We studied the properties of dissolved organic matter (DOM) in drained boreal peatland in stem-only-harvested (SOH), whole-tree-harvested (WTH), and unharvested sites. The DOM derived from both aerobic and anaerobic peat layers was divided into four different molecular-weight fractions with ultrafiltration, and the fractions were measured for the concentrations of dissolved organic C (DOC), N, and carbohydrates, aromaticity (specific ultraviolet [UV] absorbance at 254 nm), pH, and bioavailability to bacteria. The percentage of DOC in the low-molecular-weight (LMW) fraction was higher in the deeper than in the upper layer. We suggest that easily degradable LMW compounds with low aromatic character and high bioavailability had not degraded in the deeper layer under anaerobic conditions. This had produced a relative enrichment of LMW DOC in the anaerobic layer. A different relation to aerobicity was observed in alternating aerobic conditions in the upper layer alone, where the high-molecular-weight compounds seemed to increase under more anaerobic conditions. Thus the properties and processes of DOM in peatlands seem to be controlled by the aerobicity in a complicated manner. We suggest that operations that affect the water table level in peatland affect also the properties of peat DOM. The properties of DOM that could be connected to clear-cut harvest-induced changes in the recent inputs of C and N were small. Peat DOM was more N rich in harvested than in control sites. Thus it seems possible that N is susceptible to leaching after harvest in naturally relatively N-rich sites and especially in mineral form.

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