## 2 Methodology
### CO\({}_{2}\) Flux Measurements and Flux Calculation  
The closed dynamic portable chamber system was used to manually measure carbon dioxide fluxes, following methodology described in [PERSON] et al. [16]. The system measures R\({}_{\text{eco}}\) and NEE by combining an opaque and a transparent chambers, including measurements of relative humidity and temperature within each chamber and PAR in the transparent one; detailed information can be found in [PERSON] et al. [15]. Both sites, CL and CR, utilize preinstalled 20 cm high white PVC soil frames with outside dimensions of 0.75 \(\times\) 0.75 m inserted 15 cm deep into the peat (three frames per each treatment per site). These frames enable a complete sealing of the chamber headspace, complemented by the elastic rubber installed along the bottom edge of each chamber's walls.  
Chamber campaigns were conducted every 3-5 weeks (overall, five campaigns were organized in 2020 and ten in 2021) under sunny and cloudless conditions from early morning until late afternoon. The measurement duration was 1.5 min for NEE and 2.5 min for R\({}_{\text{eco}}\), with these measurements taken sequentially with a maximum interval of one minute between them. Here, R\({}_{\text{eco}}\) flux rates are assumed to represent CO\({}_{2}\) emissions corresponding to the time of NEE measurements. Hence, GPP flux rates were calculated for each plot as a sum of consecutively measured NEE and R\({}_{\text{eco}}\) fluxes [17]. The CO\({}_{2}\) concentrations measured in the chamber headspace during the closure time were corrected for the water dilution effect [18] and were used to calculate the flux rates of NEE and Reco  
Figure 1: Location of Rzecin peatland as well as CR and CL sites with climate manipulation experiments.  
as described by [PERSON] et al. [7]. The flux of GPP is defined as negative in this research using the atmospheric sciences point of view, atmosphere is the reference point, hence GPP is a reduction in the CO\({}_{2}\), expressed as negative.  
The above approach enabled the measurements of NEE and R\({}_{\text{eco}}\) fluxes on the same plot with no more than a 2 h interval between successive measurements. However, due to the reduced sun hours, only three complete rounds of measurements were taken per site during winter, while summer conditions allowed five rounds per site. The chamber system was designed to operate manually, resulting in significant gaps between measurement campaigns and a limited number of replicates per campaign due to the need for physical presence and operation in the field. Consequently, a gap-filling procedure is required to estimate CO\({}_{2}\) fluxes with the highest possible temporal resolution during the periods between campaigns.  
As described in the following sub-section, multiple models have been used to estimate GPP values for each plot during the campaign dates based on the calculated plot-specific GPP rates and PAR measured during NEE measurements. Subsequently, the estimated model parameters were employed to calculate GPP based on the PAR measured with 30 min time steps; the sum of 24 h of half-hourly GPP fluxes cumulated over the campaign day was considered as the daily GPP. When fewer than four measurements of NEE and R\({}_{\text{eco}}\) fluxes per plot were available for a campaign, data from two consecutive campaigns were merged using a moving window (e.g., first + second campaign, second + third campaign). Then, the median date of the combined dataset was selected as the reference point for the estimated model parameters.