Seasonal changes in photosynthesis, respiration and NEE of a reclaimed post-mining site at the early stage of succession

No investigations have addressed the CO₂ fluxes, including NEE (Net Ecosystem Exchange), GEE (Gross Ecosystem Exchange), and ER (Ecosystem Respiration), in extremely disturbed areas like post-mining sites. Despite its importance, there is a big gap in the existing research literature regarding these specific ecosystems. Therefore, to determine the carbon sequestration and emission potential of a restored early post-mining stage, this study was conducted in the northwestern Czech Republic during 2023.

To achieve this objective, continuous measurement of NEE was conducted using an Eddy Covariance (EC) tower from January to October 2023. The site was leveled in 2019 and planted with 1-yr-old alder (Alnus glutinosa) seedlings at a density of 10000 seedlings/ha^-1. The high-frequency raw data (10 Hz) was processed using Eddypro software (LICOR, Lincoln, NE, USA) to calculate 30-minute average fluxes of CO₂. More processing on the 30-minute data involved quality checks, despiking, u* filtering, gap filling, estimation of uncertainty in the gap-filled half-hourly data, and the partitioning of NEE into GEE and ER.

Four years after reclamation, ER (1.10 kg C m^-2) exceeded carbon assimilation (GEE = -0.85 kg C m^-2), however, the resulted NEE was near zero (0.25 kg C m^-2) for the entire 10-month period. The greatest losses of CO₂ into the atmosphere occurred during the summer (July – October), with an average daily value of 4.37 ± 1.19 g C m^-2. Meanwhile, spring with the highest rate of assimilation (April – June; average daily value of GEE -4.35 ± 0.39 g C m^-2), acted as a net sink of CO₂, with an average daily value of -0.24 ± 1.16 g C m^-2. This finding implies a high assimilation capacity of the restored post-mining site at the early stage of succession. Overall, based on our analysis, temperature and vapour pressure deficit (VPD) were identified as the main driving factor for NEE and ER, while global radiation (Rg) and temperature were found to be the most important factors for GEE.