Peat Quality and Enzymatic Activities Control the Recovery of Peatlands’ C Sink Function in Addition to Rewetting

Abstract

Due to land use intensification and drainage many peatlands have lost their C sink function. Consequently, rewetting has become an important strategy to mitigate increased greenhouse gas emissions from degraded peatlands. Whereas CO2 emissions decrease under reducing conditions upon waterlogging, CH4 production rates increase. The exact effect of rewetting may depend on the initial degree of degradation of a peatland and resulting peat quality. Therefore, the aim of this study was to elucidate waterlogging effects on C mineralization rates of peat from two contrasting sites. Near-surface peat soils from a long-term drained area and a rewetted site with newly formed floating mat were incubated under aerobic and anaerobic conditions for 90 days. CO2 and CH4 production rates were measured with weekly intervals. At the beginning and at the end of the incubation, liquid phase samples were taken and analysed for (in)organic ions, element stoichiometry, UV absorbance spectra and, DOC concentrations. When CO2 and CH4 production had reached steady states, we measured C-, N- and P-related hydrolytic enzyme activities of the peat. We expected that hydrolytic enzyme activities decrease, resulting in lower CO2 production rates, under anaerobic conditions. Furthermore, it was hypothesized that C mineralization rates of the pristine floating mat would exceed those of the degraded drained peatland due to higher availability of more labile organic matter in the former site. As expected, rewetting, as simulated by anoxic incubations, slowed CO2 production rates and activities of beta-glucosidase as compared to the oxic controls. Moreover, the availability of oxygen stimulated near-surface peat decomposition supported by a strong decrease in DOC concentrations after aerobic incubation in the degraded peat. However, the average rate of CO2 production was six times higher in the degraded drained site compared to the restored floating mat (189.84 and 29.76 μmol CO2 g dw-1 d-1, respectively). CH4 production from the long-term drained site began after 75 days of anoxic incubation and was almost negligible compared to the restored site (0.06 v. 0.46 g dw-1 d-1 after 75 days of incubation, respectively). Due to the high CO2 production rates measured at the drained site, it is unlikely that high peat recalcitrance was the cause of low CH4 production. In contrast to CO2 production rates, there were no significant differences in beta-glucosidase activities between the two sites. Probably other substrates than cellulose were involved in peat decomposition from the degraded site compared to decomposition of the floating mat. Therefore, this may either imply that degraded peat has an adapted community of microbes releasing enzymes that are able to breakdown a wide spectrum of organic sources, including aromatics. Or, alternatively, the build-up of phenolics in the Sphagnum-rich restored site inhibits hydrolytic enzyme activity and consequently leads to lower CO2 production rates. Thus, under anoxic conditions, overall low activities of hydrolytic enzymes partly supported the enzymatic latch paradigm. We have shown that rewetting slows CO2 production rates and may not result in immediate CH4 production. Moreover, peat quality and enzyme activities appear an important control on peatland restoration that requires further investigation.