Mineralization of dissolved organic carbon (DOC) and nitrogen (DON) in the soil solution of different forested stands in Flanders

Dissolved organic carbon (DOC) and nitrogen (DON) play an important role in C and N cycles in natural ecosystems. Leaching of these compounds to ground and surface waters result in loss of nutrients from the ecosystem and in eutrophication waters. Even if sorption to mineral soil is considered to be the main regulating mechanism for DOC and DON leaching (Qualls and Haines, 1992), mineralization is an important process in DOC and DON removal from the percolating soil solutions. The aim of this study is to determine to what extent mineralization is involved in the removal of DOC and DON from the percolating soil solution in forest ecosystems in Flanders.
Samples of forest floor leachate, soil solutions at different depths in the mineral soil, undisturbed soil samples and inoculum samples were collected from three different sites in the Forest Reserve of Ravels, situated in northern Antwerp (Belgium).
Forest floor leachate and soil solution collected from the field and mineral soil leachate (BC-horizon) collected in the lab were filtered through a 0.45 m filter and adjusted to a common initial DOC concentration of 60 ppm. Then, the soil solutions were dialysed in order to remove most dissolved inorganic nitrogen (NO3- + NO2- + NH3/NH4+), inoculated and incubated at 15°C for 84 days. Filtered and unfiltered subsamples were analysed in order to determine total organic carbon (TOC), total nitrogen (TN) and dissolved inorganic nitrogen (DIN) concentrations.
Both DOC and DON mineralization were found to be dependent on soil depth, vegetation type and N-deposition.
Within the mineral soil mineralization rates of DON decreased with depth below the upper A horizon, while for DOC there was a tendency for mineralization rates to increase below the A horizon. Whereas in the humus percolate the higher DOC mineralization rate was observed, DON concentration increased, meaning that dissolution prevailed over mineralization.
DOC decrease was higher in the solution colleted from Corsican pine vegetated sites than in the silver birch vegetated plot.
The observed DOC degradation rate was quite higher in the soil solution collected from the site with background N-deposition than for the plot affected by high N-deposition. Whereas no DON removal was detected in the case of elevated N-deposition, a relevant DON increase was observed in the solution from the site with background N-deposition.
After all, DOC and DON biodegradability was low (2-24 %) for all the solutions, meaning that mineralization is likely to be a minor process in removal of DOC and DON from mineral soil compared to adsorption interactions.