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Abstract

Dissolved organic matter affects fundamental biogeochemical processes in the soil such as nutrient cycling and organic matter storage. The current paradigm is that processing of dissolved organic matter converges to recalcitrant molecules (those that
resist degradation) of low molecular mass and high molecular diversity through biotic and abiotic processes. Here we demonstrate
that the molecular composition and properties of dissolved organic matter continuously change during soil passage
and propose that this reflects a continual shifting of its sources. Using ultrahigh-resolution mass spectrometry and nuclear
magnetic resonance spectroscopy, we studied the molecular changes of dissolved organic matter from the soil surface to 60 cm
depth in 20 temperate grassland communities in soil type Eutric Fluvisol. Applying a semi-quantitative approach, we observed
that plant-derived molecules were first broken down into molecules containing a large proportion of low-molecular-mass compounds.
These low-molecular-mass compounds became less abundant during soil passage, whereas larger molecules, depleted
in plant-related ligno-cellulosic structures, became more abundant. These findings indicate that the small plant-derived molecules
were preferentially consumed by microorganisms and transformed into larger microbe-derived molecules. This suggests
that dissolved organic matter is not intrinsically recalcitrant but instead persists in soil as a result of simultaneous consumption, transformation and formation.