Net ecosystem carbon balance, emissions of methane and nitrous oxide and water quality in the first four years of rewetting a cultivated peat soil site for paludiculture

Cultivated peatlands are a major source of greenhouse gas (GHG) emissions and water pollution in northern Europe, and their future management is a key issue on the path to carbon neutral societies. Conventional cultivation requires drainage, and above the drainage depth all peat is prone to decomposition with the implication that these soils have the highest emission rates per area compared to any other land use. Paludiculture is a management option in which wet-tolerant crops are produced with raised ground water levels. It is thus a GHG mitigation method that allows for slowing down peat decomposition in drained peatlands while still maintaining agricultural income for the landowner. There are tradeoffs to consider when implementing paludiculture: 1) methane emissions rise with the switch of aerobic to anaerobic decomposition, 2) slowing down decomposition reduces nutrient mineralisation from the peat and compromises productivity and 3) harvesting reduces the potential to sequester carbon to the ecosystem compared to natural wetlands. 

We experimented paludiculture at a highly degraded peat site in southern Finland with plots of willow, forage, and mixed vegetation (set-aside). We recorded the yields, emissions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and auxiliary environmental data for four years, as well as nutrient content of the soil water for two years. We will present the results of these measurements, including estimates on the net ecosystem carbon balance of each crop based on empiric models.  

Raising the ground water level to the desired depth (-20 cm) turned out to be challenging. The mean annual ground water table levels during the four study years were about 80, 40, 40 and 30 cm (the measurements of the last year are still ongoing). The preliminary results suggest that even a slight raise of the ground water level was able to slow down CO2 emissions from soil respiration, while an increase in CH4 emission partly counteracted this benefit especially when the ground water level was above 30 cm. Nitrous oxide emissions were extremely high after the initial disturbance of the site but remained at a relatively low level after that. The results will be compared to an adjacent site with an annual crop, and paludiculture as a mitigation measure discussed.