Long-term feedbacks result in the recovery of the CO2 sink in a remnant peatland following water table lowering

Peatland biological, physical and chemical properties change over time in response to the long-term water table position. Such changes complicate predicting the response of peatland carbon stocks to sustained drying. Here we use Eddy Covariance measurements of CO₂ exchange to study the effect of sustained water table lowering on peatland carbon dynamics. We compare measurements from a near-pristine peatland with those of a drying remnant, both raised bogs dominated by Empodisma robustum (Restionaceae), across two different time periods separated by a 16-year interval. We found that the remnant bog was initially a source of CO₂ following water table lowering. However, the CO₂ sink recovered and strengthened after the 16-year interval between measurements. The increase in CO₂ sink strength in the remnant bog was primarily due to increased photosynthetic uptake of CO₂, which exceeded that of the near-pristine site in both time periods. Additionally we found the loss of CO₂ via ecosystem respiration to have declined with time, however, ecosystem respiration remained elevated compared to the near-pristine site. These trends of increasing photosynthesis and declining ecosystem respiration resulted in the CO₂ sink in the dry bog reaching half the sink strength of the near-pristine bog. We consider two factors to have been key for the recovery of the CO₂ sink in the remnant bog. These were 1) resilience of the peat-forming plant community to water-table change and 2) the expansion of ericoid shrubs. Our results demonstrate that the peatland carbon sink can recover from drying over a multi-decadal timescale, but questions remain as to the long-term trajectory of dry bogs and the stability of carbon fixed after water table lowering.