Is Backfilling a Sustainable Alternative to Reduce CO2 Emissions from Swiss Degraded Peatlands?

Peat soils in Western Europe play a crucial role in carbon storage and agriculture. However, these two functions are often incompatible, as draining of peatlands, to convert them into agricultural land, leads to emissions of stored carbon, turning these carbon reservoirs into significant carbon sources. In the Three Lakes region of Switzerland, peatlands have been drained for agriculture for the last about 100–150 years. While drainage has improved agricultural use of these peatlands, it has also accelerated peat decomposition, leading to the loss of more than 2 meters of peat thickness and causing substantial CO₂ emissions.

Currently, there are no effective and sustainable measures to regenerate peatlands, aside from reflooding. In Switzerland, backfilling has emerged as an alternative approach to potentially reduce CO₂ emissions without ceasing agricultural activities. Backfilling involves the deposition of mineral material from various sources onto the soil to disconnect the peat from the surface, thereby maintaining agricultural production while protecting the already degraded organic soils.

This method has been used for over 50 years in the region to improve access for machinery in areas prone to waterlogging caused by peat mineralization, but little research has been conducted on its long-term effects on the carbon cycle or overall soil functioning. With this study, we aim at better understanding the impact of backfilling on the carbon cycle in managed peatlands. To achieve this, we measured CO₂ emissions and their radiocarbon content (¹⁴CO₂) at three locations in the Three Lakes region to assign the source of the respired organic C. In addition, the quality (DRIFT) of soil carbon from drained and drained-backfilled peat soils was determined.

Initial summer measurements showed that CO₂ emissions were over 40% higher in drained peatlands compared to their backfilled counterparts. The ¹⁴C content of the carbon respired also differed, with older carbon released from the original peatlands (up to -193 ‰, indicative of ~ 1,500 years) than from the backfilled sites (up to -115 ‰, ~ 800 years). Incubation experiments revealed that CO₂ emissions predominantly originated from deeper horizons (>40 cm), which were richer in carbon and less degraded. Comparing the original drained peat to the peat buried beneath the backfilling, we observed lower carbon content and fewer easily degradable compounds in the buried peat. Compounds such as aliphatics were largely replaced by more resistant materials, like phenolics. The difference in emissions is then, primarily attributed to the quality and quantity of the remaining carbon, which is mainly dependent on the state of peat degradation at the time the backfilling was implemented. These findings highlight the critical role of the quality and quantity of the remaining carbon stock in this system. While backfilling may help reduce CO₂ emissions by altering carbon availability in peat soils, it cannot fully stop the degradation process. Further research is needed to investigate spatio-temporal variability, potential peat compaction, and the influence of factors such as the groundwater table and the composition of the mineral layer.