Recovery of the CO2 sink in a remnant peatland following water table lowering

Abstract

Peatland biological, physical and chemical properties change over time in response to alterations in long-term water table position. Such changes complicate our ability to predict the response of peatland carbon stocks to sustained drying. In order to better understand the effect of sustained lowering of the water table on peatland carbon dynamics, we re-visited a drainage-affected bog, repeating eddy covariance measurements of CO2 flux after a 16-year interval. We found the ecosystem CO2 sink to have strengthened across the intervening period, despite a deep and fluctuating water table. This was mostly due to an increase in CO2 uptake through photosynthesis associated with increased shrub growth. We also observed a decline in CO2 loss through ecosystem respiration. These changes could not be attributed to environmental conditions. Air temperature was the only significant contemporaneous driver of monthly anomalies in CO2 fluxes, with higher temperatures decreasing the net CO2 sink via increased ecosystem respiration. However, the effect of air temperature was weak in comparison to the underlying differences between time periods. Therefore, we demonstrate that for drying peatlands, long-term changes within the ecosystem can be of primary importance as drivers of CO2 exchange. In this peatland, the ecosystem carbon sink has shown resilience to water table drawdown, with internal feedbacks leading to a recovery of the CO2 sink after a 16-year interval.