Non-growing season CO2 fluxes of a boreal fen – key drivers and responses to anomalous winter conditions

Abstract

Non-growing season carbon fluxes can contribute to a significant amount of the full-year ecosystem carbon balance. Year-round net ecosystem exchange (NEE) data from 17 years was used to analyse the non-growing season carbon dioxide (CO2) fluxes together with environmental and meteorological variables at Siikaneva fen in Southern Finland. Generally, the fen acted as a sink of CO2 with a mean annual carbon balance of -50.9 (± 39.4) g C m-2 . On average, the CO2 emissions during non-growing seasons offset 57% of the following growing seasons CO2 uptake. Two years from the 17-year dataset were a consistent net source of CO2, 2018 and 2016. In 2016, a strong respiration release during the winter-spring transition period turned the annual balance of CO2 to positive, resulting in the highest emission of CO2 during the entire study period. The period of anomalous respiration alone offset 38% of the following growing seasons CO2 uptake. Since the biological activity during non-growing season is low, the CO2 fluxes likely composed mainly of heterotrophic respiration, and thus were likely driven by soil temperature. The relative importance of air and soil temperature, vapor pressure deficit, water table depth, and photosynthetically active radiation (PAR) on NEE of CO2 was analyzed with a random forest algorithm. PAR and soil temperature were the most important drivers during non growing season, growing season and year-round periods. PAR had the highest importance during growing season while during non-growing seasons, PAR and soil temperature had an equal relative importance. A wavelet coherence analysis further revealed that the mean annual cycles of NEE and soil temperature were coherent during the summer months across the diurnal scale. For instance, NEE correlated positively (decreasing the net carbon sink) with soil temperature and lagged the response to temperature by 4-6 hours. There was no coherence between the annual cycles of soil temperature and NEE during winter. The results emerging from this thesis stress the importance of studying what controls the interannual variation in soil temperature and the possible accumulation of CO2 in soil during non-growing season, but also how they contribute to spring-time CO2 releases like the one observed in spring 2016.