The dynamic exchange of dissolved organic matter percolating through six diverse soils

The movement of dissolved organic matter (DOM) through forest soils is regulated by a suite of physicochemical and biological processes that retain, transform, and release DOM. While sorptive processes are known to limit DOM losses, there are still uncertainties about what regulates DOM composition. This study examined DOM dynamics in waters percolating through ex-situ soil cores from six diverse forest soils to determine if DOM leaching losses reflected dynamic exchange processes between fresh DOM inputs and soil surfaces or the continual stripping of surface-reactive compounds from recent DOM inputs. There was a net desorption of hydrophilic compounds into soil solutions after 10 cm soil depth that coincided with an increase in DOM biodegradability as solutions percolated to depth. There was also a limited net retention of dissolved organic nitrogen (DON) in surface soils. Taken together, these results support a dynamic exchange model of DOM dynamics where highly sorptive, hydrophobic compounds displace previously sorbed, N-rich hydrophilic compounds from soil surfaces. These soils also demonstrated fairly consistent leaching losses of dissolved organic carbon (DOC) and DON despite their variation in texture, hydraulic conductivity, and Fe and Al mineralogy, removing 72–85% of the DOC added by 50 cm depth. The strong sorption capacity of these soils may be one reason for the fairly uniform DOM chemistry leaching from these soil cores.