The future of carbon storage in calcareous fens depends on the balance between groundwater discharge and air temperature

• Published in: Catena (Giessen) 2023-10, Vol.231, p.107350, Article 107350
• Main Authors: Singh, Patrícia, Jiroušek, Martin, Hájková, Petra, Horsák, Michal, Hájek, Michal
• Format: Article
• Language: English
• Subjects: Air temperature; Carbon; Carbonate precipitation; Climate change; Decomposition; Hydrology; Soil nutrients; Soil temperature; Tea Bag Index
• ISSN: 0341-8162
• DOI: 10.1016/j.catena.2023.107350

•Tea bag method can track the contrasting carbon accumulation processes in wetlands.•Lowering the water table increases soil nitrogen, phosphorus, and potassium.•High water table and air temperature speed up carbonate accumulation.•Low water table and high soil phosphorus accelerates rooibos decomposition. Calcareous spring fens accumulate carbon-rich deposits through carbonate precipitation and slow organic-matter decomposition, which can be affected by a lowering water table. Ongoing climate change is altering the carbon balance and threatening the biota of these vulnerable ecosystems. Rising air temperatures intensify carbonate precipitation and may accelerate decomposition, which is also influenced by soil nutrients and soil temperature. These relationships complicate predictions of carbon storage in calcareous fens. Here, we measured summer mass loss and carbonate accumulation at 57 spots in 19 calcareous spring fens in the Western Carpathians using commercial green tea and rooibos, i.e., the tea bag method. Decomposition rates were determined by mass losses corrected for leaching. Structural equation modelling was used to test the causal relationships between air and soil temperature, water table, soil nutrient concentrations, and mass loss or carbonate accumulation. The results demonstrate that a lowering water table increases soil nitrogen, phosphorus, and potassium concentrations. Water table and air temperature positively affected carbonate accumulation for both types of tea bags. Rooibos decomposition rate decreased with increasing water table and decreasing soil phosphorus concentration. Overall, the role of hydrology appeared crucial for global change predictions. If increased precipitation or groundwater recharge keeps the water table high, as predicted for some areas, the rising temperature will intensify carbonate precipitation and shift the ecosystem from peat to tufa-forming. While this scenario is more conducive to maintaining biodiversity and sustainability of existing carbon sinks than an alternative scenario predicting decreasing discharge due to decreasing precipitation and increasing evapotranspiration, it depends on preserving and maintaining fens and natural landscape hydrology. This study demonstrates the utility of the tea bag method to test drivers of contrasting carbon accumulation processes in groundwater-dependent wetlands.