Biodegradability of dissolved organic carbon in boreal peatland forest is affected by water table, labile carbon, and nitrogen availability but not forest harvesting alone

Around 30% of peatland in Nordic and Baltic countries has been drained for forestry. Drained peatlands are major sources of dissolved organic carbon (DOC) and nutrients to surface waters, contributing to global warming, eutrophication, and brownification. However, the effects of forest clearcutting and changes in the water table on the biodegradation of DOC to CO₂ are poorly known. We collected peat columns from drained, uncut, and clearcut forests for a common garden experiment and exposed them to high and low water tables to study the effects of clearcutting and water table levels on DOC production and biodegradation. ¹³C-labeled glucose was added to half of the columns to study the effects of labile carbon (C) addition on DOC dynamics. We measured the concentration, quality, and biodegradation rate of DOC monthly by incubating the column porewater at 15°C. Nitrogen (N) limitation of DOC biodegradation was studied by adding ¹⁵N-labeled glycine to half of the incubated water samples. DOC concentrations decreased in the columns with both low water table and glucose addition, while clearcutting had no clear effects. The biodegradation rate of recalcitrant DOC in the later stages of the incubation increased with glycine addition but was not affected by glucose or water table. The results suggest that the biodegradation of recalcitrant DOC in these drained peatland forests is N-limited and dependent on the quality of DOC, which can be seasonally variable.