Two years of GHG emissions from reed canary grass under different harvest management intensities in a rewetting fen peatland

Drained agricultural peatlands are a large source of greenhouse gasses (GHGs) due to peat oxidation. Paludiculture, where flood-tolerant grasses are grown on rewetted peatlands, might be a potential strategy for climate change mitigation by reducing GHG emissions while maintaining biomass production. This study assessed the impact of different harvest and fertilization treatments of reed canary grass (Phalaris arundinacea, cv. Lipaula; RCG) on GHG exchange dynamics and global warming potential (GWP) in two measurement periods (5 May 2020 to 4 May 2021, and 18 May 2021 to 17 May 2022) at a fen with shallow water tables depths (annual mean WTD of -10 cm and -8 cm, respectively) and ca. 2 m deep peat. RCG was established in 2018 and in the following years management strategies with 2 or 5 cuts per year were compared with a non-harvested scenario (0-cut). Treatments involving 2 and 5 annual cuts were fertilized with 200 kg N ha^-1 yr^-1 in equal split doses for each cut while the 0-cut scenario remained unfertilized. Fluxes of CO₂, CH₄, and N₂O (only 2020-21) were measured with fortnightly intervals using the manual chamber technique and cumulative fluxes were derived by empirical models.

Yields of RCG decreased slightly over the years with 15.6, 11.5 and 8.9 t DM ha^-1 yr^-1 for the 2-cut system and 14.5, 9.4 and 8.6 for the 5-cut system in 2019, 2020 and 2021, respectively. Mean annual WTD of -13 cm in 2019 was slightly lower than the following years. In general, photosynthetic CO₂ uptake was higher in treatments with active biomass management, but carbon export in the harvested biomass offset this benefit, resulting in a near-equal net ecosystem carbon balance (NECB) across all treatments ranging from 36.0 to 43.6 and 17.1 to 28.2 t CO₂ ha^-1 yr^-1 in 2020-21 and 2021-22, respectively. The mean NECB of 22.5 t CO₂ ha^-1 yr^-1 in 2021-22 across treatments was significantly lower than the mean of 38.7 t CO₂ ha^-1 yr^-1 in 2020-21. This might be partly explained by the slightly increasing WTD due to lack of ditch maintenance and more precipitation, but the flux effect of increasing WTD on decreased peat oxidation may also be delayed by a few years. Emissions of CH₄ remained low during 2020-21 (1.1–1.9 t CO₂e ha^-1 yr^-1), while N₂O emissions were relatively high (4.0-5.7 t CO₂e ha^-1 yr^-1) without any treatment effects. In 2021-22, CH₄ emissions increased to 2.6-3.7 t CO₂e ha^-1 yr^-1 equivalent to 11.3 % of the total carbon emission in CO₂ equivalents. Although the peat field seemed uniform, large variation within treatments was seen across the experimental blocks which could be linked to differences in soil nutrient concentrations and water chemistry. Overall, it can be concluded that paludiculture and the non-managed restoration scenario exhibited comparable climate outcomes thereby offering flexibility in land-use options for peatland restoration. However, results also suggested that biomass harvest can reduce GHG emissions in the more productive area, while leaving the biomass unmanaged was advantageous in the less productive area of the field.