The potential of reed canary grass and the importance of field heterogeneity for reducing GHG emissions in a rewetting fen peatland

Abstract. Rewetting drained peatlands can reduce CO2 emissions but prevents traditional agriculture. Crop production under rewetted conditions may continue with flood-tolerant crops in paludiculture, but its effects on greenhouse gas (GHG) emissions compared to rewetting without further management are largely unknown. This study was conducted between 2021 and 2022 on a fen peatland in central Denmark established with Phalaris arundinacea L. (Reed Canary Grass) in 2018. Three harvest/fertilization management treatments (0, 2, and 5-cut) were applied with the 2-cut and 5-cut treatments receiving 200 kg N ha−1 yr−1 in equal split doses, whereas the 0-cut remained unfertilized. Measurements of CO2 and CH4 emissions were conducted biweekly under four different light intensities using a manual chamber connected to a gas analyzer. Although the mean annual water table depth (WTD) was −8 cm, indicating a rather wet peatland, the site remained a CO2 source with a mean net ecosystem C balance (NECB) of 6.6 t C ha−1 yr−1 across treatments. Methane emissions averaged 90 kg CH4-C ha−1 yr−1, equivalent to 11.7 % of NECB given as CO2 equivalents. Results showed that management marginally increased biomass production reflected by more negative gross primary productivity (GPP) in 2-cut and 5-cut compared to 0-cut. No significant treatment effect was found on NECB due to field heterogeneity reflected by differences in pore water nutrient concentrations and WTD dynamics among the studied blocks, with higher Reco corresponding to blocks where higher pore water nutrient concentrations were observed. The results indicated that GHG emissions might potentially be reduced when the biomass is harvested from the more productive peatland area in comparison with no management, whereas on the less productive area it might be beneficial to leave the biomass unmanaged. Model simulation of ecosystem respiration (Reco) using WTD data of high temporal resolution captured the variability better as compared to the use of mean annual WTD, which underestimated Reco by 18 % on average compared to the hourly WTD model. Data on pore water chemistry further improved statistical linear models of CO2 fluxes using soil temperature (Ts), WTD, ratio vegetation index (RVI) and photosynthetic active radiation (PAR) as explanatory variables. Overall, from a climate perspective the study supported biomass production compared to no management activity in rewetted fertile peatlands.