The colloidal fraction of dissolved organic matter extracted from a forest soil persists microbial decomposition

We have investigated the bacterial decomposition of dissolved organic matter (DOM) extracted from the organic layer of a boreal forest soil and filtered at a pore size of 0.2 µm. This DOM source has previously been extensively characterized and contains approximately equal amounts by carbon of a colloidal fraction, mainly composed of carbohydrates, and a fraction of molecularly dissolved DOM. Here, extracts were inoculated with soil bacteria and the decomposition of DOM was followed over a period of 2 months, during which it was analyzed with scattering methods and ¹H NMR, and by measuring the concentration of total organic carbon. A comparison was also made with dialyzed extract. Results showed that while the bacteria fully decomposed the molecular fraction within approximately two weeks, the colloidal fraction was stable with no visible decomposition within the 2 months. The results indicate the importance of distinguishing small molecules from colloidal aggregates in decomposition studies, and demonstrate the usefulness of combining scattering methods with ¹H NMR for this purpose.