litter DOM to soils sampled along a glacier forefield. 28% of added DO 14 C was leached in initial but less than 5% in mature soils. Biodegradation hardly contributed to the removal of litter-DO 14 C (<10% of added). Autoradiography showed DOM transport via preferential flow. Most of litter DO 14 C (>80%) was retained in soils, probably by sorptive retention.